The Effect of Mechanical Mold Vibration - Worcester Polytechnic

The Effect of Mechanical Mold Vibration On the Characteristics of Aluminum Alloys

by

Jayesh Deshpande

A Thesis Submitted to the Faculty

Of

Worcester Polytechnic Institute

In partial fulfillment of the requirements for the

Degree of Master of Science

In

Manufacturing Engineering

September 2006

APPROVED:

Makhlouf M. Makhlouf, Professor of Mechanical Engineering, Advisor Richard D. Sisson, Jr., Professor of Mechanical Engineering Director of Manufacturing and Materials Engineering

LIST OF FIGURES .................................................................................................................4

LIST OF TABLES ...................................................................................................................7

ABSTRACT ...........................................................................................................................9

ACKNOWLEDGEMENTS .....................................................................................................10

1. INTRODUCTION ..........................................................................................................12

2. OBJECTIVE.................................................................................................................15

3. BACKGROUND ...........................................................................................................16

3.1. VIBRATIONS ...........................................................................................................16

Ultrasonic Vibrations .....................................................................................................19

Electro-magnetic vibrations ...........................................................................................21

Mechanical Vibrations ...................................................................................................23

Definitions and Parameters ..........................................................................................................................23

The Use of Mechanical Vibrations in Casting............................................................................................25

3.2. CASTING CHARACTERISTICS OF AL-S I AND AL-CU ALLOYS ............................................29

Aluminum-Silicon Alloys ................................................................................................30

Hypoeutectic Alloys ........................................................................................................................................31

Hypereutectic Alloys ......................................................................................................................................33

Aluminum-Copper Alloys ...............................................................................................36

Alloy B206........................................................................................................................................................37

Dendrite Coherency......................................................................................................39

Determination of the Dendrite Coherency Point........................................................................................41

Hot Tearing Tendency of Aluminum Casting Alloys .........................................................44

Measurement of Hot Tearing Tendencies ..................................................................................................48

4. DESIGN OF EXPERIMENTS, MATERIALS AND PROCEDURES ...................................50

4.1. DESIGN OF EXPERIMENTS ........................................................................................50

4.2. MATERIALS............................................................................................................52

4.3. VIBRATION TABLE SETUP ..........................................................................................55

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4.4. HOT TEARING TENDENCY .........................................................................................58

Modified Crack Susceptibility Criteria .............................................................................58

The Ring Mold Test.......................................................................................................59

4.5. MICROSTRUCTURE ANALYSIS ...................................................................................61

5. RESULTS AND DISCUSSION ......................................................................................64

5.1. B206 ALLOY ..........................................................................................................64

Thermal Analysis ..........................................................................................................64

Microstructure Analysis .................................................................................................72

Ring Mold Casting ........................................................................................................80

5.2. AL-7WT%S I ALLOY .................................................................................................84



5.3 B390 ALLOY ..........................................................................................................97



6. CONCLUSIONS ............................................................................................................. 108

REFERENCES ................................................................................................................... 110

4

List of Figures

Figure 3-1 Eutectic Si Morphology (a) without ultrasonic vibration and (b) with

ultrasonic vibration. ........................................................................................................20

Figure 3-2 Effect of ultrasonic vibrations on microstructure of A356 alloy, without

(a) and with (b) ultrasonic vibrations. ..........................................................................21

Figure 3-3 Direction of vibrating force F developed by the interaction of the

alternating electric field J and the stationary magnetic field B ................................22

Figure 3-4 – Relation between amplitude and frequency. .......................................24

Figure 3-5 Aluminum rich portion of the Al-Cu phase diagram...............................37

Figure 3-6 Microstructure of B206 alloys (a) dendrites and grains (b) Al7FeCu2

needle and Al2Cu............................................................................................................39

Figure 3-7 Feeding Mechanisms (a) Mass Feeding (b) Inter-dendritic feeding ...40

Figure 3-8 The Two Thermocouple method (a) Apparatus (b) Typical output .....42

Figure 3-9 (a) Experimental setup and (b) typical output data of the rheological

determination method....................................................................................................43

Figure 3-10 (a) Schematic of the direct shear cell (b) Typical output data. ..........44

Figure 3-11 Equilibrium distribution of liquid at grain boundaries. .........................45

Figure 3-12 Typical design of ring mold used for hot tear testing..........................49

Figure 4-1 range of vibration parameters ...................................................................52

Figure 4-2 Vibration table setup (a) schematic (b) photographs ............................56

Figure 4-3 Schematic of the thermal analysis setup for (a) B206, Al-7%Si (b)

B390 .................................................................................................................................58

Figure 4-4 Ring mold used for hot tear testing ..........................................................60

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Figure 4-5 Locations of the samples used for microsturucture analysis (a) B390

...........................................................................................................................................63

Figure 5-1 Thermal profile of the B206 with no vibrations (Sample#1) .................65

Figure 5-2 Thermal profile of B206 with mold vibrations at 1.5g (Sample #2) .....65

Figure 5-3 Thermal profile of B206 with mold vibrations at 2g (Sample #3).........66

Figure 5-4 Thermal profile of B206 with mold vibrations at 3g (Sample #4).........66

Figure 5-5 Comparison between thermal profile at center of the castings ...........67

Figure 5-6 Comparison between thermal profile at center of the castings ...........68

Figure 5-7 Variation in TEdge-TCenter with the chance in vibration intensity ...........68

Figure 5-8 Determination of dendrite coherency point.............................................69

Figure 5-9 Temperature Vs Fraction of solid curve for B206 alloy.........................70

Figure 5-10 As-cast grain structure of B206 alloy with no vibrations

(i),(ii)Horizontal section (iii) vertical section................................................................72

Figure 5-11 As Cast grain structure of B206 with vibration at 1.5g (i),(ii)

Horizontal section (iii) Vertical section ........................................................................73

Figure 5-12 As Cast grain structure of B206 with vibration at 2g (i),(ii) Horizontal

section (iii) Vertical section ...........................................................................................74

Figure 5-13 As Cast grain structure of B206 with vibration at 3g (i),(ii) Horizontal

section (iii) Vertical section ...........................................................................................75

Figure 5-14 Effect of mechanical vibrations on As-cast (i) grain size (ii) Dmax and

(iii) compactness of B206 alloy ....................................................................................77

Figure 5-15 Effect of vibrations on CSCb of B206 alloy ...........................................80

Figure 5-16 Ring casting of B206 without vibrations ................................................81

Figure 5-17 Ring casting of B206 with vibrations (a) photograph (b) X-ray..........81

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Figure 5-18 Thermal profile of Al-7%Si alloy with no mold vibrations ...................85

Figure 5-19 Thermal profile of B206 alloy cast with mold vibrations at 2g ...........85

Figure 5-20 Thermal profile of B206 alloy with mold vibrations at 3g ....................86

Figure 5-21 Comparison between the thermal profiles at the center of the

castings ............................................................................................................................86

Figure 5-22 Comparison between ?T in vibrated and un-vibrated castings .........87

Figure 5-23 Temperature Vs fraction of solid for Al-7%Si .......................................88

Figure 5-24 Effect of vibrations on dendrite coherency point of Al-7%Si alloy ...89

Figure 5-25 Grain structure of Al-7%Si alloy solidified under no vibrations ..........89

Figure 5-26 Grain structure of Al-7%Si alloy solidified under vibrations at 2g .....90

Figure 5-27 Grain structure of Al-7%Si alloy solidified under vibrations at 2g .....90

Figure 5-28 Effect of mechanical vibrations on (i) Grain size (ii) Dmax and (iii)

compactness of al-7%Si alloy ......................................................................................92

Figure 5-29 Al-7%Si alloy solidified under vibration at 3g with mold temperature

(i) room temperature (ii) 175oC ....................................................................................93

Figure 5-30 Eutectic silicon in un-vibrated Al-7%Si sample ....................................94

Figure 5-31 Eutectic silicon in Al-7%Si sample vibrated at 2g................................94

Figure 5-32 Eutectic silicon in Al-7%Si sample vibrated at 3g................................95

Figure 5-33 SEM image of eutectic silicon particles in Al-7%Si vibrated at (i) 2g

(ii) 3g.................................................................................................................................95

Figure 5-34 Thermal profile of B390 alloy solidified without vibrations .................98

Figure 5-35 Thermal profile of B390 alloy cast while the mold is vibrated at 1.5g

...........................................................................................................................................98

7

Figure 5-36 Temperature difference between the center and edge thermocouples

in vibrated and un-vibrated B390 alloy .......................................................................99

Figure 5-37 Particle size and distribution of primary Si in B390 alloy: (a) without

vibrations, and (b) with vibrations at 1.5g (plane 1). ...............................................102


vibrations, and (b) with vibrations at 1.5g (plane 2) ................................................104

Figure 5-39 Layered growth of primary silicon particles (a) traces of layered

growth (b) different stages of growth53......................................................................106

Figure 5-40 Change in eutectic reaction time due to vibrations. ..........................107

List of Tables

Table 3-1 Literature survey on the use of vibrations on solidifying metals ...........17

Table 3-2 Typical composition of hypereutectic 390 alloy.......................................33

Table 3-3 Reactions during solidification of B390 alloy ..........................................34

Table 3-4 Nominal Composition of Alloy B206..........................................................38

Table 3-5 Sequence of reactions during solidification of B206 alloy .....................38

Table 4-1 Design of Experiments ................................................................................50

Table 4-2 Experiments’ matrix .....................................................................................51

Table 4-3 Composition of B390 alloy..........................................................................53

Table 4-4 Composition of Al-7%Si alloy.....................................................................54

Table 4-5 Composition of B206 alloy..........................................................................54

Table 4-6 Specification of mechanical shaker...........................................................55

Table 5-1 Pouring temperature, vibration intensity and superheats for each

specimen .........................................................................................................................64

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Table 5-2 Dendrite coherency point and corresponding fraction of solid for B206

alloy vibrated at different intensities ............................................................................70

Table 5-3 As-cast grain structure analysis of B206 alloy with different vibration

intensities.........................................................................................................................76

Table 5-4 Effect of mechanical vibration CSCb of B206 alloy .................................79

Table 5-5 Casting parameters for ring mold casting experiments..........................80

Table 5-6 Pouring temperature, vibration intensity and superheats for each

specimen .........................................................................................................................84

Table 5-7 Dendrite coherency point and corresponding fraction solid for Al-

7wt%Si alloy vibrated at different intensities..............................................................88

Table 5-8 Grain size, Dmax and Compactness of Al-7%Si alloy..............................91

Table 5-9 Experimental parameters for thermal analysis of B390 alloy................97

Table 5-10 Mean primary silicon particle size and count in un-vibrated and

vibrated samples ..........................................................................................................100

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Abstract

Aluminum-Silicon and Aluminum-Copper alloys are important non-ferrous casting

alloys. Different methods have been applied to improve their casting

characteristics, their microstructure and consequently, their mechanical

properties. Application of mechanical vibrations to the mold during solidification

of the alloy is one of these methods. In this study, the effect of controlled

mechanical vibrations on the dendrite coherency point, the hot tearing tendency,

and the microstructure of B206, B390, and binary Al-7%Si alloys was evaluated.

The dendrite coherency point was determined using the two-thermocouple

method. The hot tearing tendency was evaluated using the crack susceptibility

criterion (CSCb) and by means of measurements using a specially designed ring

mold. Microstructure characterization was performed using optical and scanning

electron microscopy coupled with image analysis. It was found that mechanical

vibrations refine the microstructure of the alloys; and, in the case of B390 alloy, it

resulted in significant improvement in the distribution of the primary silicon

particles. In the case of B206 and Al-7%Si alloys, where aluminum is the primary

phase, mechanical vibrations caused the dendrite coherency point to shift

towards lower temperature, i.e., towards higher fraction solid. This shift, together

with the refinement of the grain structure, manifested itself in significant reduction

in the incidence of hot tearing in B206 castings.

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Acknowledgements

I would like to acknowledge assistant, support and guidance I received from

these great people.

I would like to acknowledge my advisor Dr. M. M. Makhlouf for his guidance,

support and patience during this research. His help has been important not only

to my completion of the degree but also to my development as an engineer. I

have and will always feel deeply indebted for his mentorship.

I express my profound gratitude to Dr. D. Apelian, director, MPI for his

encouragement and guidance. I could not have asked for a better treasure than

to have the privilege to be influenced by these two fine men throughout my

experience at WPI.

I would also like to thank Dr. R. D. Sisson Jr., director of Materials and

Manufacturing engineering program for his mentorship.

I am also indebted to research staff at MPI who generously gave their time and

expertise to assist me during this work. To mention that Dr. Sujoy Chaudhury has

been extremely helpful to me would be an egregious understatement. I don’t

remember a single day without a knock on his office door with a query. I truly

appreciate his patience in explaining me myriad of queries (sometimes really

stupid!), on solidification, aluminum alloys and almost everything related to

metallurgy. Thanks are also extended to Dr. Libo Wang for his help in the casting

part of this work, Mr. Matthew Diehm of Consolidated Metco and Michael

O’Donnell of Washburn Machine Shops for their help in fabrication of the ring

mold.

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I would like to thank members of ACRC consortium for their guidance and

financial support for this work. I acknowledge Karl Raatikainen for all the help

and co-operation he extended during my thesis work.

I would like to thank Virendra and other fellow students for all the good time we

spent in and out of the Washburn and for the help getting home from some of

those good times.

I would like to thank my father Umesh for introducing me to the beauty of metal

casting, my mother Madhavi for her constant encouragement and support

throughout my life. I wish my mother could have seen this, but such was not to

be. Instead, her memories alone must suffice and I take comfort in the fact that

she is constantly watching me and her blessings will be with me forever.

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1. Introduction

In 1857, the noted British author Charles Dickens, known for his social

sensibilities wrote, “Within the course of the last two years ... a treasure has

been divined, unearthed and brought to light ... what do you think of a metal as

white as silver, as unalterable as gold, as easily melted as copper, as tough as

iron, which is malleable, ductile, and with the singular quality of being lighter than

glass? Such a metal does exist in considerable quantities on the surface of the

globe.” “The advantages to be derived from a metal endowed with such qualities

are easy to be understood. Its future place as a raw material in all sorts of

industrial applications is undoubted, and we may expect soon to see it, in some

shape or other, in the hands of the civilized world at large.” Dickens’ forecast was

indeed a proven truth as aluminum has found application virtually in every

market.

Being a light metal, aluminum has been instrumental in developing lightweight

fuel-efficient transportation systems. In North America, new automobiles have on

an average more than 300lbs of aluminum. More than 40 automobiles carry more

than 500lbs of aluminum in the form of various structural and transmission

components. Automobiles back in 1973 contained less than a quarter by weight

of the aluminum than what today’s automobiles contain1. As Aluminum’s

cost/benefit ratio continues to improve, no doubt aluminum will continue to play a

pivotal role in the development of safer, cleaner and lighter automobiles.

High performance applications, such as the automobile, call for stronger, more

formable aluminum alloys. In order to achieve better performance from an alloy,

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manufacturers may alter the alloy composition or the manufacturing process. The

alterations can be done by means of addition of alloying elements, by means of

changes to the component design, or by means of changing the processing

parameters.

In the present work, the effect of mold vibrations on the characteristics of three

aluminum alloys; namely B206, B390, and Al-7wt%Si is evaluated. B206 is an Al-

Cu alloy characterized by its high strength. It is typically used in high strength

structural components. B390 alloy is a hypereutectic Al-Si alloy characterized by

high wear resistance. It is typically used in high wear resistance applications

such as in ring gears, engine blocks, pistons etc. Al-7wt%Si is a hypoeutectic

binary alloy. Composition wise, it is related to A356. A356 has many automotive

applications including cylinder heads and intake manifolds2.

Experimentation with mold vibration in order to alter the as-cast microstructure of

cast components date back to 1868. In one of the earlier investigations, Chernov

found that application of mechanical vibration during solidification of steel caused

refinement of austenite 3. More recent investigations by Abu-Dheir et al 4 shows

an effect of mechanical vibrations on the morphology of silicon in Al-Si alloys,

which manifests itself in significant enhancement of mechanical properties. Also

recent work by Dommaschk 5 showed that a refined grain structure of Al-Si alloys

could be obtained by mold vibration.

In addition to the Introduction (Chapter 1), this thesis contains 5 chapters, which

are:

Chapter 2: Objective, which clearly lists the objectives of the work.

14

Chapter 3: Background, which presents a review of the open literature on the

application of mold vibrations to solidifying melts and the effects of vibrations on

the casting characteristics of aluminum alloys.

Chapter 4: Design of Experiments and Procedures, which details the design

of experiments, and the materials and procedures used in the various

experiments.

Chapter 5: Results and Discussion, which presents and discusses the results

obtained, and correlates them with the understanding available from the

literature.

Chapter 6: Conclusions, which summaries the findings and recommends

further work.

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2. Objective

The objective of this thesis is to investigate the effect of mechanical mold

vibration on the following:

1. The casting characteristics of casting alloys, including:

1.1. The tendency of the alloy to hot tear during solidification as evaluated

from

1.1.1. The Crack Susceptibility Criterion (CSCb)

1.1.2. Hot tear observations using a ring mold

1.2. The dendrite coherency temperature

2. The as-cast microstructure of the alloy, including:

2.1. The morphology of silicon particles including eutectic Si particles, as well

as primary Si particles (in the case of hypereutectic alloys)

2.2. The as-cast grain size

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3. Background

In this section, a review of vibration melt treatment and characteristics of

aluminum alloys is presented. Section 3.1 discusses the fundamentals of

vibrations, the parameters associated with vibrations, and the methods available

for application of vibrations to metallic melts. Section 3.2 reviews previous work

on the application of vibrations to melts Section 3.3 is devoted to discussing the

relevant casting characteristics of Al alloys particularly, Al-7wt%Si, B390 and

B206 alloys.

3.1. Vibrations

In general, vibration is the motion of the particles of an elastic body or medium in

alternate ly opposite directions from the position of equilibrium, periodically in

time. Pillai et. al. have published an extensive survey of the different methods of

vibrations used on solidifying metals and their effect on the final structure6. Table

3.1 presents a summary of their survey.

17

Table 3-1 Literature survey on the use of vibrations on solidifying metals6.

Aluminum Alloy Source of Vibration Effect of vibrations Al Ultrasonic Degassing Pure Al Rectilinear vibrations by

transforming rotary motion of a DC motor, 100 cycles/min (2Hz)

Grain refinement Reduction in pipe formation Reduced solidification time

Eutectic Al-Si alloy Low frequency vibration Coarsening of Mg and Na modified alloy Refinement of Sr modified eutectic Si

Al-Si alloys, AK9(hypo) and AK11 (eutectic)

Electromagnetic stirring (EM) during continuous casting

Lower porosity by the factor of 3 Higher UTS and % elongation Modified eutectic only inh EM mold without modifier addition

Al–20Si

Low frequency melt agitation Significant reduction in gas content

Hypoeutectic Al7Si hypereutectic Al20Si

Electromagnetic stirring Reduction in Si segregation in hypereutectic alloy Promotion of dendrite fragmentation in hypoeutectic alloy Reduced axial porosity and hence sound core of the ingot

Al–Ti, Al alloy , Al–Si alloy, Al–Cu alloy

Vigorous agitation of mould Vibration to the dies

Formation of fine grains Grain refinement Improved surface finish Reproduction of thin sections Dispersion of porosity and oxides Coarsening of secondary precipitating phases Segregation due to constitutional under cooling not prevented

Light alloy (500 kg of Al)

Ultrasonic vibration Improves degasification Reduced degassing time of a few minutes with scavenging cum vibration against 30 min with scavenging alone Suppression of pipe Reduction in hot tearing tendency Reduction in porosity and shrinkage concentration in a spot

18

Reduction in gravity segregation of FeAl3

Al–8.5Si–1.75Cu–0.35Mg– 0.4Mn–0.55Fe

Ultrasonic at 990K Increased hardness with decreased fluctuations in different parts of die cast parts Formation of homogeneous fine structures of _ and _+Si phases Enhancement in density 0.001–0.01 g/cm3

Al alloys Magnetic field/passing AC through the melt

Grain refinement

Continuous cast Al–Zn–Mg–Cu ingot

Ultrasonic Grain refinement Decreased hydrogen especially with low Fe content alloys

AK9 Al casting

Ultrasonic 20 KHz Ti alloy emitter immersed inside the melt at 1013K

Maximum enhancement in mechanical properties after 10 min of treatment UTS increases from 164 to 181Mpa % Elongation increases from 2.8 to 3.1 Reduction in H2 content and size of oxide inclusions by a factor ~1.5 and ~3

Al–5Mg

Electromagnetic field Reduced diffusion layer thickness in front of the solidification front Increased concentration of alloying elements in solid solution Decreased volume of non equilibrium eutectics Refinement of dendritic microstructure and fine distribution of non equilibrium eutectic phases

Al–6/11/15 Cu Mechanical vibration Refinement and uniform distribution of primary Al Primary grains contact/connect each other forming a complicated inter twist morphology Improvement in tensile strength with increasing amplitude of Vibration

19

Al alloy

Combined effect of internal variable magnetic field and passing of AC through the molten metal

Refinement of structure Dispersion of inclusions Elimination of modifier addition

Al alloy with Zr addition

Ultrasonic treatment during continuous casting

Formation of sub dendritic grains (≈0.1 mm) against coarser grains (0.8–1.5 mm) without treatment leading to enhanced plasticity

Al–5Mg Al.4.5% Cu

Electromagnetic generator at 30 and 150 Hz, 0.05–5.52mm amplitude, 1–120 g peak acceleration

Extensive grain refinement especially with high accelerations

,Al–4.5Cu

Rectilinear vibration by transforming rotary motion of a DC motor, 100 cycles/min (2 Hz)

Elimination of most of the shrinkage Grain refinement occurs but similar to pure Al, rapid initially and slower later Reduced solidification time Si and Fe phases become less acicular

Al12.3Si

Varying frequencies of 15–41.7 Hz and amplitude of 0.125 –0.5mm With increasing vibration time

Increasing frequency and amplitude resulted in grain refinement and reduced pipe Coarsening of eutectic silicon in unmodified and sodium modified Coarsening of primary silicon

Forced vibrations can be applied to a melt via a variety of methods including

ultrasonic, electromagnetic, and mechanical methods.

Ultrasonic Vibrations

A number of researchers haves used ultrasonic vibrations for melt treatment.

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Eskin summarized the effect of ultrasonic treatment on light alloys 7,8. Works of

various researchers demonstrate that ultrasonic vibrations can be used for

cavitation, melt degassing, fine filtration of melts (the USFIRALS process), non-

dendritic solidification, improved semi-solid deformation, spatial solidification and

for the production of aluminum alloys with low-solubility components7.

Xu et al9 found that ultrasonic treatment is an effective method for degassing

aluminum melts. Also Jian, et al found that ultrasonic vibrations could be used to

refine eutectic silicon in hypoeutectic Al-Si alloys 10. Figure 3-1 shows the effect of

ultrasonic vibrations on the morphology of eutectic Si. Figure 3 -2 shows the

effect of ultrasonic vibrations on the grain structure of A356 alloy.

(a) (b)

Figure 3-1 Eutectic Si Morphology (a) without ultrasonic vibration and (b) with ultrasonic

vibration10.

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(a) (b)

Figure 3-2 Effect of ultrasonic vibrations on microstructure of A356 alloy, without (a) and

with (b) ultrasonic vibrations11.

Although ultrasonic vibrations have shown favorable effects on the solidification

characteristics of aluminum alloys, its commercial applications are constrained

mainly because of the difficulties encountered in the use of ultrasonic instruments

on the foundry floor11.

Electro-magnetic vibrations

As the name suggests, electro-magnetic vibrations typically involve two different

force fields, a stationary magnetic field and an alternating electric field. If a

stationary magnetic field with a magnetic flux density B and an alternating

electrical field with a frequency f and current density J is applied to a melt, a

vibrating electromagnetic body force with a density F = J X B is induced inside

the melt. This force sets the particles inside the melt into vibration motion with a

frequency equal to the frequency of the alternating electrical field, vibrating

perpendicular to the plane of J and B12. Another electro-magnetic force is formed

inside the melt due to the applied magnetic force and the induced force, This

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force is partly rotational and stirs the melt12. Figure 3-3 illustrates the relationship

between these forces.

Figure 3-3 Direction of vibrating force F developed by the interaction of the alternating

electric field J and the stationary magnetic field B12.

Zong13 reported that low frequency electromagnetic vibrations could be used to

grain refine, to eliminate micro segregation, and to avoid cracks and improve the

as-cast surface quality of alloys. Yoon et al 14 found that electromagnetic

vibrations reduce the grain size of primary silicon. They attributed this

phenomenon to the collision of primary Si particles with one another. Mizuki et

al15 imposed electro-magnetic vibrations on an Al-7wt%Si alloy and found that

with increasing the intensity of the vibrations, the primary a-Al dendrites

approached a globular shape of about 25 µm in size. Mizuki et al also found that

in Al-17wt%Si, the primary Si particles were refined to 5 µm at a frequency

nearing 1 kHz. The level of refinement increased with the frequency of

vibration16. They attributed this phenomenon to collapsed dendrite arms due to

micro-explosions and stirring in the melt. Various researchers17,18,19 reported

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refined and uniform grain structure, refined primary and eutectic Si, and improved

surface quality of castings due to electromagnetic vibrations.

Mechanical Vibrations

In this method, the entire mold is set into vibration by means of a vibration

source. Although the use of mechanical vibrations allows limited degrees of

freedom to the operator, it is the most promising method of applying vibrations to

solidifying melts due to its simplicity and the ruggedness of the equipment

needed for inducing vibrations.

Definitions and Parameters

Vibration - A periodic motion of the particles of an elastic body or medium in

alternately opposite directions from the position of equilibrium when that

equilibrium has been disturbed.

Amplitude - The severity of the vibration. Amplitude can be represented in

several forms:

– Peak-to-peak

– Zero-to-zero

– Average Value

– Root Mean Square Value

For the purposes of this work, all values of amplitude are represented in the form

of Root Mean Square Value (RMS).

Frequency - The number of cycles that a system will perform in a unit time. It is

usually measured in Hertz (Hz).

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Acceleration - The rate of change of velocity with time (given as dv/dt or d2x/dt2).

For the purposes of this work, acceleration is given in units of gravity. Equation 1

relates acceleration to vibration amplitude and vibration frequency20.

20511.0 DfG ×= (1)

Where: G is the acceleration in units of gravity (1g= 32.3 ft/s2), D is the

displacement or double amplitude (inches), and f is the frequency of vibrations

(cycles/sec or Hz).

Figure 3-4 is a plot of Equation 1 and shows the relationship between amplitude

and frequency.

Figure 3-4 – Relation between amplitude and frequency.

25

The Use of Mechanical Vibrations in Casting

Sokoloff4 was probably the first to report on the use of mechanical vibrations for

grain refinement. Campbell21 reported that mechanical vibration causes

improvement in mechanical and corrosion properties of alloys. Mechanical

vibrations have also been linked to the reduction or complete removal of the

tendency for pipe formation in ingots of pure metals 22. Figure 3.6 shows

fragmentation of the dendrites5 caused by mechanical vibrations during

solidification of NH4Cl-H2O.

Figure 3-5 Dendrite fragmentation while solidification of NH4Cl-H2O under vibrations [5].

Dommaschk et al studied the effect of vibrations on pure Aluminum,

Al7wt%SiMg, and Al12%wtSi alloys along with other non-ferrous alloys. Their

research focused on the effect of mechanical vibrations on grain refinement and

mechanical properties. They observed that the cooling rate and the degree of

grain refinement increase with the intensity of vibrations, and the grain size

becomes more homogenous. The effect of mechanical vibrations on the

solidification behavior of pure Aluminum is shown in Figure 3-6.

No vibrations With vibrations

26

Dommaschk et al also reported that the dependence of the castings’ wall

thickness on casting characteristics could be minimized with the use of

mechanical vibrations 23, 5.

Figure 3-6 Effect of mechanical vibrations on the cooling curve of pure aluminum23.

Pillai et al used very low frequency vibrations (100 and 200 cycles per minute) to

study its effect on A356 and Al12Si alloy. They concluded that mechanical

vibrations improve the density, hardness, UTS, and elongation of the cast

components. They attributed these improvements to the enhanced coagulation of

hydrogen bubbles and their escape from the melt brought about by vibrating the

mold. Thus porosity was reduced and wetting of the mold walls by the melt was

enhanced, this in turn promoted faster heat transfer and fragmentation of the

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solids formed on the mold wall [6]. However the method that Pillai et al used for

generating the low frequency vibrations (hand tapping and mold tilting) is highly

impractical in a produc tion foundry environment. Kokatepe et al applied

vibrations of 15 to 41.7 Hz frequency and 0.125 to 0.5 mm amplitude to Al12.3Si

alloy ingots poured in a graphite mold3. They found that at 41.7 Hz, the

solidification time of the casting was reduced by 24%, pipe volume was reduced

by 55%, and grain size was reduced by 52% as compared to the un-vibrated

casting. See Figure 3 -7.

(a) (b)

(c)

Figure 3-7 Effect of mechanical vibrations on (a) pipe volume, (b) pipe depth, and (c)

grain size of Al12.3Si ingot casting3.

28

But Kokatepe et al also reported that the vibrations caused coarsening of the

eutectic silicon due to an increase in diffusivity of silicon in the liquid caused by

the vibrations. Kokatepe et al attributed the observed grain refinement to mainly

the fragmentation of dendrites and the growing crystallites during the early

stages of solidification.

Abu Dheir et al used an electromagnetic shaker to induce mechanical vibrations

in a permanent mold 24, 4. They vibrated the mold at frequencies ranging from

100Hz to 2 kHz and amplitudes ranging from 3.73µm to 199µm, and recorded

the thermal history at different points in the mold. Their observation with AA356

alloy reveals that vibration homogenizes the temperature distribution in the mold

and promotes a faster cooling rate. This manifested itself in a more uniform

dendrite structure and less porosity in the castings. Abu Dheir et al observed

fragmentation of the dendritic structure in Al12.5Si. They found that the degree of

fragmentation increased with the amplitude of vibration. They also reported that

the eutectic structure transformed from the typical flaky structure to a more

fibrous structure with increasing amplitude up to 149µm (See Figure 3-8).

Beyond 149µm, the fibrous eutectic silicon agglomerated to form a structure of

coarse flakes. Abu Dheir et al also reported that certain mechanical properties

were affected by the vibrations including a 19 to 68 in percent increase in

elongation and a slight increase (3%) in UTS.

29

(a)

(b)

Figure 3-8 Morphology of Eutectic Silicon (a) without vibrations (b) with vibrations at a

frequency 100Hz and amplitude 149µm 4.

3.2. Casting Characteristics of Al-Si and Al-Cu Alloys

Al-Si and Al-Cu alloys constitute a large portion of the commercial foundry

aluminum alloys. This Chapter is a review of the casting characteristics of these

alloys.

30

Aluminum-Silicon Alloys

Aluminum-Silicon alloys are by far the most important commercial casting alloys

mainly due to their superior casting characteristics as compared with other alloys.

Al-Si alloys have excellent castability, machinability, and corrosion resistance. As

shown in Figure 3 -9 aluminum casting alloys can be classified based on their

Silicon content into three groups: hypoeutectic alloys, in which the Silicon content

is between 5 and 10%; eutectic alloys; in which the Silicon content is between 11

and 13%; and hypereutectic alloys; in which the Silicon content is between 14

and 20%25. Al-Si alloys can be cast using various processes including high

pressure die casting, permanent mold casting, sand casting, lost foam casting,

etc. Aluminum and silicon form a simple eutectic system with limited solid

solutions.

Figure 3-9 The Aluminum rich portion of the Al-Si phase diagram.

31

The eutectic temperature and composition are respectively 577oC and 12.6wt%

Silicon. At the eutectic temperature, the aluminum and silicon solid solutions

contain 1.65%wt Silicon and 0.17wt% Aluminum26.

Hypoeutectic Alloys

Hypoeutectic Al-Si alloys are characterized by good castability, corrosion

resistance and pressure tightness. Binary Al-Si alloys show some response to

heat treatment. The aluminum phase can be moderately supersaturated with

silicon by rapid cooling. Much greater strengthening can be done by the addition

of Copper, Magnesium or Nickel. Addition of Copper imparts high strength and

improved machinabiity but reduced ductility and corrosion resistance27.

Binary Al-7%wt Si Alloy

Binary hypoeutectic Aluminum-Silicon alloys have good castability and corrosion

resistance 2. If the Iron content is kept at a minimum, they also show good

ductility.

Microstructure of Al-7wt%Si Alloy

The microstructure of binary Al-7wt%Si alloy consists of a-Aluminum dendrites

and eutectic Silicon particles. Due to limited solid solubility, silicon prominently

occurs as elemental particles in the eutectic. The microstructure greatly depends

on the solidification rate and the presence of modifying elements such as

Titanium and Stontium. A low solidification rate produces large flakes of silicon,

large dendritic cells and large dendrite arm spacing, while a high solidification

rate produces small dendritic cells, and small dendrite arm spacing, and the

32

eutectic Silicon assumes a fibrous morphology. Figure 3-10 and Figure 3 -11

show representative microstructures of unmodified and modified Al-7%wtSi alloy.

Figure 3-10 a-Aluminum particles in (a) unmodified (b) modified hypoeutectic Al-Si

alloy28

Figure 3-11 Eutectic Si morphology in (a) unmodified and modified with (b) 47 ppm Sr

(c) 156 ppm Sr (d) 720 ppm Sr 29.

33

Hypereutectic Alloys

Hypereutectic Al-Si alloys are characterized by good fluidity, excellent wear

resistance, and low thermal expansion. Addition of Copper and Magnesium to

these alloys makes them heat-treatable and enhance their high temperature

properties.

B390 Alloy

The chemical composition of 390 alloy is shown in Table 3-2.

Table 3-2 Typical composition of hypereutectic 390 alloy

Solidification Reactions in 390 Alloy

During solidification of 390 alloy, primary Silicon forms first. As solidification

progresses, Aluminum dendrites develop in the melt. The presence of Iron in the

alloy promotes the formation of the Al5FeSi phase. Manganese in the alloy

suppresses the formation of this brittle phase and forms Al15(Mn,Fe)3Si2 instead.

This phase is often termed “Chinese script” and has a complex morphology.

Table 3-3 shows the sequence of reactions during solidification of 390 alloy.

Si Mg Mn Fe Cu Zn Al

16-18 0.15-0.65 0.5max 1max 4-5 1.4max Balance

34

Table 3-3 Reactions during solidification of B390 alloy 25.

# Reaction Suggested Temperature °C

1 Formation of primary Si 667.81

2 Development of dendrite network 557

3 Liquid → Al +Si+ Al5FeSi 575

4 Liquid → Al +Si+ Al15(Mn,Fe)3Si2 573

5 Liquid → Al +Si +Mg2Si 555

6 Liquid + Mg2Si → Al +Si+ Al2Cu + Al5Mg8Cu2Si6 512

7 Liquid → Al +Si+ Al2Cu + Al5Mg8Cu2Si6 507

Microstructure of 390 Alloy

The microstructure of 390 alloy consists of p rimary Silicon particles in a matrix of

Aluminum and Silicon eutectic. The morphology of the primary Silicon greatly

depends on the imposed temperature gradient, and the presence of nucleating

agents and impurities29. Several morphologies of primary Silicon have been

identified. The most common among them are star-shaped, polyhedral, plate-like,

and dendritic. See Figure 3-12.

The morphology of the eutectic silicon in hypereutectic Al-Si alloys is similar to

that in hypoeutectic Al-Si alloys. Al5FeSi, Al5Mg8Cu2Si6 phases, particles of

Al2Cu, and Al15(Mn,Fe)3Si2 can be seen in the eutectic area 25. See Figure 3-13.

35

(a) (b)

(c) (d)

Figure 3-12 Morphologies of primary silicon particles in hypereutectic Al-Si alloys (a)

polyhedral with multiple twin planes, (b) dendritic, (c) plate-like30, (d) star-shaped.

36

Figure 3-13 Morphology of the eutectic phases in hypereutectic Al-Si alloys.

Aluminum-Copper Alloys

Aluminum-Copper alloys have been associated with the aerospace industry since

its inception. In the very first flight in 1903, the Wright brothers used an Aluminum

8wt%Copper alloy for the crankcase of their engine 31. Copper is one the most

important alloying elements for Aluminum due to its relatively high solubility in

Aluminum and its strengthening effect. Copper, as a principle alloying element in

Aluminum, imparts substantial precipitation hardening characteristics and

excellent elevated temperature properties to the alloy. Often Copper is

accompanied by Magnesium in order to accelerate and increase age hardening

37

at room temperature 26. Often Manganese is added to Al-Cu alloys to mitigate the

detrimental effects of impurities such as Iron and Silicon 25.

The eutectic temperature in the Al-Cu system is 548°C and the eutectic

composition is 33.2wt% Cu26. The eutectic phases that separate from the liquid in

the solidification reaction are ?-CuAl2, which is an intermetallic phase containing

53.5wt% Cu, and the aluminum solid solution, which contains 5.65wt% Cu.

Figure 3-14 shows the Aluminum rich portion of the Al-Cu phase diagram.

33.2%

548oC

5.7%

Figure 3-5 Aluminum rich portion of the Al-Cu phase diagram32.

Alloy B206

Alloy B206 is an important Aluminum-Copper alloy with Copper content between

4.5% and 5.5%. Table 3-4 gives the nominal composition of B206 alloy.

38

Table 3-4 Nominal Composition of Alloy B206

Cu Mg Mn Fe Si Zn Al

4.2-5.0 0.15-0.35 0.2-0.5 0.15 max 0.054 0.014 Balance

Solidification Reactions and Microstructure of B206 Alloy

During the solidification of B206 alloy, a dendritic network of a-Al forms first. Over

the course of solidification, considerable thickening of these dendrites occurs. An

intermetallic phase with the chemical composition Al6 (MnFeCu) forms from the

liquid; later via a peritectic reaction it transforms to Al20Mn3Cu 2. Table 3-5 shows

the complete sequence of reactions during the solidification of B206 alloy.

Table 3-5 Sequence of reactions during solidification of B206 alloy25.

During the solidification of B206 alloy, a network of coherent dendrites forms at

about 10-15oC below the liquidus temperature and at about 0.30-0.35 fraction

solid. The remaining 0.65-0.70 fraction liquid has to solidify in the interdendritic

region over 170-180°C. Hence, castings made from B206 alloy are very

susceptible hot tearing25. Figure 3-6 shows the microstructure of B206 alloy, and

Section 3.2.4 reviews the concept of hot tearing and the tendency of Aluminum

alloys to hot tear.

# Reaction Suggested

Temperature

(°C)

1 Development of dendrite network 651 - 649

2 Liquid → Al + Al6(MnFeCu) 649

3 Liquid + Al6(MnFeCu) → Al + Al20Mn3 Cu 2 616

4 Liquid → Al +Al2Cu + Al20Mn3Cu2 + Al7FeCu2 537

5 Liquid → Al+Mg2Si + Al2Cu + Al2MgCu 512

39

(a)

(b)

Figure 3-6 Microstructure of B206 alloys (a) dendrites and grains (b) Al7FeCu2 needle

and Al2Cu25.

Dendrite Coherency

The dendrite coherency point (Tdc) is defined as a temperature during

solidification at which dendrites in the solidifying melt forms a coherent network.

The fraction of solid formed in the casting at this temperature is called the

40

coherency solid fraction (fsdc). During the initial stages of solidification, the

dendrites are not interconnected and they are free to move as shown in Figure 3-

16(a). Figure 3-16(b) shows interconnected dendrite in white and inter-dendritic

liquid in grey. This feeding mechanism prevails after the dendrite coherency

point. Consequently, in order to compensate for solidification shrinkage, the melt

has to take more treacherous inter-dendritic path and contraction induced

stresses developed in the continuous solid network, can result in casting defects

like hot tearing, shrinkage porosity, gas porosity and macro-segregation33.

Figure 3-7 Feeding Mechanisms (a) Mass Feeding (b) Inter-dendritic feeding34

Spencer35 observed that continuous stirring of melt during solidification results in

coherency at higher solid fractions and resultant microstructure shows smaller

and more round dendrites. Thus, he related shift in coherency point to the

dendrite morphology. Claxton36 concluded that dendrite coherency point is an

important characteristic influencing castability of Aluminum wrought alloys.

41

Determination of the Dendrite Coherency Point

Three methods have been developed for measuring the dendrite coherency point

of alloys. These are (1) the Two Thermocouple method (2) the Direct Shear Cell

method, and (3) the Rheological Determination method. These methods are

reviewed in some detail in the next sections.

The Two Thermocouple Method

This method was developed by Backerud et al37 and utilizes two thermocouples,

one placed at the center and the other at the wall of a cylindrical mold. As

solidification progresses, new phases first nucleate at the wall of the crucible and

then progress to the center. Consequently, the latent heat of solidification is

released near the crucible’s wall before it is released at the center of the crucible.

The difference in temperature between the two thermocouples is recorded.

Initially, solidification raises the temperature of the thermocouple placed at the

crucible’s wall while the region surrounding the thermocouple at the center of the

crucible is still in the liquid phase. At this point, the temperature difference curve

rises. The heat wave gradually travels towards from the crucible’s wall towards

the crucible’s center, and the temperature difference curve drops until a minimum

value is reached. At this point, nucleation of the primary phase is complete. After

this point the temperature difference curve rises again. The point at which the

curve drops to its minimum value is the dendrite coherency point. After the

dendrite coherency point, thickening of the dendrite occurs laterally. Figure 3-8

shows a typical setup and the output data of the Two Thermocouple method.

42

Figure 3-8 The Two Thermocouple method (a) Apparatus (b) Typical output34.

The Rheological Determination Method

This method was developed by Cha40 in 1994, and involves a rotating stirrer (or

vane) connected to a commercial rheometer immersed in the solidifying melt

along with a thermocouple. The rheometer measures the torque required to

maintain a constant rotation speed of the stirrer in the solidifying melt. The torque

is recorded as a function of time and temperature. Veldman et al 34 used this

method to measure the dendrite coherency point in terms of solid fraction. They

plotted the torque required to maintain a constant rotational speed vs. fraction of

solid. They concluded that for Al-Si-Cu alloys, the dendrite coherency point is

dependent on the silicon content and it is independent of the cooling rate and the

copper content. Figure 3-18 shows the Veldman et al apparatus and the

torque/temperature vs. fraction of solid curve.

43

Figure 3-9 (a) Experimental setup and (b) typical output data of the rheological

determination method34.

The Direct Shear Cell Method

This method was developed by Nabulsi36 and utilizes a shear cell to measure the

shear response of the semi-solid material as shown in Figure 3-19(a). The pulling

arm is connected to a universal tensile testing machine by means of an

arrangement of pulleys. The melt is maintained in an isothermal condition

throughout the experiment, and several experiments each at a different

temperature are performed in order to obtain the shear cell response vs.

temperature curve shown in Figure 3-19(b).

44

(a)

(b)

Figure 3-10 (a) Schematic of the direct shear cell (b) Typical output data38.

Hot Tearing Tendency of Aluminum Casting Alloys

Hot tearing is a casting defect that is formed when the tensile stress developed in

the solidifying casting exceeds the mechanical strength of the casting. It is also

referred to as hot shortness or hot cracking. It can be observed as an irreversible

tear in the solidified casting.

45

In general, hot tears can form in certain alloy systems during the terminal stages

of solidification when a liquid film is distributed along the grain boundaries and

the interdendritic regions. At this stage, shrinkage strains across the partially

solidified boundaries can become appreciable. If the terminal liquid is distributed

along the boundaries as a continuous film, the strains cannot be accommodated

and the boundaries separate to form a crack. In this sense, hot tearing may be

regarded as a special case of liquid metal embrittlement where significant loss of

ductility occurs when an alloy is heated above its solidus temperature and liquid

forms. Consequently, the temperature range over which the alloy solidifies, and

the characteristics (particularly surface tension) of the liquid that exists at the

terminal stages of solidification are primary factors that control the susceptibility

of the alloy to hot tearing. Surface tension forces play an important role in liquid

metal embrittlement. If γSL is the interfacial free energy between solid and liquid,

and if γSS is the interfacial free energy between two solid grains, then the dihedral

angle (θ) of the liquid film in the grain boundary depends on the ratio LSSS γγ

(See Figure 3-20). When LSSS γγ ≥ 2, then θ = 0, and the liquid completely wets

the solid grains.

2θ

γSL

γSL

γSS = 2 γSLcosθ γSS

boundary

Grain

Liquid metal

Figure 3-11 Equilibrium distribution of liquid at grain boundaries.

46

Solute redistribution also plays an important role in the sense that it affects the

solidification temperature range and the amount of terminal liquid.

Clyne and Davies39 reasoned that liquid feeding and mass feeding would readily

occur at liquid fractions between 0.1 and 0.6; and therefore they defined the time

spent in this range as recovery time, tr. They also reasoned that, at very low

liquid fractions, the alloy is too strong to tear so that the region in which the alloy

is vulnerable is where the fraction liquid is less than 0.1, but not zero. Based on

this reasoning, they chose the liquid fraction range between 0.01 and 0.1 to be

the vulnerable region and they defined the time spent in this range as tv.

For a given alloy, the times spent in the two regions (tr and tv) may be estimated

by using the Schile Equation to obtain a plot of liquid fraction as a function of

temperature, and cooling curve measurements to provide temperature as a

function of time. From these two plots, a plot of liquid fraction vs. time can be

developed and the Crack Susceptibility Criterion (CSC) for the alloy can be

determined using equation 1.

T r

T vCSC = (1)

An important assumption implicit to the Clyne and Davies theory is that a hot tear

is a uniaxial tensile failure in a weak material. In light of many evidences put forth

in the recent literature, this assumption is almost certainly true.

Campbell40 correlated the concept strain concentration, as defined by Pellini, to

the hot tearing as follows: If the length of the casting is L, and if the alloy has a

coefficient of thermal expansionα, then during cooling of the casting by ∆T from

the liquidus temperature it will contract by an amount α(∆T)L. If all this

47

contraction occurs in a hot spot of length l, then the strain in the hot spot is given

by

lTL∆= αε (2)

If the grain size is a, then the number of grains in the hot spot is l/a; and if we

divide the strain, ε, by the number of grain boundaries in the hot spot, then we

have the strain per grain boundary that is acting to open the hot tear

2

)(l

LaT∆= αε (3)

If all the castings are made in the same mold, then L and l will not change from

casting to casting, and we can also assume that α does not change significantly

from alloy to alloy, so Equation (3) may be re-written as

aT )(∆∝ε (4)

Combining Equation (4) with Equation (1) gives a modified Crack Susceptibility

Criterion, CSCb that can account for the effect of grain size on the susceptibility to

hot tearing

( )aTtt

CSCr

vb ∆∝

(5)

Many theories have been put forth over the years attempting to predict

susceptibility to hot tearing as a function of alloy composition. The theory

presented here is one of the few serious contenders. However, the ability to deal

with all of the aspects of solidification across an alloy system is a difficult

preposition for any one theory. Consequently, the theory is most useful for binary

48

alloys. Nevertheless, it is useful in a comparative study of the potential that

vibrations may have in minimizing the incidence of hot tearing in complex alloys.

Measurement of Hot Tearing Tendencies

Testing of an alloy for its susceptibility to hot tearing is a very important task and

several methods have been suggested over the years to gauge the tendency of

alloys to hot tear. These methods can be broadly classified into the following

categories:

1. Tests using mechanical techniques – these include tensile test at high

temperature41, direct chill casting tensile test42, stress and strain

measurement of ‘C’ shaped castings43, variable tensile strain tests44, tests

using, electrical resistance methods45, and optical emission methods46.

2. Tests using observation of the hot tears47,48 – these include the Flanged Bar

test, the Cylindrical Bar test, the Ball-Bar test, the I-Beam Casting test, the

Cold Finger test, the C-Bar Casting test, the U Casting test, the N-Tec Hot

Tear Mold test, and the Ring Mold test.

Most of these tests use castings that are constrained at one end so that feeding

is restricted while solidification occurs. When the stress exceeds the tensile

strength of the solidifying casting, hot tears form at the hot spot. It is important to

realize that because of the unique geometry of each mold, it is not possible to

compare results from one test to the other. Among all these tests, the Ring Mold

test, shown schematically in Figure 3-22 is the most widely used because of its

simplicity48.

49

Figure 3-12 Typical design of ring mold used for hot tear testing48.

50

4. Design of Experiments, Materials and Procedures

4.1. Design of Experiments

In order to evaluate effects of vibrations on aluminum based alloys, several

experiments were performed. Table 4-1 summarizes design of experiments.

Table 4-1 Design of Experiments

Independent Variables Dependant Variables

Alloy type:

− Al 7%Si

− B390

− B206

Vibration Parameter g:

− ≈ 0

− g1*

− g2*

− g3*

Casting characteristics:

Tendency to hot tear

Dendrite coherency temperature

Crack Susceptibility Criterion (CSCb)

Microstructure:

Size and morphology of grains

Size and morphology of eutectic Si

Distribution of primary Si

*g1,g2 and g3 are the vibration acceleration values in terms of gravity units. It is a combination of vibration amplitude and frequency. 1g=0.0511Df 2.

51

Constants Value

Melt super heat for given alloy N/A*

Hydrogen content of the melt <0.12 ml H/100g alloy

Grain refiner ≈ 0.00

Chemical modifier ≈ 0.00

Vibration time Pouring to complete solidification

* amount of superheat differs with alloy but constant for given alloy

Alloys used for this work were chosen to address specific issues associated with

their castings. Hence, partial factorial analysis of Table 4-1 was done.

Table 4-2 shows experiments’ matrix.

Table 4-2 Experiments’ matrix

Alloy Objective

B206 B390 Al-7%Si

Thermal Analysis v v Grain size v

Hot Tearing

Ring mold v Grain Size v

Primary Silicon v Microstructure

Eutectic Silicon v

52

The shaded area in Figure 4-1 shows the range of vibration parameters used in

this work. The magnitude of the vibration parameter g was dictated by the

capacity of the vibration table.

Figure 4-1 range of vibration parameters

4.2. Materials

Recall that the objective of this work is to characterize the effect of mechanical

vibrations on the primary as well as eutectic silicon, and on the hot tearing

tendency of Aluminum alloys. In order to meet these objectives, three different

Aluminum alloys were used.

Alloy B390 is a hypereutectic alloy that contains an appreciable amount of

primary silicon and eutectic silicon. Depending on solidification conditions, this

alloy may exhibit an irregular size and distribution of primary silicon, and without

53

proper chemical modification, coarse primary silicon renders B390 unusable for

many applications. The compositional analysis was done using spark

transmission spectrometerI. Table 4-3 shows composition of the B390 alloy used

for this work.

Table 4-3 Composition of B390 alloy

*elements with wt% less than 0.01 are not shown

Al-7%Si is a binary alloy with appreciable eutectic silicon content. This alloy was

prepared using high purity aluminum (99.99%) and Al-50%Si master alloy. The

high purity aluminum was melted and maintained at 8000 C in an electrical

resistance furnace. Al-50%Si master alloy was added to the melt and stirred

using a graphite rod. After confirming the composition means of spark emission

spectrometry, the alloy was poured in the form of ingots. Table 4-4 shows

composition of the Al-7% alloy used for this work.

I Spectro Analytical Instruments, Spectromax Spark Spectrometer LMXM3, Boschstr. 10, 47533 Kleve, Germany.

Composition (wt%) Alloy

Si Mg Mn Fe Cu Zn Al

Target 16-18 0.15-0.65 0.5max 1max 4-5 1.4max Balance B390

Measured 17.88 0.49 0.24 0.87 4.35 0.81 Balance

54

Table 4-4 Composition of Al-7%Si alloy


Alloy B206 is an Al-Cu alloy. This alloy is very prone to hot tearing during

solidification. A Commercial grade B206 with the chemical composition shown in

Table 4-5 was used for this work. The composition was obtained using spark

emission spectrometry.

Table 4-5 Composition of B206 alloy


Cu Mg Mn Fe Si Sn Al

Target 4.2-5 0.15-.035 0.2-0.5 <0.15 <0.1 <0.05 Balance B206



It should be noted that in order to accentuate effect of mechanical vibrations on

grain refinement and modification, no chemical grain refiner (e.g. Ti ) and

modifiers (e.g. P for primary silicon, Sr for eutectic silicon) are used.


Si Mg Ti Fe Cu Zn Al

Target 7 <0.01 <0.01 <0.01 <0.01 <0.01 Balance Al-7%Si


55

4.3. Vibration table setup

A BRFCCD-36 AGREEII Mechanical Shaker with the specification shown in

Table 4-6 used to vibrate the molds.

Table 4-6 Specification of mechanical shaker

Also, a Ceramic shear ICP accelerometer III with panel meter was used for the

measurement of vibrations parameters. An insulating plate was sandwiched

between table surfaces to protect vibration table from heat. Figure 4-2 shows

schematic and photographs of the vibration table setup.

II Lab equipments Inc. , Franklin Park IL USA III Model 603C10, IMI sensors, Depew, NY, USA

Parameter Specification

Frequency 8Hz to 60Hz Amplitude 2.03mm max

Maximum payload 500lbs Maximum acceleration 3.2g

56

Signal Conditioner

Thermocouples Fixture

Plate

Insulator

Panel Meter

Test Specific

Mold

Accelerometer

Temperature Data Acquisition

Vibration Table

Mechanical Shaker AccelerometerMechanical Shaker Accelerometer

(a)

(b)

Figure 4-2 Vibration table setup (a) schematic (b) photographs

57

Melt Preparation – In each case, about forty pounds of the alloy was melted in a

clean silicon carbide crucible in an induction furnace. The melt was degassed

with argon gas using a rotating impeller degasser until the hydrogen level in the

melt is below 0.12 ml of H/100g of alloy. The hydrogen level will be monitored

using an Alscan unit. No grain refiner or chemical modifier was added. . The

alloy chemical composition will be verified using a spark emission spectrometerIV.

Thermal Analysis - Thermal analysis was performed using the two

thermocouples technique. A clean crucible coated with boron nitride was used. In

order to avoid relative vibrations between the crucible and the vibration table, a

fixture arrangement was devised and used. Two holes were drilled through

fixture to accommodate thermocouples. Two calibrated, K-type thermocouples

were set in a crucible by means of the fixture arrangement. One thermocouple

was placed neat the wall of the crucible and its temperature readings are

designated Tedge. The other thermocouple is placed at the center of the crucible

and its temperature readings are designated Tcenter The alloy was poured into the

crucible when its temperature reached 700oC. Data form the thermocouples was

recorded using a data acquisition system. . Based on this thermal profile, the

dendrite coherency temperature and the corresponding fraction of solid will be

determined as per the method suggested by Bäckerud et al25.

Figure 4-3 shows a schematic representation of the arrangement

IV Spectro Analytical Instruments, Spectromax Spark Spectrometer model LMXM3, Boschstr. 10,

47533 Kleve, Germany.

58

Figure 4-3 Schematic of the thermal analysis setup for (a) B206, Al-7%Si (b) B390

4.4. Hot Tearing Tendency

The Effect of vibrations on the hot tearing tendency was evaluated with two

routes.

Modified Crack Susceptibility Criteria

Equation (5) gives the Modified Crack Susceptibility Criteria (CSCb) used to

quantify the tendency of the alloy to heat tear during solidification. This

relationship is based on the analysis presented in Section 3.2

( )aTtt

CSCr

vb ∆∝

(5)

Thermocouples

63 101

76

Fixtures

152

114

101

Fixtures

Thermocouple

59

Where tv is the time taken for 0.1fraction of liquid to 0.01 fraction of liquid, tr is the

time taken for fraction of liquid at 0.6 to 0.1 fraction of liquid and ?T is the

temperature difference between the temperature at solidus and the temperature

at liquidus, and a is as cast grain size.

The phenomenon of hot tearing is certainly related to the feeding characteristics

of an alloy. At the temperature above dendrite coherency point, the casting is

subjected to the mass feeding. As difficulties in the feeding for contracting solids

arise after the dendrite coherency point, the stress in the casting really starts

developing after the dendrite coherency point. Considering ∆T as the

temperature difference between dendrite coherency point and the solidus would

be more appropriate in the estimation of the strains in a casting. Incorporating

dendrite coherency temperature with Equation (5), gives Equation (6).

aTTtt

CSC sdc

r

vb )( −∝ (6)

The dendrite coherency and solidus temperatures, as well as the solidification

range are obtained from the cooling curve. The change in fraction liquid with

solidification time was obtained from a Scheil analysis using the thermodynamic

software PANDATV.

The Ring Mold Test

A ring mold was used for hot tear testing. The design of the mold was based on

the design from Singer 49 and was adapted to the restrictions of the present work.

V PANDAT, CompuTherm LLC, Madison, WI, 53719 USA

60

The mold is consists of two parts, central core and outer part. Both the parts

were machined from 4140 steel using vertical machining center.

Figure 4-4 Ring mold used for hot tear testing

During the solidification core resists solidification contraction of the solidifying

melt. If the tensile stresses developed in the casting, exceeds strength of the

solidifying body, hot tear develops parallel to the direction of solidification

growth50.

Prior to casting, the mold was coated with boron nitrite to prevent any

contamination. The outer part of the mold was preheated to 175o C while central

core was not preheated. The pouring temperature was kept constant at 800o C.

The melt was poured to the height of 1.5inch. At least three iterations with each

parameter were done to ensure consistency. After pouring, casting was allowed

to solidify under vibrations. Solidified casting was extracted from the mold and

allowed to cool to room temperature. The length of the crack was measured after

it reaches to room temperature. The area of the crack was assumed to be

61

rectangular and was calculated from the length and depth measurements to give

an approximate quantitative measure of the alloy’s susceptibility to hot tearing.

4.5. Microstructure Analysis

Microstructure Analysis Samples for microstructure analysis were taken from the

castings. For each vibration condition, 3 samples were taken from the central

horizontal plane of the casting and 3 samples were taken from the central vertical

plane. Figure 4-5 shows location of the samples. Samples were first cut and

ground using standard metallographic procedures. After grinding samples were

polished using 1 µm, and 0.05 µm Alumina suspension in water. Final polishing

was done using silica suspension. Between each step, samples were thoroughly

cleaned. Various etchants were used to reveal micro and macro structure. A

macro etchant, namely, 5vol%HF, 20vol%HCL, 20vol%HNO3, 55vol%H2O was

used to reveal the grain structure of B206 and Al-7%Si. The B390 samples were

electro-etched with Ethyl Alcohol (60vol%) Perchloric Acid (20vol%)

Glycol(20vol%). A Nikon inverted epiphot microscope fitted with digital camera

was used for optical microscopy. Captured images were analyzed using

microGOP 2000 image analysis system. For grain size measurement, atleast 500

grain-counts were taken from each sample and the equivalent diameter method

was employed to calculate grain size. In order to characterize the size and

distribution of primary silicon in B390, a random strip 600µm in width and

spanning the entire sample length was selected as a representative of the

sample. Every primary silicon particle in the strip was analyzed for shape and

62

size. A JEOL 840 scanning electron microscope (SEM) at an accelerating

voltage of 15keV using a LaB6 electron source was used for all SEM work.

63

Figure 4-5 Locations of the samples used for microsturucture analysis (a) B390

(b) B206 and Al-7% Si

152mm 50 mm

50 mm Plane 1

Plane 2

101mm

63 mm Plane 1

Plane 2

(a)

(b)

64

5. Results and Discussion

This section presents results of the experiments performed as per design of

experiments. This section is divided into three parts, each part devoted to one

alloy. Also, results were correlated with the understanding available from the

literature sources.

5.1. B206 alloy

Thermal Analysis

The two-thermocouple method was used for thermal analysis and Table 5-1

shows the pouring temperature, and the vibration intensity as well as the

measured and calculated superheats.

Table 5-1 Pouring temperature, vibration intensity and superheats for each specimen

Calculated from “Pandat”

Measured from Cooling Curve

# Vibration intensity

(g)

Pouring Temperature

(°C)

Liquidus (°C)

Superheat (°C)

Liquidus (°C)

Superheat (°C)

1 0 740 646.7 93.3 652.1 87.9 2 1.5 740 646.7 93.3 650.6 89.4 3 2 740 646.7 93.3 650.3 89.7 4 3 740 646.7 93.3 651.2 88.8

Figure 5-1 to Figure 5-2 show the temperature Vs Time corresponding to the

samples in Table 5-1. It is apparent from these figures that the solidification rate of

B206 alloy increases with increasing the vibrations’ intensity.

65

Figure 5-1 Thermal profile of the B206 with no vibrations (Sample#1)

Figure 5-2 Thermal profile of B206 with mold vibrations at 1.5g (Sample #2)

Tedge

Tcenter

Tedge

Tcenter

66

Figure 5-3 Thermal profile of B206 with mold vibrations at 2g (Sample #3)

Figure 5-4 Thermal profile of B206 with mold vibrations at 3g (Sample #4)

Tedge

Tcenter

Tedge

Tcenter

67

Figure 5-5 further highlights this point and shows the effect of the intensity of

mold vibrations on the central thermocoup le. Similarly, Figure 5-6 shows the

effect of the intensity of mold vibrations on the edge thermocouple. This

observed increase in cooling rate with the intensity of mold vibrations may be

attributed to an increase in the forced convection in the melt brought about the

increased vibration levels. Figure 5 -7 shows the effect of the intensity of mold

vibration on (TEdge-TCenter).

Figure 5-5 Comparison between thermal profile at center of the castings

68

Figure 5-6 Comparison between thermal profile at center of the castings

Figure 5-7 Variation in TEdge-TCenter with the chance in vibration intensity

69

The findings indicate that up to about 2g intensity of vibration, the vibrations

decrease the temperature difference between the mold edge and center (? T),

thus enabling the casting to solidify more uniformly. At about 3g intensity of

vibration, the ∆T significantly to around 27oC before it drops again towards the

end of solidification. Following Backeraud et. al., the dendrite coherency point

was calculated by finding the absolute minimum in the ? T vs. time curve and

imposing it on the temperature vs. time curve to determine the dendrite

coherency temperature. This is shown in Figure 5-8.

Figure 5-8 Determination of dendrite coherency point

In order to get fraction of solid formed at the time of dendrite coherency, a

fraction of solid Vs temperature curve was generated using thermodynamic

simulation software, PANDAT. Sheil’s equation was used for simulation of

solidification process. The dendrite coherency temperature obtained from the

70

thermal analysis was correlated to corresponding fraction of solid. Figure 5-9

shows the curve generated using PANDAT.

Figure 5-9 Temperature Vs Fraction of solid curve for B206 alloy

Table 5-2 shows calculated dendrite coherency temperature and corresponding

fraction of solid for the B206 alloy sample vibrated at different frequencies.

Table 5-2 Dendrite coherency point and corresponding fraction of solid for B206 alloy

vibrated at different intensities No vibrations 1.5g 2g 3g

Temperature at dendrite coherency oC 645.3 644.5 644.9 642.74

Fraction of Solid at dendrite coherency 0.099 0.1507 0.1257 0.25

Although vibrations do not affect dendrite coherency temperature considerably,

the amount solid at dendrite coherency was increased from 9.99% to 25% at

71

highest vibration intensity. This can be attributed to possibility of dendrite

remelting due to the fluctuations in temperature caused by vibrations. Abu-Dheir4

et al reported observation of broken dendrites in the final microstructure. The

breaking of dendrites probably happen at the early stages of solidification due to

the low strength of the dendrites at higher temperature. This can also cause

delay in formation of coherent dendrite network.

The dendrite coherency point denotes transition from mass feeding to more

treacherous inter-dendritic feeding. Shift in dendrite coherency point towards

lower temperature or higher fraction of solid is expected to manifest in lowering of

feeding related casting defects like shrink porosity, hot tearing etc.

72

Microstructure Analysis

Figures 5-10 to 5-13 show the grain structure of specimens obtained from the

samples described in Table 5 -1.

Figure 5-10 As-cast grain structure of B206 alloy with no vibrations (i),(ii)Horizontal

section (iii) vertical section

(i) (ii)

(iii)

73

(i)

(ii)

(iii)

Figure 5-11 As Cast grain structure of B206 with vibration at 1.5g (i),(ii) Horizontal

section (iii) Vertical section

74

(i)

(ii)

(iii)

Figure 5-12 As Cast grain structure of B206 with vibration at 2g (i),(ii) Horizontal section

(iii) Vertical section

75

(i)

(ii)

(iii)

Figure 5-13 As Cast grain structure of B206 with vibration at 3g (i),(ii) Horizontal section

(iii) Vertical section

Careful examination of Figures 5 -10 to 5-13 reveals that the morphology of the

grains changes from a predominantly dendritic to a more globular structure, and

the grains become finer as the intensity of vibrations is increased. Table 5-3

shows the calculated grain size, maximum grain diameterVI, and grain

VI Calculated as the largest distance between two points on a grain boundary.

76

compactnessVII. For better visualization, the same information is shown in Figure

5-14 in graphical form.

Table 5-3 As-cast grain structure analysis of B206 alloy with different vibration intensities

no vibration 1.5g 2g 3g

Grain Size µm

(Std. Div.)

218.26

(103.73)

158.22

(79.83)

82.8

(37.73)

79.56

(43.9)

Dmax µm

(Std. Div.)

308.93

(143.33)

239.57

(116.19)

114.04

(51.33)

110.5

(45.3)

Compactness

(Std. Div.)

5.11

(2.55)

4.4

(3.34)

3.12

(1.73)

3.0

(1.95)

VII Calculated as P2/A*4π. Grain compactness is a measure of roundness, 1 denotes perfect circle

77

Figure 5-14 Effect of mechanical vibrations on As-cast (i) grain size (ii) Dmax and (iii)

compactness of B206 alloy

No Vibrations

1.5g 2g 3g

(i)

No 1.5g 2g 3g

(ii)

No Vibrations

1.5g 2g 3g

(iii

78

It can be inferred that approximately 3:1 reduction in the as cast grain size can

be achieved with the application of vibrations. This is in agreement with

conclusions of previous research works listed in section 3.1. Kocatepe3 reasoned

different mechanisms for this grain refinement.

Dendrite Fragmentation

In this mechanism, the growing dendrites are subjected to constant impact from

the surrounding liquid due to the movement generated by the vibrations. In the

early stages of solidification, these dendrites have little strength, and hence they

fragment easily. Moreover, vibrations cause detachment of the parts of the solid

layer that forms on the interior surface of the mold. These solid particles are

carried to the internal regions of the liquid by convection currents where they act

as nuclei for the next solidifying particles. This also explains the formation of

uniform and equiaxed grains throughout the casting when the mold is vibrated.

Dendrite Re-melting

In this mechanism, vibrations cause fluctuations in temperature by causing liquid

metal movement. It is envisioned that a growing dendrite in a relatively cool

region of the solidifying casting is carried away to a relatively hotter region of the

casting over these currents. There, the dendrite begins to re-melt at the necks of

its arms.

Although other mechanisms have been proposed to explain vibration induced

grain refinement, such as cavitations, and reduction in solidification time, dendrite

79

fragmentation seems to be the mechanism underlying grain refinement of B206

alloy.

Calculation of the Crack Susceptibility Criteria

The Modified Crack Susceptibility Criteria (CSCb) for B206 alloy vibrated at

different vibration intensities was determined using data from the cooling curves

obtained from thermal analysis (Figures 5-1 to 5-4), temperature vs. fraction

liquid data obtained from thermodynamic software PANDAT (Figure 5-9), and the

grain size measurements (Table 5-3). Table 5.4 shows the calculated CSCb

values for B206 alloy.

Table 5-4 Effect of mechanical vibration CSCb of B206 alloy

No

vibrations 1.5g 2g 3g

Grain Size(mm) 0.2183 0.1582 0.0822 0.079

Temperature at Dendrite coherency 645.3 644.5 644.9 642.75

Temperature at 0.6 fraction of liquid 630.81 630.81 630.81 630.81



Time taken from 0.6 to 0.1 f(L) 198.6 198.6 194 133.7

Time taken from 0.1 to 0.01f(L) 144 147.6 141.2 134.3

Tdc-Tsolid 140.97 140.17 140.57 138.42

CSCb 22.31 16.48 8.41 10.98

80

Figure 5-15 Effect of vibrations on CSCb of B206 alloy

Ring Mold Casting

The effect of mold vibrations on the incidence of hot tearing was evaluated using

the ring mold test and the casting parameters shown in Table 5-5.

Table 5-5 Casting parameters for ring mold casting experiments

Alloy Vibrations

Parameter

Mold

Temperature (oC)

Pouring

temperature (oC)

Superheat

(oC)

B206 No vibrations 175 800 154

B206 2g 175 800 154

Figure 5-16 shows a photograph of a typical casting made in the ring mold

without mold vibration. Figure 5-17 shows X-ray images castings of a typical

casting made with the mold vibrated at 2g.

81

Figure 5-16 Ring casting of B206 without vibrations

(a)

(b)

Figure 5-17 Ring casting of B206 with vibrations (a) photograph (b) X-ray

Hot tears

5 mm

82

Although the casting did not separate into two halves, complete circumferential

cracks formed in the castings made without mold vibration. On the other hand,

the castings made while the mold was vibrated at 2g show only hairline cracks. In

order to quantify the size of the cracks, x-ray imaging was performed on the

castings and an approximate crack area was calculated from the product of the

crack length and crack depth. In the case of the castings made without mold

vibration, the average crack area was calculated to be 1.2 in2, On the other hand,

in the case of the castings made while the mold was vibrated at 2g the average

crack area was calculated to be 0.5 in2. These observations are in agreement

with the CSCb calculations.

These observations are in agreement with the CSCb calculations. During the

thermal analysis, it was observed that the fraction of solid at the dendrite

coherency point increases with the application of vibrations, i.e., there is less

liquid left in the casting after the dendrite coherency point. At the dendrite

coherency point, the feeding mechanism changes from mass feeding to

interdendritic feeding. The contraction during solidification of the remaining liquid

has to be compensated for through a more treacherous interdendritic path. This

has a significant effect on defects such as hot tears. Hence it can be inferred that

with the application of vibrations, better feeding can be achieved resulting in

improvement in the hot tearing tendency of the alloy.

As seen in CSCb calculations grain size has a significant impact on hot tearing

tendency of an alloy. Smaller and less dendritic grain structure can promote

better interdendirtic feeding making the alloy less prone to hot tearing.

83

As mentioned before, CSCb is based on the theory of strain concentration. More

the number of grain boundaries at the hot spot, lesser is the concentration of

strain at one grain boundary. With the application of vibrations the grain size was

reduced significantly. Hence the reduction in the crack area demonstrated in the

ring casting is probably due to the better feeding facilitated by the application of

vibrations as well as lesser strain concentrations due to refined grain size.

84

5.2. Al-7wt%Si Alloy

Thermal Analysis

The Two-Thermocouples method was used for thermal analysis and Table 5-6


measured and calculated liquidus and superheat temperatures.

Table 5-6 Pouring temperature, vibration intensity and superheats for each specimen

Calculated from “Pandat”

Measured from Cooling Curve

# Vibration Parameter

(g)

Pouring Temperature

(°C)

Liquidus (°C)

Superheat (°C)

Liquidus (°C)

Superheat (°C)

1 0 700 616.48 83.52 615.1 84.9 2 2 700 616.48 83.52 613.2 86.8 3 3 700 616.48 83.52 615.3 84.7

Figures 5-17 to 5-19 show the temperature vs. time profiles corresponding to the

samples in Table 5-6. It is apparent from these Figures that the solidification rate

of Al-7wt%Si alloy increases with increasing the vibrations’ intensity. Figure 5 -20

further highlights this point and shows the effect of the intensity of mold vibrations

on the central thermocouple. Similar to the case of B206 alloy, this observed

increase in cooling rate with the intensity of mold vibrations may be attributed to

an increase in the forced convection in the melt brought about the increased

vibration levels. These results are in agreement with work done by Abu-Dheir4

and Kokatepe3.

85

Figure 5-18 Thermal profile of Al-7%Si alloy with no mold vibrations

Tedge

Tcenter

Figure 5-19 Thermal profile of B206 alloy cast with mold vibrations at 2g

Tedge

Tcenter

86

Figure 5-20 Thermal profile of B206 alloy with mold vibrations at 3g

Figure 5-21 Comparison between the thermal profiles at the center of the castings

Tedge

Tcenter

87

Figure 5-22 Comparison between ?T in vibrated and un-vibrated castings

The dendrite coherency point was calculated following the procedure described

for B206 alloy in Section 5.1. Figure 5-23 shows the simulated temperature vs.

fraction solid curve for the Al-7wt%Si alloy generated using PANDAT, and Table

5-7 shows the calculated dendrite coherency temperature and corresponding

fraction solid for samples vibrated at different intensities.

88

Figure 5-23 Temperature Vs fraction of solid for Al-7%Si

Table 5-7 Dendrite coherency point and corresponding fraction solid for Al-7wt%Si alloy

vibrated at different intensities.

No

vibrations

2g 3g

Temperature at dendrite coherency oC 612.8 609.6 608.1

Fraction of Solid at dendrite coherency 0.081 0.142 0.167

Examination of Table 5-7 reveals shows vibrations delay dendrite coherency by

as much as 4°C. This causes an 8% increase in fraction solid at dendrite

coherency. Similar to the case of B206 alloy, the change in the dendrite

coherency point with mold vibration during casting Al-7wt%Si alloy can be

attributed to fragmentation of the dendrites during the early stages of

solidification.

89

Figure 5-24 Effect of vibrations on dendrite coherency point of Al-7%Si alloy


Figures 5-23 to 5-25 show the grain structure of specimens obtained from the

samples described in Table 5 -6.

Figure 5-25 Grain structure of Al-7%Si alloy solidified under no vibrations

90

Figure 5-26 Grain structure of Al-7%Si alloy solidified under vibrations at 2g

Figure 5-27 Grain structure of Al-7%Si alloy solidified under vibrations at 2g

Careful examination of Figures 5 -23 to 5-25 reveals that the grains become finer

as the intensity of vibrations is increased. Table 5-8 shows the calculated grain

size, maximum grain diameterVIII, and grain compactness IX. For better

visualization, the same information is shown in Figure 5 -26 in graphical form.

VIII Calculated as the largest distance between two points on a grain boundary.

IX Calculated as P2/A*4π. Grain compactness is a measure of roundness, 1 denotes perfect circle

91

Table 5-8 Grain size, Dmax and Compactness of Al-7%Si alloy

no vibration 2g 3g

Grain Size µm

(Std. Div.)

1241.00

(172.03)

810.8

(157.92)

165.2

(92.9)

Dmax µm

(Std. Div.)

1472.4

(143.33)

937.73

(51.33)

210.88

(45.3)

Compactness

(Std. Div.)

7.11

(2.55)

6.2

(1.73)

3.3

(1.95)

These findings are in agreement with the findings of previous researchers. For

example, Burbure et al49 reported grain refinement in Al-12wt%Si that was

vibrated at 3g. Unfortunately they did not quantify their findings. Also, Pillai et al6

and Kokatepe et al4 reported grain refinement in Al-Si alloys. Kokatape reported

grain refinement by Similar to the case of B206 alloy, the observed grain

refinement may be attributed to dendrite fragmentation during the early stages of

solidification.

92

(i)

(ii)

(iii)

Figure 5-28 Effect of mechanical vibrations on (i) Grain size (ii) Dmax and (iii)

compactness of al-7%Si alloy

93

Figure 5-27 shows macro-structure of Al-7%Si alloy cast while the mold was

vibrated at 3g. A transition between coarse grain structure (at the wall) and

refined grain structure (at the center) is clear. This transition is caused by a

relatively fast solidification rate at the wall, which shortens the effective vibration

time. Burbure et al. observed a similar transition and reported that it does not

happen in relatively large castings.

(i) (ii)

Figure 5-29 Al-7%Si alloy solidified under vibration at 3g with mold temperature (i) room

temperature (ii) 175oC

Figure 5-28 shows the macrostructure of Al-7wt%Si alloy solidified while the mold

was vibrated at 3g. Casting (i) was solidified in a mold that was not preheated. In

this case, the casting solidifies rapidly and the effective vibration time near the

casting surface is not sufficient to cause significant grain refinement. On the

hand, the center of the sample, which remained liquid for a longer period of time,

shows significant refinement. Sample (ii) was solidified in a mold that was

preheated to 175°C. In this case, the casting solidifies slowly and the effective

vibration time near the casting surface is sufficient to cause significant grain

refinement. Consequently, compared to sample (i), sample (ii) shows significant

94

reduction in the size of the region with coarse grains. Burbure’s observation

regarding a more uniform grain size in large castings may be explained along

these lines: Larger castings allow more effective vibration time.

Figures 5-29 to 5-31 show the eutectic silicon in Al-7wt%Si samples that were

solidified under different vibration intensities.

Figure 5-30 Eutectic silicon in un-vibrated Al-7%Si sample

Figure 5-31 Eutectic silicon in Al-7%Si sample vibrated at 2g

95

Figure 5-32 Eutectic silicon in Al-7%Si sample vibrated at 3g

(i) (ii)

Figure 5-33 SEM image of eutectic silicon particles in Al-7%Si vibrated at (i) 2g (ii) 3g

Examination of Figures 5-29 to 5 -31 shows castings vibrated at 2g exhibit

shortening and a reduction in size of their eutectic Silicon flakes compared to the

un-vibrated castings. On the other hand, castings vibrated at 3g show thickening

of their eutectic Silicon flakes compared to the un-vibrated castings and the

castings vibrated at 2g. These findings are in agreement with previous findings

by Abu-Dheir4, Pillai6, and Burbure51. The observed shortening of the eutectic

silicon particles with increased vibration intensity can be attributed to their

96

fragmentation during the early stages of solidification. The observed thickening of

the flakes with increased vibration intensity can be attributed to an increase in

diffusion rate brought about by the enhanced mixing, and also to agglomeration

of eutectic silicon particles. Kokatepe4 reported that the degree of coarsening

increases with increasing mold temperature, where effective vibration time is

more due to longer solidification time.

97

5.3 B390 alloy

Thermal Analysis

The Two-Thermocouples method was used for thermal analysis and Table 5-9


measured and calculated liquidus and superheat temperatures.

Table 5-9 Experimental parameters for thermal analysis of B390 alloy

Calculated from

“Pandat”

Measured from

Cooling Curve #

Vibration

Parameter

(g)

Pouring

Temperature

(°C)

Liquidus*

(°C)

Superheat

(°C)

Liquidus**

(°C)

Superheat

(°C)

1 0 740 646.7 93.3 648.1 91.9

2 1.5 740 646.7 93.3 648.1 91.9

Figure 5-34and Figure 5-35 show the temperature vs. time profiles corresponding

to the samples in Table 5-6. Figure 5-36 shows the change in the magnitude of

(TEdge-TCenter) with change in vibration intensity. Note that similar to the case of

B206 alloy, vibrations decrease the temperature difference between the mold

edge and center (? T), thus enabling the casting to solidify more uniformly.

98

Figure 5-34 Thermal profile of B390 alloy solidified without vibrations

Figure 5-35 Thermal profile of B390 alloy cast while the mold is vibrated at 1.5g

99

Figure 5-36 Temperature difference between the center and edge thermocouples in

vibrated and un-vibrated B390 alloy


A random strip 600µm in width and spanning the entire sample length was

selected as a representative area where measurements of the size and

distribution of the primary silicon particles was performed. See Figure 4-5.

Figure 5-37 shows the primary silicon size and distribution in plane 1 for (a) when

the mold is not vibrated, and (b) when the mold is vibrated at 1.5g. Similarly,

Figure 5-38 shows the primary silicon size and distribution in plane 2 for (a) when

the mold is not vibrated, and (b) when the mold is vibrated at 1.5g. In Figure

5-37and Figure 5-38, for the primary Si size profile, each bar represents a 600µm

100

× 2000µm area, and for the particle size distribution profile, each bar represents

a 600µm × 1000µm area.

Table 5-10 shows the mean Silicon particle count and the mean Silicon particle

size from castings with no mold vibration and castings with the mold vibrated at

1.5g.

Table 5-10 Mean primary silicon particle size and count in un-vibrated and vibrated

samples

No Vibrations With vibrations at 1.5g

Plane 1 4.12 18.4 Mean particle Count

Plane 2 5.17 8.1

Plane 1 230.56 162.01 Mean particle size (Dmax)

Plane 2 192.83 190.01

101

Primary Si size

Primary Si distribution

Par

ticle

siz

e (m

icro

ns)

0

1 0 0

2 0 0

3 0 0

4 0 0

mea

n (m

icro

ns)

2 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 62 3 0 . 5 6

N

0

2

4

6

8

1 0

1 2

1 4

1 6

Mea

n

4 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 24 . 1 2

(a)

102


vibrations, and (b) with vibrations at 1.5g (plane 1).

Par

ticle

siz

e (m

icro

ns)

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

mea

n (m

icro

ns)1 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 11 6 2 . 0 1

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5

N

0

5

1 0

1 5

2 0

2 5

3 0

mea

n1 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 41 8 . 4


Primary Si size

(b)

103


Parti

cle

size

(mic

rons

)

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

mea

n (m

icro

ns)

1 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 31 9 2 . 8 3

N

0

5

1 0

1 5

2 0

2 5

mea

n

5 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 55 . 1 7 5

Primary Si size

(a)

104


vibrations, and (b) with vibrations at 1.5g (plane 2)

Primary Si size

Parti

cle

size

(mic

rons

)

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

mea

n (m

icro

ns)

1 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 51 9 0 . 0 7 2 5

N

0

5

1 0

1 5

2 0

2 5

mea

n

8 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 78 . 7

Primary Si Distribution

(b)

105

During solidification of hypereutectic Al-Si alloys, such as B390, primary Silicon

nucleates first at the alloy’s liquidus temperature and continues to grow until the

nucleation of eutectic silicon. The density of the growing primary Silicon particles

is less than that of the melt. If the cooling rate is slow, ample time may be

available for these particles to float towards the melt’s surface causing Silicon

particle segregation in castings. Figure 5-37clearly demonstrates this

phenomenon. In Figure 5-37(a), a region at the center of the casting is completely

deficient of primary Silicon particles. Where as, in Figure 5-37(b), the distribution

of primary Silicon particles tends to become more uniform.

In plane 1 of the casting, the mean primary Silicon particle size decreased from

230.56 µm to 160.01 µm when the mold was vibrated. On the other hand, there

was only a marginal decrease in the mean primary Silicon particle size in plane 2

(from 192.83 µm to 190.01µm). This is due to the fact that the region around

plane 2 solidifies relatively quickly, and therefore the effective vibration time is

short.

The observed refinement of primary silicon particles may be explained as follows.

During the initial stages of solidification, primary Silicon particles nucleate on the

surface of the mold. Mold vibrations may cause these particles to separate from

the mold walls and to be carried inwards towards the center of the casting where

they become nucleation sites for new particles, thus increasing the overall

nucleation rate at the expense of the growth rate of primary Silicon particles.

The observed refinement of primary silicon particles may be explained as follows.

1. During the initial stages of solidification, primary Silicon particles nucleate

on the surface of the mold. Mold vibrations may cause these particles to

106

separate from the mold walls and to be carried inwards towards the center

of the casting where they become nucleation sites for new particles, thus

increasing the overall nucleation rate at the expense of the growth rate of

primary Silicon particles

2. The primary silicon particles follow layered growth mechanism50. This can

be seen in Figure 5-39(a) which shows traces of layered growth. These

traces are formed due to the disturbances in the silicon layering process

caused by the convection currents present in the casting. Also, Figure

5-39(b) shows 3 stages of the primary silicon particle growth. It can be

clearly inferred that primary silicon growth is a time dependent

phenomenon.

Figure 5-39 Layered growth of primary silicon particles (a) traces of layered growth (b)

different stages of growth53

(a) (b)

107

Also, it is a well known fact that growth and coarsening of primary silicon

particles continues till the Al-Si eutectic reaction51.

Now, Figure 5-40 shows Al-Si eutectic reaction starting time for non-

vibrated and sample solidified under vibrations at 1.5g. At the beginning of

analysis, starting times for solidification reaction for each curve were

normalized with respect to each other. The eutectic reaction starting point

is observed as rapid change in cooling rate. Those points are projected on

time axis to get eutectic reaction starting time. It was observed that in

samples without vibrations eutectic reaction starts at 222 seconds after

the beginning of solidification. While under the similar experimental

conditions and under vibrations at 1.5g, eutectic reaction starts at 105

seconds. This difference in time indicates that, in case of solidification

under vibrations, growth time for primary silicon was suppressed.

Consequently, this reflects in the refinement of primary silicon particles in

final microstructure.

Figure 5-40 Change in eutectic reaction time due to vibrations.

108

6. Conclusions

The effects of mechanical mold vibration on casting characteristics of Al-based

alloys were evaluated. The materials used in the study were chosen so as to

address specific problems associated with casting Al-based alloys. B206 alloy

was chosen to study the effect of mold vibration on hot tearing, B390 alloy was

chosen to study the effect of mold vibration on the size and distribution of primary

silicon particles, and an Al-7wt%Si binary alloy was chosen to study the effect of

mold vibration on as-cast grain size and on the morphology of eutectic silicon.

It was found that:

− Mechanical mold vibration has an effect on the dendrite coherency point of

hypoeutectic Al-based alloys (B206 alloy and Al-7wt%Si alloy). In these

alloys, the dendrite coherency point tends to shift towards lower temperatures

(higher fraction solid). Based on this finding, it is believed that mechanical

mold vibrations may lower feeding related defects such as shrink porosity and

hot tearing in castings made from these alloys.

− A modified Crack Susceptibility Criterion (CSCb) was developed and used to

quantify the effect of mechanical mold vibrations on the hot tearing tendency

of B206 alloy. It was found that mechanical mold vibrations significantly lower

the CSCb of B206 alloy. This finding was verified by observation of hot tears

using the Ring Mold test.

− Mechanical mold vibration has a significant effect on the grain structure of

hypoeutectic Aluminum-based alloys. In these alloys the equivalent grain

109

diameter, Dmax, was reduced by the vibrations, and the grains became more

compact.

− Mechanical mold vibration has a significant effect on the primary Silicon

particles in hypereutectic Al-Si alloys (B390 alloy). The primary Silicon

particles show considerable refinement and their distribution tends to become

more uniform when the casting is solidified while being vibrated. It is believed

that the application of vibrations to the mold during solidification of

hypereutectic Al-Si alloys limits the growth time and disrupts the layered

growth process of primary Silicon particles, thus influencing their size.

− Finally, with the application of mechanical mold vibrations, refinement of the

primary Si particles can be obtained without the use of chemical additives.

For example, Phosphorous is usually added to refine primary Silicon. But then

eutectic silicon modifiers like strontium or sodium cannot be added because

Phosphorus tends to poison their effect by forming the phosphide of these

elements. With the application of vibrations, Phosphorous additions may not

be required and Sr may then be used to modify the eutectic Si resulting in an

alloy with both excellent strength and ductility (from the modified eutectic Si)

and excellent wear resistance (from the refined primary Si).

110

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