The Effect of Solutionizing Heat Up Rate and Quench Rate on ...

The Effect of Solutionizing Heat Up Rate and Quench Rate on the Grain Size and Fracture Mode of a

6061 Alloy Pressure Vessel

by

KYLE EVAN KULPINSKI

Submitted in partial fulfillment of the requirements

For the degree of Master of Science

Thesis advisor: Prof. John J. Lewandowski

Department of Materials Science and Engineering

CASE WESTERN RESERVE UNIVERSITY

May, 2012

Kyle Kulpinski

Master’s

John Lewandowski

Gary Michal

David Schwam

4.11.12

Dedication

To my parents and my sister, for their love, support, and guidance these many years.

1

Contents

Table of Contents

List of Tables

List of Figures

Acknowledgements

Abstract

1 Background & Introduction 11

1.1 Introduction to Aluminum Alloys……………………………………………………... 11

1.2 Introduction to Pressure Vessels………………………………………………………. 14

1.3 Heat Treatment of 6061 Aluminum…………………………………………………… 15

1.3.1 Work Hardening…………………………………………………………… 15

1.3.2 Annealing/Homogenization and Recovery……………………………….. 16

1.3.3 Grain Growth & Recrystallization………………………………………… 17

1.3.4 Solution Heat Treating…………………………………………………….. 18

1.3.5 Role of Quenching…………………………………………………………. 19

1.3.6 Aging………………………………………………………………………. 20

1.4 Goals of Investigation…………………………………………………………………. 21

2 Experimental Procedures 23

2.1 Material Chemistry……………………………………………………………………. 23

2.2 Product Forms & Specimen Geometry……………………………………………….. 23

2.3 Strain Measurements………………………………………………………………….. 27

2.4 Microstructure & Metallography……………………………………………………… 27

2.5 Heat Treatments……………………………………………………………………….. 30

2.5.1 Standard Treatment……………………………………………………….. 30

2.5.2 Salt Bath Solution Heat Treatment………………………………………… 30

2.5.3 Induction Heating of As-Spun Material…………………………………… 31

2.5.4 Quenching…………………………………………………………………. 33

2.5.5 Aging………………………………………………………………………. 33

2

2.6 Mechanical Testing……………………………………………………………………. 33

2.6.1 Tension Testing……………………………………………………………. 33

2.6.2 Hardness Testing…………………………………………………………... 34

2.6.3 4 Point Bending……………………………………………………………. 34

2.6.4 Controlled Surface Strain Over Mandrel………………………………….. 36

2.7 Surface Analysis………………………………………………………………………. 37

2.8 Disturbed Layer Analysis……………………………………………………………… 37

2.9 Void-Like Features & Iron-Containing Particle Analysis……………………………... 37

3 Experimental Results and Analysis 39

3.1 Strain Measurements…………………………………………………………………… 39

3.2 Tension Results………………………………………………………………………… 39

3.3 Hardness & Grain Size…………………………………………………………………. 41

3.4 Disturbed Layer Results………………………………………………………………... 44

3.5 Void-Like Features & Iron-Containing Particles……………………………………….. 45

3.6 Images of Bend Results………………………………………………………………… 47

3.6.1 Standard Commercial Treatment…………………………………………… 48

3.6.2 Salt Bath SHT with Oil Quench…………………………………………….. 50

3.6.3 Salt Bath SHT with Boiling Water Quench…………………………………. 52

3.6.4 Salt Bath SHT with Warm Water Quench…………………………………... 55

3.6.5 Salt Bath SHT with Ice Water Quench……………………………………… 58

3.6.6 Induction SHT with Warm Water Quench………………………………….. 61

3.6.7 Standard Commercial Heat Treated 7” OD Specimen……………………… 63

4 Discussion 65

4.1 Tension Results…………………………………………………………………………. 65

4.2 Iron-Containing Particles & Void-Like Features……………………………………….. 65

4.3 Fracture of Small Diameter Material…………………………………………………… 68

4.4 Slow Bend ID Surface Cracks………………………………………………………….. 70

4.5 Quench Rate Effects……………………………………………………………………. 73

4.6 Solutionizing Heat Up Rate…………………………………………………………….. 75

4.7 Grain Size Effects on Fracture Mode…………………………………………………… 79

5 Conclusion 84

3

6 Future Studies 86

A Appendix 87

A.1 Standard SHT Bend Images……………………………………………………………. 87

A.2 Salt Bath SHT Bend Images……………………………………………………………. 92

A.3 Induction SHT Bend Images……………………………………………………………. 114

A.4 Small Diameter “Banana-Shaped” Bend Images……………………………………….. 126

A.5 Small Diameter Transverse Bend Images………………………………………………. 147

4

List of Tables

Table 1.1- Aluminum Alloy series [1] …………………………………………………………….. 11

Table 1.2- Nominal Compositions for 6xxx series [6]…………………………………………….. 13

Table 1.3 - Strain hardening factors for work hardened aluminum [10]…………………………... 16

Table 1.4 - Resulting strength as a function of solution treating temperature [12]………………... 18

Table 2.1- Chemistry for received material of large (10") and small (7") diameter heads………… 23

Table 2.2- Wecks Reagent composition……………………………………………………………. 28

Table 2.3- Composition of salt bath used for SHT………………………………………………… 30

Table 2.4 - Quench media and their associated quench rate……………………………………. 33

Table 3.1 - Hardness values for “banana-shaped” specimens……………………………………… 43

5

List of Figures

Figure 1.1- Aluminum Temper Designations [4]……………………………………………………. 12

Figure 1.2- Diagram of principal stresses present in a pressure vessel head [8]……………………. 14

Figure 1.3 - Image of small grains transitioning to large grains along a cylinder head…………….. 17

Figure 1.4- Effect of Quench rate on tensile strength for various aluminum alloys [10]…………… 19

Figure 1.5 - Effect of Aging time and temperature on 6061 [11]…………………………………… 20

Figure 1.6 - Schematic of extrusion process for large diameter tubes for use in the production of pressure vessels……………………………………………………………………………………… 22

Figure 1.7- Schematic of extrusion process from plate material for small diameter tube for use in the production of pressure vessels………………………………………………………………………. 22

Figure 2.1- Received cylinder (left) and dome head (right) with fiber wrapping still attached……. 24

Figure 2.2- Band of large grains along region of interest of received cylinder head………………. 24

Figure 2.3- Dome head with ROI marked with X's (left) and sectioned dome with tensile specimens and “banana-shaped” specimens visible (right)…………………………………………………………. 25

Figure 2.4 - Schematic of cylinder head with specimen locations marked, dotted lines indicate region of interest………………………………………………………………………………………………. 25

Figure 2.5- Dimensions for tensile specimens (top), longitudinal “banana-shaped” specimens (middle), and transverse specimens (bottom)…………………………………………………………………. 26

Figure 2.6- Section of cylinder head after spinning operation with markings visible……………… 27 Figure 2.7-Surfaces polished marked with red lines for “banana-shaped” specimens (top) and tensile specimens (bottom)…………………………………………………………………………………. 28

Figure 2.8- Metallography of As Spun ROI (top) and standard solution heat treatment (bottom)…. 29

Figure 2.9 - Metallography of 7" OD ROI after spinning operation and standard heat treatment…. 29

Figure 2.10 - Furnaces used for salt bath solution heat treatments…………………………………. 31

Figure 2.11- Induction furnace used for SHT, samples placed in area indicated by red arrow…….. 32

Figure 2.12- Locations of hardness indents taken………………………………………………….. 34

Figure 2.13- Example of 4 point bend test…………………………………………………………. 35

Figure 2.14- Schematic of mandrel distances for slow 4 point bend test………………………….. 36

Figure 2.15- Schematic of controlled surface strain bent over a mandrel…………………………. 36

Figure 2.16 - Close up SEM image of fracture surface with void-like features present on fracture surface……………………………………………………………………………………………… 38

Figure 2.17- Locations of Images taken (top) and example of filtering results from backscattered image taken of polished surface…………………………………………………………………………... 38

Figure 3.1 Strain results from measurements of dimensional changes of marked lines after spinning operation. Thickness denotes strain perpendicularly to the strain described as spacing.…………. 39

6

Figure 3.2- Laser confocal images of a fast (10-2/s) strain rate sample…………………………….. 40

Figure 3.3- Laser confocal images of a slow (10-6/s) strain rate sample…………………………… 40

Figure 3.4 - Comparison in metallography between a standard solution heat treated sample (left) and an induction solution heat treated sample (right)……………………………………………………… 41

Figure 3.5- Grain Size results for various heat treatments and quenches………………………….. 41

Figure 3.6- TEM cross section of the disturbed layer present at ID surface of 10” OD cylinder

head………………………………………………………………………………………………… 44

Figure 3.7 – TEM cross section of the lack of obvious disturbed layer not featured on the ID

surface of 7” OD cylinder head.......................................................................................................... 44

Figure 3.8 - XEDS of iron-containing particles (white)……………………………………………. 45 Figure 3.9- Overview of iron-containing particles and void-like features at the ID surface and

mid-point of the through thickness for 7" and 10" cylinder heads…………………………………. 46

Figure 3.10- Example of a crack feature (indicated by arrow) formed during a slow bend test to 560 MPa of a 10” OD cylinder head “banana-shaped” specimen………………………………………. 47

Figure 3.11 - ID surface of commercial standard heat treatment after slow bend test……………... 48

Figure 3.12 - Fracture surface of a sample given the standard heat treatment and quench, and

subjected to a controlled strain to ε = 0.135………………………………………………………… 49

Figure 3.13- ID surface of a “banana-shaped” specimen SHT in a salt bath and quenched in oil….. 50

Figure 3.14 - Fracture surface of a sample SHT in salt, quenched in oil, and subjected to a

controlled strain to ε = 0.135……………………………………………………………………….. 51

Figure 3.15- ID surface of a “banana-shaped” specimen SHT in salt and quenched in boiling water upon completion of a slow bend test to 560 MPa…………………………………………………... 52 Figure 3.16 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in boiling water subjected to a controlled strain to ε = 0.135………………………………………………………… 53

Figure 3.17 - Fracture surface of a sample SHT in salt, quenched in boiling water, and broken open to reveal fracture surface………………………………………………………………………………. 54

Figure 3.18 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in 40°C

water upon completion of a slow bend test to 560 MPa……………………………………………. 55 Figure 3.19 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in 40°C water subjected to a controlled strain to ε = 0.135………………………………………………………… 56

Figure 3.20 - Fracture surface of a sample SHT in salt, quenched in 40°C water, and broken open

to reveal fracture surface……………………………………………………………………………. 57

Figure 3.21 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in ice water

upon completion of a slow bend test to 560 MPa………………………………………………….. 58 Figure 3.22 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in ice water subjected to a controlled strain to ε = 0.135……………………………………………………….. 59

7

Figure 3.23 - Fracture surface of a sample SHT in salt, quenched in ice water, and broken open to reveal fracture surface……………………………………………………………………………… 60

Figure 3.24 - ID surface of a “banana-shaped” specimen SHT in an induction furnace and

quenched in 40°C water upon completion of a slow bend test to 560 MPa……………………….. 61 Figure 3.25 - ID surface of a “banana-shaped” specimen SHT in an induction furnace and

quenched in 40°C water subjected to a controlled strain to ε = 0.135…………………………….. 62

Figure 3.26- ID surface of a “banana-shaped” specimen given the standard heat treatment and

quench by collaborator upon completion of a slow bend test to 560 MPa………………………… 63

Figure 3.27 - ID surface of a “banana-shaped” specimen given the standard heat treatment and

quench by collaborator upon completion of a controlled strain to ε = 0.135……………………… 64

Figure 4.1- Comparison of grain sliding behavior between samples at slow strain rate

(ε = 10-2/s) and fast strain rate (ε = 10-6/s)…………………………………………………………. 65

Figure 4.2 - Schematic of image locations and orientations for images presented

in Figures 4.3-4.5…………………………………………………………………………………… 66

Figure 4.3- Image with iron-containing particles filtered out in the longitudinal orientation

at mark #13 in the unstrained cylinder wall………………………………………………………... 67

Figure 4.4 Image with iron-containing particles filtered out in the longitudinal orientation

at mark #5 in the region of interest…………………………………………………………………. 67

Figure 4.5 - Image with iron-containing particles filtered out in the longitudinal orientation

at mark #2 in the cylinder head…………………………………………………………………….. 68

Figure 4.6 - Fracture surface of a 7" “banana-shaped” specimen upon completion of a

controlled surface strain to ε = 0.270………………………………………………………………. 69 Figure 4.7 - Grain size change for the through thickness in the ROI for a 7" cylinder head.

Sample anodized by Innoval Technologies…………………………………………………………. 69

Figure 4.8 - (a) Standard Commercial (b) Salt SHT with oil quench (c) Salt SHT with boiling water quench (d) Salt SHT with warm water quench (e) Salt SHT with ice water quench (f) Induction SHT with warm water quench……………………………………………………………………………. 71 Figure 4.9- Crack present on the ID surface of a “banana-shaped” specimen

from a 7" cylinder head tested in slow 4 point bending to 560 MPa……………………………..... 72

Figure 4.10 - Crack present on the ID surface of a “banana-shaped” specimen

from a 7" cylinder head strained to ε = 0.135 via a controlled surface strain over a mandrel……… 72

Figure 4.11 - Overview of the ID surface of samples SHT in a salt bath after controlled strain to ε=0.135 of (a) ice quench (b) warm water quench (c) boiling water quench (d) oil quench……….. 74

Figure 4.12 - Fracture surface of a large diameter “banana-shaped” specimen SHT in a salt bath,

and quenched in ice water………………………………………………………………………….. 75

8

Figure 4.13 - Fracture surface of a large diameter “banana-shaped” specimen SHT in a salt bath,

and quenched in oil………………………………………………………………………………… 75

Figure 4.14- Time to solution heat treat temperature (535°C) for Induction furnace and salt bath.. 76

Figure 4.15 - Local melting features on the fracture surface of an induction heat treated sample… 77

Figure 4.16 - Overheated induction heat treated sample yields small grains and melted features. Sample etched with Wecks Reagent……………………………………………………………….. 78

Figure 4.17 - ID surface of an overheated induction sample upon completion of a controlled

surface strain……………………………………………………………………………………….. 78

Figure 4.18 - Images of the ID surface for (a) Standard commercial treatment (b) Salt SHT (c) Induction SHT……………………………………………………………………………………… 79

Figure 4.19 - Dogbone tensile specimens with varying degrees of strain imparted prior to

SHT [17]……………………………………………………………………………………………. 80

Figure 4.20 - Relation of grain size to fracture mode in slow strained specimens of Al-Mg-Si alloy systems [20]………………………………………………………………………………………… 82

Figure 4.21 - Fracture surfaces of “banana-shaped” samples upon completion of a controlled

surface strain test to ε = 0.135 of a (left) standard SHT (right) induction SHT.……………………. 83

9

Acknowledgements

I would like to recognize the following for making this project possible: Dr. John Lewandowski for his time, support, and guidance during the course of this project. Dr. Henry Holroyd for his time, support and his shared considerable knowledge base. The thoughts and advice shared made this project possible. Prof. Michal and Prof. Schwam for the use of their groups equipment. Chris Tuma for his patience, expertise and aid in mechanical testing. Rich Tomazin for his help fixing the induction furnace and his aid in carrying experiments out. The machine shop for their endless work producing samples in a professional and timely manner. Our commercial collaborator, for supplying the cylinder material used for testing. Work was supported by multiple industrial collaborators

10

The Effect of Solutionizing Heat Up Rate and Quench

Rate on the Grain Size and Fracture Mode of a 6061 Alloy Pressure Vessel

Abstract

By

KYLE E. KULPINSKI

Current production techniques for large diameter 6061 alloy seamless pressure vessels can lead to

cylinder heads with regions of large (1mm+) grains that fracture in a low ductility intergranular fashion

along the circumference of the head during proof tests. This phenomena and fracture mode was

reproduced in commercially produced cylinders by a combination of slow strain rate (10-6/s) 4-point

bending and controlled surface strain experiments. As-spun heads were commercially produced and

given solution heat treatments at CWRU via either a salt bath or an induction coil, the former followed

by quenching at rates from 180°C/s to 18°C/s. The effects of these different heat treatments on the

resulting grain size, crack resistance and fracture mode were compared to the standard commercial

treatments. Induction heated samples produced grain sizes below 150 µm and exhibited a significantly

higher crack resistance, fracturing in a transgranular ductile manner.

11

Chapter 1

Background & Introduction

1.1 Introduction to Aluminum Alloys

Aluminum and its alloys are one of the most prevalent metallic systems in use around the world for a

wide variety of applications. Aluminum is the third most abundant element and through its ore (found

in over 270 different naturally occurring minerals), the most abundant metal found in the Earth’s crust

[1]. Its combination of density, ductility, cost, recycling ability and corrosion resistance is nearly

unmatched. However, pure aluminum does not possess great strength, typically exhibiting yield

strengths at room temperature of around 20 MPa, and an ultimate tensile strength between 30-70 MPa

when tested at room temperature [2]. These strength limitations limit its use primarily to cladding

applications. The vast majority of aluminum consumed is in the form of cast and wrought alloys. Cast

aluminum alloys are limited in available shapes and strength, while wrought alloys are used in greater

quantities in engineering applications. Wrought aluminum alloys are classified in families by the

Aluminum Association International Alloy Designation H35.1 [3]. These families are differentiated by

the alloying elements present and the summary of the families is presented in Table 1.1.

Series Primary alloying elements 1xxx Few alloying elements present 2xxx Cu 3xxx Mn 4xxx Si 5xxx Mg 6xxx Si & Mg 7xxx Zn 8xxx Misc. 9xxx Reserved for future uses

Table 1.1- Aluminum Alloy series [1]

12

Each family offers different advantages depending on the desired application, while the 1xxx family

performs similarly in mechanical tests to pure aluminum. In addition, there exist various temper

designations defined by ANSI standards, which are summarized in Figure 1.1.

Figure 1.1- Aluminum Temper Designations [4]

13

The material used in this project is classified as an aluminum-magnesium-silicon alloy, more

specifically a 6061 alloy. Table 1.2 gives an overview of the nominal compositions of 6xxx series

alloys, including 6061 alloy. The 6061 alloy shares some general characteristics with other 6xxx alloys

in that it is heat treatable, offers high corrosion resistance and formability, while exhibiting moderate

strength [5].

Alloy name UNS no. Si (%) Fe (%) Cu (%) Mn (%) Mg (%) Cr (%) Zn (%)

Ti (%) others total (%)

6003 A96003 0.35-1.0 0.6 0.1 0.8 0.8-1.5 0.35 0.2 0.1 0.15

6005 A96005 0.6-0.9 0.35 0.1 0.1 0.40-0.6 0.1 0.1 0.1 0.15

6005A A96005A 0.50-0.9 0.35 0.3 0.5 0.40-0.70 0.3 0.2 0.1 0.15

6009 A96009 0.6-1.0 0.5 0.15-0.6 0.20-0.8 0.40-0.8 0.1 0.25 0.1 0.15

6010 A6010 0.8-1.2 0.5 0.15-0.6 0.20-0.8 0.6-1.0 0.1 0.25 0.1 0.15

6013 A96013 0.6-1.0 0.5 0.6-1.1 0.20-0.8 0.8-1.2 0.1 0.25 0.1 0.15

6020 A96020 0.40-0.9 0.5 0.30-0.9 0.35 0.6-1.2 0.15 0.2 0.15 0.15

6022 A96022 0.8-1.5 0.05-0.20 0.01-0.11 0.02-0.10 0.45-0.7 0.1 0.25 0.15 0.15

6053 A96053 20 0.35 0.1 1.1-1.4 0.15-0.35 0.1 0.15

6060 A96060 0.30-0.6 0.10-0.30 0.1 0.1 0.35-0.6 0.05 0.15 0.1 0.15

6061 A96061 0.40-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.35 0.25 0.15 0.15

6063 A96063 0.20-0.6 0.35 0.1 0.1 0.45-0.9 0.1 0.1 0.1 0.15

6066 A96066 0.9-1.8 0.5 0.7-1.2 0.6-1.1 0.8-1.4 0.4 0.25 0.2 0.15

6070 A96070 1.0-1.7 0.5 0.15-0.40 0.40-1.0 0.50-1.2 0.1 0.25 0.15 0.15

6082 A96082 0.7-1.3 0.5 0.1 0.40-1.0 0.6-1.2 0.25 0.2 0.1 0.15

6101 A96101 0.30-0.7 0.5 0.1 0.03 0.35-0.8 0.03 0.1 0.15

6105 A96105 0.6-1.0 0.35 0.1 0.15 0.45-0.8 0.1 0.1 0.1 0.1

6111 A96111 0.6-1.1 0.4 0.50-0.9 0.10-0.45 0.50-1.0 0.1 0.15 0.1 0.15

6151 A96151 0.6-1.2 1 0.35 0.2 0.45-0.8 0.15 0.25 0.15 0.15

6162 A96162 0.40-0.8 0.5 0.2 0.1 0.7-1.1 0.1 0.25 0.1 0.15

6201 A96201 0.50-0.9 0.5 0.1 0.03 0.6-0.9 0.03 0.1 0.15

6262 A96262 0.40-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.14 0.25 0.15 0.1

6351 A96351 0.7-1.3 0.5 0.1 0.40-0.8 0.40-0.8 0.2 0.2 0.15

6463 A96463 0.20-0.6 0.15 0.2 0.05 0.45-0.9 0.05 0.15

6560 A96560 0.30-0.7 0.10-0.30 0.05-0.20 0.2 0.20-0.6 0.05 0.15 0.1 0.15

6951 A96951 0.20-0.50 0.8 0.15-0.40 0.1 0.40-0.8 0.2 0.15

Table 1.2- Nominal Compositions for 6xxx series [6]

14

Typical 6061 applications include welded structural members (i.e. automotive frames), rail road cars,

pipelines and of interest in this study, pressure vessels [7].

1.2 Introduction to Pressure Vessels

A pressure vessel is a closed container in which liquids or gasses are stored at a pressure different than

the surrounding pressure. Uses range from small containers of compressed oxygen to industrial tanks

holding propane. There are two principal stresses affecting the pressure vessel, longitudinal or

meridional, and latitudinal or hoop stress. These stresses are indicated in Figure 1.2, where stresses

along the meridional axis cause cracks to form perpendicular to the stress along the circumference of

the head. Stresses along the Latitudinal axis would cause cracks to form running longitudinally up and

down the vessel walls. The cracks under investigation in this report are due to longitudinal stresses.

Figure 1.2- Diagram of principal stresses present in a pressure vessel head [8]

These stresses can be calculated through the equations given below [9].

𝜎𝜃 =𝑝(𝑟 + 0.6𝑡)

𝑡𝑡

15

𝜎𝑙𝑙𝑙𝑙 =𝑝(𝑟 − 0.4𝑡)

2𝑡𝑡

1.3 Heat Treatment of 6061 Aluminum

In order for 6061 aluminum alloys to achieve the strengths needed for practical applications, they must

be heat treated. Heat treatments have a variety of steps that influence the final material properties.

These steps include homogenization, solution heat treatments, quenching, and aging. Work hardening

as well as cold working can also increase the strength. These various treatments/processes are

summarized below.

1.3.1 Work Hardening

In the case of wrought aluminum, deformation processing the aluminum into its desired shape imparts a

certain amount of work into the material. Whether desired or not, this imparted work can change

mechanical properties such as tensile strength, hardness, shear strength, creep strength and notch tensile

strength [1]. Aluminum gains improvements in these areas while sacrificing ductility. The work

hardening characteristics of aluminum alloys are very dependent on the type of alloy system and heat

treatment. The stress-strain curves of work hardened aluminum can be modeled by

𝜎 = 𝑘𝜖𝑙

Where 𝜎 is the true stress, 𝑘 is the stress at unit strain, 𝜖 is the true strain, and 𝑛 is the strain hardening

exponent. Values of the strain hardening exponent for various alloys including 6061 are reproduced in

Table 1.3.

16

Table 1.3 - Strain hardening factors for work hardened aluminum [10]

Microstructurally, aluminum is able to reach these mechanical strengths through work hardening

through crystallographic slip. Transmission electron microscopy can be used to image the

accumulation of dislocations after work has been imparted, revealing the increased dislocation

densities. Lattice distortions and the interaction stresses between dislocations result in strain hardening

in the material [10].

1.3.2 Annealing/Homogenization and Recovery

While strain hardening wrought aluminum can increase some desirable properties, annealing at

intermediate temperatures can relieve the accumulation of dislocations, thereby reducing the strength

and enabling more deformation strain to be imparted to the material. In high purity aluminum,

annealing can take place at near room temperature, resulting in a material that is strain free. However,

most commercial aluminum alloys only undergo strain relief at elevated temperatures. The first

changes in the material are typically a decrease in the number of dislocations present, while other

dislocations are rearranged into sub-grain structures. These changes cause a corresponding decrease in

properties such as strength and hardness [1]. Homogenization is a heat treatment similar to annealing,

which occurs at a temperature above the solubility limit of aluminum [1].

17

1.3.3 Grain Growth & Recrystallization

During recrystallization, visually resolvable grains first begin to appear. These grains have a structure

that has very few dislocations present. For recrystallization to occur, the material must be at either an

elevated temperature or held for a longer time when compared to the recovery process.

Recrystallization rate can be mathematically modeled by the equation

1𝑡

= 𝑘𝑒−𝑎 𝑇�

Where k and a (can be replaced by Q/R where R is the gas constant and Q is similar to activation

energy) are material dependent values, t is time, and T is the absolute temperature. While aluminum

recrystallization follows this formula relatively well, dispersed secondary phases and other alloying

elements present can interact to speed up or slow down the recrystallization process [10].

Perhaps the most important factor in the grain size after recrystallization is the amount of cold

work and corresponding dislocations initially present. For a specific material/part history, there exists a

critical strain that will produce the largest grains, which manifests in cylinder heads going from the

wall to the top of the head. A general illustration of this phenomenon in a sectioned cylinder head (of a

different manufacturing route) is given in Figure 1.3.

Figure 1.3 - Image of small grains transitioning to large grains along a cylinder head

7 mm

18

The recrystallized grain size is also affected by the alloying elements present. Elements such as copper,

iron, magnesium and manganese can decrease grain size in some aluminum alloys. For the studied

6061 alloy, titanium, strontium, and small amounts of chromium have been shown to help refine the

grain size, while the presence of iron and a large excess of chromium can decrease the efficiency of

these refiners [11].

1.3.4 Solution Heat Treating

Solution heat treatments serve an important role in aluminum processing. The purpose of this step is to

dissolve as much of the solutes into solid solution as possible within the aluminum matrix. This step

must balance the need to reach a high enough temperature to dissolve the precipitates while avoiding

partial melting and overheating. Depending on the alloy, temperatures in the range of 485ºC - 560ºC

can be used for solution heat treatments. 6061 alloys are typically solution heat treated around 530ºC

[12]. Table 1.4 shows the effect of some solution treating temperatures on the resulting strengths.

Table 1.4 - Resulting strength as a function of solution treating temperature [12]

Solution heat treat time depends on both the type of alloy as well as the geometry of the part.

Additionally, the time held at temperature affects the final grain size of the part. Once the solutes have

19

been put back into solution, quenching is used to create a supersaturated solution to facilitate

subsequent precipitation hardening.

1.3.5 Role of Quenching

Quenching is the process of cooling down the material to a lower temperature (typically room

temperature) after solution heat treatment. This cooling process is used to preserve the solid solution.

Typically, the highest possible strengths are obtained with the fastest quenching rates, though this is not

always the case. However, rapid quenching can sometimes distort the material, potentially cracking or

rendering the part unusable due to its new geometry. Conversely, while a slow quench may not crack

or distort a sample, it may allow constituents out of the solid solution and decrease the overall strength

and hardness of the material. If the cooling rate is significantly slow, a large decrease in toughness can

result. Figure 1.4 shows examples of the effect cooling rates have on tensile strength for various

aluminum alloys, including a 6061 alloy.

Figure 1.4- Effect of Quench rate on tensile strength for various aluminum alloys [10]

20

1.3.6 Aging

Another processing method to increase mechanical characteristics is artificial aging. While many

aluminum alloys can be “naturally” aged (i.e. at room temperature), this process takes considerable

time before any appreciable increase in properties is observed [12]. The more common commercial

method is artificial aging, where the quenched material is held at a temperature ranging from 90-210ºC.

The desired temperature and time at the set temperature depends on the alloy used. The 6061 type

alloy is typically aged between 150-170ºC for a balance between time spent on a part and strength

gained [12]. Figure 1.5 shows the change in tensile strength for various temperature ages as a function

of time at temperature for the 6061 alloy system.

Figure 1.5 - Effect of Aging time and temperature on 6061 [11]

21

1.4 Goals of Investigation

While there exists a long history of optimizing heat treatments for aluminum alloys, specific parts need

differing series of treatments due to varying processing routes and geometries. For large diameter

seamless pressure vessels made of 6061, current production techniques lead to cylinders that may pass

traditional qualification tests but exhibit material failures at service level loads. Figure 1.6 illustrates

the production method for the base tube material of large diameter pressure vessels, while Figure 1.7

illustrates the production of the base tube for small diameter pressure vessels. The head of the pressure

vessel can be produced by a variety of different techniques such as spinning, hot forming and cold

forming. In this commercial product the heads were produced by a hot spinning operation, in which

the tube itself was spinning about its longitudinal axis, and a spinning wheel was used to form the head

in successive passes.

Various products produced via this approach have failed proof test prior to product release. Previous

attempts to alleviate this problem failed to even replicate the failure mechanism. Therefore, the goals

of this project were to first replicate the failures experienced in service using cylinder head material

provided by a commercial supplier. Mechanical testing (4 point bending and controlled straining of

material) and SEM imaging were used for documentation. The second goal was to solve the problem

by developing potential commercially viable processing methods by studying the effects of varying

solution heat treatment heat up rate, quench rate, spin temperature, and the addition of an annealing

step to eliminate the failure problem. Various microstructure characterization techniques and

mechanical testing were used to document the observations.

22

Figure 1.6 - Schematic of extrusion process for large diameter tubes for use in the production of pressure vessels

Figure 1.7- Schematic of extrusion process from plate material for small diameter tube for use in the production of pressure vessels

23

Chapter 2

Experimental Procedures

2.1 Material Chemistry

Two variants of 6061 aluminum alloys were used to study the effects of processing changes on

microstructure, strength, fracture behavior and crack resistance. Material of chemistry A (Table 2.1)

was used in 7 inch diameter pressure vessels and was supplied in plate form that was made into a tube

by the operation (at room temperature) described in Figure 1.7. This material was annealed during the

cold drawing operation, and then underwent a spinning operation to produce the head. Material of

chemistry B (Table 2.1) was used in 10 inch diameter pressure vessels. This material was supplied in

tube form prior to heading.

Liner Excess Si Fe Cu Mg Mn Si Cr A 7" OD -0.073 0.48 0.21 0.94 0.01 0.685 0.155 B 10" OD 0.021 0.18 0.32 0.89 0.01 0.62 0.064

Table 2.1- Chemistry for received material of large (10") and small (7") diameter heads

2.2 Product Forms and Specimen Geometries

Figure 2.1 shows show the manufactured dome heads. Three different locations were removed from

each of the domes to capture the region that exhibited excessive grain growth and the locus of failure as

illustrated in Figure 2.2

24

Figure 2.1- Received cylinder (left) and dome head (right) with fiber wrapping still attached

Figure 2.2- Band of large grains along region of interest (ROI) of received cylinder head

In order to guarantee that testing was conducted in appropriate zones, the Region of Interest (ROI) was

marked along the outer circumference of the head in an inch long demarcation (Figure 2.3). Figure 2.4

gives an overall schematic of the locations of samples removed from the cylinder head. Tensile

25

specimens were removed from the ROI with dimensions labeled in Figure 2.5 in order to measure

tensile properties as well as document strain rate effects on accommodation of deformation.

Figure 2.3- Dome head with ROI marked with X's (left) and sectioned dome with tensile specimens and “banana-shaped” specimens visible (right)

In addition, longitudinal samples were removed from the head to the wall to capture the transition

region in the same orientation as the tensile specimens. These “banana-shaped” samples were machined

to the dimensions represented in Figure 2.5. Transverse samples were taken in a perpendicular

orientation to the “banana-shaped” and tensile specimens and were machined to the dimensions in

Figure 2.5.

Figure 2.4 - Schematic of cylinder head with specimen locations marked, dotted lines indicate region of interest

26

Figure 2.5- Dimensions for tensile specimens (top), longitudinal “banana-shaped” specimens (middle), and transverse specimens (bottom)

27

2.3 Strain Measurements

Prior to the material undergoing the heading process, the tube ID was marked with indelible ink lines

numbered from 1 to 13. These lines were spaced 1” apart and drawn so they were initially 1” wide.

Figure 2.6 shows the markings following the heading operation. The #1 mark is not visible as the

heading process results in producing its location in the snout of the head.

Figure 2.6- Section of cylinder head after spinning operation with markings visible

The change in width and spacing of the markings were measured and tabulated, the results of which

were used to estimate the strain imparted during the spinning at different regions of the spun head.

2.4 Microstructure & Metallography

Metallography was carried out on samples of varying condition. The tension samples were polished

but not etched. Figure 2.7 shows the sides to be polished on the longitudinal (banana-shaped) and

tension samples.

2 in

28

Figure 2.7-Surfaces polished marked with red lines for “banana-shaped” specimens (top) and tensile specimens (bottom)

Samples were polished to 0.05 µm through steps of SiC paper, diamond suspension (water based), and

finally a 0.05 µm Al2O3 suspension. The polishing process was based on the work of George F. Vander

Voort on aluminum alloys [13]. Voids and iron-containing inclusions were imaged in the as-polished

state. Grain size analysis was conducted on samples that were submerged in Weck's Reagent (a color

tint etch) for 15-25 seconds at room temperature. Table 2.2 gives the composition of Weck’s Reagent.

Selected samples were polished, etched with Barkers Reagent and anodized to reveal grain size at

Innoval Technologies. Images were taken at 50x up to 400x on an optical microscope, and grain sizes

were calculated using the lineal intercept method [14].

Wecks Reagent 100 mL H2O 4g KMnO3

1g NaOH

Table 2.2- Wecks Reagent composition

29

Figure 2.8- Metallography of As Spun ROI (top) and standard solution heat treatment (bottom), samples anodized at Innoval Technologies

Figure 2.9 - Metallography of 7" OD ROI after spinning operation and standard heat treatment, sample anodized at Innoval Technologies

30

2.5 Heat Treatments

The effects of changes in heat treatment on microstructure and properties were obtained on cylinders

heat treated by a commercial collaborator. In addition, heat treatments were conducted on cylinders

provided in the as-spun condition by the same collaborator that had received no prior heat treatments.

2.5.1 Standard Treatment

Some heads provided by the commercial collaborator were also given a solution heat treatment and age

prior to CWRU receiving them. These samples had a history as follows; a 2 hour SHT to 535°C in an

air furnace, a cold water quench at ~100°C/s, a 1 hour hold at room temperature, and a 4 hour age at

177°C. These samples are designated as Collab in Table 3.1. This standard treatment was given to

both 10” and 7” OD cylinders.

2.5.2 Salt Bath Solution Treatment

In some cases where as-spun material was treated, SHT was conducted in a salt bath. Table 2.3

describes the composition of the salt bath. The salt bath was held at 535ºC. Individual samples were

immersed in the salt bath and held at temperature for 10 minutes, and then quenched in one of 4

different quench mediums; Oil, boiling water, water at 40ºC, and Ice water. Temperature was

monitored with a thermocouple press fitted into hole drilled an inch into the sample and recorded with

software (Figure 2.10).

Molten Salt Composition 30% BaCl2

50% CaCl2

20% NaCl

Table 2.3- Composition of salt bath used for SHT

31

Figure 2.10 - Furnaces used for salt bath solution heat treatments

2.5.3 Induction Heating of As-Spun Material

Samples undergoing SHT via induction heating utilized an induction coil from a vacuum induction

casting unit. A single sample was placed in the coil, held by the thermocouple press fitted in the

sample. The individual sample was heated to 535°C and immediately quenched. The induction

furnace was operating between 20-30 kilowatts during heat up. Temperature was monitored with a

thermocouple press fitted an inch into the end of the sample and recorded with software. Induction

solution heat treatments were completed on 10” OD cylinder head material only. Figure 2.11 shows the

induction furnace used and indicates where the samples were placed in the coil.

32

Figure 2.11- Induction furnace used for SHT, samples placed in area indicated by red arrow

33

2.5.4 Quenching

The effects of quench rate on microstructure and subsequent performance were also studied. 4 quench

rates were tested, ranging from 18°C/s to 180°C/s. Temperature changes were recorded with a

thermocouple placed in the sample and linked to a computer with data acquisition at 5 Hz. Quench

rates were calculated by the average slope of the time-temperature trace from 535°C to 300°C. The

quench media and associated quench rates are given in Table 2.4

Quench Medium Quench Rate (°C/s) Oil 18

Boiling Water (100°C) 30 Warm Water (40°C) 100

Ice Water (0°C) 180

Table 2.4 - Quench media and their associated quench rate

2.5.5 Aging

Samples were aged in a BlueM furnace held at 177ºC for 5 hours. Select samples were held at 177ºC

for >5 hours. The aging curve for 6061 alloys indicates the alloy is relatively insensitive to aging times

between 3 hours and 1 week (Figure 1.5).

2.6 Mechanical Testing

Following the heat treatments and aging, mechanical tests were carried to determine their effects on

crack initiation and growth. These tests consisted of hardness testing, 4 point bending, strain over a

mandrel, tension testing and notch toughness tests, as described below.

2.6.1 Tension Testing

Tension tests on the small tension samples were carried out on an Instron 1125 (screw driven) machine,

34

with data recorded by Test Works 4 testing software. Due to the small area and size of the testing

sample, extension and strain were measured with a digital caliper. Individual tension samples were

pulled at slow (10^-6/s) and fast (10^-2) rates. Samples were pulled to specified and sequential strains

followed by imaging of the surface deformation with a Laser Confocal microscope after each strain

step.

2.6.2 Hardness Testing

Rockwell B hardness tests were carried out in a manner corresponding to ASTM standard E18 [15]. A

100 kg force was applied to a flat surface of a specimen by a 1/16” steel ball. Figure 2.12 gives the

locations of the 5 indentations that were taken ranging from end to end of the specimen, with 3 points

within the specified region of interest. Only samples that had undergone SHT and the subsequent aging

process were tested.

Figure 2.12- Locations of hardness indents taken

2.6.3 4 Point Bending

4 point bending tests were carried out on “banana-shaped” samples in the manner shown in Figure

2.13. The sample was placed so that the ID surface was strained in tension. As shown in Figure 2.13,

special fixtures were made to accommodate the disparate dimensions. Span lengths are given in Figure

2.14. “Banana-shaped” samples were machined flat to a thickness of < 4 mm as shown in Figure 2.14.

Samples were preloaded to ¼ of their calculated stress goal of 560 MPa. Loads were calculated with

the equation

35

𝜎 =3𝐹(𝐿 − 𝐿𝑖)

2𝑏𝑑2

Where F is the load, L is the length of the wider span, Li is the length of the shorter span, b is the width

of the sample, and d is the thickness of the sample. Once preloaded, samples were slowly loaded at a

strain rate of 10-6/s. Machines and software used for these tests were as follows;

Figure 2.13- Example of 4 point bend test

• MTS Model 810 with 458 controller and DATAQ data acquisition system with 20 kip load cell

• 2 x MTS Model 810 with 458 controller and DATAQ data acquisition system with 50 kip load

cell

• Instron model 1361 with Testworks 4 software

• MTS model 810 with 458 controller (mounted horizontally) with labVIEW data acquisition

system.

36

Figure 2.14- Schematic of mandrel distances for slow 4 point bend test

2.6.4 Controlled Surface Strain Over Mandrel

Once the “banana-shaped” samples had reached 560 MPa, they were unloaded and were imaged in the

SEM, they were soaked in liquid nitrogen and then bent over a mandrel to a specified surface strain of ε

= 0.135. The strain in such an experiment can be roughly calculated as follows

𝜀 = 𝑡𝑑

Where 𝜀 is equal to surface strain, 𝑡 is sample thickness, and 𝑑 is mandrel diameter. The procedure

involved first soaking the samples in a cup of liquid nitrogen, followed by bending around a mandrel of

a specified diameter depending on the “banana-shaped” sample thickness. The result of this process is

shown in Figure 2.15

Figure 2.15- Schematic of controlled surface strain bent over a mandrel

37

2.7 Surface Analysis

After the completion of a mechanical test, samples were imaged with one of 2 machines. Tensile

samples were imaged with a Olympus BX62 with scan head FV1000 laser scanning confocal

microscope with Nomarski contrast. “Banana-shaped” specimens and notched specimens were imaged

with an Environmental Dual-Beam system FEI Quanta 200 3D SEM. These images were taken at

either 15.00 kV or 20.00 kV with a spot size of 4.0, and a scan time of 30 µs.

2.8 Disturbed Layer Analysis

In order to investigate whether there existed a “disturbed” layer on the inner diameter of a finished

cylinder from our commercial collaborator, TEM analysis was completed by Innoval Technologies. A

sample of commercially heat treated cylinder material was provided to Innoval to be analyzed. This

material had a cross section cut with an ultramicrotome in the ROI.

2.9 Void-Like Features & Iron-Containing Particle Analysis

Initial fracture images showed evidence of void-like features present within the material. This is

illustrated in Figure 2.16. To better image void-like features and iron-containing particles,

backscattered electron images of a polished surface were taken with a FEI Quanta SEM operated at

15.00 kV. These BSE images were analyzed with ImageJ by adjusting the threshold settings to filter

out either everything not black (voids), or not white (iron-containing particles). Figure 2.17 shows a

backscattered image taken at 15.00 kV, along with its associated images with iron-containing particles

filtered out, and another with void-like features filtered out.

38

Figure 2.16 - Close up SEM image of fracture surface with void-like features present on fracture surface

With these images, ImageJ provides an “Analyze Particles” function which provides data on area

fraction, average particle size and total number of particles. The void-like features and iron-containing

particles were compared to those present in the 7” OD cylinder heads.

Figure 2.17- Locations of Images taken (top) and example of filtering results from backscattered image taken of polished surface

39

Chapter 3

Experimental Results and Analysis

3.1 Strain Measurements

Strain results are given in Figure 3.1. The region of interest corresponds to the locations of greatest

strain.

Figure 3.1- Strain results from measurements of dimensional changes of marked lines after spinning operation. Thickness denotes strain perpendicularly to the strain described as spacing

3.2 Tension Results

Tension samples were strained to similar strains at different strain rates to determine the

accommodation of deformation. Figure 3.2 illustrates the results from a tension test at a fast strain rate.

Significant grain sliding is not observed until a strain of 3.1%. Figure 3.3 illustrates the results from a

tension test at a slow strain rate. The slowly strained sample exhibits a higher degree of grain sliding

behavior than the fast strain rate sample.

40

Figure 3.2- Laser confocal images of a fast (10-2/s) strain rate sample

Figure 3.3- Laser confocal images of a slow (10-6/s) strain rate sample

41

3.3 Hardness & Grain Size

Figure 3.4 gives a comparison between a standard heat treated sample and an Induction heat treated

sample. The grain size changes from >1 mm at the ID surface for the standard treatment, to < 100 µm

for an induction solution heat treated sample. Figure 3.5 summarizes the grain size results for the

various treatments. Quench rate had less effect than SHT heat up rate did on the grain size of the

samples. The factor with the largest effect on grain size was found to be the heat up rate to solution

heat treat temperature.

Figure 3.4 - Comparison in metallography between a standard solution heat treated sample (left) and an induction solution heat treated sample (right). Sample on left anodized by Innoval Technologies.

Sample on right etched with Wecks Reagent.

Figure 3.5- Grain size results for various heat treatments and quenches

0

200

400

600

800

1000

1200

0 50 100 150 200

ID G

rain

Siz

e (µ

m)

Quench Rate (°C/s)

Salt Bath SHT

Air SHT

Commercial StandardTreatment

Induction SHT

7" OD w/ standard SHT

42

Hardness values of the various combinations of SHT and quench rate are summarized in Table 3.1.

Samples quenched at the fastest rate (180ºC/s) exhibited the highest hardness values, while samples

SHT via induction methods had a higher standard deviation and lower average hardness values.

43

Sample ID Heat Medium Quench Rate (°C/s) Avg. Hardness (HRB) Collab Air 100 63 ± 1.48

B6 Salt 100 65 ± 1.56 B7 Salt 100 64 ± 2.10 B8 Salt 100 65 ± 1.75 A6 Salt 30 62 ± 1.04 A7 Salt 30 61 ± 2.07 A8 Salt 30 60 ± 1.88 B9 Salt 180 68 ± 1.11 A9 Salt 180 67 ± 2.87 A10 Salt 18 63 ± 2.11 B10 Salt 18 64 ± 1.87 B11 Salt 18 59 ± 0.71 B12 Salt 18 60 ± 1.48 A11 Salt 180 56 ± 1.30 B3 Salt 30 63 ± 1.22

A12 Air 180 67 ± 2.07 A13 Air 180 68 ± 1.22 A14 Air 180 64 ± 2.77 B13 Air 18 63 ± 1.73 B14 Air 18 60 ± 1.48 B15 Air 18 63 ± 0.80 A15 Air 100 65 ± 0.84 A16 Air 100 65 ± 0.45 A17 Air 100 64 ± 0.84 B16 Air 30 62 ± 2.77 B17 Air 30 63 ± 1.14 B21 Air 30 63 ± 1.67 A18 Air (4hrs) 100 N/A A19 Air (4hrs) 100 61 ± 1.82 B18 Salt (2 hrs) 100 64 ± 1.09 B19 Salt (2 hrs) 100 64 ± 1.30 I1 Induction (3 min) 100 58 ± 3.19 I2 Induction (2 min) 100 58 ± 2.30 I3 Induction (45 sec) 100 58 ± 4.08 I4 Induction + Air (535°C) 100 56 ± 3.87 I5 Induction 100 56 ± 4.01 I6 Induction 100 53 ± 3.78 I7 Induction 100 59 ± 4.18 I8 Induction + Air (550°C) 100 59 ± 3.92

Table 3.1 - Hardness values for “banana-shaped” specimens

44

3.4 Disturbed Layer Results

TEM analysis of the ID surface layer by Innoval Technologies, Inc. in the ROI found the presence of a

shallow (2 µm) disturbed layer on the 10” OD cylinders. This is presented in Figure 3.6. Figure 3.7

illustrates that no disturbed layer was found on the ID surface of the 7” OD cylinder heads.

Figure 3.6- TEM cross section of the disturbed layer present at ID surface of 10” OD cylinder head

Figure 3.7 – TEM cross section of the lack of obvious disturbed layer not featured on the ID surface of

7” OD cylinder head

45

3.5 Void-Like Features and Iron-Containing Particles

Image analysis of iron-containing particles and void-like features at the ID and midpoint of the through

thickness were completed for the 10” OD cylinder heads and 7” OD cylinder heads. Figure 3.8 gives

an XEDS report on the white particles in the image, revealing that they are iron-containing. Void-like

features were found to be more numerous on the ID surface for the 10” cylinder heads when compared

to the same location in the 7” cylinder heads. In the mid-section of the through thickness the area

fraction of void-like features was found to be identical in both the 7” and 10” cylinder heads. The

concentration of iron-containing particles was found to be higher in the 7” cylinder heads as compared

to the 10” cylinder heads in both locations imaged, which was supported by the chemical analysis.

Figure 3.9 gives an overview of these results.

Figure 3.8 - XEDS of iron-containing particles (white)

46

Figure 3.9- Overview of iron-containing particles and void-like features at the ID surface and mid-

point of the through thickness for 7" and 10" cylinder heads

47

3.6 Images of Bend Results

The images taken of the ID surface after slow bending to 560 MPa revealed the formation of some

small, shallow cracks. These cracks were found on all samples tested to 560 MPa. Figure 3.10

illustrates an example of a crack found after undergoing a slow 4 point bend test.

Figure 3.10- Example of a crack feature (indicated by arrow) formed during a slow bend test to 560 MPa of a 10” OD cylinder head “banana –shaped” specimen

48

3.6.1 Standard Commercial Treatment

Figure 3.11 gives an overview of the ID surface of a commercial standard heat treated sample upon

completion of a slow bend test to 560 MPa. Small, shallow cracks are visible throughout the ID

surface of the region of interest, indicated by arrows in Figure 3.10b, c, and d.

Figure 3.11 - ID surface of commercial standard heat treatment after slow bend test

A

D C

B

49

Figure 3.12 captures the fracture surface of the sample presented in Figure 3.11 upon completion of the

controlled strain test. The sample has fractured through the thickness of the sample into two pieces.

The top half of the fracture surface closest to the ID surface shows evidence of smooth, IG brittle

fracture in Figure 3.12c. Void-like features are present on the smooth IG brittle surfaces, indicated by

the arrow in Figure 3.12d

Figure 3.12 - Fracture surface of a sample given the standard heat treatment and quench, and subjected to a controlled strain to ε = 0.135

C D

B A

ID Surface

ID Surface

50

3.6.2 Salt Bath SHT with Oil Quench

Figure 3.13 gives an overview of the ID surface of a “banana-shaped” specimen that was solution heat

treated in a salt bath to 535°C and quenched in oil after completion of a slow bend test to 560 MPa.

The figure shows evidence of small narrow cracks on the surface throughout the region of interest after

the completion of a slow bend test to 560 MPa. Cracks are only ~5 µm wide with lengths up to 600

µm.

Figure 3.13- ID surface of a “banana-shaped” specimen SHT in a salt bath and quenched in oil

A B

C D

51

Figure 3.14 captures the fracture surface of the sample presented in Figure 3.13 upon completion of the

controlled strain test. The sample has fractured through the thickness of the sample into two pieces.

The top half of the fracture surface closest to the ID surface shows evidence of smooth, IG brittle

fracture in Figure 3.13b. Void-like features are present on the smooth IG brittle surfaces, indicated by

the arrow in Figure 3.13d

Figure 3.14 - Fracture surface of a sample SHT in salt, quenched in oil, and subjected to a controlled

strain to ε = 0.135

A B

C D

ID Surface

52

3.6.3 Salt Bath SHT with Boiling Water Quench An overview of the ID surface after a slow bend to 560 MPa of a “banana-shaped” specimen solution

heat treated in a salt bath and quenched at 30°C/s is presented in Figure 3.15a. Small, shallow cracks

are evident throughout the surface of the region of interest after completion of the slow bend test,

indicated by arrows in Figure 3.15b and d.

Figure 3.15- ID surface of a “banana-shaped” specimen SHT in salt and quenched in boiling water

upon completion of a slow bend test to 560 MPa

A

C

B

D

53

An overview of the ID surface after a controlled strain around a mandrel of the “banana-shaped”

specimen from Figure 3.15 is presented in Figure 3.16a. A large and deep crack indicated by the arrow

is evident in Figure 3.14a in addition to several smaller secondary cracks in the area circled. Figure

3.16c shows evidence of a smooth, IG brittle fracture surface, while void-like features are present on

the fracture surface in Figure 3.16d, indicated by an arrow.

Figure 3.16 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in boiling water

subjected to a controlled strain to ε = 0.135

A

C D

B

54

The “banana-shaped” specimen from 3.16 was broken open to reveal the fracture surfaces through the

thickness. Figure 3.17a illustrates an overview of the fracture surface. Figure 3.17b and 3.17c

illustrate evidence of a smooth IG brittle fracture surface. Figure 3.17d is a high magnification view of

the surface and presents void-like features on the fracture surface indicated by the arrow.

Figure 3.17 - Fracture surface of a sample SHT in salt, quenched in boiling water, and broken open to

reveal fracture surface

ID Surface ID Surface

B A

C D

55

3.6.4 Salt Bath SHT with Warm Water Quench An overview of the ID surface after a slow bend to 560 MPa of a “banana-shaped” specimen solution

heat treated in a salt bath and quenched at 100°C/s is presented in Figure 3.18a. Small, shallow cracks


indicated by arrows in Figure 3.18b, c, and d.

Figure 3.18 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in 40°C water

upon completion of a slow bend test to 560 MPa

D C

B A

56

Figure 3.19a is an overview of the ID surface after a controlled strain around a mandrel of a “banana-

shaped” specimen initially presented in Figure 3.18. A large and deep crack indicated by the arrow is

evident in Figure 3.19a in addition to several smaller secondary cracks seen in the area circled. Figure

3.19c shows evidence of a smooth, IG brittle fracture surface, while void-like features are present on

the fracture surface in Figure 3.19d, indicated by an arrow.

Figure 3.19 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in 40°C water

subjected to a controlled strain to ε = 0.135

A

C D

B

57

The “banana-shaped” specimen from Figure 3.19 was broken open to reveal the fracture surfaces

through the thickness. Figure 3.20a illustrates an overview of the fracture surface, with secondary

cracks visible in the area circled. Figure 3.20c illustrates evidence of a smooth IG brittle fracture

surface. Figure 3.20d is a high magnification view of the surface and presents void-like features on the

fracture surface indicated by the arrow.

Figure 3.20 - Fracture surface of a sample SHT in salt, quenched in 40°C water, and broken open to

reveal fracture surface

ID Surface

ID Surface

ID Surface

D C

B A

58

3.6.5 Salt Bath SHT with Ice Water Quench

Figure 3.21a gives an overview of the ID surface after a slow bend to 560 MPa of a “banana-shaped”

specimen solution heat treated in a salt bath and quenched at 180°C/s. Small, shallow cracks are

evident throughout the surface of the region of interest after completion of the slow bend test, indicated

by arrows in Figure 3.21c and 3.21d.

Figure 3.21 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in ice water upon completion of a slow bend test to 560 MPa

C D

B A

59


shaped” specimen initially presented in Figure 3.21. A small crack indicated by the arrow is evident in

Figure 3.22a. Figure 3.22c gives a higher magnification view of the crack from 3.22a. Figure 3.22d

shows evidence of both an IG brittle fracture surface indicated by the arrow and a small area of ductile

fracture indicated by the circle.

Figure 3.22 - ID surface of a “banana-shaped” specimen SHT in salt and quenched in ice water subjected to a controlled strain to ε = 0.135

D C

B A

60

The “banana-shaped” specimen from Figure 3.21 was broken open to reveal the fracture surfaces

through the thickness. Figure 3.23a illustrates an overview of the fracture surface. Figure 3.23c

illustrates evidence of a smooth IG brittle fracture surface. Figure 3.23d is a high magnification view

of the surface and presents void-like features on the fracture surface indicated by the arrow.

Figure 3.23 - Fracture surface of a sample SHT in salt, quenched in ice water, and broken open to reveal fracture surface

ID Surface

ID Surface

C D

B A

61

3.6.6 Induction SHT with Warm Water Quench


specimen solution heat treated in an induction furnace and quenched at 100°C/s. Small, shallow cracks


indicated by arrows in Figure 3.24c and 3.24d.

Figure 3.24 - ID surface of a “banana-shaped” specimen SHT in an induction furnace and quenched in 40°C water upon completion of a slow bend test to 560 MPa

D C

B A

62


shaped” specimen initially presented in Figure 3.24. Small cracks indicated by the arrow are evident in

Figure 3.25c. Figure 3.25d gives a higher magnification view of the surface cracks, showing several

narrow, shallow cracks indicated by arrows.

Figure 3.25 - ID surface of a “banana-shaped” specimen SHT in an induction furnace and quenched in

40°C water subjected to a controlled strain to ε = 0.135

D C

B A

63

3.6.7 Standard Commercially Heat Treated 7” OD Specimen


specimen given the standard heat treatment and quench by our collaborator. Small, shallow cracks are

evident throughout the surface of the region of interest after completion of the slow bend test, oriented

perpendicularly to those seen in the 10” OD specimens, indicated by arrows in Figure 3.26c and 3.26d.

Figure 3.26- ID surface of a “banana-shaped” specimen given the standard heat treatment and quench by collaborator upon completion of a slow bend test to 560 MPa

D C

B A

64


shaped” specimen initially presented in Figure 3.26. Small cracks indicated by the arrow are evident in

Figure 3.27c.

Figure 3.27 - ID surface of a “banana-shaped” specimen given the standard heat treatment and quench by collaborator upon completion of a controlled strain to ε = 0.135

D C

B A

65

Chapter 4

Discussion

4.1 Tension Results

Tension tests at different strain rates (ε = 10-2/s and ε = 10-6/s) resulted in a drastic difference in the how

much the sample was strained before grain sliding behavior was viewed, as well as the amount of grain

sliding at similar strains. Figure 4.1 illustrates the difference in grain sliding between the two different

strain rates at a similar overall amount of strain. Similar findings have been reported for Al-Mg-Si

alloys with large grain sizes [16].

Figure 4.1- Comparison of grain sliding behavior between samples at slow strain rate (ε = 10-2/s) and fast strain rate (ε = 10-6/s)

4.2 Iron-Containing Particles & Void-Like Features

The area fraction of void-like features found in the ID surface of the large diameter cylinder heads was

found to be double that of those found on the ID surface of the small diameter heads (Figure 3.9).

However, the area fraction was found to be identical in the middle section of both the large diameter

and small diameter cylinder heads, and this area in both diameter heads exhibited an intergranular

66

brittle fracture mode. This could potentially be caused by slight overheating during the cylinder head

spinning process resulting in some incipient melting. Current production techniques for the large

diameter heads call for spinning operations to alternate with periodic reheating by a blow torch, with no

method for monitoring the actual temperature of the material during the heading operation. The

combination of reheating and heat generated by the straining of the material might easily cause the

temperature of the material to rise to close to the materials melting temperature. Additionally, the

diameter of these void-like features grows when local liquation occurs during overheating of an

induction SHT, roughly doubling in size from 8 µm to 15 µm. The analysis of the iron-containing

particles confirmed that there was indeed more iron in the small diameter cylinder material when

compared to the large diameter cylinder material. These particles were found to orient along with the

applied strain during the heading operation, and were found to be broken up along the OD surface.

Figure 4.2 gives a schematic of image locations for Figures 4.3-4.5, which illustrate how the iron-

containing rod-like particles are oriented together longitudinally in the cylinder wall (Figure 4.3),

orientated at a roughly 45° angle in the region of interest along with a larger quantity of broken up

particles (Figure 4.4), and nearly all the rod-like particles are broken up in Figure 4.5.

Figure 4.2 - Schematic of image locations and orientations for images presented in Figures 4.3-4.5

67

Figure 4.3- Image with iron-containing particles filtered out in the longitudinal orientation at mark #13 in the unstrained cylinder wall

Figure 4.4 Image with iron-containing particles filtered out in the longitudinal orientation at mark #5 in the region of interest

68

Figure 4.5 - Image with iron-containing particles filtered out in the longitudinal orientation at mark #2 in the cylinder head

4.3 Fracture of Small Diameter Material

While nearly all samples from the 7” cylinder head resisted fracture upon completion of the controlled

strain over a mandrel, one sample was fractured. This sample was tested at a rate of 10-7/s during the

slow bend test, and strained to a surface strain of ε = 0.270. Figure 4.6 illustrates the ID surface cracks

in a 90° orientation to loading, as well as a fracture surface that shows ductile features until greater than

1 mm into the material before transitioning to an intergranular brittle mode.

69

Figure 4.6 - Fracture surface of a 7" “banana-shaped” specimen upon completion of a controlled surface strain to ε = 0.270

This fracture mode transition occurs at a similar depth to a grain size change from 118 µm to 587 µm.

Figure 4.7 illustrates this change in grain size.

Figure 4.7 - Grain size change for the through thickness in the ROI for a 7" cylinder head. Sample anodized by Innoval Technologies

70

4.4 Slow Bend ID Surface Cracks

While all samples performed similarly upon completion of the slow bend test, the results of the

controlled strain over a mandrel differed dramatically. Figure 4.8a illustrates the performance of a

standard commercially treated sample upon completion of the controlled strain. The presence of small,

thin, shallow cracks on the ID surface of all slow bend tested samples suggests that these cracks are

likely related to the thin “disturbed” layer or the oxide layer found on the ID surface of the cylinder

head and are unlikely to disappear regardless of grain size near the surface. Figure 4.8b-f illustrates the

presence of these cracks (arrowed) on the ID surface of several samples of large diameter material with

different heat treatments and quenches.

71

Figure 4.8 - (a) Standard Commercial (b) Salt SHT with oil quench (c) Salt SHT with boiling water quench (d) Salt SHT with warm water quench (e) Salt SHT with ice water quench (f) Induction SHT

with warm water quench

72

These small thin cracks are found on the ID surface of the of the 7” cylinder heads as well, however

they are oriented perpendicularly to those found on the ID surface of the 10” cylinder heads. Figure 4.9

depicts an example of cracks (indicated by arrow) found after completion of the slow bend test. When

put through a controlled strain over a mandrel, these cracks resist opening up, depicted by the arrow in

Figure 4.10.

Figure 4.9- Crack present on the ID surface of a “banana-shaped” specimen from a 7" cylinder head

tested in slow 4 point bending to 560 MPa

Figure 4.10 - Crack present on the ID surface of a “banana-shaped” specimen from a 7" cylinder head strained to ε = 0.135 via a controlled surface strain over a mandrel

73

4.5 Quench Rate Effects

While the quench rate was found to have far less significant effect than solution heat up rate on the

resulting grain size, the performance of the samples was significantly affected. Figure 4.11 illustrates

the difference in crack growth resistance in samples solution heat treated in a salt bath with different

quenches. In Figure 4.11a, the oil quenched sample broke completely in half before completing the

controlled strain, while in Figure 4.11d the ice water quenched sample remained intact. The difference

in crack susceptibility between samples quenched in boiling water and samples quenched in warm

water was negligible. While the ice water quench result is encouraging, it is currently impractical for

commercial use as it would be highly difficult to achieve similar quench rates (~180°C/s) when

quenching entire cylinders.

74

Figure 4.11 - Overview of the ID surface of samples SHT in a salt bath after controlled strain to

ε=0.135 of (a) ice quench (b) warm water quench (c) boiling water quench (d) oil quench Additionally, by increasing the quench rate to 180°C/s the fracture mode transitions to a more ductile

mode. Figure 4.12 displays the fracture surface of a “banana-shaped” specimen quenched in ice water.

This sample was broken open, and the surface shows very little evidence of intergranular brittle

fracture. Conversely, Figure 4.13 illustrates the fracture surface of a “banana-shaped” specimen

quenched in oil. This fracture surface looks to be almost entirely smooth, with little evidence of a

ductile mode of fracture until well through the thickness of the material.

75

Figure 4.12 - Fracture surface of a large diameter “banana-shaped” specimen SHT in a salt bath, and

quenched in ice water

Figure 4.13 - Fracture surface of a large diameter “banana-shaped” specimen SHT in a salt bath, and

quenched in oil

4.6 Solutionizing Heat Up Rate

Figure 4.14 illustrates the time to temperature (535°C) it takes for the two solution heat up methods.

76

The salt bath takes up to 5 minutes to reach the desired temperature, significantly slowing down once it

reaches 400°C. In contrast, the induction furnace takes ~45 seconds at a steady, linear rate of

11.88°C/s. This time to temperature is more than 50 times as fast as the standard commercial treatment

performed by our collaborator. By reaching the solution heat treatment temperature in 45 seconds, the

time for grains to recrystallize and grow is diminished. Though it takes up to 45 minutes to reach the

desired temperature in the current commercial process, it is likely that grain growth is beginning

slightly before reaching 535°C.

Figure 4.14- Time to solution heat treat temperature (535°C) for Induction furnace and salt bath

Temperature control is paramount for the heat up rate of the induction furnace. 6061 aluminum alloy

has a melting temperature around 580°C. If the temperature goal is 535°C with a heat up rate of

12°C/s, a few seconds miss can cause some local liquation. Figure 4.15 illustrates liquation features

from a SEM image of the fracture surface. Figure 4.16 depicts features of liquation at the ID surface of

an induction SHT sample despite forming small grains. Figure 4.17 illustrates the regression of crack

resistance in a sample that has been overheated.

0100200300400500600

0 100 200 300

Tem

pera

ture

(°C

)

Time (Seconds)

Induction vs. Salt

SaltInduction

77

Figure 4.15 - Local melting features on the fracture surface of an induction heat treated sample

78

Figure 4.16 - Overheated induction heat treated sample yields small grains and melted features. Sample etched with Wecks Reagent

Figure 4.17 - ID surface of an overheated induction sample upon completion of a controlled surface strain

79

4.7 Grain Size Effects on Fracture Mode The material feature that was found to have the largest effect on crack growth resistance was the grain

size, in particular the grain size near the ID surface. Figure 4.18 illustrates the change in performance

for samples SHT at different rates with the quench rate kept constant.

Figure 4.18 - Images of the ID surface for (a) Standard commercial treatment (b) Salt SHT (c) Induction SHT

80

The cause of the large growth in grains is also related to the strain imparted to the cylinder head before

undergoing solution heat treatments. Figure 4.19 illustrates the effect imparted strain has on the growth

of grains, and is the result of studies completed and reported in an unreleased report by Alcan

International [17].

Figure 4.19 - Dogbone tensile specimens with varying degrees of strain imparted prior to SHT [17]

81

Other research has found similar results in the change of fracture mode accompanying changes in grain

size. Previous work done on Al-Mg-Si alloys has observed a transition from intergranular fracture to

ductile fracture by reducing grain size albeit by different methods. One method used to achieve lower

grain size was the addition of Manganese. One effect of this addition caused the grain size to

decreased, and found that the Mn phase as well as the lower grain size caused stress concentrations to

decrease in magnitude enough to suppress decohesion and thus intergranular fracture from occurring

[18]. Another method of indirectly reducing grain size was achieved by adding either manganese or

Cr-containing dispersoids. The slight reduction of grain size caused by the disperoids increased the

ductility and toughness of the tested material, increased the plastic zone size, while relieving local

strain and causing a change in fracture mode from intergranular to a mixed mode of intergranular and

ductile fracture [19].

82

Figure 4.20 - Relation of grain size to fracture mode in slow strained specimens of Al-Mg-Si alloy

systems [20] Figure 4.20 [20] is a summary of data relating grain size to fracture mode of various Al-Mg-Si

aluminum alloys at slow strain rates over the last 30 years. This figure was taken from an upcoming

paper by Holroyd and Lewandowski and reprinted here with permission. This transition of fine-

grained material fracturing in a transgranular ductile mode to a large-grained material fracturing in an

intergranular low ductility mode is also seen in the 6061 alloy cylinder heads studied in this

investigation. Figure 4.21 illustrates the difference in fracture surfaces between “banana-shaped”

samples differing only in solution heat treat up rate and tested in an identical manner.

83

Figure 4.21 - Fracture surfaces of “banana-shaped” samples upon completion of a controlled surface strain test to ε = 0.135 of a (left) standard SHT (right) induction SHT

84

Chapter 5

Conclusions

The effects of solution heat treatment heat up rate on the resulting grain size and fracture characteristics

of 6061 alloy pressure vessel cylinder heads were studied in this investigation by applying

metallography and a suite of consistent mechanical tests of slow strain rate 4 point bending, controlled

surface strain, tension tests, and hardness tests. Through these tests results were found to be as follows:

1. Strain rates below (10-6/s) are needed to promote the type of intergranular low ductility

intergranular fracture found to occur during service in large grained 6061 alloy cylinder heads.

2. Current production methods produce cylinder heads of varying diameters by using tube stock

created by 2 different methods. The tube stock used in small diameter cylinders results in a 1

mm depth of fine (<170 µm) grains on the ID surface. The tube stock used for large diameter

(10”+) cylinder heads results in large (>1 mm) grains at the ID surface. This results in material

that behaves differently upon completion of the mechanical tests. The small diameter cylinders

with standard heat treatments are resistant to slow strain rate low ductility intergranular fracture,

while the large diameter cylinders with standard heat treatments are prone to slow strain rate

low ductility intergranular fracture.

3. Void-like features were found on the fracture surfaces, in twice the concentration on the ID

surface of the large diameter material as opposed to the small diameter material. The middle

section of both large and small diameter had identical concentrations of voids and both

exhibited low ductility intergranular fracture.

4. The grain size and resulting fracture mode of the studied material was shown to be affected to

varying degrees by both SHT heat up rate and quench rate.

85

5. The quench rate needed (180°C/s) to significantly reduce the propensity of intergranular brittle

fracture has not yet been commercially achieved.

6. Increasing the SHT heat up rate to reach temperature in 5 minutes as opposed to 45 minutes

reduces the grain size from >1mm to ~280 µm. This grain size is still large enough to promote

low ductility intergranular fracture.

7. By SHT in an induction furnace and reaching SHT temperature in 45 seconds, the grain size

was reduced to ~100 µm. This grain size is small enough to promote transgranular ductile

fracture and significantly enhances crack resistance in the applied mechanical tests.

8. The fracture mode transition was found to be in agreement with literature results on similar

material tested at slow strain rates.

9. Temperature control during SHT is important at higher rates. Slightly overheating causes local

liquation near the surface and causes void-like features to increase in size. These lead to a

decrease in crack resistance and the formation of large cracks during the slow 4 point bend

tests.

86

Chapter 6

Future Studies

The work completed and presented in this thesis points to a potential solution to the material problem

that large diameter 6061 alloy pressure vessels face. Additionally, this work raises questions regarding

the properties of 6061 aluminum under slow strain rate conditions. Future work should include:

1. Trial solution heat treatments with fast heat up rates on complete cylinders/cylinder heads and

associated bend testing on removed “banana-shaped” specimens.

2. Test temperature limits for additional solution heat treatment and its effect on grain growth.

3. Induction heat treat to temperatures below 535°C to see resulting grain size, hardness and

resulting fracture mode.

4. Effects of holding at solution heat treat temperature for a given time immediately after fast

solution heat up.

5. Acoustic emission monitoring of samples in slow bend tests to monitor sub-surface cracking.

6. Notched transverse samples held at various loads to test tendency for potential sub-surface

cracking in other orientations.

7. Effect of homogenization temperature on large diameter tube stock and its effect on resulting

grain size.

8. Conduct additional microstructure (i.e. TEM, texture) measurements to determine mechanisms

of grain size refinement/growth associated with SHT rate.

87

Appendix A

Bend Images

A.1 Standard SHT Bend Images

88

89

90

91

92

A.2 Salt Bath SHT Bend Images

Sample ID

Heat Medium

Quench Medium

Quench Rate (°C/s)

Time Between Quench and Heat Treatment (hours)

Avg. Hardness

(HRB) A10 Salt Oil 18 1 63 ± 2.11 B10 Salt Oil 18 0.5 64 ± 1.87 B11 Salt Oil 18 4 59 ± 0.71 A6 Salt 100°C Water 30 4 62 ± 1.04 B6 Salt 40°C Water 100 1 65 ± 1.56 A9 Salt 0°C Water 180 1 67 ± 2.87 A11 Salt 0°C Water 180 24 56 ± 1.3 B9 Salt 0°C Water 180 0.5 68 ± 1.11

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

A.3 Induction SHT Bend Images

Sample ID

Heat Medium

Addition Air SHT

Temp (°C)

Final Temp

Recorded (°C)

Quench Rate (°C/s)

Time Between Quench and Heat Treatment (hours)

Avg. Hardness

(HRB) I1 Induction

539 100 1 58 ± 3.19

I2 Induction

510 100 1 58 ± 2.30 I3 Induction

510 100 1 58 ± 4.08

I4 Induction 535 540 100 1 54 ± 3.87 I5 Induction

534 100 1 56 ± 4.01

I6 Induction

531 100 1 53 ± 3.78 I7 Induction

535 100 1 59 ± 4.18

I8 Induction 550 532 100 1 59 ± 3.92

115

116

117

118

119

120

121

122

123

124

125

126

A.4 Small Diameter “Banana-Shaped” Bend Images

Sample ID

Heat Medium

Quench Medium

Quench Rate (°C/s)

Time Between Quench and

Heat Treatment

(hours)

Avg. Hardness

(HRB)

Flash Anneal

(°F)

Spin Temp (°F)

1.4 Salt Oil 18 1 56 ± 1.92 790 750

6.2 Air (Commercial)

Fast (Commercial) 100 2 62 ± 1.32 890 750

7.4 Salt Oil 18 1 59 ± 1.10 890 750


Fast (Commercial) 100 2 62 ± 1.97 N/A N/A





16.4 Salt Oil 18 1 54 ± 2.5 890 500 20.1 Salt Fast 100 1 63 ± 1.10 790 500 20.4 Salt Oil 18 1 60 ± 1.14 790 500


Fast (Commercial) 100 2 65 ± 1.74 790 500

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

A.5 Small Diameter Transverse Samples

Sample ID

Heat Medium

Quench Medium

Quench Rate (C/s)

Time Between Quench and

Heat Treatment

(hours)

Avg. Hardness

(HRB)

Flash Anneal

(°F)

Spin Temp (°F)

1.T Salt Oil 18 1 56 ± 1.92 790 750 5.T Salt Oil 18 1 59 ± 1.62 890 750 7.T Salt Oil 18 1 59 ± 1.10 890 750

9.T Air

(Commercial) Fast

(Commercial) 100 2 62 ± 1.97 N/A N/A 20.T Salt Oil 18 1 60 ± 1.14 790 500

148

149

150

151

152

153

154

155

156

157

158

References

[1] G E Totten and D S MacKenzie, editors. Handbook of Aluminum, volume 1. Marcel Dekker,

New York, 2003.

[2] J R Davis. Aluminum and Aluminum Alloys (ASM Specialty Handbook). American Society for

Materials, 1993.

[3] J C Benedyk. International Temper Designation Systems for Wrought Aluminum Alloys: Part I

– Strain Hardenable (H Temper) Aluminum Alloys. Light Metal Age, 2009.

[4] I J Polmear. Light Alloys: Metallurgy of the Light Metals. Edward Arnold, London, 2nd edition.

- Temper designations

[5] M Kutz. Handbook of Materials Selection. John Wiley & Sons, Inc, New York, 2003

[6] J G Kaufman. Aluminum Alloy Database. Knovel, 2009

[7] D G Altenpohl. Aluminum: Technology, Applications, and Environment. The Aluminum

Association, Washington DC, 1998.

[8] D R Moss. Pressure Vessel Design Manual. Elsevier, 3rd Edition, 2004.

[9] G E Dieter. Mechanical Metallurgy. SI Metric Edition, London, 1988.

[10] J E Hatch. Aluminum: Properties and Physical Metallurgy. American Society for Metals, 1984

[11] V T Vacariu et al. The Effect of Grain Refiners for Aluminum Alloy Containing Mn-Ti-Sr

Dispersoid. Materials Science Forum. 519-521:449-454, 2006.

[12] H Chandler. Heat Treater’s Guide – Practices and Procedures for Nonferrous Alloys. ASM

International, Materials Park, OH, 1996.

[13] G F Vander Voort. Metallographic Etching of Aluminum and Its Alloys. Buehler Ltd. Lake

Bluff, Illinois.

[14] G F Vander Voort et al. Comments on Grain Size Measurements Using the Point-Sampled

Intercept Technique. Scripta Metallurgica et Materialia. 26:1655-1660, 1992.

159

[15] ASTM E18 – Standard Test Methods for Rockwell Hardness of Metallic Materials. Annual

Book of ASTM Standards, 2007.

[16] Lewandowski et al. Effects of lead on the sustained-load cracking of Al-Mg-Si at ambient

temperatures. Materials Science and Engineering. 96:185-195, 1987.

[17] Alcan International, Unreleased Alcan International Report.

[18] J M Dowling and J W Martin. The Influence of Mn Additions On The Deformation Behaviour

Of An Al-Mg-Si Alloy. Acta Metallurgica. 24:1142-1153. Great Britain, 1976.

[19] K C Prince and J W Martin. The Effects of Dispersoids Upon the Micromechanisms of Crack

Propagation in Al-Mg-Si Alloys. Acta Metallurgica. 27:1401-1408. Great Britain, 1979.

[20] NJH Holroyd and JJ Lewandowski. Manuscript in preparation, 2012.

Date post:	20-Jan-2023
Category:	Documents
Upload:	khangminh22
View:	0 times
Download:	0 times

The Effect of Solutionizing Heat Up Rate and Quench Rate on ...

Documents