INVESTIGATION OF PHYSICAL AND MECHANICAL PROPERTIES OF Ti ALLOY (Ti-6Al-4V) UNDER PRECISELY...

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),

ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 116-127© IAEME

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INVESTIGATION OF PHYSICAL AND MECHANICAL

PROPERTIES OF Ti ALLOY (Ti-6Al-4V) UNDER

PRECISELY CONTROLLED HEAT TREATMENT

PROCESSES

1K P Anil Rajagopal,

2Ajin Mathew Jose,

3Ajin Soman,

4Christo J Dcruz,

5Nived Sankar N,

6Syamraj S,

7Vimalkumar P

1-7

Department of Mechanical Engineering, College of Engineering Thalassery, Kannur

ABSTRACT

Titanium and titanium alloys are metals that contain a mixture of titanium and other chemical

elements. Such alloys have very high tensile strength and toughness. They are light in weight, have

extraordinary corrosion resistance and ability to withstand extreme temperatures. Its applications

include military applications, medical devices, connecting rods on expensive sports cars and

consumer electronics. Titanium and Titanium Alloys are heat treated in order to reduce residual

stresses developed during fabrication (stress relieving), produce an optimum combination of

ductility, machinability, and dimensional and structural stability (annealing) increase strength

(solution treating and aging), optimize special properties such as fracture toughness, fatigue strength,

and high-temperature creep strength. Our objective is to investigate physical and mechanical

properties of Ti-6Al-4V under precisely controlled heat treatment process i.e., under different

combinations of heat treatment process with different cooling rates. Specimen used is 22mm Ti-6Al-

4V plate. Microstructure analysis is also done.

Keywords: Titanium Alloys, Light Weight, Heat Treatment, Microstructure, Aerospace Applications

I. INTRODUCTION

Titanium (Symbol Ti, melting point: 1,670 °C, density 4.5 g/cm3) is the fourth most

abundant industrial metal in the earth’s crust (0.61%), after aluminum (8.14%), iron (5.12%), and

magnesium (2.10%). Although unalloyed titanium metal is soft and exhibits low strength, its alloys

demonstrate exceptional mechanical properties. The uses of commercially pure titanium are limited

to applications where moderate strength, high corrosion resistance, and good weld ability are desired.

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND

TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 6, Issue 2, February (2015), pp. 116-127

© IAEME: www.iaeme.com/IJMET.asp

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The remarkable properties of titanium alloys with regards their high strength, wear resistance, and

low density are well known in aerospace related engineering circles. The remarkable properties of

titanium alloys with regards their high strength, wear resistance, and low density are well known in

aerospace related engineering circles. Beyond the aerospace sector, the usefulness of titanium alloys

is also being realized in other industrial sectors that include petroleum refining, chemical and food

processing, surgical implantation (biomedical industry), nuclear waste storage, automotive and

marine applications.

Commercially pure titanium has an all-alpha structure and demonstrates superior resistance

to corrosion but inferior mechanical properties as compared to titanium alloys. Compared with beta

titanium alloys, alpha titanium alloys are superior in heat resistance and weldability but inferior in

strength and workability. Beta titanium alloys are alloys which are solution strengthened by adding

beta structure stabilizers. An all-beta structure at room temperature can be obtained by rapidly

cooling the specimen through solution treatment. Alpha phase precipitates in an all-beta structure by

aging treatment. Alloys having a beta structure with precipitated alpha phase exhibit excellent

strength. Two phase α+β alloys with a dispersion of the beta form in the alpha phase exhibit

properties of each phase.

II. HEAT TREATMENT OF Ti AND Ti ALLOYS

Titanium and titanium alloys are heat treated in order to:

• Reduce residual stresses developed during fabrication (stress relieving)

• Produce an optimum combination of ductility, machinability, and dimensional and structural

stability (annealing)

• Increase strength (solution treating and aging)

• Optimize special properties such as fracture toughness, fatigue strength, and high-temperature

creep strength.

STRESS RELIEVING

Titanium and titanium alloys can be stress relieved without adversely affecting strength or

ductility. Stress-relieving treatments decrease the undesirable residual stresses that result from first,

non uniform hot forging or deformation from cold forming and straightening, second, asymmetric

machining of plate or forgings, and, third, welding and cooling of castings. The removal of such

stresses helps maintain shape stability and eliminates unfavorable conditions, such as the loss of

compressive yield strength commonly known as the Bauschinger effect.

When symmetrical shapes are machined in the annealed condition using moderate cuts and

uniform stock removal, stress relieving may not be required. Compressor disks made of Ti-6Al-4V

has been machined satisfactorily in this manner, conforming with dimensional requirements. In

contrast, thin rings made of the same alloy could be machined at a higher production rate to more

stringent dimensions by stress relieving 2 h at 540°C (1000°F) between, rough and final machining.

Separate stress relieving may be omitted when the manufacturing sequence can be adjusted to use

annealing or hardening as the stress-relieving process. For example, forging stresses may be relieved

by annealing prior to machining.

ANNEALING

The annealing of titanium and titanium alloys serves primarily to increase fracture toughness,

ductility at room temperature, dimensional and thermal stability, and creep resistance. Many titanium



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alloys are placed in service in the annealed state. Because improvement in one or more properties is

generally obtained at the expense of some other property, the annealing cycle should be selected

according to the objective of the treatment.

Common annealing treatments are:

• Mill annealing

• Duplex annealing

• Recrystallization annealing

• Beta annealing

Mill annealing is a general-purpose treatment given to all mill products. It is not a full anneal

and may leave traces of cold or warm working in the microstructures of heavily worked products,

particularly sheet.

Duplex annealing alters the shapes, sizes, and distributions of phases to those required for

improved creep resistance or fracture toughness. In the duplex anneal of the Corona 5 alloy, for

example, the first anneal is near the β transus to globularize the deformed α and to minimize its

volume fraction. This is followed by a second, lower-temperature anneal to precipitate new lenticular

(acicular) α between the globular α particles. This formation of acicular α is associated with

improvements in creep strength and fracture toughness.

Recrystallization annealing and β annealing are used to improve fracture toughness. In

recrystallization annealing, the alloy is heated into the upper end of the α-β range, held for a time,

and then cooled very slowly. In recent years, recrystallization annealing has replaced β annealing for

fracture critical airframe components.

(Beta) Annealing. Like recrystallization annealing, Annealing improves fracture toughness.

Beta annealing is done at temperatures above the β transus of the alloy being annealed. To prevent

excessive grain growth, the temperature for β annealing should be only slightly higher than the β

transus. Annealing times are dependent on section thickness and should be sufficient for complete

transformation. Time at temperature after transformation should be held to a minimum to control β

grain growth. Larger sections should be fan cooled or water quenched to prevent the formation of a

phase at the β grain boundaries.

SOLUTION TREATMENT AND AGEING

A wide range of strength levels can be obtained in α-β or β alloys by solution treating and

aging. With the exception of the unique Ti-2.5Cu alloy (which relies on strengthening from the

classic age-hardening reaction of Ti2Cu precipitation similar to the formation of Guinier-Preston

zones in aluminum alloys), the origin of heat-treating responses of titanium alloys lies in the

instability of the high-temperature β phase at lower temperatures. Heating an α-β alloy to the

solution-treating temperature produces a higher ratio of β phase.

This partitioning of phases is maintained by quenching; on subsequent aging, decomposition

of the unstable β phase occurs, providing high strength. Commercial β alloys generally supplied in

the solution-treated condition, and need only to be aged. After being cleaned, titanium components

should be loaded into fixtures or racks that will permit free access to the heating and quenching

media. Thick and thin components of the same alloy may be solution treated together, but the time at

temperature is determined by the thickest section. Time/temperature combinations for solution

treating are given in Table 1. A load may be charged directly into a furnace operating at the solution-

treating temperature. Although preheating is not essential, it may be used to minimize the distortion

of complex parts. Solution treating of titanium alloys generally involves heating to temperatures

either slightly above or slightly below the β transus temperature.



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The solution-treating temperature selected depends on the alloy type and practical

considerations briefly described below.

β (Beta) alloys are normally obtained from producers in the solution-treated condition. If reheating

is required, soak times should be only as long as necessary to obtain complete solutioning. Solution-

treating temperatures for β alloys are above the β transus; because no second phase is present, grain

growth can proceed rapidly. α-β (Alpha-beta) alloys. Selection of a solution-treatment temperature

for α-β alloys is based on the combination of mechanical properties desired after aging. A change in

the solution-treating temperature of α-β alloys alters the amounts of β phase and consequently

changes the response to aging.To obtain high strength with adequate ductility, it is necessary to

solution treat at a temperature high in the α-β field, normally 25 to 85°C (50 to 150°F) below the β

transus of the alloy. If high fracture toughness or improved resistance to stress corrosion is required,

β annealing or β solution treating may be desirable. However, heat treating α- alloys in the β range

causes a significant loss in ductility. These alloys are usually solution heat treated below the β

transus to obtain an optimum balance of ductility, fracture toughness, creep, and stress rupture

properties.

III. Ti-6Al-4V : COMPOSITION, TYPICAL PROPERTIES AND USES

COMPOSITION

Table 1. Composition of Ti-6Al-4V

COMPONENT WT %

Carbon 0.08

Iron 0.03

Nitrogen 0.05

Aluminium 5.5-6.75

Oxygen 0.20

Vanadium 3.5-4.5

Hydrogen 0.015

Yttrium 0.005

Titanium Balance

Other 0.40

TYPICAL PROPERTIES

Table 2. Typical properties of Ti-6Al-4V

PROPERTY VALUE

Density 4.43g/cc

Hardness 334BHN

UTS 950 MPa

Yield strength 880MPa

Elongation 14 %

Reduction in area 36%

Fatigue strength 240 MPa

Melting point 1604-1660 ºC

Beta transus 980 ºC



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USES

Ti 6Al-4V is known as the "workhorse" of the titanium industry because it is by far the most

common Ti alloy, accounting for more than 50% of total titanium usage. It is an alpha+beta alloy

that is heat treatable to achieve moderate increases in strength. Ti 6Al-4V is recommended for use at

service temperatures up to approximately 350°C (660°F). Ti 6Al-4V offers a combination of high

strength, light weight, formability and corrosion resistance which have made it a world standard in

aerospace applications.

Ti 6Al-4V may be considered in any application where a combination of high strength at low

to moderate temperatures, light weight and excellent corrosion resistance are required. Some of the

many applications where this alloy has been used include aircraft turbine engine components, aircraft

structural components, aerospace fasteners, high-performance automotive parts, marine applications,

medical devices, and sports equipment. Ti-6Al-4V isthe alloy most commonly used in wrought and

cast forms. Palladium or ruthenium can be added for increased corrosion resistance. Most properties

are affected by the microstructure, which is determined by the thermo-mechanical history. It is

highly resistant to general corrosion in sea water. This alloy is available in most common product

forms including billet, bar, wire, plate, and sheet. Ti-6Al-4V has Excellent biocompatibility,

especially when direct contact with tissue or bone is required. Ti-6Al-4V's poor shear strength makes

it undesirable for bone screws or plates. It also has poor surface wear properties and tends to seize

when in sliding contact with itself and other metals. Surface treatments such as nitriding and

oxidizing can improve the surface wear properties.

IV. DESIGN OF HEAT TREATMENT PROCESSES

• 3 types of heat treatment processes-ANNEALING, SOLUTION TREATMENT AND AGEING

• These processes are done with different combinations of cooling rate to get desired results.

• 4 processes are designed to get the results with minimum deviation of microstructure

• Furnace used is pit furnace.

Process 1: Annealing

• Charge -Ti-6Al-4V Plate

• Quantity -1

• Size -143×91×22 mm

• Type of furnace - Pit Furnace

• Raw Material - Ti-6Al-4V

• Set Temperature - 730°C

• Soaking Time - 1 hr 40 min

Ti-6Al-4V Plate is annealed to 730°C±10°C. After attaining this temperature, the material is

soaked to 1 hr 40 min followed by furnace cooling up to 565°C then air cooled to room temperature.



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Process 2: Solution Treatment and Ageing

SOLUTION TREATMENT


• Quantity -1

• Size -143×91×22 mm

• Type of Furnace - Pit Furnace

• Set Temperature - 700ºC

• Soaking Time -1 hr

• Temperature raised to 955ºC.

• Soaking Time -1 hr 40 min

Temperature raised to 970ºC before taking out of furnace to minimize the loss of heat before

quenching. Here water is used as quenching medium.

AGEING


• Quantity -1



• Soaking Time -8 hrs

Material is kept for ageing in pit furnace at 510ºC with a soaking time of 8 hrs and is air cooled to

room temperature.


SOLUTION TREATMENT

• Charge - Ti-6Al-4V Plate

• Quantity -1

• Size - 143×91×22 mm



• Soaking Time - 1 hr




quenching. Here 10% Ice Brine Solution is used as quenching medium. 11000 Ltr tank contains 10%

salt with ice cubes.



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AGEING

• Charge - Ti-6Al-4V Plate

• Quantity - 1



• Soaking Time - 8 hrs

Material is kept for ageing in pit furnace at 510ºC with a soaking time of 8 hrs and is air

cooled to room temperature.

Process 4: Solution Treatment Ad Ageing

SOLUTION TREATMENT


• Quantity -1

• Size -143×91×22 mm



• Soaking Time - 1 hr




quenching. Here 9.09% Ice Brine Solution is used as quenching medium, but to increase the cooling

rate, the quantity of ice used is increased. 11000 Ltr tank contains 60 no.s of ice blocks each

weighing 50 kg and 1000 kgs of salt.

• Water temperature before quenching : 13 ºC

• Water temperature After quenching : 14 ºC

• Quenching Delay : 18 sec

AGEING


• Quantity - 1


• Set Temperature -510ºC

• Soaking Time - 8 hrs

Material is kept for ageing in pit furnace at 510ºC with a soaking time of 8 hrs and is air cooled

to room temperature.



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IV. MECHANICAL TEST REPORT


Speci Dia. Area UTS PS EI% RA

men in MPa MPa %

mm²

22 mm (T)PLATE Condition: Annealing (730ºC)

1 6.2 30.2 927 892 13 38

2 6.23 30.49 939 905 15 36

3 6.24 30.59 946 903 17 44


Spe Dia in Area UTS PS % %

cime mm in Mpa Mpa El RA

n mm2

22mm (T)PLATE Condition: ST

1 6.09 29.1 1055 933 10 36

2 6.12 29.4 1046 940 14 37

3 6.17 29.9 987 857 16 48

22mm (T)PLATE Condition: STA

1 6.16 29.8 1054 972 13 34

2 6.21 30.3 1067 976 16 42

3 6.17 29.9 1081 1006 14 44


Spe Dia in Area UTS PS % %

cime mm in Mpa Mpa El RA

n mm2


1 12.30 118.8 1132 1019 13 37

2 12.31 119.0 1111 961 12 41

3 12.30 118.8 1138 1108 13 35


1 12.44 121.5 1192 1136 10 39

2 12.39 120.6 1169 1148 12 39

3 12.31 119.0 1177 1111 13 40



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Process 4: Solution Treatment and Ageing Spe Dia Area UTS PS % %R

cim in in Mpa Mpa El A

en mm mm2


1 12.33 119.4 1070 953 11 22

2 12.31 119.0 1091 1019 11 32

3 12.34 119.6 1070 987 13 38


1 12.32 119.2 1124 1080 12 22

2 12.40 120.8 1137 1100 11 21

3 12.44 121.4 1145 1084 11 26

Where,

UTS - Ultimate tensile strength

PS - Proof stress

EI - Elongation

RA - Reduction in area

ST - Solution treated condition

STA - Solution treated & aged condition

V. MICROSTRUCTURE REPORT


Disposition: Homogeneousequiaxed primary α intransformed β matrix.


ST CONDITION



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Disposition: Micro-structure revealed equiaxed αin a matrix of α’ (Martensite).

STA CONDITION

Disposition: Microstructure revealed. Fine acicular α grains in transformed β matrix. Acicularity

revealed in structure due to cooling rate of the quenchant.


ST CONDITION

Disposition: Microstructure revealed equiaxedprimary α and α’ (Martensite) in transformed β matrix

STA CONDITION

Disposition: Fine grains of primary α intransformed β matrix.

Process 4: Solution Treatment and Ageing St Condition



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Disposition: Structure revealed equiaxed primary αgrains in a matrix of α’. Intermittent grain

boundary α is also seen in some locations.

STA CONDITION

Disposition: Microstructure observation carried outon plate of size 143x91x22mm. Grain boundary

α, blocky α seen in many locations Structure consists of primary α grains in a matrix of transformed

β containing coarse and acicular α. Near-equiaxed α’ grains are also seen in many locations..

VI. RESULTS

A comparison of mechanical test report and microstructure report of all the four processes

revealed that heat treatment processes(Annealing , solution treatment and ageing) done on Ti-6Al-

4V alloy with different cooling rates had an positive impact on its mechanical properties and

microstructure. Mechanical properties gradually increased from process 1 to process 3, but it went

down in process 4.This may be due to higher cooling rate achieved by increasing the number of ice

cubes. Hence, we can conclude that process 3 is the best heat treatment process that gives optimum

mechanical properties and microstructure.

VII. CONCLUSION

Ti-6Al-4V alloy is widely used in aerospace and aeronautic industries. . They are light in

weight, have extraordinary corrosion resistance and ability to withstand extreme temperatures. Ti-

6Al-4V is alos used to manufacture the domes (combustion chamber) of cryogenic engines. Very

high temperature will be produced in these domes. So such heat treatment processes done Ti-6Al-4V

helps it to withstand higher temperature of combustion. It also improves its tensile strength and

toughness.

VII. REFERENCES

1. Effect of heat treatment on mechanical properties of Ti–6Al–4V B.D. Venkatesh,D.L. Chen∗,

S.D. Bhole. Department ofMechanical and Industrial Engineering, Ryerson University, 350

Victoria Street, Toronto, Ontario M5B 2K3, Canada

2. Failure analysis and optimization of thermo mechanical process parameters of Ti alloy (Ti-

6Al-4V) fasteners for aerospace applications Vartha Venkateswarlu*, Debashish Tripathy,

Rajagopal, K. Thomas Tharian,P.V. Venkitakrishnan, Liquid Propulsion Systems Center,

ISRO, Trivandrum 69554

3. A calorimetric study on Ti-6Al -4V alloy, S. Manikandan1*, S. Ramanathan2 and

Ramakrishnan. 1Department of Mechanical Engineering, Annamalai University, India



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4. Effect of heat treatment process on tribological behavior of Ti-6Al-4V alloy, Sabry S

Youssef1*, Khaled M Ibrahim2 and Mohammad Abdel-Karim1

5. U. D. Gulhane, S. B. Mishra, P. K. Mishra, “Enhancement of Surface Roughness of 316l

Stainless Steel and Ti-6al-4v Using Low Plasticity Burnishing: Doe Approach” International

Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 1, 2012, pp.

150 - 160, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

6. Saravanan P Sivam, Dr. Antony Michael Raj and Dr. Satish Kumar S, “Influence Ranking of

Process Parameters In Electric Discharge Machining of Titanium Grade 5 Alloy Using Brass

Electrode” International Journal of Mechanical Engineering & Technology (IJMET), Volume

4, Issue 5, 2013, pp. 71 - 80, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

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