INVESTIGATION OF PHYSICAL AND MECHANICAL PROPERTIES OF Ti ALLOY (Ti-6Al-4V) UNDER PRECISELY CONTROLLED HEAT TREATMENT PROCESSES

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1 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) International Journal of Mechanical Engineering and Technology (IJMET), ISSN (Print), ISSN (Print) ISSN (Online) Volume 6, Issue 2, February (2015), pp IAEME: Journal Impact Factor (2015): (Calculated by GISI) IJMET I A E M E INVESTIGATION OF PHYSICAL AND MECHANICAL PROPERTIES OF Ti ALLOY (Ti-6Al-4V) UNDER PRECISELY CONTROLLED HEAT TREATMENT PROCESSES 1 K P Anil Rajagopal, 2 Ajin Mathew Jose, 3 Ajin Soman, 4 Christo J Dcruz, 5 Nived Sankar N, 6 Syamraj S, 7 Vimalkumar 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. 116

2 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 117

3 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 solutiontreating 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. 118

4 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. Solutiontreating 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 TYPICAL PROPERTIES Table 1. Composition of Ti-6Al-4V COMPONENT WT % Carbon 0.08 Iron 0.03 Nitrogen 0.05 Aluminium Oxygen 0.20 Vanadium Hydrogen Yttrium Titanium Balance Other 0.40 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 ºC Beta transus 980 ºC 119

5 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 mm Type of furnace - Pit Furnace Raw Material - Ti-6Al-4V Set Temperature 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. 120

6 Process 2: Solution Treatment and Ageing SOLUTION TREATMENT Charge -Ti-6Al-4V Plate Quantity -1 Size 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 Charge -Ti-6Al-4V Plate Quantity -1 Type of Furnace - Pit Furnace 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. Process 3: Solution Treatment and Ageing SOLUTION TREATMENT Charge - Ti-6Al-4V Plate Quantity -1 Size 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 10% Ice Brine Solution is used as quenching medium Ltr tank contains 10% salt with ice cubes. 121

7 AGEING Charge - Ti-6Al-4V Plate Quantity - 1 Type of Furnace - Pit Furnace 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. Process 4: Solution Treatment Ad Ageing SOLUTION TREATMENT Charge -Ti-6Al-4V Plate Quantity -1 Size 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 9.09% Ice Brine Solution is used as quenching medium, but to increase the cooling rate, the quantity of ice used is increased 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 Charge -Ti-6Al-4V Plate Quantity - 1 Type of Furnace - Pit Furnace 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. 122

8 IV. MECHANICAL TEST REPORT Process 1: Annealing Speci Dia. Area UTS PS EI% RA men in MPa MPa % mm² 22 mm (T)PLATE Condition: Annealing (730ºC) Process 2: Solution Treatment and Ageing Spe Dia in Area UTS PS % % cime mm in Mpa Mpa El RA n mm 2 22mm (T)PLATE Condition: ST mm (T)PLATE Condition: STA Process 3: Solution Treatment and Ageing Spe Dia in Area UTS PS % % cime mm in Mpa Mpa El RA n mm 2 22mm (T)PLATE Condition: ST mm (T)PLATE Condition: STA

9 Process 4: Solution Treatment and Ageing Spe Dia Area UTS PS % %R cim in in Mpa Mpa El A en mm mm 2 Where, 22mm (T)PLATE Condition: ST mm (T)PLATE Condition: STA 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 Process 1: Annealing Disposition: Homogeneousequiaxed primary α intransformed β matrix. Process 2: Solution Treatment and Ageing ST CONDITION 124

10 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. Process 3: Solution Treatment and Ageing 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 125

11 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 A calorimetric study on Ti-6Al -4V alloy, S. Manikandan1*, S. Ramanathan2 and Ramakrishnan. 1Department of Mechanical Engineering, Annamalai University, India 126

12 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 , ISSN Print: , ISSN Online: 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 , ISSN Print: , ISSN Online: