Influence of Diffusion Bonding Pressure on Microstructural Characteristics and Mechanical Properties of Thin Sheets of Ti64 Alloy

Joints

T.Pragatheswarana*, S.Rajakumara, V.Balasubramaniana, Vijay Petleyb, Shweta Vermab

aCentre for Materials Joining and Research, Annamalai University, Annamalai Nagar, Tamilnadu, India. PIN:608002bGas Turbine Research Establishment, DRDO, Bengaluru, Karnataka, India. PIN:560093

A B S T R A C T

In this present research work, thin sheets of Ti alloy (Ti6Al4V) which is having a thickness of 2 mm were diffusion bonded with the aim of evaluating the effect of bonding pressure on the microstructural and mechanical properties of the joints. The bonding pressure was varied between 5 MPa and 20 MPa. The bonding temperature and holding time were kept constant at 850 ºC and 30 minutes respectively. Microstructural characteristics were analyzed through optical microscopy and scanning electron microscopy. The mechanical properties of the diffusion bonded joints were evaluated through lap shear test, ram tensile test and micro hardness test. The mechanical properties are correlated with the microstructures and interface hardness to reveal the influence of bonding pressure. Interfacial bonding ratio and thickness reduction ratio were determined and compared with the mechanical properties of the Ti6Al4V diffusion bonds. From this investigation, it was found that the bonding pressure plays a predominant role in producing the void free diffusion bonds and controlling the thickness reduction ratio.

Keywords: Diffusion bonding, Titanium, preesure, micrstructure, hardness

* Corresponding author. Tel.: +91-9629199456 E-mail address: thirupragathesh@gmail.com

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1. Introduction

Titanium (Ti) and its alloys have gained significance for the past few decades in the manufacturing sectors of many engineering fields such as aerospace, chemical industries and power plants [1]. Ti got its importance because not only it is strong but also it is lighter than most of the other high strength materials such as steels and nickel base alloys. Other special characteristics of Ti such as low specific gravity, good bio compatibility, high corrosion resistance and non-magnetic property made this material a prominent one [2, 3]. In aircraft industries, the components such as hollow vane blade constructions, panel structures are usually made by Ti fabrications [4]. These varying fabrications requires both bulk formed material and joints of thin sheets. Joining of Ti sheet metals as well as bulk metals can be done by a special joining technique namely diffusion bonding in order to achieve good quality joints. In this diffusion bonding process, the surface of the workpieces are bonded together in order to produce the joint by the application of pressure at elevated temperature. The applied temperature range must be between 0.5-0.8 Tm,melting temperature of the workpiece material [5]. As this process does not involve any melting in the joint interface it is predominantly recognized as a solid state joining process. During the joining of dissimilar materials, there is a necessity of using an interlayer material at the joint interface to strengthen the interface and also to remove residual stress concentration at the interface [6, 7]. But in the case of similar material joints, it is found from the previous works that the bonds without interlayer produce good properties than the bonds with interlayers especially in Ti fabrications. The diffusion bonding process is governed by three important process variables such as bonding temperature, bonding pressure and holding time [8-10]. One of the challenging aspect of diffusion bonding is to study the influence of those process parameters. Among all the three parameters, it is believed that bonding pressure plays a significant role in terms of the joint strength and deformation characteristics. So, this present research work concentrates on one of the most influential parameter i.e. bonding pressure and its influence on the microstructural characteristics and mechanical properties such as bonding strength, shear strength and micro hardness. A correlation was performed by comparing the mechanical properties of the diffusion bonded joints with the thickness reduction ratio and interfacial bonding ratio.

2. Experimental work

In this research work, diffusion bonding operation was performed on a high temperature diffusion bonding machine which is having a maximum operating temperature of 1000°C and equipped with a hydraulic pressurizing system which can produce a maximum axial force of 10 tonnes. The base material used in this research work was the Ti6Al4V alloy thin sheets having a thickness of 2mm. The bonding specimens were cut from rolled sheets of Ti6Al4V having dimensions of 50×50×2 mm3. The base material details such as chemical composition and mechanical properties are shown in the Table 1 and Table 2.

Table 1- Chemical composition (wt. %) of the base material Ti6Al4V

Al V Fe C O N H Ti

6.2 4.15 0.01 0.03 0.15 0.01 0.003 Balance

Table 2- Mechanical properties of the base material Ti6Al4V

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The bonding specimens were cleaned with acetone in order to get rid of

the surface contaminants and polished finely to reduce the surface roughness as much as possible (0.6 µm) since it is very important to achieve successful bonds free of defects. The diffusion bonding experiments were carried out inside a vacuum chamber at various bonding pressures such as 5 MPa, 10 MPa, 15 MPa, and 20 MPa. Other parameters such as bonding temperature and holding time were kept constant at 850 ºC and 30 minutes respectively.

Fig. 1 - Specimen design (a) Lap shear test (b) Ram tensile test

After the diffusion bonding process, the specimens were cut into different profiles by wire cut EDM process for different tests such as lap shear test, ram tensile test and metallography through optical microscopy (OM) and scanning electron microscopy (SEM). The design for the specimen preparation for the lap shear test and ram tensile test are shown in the Fig. 1 (a) and (b). The actual specimens made for ram tensile and lap shear test are shown in the Fig. 2 (a) and (b). The metallurgical examinations were followed by Vickers micro hardness test which was carried out with a load of 50g at a dwell time of 15 seconds and the indentations were made on the diffusion bonded samples at an interval of 200 µm.

Fig. 2- Fabricated test specimen a) Lap shear b) Ram tensile

The thickness of the specimen was measured before and after the

bonding process and then thickness reduction ratio was determined for

all the diffusion bonded joints. The thickness reduction ratio was

determined by the equation 1.

Thickness reductionratio=b0−b

b0× 100

where, ‘b0’ is the thickness of the material before bonding and ‘b’ is the

thickness of the material after bonding. Likewise, in order to quantify

the void free bond, the interfacial bonding ratio was determined for all

the joints through OM using the equation 2.

Interfacialbonding ratio=l0−l

l0×100

where ‘l0’ is the total length of the interface and ‘l’ is the total length of

the voids along the interface

3. Results

3.1 Microstructural characterizationThe main defect usually encounters in diffusion bonding is

voids or gaps in the bond interface as it affects the bond quality by reducing its strength. Fig. 3 shows the microstructures of the diffusion bonds characterized through optical microscope. The bond interface was marked by an arrow mark in the Fig. 3. From the Fig. 3 (a) it was observed that the grains are small, elongated and equiaxed throughout the matrix. But at the interface visible number of small voids can be observed. The microstructure examination through SEM reveals clearer morphology of the voids present in the bond interface region which is shown in the Fig. 4. The marking in the Fig. 4 (a) indicates the voids along the interface and in the Fig. 4 (b) indicates the void free interface.3.2 Mechanical properties

The mechanical properties of the diffusion bonded Ti6Al4V thin sheets were evaluated by mechanical tests such as lap shear test, ram tensile test and microhardness test. The lap shear test and ram

(2)

(1)

Material

Yield Tensile

Strength (MPa) at

0.2% strain

Ultimate Tensile

Strength (MPa)

Elongation (%) at

25mm GL

Hardness (Hv)

at 50g load

Ti6Al4V 985 1068 16 348

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tensile test results are presented in Table 3. The fractured specimens of ram tensile test and lap shear test are in the Fig. 5.

Fig. 3- Optical micrographs at different bonding pressures (a) 5MPa

(b)10 MPa (c)15 MPa and (d) 20 MPa

Fig. 4-: SEM microstructures at different bonding temperatures (a) 5MPa and (b) 15 MPa

Table 3- Mechanical Properties of the diffusion bonds at different pressuresExp

. No.

Bonding

Pressure, MPa

Bonding strength, MPa

Lap shear

strength, MPa

Interfacial bonding ratio, %

Thickness

reduction ratio, %

1. 5 148 161 80 5

2. 10 168 189 90 8

3. 15 175 194 100 10

4. 20 165 184 100 15

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Fig. 5- After test specimens of (a) Lap shear and (b) Ram tensile tests

Fig. 6- Hardness variation along the thickness directionThe maximum lap shear strength attained is 194 MPa at a bonding pressure of 15 MPa and minimum value is 161 MPa at 5 MPa. The maximum bonding strength from ram tensile test attained is 175 MPa at a bonding pressure of 15 MPa and minimum value is 148 MPa at 5 MPa. The hardness comparison chart is shown in the Fig. 6. The maximum peak hardness recorded in the interface is 376 Hv for 15 MPa. For 5 MPa, the hardness recorded at the interface was 364 Hv which is very low compared to the other conditions.

4. Discussion

4.1 Bond Quality The quality of the bonds was determined by the

microstructural examination through OM and SEM. From the observations, it was found that the bonds produced at 5 MPa 10 MPa, 15 MPa and 20 MPa pressures have good microstructures except presence of few voids at the interface. At 15 MPa and 20 MPa, the bond interface is completely free of voids. The bonds produced at 5 MPa and 10 MPa have very few voids present along the interface of the joints. The effect of these voids can be determined by performing a statistical analysis. For quantifying the amount of bonding in the joints, the interfacial bonding ratio was calculated and compared with the mechanical properties which is given in the Fig. 7. From the correlation it is confirmed that all the mechanical properties such as shear strength, bonding strength and hardness increases with increase in the interfacial bonding ratio. This was supported by the SEM images (Fig. 4) taken from 5 MPa and 15 MPa bonds where it is easy to identify the void present regions and void free regions.

Fig. 7- Plot of mechanical properties vs bonding pressure, interfacial bonding ratio and thickness reduction ratio

4.2 Thickness Reduction RatioThough the mechanical properties can be improved by

attaining 100% interfacial bonding ratio, the mechanical properties seem to be reducing after 15 MPa which is shown in the Table 3. There comes the effect of pressure on the deformation of the material during the diffusion bonding process. It is obvious that when a material experiences high pressure at an elevated temperature will be affected by the deformation which causes the thickness reduction of the material. Thus here to quantify the amount of deformation, the thickness reduction ratio was determined and correlated with the mechanical properties which is shown in the Fig. 7. It was observed that increasing the bonding pressure, increases the thickness reduction ratio. The reduction of the mechanical properties at 20 MPa bonding pressure show s that the deformation of the material during diffusion bonding must be controlled. The thickness reduction limit should be kept within 10% otherwise the mechanical properties will reduce drastically.

5. Conclusions

Diffusion bonding pressure is a very important bonding parameter since it influences the formation of voids at the interface region predominantly

The achievement of 100% interfacial bonding ratio enhances the mechanical properties such as shear strength, bonding strength and hardness of Ti6Al4V diffusion bonds

Thickness reduction ratio must be kept controlled within 10% in order to maximize the mechanical properties of the Ti6Al4V diffusion bonds

The maximum values of mechanical properties such as shear strength, bonding strength and hardness achieved were 194 MPa, 175 MPa and 376 Hv respectively at a bonding pressure of 15 MPa

6. Acknowledgement

The authors are very grateful to Gas Turbine Research Establishment (GTRE), Ministry of Defence for providing financial support under the Contract Acquisition Research Support (CARS) scheme (Grant No. GTRE/MMG/BMR1/1022/16/CARS/A/16).

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