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Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer R. Kumar a, M. Balasubramanian b,⇑ a b
AVC College of Engineering, Tamilnadu 609305, India RMK College of Engineering and Technology, Tiruvallur, Tamilnadu 601206, India
a r t i c l e
i n f o
Article history: Received 20 August 2019 Received in revised form 5 October 2019 Accepted 7 November 2019 Available online xxxx Keywords: Diffusion bonding Friction welding Mechanical tests Joint strength Titanium alloy
a b s t r a c t Joining of dissimilar materials by diffusion bonding and friction welding have been considered for investigation for comparing the process output. The authors have made an attempt to compare the performance of the joints fabricated by these two processes, considering that both processes involve solid state joining, used in the joining of cylindrical work pieces. Cylindrical work pieces are often joined using the diffusion bonding process. But the friction welding process can be effectively used to join particularly dissimilar joints for better joint strength. An attempt has been made to join commercial rods of Ti-6Al-4V and stainless steel 304L with copper as the intermediate metal using friction welding. The strength of the joint was evaluated in comparison to diffusion bonding. The friction welded joints showed better joint strength and the processing time was also minimum when compared to diffusion bonding. The joint strength attained by friction welding was 520 MPa as against 282 MPa by diffusion bonding for the joints under consideration. The bonding time for friction welding is as low as 36 min as against 127 min for diffusion bonding. Further between both the processes, friction welding is suggested as an effective and efficient process to join the dissimilar combination under investigation. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
1. Introduction Advancement in newer technology and newer material development is the order of the day. Ti-6Al-4V parts are most preferred for their noteworthy properties like less weight, high strength, commendable weldability and anti-corrosive character. The assembly of titanium (Ti) alloy with a dissimilar combination disclose considerable attention for achieving loftier quality joint. Beta (b) titanium alloys reveal outstanding fatigue properties and are the best for applications such as torsion bar, drive shaft, road arms, road wheels, and housing in armed trucks and automobile. Ti-6Al4Vs are prevalent in transmission shaft, which shows considerable performance in cyclic loading [1]. Stainless steel with low cost, high strength and anti-corrosive nature makes it ideally suitable for bio-medical, automobile and high-pressure pipeline application. A stainless steel tube with pure titanium tube was successfully friction welded with niobium (Nb) as interlayer. A joint strength of 357 MPa was achieved and failure ⇑ Corresponding author. E-mail address: [email protected] (M. Balasubramanian).
occurred in Ti-Nb side away from the interface [2]. Diffusion bonded Ti-6Al-4V and stainless steel 304 rods with pure copper foil sustained a maximum shear strength and tensile strength of 162.3 MPa and 282 MPa respectively. Interface structure was found to be the factor determining the bonding strength. [3]. Grade 2 titanium and low carbon steel were diffusion bonded with a copper foil of 100 mm as an interlayer. The joints exhibited very low strength when the diffusion temperature was 800 °C. The contact of the bonded surfaces was seen to be poor while the thermal excitation was not adequate for effecting a total amalgamation of the coupling surfaces at this low bonding temp. [4]. Pure titanium and stainless steel diffusion bonded resulted in weak joints when bonding temperatures were lower or higher than the optimum temperature. This was the result of a lack of inter diffusion of each base metal as also excess interdiffusion of the base metal with enhanced intermetallic phases and also the generation of internal stress respectively [5]. The close packed face centered cubic structure of the materials under consideration has provided freedom for the AISI 304/Cu interface from intermetallic compounds, while there was increase in shear strength with increase in process temperature and duration of the holding [6].
https://doi.org/10.1016/j.matpr.2019.11.047 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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Direct joining of Ti-6Al-4V and stainless steel resulted in failure of most of the joints under drop tests. A quality welded joint between the two materials is impossible when the values of friction time and axial pressure are lower. Development of an inadequate heat during the welding process causes inadequacy in the copper foil interaction with two contact surfaces resulting in a poor joint [7]. Pure titanium and stainless steel joint were attempted with an interlayer of copper and nickel. This yielded a good bonding strength [8]. Commercially pure titanium and stainless steel 304 materials were investigated with nickel, vanadium and tantalum as intermediate materials. Out of the interlayer materials considered, nickel, as interlayer, retained a bonding strength of 410 MPa when compared with other materials used as interlayer materials [9]. Absence of interlayer, in joining maraging steel with nickel (Ni) as intermediate layer showcased poor strength and low impact toughness. This is mainly due to the content of fine grain size in the welds, thermo-mechanical working during the process of welding [10]. Titanium and stainless steel 304L friction welded [11] with Ni as an interlayer attained a maximum tensile strength of 295 MPa. Increase in the strength of the weld was seen with longer friction time and formation of thick intermetallic compounds like TiNi3, and Ti2Ni at the interface. Effective friction welded joint of stainless steel and copper was done at three different speeds. The best joint strength of 238 MPa was achieved at a friction pressure of 33 MPa [12]. The formation of excess intermetallic layer at the joint interface with increased HAZ (heat affected zone) was seen as the result of larger friction. This type of dissimilar joint under higher friction pressure and low upset pressure produced a good tensile strength [13]. A high tensile strength at 2 s friction time was reached with titanium and nickel combination with copper as interlayer, followed by a decrease in strength. Thickness of the interlayer and the extent of formation of intermetallic phases are perhaps the cause for this [14]. Titanium aluminide rod prepared by investment casting was [15] friction welded with an oxygen free copper rod. The assembly was then joined to AISI4140 structural steel. During the process, copper acted as a very good candidate for interlayer, avoiding the transformation of martensite and crack development. Grade 2 titanium and commercial stainless steel 321 was joined by friction welding. Acquisition of a higher mechanical property under higher upset pressure condition was seen. The smaller grain size and narrower thickness of reaction layer explain this phenomenon [16]. Maximum strength of 400 MPa was obtained for a titanium-stainless steel 304L friction welded joint at a friction pressure of 100 MPa. Failure occurred at titanium base material. This was the result of poor bend ductility and establishment of brittle intermetallics and residual stresses [17]. Ti-6Al-4V and 304L stainless steel friction welded with copper as an interlayer reported good strength with minimum interlayer thickness [18]. Stainless steel face coated with nickel by pulse electro deposition as interlayer showcased poor joint strength and the coating peeled off during the process at high friction pressure. The process was repeated by using AlSi CNT composite coating by thermal spray technique as interlayer in between the materials. The joint resulted in the formation of hard intermetallic between the surfaces due to hard interlayer composite [19]. Rotary friction welding of a high strength metastable beta titanium alloy Ti-10V-2Fe-3Al was attempted. Welds were also subjected to solution treatment [20]. Microstructural evolution and characterization of interfacial reactions during friction welding of commercially pure titanium and 304 stainless steel using a Ni interlayer were investigated. The weld interface of titanium to interlayer has formed with three types of layers which are consisting of TiNi, TiNi3 and Ti2Ni, intermetallic compounds. Tensile tests indicated that the failure in the joints occurred by formation and propagation of the crack mostly
along the titanium–nickel interlayer interface, through the Ti–Nitype intermetallic layers [21]. Friction welding of incompatible materials is always difficult due to lot of problems faced during the process. This area of research with Ti-6Al-4V with stainless steel joining is always in demand. This combination of dissimilar direct joint was not successful, and had failed more often due to hard interface formation and migration of carbon from steel to titanium alloy. The joining of Ti-6Al-4V and stainless steel seems to be not easy. Based on the above literature and an assessment made, the Ti-6Al-4V and 304L stainless steel with a copper interlayer of diffusion bonded joint and friction welded joint was compared for their performance.
2. Materials and methods Diffusion bonding of Ti-6Al-4V and stainless steel rods with copper interlayer and friction welding of the same is compared for bonding strength and microstructure.
2.1. Diffusion bonding of the Ti-6Al-4V/copper/stainless steel Commercially pure titanium and PH (precipitation hardened) stainless steel rod of 30 mm length and 15 mm diameter was fabricated to size using the machining process. The surfaces were then prepared with standard metallographic techniques, cleaned with acetone and dried before being placed into the welding chamber. The process was carried out at 800–1000 °C with steps of 50 °C for 3.6 ks under 3.5 MPa uni-axial load in (4–6) 10 3 Pa vacuum and 50, 60 and 70 min of holding time [4].
2.2. Friction welding of Ti-6Al-4V/Copper/stainless steel The materials considered during experimentation were commercial rolled rods of grade-5 Ti-6Al-4V, stainless steel 304L and oxygen free copper of diameter 20 mm. The materials were prepared to a length of 100 mm each using the machining process. The surfaces were then prepared with standard metallographic techniques, cleaned and dried before being set into the friction welding machine. The elements present in the base metal were configured in accordance with ASME-E-1086-2008 standard at room temperature. A friction pressure of 30 bar and upset pressure of 100 bar was fixed for the copper and titanium alloy joint by trials. The 30 bar friction pressure provided a better heating between the titanium alloy and copper faying surface. Details are presented in Tables 1(a) and 1(b). The samples prepared were fixed in the KUKA friction welding machine with a feed rate of 0.5 mm/s and 1125 RPM. The joints were prepared in two steps. The first step was accomplished by joining stainless steel and copper with 8 N/ mm2 friction pressure and 14 N/mm2 upset pressure. This is showed in Fig. 1b. Following this, final joint is established in the second step by joining Ti-6Al-4V and copper with 12 N/mm2 friction pressure and 40 N/mm2 upset pressure by maintaining the copper length at 12 mm before joining.
2.3. Hardness test Preliminary investigation was done on the joints for assessment of the bonding strength by performing the drop test. All the joints passed the drop test and were ready for further investigation. This is exhibited in Fig. 2. The welded specimens were tested for hardness from the welded region to the parent metal region by microhardness testing machine INNOVATES vickers micro hardness with a load of 0.01–2 kgf.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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R. Kumar, M. Balasubramanian / Materials Today: Proceedings xxx (xxxx) xxx Table 1a Chemical composition (Wt.%) of SS304L. C
Si
Mn
P
S
Cr
Mo
Ni
N
Fe
0.030
0.36
1.58
0.038
0.022
18.37
0.13
8.28
0.033
Bal
Table 1b Chemical composition (Wt.%) of Ti-6Al-4V. C
Si
Al
V
Ni
Fe
Ti
0.030
0.010
6.3
4.3
0.01
0.05
Bal.
Fig. 1. Schematic representation of processes.
Fig. 2. Friction welded samples.
2.4. Tensile test Ti-6Al-4V/Copper/stainless steel welded samples were prepared for tensile testing as per ASTM standard with an initial gauge length of 50 mm and diameter of 12.5 mm. The test was executed at the room temperature of 24.4 °C by an MTS make universal testing machine of 1000 KN capacity. The test samples are shown in Fig. 3.
Fig. 3. Tensile tested samples.
2.5. Microstructure study For micro structural examination, the samples were cut from the welded sample in the middle of the welded region by wire cut EDM. The specimen required being taken from the region, adequately representing the structure and other features of the entire
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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sample under consideration. The fine scratches caused by the grinding operation were removed by polishing with diamond powders. The etchants used were Kroll’s reagent for titanium alloy, potassium dichromate for copper and glyceregia solution for stainless steel. The testing was conducted with de-wintor inverted trinocular metallurgical microscope with 150 magnification. The micrographs are shown in Fig. 4(a–f).
3. Result and discussion 3.1. Hardness survey When Ti-6Al-4V/Copper/stainless steel 304 diffusion bonded samples were evaluated for micro hardness distribution (Fig. 5) in the interface, the stainless steel region experienced an
Fig. 4. (a–f) Micro structural observation at interface.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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the fractured face after the tensile test shows the major diffusion of copper on both the sides. The fractured face showed more copper diffused on the Ti-6Al-4V side and stainless steel side. Processing time of diffusion bonding and friction welding is provided in Table 2. 3.3. Microstructure analysis
Fig. 5. Hardness of diffusion bonded joints.
insignificant increase in hardness due to copper interlayer. In Ti6Al-4V zone, intermetallic formation was evidenced along with high temperature and holding time as a result of sudden increase in hardness due to the presence of copper. Hardness was found to increase from 400 to 700 HV in titanium side and 300 to 400 HV at the joint interface region. Details of the hardness distribution across the samples of friction welded joints are presented in Fig. 6. The hardness at the Ti-6Al-4V and interface varied from 360 HV to 370 HV and 290 HV to 300 HV respectively. Friction pressure and friction time are the main causes for variation in hardness.
3.2. Evaluation of tensile test The diffusion of copper on both the sides of the joint was good due to lesser interlayer thickness. There was a gradual increase in tensile strength with decrease in interlayer thickness. The tensile strength ranged between 80.4 and 520 MPa. The max tensile strength obtained was 520 MPa for an interlayer thickness (ILT) of 0.65 mm. The tensile strength was noted with interlayer thickness between 0.65 mm and 0.72 mm. A macro examination of
Fig. 4(a-b) shows the microstructure friction welded titanium alloy and stainless steel. The microstructure shows fine uniform grains of alpha and beta formed at the grain boundaries of alpha. The microstructure of the stainless steel was seen with uniform grains of austenite throughout the matrix. The stainless steel in the solution was in annealed condition and alloys carbides were not present at the grain boundary. Fig. 4(c) shows the microstructure of the three zones of Ti-6Al4V, copper and stainless steel 304L. Fig. 4(d) shows the copper interface with stainless steel. The microstructure of stainless steel remained unchanged, but the microstructures of copper have become finer as a result of rapid heating and cooling. Fig. 4(e) shows the interfacial zone with a substantial flow of grains due to frictional heat and stress. Fig. 4(f) shows the interface zone of copper and titanium. The copper at the diffusion zone shows fine grains resulting from rapid heating and cooling. The diffusion zone is dark in color are consists of constituents of both copper and titanium. The Ti-6Al-4V microstructure shows the presence of an acicular a phase transformed into a b phase and formation of some a’ due to the temperature gradient. 3.4. EDS and XRD analysis The titanium alloys and steel weldment suffering from the brittleness of resulting intermetallic compounds like Ti-Fe and Ti-Cr is common knowledge. Hence energy dispersive X-ray (EDS) analysis was performed for understanding the formation of intermetallics and to find out the distribution of elements at the interface region. The microstructure of the stainless steel and copper interface region reveals the presence of some intermetallics. The solubility of copper, 14.52% by weight is clearly seen in the stainless steel region. The solubility of copper in the Ti-6Al-4V side as 64.3% which is more than any other elements is also evident. Cu-Ti intermetallic phases with increasing Cu content is seen (Fig. 7) while the low fusion point of copper encourages improved contact area in the mating surfaces. 3.5. Effect of friction pressure and upset pressure Many trials helped the fixing of the friction pressure and upset pressure in the stainless steel and the copper joint at 20 bar and 35 bar respectively, and as providing the best result and passing the drop test. The adequacy of the 20 bar friction pressure for the heat generated between the stainless steel and copper surface,
Table 2 Processing time – diffusion bonding and Friction welding. Sl. No.
Description of work
1 2 3 4 5 Fig. 6. Hardness of friction welded joints.
Time (Min) Diffusion bonding
Friction welding
Preparation of specimen, including polishing Loading and Setting time DB holds Time/FW time Unloading Time Cleaning of flash and finishing
20
16
10 90 03 04
10 06 02 02
Total time
127
36
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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Fig. 7. EDS analysis at stainless steel/copper and copper/Titanium interface region.
and the ability of the 35 bar upset pressure providing better bonding are palpable. A significant role is played by higher friction pressure in providing the heat found necessary for the parent metal. A higher strength of the joint resulted from this. The upset pressure which was comparatively high created a good bonding between titanium and copper. 3.6. Effect of friction time An increase in strength is seen with increase in contact time. The amount of heat generated is the reason for this phenomenon. Increase in friction time is followed by increase in heat generated. This causes deformation in the interlayer to a great extent, causing the formation of thin interlayer which produces a higher tensile strength. The increase in friction time is followed by a decrease in interlayer thickness. Such increase will also mean greater strength for the joint, while lower friction time means less strength. 3.7. Effect of upset time With increase in upset time, there is increase in tensile strength also. This is the result of good bonding between the base metals and the interlayer metal. This bonding causes a high rate of diffusion of copper in the interface. Increased upset time resulted in a hard surface in the copper region compared to the base metals. Absorption of more heat due to the greater thickness of the copper interlayer was the reason for this, requiring more time for cooling, whereas less thickness required less time for cooling. The excess copper metal pumped out as flash, which is not available in the welded region explains this. Tensile strength of 80 MPa was accomplished for a lower upset time of 2 s. With a gradual increase in the upset time, there was decrease in the interlayer thickness to 0.65 mm but there was an increase in tensile strength. 4. Conclusions The conclusion drawn from the comparative study and analysis performed on Ti-6Al-4V and stainless steel 304L with copper interlayer by diffusion bonding and friction welding are a) The diffusion bonding process helps achievement of a maximum shear strength of 163 MPa and tensile strength of 282 MPa, while the yield from the friction welding process for the material under investigation was 520 MPa. b) Variations in hardness seen in friction welded joints is less at the interface region and resulted in formation of 320 HV close to the boundary. In diffusion bonding, the region
adjacent to the interface has a maximum hardness of 240 HV arising from high temperature and longer period of holding time. c) The total processing time for friction welding is as low as 36 min as against 127 min for diffusion bonding.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] S. Jonathan, G. Montgomery, H. Martin, Brij Roopchand Wells, Low-cost titanium armors for combat vehicles, Miner. Met. Mater. Soc. 49 (5) (1997) 4547. [2] M.J. Cola, R.M. Dickerson, R.C. Gentzlinger, D.F. Teter, Dissimilar welds for the apt superconducting cavity’s cryogenic plumbing system, Proceedings of the Workshop on RF Superconductivity, Santa Fe, New Mexico, USA, 1999. [3] K. Shuying Liu, Xu. Uan, Guangbao Liu, Congcong Cao, Zhonghao Heng, The evolution characteristics and numerical analysis of diffusion bonding interface structure of titanium alloy/Cu/stainless steel, Rev. Adv. Mater. Sci. 33 (2013) 224–231. [4] A. Elrefaey, W. Tillmann, Solid state diffusion bonding of titanium to steel using a copper base alloy as interlayer, J. Mater. Process. Technol. 209 (2009) 2746–2752. [5] Debasis Poddar, Solid-state diffusion bonding of commercially pure titanium and precipitation hardening stainless steel, IJROTE 1 (5) (2009) 93–99. [6] N. Ozdemir, B. Bilgin, Interfacial properties of diffusion bonded Ti-6Al-4V to AISI 304 Stainless steel by inserting a Cu interlayer, Int. J. Adv. Manuf. Technol. 41 (2009) 519–526. [7] A. Cosmin Groza, A. Ion Mitelea, Marcela Elena Dimian, Mechanical and structural characteristics of friction welded TI-6AL-4V, X5CRNI18-10 dissimilar joints, Metal 5 (2011) 18–20. [8] M. Shamanian, M. Salehi, A. Saatchi, T.H. North, Influence of Ni interlayer on the mechanical properties of Ti6Al4V/(WC-Co) friction weld, Mater. Manuf. Process. 18 (4) (2003) 581–598. [9] M. Ashfaq, K. Prasad Rao, H. Khalid Rafi, B.S. Murty, H.C. Dey, A.K. Bhaduri, Friction welding of titanium to 304L stainless steel using interlayer Ó Carl HanserVerlag, Munchen Prakt. Metallogr. 48 (4) (2011) 188–207. [10] G. Madhusudhan Reddy, P. Venkataraman, Role of nickel as an interlayer in dissimilar metal friction welding of maraging steel to low alloy steel, J. Mater. Process. Technol. 212 (1) (2012) 66–77. [11] C.H. Muralimohan, V. Muthupandi, K. Sivaprasad, Properties of friction welding Titanium-stainless steel joint with a nickel interlayer, Procedia Mater. Sci. 5 (2014) 1120–1129. [12] C. Shanjeevi, S. Satishkumar, P. Sathiya, Evaluation of mechanical and metallurgical properties of dissimilar materials by friction welding, Procedia Eng. 64 (2013) 1514–1523. [13] Masaaki Kimura, Yoshitaka Saitoh, Masahiro Kusaka, Koichi Kaizu, Akiyoshi Fuji, Effect of friction welding condition and weld faying surface properties on tensile strength of friction welded joint between pure titanium and pure copper, JSMME 12 (2011) 849–865. [14] F. Sassani, J.R. Neelam, Friction welding of incompatible materials, Weld. Res. Suppl. (1988) 264s–270s.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
R. Kumar, M. Balasubramanian / Materials Today: Proceedings xxx (xxxx) xxx [15] Won-Bae Lee, Young-Jig Kim, Seung-Boo Jung, Effect of copper inset layer on properties of friction-welded joints between TiAl and AISI 4140 structural steel, Intermetallics 12 (2004) 671–678. [16] Won-Bae Lee, Young-Jig Kim, Seung-Boo Jung, Effect of Microstructure on mechanical properties of friction welded joints between Ti and AISI321 stainless steel, Mater. Trans. 45 (9) (2004) 2805–2811. [17] H.C. Dey, M. Ashfaq, A.K. Bhaduri, K. Prasad Rao, Joining of titanium to 304L Stainless steel by friction welding, J. Mater. Process. Technol. 209 (2009) 5862– 5870. [18] R. Kumar, M. Balasubramanian, Experimental investigation of Ti-6Al-4V titanium alloy and stainless steel 304L friction welded with copper interlayer, Defence Technol. 11 (2015) 65–75.
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[19] R. Kumar, M. Balasubramanian, Comparative study of Ti alloy and stainless steel 304L friction welded joint with different interlayer process methods, Appl. Mech. Mater. 766–767 (2015) 739–744. [20] D.B.A. Sagara, B. Vikas, B. Sahaa, N. Narasaiah, P. Jayapal, G.D.J. Ram, M.S.K. Rao, Study of microstructure and mechanical properties of friction welded metastable beta titanium alloy titan, Mater. Today: Proc. 5 (9) (2018) 20760–20768. [21] M. Cheepu, V. Muthupandi, W.S. Che, Interface microstructural characterization of titanium to stainless steel dissimilar friction welds, in: The Minerals, Metals & Materials Series (Ed.), TMS 2019 148th Annual Meeting & Exhibition Supplemental Proceedings. The Minerals, Metals & Materials Series, Springer, Cham, 2019.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer R. Kumar a, M. Balasubramanian b,⇑ a b
AVC College of Engineering, Tamilnadu 609305, India RMK College of Engineering and Technology, Tiruvallur, Tamilnadu 601206, India
a r t i c l e
i n f o
Article history: Received 20 August 2019 Received in revised form 5 October 2019 Accepted 7 November 2019 Available online xxxx Keywords: Diffusion bonding Friction welding Mechanical tests Joint strength Titanium alloy
a b s t r a c t Joining of dissimilar materials by diffusion bonding and friction welding have been considered for investigation for comparing the process output. The authors have made an attempt to compare the performance of the joints fabricated by these two processes, considering that both processes involve solid state joining, used in the joining of cylindrical work pieces. Cylindrical work pieces are often joined using the diffusion bonding process. But the friction welding process can be effectively used to join particularly dissimilar joints for better joint strength. An attempt has been made to join commercial rods of Ti-6Al-4V and stainless steel 304L with copper as the intermediate metal using friction welding. The strength of the joint was evaluated in comparison to diffusion bonding. The friction welded joints showed better joint strength and the processing time was also minimum when compared to diffusion bonding. The joint strength attained by friction welding was 520 MPa as against 282 MPa by diffusion bonding for the joints under consideration. The bonding time for friction welding is as low as 36 min as against 127 min for diffusion bonding. Further between both the processes, friction welding is suggested as an effective and efficient process to join the dissimilar combination under investigation. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
1. Introduction Advancement in newer technology and newer material development is the order of the day. Ti-6Al-4V parts are most preferred for their noteworthy properties like less weight, high strength, commendable weldability and anti-corrosive character. The assembly of titanium (Ti) alloy with a dissimilar combination disclose considerable attention for achieving loftier quality joint. Beta (b) titanium alloys reveal outstanding fatigue properties and are the best for applications such as torsion bar, drive shaft, road arms, road wheels, and housing in armed trucks and automobile. Ti-6Al4Vs are prevalent in transmission shaft, which shows considerable performance in cyclic loading [1]. Stainless steel with low cost, high strength and anti-corrosive nature makes it ideally suitable for bio-medical, automobile and high-pressure pipeline application. A stainless steel tube with pure titanium tube was successfully friction welded with niobium (Nb) as interlayer. A joint strength of 357 MPa was achieved and failure ⇑ Corresponding author. E-mail address: [email protected] (M. Balasubramanian).
occurred in Ti-Nb side away from the interface [2]. Diffusion bonded Ti-6Al-4V and stainless steel 304 rods with pure copper foil sustained a maximum shear strength and tensile strength of 162.3 MPa and 282 MPa respectively. Interface structure was found to be the factor determining the bonding strength. [3]. Grade 2 titanium and low carbon steel were diffusion bonded with a copper foil of 100 mm as an interlayer. The joints exhibited very low strength when the diffusion temperature was 800 °C. The contact of the bonded surfaces was seen to be poor while the thermal excitation was not adequate for effecting a total amalgamation of the coupling surfaces at this low bonding temp. [4]. Pure titanium and stainless steel diffusion bonded resulted in weak joints when bonding temperatures were lower or higher than the optimum temperature. This was the result of a lack of inter diffusion of each base metal as also excess interdiffusion of the base metal with enhanced intermetallic phases and also the generation of internal stress respectively [5]. The close packed face centered cubic structure of the materials under consideration has provided freedom for the AISI 304/Cu interface from intermetallic compounds, while there was increase in shear strength with increase in process temperature and duration of the holding [6].
https://doi.org/10.1016/j.matpr.2019.11.047 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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R. Kumar, M. Balasubramanian / Materials Today: Proceedings xxx (xxxx) xxx
Direct joining of Ti-6Al-4V and stainless steel resulted in failure of most of the joints under drop tests. A quality welded joint between the two materials is impossible when the values of friction time and axial pressure are lower. Development of an inadequate heat during the welding process causes inadequacy in the copper foil interaction with two contact surfaces resulting in a poor joint [7]. Pure titanium and stainless steel joint were attempted with an interlayer of copper and nickel. This yielded a good bonding strength [8]. Commercially pure titanium and stainless steel 304 materials were investigated with nickel, vanadium and tantalum as intermediate materials. Out of the interlayer materials considered, nickel, as interlayer, retained a bonding strength of 410 MPa when compared with other materials used as interlayer materials [9]. Absence of interlayer, in joining maraging steel with nickel (Ni) as intermediate layer showcased poor strength and low impact toughness. This is mainly due to the content of fine grain size in the welds, thermo-mechanical working during the process of welding [10]. Titanium and stainless steel 304L friction welded [11] with Ni as an interlayer attained a maximum tensile strength of 295 MPa. Increase in the strength of the weld was seen with longer friction time and formation of thick intermetallic compounds like TiNi3, and Ti2Ni at the interface. Effective friction welded joint of stainless steel and copper was done at three different speeds. The best joint strength of 238 MPa was achieved at a friction pressure of 33 MPa [12]. The formation of excess intermetallic layer at the joint interface with increased HAZ (heat affected zone) was seen as the result of larger friction. This type of dissimilar joint under higher friction pressure and low upset pressure produced a good tensile strength [13]. A high tensile strength at 2 s friction time was reached with titanium and nickel combination with copper as interlayer, followed by a decrease in strength. Thickness of the interlayer and the extent of formation of intermetallic phases are perhaps the cause for this [14]. Titanium aluminide rod prepared by investment casting was [15] friction welded with an oxygen free copper rod. The assembly was then joined to AISI4140 structural steel. During the process, copper acted as a very good candidate for interlayer, avoiding the transformation of martensite and crack development. Grade 2 titanium and commercial stainless steel 321 was joined by friction welding. Acquisition of a higher mechanical property under higher upset pressure condition was seen. The smaller grain size and narrower thickness of reaction layer explain this phenomenon [16]. Maximum strength of 400 MPa was obtained for a titanium-stainless steel 304L friction welded joint at a friction pressure of 100 MPa. Failure occurred at titanium base material. This was the result of poor bend ductility and establishment of brittle intermetallics and residual stresses [17]. Ti-6Al-4V and 304L stainless steel friction welded with copper as an interlayer reported good strength with minimum interlayer thickness [18]. Stainless steel face coated with nickel by pulse electro deposition as interlayer showcased poor joint strength and the coating peeled off during the process at high friction pressure. The process was repeated by using AlSi CNT composite coating by thermal spray technique as interlayer in between the materials. The joint resulted in the formation of hard intermetallic between the surfaces due to hard interlayer composite [19]. Rotary friction welding of a high strength metastable beta titanium alloy Ti-10V-2Fe-3Al was attempted. Welds were also subjected to solution treatment [20]. Microstructural evolution and characterization of interfacial reactions during friction welding of commercially pure titanium and 304 stainless steel using a Ni interlayer were investigated. The weld interface of titanium to interlayer has formed with three types of layers which are consisting of TiNi, TiNi3 and Ti2Ni, intermetallic compounds. Tensile tests indicated that the failure in the joints occurred by formation and propagation of the crack mostly
along the titanium–nickel interlayer interface, through the Ti–Nitype intermetallic layers [21]. Friction welding of incompatible materials is always difficult due to lot of problems faced during the process. This area of research with Ti-6Al-4V with stainless steel joining is always in demand. This combination of dissimilar direct joint was not successful, and had failed more often due to hard interface formation and migration of carbon from steel to titanium alloy. The joining of Ti-6Al-4V and stainless steel seems to be not easy. Based on the above literature and an assessment made, the Ti-6Al-4V and 304L stainless steel with a copper interlayer of diffusion bonded joint and friction welded joint was compared for their performance.
2. Materials and methods Diffusion bonding of Ti-6Al-4V and stainless steel rods with copper interlayer and friction welding of the same is compared for bonding strength and microstructure.
2.1. Diffusion bonding of the Ti-6Al-4V/copper/stainless steel Commercially pure titanium and PH (precipitation hardened) stainless steel rod of 30 mm length and 15 mm diameter was fabricated to size using the machining process. The surfaces were then prepared with standard metallographic techniques, cleaned with acetone and dried before being placed into the welding chamber. The process was carried out at 800–1000 °C with steps of 50 °C for 3.6 ks under 3.5 MPa uni-axial load in (4–6) 10 3 Pa vacuum and 50, 60 and 70 min of holding time [4].
2.2. Friction welding of Ti-6Al-4V/Copper/stainless steel The materials considered during experimentation were commercial rolled rods of grade-5 Ti-6Al-4V, stainless steel 304L and oxygen free copper of diameter 20 mm. The materials were prepared to a length of 100 mm each using the machining process. The surfaces were then prepared with standard metallographic techniques, cleaned and dried before being set into the friction welding machine. The elements present in the base metal were configured in accordance with ASME-E-1086-2008 standard at room temperature. A friction pressure of 30 bar and upset pressure of 100 bar was fixed for the copper and titanium alloy joint by trials. The 30 bar friction pressure provided a better heating between the titanium alloy and copper faying surface. Details are presented in Tables 1(a) and 1(b). The samples prepared were fixed in the KUKA friction welding machine with a feed rate of 0.5 mm/s and 1125 RPM. The joints were prepared in two steps. The first step was accomplished by joining stainless steel and copper with 8 N/ mm2 friction pressure and 14 N/mm2 upset pressure. This is showed in Fig. 1b. Following this, final joint is established in the second step by joining Ti-6Al-4V and copper with 12 N/mm2 friction pressure and 40 N/mm2 upset pressure by maintaining the copper length at 12 mm before joining.
2.3. Hardness test Preliminary investigation was done on the joints for assessment of the bonding strength by performing the drop test. All the joints passed the drop test and were ready for further investigation. This is exhibited in Fig. 2. The welded specimens were tested for hardness from the welded region to the parent metal region by microhardness testing machine INNOVATES vickers micro hardness with a load of 0.01–2 kgf.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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R. Kumar, M. Balasubramanian / Materials Today: Proceedings xxx (xxxx) xxx Table 1a Chemical composition (Wt.%) of SS304L. C
Si
Mn
P
S
Cr
Mo
Ni
N
Fe
0.030
0.36
1.58
0.038
0.022
18.37
0.13
8.28
0.033
Bal
Table 1b Chemical composition (Wt.%) of Ti-6Al-4V. C
Si
Al
V
Ni
Fe
Ti
0.030
0.010
6.3
4.3
0.01
0.05
Bal.
Fig. 1. Schematic representation of processes.
Fig. 2. Friction welded samples.
2.4. Tensile test Ti-6Al-4V/Copper/stainless steel welded samples were prepared for tensile testing as per ASTM standard with an initial gauge length of 50 mm and diameter of 12.5 mm. The test was executed at the room temperature of 24.4 °C by an MTS make universal testing machine of 1000 KN capacity. The test samples are shown in Fig. 3.
Fig. 3. Tensile tested samples.
2.5. Microstructure study For micro structural examination, the samples were cut from the welded sample in the middle of the welded region by wire cut EDM. The specimen required being taken from the region, adequately representing the structure and other features of the entire
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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sample under consideration. The fine scratches caused by the grinding operation were removed by polishing with diamond powders. The etchants used were Kroll’s reagent for titanium alloy, potassium dichromate for copper and glyceregia solution for stainless steel. The testing was conducted with de-wintor inverted trinocular metallurgical microscope with 150 magnification. The micrographs are shown in Fig. 4(a–f).
3. Result and discussion 3.1. Hardness survey When Ti-6Al-4V/Copper/stainless steel 304 diffusion bonded samples were evaluated for micro hardness distribution (Fig. 5) in the interface, the stainless steel region experienced an
Fig. 4. (a–f) Micro structural observation at interface.
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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the fractured face after the tensile test shows the major diffusion of copper on both the sides. The fractured face showed more copper diffused on the Ti-6Al-4V side and stainless steel side. Processing time of diffusion bonding and friction welding is provided in Table 2. 3.3. Microstructure analysis
Fig. 5. Hardness of diffusion bonded joints.
insignificant increase in hardness due to copper interlayer. In Ti6Al-4V zone, intermetallic formation was evidenced along with high temperature and holding time as a result of sudden increase in hardness due to the presence of copper. Hardness was found to increase from 400 to 700 HV in titanium side and 300 to 400 HV at the joint interface region. Details of the hardness distribution across the samples of friction welded joints are presented in Fig. 6. The hardness at the Ti-6Al-4V and interface varied from 360 HV to 370 HV and 290 HV to 300 HV respectively. Friction pressure and friction time are the main causes for variation in hardness.
3.2. Evaluation of tensile test The diffusion of copper on both the sides of the joint was good due to lesser interlayer thickness. There was a gradual increase in tensile strength with decrease in interlayer thickness. The tensile strength ranged between 80.4 and 520 MPa. The max tensile strength obtained was 520 MPa for an interlayer thickness (ILT) of 0.65 mm. The tensile strength was noted with interlayer thickness between 0.65 mm and 0.72 mm. A macro examination of
Fig. 4(a-b) shows the microstructure friction welded titanium alloy and stainless steel. The microstructure shows fine uniform grains of alpha and beta formed at the grain boundaries of alpha. The microstructure of the stainless steel was seen with uniform grains of austenite throughout the matrix. The stainless steel in the solution was in annealed condition and alloys carbides were not present at the grain boundary. Fig. 4(c) shows the microstructure of the three zones of Ti-6Al4V, copper and stainless steel 304L. Fig. 4(d) shows the copper interface with stainless steel. The microstructure of stainless steel remained unchanged, but the microstructures of copper have become finer as a result of rapid heating and cooling. Fig. 4(e) shows the interfacial zone with a substantial flow of grains due to frictional heat and stress. Fig. 4(f) shows the interface zone of copper and titanium. The copper at the diffusion zone shows fine grains resulting from rapid heating and cooling. The diffusion zone is dark in color are consists of constituents of both copper and titanium. The Ti-6Al-4V microstructure shows the presence of an acicular a phase transformed into a b phase and formation of some a’ due to the temperature gradient. 3.4. EDS and XRD analysis The titanium alloys and steel weldment suffering from the brittleness of resulting intermetallic compounds like Ti-Fe and Ti-Cr is common knowledge. Hence energy dispersive X-ray (EDS) analysis was performed for understanding the formation of intermetallics and to find out the distribution of elements at the interface region. The microstructure of the stainless steel and copper interface region reveals the presence of some intermetallics. The solubility of copper, 14.52% by weight is clearly seen in the stainless steel region. The solubility of copper in the Ti-6Al-4V side as 64.3% which is more than any other elements is also evident. Cu-Ti intermetallic phases with increasing Cu content is seen (Fig. 7) while the low fusion point of copper encourages improved contact area in the mating surfaces. 3.5. Effect of friction pressure and upset pressure Many trials helped the fixing of the friction pressure and upset pressure in the stainless steel and the copper joint at 20 bar and 35 bar respectively, and as providing the best result and passing the drop test. The adequacy of the 20 bar friction pressure for the heat generated between the stainless steel and copper surface,
Table 2 Processing time – diffusion bonding and Friction welding. Sl. No.
Description of work
1 2 3 4 5 Fig. 6. Hardness of friction welded joints.
Time (Min) Diffusion bonding
Friction welding
Preparation of specimen, including polishing Loading and Setting time DB holds Time/FW time Unloading Time Cleaning of flash and finishing
20
16
10 90 03 04
10 06 02 02
Total time
127
36
Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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R. Kumar, M. Balasubramanian / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 7. EDS analysis at stainless steel/copper and copper/Titanium interface region.
and the ability of the 35 bar upset pressure providing better bonding are palpable. A significant role is played by higher friction pressure in providing the heat found necessary for the parent metal. A higher strength of the joint resulted from this. The upset pressure which was comparatively high created a good bonding between titanium and copper. 3.6. Effect of friction time An increase in strength is seen with increase in contact time. The amount of heat generated is the reason for this phenomenon. Increase in friction time is followed by increase in heat generated. This causes deformation in the interlayer to a great extent, causing the formation of thin interlayer which produces a higher tensile strength. The increase in friction time is followed by a decrease in interlayer thickness. Such increase will also mean greater strength for the joint, while lower friction time means less strength. 3.7. Effect of upset time With increase in upset time, there is increase in tensile strength also. This is the result of good bonding between the base metals and the interlayer metal. This bonding causes a high rate of diffusion of copper in the interface. Increased upset time resulted in a hard surface in the copper region compared to the base metals. Absorption of more heat due to the greater thickness of the copper interlayer was the reason for this, requiring more time for cooling, whereas less thickness required less time for cooling. The excess copper metal pumped out as flash, which is not available in the welded region explains this. Tensile strength of 80 MPa was accomplished for a lower upset time of 2 s. With a gradual increase in the upset time, there was decrease in the interlayer thickness to 0.65 mm but there was an increase in tensile strength. 4. Conclusions The conclusion drawn from the comparative study and analysis performed on Ti-6Al-4V and stainless steel 304L with copper interlayer by diffusion bonding and friction welding are a) The diffusion bonding process helps achievement of a maximum shear strength of 163 MPa and tensile strength of 282 MPa, while the yield from the friction welding process for the material under investigation was 520 MPa. b) Variations in hardness seen in friction welded joints is less at the interface region and resulted in formation of 320 HV close to the boundary. In diffusion bonding, the region
adjacent to the interface has a maximum hardness of 240 HV arising from high temperature and longer period of holding time. c) The total processing time for friction welding is as low as 36 min as against 127 min for diffusion bonding.
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Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047
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Please cite this article as: R. Kumar and M. Balasubramanian, Analysis and comparison of diffusion bonded and friction welded Ti-6Al-4V and stainless steel joints with copper as interlayer, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.047