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Influence of beam current on microstructures and mechanical properties of electron beam welding-brazed aluminum-steel joints with an Al5Si filler wire
Vacuum 141 (2017) 281e287
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Influence of beam current on microstructures and mechanical properties of electron beam welding-brazed aluminum-steel joints with an Al5Si filler wire Ting Wang a, b, *, Yongyun Zhang a, Xiaopeng Li b, **, Binggang Zhang b, Jicai Feng a, b a b
Harbin Institute of Technology at Weihai, Shandong Provincial Key Laboratory of Special Welding Technology, Weihai 264209, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 October 2016 Received in revised form 17 April 2017 Accepted 18 April 2017 Available online 19 April 2017
Exploratory experiments of Electron beam welding-brazing (EBW-brazing) of 304 stainless steel (304 SS) to commercial pure aluminum (CP-Al) with an Al5Si filler wire under different beam currents were carried out. The microstructural characteristics of the joints were analyzed by SEM and XRD methods. Tensile strengths and nanoindentation were performed to evaluate the mechanical properties of the joints. The results indicated that beam current affected the thickness of the IMC layer and weld appearances, then further affected the tensile strengths of joints. The major intermetallic compound (IMC) formed at the connection layer was determined to be Al8Fe2Si with a Young's modulus of 223.2 GPa and nanohardness of 9.5 GPa. The highest tensile strength of the joints was 93 MPa under an optimum welding parameter, which was approximately equal to 83% of that of CP-Al. © 2017 Published by Elsevier Ltd.
Keywords: 304 stainless steel Pure aluminum Electron beam welding-brazing Microstructure Mechanical properties
1. Introduction Owing to high strength, excellent corrosion resistance as well as relatively low cost, stainless steels were commonly used in industrial production [1e3]. Meanwhile, aluminum alloys were widely applied into aerospace and automobile industries due to their high specific strength, good ductility, high thermal conductivity and excellent corrosion resistance [4e6]. To make full use of the advantages of these two materials simultaneously, the Al-Steel hybrid joints were gradually used in the auto industry for the light weight. Hence, feasible and reliable joining methods of 304 SS to aluminum were particularly imperative. The formation of brittle IMCs became the main problem in joining of these two metals [7e9]. Various of welding methods were attempted to solve the problem, such as friction stir welding [10], resistance spot welding [11], diffusion bonding [12], surface activated bonding [13] and vacuum brazing [14], etc. Among them, solid state joining was restricted for the limited joint types and long
* Corresponding author. Harbin Institute of Technology at Weihai, Shandong Provincial Key Laboratory of Special Welding Technology, Weihai 264209, China. ** Corresponding author. E-mail addresses: [email protected] (T. Wang), [email protected] (X. Li). http://dx.doi.org/10.1016/j.vacuum.2017.04.029 0042-207X/© 2017 Published by Elsevier Ltd.
duration [15]. Vacuum brazing was possible for joining aluminum to steel, yet the poor fatigue performance of the joint limited its industrial use. The fusion welding methods were taken into account attributed to the relatively simple welding process. However, handicaps in controlling the heat input should be considered, because the severe growth of IMCs in high heat input would lead to welding fracture. Therefore, welding-brazed steel to aluminum, similar welding process as fusion welding but a lower heat input than fusion welding, was extensively investigated in recent years. Lee et al. reported that the tensile strengths of the joints were related to the thicknesses and type of IMCs layers. As well, the thickness of the IMCs layer was influenced by welding parameters in the laser beam welding-brazed joint of aluminum to steel [16]. As another approach, Murakami et al. proposed that the thickness of IMCs layer increased with the decrease of welding speed in MIGbrazing of aluminum to steel and the thickness of IMCs layer also influenced fracture modes of welded joints [17]. Furthermore, Zhang et al. found that CMT welding process can decrease the thickness of IMCs layer to 7e8 mm on account of the low heat input as a modified MIG welding process [18]. For the precisely controlled welding parameters, vacuum condition and high energy density, EBW-brazing was considered to be a preferred welding method to join dissimilar metals [19e22]. Al-Si
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alloy was usually used as a filler metal during the welding-brazing process of Al to steel because of its superb hot cracking resistance and fluidity. Besides, Roulin et al. revealed that Si addition can restrain the growth of IMCs in furnace brazed aluminum to stainless steel joint, and the Fe-Al-Si layer was produced earlier than FeAl3 layer [23]. Wang et al. found that Si addition would restrict growth of the IMC layer, and changed the proportion of the Al-Fe IMCs in laser welded AA6061-carbon steel joints by using of Al5Si intermediate layer [24]. Consequently, in the present paper, electron beam welding-brazing with a filler wire method was firstly used to join CP-Al with 304 SS in a vacuum environment. Al5Si wire was chosen as filler metal and the influence of the beam current on the microstructures and mechanical properties of the joints was studied.
Table 2 Basic physical properties of CP-Al and 304 SS. Base metal
CP-Al 304 SS
Melting point
Density
Coefficient of linear expansion
/(K)
/(g$cm3)
(106$K1)
931 1723
2.67 7.98
24 16.6
Table 3 Basic chemical properties of CP-Al and 304 SS. Base metal
Lattice type
Atomic radius/(nm)
Electronic structure
CP-Al 304 SS
FCC BCC
0.143 0.127
3s23p1 3d64s2
2. Experimental 304 stainless steel (304 SS) and commercial pure aluminum (CPAl) plates with dimensions of 100 25 1 mm and 100 25 1.5 mm, respectively, were selected as the base material for this experiment. A groove with an angle of 45 on the aluminum side was adopted to increase the contact area of the filler wire droplet and base metal, in order to achieve a stable connection. Al5Si filler wire with a diameter of 0.8 mm was used as the filler metal and the chemical composition was shown in Table 1. The physical and chemical properties of CP-Al and 304 SS were given in Table 2 and Table 3, respectively. Large differences of physical and chemical properties between Al and steel existed, which would lead to metallurgical incompatibility during the welding process and worsen the joining ability. Base metals were mechanically and chemically cleaned before welding. Then the welding experiments were conducted under a vacuum degree of 5 102 Pa. The electron beam was focused on the CP-Al side with a beam offset of 0.3 mm. The accelerating voltage was controlled at 10 kV and welding speed was 150 mm/min. The beam current was chosen as three values of 40 mA, 52 mA and 65 mA for the comparative experiments. The schematic diagram of the welding process was shown in Fig. 1. The Al5Si wire was fed in the front of the electron beam spot along the welding direction with a feeding angle of 45 and feeding speed of 0.6 m/min. To prepare the metallographic and tensile specimens, the joints were sampled by wire-cut electric discharge machine perpendicular to the welding direction. The metallographic sample was etched with HF þ HCL þ HNO3 þ H2O in 1:1.5:2.5:95 proportion (in volume ratio). Cross sections and microstructures of the joints were observed by scanning electron microscopy (Hitachi S-4700) and optical microscope (OLYMPUS DSX-510), respectively. The composition of the IMCs layer at the interface between steel and aluminum was established by scanning electron microscopy (SEM) including Energy Dispersive Spectroscopy (EDS) with spot and line scanning modes. The tensile tests were conducted with the universal electronic material testing machine (INSTRON MODEL 1186) at the displacement velocity of 1 mm/min. The location of tensile samples in the joint was shown in Fig. 2 with the length and width of 50 mm and5 mm, respectively. Five tensile samples were prepared for each set of parameter. The fracture surfaces of the joints were also analyzed by SEM. The X-ray diffraction (XRD) was carried out on the fractured surface in both the two sides in step mode
Table 1 Chemical composition of Al5Si filler wire (wt,%). Si
Cu
Mg
Fe
Al
5
0.05
0.10
0.4
Bal.
Fig. 1. Schematic diagram of EBW-brazing process with filler wire.
between 10 and 90 (in 2q) with the total accumulation time of about 40 min (0.03 $s1). The nanohardness and Young's modulus of the IMC layer were evaluated by nanoindentation tester (Demesion 3100) at the load of 20 mN and a dwell time of 10 s. 3. Results and discussion 3.1. Surface appearances and microstructures of the joints 3.1.1. Surface appearances The surface appearances of the welded joints at different beam currents were presented in Fig. 3. As observed in Fig. 3 (a) that discontinuous weld formation was observed in the joint welded at the lowest beam current of 40 mA in this paper. This was because that the filler wire would not be molten continuously into the weld pool when the heat input was too lower [25]. With the increase of heat input by adjusting the beam current to 52 mA, drop transfer from filler wire to weld pool turned to be continuous, which resulted in a better weld surface appearance as shown in Fig. 3 (b). However, when the beam current was increased to 65 mA, high heat input made a certain amount of 304 SS adjacent to the contact face melt into the fusion pool, which would lead to more brittle IMCs formation in the weld and deteriorate the mechanical properties of the joint. As a result, cracking occurred in this joint as seen in Fig. 3 (c). The similar phenomenon was also observed in the joints welded by other welding methods when the IMCs layers were too thick [7,10]. 3.1.2. Macrostructure and microstructures of the cross sections The macrostructure of the cross section of the welded joint with optimum surface appearance under a beam current of 52 mA was shown in Fig. 4. The cross section was divided into two parts:
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Fig. 2. Schematic diagram of tensile samples.
Fig. 3. Surface appearances of the EBW-brazed Al/Steel butt joints at beam currents of (a) 40, (b) 52 and (c) 65 mA.
Fig. 4. Typical macrostructure of the cross section of the welded joint under the beam current of 52 mA in SEM.
brazing seam near 304 SS and fusion seam near CP-Al base metal. As displayed in Fig. 4 that molten aluminum extended to the upper surface of 304 SS due to its good wettability with Al5Si filler wire in vacuum atmosphere without any additional flux [26] or cladding layer [27]. Some tiny pores were found in the fusion seam. It was attributed to the lower critical hydrogen partial pressure and the larger heat conductivity coefficient of Al as displayed in Ref. [28]. In
the cooling process of the molten pool, the viscosity of liquid metal gradually increased along with the solidification process [29]. When the escaping velocities of bubbles were lower than the cooling speed of liquid metal, they would remain in the weld and formed welding pores after solidification. Fig. 5 displayed the microstructures of different zones in the cross section of the welded joint designated as A, B, C and D in Fig. 4. The interface structures at different locations displayed distinctive characteristics. The upper intermetallic layer (Zone A) between weld metal and 304 SS shown in Fig. 5(a) was characterized by plenty of plate-like compounds and crystal whiskers growing perpendicular to interface with a length of approximately 40 mm and a width of no more than 1 mm. Fig. 5(b) revealed that the corner interface as Zone B shown in Fig. 4 was made up of spindly nail-like bars and a small quantity of plate-like structures similar as Zone A, but with a smaller size. Whereas the vertical interface (Zone C) mainly contained continuous IMCs layer structure. The amount and size of nail-like bars in this region reduced significantly. The decrease of IMCs from the top to the bottom of the interface was relative to the energy decay of the electron beam in the vertical direction. The matrix of the fusion seam near CP-Al as shown in
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Fig. 5. SEM microstructures of (a) Zone A, (b) Zone B, (c) Zone C and (d) Zone D in Fig. 3.
Fig. 5(d) was typical solidification structure of a-Al with Al-Si eutectics distributed on grain boundaries. The compositions of the various phases in interfacial layers were further analyzed according to SEM-EDS consequences. In previous studies on the joining of Al to steel by other researchers [12e18], intermetallic compounds such as FeAl3 and Fe2Al5 had been detected regularly. In the experiment with Al-Si filler metal [6], different Al-Fe-Si ternary phases of t1-t11 with distinct contents of Si element were generated in the joint. SEM-EDS was carried out on distinct phases with different shapes in the interface layer. The results were presented in the tables in Fig. 5. The plate-like compounds in Fig. 5 (a) were determined to be Fe2Al5. A small quantity of iron atoms dissolved into weld metal and then grew up in nucleation during the welding process and then plate-like Fe2Al5 generated owing to its lower growth-driving force. The nail-like structures and whiskers in Fig. 5 (b) were deduced as FeAl3 on account of the atomic ratio in EDS results. However, IMC layer adjacent to 304 SS was preliminarily judged as a kind of Al-Fe-Si ternary compound t5-Al8Fe2Si on the basis of EDS results. The Xray diffraction profiles of the fracture surfaces at the interface layers of Al and Fe side shown in Fig. 6, were used to further confirm the type of IMCs. And, t5-Al8Fe2Si and FeAl3 were both identified in the fracture surface. For the low content, Fe2Al5 had a low probability to be detected by X-ray. 3.2. The influence of beam current on interfacial structures Microstructures of the interface layer in the light microscope (LM) of the joints welded at beam currents of 40 mA and 52 mA were shown in Fig. 7. The thickness of interfacial layer varied along with the variation of beam current. The concentration profiles of the major elements across the base metals/weld interfaces were shown in Fig. 8. The scanning line was indicated in Fig. 5 (c). From the line scanning results and the LM pictures, the thicknesses of IMC layers in unique position of the joints under different beam currents were measured. The average thickness of the bonding
Fig. 6. X-ray diffraction profiles of the fractured surfaces of the welded joint under the beam current of 52 mA.
layer in the joints welded under different beam currents shown in Fig. 9 indicated that the thickness of the IMC layer increased obviously with the increase of beam current. When the beam current increased from 40 mA to 65 mA, the thickness of the bonding layer also increased from 7 mm to 16 mm. The increase of beam current would improve the highest temperature of the molten pool and thus increase growth rate of IMCs [18]. On the other hand, the dwell time of high temperature in molten pool lengthened with the increasing welding heat input, which can promote the interface reaction. The similar result was announced in MIG arc brazing of aluminum to steel in Ref. [16]. 3.3. Mechanical properties 3.3.1. Nanoindentation To investigate the toughness of the joint, the nanoindentation
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285
Fig. 7. The microstructures of the interface layer welded at welding speed of 150 mm/min and beam current of (a) 52 mA and (b) 40 mA.
Fig. 8. Line scanning of the element distribution across the intermetallic layer of the weld.
Fig. 10. Load-depth curves of indentations carried out in Al8Fe2Si layer.
Table 4 Nanoindentation datum of the bonding layer.
Al8Fe2Si layer
HN (GPa)
E (GPa)
hf (nm)
hmax (nm)
9.5
223.2
177.2
272.0
Fig. 9. Effects of beam current on the thickness of the intermetallic compound layer.
test on the interface layer in the welded joint under the beam current of 52 mA was carried out. The typical load-displacement curve was illustrated in Fig. 10. Table 3 presented the nanohardness (HN), Young's modulus (E), maximum displacement (hmax) and the final depth (hf) of the indentation at Al8Fe2Si layers. Table 4
Fig. 11. Effects of beam current on tensile strength of joints.
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Fig. 12. (a) Fracture location of the joint and (b) Morphology of fracture surface in SEM.
shows that the Young's modulus of Al8Fe2Si IMC layer is 223.2 GPa, higher than that of CP-Al base metal as 70.3 GPa at room temperature [30]. The nanohardness of the interface layer is 9.5 GPa, which was so high that will lead brittle fracture occurred in the joint under tensile load. Tensile tests of CP-Al base metal and welded Al/304 SS joints were performed. The impact of beam current on tensile strength was displayed in Fig. 11. As well, welded joints were all fractured at the interface layer as seen in Fig. 12. As Fig. 12 demonstrated that tensile strength firstly increased with the rise of the beam current from 40 mA to 52 mA. When beam current got up to 65 mA, cracking occurred in the joint after welding, the tensile strength was decreased to zero. Combined to the IMC layer thickness discussed above, the variation of tensile strength along with beam current was not corresponding with the increase of IMC thickness. In particular, tensile strength of the joint deceased steeply for the thickening of the IMC layer with the increase of the beam current from 52 mA to 65 mA. When the beam current dropped from 52 mA to 40 mA, tensile strength of joint declined from 93.3 MPa to 71.8 MPa though the thickness of the bonding layer increased from 7 mm to 10 mm. It also had been reported that the tensile strength could be maintained well when the thickness of the bonding layer was under 10 mm [23]. The poor wetting of molten filler wire on the 304 SS plate surface in the lower beam current was also harmful to the tensile strength. The highest tensile strength of the hybrid metal connection structures was welded under the beam current of 52 mA with desired weld appearance and modest IMC layer thickness. The highest value is 93 MPa, which is 83% of that of CP-Al base metal at room temperature. To further study the effects of the IMC layer on the tensile strength, the fracture surfaces at the aluminum side were observed in SEM as shown in Fig. 12. There were cleavage surfaces, friable particles and micro cracks in the fracture surface. EDS was performed on the friable particle as marked in Fig. 12. The results illustrated that these particles were analogous with Fe-Al-Si ternary compound, which revealed that the IMC layer was the weakest part of the joint. Due to the high Young's modulus and hardness of the Al8Fe2Si layer, it was easy to conclude that the Al8Fe2Si is a brittle phase compared with Al base metal. The microcracks were the most likely to propagate in the Al8Fe2Si layer under the stress of during the tensile tests.
4. Conclusions (1) Electron beam welding-brazing of dissimilar CP-Al to 304 SS using an Al5Si filler wire was successfully carried out. The joint with well weld appearance and modest IMC layer
thickness had the highest tensile strength of 93 MPa, up to 83% of that of CP-Al base metal. (2) Cross section of the joint was characterized by fusion weld at aluminum side and brazing seam in 304 SS side. Intermetallic compounds with unusual shapes were produced. The interfacial layer was mainly recognized as Al8Fe2Si. And a small of amount of nail-like and whiskers FeAl3 and plate-like Fe2Al5 were also detected. The fusion weld in aluminum side was mainly made up of a-Al solid solution and Al-Si eutectic structures. (3) Beam current affected the tensile strength through its effect on the thickness of the intermetallic layer and weld surface appearance. The brittle cleavage fracture in the Al8Fe2Si IMC layer of the joint during the tensile test was primarily attributed to the high hardness of Al-Fe-Si IMCs measured by nanoindentation test. Acknowledgement This work was supported by the National Natural Science Foundation of China (51405098, U1637104) and Shandong Provincial Natural Science Foundation (BS2014ZZ002). References [1] B.G. Zhang, J. Zhao, X.P. Li, J.C. Feng, Electron beam welding of 304 stainless steel to QCr0.8 copper alloy with copper filler wire, T. Nonferr. Metal. Soc. 24 (12) (2014) 4059e4066. [2] X. Tian, R.K.Y. Fu, L. Wang, P.K. Chu, Oxygen-induced nickel segregation in nitrogen plasma implanted AISI 304 stainless steel, Mat. Sci. Eng. A 316 (1) (2001) 200e204. [3] X. Zhang, E. Ashida, S. Tarasawa, Y. Anma, M. Okada, Welding of thick stainless steel plates up to 50 mm with high brightness lasers, J. Laser. Appl. 23 (2) (2011) 807e819. [4] A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. Benedictus, Recent development in aluminum alloys for aerospace applications, Mat. Sci. Eng. A 280 (1) (2000) 102e107. [5] A.M.K. Esawi, M.A.E. Borady, Carbon nanotube-reinforced aluminum strips, Compos. Sci. Technol. 68 (2) (2008) 486e492. [6] J.L. Song, S.B. Lin, C.L. Yang, G.C. Ma, H. Liu, Spreading behavior and microstructure characteristics of dissimilar metals TIG weldingebrazing of aluminum alloy to stainless steel, Mat. Sci. Eng. A 509 (1e2) (2009) 31e40. [7] S. Bozzi, A.L. Helbert-Etter, T. Baudin, B. Criqui, J.G. Kerbiguet, Intermetallic compounds in Al 6016/IF-steel friction stir spot welds, Mat. Sci. Eng. A 527 (16e17) (2010) 4505e4509. [8] S. Kobayashi, T. Yakou, Control of intermetallic compound layers at interface between steel and aluminum by diffusion-treatment, Mat. Sci. Eng. A 338 (1e2) (2002) 44e53. [9] W.B. Lee, M. Schmuecker, U.A. Mercardo, G. Biallas, S.B. Jung, Interfacial reaction in steelealuminum joints made by friction stir welding, Scr. Mater. 55 (4) (2006) 355e358. [10] T. Tanaka, T. Morishige, T. Hirata, Comprehensive analysis of joint strength for dissimilar friction stir welds of mild steel to aluminum alloys, Scr. Mater 61 (7) (2009) 756e759. [11] N. Vol, Resistance spot welding of aluminum and steel: a comparative
T. Wang et al. / Vacuum 141 (2017) 281e287 experimental study, P I Mech. Eng. B-J Eng. 217 (2003) 1355e1363. [12] H. Springer, A. Kostka, E.J. Payton, D. Raabe, A. Kaysser-Pyzalla, G. Eggeler, On the formation and growth of intermetallic phases during interdiffusion between low-carbon steel and aluminum alloys, Acta. Mater. 59 (4) (2011) 1586e1600. [13] M.M.R. Howlader, T. Kaga, T. Suga, Investigation of bonding strength and sealing behavior of aluminum/stainless steel bonded at room temperature, Vacuum 84 (2010) 1334. [14] P. Liu, Y. Li, J. Wang, J. Guo, Vacuum brazing technology and microstructure near the interface of Al/18-8 stainless steel, Mater. Res. Bull. 38 (9) (2003) 1493e1499. [15] G. Sierra, P. Peyre, F.D. Beaume, D. Stuart, G. Fras, Galvanised steel to aluminium joining by laser and GTAW processes, Mater. Charact. 59 (12) (2008) 1705e1715. [16] K.J. Lee, S. Kumai, N. Ishikawa, K. Furuya, Interfacial microstructure of a6111/ steel lap joint fabricated by defocused laser beam welding, Mater. Sci. Forum 519e521 (2006) 1119. [17] T. Murakami, K. Nakata, H. Tong, M. Ushio, Dissimilar metal joining of aluminum to steel by MIG arc brazing using flux cored wire, Isij. Int. 43 (10) (2003) 1596e1602. [18] H.T. Zhang, J.C. Feng, P. He, Interfacial phenomena of cold metal transfer (CMT) welding of zinc coated steel and wrought aluminium, Mater Sci. TechLond 24 (11) (2008) 1346e1349. [19] M.S. We˛ glowski, S. Błacha, A. Phillips, Electron beam welding e techniques and trends e review, Vacuum 130 (2016) 72e92. [20] Y. Luo, H. Ye, C. Du, H. Xu, Influence of focusing thermal effect upon AZ91D magnesium alloy weld during vacuum electron beam welding, Vacuum 86 (2012) 1262.
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Influence of beam current on microstructures and mechanical properties of electron beam welding-brazed aluminum-steel joints with an Al5Si filler wire Ting Wang a, b, *, Yongyun Zhang a, Xiaopeng Li b, **, Binggang Zhang b, Jicai Feng a, b a b
Harbin Institute of Technology at Weihai, Shandong Provincial Key Laboratory of Special Welding Technology, Weihai 264209, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 October 2016 Received in revised form 17 April 2017 Accepted 18 April 2017 Available online 19 April 2017
Exploratory experiments of Electron beam welding-brazing (EBW-brazing) of 304 stainless steel (304 SS) to commercial pure aluminum (CP-Al) with an Al5Si filler wire under different beam currents were carried out. The microstructural characteristics of the joints were analyzed by SEM and XRD methods. Tensile strengths and nanoindentation were performed to evaluate the mechanical properties of the joints. The results indicated that beam current affected the thickness of the IMC layer and weld appearances, then further affected the tensile strengths of joints. The major intermetallic compound (IMC) formed at the connection layer was determined to be Al8Fe2Si with a Young's modulus of 223.2 GPa and nanohardness of 9.5 GPa. The highest tensile strength of the joints was 93 MPa under an optimum welding parameter, which was approximately equal to 83% of that of CP-Al. © 2017 Published by Elsevier Ltd.
Keywords: 304 stainless steel Pure aluminum Electron beam welding-brazing Microstructure Mechanical properties
1. Introduction Owing to high strength, excellent corrosion resistance as well as relatively low cost, stainless steels were commonly used in industrial production [1e3]. Meanwhile, aluminum alloys were widely applied into aerospace and automobile industries due to their high specific strength, good ductility, high thermal conductivity and excellent corrosion resistance [4e6]. To make full use of the advantages of these two materials simultaneously, the Al-Steel hybrid joints were gradually used in the auto industry for the light weight. Hence, feasible and reliable joining methods of 304 SS to aluminum were particularly imperative. The formation of brittle IMCs became the main problem in joining of these two metals [7e9]. Various of welding methods were attempted to solve the problem, such as friction stir welding [10], resistance spot welding [11], diffusion bonding [12], surface activated bonding [13] and vacuum brazing [14], etc. Among them, solid state joining was restricted for the limited joint types and long
* Corresponding author. Harbin Institute of Technology at Weihai, Shandong Provincial Key Laboratory of Special Welding Technology, Weihai 264209, China. ** Corresponding author. E-mail addresses: [email protected] (T. Wang), [email protected] (X. Li). http://dx.doi.org/10.1016/j.vacuum.2017.04.029 0042-207X/© 2017 Published by Elsevier Ltd.
duration [15]. Vacuum brazing was possible for joining aluminum to steel, yet the poor fatigue performance of the joint limited its industrial use. The fusion welding methods were taken into account attributed to the relatively simple welding process. However, handicaps in controlling the heat input should be considered, because the severe growth of IMCs in high heat input would lead to welding fracture. Therefore, welding-brazed steel to aluminum, similar welding process as fusion welding but a lower heat input than fusion welding, was extensively investigated in recent years. Lee et al. reported that the tensile strengths of the joints were related to the thicknesses and type of IMCs layers. As well, the thickness of the IMCs layer was influenced by welding parameters in the laser beam welding-brazed joint of aluminum to steel [16]. As another approach, Murakami et al. proposed that the thickness of IMCs layer increased with the decrease of welding speed in MIGbrazing of aluminum to steel and the thickness of IMCs layer also influenced fracture modes of welded joints [17]. Furthermore, Zhang et al. found that CMT welding process can decrease the thickness of IMCs layer to 7e8 mm on account of the low heat input as a modified MIG welding process [18]. For the precisely controlled welding parameters, vacuum condition and high energy density, EBW-brazing was considered to be a preferred welding method to join dissimilar metals [19e22]. Al-Si
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alloy was usually used as a filler metal during the welding-brazing process of Al to steel because of its superb hot cracking resistance and fluidity. Besides, Roulin et al. revealed that Si addition can restrain the growth of IMCs in furnace brazed aluminum to stainless steel joint, and the Fe-Al-Si layer was produced earlier than FeAl3 layer [23]. Wang et al. found that Si addition would restrict growth of the IMC layer, and changed the proportion of the Al-Fe IMCs in laser welded AA6061-carbon steel joints by using of Al5Si intermediate layer [24]. Consequently, in the present paper, electron beam welding-brazing with a filler wire method was firstly used to join CP-Al with 304 SS in a vacuum environment. Al5Si wire was chosen as filler metal and the influence of the beam current on the microstructures and mechanical properties of the joints was studied.
Table 2 Basic physical properties of CP-Al and 304 SS. Base metal
CP-Al 304 SS
Melting point
Density
Coefficient of linear expansion
/(K)
/(g$cm3)
(106$K1)
931 1723
2.67 7.98
24 16.6
Table 3 Basic chemical properties of CP-Al and 304 SS. Base metal
Lattice type
Atomic radius/(nm)
Electronic structure
CP-Al 304 SS
FCC BCC
0.143 0.127
3s23p1 3d64s2
2. Experimental 304 stainless steel (304 SS) and commercial pure aluminum (CPAl) plates with dimensions of 100 25 1 mm and 100 25 1.5 mm, respectively, were selected as the base material for this experiment. A groove with an angle of 45 on the aluminum side was adopted to increase the contact area of the filler wire droplet and base metal, in order to achieve a stable connection. Al5Si filler wire with a diameter of 0.8 mm was used as the filler metal and the chemical composition was shown in Table 1. The physical and chemical properties of CP-Al and 304 SS were given in Table 2 and Table 3, respectively. Large differences of physical and chemical properties between Al and steel existed, which would lead to metallurgical incompatibility during the welding process and worsen the joining ability. Base metals were mechanically and chemically cleaned before welding. Then the welding experiments were conducted under a vacuum degree of 5 102 Pa. The electron beam was focused on the CP-Al side with a beam offset of 0.3 mm. The accelerating voltage was controlled at 10 kV and welding speed was 150 mm/min. The beam current was chosen as three values of 40 mA, 52 mA and 65 mA for the comparative experiments. The schematic diagram of the welding process was shown in Fig. 1. The Al5Si wire was fed in the front of the electron beam spot along the welding direction with a feeding angle of 45 and feeding speed of 0.6 m/min. To prepare the metallographic and tensile specimens, the joints were sampled by wire-cut electric discharge machine perpendicular to the welding direction. The metallographic sample was etched with HF þ HCL þ HNO3 þ H2O in 1:1.5:2.5:95 proportion (in volume ratio). Cross sections and microstructures of the joints were observed by scanning electron microscopy (Hitachi S-4700) and optical microscope (OLYMPUS DSX-510), respectively. The composition of the IMCs layer at the interface between steel and aluminum was established by scanning electron microscopy (SEM) including Energy Dispersive Spectroscopy (EDS) with spot and line scanning modes. The tensile tests were conducted with the universal electronic material testing machine (INSTRON MODEL 1186) at the displacement velocity of 1 mm/min. The location of tensile samples in the joint was shown in Fig. 2 with the length and width of 50 mm and5 mm, respectively. Five tensile samples were prepared for each set of parameter. The fracture surfaces of the joints were also analyzed by SEM. The X-ray diffraction (XRD) was carried out on the fractured surface in both the two sides in step mode
Table 1 Chemical composition of Al5Si filler wire (wt,%). Si
Cu
Mg
Fe
Al
5
0.05
0.10
0.4
Bal.
Fig. 1. Schematic diagram of EBW-brazing process with filler wire.
between 10 and 90 (in 2q) with the total accumulation time of about 40 min (0.03 $s1). The nanohardness and Young's modulus of the IMC layer were evaluated by nanoindentation tester (Demesion 3100) at the load of 20 mN and a dwell time of 10 s. 3. Results and discussion 3.1. Surface appearances and microstructures of the joints 3.1.1. Surface appearances The surface appearances of the welded joints at different beam currents were presented in Fig. 3. As observed in Fig. 3 (a) that discontinuous weld formation was observed in the joint welded at the lowest beam current of 40 mA in this paper. This was because that the filler wire would not be molten continuously into the weld pool when the heat input was too lower [25]. With the increase of heat input by adjusting the beam current to 52 mA, drop transfer from filler wire to weld pool turned to be continuous, which resulted in a better weld surface appearance as shown in Fig. 3 (b). However, when the beam current was increased to 65 mA, high heat input made a certain amount of 304 SS adjacent to the contact face melt into the fusion pool, which would lead to more brittle IMCs formation in the weld and deteriorate the mechanical properties of the joint. As a result, cracking occurred in this joint as seen in Fig. 3 (c). The similar phenomenon was also observed in the joints welded by other welding methods when the IMCs layers were too thick [7,10]. 3.1.2. Macrostructure and microstructures of the cross sections The macrostructure of the cross section of the welded joint with optimum surface appearance under a beam current of 52 mA was shown in Fig. 4. The cross section was divided into two parts:
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Fig. 2. Schematic diagram of tensile samples.
Fig. 3. Surface appearances of the EBW-brazed Al/Steel butt joints at beam currents of (a) 40, (b) 52 and (c) 65 mA.
Fig. 4. Typical macrostructure of the cross section of the welded joint under the beam current of 52 mA in SEM.
brazing seam near 304 SS and fusion seam near CP-Al base metal. As displayed in Fig. 4 that molten aluminum extended to the upper surface of 304 SS due to its good wettability with Al5Si filler wire in vacuum atmosphere without any additional flux [26] or cladding layer [27]. Some tiny pores were found in the fusion seam. It was attributed to the lower critical hydrogen partial pressure and the larger heat conductivity coefficient of Al as displayed in Ref. [28]. In
the cooling process of the molten pool, the viscosity of liquid metal gradually increased along with the solidification process [29]. When the escaping velocities of bubbles were lower than the cooling speed of liquid metal, they would remain in the weld and formed welding pores after solidification. Fig. 5 displayed the microstructures of different zones in the cross section of the welded joint designated as A, B, C and D in Fig. 4. The interface structures at different locations displayed distinctive characteristics. The upper intermetallic layer (Zone A) between weld metal and 304 SS shown in Fig. 5(a) was characterized by plenty of plate-like compounds and crystal whiskers growing perpendicular to interface with a length of approximately 40 mm and a width of no more than 1 mm. Fig. 5(b) revealed that the corner interface as Zone B shown in Fig. 4 was made up of spindly nail-like bars and a small quantity of plate-like structures similar as Zone A, but with a smaller size. Whereas the vertical interface (Zone C) mainly contained continuous IMCs layer structure. The amount and size of nail-like bars in this region reduced significantly. The decrease of IMCs from the top to the bottom of the interface was relative to the energy decay of the electron beam in the vertical direction. The matrix of the fusion seam near CP-Al as shown in
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Fig. 5. SEM microstructures of (a) Zone A, (b) Zone B, (c) Zone C and (d) Zone D in Fig. 3.
Fig. 5(d) was typical solidification structure of a-Al with Al-Si eutectics distributed on grain boundaries. The compositions of the various phases in interfacial layers were further analyzed according to SEM-EDS consequences. In previous studies on the joining of Al to steel by other researchers [12e18], intermetallic compounds such as FeAl3 and Fe2Al5 had been detected regularly. In the experiment with Al-Si filler metal [6], different Al-Fe-Si ternary phases of t1-t11 with distinct contents of Si element were generated in the joint. SEM-EDS was carried out on distinct phases with different shapes in the interface layer. The results were presented in the tables in Fig. 5. The plate-like compounds in Fig. 5 (a) were determined to be Fe2Al5. A small quantity of iron atoms dissolved into weld metal and then grew up in nucleation during the welding process and then plate-like Fe2Al5 generated owing to its lower growth-driving force. The nail-like structures and whiskers in Fig. 5 (b) were deduced as FeAl3 on account of the atomic ratio in EDS results. However, IMC layer adjacent to 304 SS was preliminarily judged as a kind of Al-Fe-Si ternary compound t5-Al8Fe2Si on the basis of EDS results. The Xray diffraction profiles of the fracture surfaces at the interface layers of Al and Fe side shown in Fig. 6, were used to further confirm the type of IMCs. And, t5-Al8Fe2Si and FeAl3 were both identified in the fracture surface. For the low content, Fe2Al5 had a low probability to be detected by X-ray. 3.2. The influence of beam current on interfacial structures Microstructures of the interface layer in the light microscope (LM) of the joints welded at beam currents of 40 mA and 52 mA were shown in Fig. 7. The thickness of interfacial layer varied along with the variation of beam current. The concentration profiles of the major elements across the base metals/weld interfaces were shown in Fig. 8. The scanning line was indicated in Fig. 5 (c). From the line scanning results and the LM pictures, the thicknesses of IMC layers in unique position of the joints under different beam currents were measured. The average thickness of the bonding
Fig. 6. X-ray diffraction profiles of the fractured surfaces of the welded joint under the beam current of 52 mA.
layer in the joints welded under different beam currents shown in Fig. 9 indicated that the thickness of the IMC layer increased obviously with the increase of beam current. When the beam current increased from 40 mA to 65 mA, the thickness of the bonding layer also increased from 7 mm to 16 mm. The increase of beam current would improve the highest temperature of the molten pool and thus increase growth rate of IMCs [18]. On the other hand, the dwell time of high temperature in molten pool lengthened with the increasing welding heat input, which can promote the interface reaction. The similar result was announced in MIG arc brazing of aluminum to steel in Ref. [16]. 3.3. Mechanical properties 3.3.1. Nanoindentation To investigate the toughness of the joint, the nanoindentation
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Fig. 7. The microstructures of the interface layer welded at welding speed of 150 mm/min and beam current of (a) 52 mA and (b) 40 mA.
Fig. 8. Line scanning of the element distribution across the intermetallic layer of the weld.
Fig. 10. Load-depth curves of indentations carried out in Al8Fe2Si layer.
Table 4 Nanoindentation datum of the bonding layer.
Al8Fe2Si layer
HN (GPa)
E (GPa)
hf (nm)
hmax (nm)
9.5
223.2
177.2
272.0
Fig. 9. Effects of beam current on the thickness of the intermetallic compound layer.
test on the interface layer in the welded joint under the beam current of 52 mA was carried out. The typical load-displacement curve was illustrated in Fig. 10. Table 3 presented the nanohardness (HN), Young's modulus (E), maximum displacement (hmax) and the final depth (hf) of the indentation at Al8Fe2Si layers. Table 4
Fig. 11. Effects of beam current on tensile strength of joints.
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Fig. 12. (a) Fracture location of the joint and (b) Morphology of fracture surface in SEM.
shows that the Young's modulus of Al8Fe2Si IMC layer is 223.2 GPa, higher than that of CP-Al base metal as 70.3 GPa at room temperature [30]. The nanohardness of the interface layer is 9.5 GPa, which was so high that will lead brittle fracture occurred in the joint under tensile load. Tensile tests of CP-Al base metal and welded Al/304 SS joints were performed. The impact of beam current on tensile strength was displayed in Fig. 11. As well, welded joints were all fractured at the interface layer as seen in Fig. 12. As Fig. 12 demonstrated that tensile strength firstly increased with the rise of the beam current from 40 mA to 52 mA. When beam current got up to 65 mA, cracking occurred in the joint after welding, the tensile strength was decreased to zero. Combined to the IMC layer thickness discussed above, the variation of tensile strength along with beam current was not corresponding with the increase of IMC thickness. In particular, tensile strength of the joint deceased steeply for the thickening of the IMC layer with the increase of the beam current from 52 mA to 65 mA. When the beam current dropped from 52 mA to 40 mA, tensile strength of joint declined from 93.3 MPa to 71.8 MPa though the thickness of the bonding layer increased from 7 mm to 10 mm. It also had been reported that the tensile strength could be maintained well when the thickness of the bonding layer was under 10 mm [23]. The poor wetting of molten filler wire on the 304 SS plate surface in the lower beam current was also harmful to the tensile strength. The highest tensile strength of the hybrid metal connection structures was welded under the beam current of 52 mA with desired weld appearance and modest IMC layer thickness. The highest value is 93 MPa, which is 83% of that of CP-Al base metal at room temperature. To further study the effects of the IMC layer on the tensile strength, the fracture surfaces at the aluminum side were observed in SEM as shown in Fig. 12. There were cleavage surfaces, friable particles and micro cracks in the fracture surface. EDS was performed on the friable particle as marked in Fig. 12. The results illustrated that these particles were analogous with Fe-Al-Si ternary compound, which revealed that the IMC layer was the weakest part of the joint. Due to the high Young's modulus and hardness of the Al8Fe2Si layer, it was easy to conclude that the Al8Fe2Si is a brittle phase compared with Al base metal. The microcracks were the most likely to propagate in the Al8Fe2Si layer under the stress of during the tensile tests.
4. Conclusions (1) Electron beam welding-brazing of dissimilar CP-Al to 304 SS using an Al5Si filler wire was successfully carried out. The joint with well weld appearance and modest IMC layer
thickness had the highest tensile strength of 93 MPa, up to 83% of that of CP-Al base metal. (2) Cross section of the joint was characterized by fusion weld at aluminum side and brazing seam in 304 SS side. Intermetallic compounds with unusual shapes were produced. The interfacial layer was mainly recognized as Al8Fe2Si. And a small of amount of nail-like and whiskers FeAl3 and plate-like Fe2Al5 were also detected. The fusion weld in aluminum side was mainly made up of a-Al solid solution and Al-Si eutectic structures. (3) Beam current affected the tensile strength through its effect on the thickness of the intermetallic layer and weld surface appearance. The brittle cleavage fracture in the Al8Fe2Si IMC layer of the joint during the tensile test was primarily attributed to the high hardness of Al-Fe-Si IMCs measured by nanoindentation test. Acknowledgement This work was supported by the National Natural Science Foundation of China (51405098, U1637104) and Shandong Provincial Natural Science Foundation (BS2014ZZ002). References [1] B.G. Zhang, J. Zhao, X.P. Li, J.C. Feng, Electron beam welding of 304 stainless steel to QCr0.8 copper alloy with copper filler wire, T. Nonferr. Metal. Soc. 24 (12) (2014) 4059e4066. [2] X. Tian, R.K.Y. Fu, L. Wang, P.K. Chu, Oxygen-induced nickel segregation in nitrogen plasma implanted AISI 304 stainless steel, Mat. Sci. Eng. A 316 (1) (2001) 200e204. [3] X. Zhang, E. Ashida, S. Tarasawa, Y. Anma, M. Okada, Welding of thick stainless steel plates up to 50 mm with high brightness lasers, J. Laser. Appl. 23 (2) (2011) 807e819. [4] A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. Benedictus, Recent development in aluminum alloys for aerospace applications, Mat. Sci. Eng. A 280 (1) (2000) 102e107. [5] A.M.K. Esawi, M.A.E. Borady, Carbon nanotube-reinforced aluminum strips, Compos. Sci. Technol. 68 (2) (2008) 486e492. [6] J.L. Song, S.B. Lin, C.L. Yang, G.C. Ma, H. Liu, Spreading behavior and microstructure characteristics of dissimilar metals TIG weldingebrazing of aluminum alloy to stainless steel, Mat. Sci. Eng. A 509 (1e2) (2009) 31e40. [7] S. Bozzi, A.L. Helbert-Etter, T. Baudin, B. Criqui, J.G. Kerbiguet, Intermetallic compounds in Al 6016/IF-steel friction stir spot welds, Mat. Sci. Eng. A 527 (16e17) (2010) 4505e4509. [8] S. Kobayashi, T. Yakou, Control of intermetallic compound layers at interface between steel and aluminum by diffusion-treatment, Mat. Sci. Eng. A 338 (1e2) (2002) 44e53. [9] W.B. Lee, M. Schmuecker, U.A. Mercardo, G. Biallas, S.B. Jung, Interfacial reaction in steelealuminum joints made by friction stir welding, Scr. Mater. 55 (4) (2006) 355e358. [10] T. Tanaka, T. Morishige, T. Hirata, Comprehensive analysis of joint strength for dissimilar friction stir welds of mild steel to aluminum alloys, Scr. Mater 61 (7) (2009) 756e759. [11] N. Vol, Resistance spot welding of aluminum and steel: a comparative
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