Investigation on TIG arc welding brazing of Ti/Al dissimilar alloys with Al based fillers

Transcription

1 Investigation on TIG arc welding brazing of Ti/Al dissimilar alloys with Al based fillers S. X. Lv, X. J. Jing, Y. X. Huang*, Y. Q. Xu, C. Q. Zheng and S. Q. Yang Dissimilar alloys of Ti 6Al 4V and 5A06 Al were butt joined by Al based fillers using a novel TIG welding process, referred to as keyhole arc welding brazing. The flow behaviour of weld pool was introduced, which was characterised by the formation of a keyhole under the tungsten electrode. It was found that porosity tended to be produced in the middle of the fusion line, while adding elements prevented its formation. At brazing interface, interfacial reaction at root face was enhanced, and a uniform serrated layer, identified as TiAl 3, was obtained by pure Al fillers. When Al Cu La fillers were used, block Ti 2 Al 20 La phases appeared at the interface between the TiAl 3 layer and the brazed seam. Compared to joints brazed by pure Al fillers, the formation of Ti 2 Al 20 La reduced the hardness of the interfacial layer by more than half, while Al 2 Cu increased that of the brazed seam by y50%. The tensile strength of Ti/Al joints reached 270 MPa. Keywords: Ti/Al dissimilar alloys, Welding brazing, Intermetallic compounds, Heterogeneity, Porosity Introduction Joining of Ti/Al dissimilar alloys is significant and desirable for the reduction of weight and cost in aeronautic and automotive industries. 1,2 However, there exist great difficulties due to the large differences in their crystal structures, melting points, heat conductivities and coefficients of linear expansion. Mass of brittle intermetallic compounds (IMCs) is formed seriously at the interface, which decreases the mechanical properties of Ti/Al joints. Solid state welding methods, such as diffusion bonding, 3 friction welding 4,5 and explosive welding, 6 have been used to join titanium alloys to aluminium alloys, yet joint configurations are restricted by these processes and the practical application is limited for the unfavourable cost and productivity. In recent years, beam welding brazing technique has been developed to join dissimilar alloys with large differences in melting points. Unfortunately, two major problems, including controllability of interfacial reaction and spreadability of liquid filler, have not been solved due to the local heating of beam welding brazing. 7,8 Investigations on laser welding brazing indicated that a non-conforming IMCs layer was produced along the brazing interface. Cracks propagated easily at the bottom interface of the butt joints. In order to overcome this problem, the V shaped groove and the rectangular spot laser were suggested to decrease the temperature gradient along the interface. 8 To improve the spreading of liquid filler, H. Laukant et al. 9 proposed to increase the wetting length of liquid aluminium on steel surface by the dual beam laser brazing technique. State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin , China *Corresponding author, yxhuang@hit.edu.cn Nowadays, tungsten inert gas (TIG) arc welding brazing offers a great potential for hard to weld materials. 10,11 Compared to the laser, TIG arc exhibits lower spatial temperature gradients, and hence, it is expected to improve the heterogeneity of interfacial reaction at the brazing interface. Furthermore, significant improvements in practical application are possible due to its acceptable cost. However, wetting and spreading of liquid filler are difficult in dynamic arc heating process. 12 Spreadability of liquid metal has been considered as the key factor to ensure the stability of the welding process. Some related studies showed that Si added in the filler could improve the filling ability, 1 while Cu and rare earth elements are capable of decreasing the melting point and improving the spreadability of brazing process. 13,14 Moreover, in fusion process, the keyhole in plasma arc welding is also helpful in improving spreadability of liquid metal as a result of the plasma and arc forces in the keyhole. 15,16 So far, no systematic report on joining of Ti/Al dissimilar alloys with arc welding brazing has been found in the open literatures. In this work, a novel TIG welding process, referred to as the keyhole arc welding brazing (K-AWB), was developed to enhance the spreadability of liquid metal. Joining of Ti 6Al 4V alloy with 5A06 alloy was carried out by this process. Effects of Cu and La elements on microstructures and mechanical properties of Ti/Al joints were examined. Experimental Materials and joint assemble Two kinds of fillers with a diameter of 2?5 mm were adopted, including pure Al and Al Cu La fillers. Pure Al (1100 Al), electrolysis Cu (99?9%) and La (99?99%) were melted by a medium frequency induction furnace to manufacture Al Cu La fillers. Base materials were ß 2012 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 20 March 2012; accepted 15 May 2012 DOI / Y Science and Technology of Welding and Joining 2012 VOL 17 NO 7 519

2 a cross-section of joint assembled; b flow characterisation of weld pool on groove 1 Schematic diagram of K-AWB process Ti 6Al 4V alloy and 5A06 Al alloy plates with thickness of 2?0 mm. The chemical compositions of plates and fillers are listed in Table 1. All samples were sectioned into dimensions of mm; a single Y groove was machined with a bevel angle of 45u and a root face of 0?5 mm. The surface was prepared by SiC paper and acetone before the experiment. A shaped groove in the size of 563 mm was manufactured on the copper backing plate. The plates were gripped on the copper backing plate with a gap of 2 mm. The tungsten electrode moved from the seam centreline to the Ti root face considering the difference in heat conductivity of titanium and aluminium. The cross-section of joints assembled is shown in Fig. 1a. Pure argon was used as the shielding gas to avoid oxidation and improve the spreadability of liquid fillers on double surfaces of joints. Keyhole arc welding brazing process The K-AWB process was carried out by an ac TIG welding source (Panasonic, YC-500WX4). According to the flow characterisation of weld pool on the joint s groove, the forming process of brazed joints was divided into four stages, as described in Fig. 1b. First, heated by the TIG arc, the filler was melted and transferred to the groove of butt joints, and an elliptic weld pool was formed. Second, when the tungsten electrode moved along the welding direction, the groove was heated. Thus, when the brazing speed was low, some liquid metal began to wet and spread on the Ti groove, while the remaining parts mixed with molten Al alloy. As a result, a perturbation was presented in the weld pool, as shown in Fig. 1b(2). Then, the spreading process was impeded by low temperature in front of the tungsten electrode. Liquid metals converged again after an interval. Consequently, a keyhole was formed in the liquid metals and surrounded under the tungsten electrode, as indicated in Fig. 1b(3). Finally, as the keyhole was expanded by the arc forces, the filler was fed with 30u by a special wire feeder to achieve pulsing feeding with a frequency of 0?521 Hz. Figure 1b(4) shows a new elliptic weld pool similar to that in the first stage produced by mixing molten drops with the weld pool. At the stage of keyhole formation, liquid metal spread on the back face of Ti alloy to pack it and uphill spread on the front face to react and join with Ti alloy. Meanwhile, at the Al side, it mixed with molten Al and formed a fusion joint with the help of a shaped groove. As the tungsten electrode moved through an automatic walking mechanism, the temperature of joints began to decrease and the solidification of the liquid weld pool formed. As a result, a sound welding brazing butt joint was produced. The welding parameters were welding current of 110 A, arc length of 2?5 3?5 mm and brazing speed of 75 mm min 21. Microstructure observation and mechanical test The samples were cut transversely through the butt joints and mounted with bakelite, then polished to a mirror-like surface and etched by Keller reagent. The microstructure was observed by optical microscopy (OM) and scanning electron microscopy (SEM). The chemical composition was detected by an energy dispersive spectrometer (EDS). The X-ray diffraction (XRD) analysis was performed after removing the Al layer by burnishing on SiC papers and etching with NaOH solution. A comparison of hardness among these joints was made using a Vickers hardness tester with a load of 50 g and a hold time of 10 s. The tensile strength was measured by a testing machine at a test speed of 1 mm min 21. To evaluate accurately and avoid the influence of excess weld metal, these joints were ground to flat. Results and discussion Macrostructure Figure 2 showed the typical cross-section of Ti/Al butt joints produced by Al Cu La fillers, which exhibited dual characteristics of fusion welding and brazing. The Table 1 Chemical compositions of Ti 6Al 4V, 5A06 and fillers (wt-%) Elements Al Ti Mg Cu V Mn La Ti 6Al 4V 5?526?8 Balance 3?524?5 5A06 Balance 0?02 5?826?8 0?1 0?520?8 Pure Al Balance,0?1 Al Cu La Balance 0?15 0?1 0?3 0?15 2 Science and Technology of Welding and Joining 2012 VOL 17 NO 7 520

3 2 Macroscopic cross-sectional view of Ti/Al butt joint with Al Cu La fillers K-AWB process basically melted the filler fully and the aluminium plate partially but not the titanium plate. Fusion welded joints were formed by the molten fillers and Al alloy with low melting point. No apparent melting was observed on the Ti side, and the brazed joint was produced by the molten filler and solid Ti with high melting point. It was clear that the joint had a good front and back formation, and no crack, undercut or incomplete fusion was presented. During the K-AWB process, the low brazing velocity provided enough time for the spreading of liquid metal on the front face of Ti alloy. Meanwhile, spreading on the back face was improved dramatically by the arc forces in the keyhole. In addition, high temperature in the keyhole lessened the interfacial energy of solid Ti and thereby enhanced the spreading process on Ti groove. Consequently, single face welding double face forming of Ti/Al butt joints were easily achieved by the K-AWB process. Effect of adding elements on joint microstructure The microstructure of the fusion zone made by pure Al fillers is shown in Fig. 3. There were four different zones according to the difference in microstructure: parent metal, heat affected zone, fusion line, brazed seam. Part of Al alloy was molten and mixed with liquid filler, so a fine fusion welded joint was produced. The 5A06 Al alloy contained strengthening phases including b- Al 3 Mg 2 and b9-mg 23 Al In Fig. 3a, no significant coarsening of the strengthening phases was observed by optical examinations. While a pore with a diameter of y100 mm was observed in the middle of the fusion line, further analysis indicated that strengthening phases distributed dispersedly at the upper of fusion line, as shown in Fig. 3b. However, strengthening phases disappeared, and only a-al phase was observed at the bottom. In the K-AWB process, the tungsten electrode was set over the Ti root face, and half of the Al groove was melted and mixed with liquid fillers. It was analysed that strengthening phases at the bottom were melted by high temperature in the keyhole, and Mg atoms in the strengthening phases were prone to vaporise and run away upwards. On the other hand, liberation of impurities such as hydrogen and oxygen from liquid Al resulted in gases in the seam. At the same time, viscosity of weld pool was increased by acicular IMC phases, as presented in Fig. 3d. Gases in the seam were hard to escape and concentrated in the middle of fusion line during the solidification process. Thus, the porosity was produced in the fusion line. To improve the microstructure of the fusion zone, Al Cu La fillers were manufactured to join Ti/Al dissimilar alloys by K-AWB process. Figure 4 showed OM images of fusion zone, magnified in Fig. 2, in the Ti/Al butt joint with Al Cu La fillers. No porosity was observed in Fig. 4a. Cu and La elements lower the melting point of the filler, so the heat input was not enough to melt the strengthening phases. In addition, La was a strong deoxidising agent, so it purified the seam. On the other hand, it decreased surface tension of the liquid metal, and it was beneficial for the liberation of gases. 18 Therefore, it could be concluded that porosity was avoided by the Al Cu La filler. The microstructures in the fusion line were coarse columnar structure vertical to a fusion zone; b fusion line marked with E; c fusion line marked with F; d brazed seam 3 Images (OM) of fusion zone of butt joints with pure Al fillers Science and Technology of Welding and Joining 2012 VOL 17 NO 7 521

4 a fusion zone; b magnified of central fusion line; c brazed seam near fusion line; d central brazed seam 4 Images (OM) of fusion zone in Ti/Al butt joints with Al Cu La fillers the fusion line, as shown in Fig. 4b, which was induced by the high cooling rate and preferential direction of thermal conduction. The solidification of weld pool proceeded from the fusion line to the centre of the weld pool, and the degree of supercooling decreased gradually. As a result, granular IMC phase was produced near the fusion line, while it exhibited acicular shape in the central seam, as showed in Fig. 4c and d respectively. Compared with microstructures of brazed seam shown in Fig. 3d, more IMC phases and uniform distribution were observed, which attributed to the uniformity of elements distribution and heterogeneous nucleation ability of La element. 19 Intermetallic reaction layer Ti groove with a high melting point was no or few molten during the K-AWB process, which reacted with liquid fillers to form Ti/Al brazing interface. As a local heating source, TIG arc exhibited a high temperature gradient of through thickness. Thermal cycle at interface differed from the top to the bottom, which induced the heterogeneity of interfacial reaction. Figure 5 displays microstructures of Ti/Al interface brazed with pure Al fillers, corresponding to A, B, C and D zones plotted in Fig. 2. At zones A, B and C of the Ti groove, all layers exhibited serrated morphology with a thickness of 3 6 mm. The interface at the Ti root face (zone D) showed a upper interface (zone A); b intermediate interface (zone B); c interface at bottom (zone C); d interface at root face (zone D) 5 Microstructures of Ti/Al brazing interface by K-AWB process with pure Al fillers Science and Technology of Welding and Joining 2012 VOL 17 NO 7 522

5 7 Hardness profiles across Ti/Al brazing interface a backscattered SEM image; b XRD analysis result 6 Microstructure of Ti/Al brazing interface produced by Al Cu La filler a thicker reaction layer and a lamella morphology. Furthermore, some ruptured club shape IMCs perpendicular to the interface were observed, which attributed to the electromagnetic stirring action of TIG arc. Obviously, interfacial microstructures prepared by K- AWB process had relatively more uniform distribution than that prepared by laser welding brazing. 8 During the K-AWB process, Ti groove was surrounded by the liquid metal, as indicated in Fig. 1a. The heating model at the Ti groove was mainly heat conduction from the liquid filler rather than direct heating as conventional TIG welding, which resulted in the decrease in the temperature gradient along the Ti groove. Thus, a uniform interfacial reaction layer formed. The interfacial reaction at the bottom of the Ti groove was boosted by the high temperature in the keyhole. Lamella shape layer was produced due to a relative low cooling velocity led by the radiant heat from the copper backing plate. Therefore, the Ti/Al joint produced by K-AWB process had a uniform interfacial structure. The results of the EDS analysis in compositions of Ti/ Al interface are summarised in Table 2. Point 1 referred to the interface made by pure Al filler, and the remaining point were from the joint formed by the Al Cu La filler. In order to obtain the accurate content ranges of the main elements, five measurements in each point were conducted. Point 1 was found to be approximately 22?4 26?8 at-% Ti atoms, which corresponded to that of TiAl 3 phase in the Ti Al binary diagram. 20 Thus, the layer produced by pure Al filler was TiAl 3 phase, which was in accordance with the fact that TiAl 3 had the minimum free energy of formation at the temperature range of K. 21 In contrast, after adding Cu and La in the filler, the microstructure of the brazing interface changed drastically. Figure 6 shows the backscattered SEM images of zone B at the brazing interface formed with Al Cu La filler. Two typical layers, marked by points 2 and 3, were observed in Fig. 6a. According to the elemental composition analysis, it was confirmed that the grey interfacial layer near the Ti side was TiAl 3 phase, while the white layer near the seam side was Ti 2 Al 20 La phase. Figure 6b presented the pattern of XRD results of interfacial layers. Two IMC phases were detected from the results. Accordingly, it was confirmed that the phases at the interface were composed of TiAl 3 and Ti 2 Al 20 La. Moreover, some eutectic phases, identified as Al 2 Cu, were also detected in the brazed seam. Mechanical properties Figure 7 displays the hardness profiles across the brazing interface as a function of distance. As an overall trend, the hardness near the Ti/Al interface increased as a result of adding Cu and La elements in the filler. The average hardness of brazed seam increased by y50%, from 70?23 HV for pure Al fillers to 104?25 HV for Al Cu La fillers. It ascribed to the presence of plentiful Al 2 Cu in the grain boundaries of a-al. At a distance of 10 mm at the seam side, the hardness was measured to be 68?24 and 297?13 HV for interfaces made by pure Al and Al Cu La fillers respectively, which was resulted from the presence of Ti 2 Al 20 La along the interface Table 2 Energy dispersive spectrometry analysis results of Ti/Al brazing interface (at-%) Point Compound Al Ti La V Cu 1 TiAl 3 73?24 77?58 21?42 25?76 0?71 0?92 2 TiAl 3 75?61 76?54 22?46 24?29 0?87 1?02 3 Ti 2 Al 20 La 86?56 86?86 7?05 7?82 4?46 4?52 1?17 1?57 4 Al 2 Cu 68?09 69?62 28?94 30?61 Science and Technology of Welding and Joining 2012 VOL 17 NO 7 523

6 produced by Al Cu La filler. Ti 2 Al 20 La phases at the interface increased the hardness of brazed seam, while that of interfacial layer was decreased by more than half when compared with HV of TiAl 3 layer. On the other hand, the hardness at a distance of 220 mm at the Ti side was measured to be increased slightly for joints produced by the Al Cu La filler. This region represented the diffusion zone of Al atoms in the Ti alloy. Thus, La elements reinforced the diffusion process of Al atoms, and solid solution strengthening occurred in the Ti side. The tensile test was carried out to measure the strength of Ti/Al joints. The butt joint with Al Cu La filler had a tensile strength of 270 MPa compared with 139 MPa with pure Al filler. The fracture propagated at the Ti/Al brazing interface, and the fracture surface with pure Al fillers presented a ductile characteristic. The lower strength was led by the poor bonding between interfacial layer and brazed seam. Chen et al. 8 reported that the tensile strength was 278 MPa for butt joints by the laser welding brazing. The joint produced by K- AWB process and Al Cu La filler reached the same level with those by laser welding brazing, up to,85% of the tensile strength of 5A06 Al. Conclusions In summary, Ti 6Al 4V and 5A06 Al alloy were butt joined successfully by the keyhole arc welding brazing (K-AWB) process. Porosity tended to exist in the middle of the fusion line of joints brazed by pure Al fillers, which was avoided by Al Cu La fillers for the effect of adding elements. The heterogeneity of interfacial reaction at brazing interface was improved by the K-AWB process, and a uniform interfacial structure was obtained as a serrated TiAl 3 layer with pure Al fillers. TiAl 3 layer was observed at brazing interfaces formed by two kinds of fillers respectively, while a block layer, identified as Ti 2 Al 20 La phase, was found at the interface between TiAl 3 layer and brazed seam produced by the Al Cu La filler. The hardness of brazed seam increased by y50% with the presence of Al 2 Cu in the grain boundaries, while that of interfacial layer decreased due to the formation of Ti 2 Al 20 La. The tensile strength of Ti/ Al butt joints reached 270 MPa. Acknowledgements The work was jointly supported by the National Natural Science Foundation of China (grant nos and ), the Science and Technology Innovation Research Project of Harbin for Young Scholar (grant no. 2009RFQXG050) and the Fundamental Research Funds for the Central Universities (grant no. HIT. NSRIF ). References 1. T. Takemoto and I. Okamoto: J. Mater. Sci., 1988, 23, F. Möller, M. Grden, C. Thomy and F. Vollertsen: Phys. Procedia, 2011, 12, W. H. Sohn, H. H. Bong and S. H. Hong: Mater. Sci. Eng. A, 2003, A355, M. Kimura, S. Nakamura, M. Kusaka, K. Seo and A. Fuji: Sci. Technol. Weld. Join., 2006, 10, A. Fuji: Sci. Technol. Weld. Join., 2002, 7, N. Kahraman, B. Gulenc and F. Findik: Int. J. Impact Eng., 2007, 34, K. Saida, W. Song and K. Nishimoto: Sci. Technol. Weld. Join., 2005, 10, S. H. Chen, L. Q. Li, Y. B. Chen, J. M. Dai and J. H Huang: Mater. Des., 2011, 32, H. Laukant, C. Wallmann, M. Korte and U. Glatzel: Adv. Mater. Res., 2005, 6 8, J. L. Song, S. B. Lin, C. L. Yang, C. L. Fan and G. C. Ma: Sci. Technol. Weld. Join., 2010, 15, J. C. Yan, Y. N. Li, X. S. Liu, Y. Zhang, H. C. Yu and S. Q. Yang: Mater. Sci. Technol., 2007, 23, J. L. Song, S. B. Lin, C. L. Yang, G. C. Ma and H. Liu: Mater. Sci. Eng. A, 2009, A509, A. N. Alhazaa and T. I. Khan: J. Alloys Compd, 2010, 494, S. Y. Chang, L. C. Tsao, Y. H. Lei, S. M. Mao and C. H. Huang: J. Mater. Process. Technol., 2012, 212, M. Tomsic and S. Barhorst: Weld. J., 1984, 63, H. G. Fan and R. Kovacevic: J. Phys. D: Appl. Phys., 1999, 32D, S. H. Chen, L. Q. Li, Y. B. Chen and J. H. Huang: J. Alloys Compd, 2011, 509, Z. R. Li, J. Cao, B. Liu and J. C. Feng: Sci. Technol. Weld. Join., 2010, 15, Z. Y. Li, B. Wang and H. Q. Zhang: Sci. Technol. Weld. Join., 2008, 13, I. Ohnuma, Y. Fujita, H. Mitsui, K. Ishikawa, R. Kainuma and K. Ishida: Acta Mater., 2000, 48, M. Sujata, S. Bhargava and S. Sangal: J. Mater. Sci. Lett., 1997, 16, Science and Technology of Welding and Joining 2012 VOL 17 NO 7 524