The effect of Zn interlayer on microstructure and mechanical performance during TIG overlap welding-brazing of Al to Ti

Tungsten inert gas welding-brazing technology was investigated to achieve the sound joining of Al-Ti alloys with the addition of Zn interlayer. Experiment results show that the Zn interlayer was beneficial to enhance the wettability of weld metal on Ti alloy substrate and improve the weld appearance. With low heat input, a Zn rich layer was observed near the brazing zone. As the heat input increased, TiAl3 phase was generated in fusion area and the morphology of interfacial brazing layer changed from lamellar to serrate. With the welding current increased to 85A, the micro hardness of fusion area increased and presented a great fluctuation owing to the excessive formation of TiAl3 phase. Under the optimal welding current of 75A, the sound Al-Ti weldment with the maximum tensile strength of 175MPa was obtained. Eventually, the improvement mechanism of wettability was investigated.


Introduction
The expansion of multiple materials application in vehicle industry is an efficient way to improve fuel economy and reduce greenhouse gas emissions [1][2][3]. In recent years, Al alloys are extensively used as light metallic materials with the excellent comprehensive performances, such as low density, high strength ratio, superior corrosion resistance and strong deformation ability [4][5][6]. Besides, Ti alloys with its excellent properties have attracted more attention in the pursuit of lightweight equipment [7][8][9]. It is apparent that, the dissimilar metals joining of Al-Ti can achieve the purpose of lightweight vehicle and expand the application field of the two metals. However, it is difficult to achieve sound Al-Ti joint owing to the considerable differences in chemical and physical properties, which resulted in residual stress and crack propagation [10][11][12].
A series of welding methods have been studied for the joining of Al-Ti dissimilar metals, such as laser welding (LW) [13,14], diffusion welding (DW) [15,16], friction stir welding (FSW) [17,18], brazing [19,20] and resistance spot welding (RSW) [21,22], etc Note that welding-brazing technology, as a low-cost and highefficiency joining method, was widely studied for the heterogeneous metals welding [23,24]. The most typical characteristic of welding-brazing technology is that the joint shows dual welding features of fusion welding and brazing, which was experimentally proved to be beneficial to improve joint quality. For example, zhao et al [25] investigated the evolution of joint appearances, interfacial microstructure and mechanical performance of Mgsteel joint welded by welding-brazing technology. Experimental results show that AlNi+(α-Mg+Mg 2 Ni) eutectic formed at the seam area and bulky Mg 2 Ni intermetallic compound occurred in the matrix of Mg-Ni eutectic. With optimized welding parameters, the sound Mg-steel weldment with the maximum tensile-shear load of 230 N mm −1 was obtained, reaching an 88.5% joint efficiency relative to Mg alloy parent material. Zhang et al [26] studied the effect of heat input on microstructure and joining strength of Ti-Al butt joints obtained by double-sided arc welding-brazing process. According to Zhang, acceptable TiAl6V4/5A06 weldment with fine front and back surface appearances were obtained by welding-brazing technology. And the morphology, composition and thickness of intermetallic compounds evolved with the increase of welding heat input. Under welding current of 80-90A, welding speed of 15 mm s −1 and TIG position of 0mm, the Ti-Al joint had optimal joining strength of 240.3 MPa.
The literatures reveal that welding-brazing technology is beneficial to achieve reliable joining between heterogeneous metals with considerable physical and chemical properties. However, Wang et al [27] reported that during the welding-brazing of Al-Ti, liquid Al alloy molten metal was easy to oxidize and showed inferior wetting and spreading properties on the surface of Ti alloy base metal, which resulted in poor weld appearance and low welding strength. Eventually, in the present study, a Zn interlayer was used to assure fine spreading ability and weld performance. According to Dharmendra et al [28], Zn interlayer was successfully applied to enhance the wetting and spreading ability of molten pool during the welding-brazing of Al-steel. It is evident that the addition of alloying elements is a momentous topic for further research.
In the present paper, dissimilar metals of Al-Ti were joined in an overlap configuration by TIG weldingbrazing process. And the Zn foil was used as interlayer metal. The mechanical performances and microstructural evolution of Al-Ti joints were investigated by means of mechanical testing and microscopy techniques.

Materials and methods
In the present study, 5052 Al alloy and Ti6Al4V alloy were used as base materials during the TIG welding-brazing experiments. The tensile strength of 5052 Al alloy and Ti6Al4V alloy are respectively 225 MPa and 850 MPa. The dimensions of above two parent plates were machined to 70mmx60mmx1mm and the oxide layer was polished (800# sandpaper) prior joining experiment. The j0.8 Al-5Si alloy wire was applied as filler metal. Table 1 presents the chemical compositions of all base materials. Figure 1 depicts that the Al alloy base metal was fixed on the top of Ti alloy in the lap joint configuration. An YC-300 TIG machine was applied to complete the weldingbrazing experiment. The 30-μm-thick Zn foil was selected as interlayer and placed between Al-Ti plates. During the welding experiment, tungsten electrode center of welding gun irradiate the edge of aluminum alloy plate. Under the irradiation of electric arc, the Al base metal, filler wire and Zn foil were melted and wetted on the top surface of Ti alloy plate to form the welding joint. Argon was used to prevent oxidation of molten metal. The welding parameters used in the present welding-brazing experiment are presented in table 2.
After welding experiments, the Al-Ti joints were cut perpendicular to the welding progress for microstructure observation and joining strength testing. The weld cross section of joint was ground and polished with different grades of sandpapers, and subsequently observed by an optical microscope (OM) and scanning electron microscope (SEM) to reveal weld forming characteristics. A dwell time of 10 s and a load of 250 g were adopted during Vickers hardness testing. All indentations were sufficiently spaced to avert the formation of strain field induced by adjacent indentations. The tensile samples were polished with sandpapers and subsequently tested with tensile testing machine at room temperature with a constant speed of 1.5 mm min −1 . For each welding parameters, 5 repeated experiments were carried out to obtain the final Table 1. Chemical compositions of parent metal, interlayer and welding wire (wt%).

Elements
Al mechanical properties data. Figure 2 shows the schematic drawing of tensile specimen. SEM equipped with energy dispersive x-ray spectrometer (EDS) was used to investigate the microscopic features. Figure 3 presents the typical macro morphologies of cross sections of TIG welded-brazed Al-Ti joints obtained with and without Zn interlayer. As shown in figure 3(a), the Al-Ti joint without the aid of Zn interlayer showed poor welding features. It is apparent that the molten Al alloy base metal and welding wire did not satisfactorily wet the solid Ti alloy, which resulted in excessive accumulation of molten metal. Nevertheless, in the case of Al-Ti joint with addition of Zn interlayer, the weld seam has uniform morphology and good wettability, as indicated in figure 3(b).

Weld appearance
Previous literatures indicate that alloying elements have a great influence on the wettability and spreadability of liquid metals. For example, Li et al [29] reported that the presence of Zn coating enhanced the wetting of liquid welding wire on the steel substrate. In this paper, a 30-μm-thick Zn foil was used as interlayer to hopefully improve weld forming and joining quality. In order to determine the effect of interlayer addition on weld forming, weld wetting angles and weld widths of Al-Ti joints were measured. Figure 4 shows the schematic diagram of measurement method and the results were illustrated in figure 5. Figure 5 indicates that the wetting angle (θ) of Al-Ti joint with addition of Zn interlayer was less than that of joint without Zn foil, which resulted in the increase of weld width. Besides, it can be found that the weld width of Al-Ti joint increased along with the   welding current enhancement. During the welding process, the current determined the effective heat input to base metal. With low heat input, the molten base metal and wire could not adequately wet the Ti alloy and the measured wetting angle was more than 90 degree. With the increase of heat input, both the fluidity of molten metals and the surface activity of Ti alloy increased, which resulted in the decrease of joint wetting angle. Figure 6 shows the typical macroscopic morphology of the weld cross section. Note that the TIG weldingbrazing Al-Ti joint was characterized with dual features of brazing and fusion welding. The Al alloy base plate melted and mixed with molten welding wire, and eventually cooled to room temperature to form a fusion zone, indicated by region A. On the other hand, solid Ti alloy metal reacted with the molten Al alloy base metal and welding wire to form the interfacial reaction zone, i.e., brazing zone (indicated by B). Figure 7 shows the typical backscattered scanning electron microscope (BSE) images of fusion zone produced under Zn interlayer and various welding current. It can be found that the welding current has obvious influence on the microstructure evolution of weld seam. With low heat input (55A), the filler wire and low-melting-point Al alloy base metal received most of the arc heat. And there was not enough heat to complete the melting and sufficient diffusion of Zn, which resulted in the Zn concentration in the grain boundaries near Al-Ti interface, as shown in figure 7(a). With the increasing of heat input (65A), the diffusion of Zn element was strengthened under the stirring action of electric arc. But the Zn diffusion process was still not fully carried out, as presented in figure 7(b). As the current increased to 75A, the microstructure of fusion zone was characterized by sufficient   diffusion of Zn, as indicated in figure 7(c). However, a mass of clubbed intermetallic compounds were formed in the weld seam. EDS results (table 3) revealed that the intermetallic compounds were TiAl 3 phase, which was consistent with previous literatures. The formation mechanism of TiAl 3 phase was mainly owing to slightly dissolution of Ti from the top surface of Ti alloy base metal to molten metal under the action of enhanced heat input. Under the current of 85A, the number of clubbed intermetallic remarkably increased, attributed to the increased dissolution of Ti, as shown in figure 7(d). Figure 8 presents the typical EDS area testing results of weld seam obtained with Zn interlayer and various welding parameters. It can be found that Zn atoms aggregated in grain boundary regions as the heat input was insufficient. Conversely, the diffusion of Zn atoms was accelerated by adequate heat input, and finally the Zn atoms were evenly distributed in the weld seam, as presented in figure 8(b). The EDS testing results are consistent with that of microstructural observations. Figure 9 shows the BSE images of interfacial reaction zone of Al-Ti joints produced with Zn interlayer and different current. It is apparent that the welding current plays a momentous role in the microstructure features of interfacial reaction layer. Figure 9(a) illustrates that a continuous and thin reaction layer with thickness of  2.2 μm was formed in the brazing zone. The reaction layer has a sheet structure and without any bulge. Figures 9(b) and (c) indicate that the thickness of reaction layer increases with the increase of heat input and the morphology of reaction layer changed from lamellar to serrate. However, under current of 85A, excessive heat input leads to a significant increase in the thickness of the reaction layer, as presented in figure 9(d). Besides, it can be found that some ruptured clubbed intermetallic compounds perpendicular to the Al/Ti interface were  observed, attributed to the electromagnetic stirring effect of electric arc on molten pool. It can be inferred that the excess intermetallic compounds could degrade joint performance. EDS testing was used to detect the compositions of interfacial reaction layer and the results were listed in table 4. Based on EDS results, only Ti element and Al element were detected in the reaction layer. The ration of aluminum to titanium is about 3 to 1, indicating the formation of TiAl 3 intermetallic compound. The EDS results are consistent with the reaction between liquid Al alloy and solid Ti alloy reported by Chen et al [12].

Micro hardness
The typical micro hardness distribution of Al-Ti joints with different current were measured and illustrated in figure 10. As presented in figure 10(a), it can be found that the micro hardness of Ti alloy base metal was about 120 HV. But the micro hardness of interfacial reaction layer increased to 153 HV, owing to the formation of TiAl 3 intermetallic compound. And the hardness value of fusion zone approximately remained at 95 HV. Figure 10(b) presents the micro hardness distribution of Al-Ti joint under current of 85A. Obviously, the micro hardness values of brazing zone and Ti alloy base metal were similar to that of figure 10(a). However, the marked difference is that the micro hardness of the fusion welding area presented a great fluctuation. Microstructure observations indicated that a large amount of intermetallic compounds were generated in the fusion welding area of joint under current of 85A, resulted in the evolution of micro hardness values. Figure 11 shows the tensile strength of Al-Ti joints welded under different current. It is clear that the heat input markedly affected the mechanical performance of Al-Ti weldments. Under the current of 55A, the Al-Ti joint showed poor tensile strength (96 MPa) owing to deficient metallurgical bonding. However, the tensile strength presented an enhanced tendency as the welding current improved from 55A to 75A, attributed to the strengthening metallurgical bonding. Under optimized welding current of 75A, the maximum tensile strength of 175 MPa was obtained, representing 78% joint efficiency relative to Al alloy parent material. Under the further increase of welding current (85A), the tensile strength reduced to 132 MPa. The above microstructure analysis indicated that the decrease of mechanical performance was mainly attributed to the excessive formation of brittle TiAl 3 intermetallic compound.

Discussion
With the addition of Zn foil and optimized welding parameters, the Al-Ti joints with sound surface morphology and excellent mechanical performance were obtained. It is obvious that the addition of Zn interlayer effectively improved the wetting and spreading of weld liquid metal on solid Ti alloy plate. Previous literatures reveal that the wetting between solid and liquid metals follows the Young equation: where θ is the wetting angle, σ sg is the interfacial tension of solid-gas, σ sl is the interfacial tension of solid-liquid, and σ lg is the interfacial tension of liquid-gas (as shown in figure 4). With the assistance of Zn interlayer, the lowmelting-point Zn foil melt and even partially evaporated under the heat input of argon arc. Gaseous Zn dissolved into the molten metal and resulted in the formation of relative vacuum at the solid-liquid interface area of the weld. Eventually, σ lg decreased with the increase of external pressure, considering the inertia of system pressure balance. The above formula 1 shows that wetting angle θ decreased owing to the decrease of σ lg , indicating the improvement of wetting performance. On the other hand, Al-Zn binary phase diagram reveals that the dissolution of Zn interlayer in liquid Al alloy decreased the solidus temperature of molten metal from 923K to about 613K. The solidus temperature of Al alloy (923K) induced considerable temperature difference between the solidification and working temperature of Al-Zn alloy. As reported by Wang et al [27], the above temperature variation is equivalent to improve the wetting temperature and strengthen the spread ability of molten metal.

Conclusions
Through investigating the effect of Zn interlayer on TIG welding of Al alloy and Ti alloy by characterization of wetting angle, microstructure features, microhardness distribution and mechanical performance, the following conclusions can be obtained: i. TIG welding-brazing technology was investigated to achieve the reliable joining of Al-Ti dissimilar alloys. And the addition of Zn interlayer was proved beneficial to improve the wettability of weld metal on Ti alloy substrate.
ii. With low heat input, a Zn rich layer was observed near the brazing zone. As the heat input increased, the morphology of interfacial reaction layer changed from lamellar to serrate.
iii. As the welding current increased from 75A to 85A, the micro hardness of fusion area increased and presented a great fluctuation owing to the formation of TiAl 3 phase. Under the current of 75A, the sound Al-Ti weldment with the maximum tensile strength of 175 MPa was obtained.