A novel method for resistance spot welding between steel and aluminum alloy

Transcription

1 A novel method for resistance spot welding between steel and aluminum alloy Ranfeng Qiu 1, 2, Jiuyong Li 1, Lihu Cui 1, Hongxin Shi 1, 2, Yangyang Zhao 1 (1. School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang ,China; 2. Collaborative Innovation Center of Nonferrous Metals, He'nan Province, Luoyang , China) Abstract: A new jointing method, termed resistance spot welding with embedded composite electrodes, was tried to weld steel to aluminum alloy. A reaction layer was observed at the welding interface and its thickness at the interface presented a bimodal distribution. A tensile shear load of a maximum of 6.28 kn was obtained at a welding current of 28 ka. The results reveal that resistance spot welding with embedded composite electrodes is an effective method for restraining the interfacial reaction products growth in the welding central region. Key words: resistance spot welding; aluminum alloy; steel; embedded composite electrodes DOI: / j.issn Prof. Ranfeng Qiu qiurf1221@163.com Ranfeng Qiu was born in He received his PhD in Mechanical Engineering from Kumamoto University in He is currently a Vice-Professor at the School of Materials Science and Engineering, He'nan University of Science and Technology. His research interests are resistance spot welding of dissimilar materials. He has published over 50 refereed papers in international journals and conferences. 0 Introduction To reduce pollution and save energy, it is attractive to make car bodies lighter by introducing aluminum alloy parts as substitutes for the previous steel structures. Therefore, the joining between steel and aluminum alloy is unavoidable. However, joining between the two kinds of materials by fusion welding methods faces to technological and metallurgical limitations, because of the large difference in physical and thermal properties between steel and aluminum alloy. Accordingly, various pressure welding methods have been used to achieve optimal results when welding aluminum alloys and steel, such as diffusion welding [1-2], friction welding [3-4], explosive welding [5], friction stir welding [6-8], ultrasonic welding [9-10], and resistance spot welding [11-16]. As a result, it is well known that brittle reaction products, which formed at the welding interface, would deteriorate the tensile strength of steel/ aluminum alloy joint. In the case of resistance spot welding of steel/aluminum alloy, the authors have found that the thickness of the reaction layer is thin at the peripheral region and it increases as approaching to the center of the weld, and that the interfacial reaction layer whose thickness exceeded 1.5 μm can deteriorate the mechanical properties of the joints [17-18]. Therefore, a sound resistance spot welded steel/aluminum alloy joint would be fabricated by a welding process which is helpful to suppress the growth of reaction products in the central region of the weld. In view of this, a new method termed resistance spot welding with embedded composite electrodes for joining steel/aluminum alloy is put forward in the present study. The purpose is to better understand the weldability of steel and aluminum alloy, and to provide some foundation for improving resistance spot welded steel/aluminum alloy joint properties. 1 Resistance spot welding with embedded composite electrodes Resistance spot welding is a joining process based on the heat source obtained from Joule s effect of the resistance and electric current flow through the sheets held together by the electrode force, in which the coalescence occurs at the spot area in the faying surfaces. Electrodes are the important carrier for resistance spot welding. Fig.1(a) and ( b) shows the embedded composite electrode developed in this study. Its production process is as follows: A tungsten rod was embedded in drilled cylindrical embryo piece of CuCrZr alloy, and preheated to 680 using the high temperature box type resistance furnace. The preheated copper block with tungsten rod was placed in the mold cavity, upsetting extrusion was performed. Extrusion force was 50 kn; extrusion ratio was 25:1. After extrusion, the embryo piece was machined until the shape and dimension to meet the design requirements. 146

2 Fig.1(c) shows the schematic diagram of resistance spot welding with embedded composite electrodes. Because the resistance of tungsten at the central region of embedded composite electrode is higher than copper alloy at the outer edge of embedded composite electrode, most of the curent would flow through the outer edge of embedded composite electrode based on Joule s law during spot welding. Namely, utilizing the embedded composite electrodes during resistance spot welidng can make welding zone current gather in ring shape (henceforth calls ring-gathered current ) as shown in Fig.1(c). In this way, restraining the interfacial reaction products growth in the welding central region would be realized. After welding, the tensile shear tests were performed on AG-1250kN tensile testing machine under a cross-head velocity of m/s at room temperature. The weld joints were cut perpendicular to the faying surface through the weld center and cross-section observation experiment was conducted after grinding and polishing. The interfacial microstructure of the joint was investigated using a scanning electron microscope (SEM). And the chemical compositions of the reaction products were determined using an energy dispersive X-ray spectroscopy (EDS). Besides, the tensile shear load and nugget diameter were evaluated by the average value of five specimens per condition. 3 Results and discussion Fig.2 shows the optical micrograph of the cross-section of joint welded under the condition of welding current of 22 ka. The following characterizations can be seen. First, a white acetabuliform nugget was observed at the side of aluminum alloy. Although few welding current flowed through the center of welding zone where correspond with the tungsten rod contained in the electrode during spot welding, there still happened remelting at the aluminum alloy side. This is because the thermal conductivity of aluminum alloy is larger and its melting point is lower. The heat is easy diffused to the center of welding zone from the peripheral region by heat conduction, which caused aluminum alloy also melted at there. Since the generated heat was larger due to higher resistance at the steel side, the nugget diameter of aluminum side adjacent the welding interface is larger. However, its diameter is getting smaller away from the interface. Therfore, the acetabuliform nugget Fig.1 Side view (a) and top view (b) of embedded composite electrode; schematic diagram of resistance spot welding with embedded composite electrodes (c) present at the aluminum alloy side of joint. 2 Experimental materials and procedures The materials used in this study were 1.0 mm thick mild steel Q235 sheet and aluminum alloy A6061 sheet with thickness of 2.0 mm. The nominal compositions are listed in Table 1. The sheets were cut in the size of 100 mm 30 mm. After washing by anhydrous ethanol and drying, lap joint configuration was prepared. They were welded using DM-200 moderate frequency inverter resistance spot welding machine. The welding current was changed every 2 ka between 10 and 32 ka at the fixed electrode force of 2kN and welding time of 0.2 s in the welding process. The tip diameter of embedded composite electrode used in this experiment was 6mm, and the diameter of inlaid tungsten rod was 4mm. Table 1 Chemical composition of materials (wt.%) Matterial w (Cr ) w (Cu) w (Ti) w (Mg) w (V) w (C) A Steel Matterial w (P) w (S) w (Si) w (Mn) w (Fe) w (Al) A Bal. Steel Bal. Fig.2 Appearance of cross-section joint Second, two kinds of zone with various contrast was observed at the steel side. One is an ashen zone at the center of welding zone, where no remelting indication is observed. Its size is coincident with the diameter of tungsten rod contained in the electrode. The other is a zone with deep contrast on both sides of the ashen zone, which is a fusion zone. In geometry aspect, the deep zone is a ring shape. Henceforth it is called ring-nugget. At the steel side, the formation of ring-nugget is considered to be due to ring-gathered current during resistance spot welding. Compared with aluminum alloy, the thermal conductivity of steel is lower and its melting point is higher. At the center of welding zone, the heat got by conduction is not so enough as to melt steel. This is main reason for the formation of ring-nugget. Third, the diameter of nugget at welding interface was larger than the electrode tip diameter (6mm) as shown in Fig.2. Under the electrode pressure, the base metal sheets undergoed deformation during welding, which caused the increase of contact area between 147

3 base metal sheet and electrode, and then resulted in the formation of larger nugget. Fourth, several voids were found at the center of the acetabuliform nugget. This is considered to be due to the shrinkage result from the solidification of molten metal. During the heating process in resistance spot welding, the expansion of molten aluminum metal was constrained by the surrounding solid metal and subjected to shrinkage strain, Since the shrinkage strain cause insufficient aluminum in the molten weld cavity, the subsequent solidification of molten metal formed the voids at the nugget center, which is the last solidified area. The welding interfacial zone was observed by SEM. Fig.3 shows SEM images of the interfacial region of the A6061/Q235 joint. Images (a) to (e) indicate the typical morphology of the A6061/Q235 interface at the positions ( A to E ) in Fig.2, respectively. As shown in Fig.3, a reaction layer was observed at the welding interface. As shown in the Fig.3, the thickness of the reaction layer was varying along the welding interface. The reaction layer, which formed in the interfacial area corresponding to the ring-nugget, is thicker. In this case, the reaction layer adjacent to the steel exhibits a tongue-like morphology and adjacent to the aluminum alloy shows a serrated-like morphology as shown in Fig.3( b), (c) and (d). On the other hand, the reaction layer generated in the periphery and center zone of the joining interface is thinner. Its both side are relatively flat as shown in Fig.3(a) and (e). In order to determine the chemical composition of of the reaction layer, EDS spot analysis was carried out on four spots as shown in Fig.3. Table 2 gives the atomic ratio of Al to Fe based upon EDS analysis results. It clearly indicates the M point adjacent to steel has the chemical composition of Fe 2 Al 5, whereas the N point adjacent to aluminum alloy has the chemical composition of FeAl 3 in the thicker reaction layer. Namely, the thicker reaction layer is composed of tongue-like Fe 2 Al 5 adjacent to the steel and serrated-like FeAl 3 adjacent to the aluminum alloy. The corresponding EDS results indicate that the P and Q spots have nearly the same chemical composition with an Al:Fe atomic ratio of approximately 3:1, which means the thinner reaction layer is composed of FeAl 3. The interfacial characterization found in this study is similar to the results obtained in the previously literature [17, 19]. Table 2 EDS analysis results (at.%) Element w (M) w (N) w (P) w (Q) Al Fe The observation results show that the thickness variation of reaction layer along the interface direction is non-monotonous and complex. The reaction layer is getting thicker from A to C spot at the interface as shown in Fig.2, and then turning thinner. The thickness of reaction layer at the A, C and E spot is approximately 1.0, 3.25 and 0.75 μm, respectively. Therefore, the thickness of reaction layer at the interface presents a bimodal distribution, if seen together with the left half of the interface. Fig.3(f) shows the diagrammatic drawing of reaction layer thickness distribution at the interface. Here, the zone where the aforementioned thicker reaction layer is named for ring-zone. From the point geometric position, the ring-zone and the ring-nugget formed in the steel is corresponding. In other words, the ring-zone locates in the interface under the ring-nugget as shown in Fig.2. Apparently, the thickness of reaction layer formed in the ring-zone is larger, and the thickness of reaction layer formed in the center and periphery region of the joint interface is thinner. The distribution of reaction layer at the interface obtained in this study is different from that of the Al/steel joint welded by commonly electrodes [16, 20]. This is considered to be due to the role of composite electrodes during welding. It is well known that interfacial reaction layer thickness is a function of interaction time and temperature and that is described by the Arrhenius equation. The reaction layer thickness variation in this study may be analyzed using the Arrhenius equation, although thermal phenomenon during resistance spot welding is complex, in which interaction time depends on the position Fig.3 (a~e) SEM images of the interfacial zone, (f) diagrammatic drawing of reaction layer thickness distribution at the interface 148

4 at the welding interface and the temperature of every point at the welding interface varies with welding time. The ring-zone was a heating zone in the study, because the welding current (ring-gathered current) flowed through the zone during spot welding. The temperature was higher and the interaction time was longer. Therefore, the reaction layer formed at the zone is thicker. At the center zone of the joining interface, few current flows through the zone during welding because the resistance of tungsten at the central region of embedded composite electrode is higher than copper alloy at the outer edge of embedded composite electrode. The zone was heated by the conductive heat diffused from the ring-zone. This resulted in the temperature was lower at the center zone of the joining interface. Therefore, the reaction layer formed at the zone is thinner. Similarly, the reaction layer formed at the periphery zone of the joining interface is also thinner. The reaction layer thickness distribution at the interface reveals that restraining the interfacial reaction products growth in the welding central region can be realized by use of resistance spot welding with embedded composite electrodes. Although the results demonstrate that the expected goal in the study has been achieved, the relationship between the width of ring-zone and the diameter of tungsten rod embedded composite electrode is still need to discuss in the future. Fig.4 shows the effect of welding current on the nugget diameter and tensile shear load of the joint. Here, the nugget diameter was measured on the fractured surface of aluminum alloy side after the tensile shear testing of joint. The nugget diameter increased with the increasing of welding current. In resistance spot welding process, the heat input increased with the increasing of welding current based on Joule s law. This resulted in increasing of nugget diameter. In this study, the nugget diameter is in the range from 6.24 to 12.9 mm and meets the relevant standards requirement of D > 4t 0.5 (D is nugget diameter, t is thickness of sheet) [21]. As shown in Fig.4, the tensile shear load increased with the increasing of welding current. But the increase rate was larger in the welding current range from 10 to 22 ka, and then became slighter above the welding current of 22kA. The maximum tensile shear load of 6.28kN was obtained with a welding current of 28kA. Moreover, the fracture type of the joints varied depending on the welding current. Shear and plug fracture were observed in the range of 10~22 ka and 24~32kA of the welding current, respectively. The fracture occurred in aluminum alloy side in the case of plug fracture. When the welding current is low, the tensile shear load is rapidly increased since nugget diameter increased with the increasing of welding current. With increase in welding current, the nugget diameter still increased but the thickness of aluminum alloy sheet in the welding area decreased. The former would facilitate the increase of joint tensile shear load but the latter is the opposite [22], the combined action of both is the reason why tensile shear load changed slightly in range of high welding current. Fig.4 Effect of welding current on the nugget diameter and tensile shear load of the joint 4 Conclusions In the present study, steel and aluminum alloy were welded using the novel method of resistance spot welding with embedded composite electrodes. The results were evaluated by studying the interfacial microstructure and tensile-shear load of joints. The salient results obtained from this study are as follows: (1) At the side of aluminum alloy and steel of joint welded by resistance spot welding with embedded composite electrodes, a acetabuliform nugget and ring-nugget were observed, respectively. (2) A reaction layer was observed at the welding interface of A6061/Q235 joint welded by resistance spot welding with embedded composite electrodes; its thickness at the interface presented a bimodal distribution. (3) The maximum tensile shear load of 6.28 kn was obtained with welding current of 28kA. (4) Restraining the interfacial reaction products growth in the welding central region can be realized by use of resistance spot welidng with embedded composite electrodes. Acknowledgements This work was supported by the Natural Science Foundation of China (U ), Henan Province Support Plan of Universities and Colleges Innovation Talents (16HASTIT050), Henan Province International Science and Technology Cooperation Projects ( ). References: [1] [2] Ogura Tomo, Umeshita Hidetaka, Saito Yuichi, et al. Characteristics and estimation of interfacial microstructure with additional elements in dissimilar metal joints of aluminum alloys to steel. 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