WELDING RESEARCH Failure Mode Transition of TripleThinSheet Aluminum Alloy Resistance Spot Welds under TensileShear Loads The failure mechanism of threesheet 6061T6 aluminum alloy resistance spot welds was investigated BY Y. LI, Y. ZHANG, Z. LUO, H. SHAN, Y. Q. FENG, AND Z. X. LING DECEMBER 2016 / WELDING JOURNAL 479-s Introduction Resistance spot welding (RSW) has been widely used in the manufacturing of aerospace, electronics, and especially the automotive industry because of its high productivity, flexibility, and suitability. The automotive industry makes extensive application of RSW with typically between 2000 and 5000 spot welds in a motor vehicle (Ref. 1). It can be argued that the mechanical performance of the spot welds determines the vehicle crashworthiness, which is defined as the capability of a vehicular structure to provide adequate protection to its passengers during a crash (Ref. 2). Nowadays, with the demand for lightweight vehicular structures, three-sheet RSW is increasingly exerted in some complex structures, such as front longitudinal rails, A-, B-, and Cpillars, and the bulkhead to the inner wing (Ref. 3). Compared to two-sheet RSW, joining three sheets is more complicated because of the extra interface introduced and different material and sheet thickness combinations. Therefore, it is important to understand the nugget growth and failure behavior of the three-sheet spot weld joint. Some researchers have investigated the nugget growth and mechanical behavior of three-sheet RSW. Harlin et al. found that the position of the initial heat generation is independent of material thickness and stack configuration (Ref. 4). They also found that increasing the electrode force leads to a shift in the initial position of the weld nugget formation from the sheet/sheet interface to the center of the middle sheet (Ref. 5). Nielsen et al. studied the weldability of thin, lowcarbon steel to two thicker, highstrength steels through factorial experimentation and statistical analysis. They found that it is feasible to obtain a good weld with acceptable weld strengths (Ref. 6). Pouranvari and Marashi investigated weld nugget development during RSW of three steel sheets of equal thickness. They found a critical sheet thickness of 1.5 mm at which the size of the fusion zone at the sheet/sheet interface is nearly equal to that of the fusion zone at the geometrical center of the joint (Ref. 7). They also investigated the failure behavior of three-sheet low-carbon steel under a different joint type and pointed out that the joint design significantly affects the mechanical properties and the tendency to fail in the interfacial failure mode (Ref. 2). Many other studies using finite element (FE) simulation investigated the nugget formation process of threesheet RSW. Shen et al. performed a coupled electrical-thermal-mechanical model to predict the weld nugget formation process of RSW of three steel sheets of unequal thicknesses (Ref. 3). Lei et al. built a two-dimensional FE model considering the thermal-electrical coupling for the RSW process of mild steel (Ref. 8). Ma and Murakawa developed an FE program considering ABSTRACT This paper investigates the failure mode transition of triplethinsheet aluminum alloy resistance spot welds under tensileshear loads. Two stackups, i.e., 1.0/1.0/1.0 mm and 1.5/1.0/2.0 mm, were examined. The tensileshear tests were performed for four different joint designs for each stackup. The failure process and failure mode transition of the four types of joints in the 1.0/1.0/1.0 mm stackup were investigated through stepbystep experimental methods. An analytical model, which is suitable for the threesheet aluminum alloy resistance spot weld, was proposed to ensure the pullout failure mode. The results showed that a type of columnar grain, which has large secondary dendrite arm spacing, was the weak area in the threesheet aluminum alloy resistance spot welds. The critical weld button size required to ensure the pullout failure mode was obtained. The proposed analytical models can be used to predict the critical button size for threesheet aluminum alloy resistance spot welds. KEYWORDS • Resistance Spot Weld • ThreeSheet Spot Welding • Aluminum Alloy • Failure Mode Y. LI, Y. ZHANG, Z. LUO, H. SHAN, Y. Q. FENG, and Z. X. LING are with the School of Materials Science and Engineering, Tianjin University, Tianjin, China. Z. LUO is also with the Collaborative Innovation Center of Advanced Ship and Deep Sea Exploration, Shanghai, China.
Welding Journal | December 2016
To see the actual publication please follow the link above