WELDING RESEARCH the coupling of electrical, thermal, and mechanical fields to study the nugget formation in RSW of high-strength steels (Ref. 9). Although many studies have been performed on the weld growth process of three-sheet spot welds, these researchers have all focused on mild or high-strength steels. The usage of aluminum alloys in the automotive industry is gradually increasing due to its light weight, good formability, and high corrosion resistance (Ref. 10). However, very little work in open literature has studied the RSW of multiple aluminum alloy sheets. Although Li et al. investigated the weld growth mechanism of three-sheet RSW for aluminum alloys (Ref. 11), the failure behavior of the spot welds was not the focus of the paper, especially the failure transition mode, which is considered an important characteristic of the spot weld joint. The present article investigates the failure mechanism of three-sheet 6061-T6 aluminum alloy resistance spot welds (RSWs), especially the failure mode transition behavior of the spot welds. Four types of joints are designed in this paper. The mechanical properties of three-sheet RSWs are also investigated. Experimental Procedures In this study, 6061-T6 aluminum alloy sheets with thicknesses of 1, 1.5, and 2.0 mm were used. Table 1 lists the chemical composition of the materials, while Table 2 lists the mechanical properties. Two thickness combinations were used in the experiments. From the upper electrode tip to the lower one, the two thickness combinations were 1.0/1.0/1.0 mm and 1.5/1.0/2.0 mm, respectively. Four types of three-sheet joints for each thickness combination were designed, as shown in Fig. 1. In the Type I and II joints, only one interface bore the tensile force during the test. In the Type III and IV joints, both of the two interfaces bore the tensile 480-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 A B C D E force during the test. It is obvious that the stiffness of these joint designs and consequently the tendency of the sam- A B Fig. 1 — Joint designs of the threesheet AA6061T6 RSW: A — 1.0/1.0/1.0 mm stack; B — 1.5/1.0/2.0 mm stack. Table 1 — Chemical Composition of 6061T6 Aluminum Alloy (wt%) Si Mg Zn Cu Mn Fe Cr Ti Al 0.56 1.10 0.25 0.25 0.15 0.70 0.18 0.15 Balanced Table 2 — Mechanical Properties of 6061T6 Aluminum Alloy Yield Strength Tensile Strength Elongation (MPa) (MPa) (%) 285 310 6 Fig. 2 — The exemplary microstructure of the 6061T6 resistance spot weld nugget in the 1.0/1.0/1.0 mm stack (18 kA, 200 ms).
Welding Journal | December 2016
To see the actual publication please follow the link above