tive behavior of large strains of resistance spot weld nuggets produced in dissimilar aluminum alloy combinations. The dissimilar alloy combinations included die cast Aural-2-T7 to die cast Aural-2-T7, die cast Aural-2-T7 to AA5754-O, and die cast Aural-2-T7 to AA6022-T4. Also included were the heat-affected zones (HAZs) for the Aural 2-T7 and AA6022-T4 combination. In addition to microhardness testing and metallography, advanced material characterization tools such as focused ion beam (FIB), transmission electron microscopy (TEM), and associated techniques, e.g., energy-dispersive spectroscopy (EDS) were used to characterize and understand the measured constitutive behavior of the weld nuggets and HAZ. Experimental The materials used in the present study are typical for automotive body applications and were 3-mm-thick Aural-2-T7 (AlSi10Mg-T7) highpressure die cast sheets, 2-mm-thick AA5754-O (Al-Mg3 solid-solution hardened alloy), and AA6022-T4 (Al-Si-Mg precipitation-hardened alloy) wrought alloy rolled sheets. The chemical composition and nominal mechanical properties, respectively, of these materials are shown in Tables 1 and 2. Tensile tests were carried out per ASTM E557-10 (Ref. 14) at a nominal cross-head movement speed of 1 mm/min to obtain the baseline tensile stress-strain curves of the base alloy sheets. General Motors’s proprietary MRD resistance spot welding process was used to weld three types of aluminum dissimilar spot welds, namely, Aural 2-T7 to Aural-2- T7, Aural-2-T7 to AA5754-O, and Aural 2-T7 to AA6022- B Fig. 1 — Shear specimen design for the following: A — Aural2T7 to Aural2T7 and Aural2T7 to AA5754O spot welds; B — Aural2T7 to AA6022T4 spot welds where the AA6022T4 side was removed; C — HAZ at the Aural2T7 or AA6022T4 side where the other pieces were removed. T4. Weld coupons were 150 mm long and 38 mm wide. Spot welding was done using a medium-frequency DC (MFDC) weld control and transformer. The MRD electrode is made from C15000 copper alloy and has a 12-mm face diameter with a 25-mm radius of curvature and 5 upstanding rings. The spot welding schedules are shown in Table 3. Some of the aluminum spot welds underwent “baking” to simulate the thermal history experienced during the electrophoretic painting process, i.e., e-coat employed within the assembly plant paint shop, i.e., a thermal cycle of 30 min in total at 177C. For the sake of simplicity, specimens subjected to the e-coat bake WELDING RESEARCH simulation are denoted as “baked samples,” whereas those not subjected to the bake simulation are denoted as “as-welded samples.” Some of the spot weld specimens were sectioned through the cross section perpendicular to the length direction and mounted for microhardness measurements and macrostructure observations following standard metallographic procedures. Shear specimens (Fig. 1) were made with the shear zone located at the center of the weld nugget for weld nugget shear test specimens, or 6 mm away from the center of the weld nugget for the HAZ shear test specimens. The depth of the shear zone shown in JULY 2016 / WELDING JOURNAL 249-s A C Table 1 — Nominal Chemical Composition of Aural2T7, AA5754O, and AA6022T4 (wt%) Element Si Mn Mg Fe Ti Sr Cu Cr Zn Al Aural2T7 9.5–11.5 0.3–0.6 0.1–0.4 ≤ 0.25 ≤ 0.1 0.01–0.018 — — — Bal. AA5754O ≤ 0.4 ≤ 1.0 2.6–3.6 ≤ 0.4 ≤ 0.15 — ≤ 0.1 ≤ 0.30 ≤ 0.20 Bal. AA6022T4 0.7–1.5 ≤ 0.35 0.25–0.75 ≤ 0.30 ≤0.15 — ≤ 0.25 ≤ 0.10 ≤ 0.20 Bal.
Welding Journal | July 2016
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