nificantly greater strength than that found for the base metal substrate. Figures 9 and 10 also reveal that the effect of paint bake cycles on the tensile stress-strain curves for the Aural- 2-T7 to AA5754 and Aural-2-T7 to AA6022 weld nuggets is indeed negligible, as the differences are within the measurement error. Tensile stress-strain curves for the HAZ are also shown in Figs. 8–10. For all three stack-up combinations, the measured tensile stress-strain curves in the Aural-2-T7 HAZ are slightly lower compared to that of Aural-2-T7 base metal regardless of testing conditions, i.e., whether as-welded or baked. This means the HAZ in the Aural-2-T7 side is indeed insensitive to the paint bake simulation. It is interesting, however, to note that one sees a significant increase in yield strength of the HAZ in AA6022- T4 (Fig. 10) compared to the base metal substrate, but a corresponding decrease in work hardening of the HAZ compared to the base material. This is attributed to the heat input of the welding operation. After the paint bake cycle, an appreciable increase in yield strength is observed although the work hardening remains similar to that observed in the as-welded condition. This was likely due to age hardening as the bake temperature of 177C is close to the solution aging temperature of 180C for AA6022-T4 (Refs. 12, 16). These observations are consistent with those from the microhardness testing but quantitative tensile stress-strain curves were directly measured from the areas of interest within the corresponding spot welds. The microhardness measurements in the current work for the AA5754-O in as-welded and as-baked conditions indicate no HAZ, although metallographic analysis of AA5754 welds by Zhang and Senkara does indicate a region adjacent to the weld having darkened and widened grain boundaries indicative of a HAZ (Ref. 17). Figure 11 shows the FIB images of the microstructure near the faying interface of the Aural-2-T7 and weld nugget for all three spot welds. From Fig. 11, it is seen that the primary aluminum grains and pure Si for all WELDING RESEARCH three weld nuggets are much finer compared to the die cast Aural-2-T7 base metal where typical spheroidized and isolated globules of Si are present, indicating the addition of Sr as a modifier in its chemistry (Ref. 18). The presence of Sr is confirmed in the EDS elemental maps of Aural- 2-T7 alloy by TEM — Fig. 12. Nevertheless, the rapid cooling and solidification produced by resistance spot welding leads to grain refinement in the weld nugget. This is postulated as the main strengthening mechanism responsible for the observed increase in strength of the weld nuggets. All three weld nuggets were analyzed by TEM and the results are presented in Figs. 13–16. From the TEM results of the Aural-2-T7 to Aural-2- T7 weld nugget (Fig. 13), it is seen that the structure consists of primary aluminum surrounded by an aluminum-silicon eutectic containing magnesium, manganese, and iron. The grain boundaries for this weld nugget are free of intermetallic particles, but the grain size is much finer compared to the Aural-2-T7 base metal alloy. The TEM results for the Aural-2- T7 to AA5754-O weld nuggets are shown in Fig. 14. These weld nuggets are primarily composed of material from the Aural-2-T7 substrate — Fig. 4. Figure 14 reveals primary aluminum surrounded by a more complex eutectic structure. Intermetallic particles are clearly seen in the aluminum silicon-rich eutectic regions, e.g., AlMgSi phase (Al0.45Mg1.55Si) JULY 2016 / WELDING JOURNAL 253-s Fig. 8 — Measured effective stresseffective strain of Aural2T7 to Aural2T7 spot welds. Fig. 9 — Measured effective stresseffective strain of Aural2T7 to AA5754O spot welds. Fig. 10 — Measured effective stresseffective strain of Aural2T7 to AA6022T4 spot welds.
Welding Journal | July 2016
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