Introduction Resistance spot welding (RSW) has been the dominant welding process employed by the automotive industry for joining of steel sheet for body structures because of its inherently low cost and high speed. Facing increasing demand from the automotive industry to utilize aluminum alloy sheets for mass reduction and increased fuel economy, it is thus desirable to continue using RSW as the joining process to save considerable costs in capital investment and facility upgrades that would be required with the introduction of other possible joining techniques in existing RSWbased body shops (Refs. 1, 2). However, the presence of insulating oxide layers on the aluminum alloy material surface presents significant problems in obtaining consistent spot welding processes. This problem has been addressed with the invention of the multiring domed (MRD) electrode (Refs. 3–5). This process is currently used to weld automotive structures and closures including the 2014 Corvette Stingray (Ref. 5) and 2017 Cadillac CT6, as well as numerous hoods, doors, and liftgates. Quantitative determination of the performance of aluminum alloy spot welds is critical to safety and structural life prediction (Ref. 6). Traditional tensile tests such as lap shear, crosstension, and peel tests can report the maximum load and load-displacement curves for the tested nugget size and weld schedule. However, such loaddisplacement curves cannot be directly converted into stress-strain curves of the specific weld nugget or heat-affected zone (HAZ) material given the fact that the actual area of the weld nugget is unknown and because of the complex stress state of the weld nugget. The actual stress state of the weld nugget is much more complex than those applied during simple tensile tests and depends on the base materials, nugget size, loading condition, and failure mode (Refs. 7, 8). What is desired is a general method to evaluate the material properties of the weld nugget and/or surrounding heat-affected zone HAZ that could then be used to extrapolate joint behavior in both simple tensile tests and, eventually, vehicle structures. A new shear test specimen and test procedure coupled with digital image correlation was recently developed by Kang et al. (Refs. 9, 10) and have been included in the latest ASTM Standard B831-14 (Ref. 11). Compared to the previous versions of ASTM B831, a pair of notches were added in the shear zone in the thickness direction ensuring a simple shear state in the shear zone in the new shear test specimen (Ref. 10) and digital image correlation was introduced to directly measure the shear strain in the shear zone. This new shear test specimen and test procedure have been used to successfully measure the constitutive behavior of aluminum sheet materials (Ref. 9) and resistance spot weld nuggets of aluminum alloy wrought sheets at large strains (Refs. 12, 13). In this contribution, a newly developed shear test and test procedure was used to directly measure the constitu- WELDING RESEARCH Characterization of Constitutive Behavior of Dissimilar Aluminum Alloy Resistance Spot Welds All of the weld nuggets produced during testing proved to have a higher yield strength than the base metal due to grain refinement that occurs during resistance spot welding BY J. KANG, B. SHALCHI-AMIRKHIZ, Y. CHEN, D. R. SIGLER, AND B. E. CARLSON ABSTRACT The constitutive behavior of resistance spot weld nuggets and heataffected zones was directly measured in samples made from three different dissimilar aluminum alloy stackups that included die cast Aural2T7 to Aural2T7, Aural2T7 to AA5754O, and Aural2T7 to AA6022T4 using modified shear testing coupled with digital image correlation. The results show that all of the weld nuggets produced with the Aural2T7 die cast material have higher yield strength than the base Aural2T7 substrate due to grain refinement that occurs during the resistance spot welding process. Further strengthening is obtained in the Aural2T7 to AA5754O and Aural2T7 to AA6022T4 dissimilar spot welds due to the formation of intermetallic particles at grain boundaries as revealed by TEM observations. KEYWORDS • Constitutive Behavior • Shear Test • Resistance Spot Welding • Aluminum Alloys • Mechanical Properties • Transmission Electron Microscopy J. KANG (jidong.kang@canada.ca), B. SHALCHIAMIRKHIZ, and Y. CHEN are with CanmetMATERIALS, Hamilton, ON, Canada. D. R. SIGLER and B. E. CARLSON are with General Motors Global R&D Center, Warren, Mich. 248-s WELDING JOURNAL / JULY 2016, VOL. 95
Welding Journal | July 2016
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