The robustness of Al-steel resistance spot welding process

Several types of materials are commonly used in automotive body structures, driving the need for joining of dissimilar material joints of aluminum alloys to steels by various techniques including resistance spot welding. Al-steel spot welds can exhibit variable structures and properties, even when produced with the same welding schedule, welding equipment, and with the same combination of materials. Laboratory test conditions can correct for this variability; however, this is not always a realistic representation of production conditions. This study evaluated the influence of production conditions such as gaps between sheet metal, angles between the sheet metal and welding electrode, changes in current, and cap length on the microstructural and mechanical properties of Al-steel spot welds. A designed experiment was used to weld tensile (lap) shear, coach peel, and metallographic specimens under various combinations of production conditions. Comparative t-test and quadratic regression statistical analyses were performed to quantify the production conditions that significantly affect the robustness of the Al-steel spot welding process. Compared to analogous Al-Al specimens, the Al-steel tensile shear specimens exhibited greater strengths and were robust to being produced under various production conditions. However, the Al-steel coach peel specimens were sensitive to some of the production conditions, especially those that combined off normal sheet angles and gaps between the metal sheets. These differences in behavior are discussed, including the underlying fundamental metallurgical and physical mechanisms.