Tensile and Fatigue Behaviour of AA6022-T4 to IF Steel Resistance Spot Welds

Abstract

Vehicle light weighting plays an integral role in achieving the mandatory fuel efficiency requirements as established in the Corporate Average Fuel Economy Standards (CAFE), USA. To optimize vehicle structures for both performance and mass savings, materials such as aluminum alloys may be used in combination with steels, especially advanced high strength steels. Although joining dissimilar materials is a challenge, General Motors (GM) has recently succeeded in developing a proprietary resistance spot welding process using a multi-ring, domed (MRD) electrode geometry that is capable of producing welds between aluminum alloys and steel materials with acceptable joint strength. In this work, tensile and fatigue properties are presented of welds produced between dissimilar materials in lap-shear configuration. The dissimilar metal resistance spot welds (RSWs) were composed of wrought aluminum 1.2-mm thick AA6022-T4 sheet and 2.0-mm thick IF steel. For comparison, similar metal lap-shear RSWs were made and tested of 1.2-mm and 2.0-mm thick AA6022-T4. Average lap-shear strengths of 4400 N and 3200 N were observed for AA6022-T4 to IF steel welds and AA6022-T4 to AA6022-T4 welds, respectively. In load-controlled fatigue testing, limited scatter in fatigue life was observed for both stack-ups tested at maximum fatigue loads below 1500 N. The overall fatigue life was lower for the AA6022-T4 to AA6022-T4 welds compared to AA6022-T4 to IF steel welds. Failure modes for both stack-ups, particularly at long lives, were primarily a result of crack growth through the thickness direction in the AA6022-T4 sheet around the outer edge of the weld nugget. The superior performance of the AA6022-T4 to IF steel welds was most likely primarily due to the larger size of the weld nuggets created in that joint. Also contributing to the superior performance of the Al-steel welds was a more favourable notch root opening geometry that reduced stress concentration as well as a microstructure in the region of the crack path that contained fine, columnar grains alloyed with iron that SEM analyses indicated slowed fatigue crack growth. Using the structural stress concept, it was found that the fatigue life of lap-shear AA60222-T4 to AA6022-T4 and AA6022-T4 to IF steel welds falls onto a master curve indicating that the nugget size dominates the fatigue life.