Microstructure and Shear Strength of Novel Aluminum to Steel Resistance Spot Welds

Resistance spot welds were produced between dissimilar 1.2-mm-thick wrought aluminum alloy AA6022-T4 and 2.0-mm-thick interstitial free (IF) steel with acceptable joint strength using symmetric and asymmetric electrode combinations that incorporate a multiring domed (MRD) electrode and multiple solidification weld schedules. The focused ion beam and transmission electron microscopy results indicated the intermetallic layer that was created consisted of two distinct layers, i.e., a needle-like FeAl3 adjacent to the AA6022-T4 sheet and a tongue-like Fe2Al5 adjacent to the IF steel sheet. A new mini-shear test was developed to directly measure the shear strength of the intermetallic layer formed at the interface of these welds produced using symmetric and asymmetric welding electrodes. The results showed the intermetallic layer near the aluminum weld nugget periphery, which is the region critical for joint strength, had an intermetallic layer less than 2 μm thick and an average shear strength of 64 MPa, irrespective of the welding electrodes used. In addition, the tensile shear strengths of the aluminum-steel welds were greater than comparable aluminum-aluminum resistance spot welds.