Lightweight body structures are essential for improving the fuel economy of internal combustion engine vehicles that account for about 15% of national energy use. Moreover, these structures are vital for increasing the driving range of electric vehicles by offsetting the weight of power systems such as batteries and electric motors. To create these structures, lightweight materials such as aluminum alloys (Al) have been incorporated into the vehicle design where they are joined to advanced high strength steels (AHSS). Joining of Al to AHSS is generally difficult due to the metallurgical incompatibility as well as the large difference in physical properties between the two metals. A variety of processes have been developed for connecting Al to AHSS, ranging from resistance spot welding, impact welding to self-pierce riveting. Many published studies are focused on mechanical properties under quasi-static loading. As vehicles undergo cyclic loading throughout their lifetime, fatigue properties of these welds are a vital consideration for the durability of the dissimilar structures made of Al and AHSS.
In this study, Ultrasonic interlayered Resistance Spot Welding (Ulti-RSW) was used to join Al 6022 T4 to Zn-coated JAC590R steel. The welding process consisted of two steps. In the first step, an AISI 316L stainless steel interlayer was welded to Al 6022 T4 by ultrasonic spot welding, a solid-state welding process. Then in the second step, the interlayer side of Al 6022 T4 was welded to JAC590R steel using the standard resistance spot welding. A unique feature of the Ulti-RSW joint is two nuggets formed: one formed due to local melting of JAC590R steel and 316L, and the other formed due to Al brazing over the unmelted 316L. This process limits local peak temperature and thus mitigates formation of brittle intermetallics at the Al-steel interface. The welding time, current and force were found to significantly affect the nugget size and the joint tensile shear strength. These parameters were optimized to produce the tensile shear strength above 5 kN while minimizing the occurrence of expulsion.
Constant-amplitude cyclic fatigue testing in the tensile shear configuration was performed to generate a load range vs. number of cycles to failure curve for the dissimilar joints welded by Ulti-RSW. The curve spanned from 10,000 cycles to failure to runout at 10 million cycles. The effect of the Al-Fe intermetallic layer on the fatigue properties was studied through analyzing the fracture surfaces in scanning electron microscopy (SEM). Moreover, a new joint efficiency parameter was established that accounted for different base metal materials and joint geometries. Using this parameter, the fatigue properties of dissimilar joints produced by Ulti-RSW were compared to those by other processes.