Fatigue of Cold Spray-Assisted Resistance Spot Welding of Aluminum to Magnesium

Abstract

Direct fusion welding of Al to Mg creates brittle intermetallics (IMCs) that restrict the application of these joints in structural applications. An interlayer foil can be employed to prevent intermetallic formation allowing successful resistance spot weld joint. However, the presence of a foil increases the part count, and is difficult to place during welding. Cold spray coating is a promising solid-state coating deposition technology used here to provide an enhanced interface to facilitate joining Al to Mg sheets by means of resistance spot welding. The ability of cold spray coating technology to deposit an intermediate layer on a substrate surface using powder material is shown to be beneficial in restricting the formation of the brittle intermetallic between Al and Mg. In this study, Ni powder has been successfully coated onto Al and Mg sheets. The Ni-coated coupons were then welded via resistance spot welding using optimized welding parameters. Successful welds were produced using 27 kA for 15 cycles in 2 pulses with 5 delay cycles between pulses. Metallurgical bonding between the Al, Mg, and Ni coating in the fusion zone were revealed by using scanning electron microscopy. It is shown that the bonding between the three elements prevents the formation of Al-Mg IMCs and restricts formation of deleterious IMCs. Fatigue test was then performed to evaluate the joint performance under cyclic load. The develop weld survived 2M cycles at maximum cyclic load of 1.28 kN at R=0.2. While there are no reports on fatigue of direct weld of Al/Mg, the monotonic tensile strength of lap shear samples of the developed weld is five-fold more than the direct Al/Mg welds, showing significant improvement.