Abstract
Lightweight structural applications of magnesium and aluminum alloys inevitably necessitate welding and joining, especially dissimilar welding between these alloys. The objective of this study was to examine the feasibility of joining ZEK100 Mg alloy to Al6022 alloy via ultrasonic spot welding, focusing on effects of welding energy. An interface diffusion layer consisting of α-Mg and Al12Mg17 eutectic structure was observed to form, with its thickness increased from ~0.5 µm to ~30 µm with increasing welding energy from 500 J to 2000 J. The tensile lap shear peak load or strength and critical stress intensity of the welded joints first increased and then decreased with increasing welding energy, with their peak values achieved at 750 J. Fatigue life of the joints made at 750 J and 2000 J was equivalent at the lower cyclic loading levels, while it was longer for the joints made at 750 J at the higher cyclic loading levels. Fatigue fracture mode changed from interfacial failure to mainly transverse-through-thickness crack growth with decreasing cyclic loading level, which corresponded well to the bi-linear characteristic of S-N curves. Crack initiation basically occurred at the weld nugget border and at the interface between the two sheets, which can be understood via a theoretical stress analysis.
Highlights
The transportation industry is increasingly adopting lightweight structural materials in the manufacture of auto-body structures, which is deemed as one of the most important strategies to improve fuel efficiency and reduce anthropogenic climate-changing, environment-damaging, human death-causing
With increasing temperature owing to the high frequency rubbing/vibration, the inter-diffusion intensified in the adhesion region, intermetallics would form during a sufficiently rapid kinetic reaction [22,24]
When the welding energy rose to 750 J, a thin continuous but irregular diffusion layer with an average thickness of about 3~5 μm could be seen, as indicated by an arrow in Figure 2b, owning to the increase in temperature and strain rate at the interface caused by longer time friction/rubbing
Summary
The transportation industry is increasingly adopting lightweight structural materials in the manufacture of auto-body structures, which is deemed as one of the most important strategies to improve fuel efficiency and reduce anthropogenic climate-changing, environment-damaging, human death-causing Magnesium and aluminum alloys are being considered as excellent candidates for the lightweight structural applications due to their low density, high specific strength, and superior damping capacity [7,8,9,10,11,12,13]. Such applications inevitably entail the welding and joining of Al and Mg alloys. Due to the rapid formation of brittle intermetallic compounds (IMCs) at the weld interface, caused by the high mutual diffusivities in these two materials even at relatively low temperatures, welding of Al to Mg is tremendously challenging, especially in the fusion welding processes [14,15,16,17]
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