Microstructure of Al-Mg dissimilar weld made by cold metal transfer MIG welding

A modified metal inert gas welding process called cold metal transfer was successfully applied to dissimilar Mg and Al welding with AlSi5 filler metal. Optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction were used to characterise the microstructure, alloy element distribution and phase constituents of the welded joint. The microhardness distribution was determined by microhardness tester on the fusion zone. The super low heat input and the addition of Si inhibited the creation and growth of brittle intermetallic compound in weld metal, which improved the strength of the joint. An obvious multilayer microstructure, which comprised of solid solution, eutectic structure, Mg17Al12, and Mg2Al3 layer, was observed in the fusion zone adjacent to the Mg substrate. It degraded the strength, leading to the fracture of the joint in tensile test. The fracture morphology presented the typical brittleness fracture mode.