GTA and EB welding of AM50 and AZ31 cast magnesium alloys: Microstructure and mechanical properties

Gas Tungsten Arc and Electron Beam welding techniques have been used to study the weldability of cast Mg-AM50 and Mg-AZ31 magnesium alloys. The microstructural changes that occurred during welding were examined by optical microscopy and under the scanning electron microscope. Secondary and backscattered electron imaging giving pronounced compositional contrast was used, in combination with energy dispersive and wavelength dispersive spectroscopy microanalysis. Mechanical properties were determined by standard tensile tests on small-scale specimens. The tension test specimens machined from the GTA and EB weld metal exhibit improved mechanical properties, in both strength and ductility, as compared to the same properties measured on transverse and base metal specimens. This is due to the refinement of the microstructure caused by the rapid melting and cooling of the fusion zone. The higher the cooling rates the larger the increase in the mechanical properties of the weld metal in the fusion zone (FZ). It was found that GTA welding of Mg-AM50 and Mg-AZ31 plates creates a large heat affected zone (HAZ) that surrounds the FZ. The major part of the HAZ, located adjacent to the fusion line, is actually a partially melted zone (PMZ) in which most of the grain boundaries and interdendritic regions were fully melted during the welding process. This in turn, causes two major effects: (i) the melted grain boundaries might act as channels to transport the melt to the root face resulting in shrinkage microporosity in the FZ and HAZ, (ii) after resolidification a continuous brittle beta-phase (Mg-17(Al,Zn)(12)), is observed along the grain boundaries. The continuous brittle beta-phase found along the grain boundaries in the PMZ decreases markedly the strength and the ductility of the joint and grain boundary failure of the transverse specimens was always observed. Welding of the alloys by electron beam technique, creates a very small HAZ in the order of 80mum to 250mum. The transverse tension test specimens exhibit mechanical properties comparable to those of the base metal. The rapid melting and subsequent resolidification during EB welding induces microstructural refinement which increase both the strength and the ductility of the resolidified FZ even more than that of the GTA's FZ. The results were analyzed on the basis of current solidification theories. It was concluded that the source of the grain boundary embrittlement is caused by the coring effect during solidification of the cast ingot. Indeed after thermo-mechanical processing, i.e. rolling of the AM50 and AZ31 Mg alloys in which the coring effect is eliminated, no grain boundary failure was observed. Further work is currently being performed on this issue.