The microstructure of CO2 laser welds in an Al-Fe-V-Si alloy

Autogenous, bead-on-plate CO2 laser welding of an Al-8.5wt.%Fe-1.2wt.%V-1.7wt.%Si alloy was performed on 2 mm thick sheet. The microstructural characteristics of the fusion zone were investigated using optical, scanning and transmission electron microscopies, and the fusion zone microstructure was compared with that of the base metal. In the as-welded condition the fusion zone microstructure consisted of faceted precipitates around 10 μm in size, embedded in a cellular-dendritic α-Al matrix with a submicrometre intercellular phase. This fusion zone structure was significantly different from that of the powder-metallurgy base alloy which contained Al12(Fe, V)3 dispersoids in a fine grained α-Al matrix. Detailed electron microscopy showed that the faceted precipitates in the fusion zone have the Alm Fe-type crystal structure (m ≈ 4) and are enriched in Si, Fe and V compared with the α-Al. A crystalline Fe- and Si-rich phase formed at the cell boundaries. Over the range of conditions examined, changing the welding speed did not alter significantly the microstructural development in the fusion zone. As expected, the presence of coarse intermediate phase particles meant that the fusion zone had a significantly lower room temperature Vickers hardness than the base metal.