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 microstructur al 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-wel ded condition the fusion zone microstructure consisted of faceted prec ipitates around 10 mu m in size, embedded in a cellular-dendritic alph a-Al matrix with a submicrometre intercellular phase. This fusion zone structure was significantly different from that of the powder-metallu rgy base alloy which contained Al-12(Fe, V)(3)Si dispersoids in a fine grained alpha-Al matrix. Detailed electron microscopy showed that the faceted precipitates in the fusion zone have the AlmFe-type crystal s tructure (m approximate to 4) and are enriched in Si, Fe and V compare d with the alpha-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 developm ent in the fusion zone. As expected, the presence of coarse intermedia te phase particles meant that the fusion zone had a significantly lowe r room temperature Vickers hardness than the base metal.