Fusion Boundary Microstructure Evolution in Aluminium Alloys

Abstract

Aluminium alloys exhibit a variety of microstructures within the fusion zone adjacent to the fusion boundary. Under conventional weld solidification conditions, epitaxial nucleation occurs off grains in the heat-affected zone (HAZ) and solidification proceeds along preferred growth directions. In some aluminium alloys, such as those containing Li and Zr, a non-dendritic equiaxed grain zone (EQZ) has been observed along the fusion boundary that does not appear to nucleate epitaxially from the HAZ substrate. The EQZ in aluminium alloy 2195 has been the subject of considerable study because of its susceptibility to cracking during initial fabrication and repair. Fusion boundary behaviour was studied in experimental alloy compositions which were produced by making bead on plate welds using an alloy 5454-H32 base metal and 5087 or 5025 filler metals. These filler metals contain zirconium and scandium additions, respectively, and were expected to influence nucleation and growth behaviour. For comparison purposes fusion boundary behaviour was studied in commercially available aluminium alloys 5454-H34, 6061-T6, and 2219-T8. In the context of the alloys, compositions and substrate conditions examined a mechanistic model for EQZ zone formation is proposed. This model can be helpful in adjusting base metal compositions and/or substrate conditions to control fusion boundary microstructure.