Intermetallic and dispersoid structures in AA3104 aluminium alloy during two-step homogenisation

During homogenisation of the AA3104 cast ingot, a phase transformation of intermetallic particles from β-Al₆(Fe,Mn) orthorhombic phase to harder α-Al_x(Fe,Mn)₃Si₂ cubic phase occurs. The large constituent intermetallic particles, regardless of phase, assist in the recrystallisation nucleation process through particle stimulated nucleation (PSN). Ultimately, this helps to refine grain size. The sub-micron dispersoids act to impede grain boundary migration through a Zener drag mechanism. For this reason, the dispersoids that form during homogenisation are critical to the recrystallisation kinetics during subsequent rolling, with smaller dispersoids being better suited to instances where the minimisation of recrystallisation is required during hot rolling. This work simulates an industrial two-step homogenisation practice with variations in the peak temperature of the first step between 560 °C and 580 °C. The effect of this temperature variation on the intermetallic particle phase evolution is investigated. The aim is to identify the ideal intermetallic phase balance and the dispersoid structure that are best suited for the minimisation of recrystallisation during hot rolling through maximising Zener drag and maintaining galling resistance. The results indicate a trend where an increase in homogenisation temperature from 560 °C to 580 °C yields, firstly, an increase in the volume fraction of the α-phase particles to greater than 50% of the total volume fraction at both the edge and the centre of the ingot and, secondly, it yields an increased dispersoid size. Thus, a lower temperature homogenisation practice produces a near-ideal combination of intermetallic particle phase distribution, as well as dispersoid size, which is critical for Zener drag and the minimization of recrystallisation during the hot rolling processes.