In this paper, the effect of high-current pulsed electron beam (HCPEB) on the microstructure refinement of an Al–20Si–5Mg alloy in the cross-section modified zone was studied, and a double-layer ultrafine crystal structure of the Al–20Si–5Mg alloy was formed. It was found that the cross-section modified zone was divided into three zones, namely, the remelted layer, the heat-affected zone, and the thermal stress wave-affected zone after HCPEB treatment. For the remelted layer, metastable structures were formed due to the rapid heating and cooling rates. For the heat-affected zone, the grain of the aluminum phase was refined due to the cooperative effects of shock wave (formed during an eruption event of the brittle phase), thermal-stress wave (formed during thermal expansion of the alloy surface), and quasi-static thermal stress (formed as a result of an unevenly distributed temperature gradient in the inner material) at high temperatures. For the thermal stress wave-affected zone, the grain refinement was not obvious due to the decreasing energy of the shock wave and the thermal-stress wave at low temperatures. In addition, firm evidence for the tracing of shock waves in the heat-affected zone was demonstrated for the first time and verified for the founding of the broken acicular eutectic silicon. Through this experiment, the mechanical properties of Al–20Si–5Mg alloy materials in both the remelted layer and heat-affected zone were significantly improved after HCPEB treatment.

An hypereutectic aluminium alloy with a composition of 16-18wt%Si, 4-5wt%Cu, 0.45-0.65wt%Mg, 0.3wt%Nd and Al balance (A390-0.3wt%Nd alloy) was processed by extrusion and aging at 200ºC for 10 hours. The effect of the thermomechanical processing and heat treatment on microstructure and mechanical properties was investigated for A390-0.3wt%Nd alloy. It was shown that the eutectic structure disappeared after the processing and heat treatment. In the meantime, the morphology of Al(Si,Mg) solid solution was transformed from dendritic to equiaxed and the Si particles were refned to have fne sizes of 1-10µm and dispersed in the Al(Si,Mg) matrix. Some of the Si particles became globular in shape, which can reduce stress concentration. The tensile strength of the A390-0.3wt%Nd alloy increased by 14.8% (211MPa to 242MPa) after extrusion and aging, and the elongation to fracture increased signifcantly from 0.5% to 13.3%. Thus, the mechanical properties of extruded and heat-treated A390-0.3wt%Nd alloy were improved after the extrusion and aging. The fracture mode is transformed from brittle fracture of as-cast alloy to ductile fracture of extruded and heat-treated alloy.