Utilizing powder thixoforming to fabricate heterogeneous lamella structure in aluminum alloy: Formation, strengthening and toughening mechanisms

Powder thixoforming is a promising technique to synthesize heterogeneous lamella (HL) structure in aluminum alloy to promote strength-ductility synergy by mixing softer alloy powders (Al-1.04Mg-0.56Si-0.28Cu-0.04Zn, weight fraction) and harder alloy powders (Al-13.32Zn-3.70Si-2.44Mg-1.48Cu, weight fraction) together. However, the formation mechanism of coarse grain lamella and fine grain lamella is still unclear, and the strengthening and toughening mechanisms of HL structure have not been clarified thoroughly. This work shows that coarse grain lamella results from rapid coarsening of softer alloy powders driven by reduction in surface energy, fine grain lamella comes from both solidified grains from partially remelted softer/harder alloy melts and unmelted harder alloy powders surrounded by abundant eutectics. Besides, controlling semi-solid heating duration can regulate HL structure of Al alloys effectively, and the best comprehensive properties with ultimate tensile strength of 400 MPa, yield strength of 260 MPa and elongation of 7.1% are achieved for the HL-structured Al alloy heated for 50 min. The high strength of HL-structured Al alloy is ascribed to high back stress resulting from progressive yielding from coarse grain lamella to fine grain lamella, the high toughness is due to pulling out of fine grains and bifurcation of main cracks in coarse grains.