Anomalous strengthening by supersaturated solid solutions of selectively laser melted Al-Si-based alloys

To identify the dominant contributing factor in the anomalously high strength of Al-Si-based alloys fabricated by selective laser melting (SLM), microstructural characteristics of a SLM-built Al-10Si-0.3 Mg alloy (AlSi10Mg) and their changes upon annealing at elevated temperatures were investigated. The as-built AlSi10Mg alloy exhibits a peculiar microstructure comprising of a number of columnar alpha-Al (fcc) phase with concentrated Si in solution. Numerous nano-sized particles were observed within the alpha-Al matrix. At elevated temperatures, a number of Si phase (diamond structure) precipitates consumed the solute Si in the columnar alpha-Al phase, but the microstructure of the alpha-Al matrix changed slightly. After annealing at elevated temperatures, the tensile strength of the as-built AlSi10Mg alloy substantially decreased accompanied by a reduction in the strain hardening rate. The supersaturated solid solution of the alpha-Al phase containing numerous nano-sized particles enhanced the strain hardening, resulting in the anomalous strengthening of the SLM-built AlSi10Mg alloy. The microstructural features were formed due to rapid solidification at an extremely high cooling rate in the SLM process, which provides important insights into controlling the strength of Al-Si-based alloys fabricated by SLM.