Microstructure and mechanical properties of selective laser melted Al-Zn-Mg-Si-Sc-Zr alloy with high hot-cracking resistance

A novel additive manufacturing dedicated Al-Zn-Mg-Si-Sc-Zr alloy is designed in this work, and its formability during selective laser melting (SLM) is investigated. Adding 3 % Si to the Al-Zn-Mg alloy leads to a refined microstructure and significant enhancement of hot-cracking resistance. By means of SEM, TEM and EBSD analysis some aspects of the alloy microstructure in as-built and thermal-treated states are discussed. Unlike traditional Al-Zn-Mg alloys, the main strengthening phase in the Al-Zn-Mg-Si-Sc-Zr alloy is Mg2Si, which forms a cellular structure and contributes to the favorable mechanical properties of the as-printed alloy. The as-built alloy exhibits a hardness of 138 HV, ultimate tensile strength (UTS) of 383 MPa, and elongation of 9.5 %. After solution and aging treatment, the UTS decreases to approximately 341-349 MPa, while the elongation increases to 17.6 %. However, with direct multistage aging treatment, the UTS can be increased to 409 MPa, albeit with a reduced elongation of 0.6-1.6 %. The variations in mechanical properties are attributed to the morphology of the Mg2Si phase and its coherent relationship with the Al matrix under different heat treatment processes.