Effect of heat treatments on the microstructure and mechanical properties of Al-Mg-Sc-Zr alloy fabricated by selective laser melting

Abstract

Abstract High-strength aluminum alloy with Sc-Zr is specially optimized for Selective Laser Melting (SLM). The alloy has a great potential in structurally optimized lightweight parts in the fields of aerospace, nuclear power and automobile due to its good formability, fine microstructure, and excellent mechanical properties. An appropriate heat treatment for this alloy is critical to improve its ultimate tensile strength. However, the effect of heat treatments on the microstructure and mechanical properties of SLMed Al-Mg-Sc-Zr alloy is not fully understood and heat treatment needs to be optimized. In this investigation, the effect of 500\xa0°C homogenization and 330\xa0°C ageing on the microstructure and properties of the SLMed parts were investigated. The results show that a homogenization treatment at 500\xa0°C does not increase the solubility of the alloy elements, but accelerates the precipitation of Sc/Zr elements and recrystallization. Moreover, homogenization with holding time longer than 2\xa0h results in a decrease of mechanical properties including both tensile properties and ductility compared with those of the as-built state. Ageing treatment at 330\xa0°C for 1\xa0h drives the second phase of Al3(ScxZr1-x) to fully precipitate, and long-term ageing for 8\xa0h neither leads to recrystallize nor deteriorates the properties of the alloy. A rapid ageing heat treatment at 330\xa0°C for 1\xa0h was explored, with a tensile strength of 479\xa0MPa, a yield strength of 441\xa0MPa, an elongation after break of 14.5%, and a shrinkage of 45%. Specimens of SLMed Al-Mg-Sc-Zr alloy are strengthened by fine grains whose formation is driven by extremely high temperature gradients and solidification rates, and precipitation of the second phase Al3(ScxZr1-x) caused by ageing. The Orawan bypass mechanism plays an important role in the strengthening of the aged SLMed Al-Mg-Sc-Zr alloy.