Microstructural analysis of the influences of platform preheating and post-build heat treatment on mechanical properties of laser powder bed fusion manufactured AlSi10Mg alloy

Platform preheating is a common strategy towards reducing residual stress and distortion of additively manufactured metal components by powder bed fusion – laser beam (PBF-LB) due to the lowered thermal gradient. However, platform preheating can lead to in-situ ageing for aluminium alloy components in the building process, which will affect the mechanical properties. In this work, the microstructures of PBF-LB AlSi10Mg built on preheated (150 °C) and un-preheated (ambient) platforms have been characterized in detail by various electron microscopy techniques in order to better understand the influence of platform preheating on mechanical properties. Furthermore, the influences of T5 ageing and solution treatment plus ageing (T6) on microstructure and mechanical properties have also been examined. Platform preheating to 150 °C improves strength, which is associated with the in-situ precipitation of granular or rod-shaped Si phase and needle-like β'' phase. To the best of our knowledge, this is the first report on the observation of β'' precipitates in PBF-LB Al–Si–Mg alloys in the as-built state. Upon T5 treatment, the ambient platform material exhibits significant age hardening while only marginal age hardening is observed for the 150 °C platform material. Consequently, the materials built at different platform temperatures show similar mechanical properties after T5 treatment. The age hardening response is more pronounced when a rapid solution treatment (RST, 520 °C for 10 min) is adopted instead of the conventional solution treatment (CST, 520 °C for 2 h) owing to the precipitation of a higher number density of needle-like β'' phase during ageing. The T5 treatment improves both yield strength and tensile strength, but the improvements in strength are accompanied by a reduction in ductility. On the other hand, the RST-T6 treatment substantially improves ductility and reduces the tensile strength while maintaining the yield strength for the 150 °C platform material. Based on the microstructural analysis, the influences of platform preheating and post-build heat treatment on mechanical properties of PBF-LB AlSi10Mg are discussed.