Effects of Selective Laser Melting and Post Heat Treatment on the Microstructure and Mechanical Properties of Aluminium Alloy A357

Since Selective Laser Melting (SLM) is essentially a targeted solidification process with very fast cooling rates, cast aluminium alloys such as A357 can be processed by SLM to achieve an ultrafine microstructure. However, since the microstructure after SLM is different from that after casting, the post heat treatment should also be different. We have improved the mechanical properties of SLMed A357 to a level comparable to traditional cast Al alloys by controlling the laser-fabricated microstructure via the adjustment of SLM and subsequent heat treatment parameters. The SLM process results in sub-micron sized Si cells surrounding supersaturated Al dendrites, which are ten times finer than in the cast material. During solution heat treatment (SHT), small Si particles coarsen and long fibrous Fe intermetallics are formed. Based on cast Al heat treatment parameters, hardness curves were plotted for the optimisation of temperature and time for stress relief (SR), SHT and ageing. The optimal parameters were then applied on tensile samples to determine the best ultimate tensile strength (UTS), yield strength (YS) and ductility. Within an optimised density region from SLM, small density differences were found not to affect the corresponding mechanical properties significantly. However, the tensile results did vary with the combination of laser parameters which influence the degree of powder melting and the thermal history. Therefore, the strength of bonding, the dendrite/particle characteristics, the grain structure, and the molten pool characteristics in different orientations relative to the build direction were found to contribute to the anisotropy of mechanical properties to varying degrees. In this regard, the work contributes to the optimisation of SLM and heat treatment parameters to minimise mechanical property anisotropy and maximise absolute mechanical properties to meet aerospace component property requirements.

Keywords: Selective Laser Melting; A357 aluminum alloy; microstructure; porosity; heat treatment; mechanical properties