Effect of direct aging on the microstructure and tensile properties of AlSi10Mg alloy manufactured by selective laser melting process

Abstract

The microstructure and mechanical properties of AlSi10Mg alloy manufactured via selective laser melting process were investigated. The effect of post heat treatment on microstructure and mechanical properties was also examined. Direct aging (DA) process without solution heat treatment was employed as a post heat treatment to prepare specimens. Specimens were also arranged via the conventional solution heat treatment + precipitate hardening (T6) for comparison. Initial microstructure observation results revealed that the as-built alloy showed the unique characteristic of SLM material’ microstructure having cellular structure together with molten pool. The cellular structure boundaries were found to contain eutectic Si and a small amount of nano size Si particles inside the cellular structure. After the solid solution + T6 heat treatment, the unique microstructural characteristic of the SLM as-built material disappeared, whereas the Si particles became as coarse as several μm. On the other hand, the DA heat treated alloy, even after the heat treatment, maintained the molten pool and cellular structure and contained a lot of Si precipitates (nano size) inside the cellular structure. Room temperature tensile results showed that the DA alloy possessed superior tensile strength (310 MPa) compared with the as-built and T6 alloys. The elongation of the DA alloy was 6.2%, which was about 3% lower than that of the as-built alloy. However, compared with the conventional precipitate hardening Al-Si alloy, the DA alloy's elongation was found higher. Tensile fracture surface observation found that in the as-built specimen, fine dimples were formed and cellular structure played a role in increasing the strength of the alloy. In the DA alloy, a lot of precipitated Si particles played as a major reinforcing phase. From the results above, the DA was found as a suitable heat treatment method capable of enhancing the strength of SLM AlSi10Mg alloy, and at the same time, granting a decent level of elongation.