A comprehensive and comparative study of microstructure and mechanical properties for post-process heat treatment of AlSi7Mg alloy components fabricated in different laser powder bed fusion systems

Abstract

AlSi7Mg (F357) alloy specimens were fabricated in two different laser powder bed fusion (LPBF) systems: EOS M290 and SLM 280HL. Vertical (Z) and horizontal (XY) orientations were fabricated, and five different thermal post processes were applied to samples, individually. According to ASTM F3318-18, the considered thermal conditions were as-built, stress relieved (SR1), HIP, T6 and HIP+T6. Subsequently, the individual samples were aged at 140 °C and 177 °C for 100 h and for up to 1000 h. Tensile specimens were machined down from the aged samples and tested as per ASTM E8/E8M-21. While the yield stress (YS), elongation (%), and Vickers microindentation hardness (HV) were somewhat different for the as-built components, the general trends for the different heat treatments were essentially the same. As-built and SR1 treated microstructures were dominated by microdendritic cells, while the HIP, T6 and T6 + HIP component microstructures consisted of recrystallized grains containing eutectic Si particles of various sizes and shapes within the grain interiors and the grain boundaries; which gave rise to wide-ranging mechanical properties. As an example of these widely-ranging mechanical properties, it was observed that components fabricated in the Z or build direction in the EOS system exhibited a YS, elongation, and HV of 225 MPa, 13%, and HV120, while when HIPed and unaged exhibited values of 87 MPa, 25%, and HV51, respectively. These same HIPed components when aged at 177 °C for 1000 h exhibited values of 81 MPa, 41% and HV44. The mechanical properties of the unaged, HIPed and aged fabricated in Z direction in SLM system were 85 MPa, 31%, HV51, and 80 MPa, 42%, and HV47, respectively, providing support for LPBF system fabrication compatibility. These measured mechanical property values represent a small fraction of the more than 1600 mechanical property measurements (YS, UTS, elongation, and hardness (HV)) in this study.