Rapid Solidification and Non-equilibrium Phase Constitution in Laser Powder Bed Fusion (LPBF) of AlSi10Mg Alloy: Analysi
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(e.g., 7 to 12 wt pct Si) are attractive choices of materials for weight-saving metal 3D printing applications. While possessing a low density (~ 2.7 g/cm3), these alloys exhibit relatively high strength and strain hardening capacity in the as-built (as-printed) state.[1] This is mainly owing to a high degree of microstructural refinement that can be achieved in these alloys via
HONG QIN, QINGSHAN DONG, VAHID FALLAH, and MARK R. DAYMOND are with the Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada. Contact e-mails: [email protected]; [email protected] Manuscript submitted April 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
solidification under high cooling rates.[1–4] Cooling rates as high as 106 Ks1 have been found to exist during Laser Powder Bed Fusion (LPBF)[3,5] (as opposed to much lower cooling rates attainable via conventional casting processes, e.g., ~ 300 Ks1 in strip casting of Al alloys[6]). At such high cooling rates, non-equilibrium solidification patterns can evolve in hypo-eutectic Al-Si alloys.[5,7–9] Sarreal and Abbaschian[7] showed that, at a cooling rate of 106 K s1, the volume fraction of eutectic phase and the a-Al solubility limit largely deviate from the equilibrium values (i.e., as determined by the equilibrium Al-Si phase diagram). The same rapid solidification phenomena and the corresponding phase evolution were reported by Marola et al.[4] to occur in an LPBF-processed AlSi10Mg alloy. The apparent effect of such non-equilibrium solidification conditions on the resulting microstructure can be observed in the results of a recent study by Qin et al.[2] which investigated the microstructural evolution in a LPBF-processed
AlSi10Mg alloy. They revealed an exceptionally refined cellular/dendritic structure in which the a-Al cells contain nano-sized precipitates while the cell boundary regions accommodate pockets of eutectic lamellae as well as precipitates. LPBF of AlSi10Mg alloy has been extensively studied in the past few years with a focus on process optimization for higher mechanical properties in the builds.[e.g., 1,10–15] A few of the more recent studies have attempted to explain the observed process-dependency of mechanical properties by means of characterization and analysis of microstructural evolution during LPBF. Xiong et al.[10] have addressed the ambiguous dependency of tensile properties to the build direction. This was done by correlating the change in the mechanical properties (due to the build direction) to the corresponding distribution of melt-pool boundaries with respect to the load-bearing face of the printed tensile specimens. Through analysis of fracture surfaces, they suggested the weak and vulnerable melt-pool boundaries to be the main cause of failure, the orientation of which determines the strength and ductility as a function of build direction (i.e., either horizontal or vertical). Therefore, they ruled out the significance of solidification microstructure and texture. During the analysis of mechanic
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