Gradient of precipitation phenomena in tall laser powder bed-fused AlSi7Mg samples: Effects of heated substrate and aging heat treatments

The ability of Laser Powder Bed Fusion (LPBF) to manufacture complex-shaped and semi-finished components, along with the study of metallurgical properties, improves the future design and applications of LPBF- manufactured large AlSi7Mg parts. However, due to the unique microstructure of LPBF parts and the presence of a heated substrate (build platform), the precipitation sequence and mechanical properties vary along the height of the samples. This study investigates the gradient effects induced by a build platform heated to 150 ◦C on 300 mm-tall AlSi7Mg samples after direct and artificial aging. TEM observations revealed a gradient in precipitation hardening and dislocation densities between the bottom and top regions of as-built samples, which exhibit differences in tensile strengths but not in ductility. Direct aging at 175 ◦ C -1 h successfully homogenized the tensile property values across the height of the billet. Newly formed precipitates balanced the strengthening reduction caused by coarsened precipitates. The artificial aging was performed after solution treatment at 505 ◦ C, which destroyed the very fine Si-eutectic network in few minutes. Nevertheless, peak-aging at 175 ◦C was reached earlier in samples solution treated at 505 ◦C for 4 h rather than at 505 ◦C-0.5 h, ultimately reaching comparable hardness and tensile properties values. TEM analysis showed the distribution of Mg2Si precipitate and Si particles. The quality index shows that heat treatments fail to improve the quality of LPBF-manufactured AlSi7Mg compared to cast alloy under similar conditions.