The influence of part height on the properties and microstructure of AlSi7Mg produced by laser powder bed fusion

Laser powder bed fusion (PBF-LB) is an additive manufacturing technology commonly used to manufacture AlSi7Mg components due to its ability to produce customisable parts of complex geometries. While this is true, the geometric factors associated with these complex geometries are often not considered during manufacturing, and as such, this research aims to investigate the influence of part height on the mechanical and microstructural properties of AlSi7Mg. The hardness of the as-built parts was seen to decrease 14.8% from 128 HV near the build plate to 109 HV 250 mm from the build plate. A microstructural analysis revealed that the melt pools, grain structure, and solid solution strengthening all remained consistent over the build height. An increase in residual stress along the build direction from 70 to 223 MPa from the bottom to the top of the build occurred due to in-situ annealing during the manufacturing process. Cells within the grains increased in size from 0.64 µm to 1.01 µm over the build height due to changes in thermal gradient and solidification velocity, reducing their Hall–Petch strengthening effect near the top of parts. Increased formation of solute atom clusters and GP zones was also observed nearer the build plate due to in-situ ageing, contributing to the gradient in hardness values seen over the build height. These results indicate that geometric factors, including the height of a part, need to be properly considered when PBF-LB is utilised, while also highlighting the importance of proper heat treatment as a homogenisation step.