Pulsed laser polishing of selective laser melted aluminium alloy parts

Laser polishing (LP) of aluminium alloys by re-melting is particularly challenging due to their high thermal conductivity, diffusivity and reflectivity. In this research, a novel LP strategy is proposed to improve material's re-melting by introducing a thermally insulating ceramic baseplate during nanosecond-pulsed laser polishing of selective laser melted (SLM) AlSi10Mg parts in atmospheric and argon environments. The strategy considerably improves the material's re-melting as realised via sub-surface temperature measurements. This leads to a substantial reduction in the average roughness Sa (by -80-88%) and neutral/compressive residual stresses (up to -19 MPa) when polishing in air with a laser energy density of 12 J/cm2 and 10 scanning passes. In contrast, the unpolished SLM counterparts exhibit tensile stresses, up to +55 MPa. Laser polishing, however, somewhat reduces the Al parts' bulk hardnesses (by -15-25%) compared to the as-built specimens. Heat affected zones (HAZ) in the form of Al-rich white layers up to a depth of -35 mu m beneath the LP surfaces are observed on the cross-sectional microstructures. The study reveals the importance of controlling the heat dissipation from the objects when laser polishing of thermally conductive materials to achieve the desired surface integrity properties.