Effect of Hirtisation treatment on surface quality and mechanical properties of AlSi10Mg samples produced by laser powder bed fusion

Laser Powder Bed Fusion (L-PBF) provides a larger design freedom than conventional manufacturing techniques. Surface post-processing is widely investigated to improve the roughness of as built L-PBF parts by reducing powder sintered to the surface and the staircase effect, but it becomes even more important to guarantee a smooth surface if support structures are included for tilted overhanging structures. This study focuses on L-PBF AlSi10Mg samples with a building angle of 30(degrees) with respect to the baseplate. This building angle inevitably requires support structures. The study investigates if a hybrid surface treatment combining chemical and electrochemical polishing, called Hirtisation (R), removes the support structure and to what extent it improves the surface roughness of samples with a 30(degrees) building angle. Currently, the results of surface treatments are mostly reported on vertically built samples while down-facing areas of samples with a lower building angle are typically the roughest areas of the samples causing crack initiation in fatigue. Roughness, defect population, residual stresses, microstructure, fatigue, and tensile properties are studied to investigate correlations with the physical effect of Hirtisation. A porous zone is observed in the down-facing area close to the support structures of as built samples. Hirtisation removed successfully the support and the porous area. In the down-facing roughest area, the average values of the average roughness, mean roughness depth and deepest valley depth are reduced by respectively 56,5%, 58,7% and 51,5% by Hirtisation compared to the as built state. The deepest valley measured reduced from 155,5 mu m to 74,4 mu m. Hirtisation combined with annealing increased the fatigue resistance at a stress amplitude of 120 MPa with a factor 3,1 compared to the as built state and with a factor 2,7 compared to the annealing state. The improved fatigue resistance is linked to halving the deepest valley depth as well as a change in the residual stress at the outer surface from tensile stresses after annealing to a small compressive stress after Hirtisation which counteracts the tensile load in fatigue. Hence, Hirtisation reduces roughness, inverses a tensile residual stress at the outer surface to a small compressive residual stress. In addition, the chemicalelectrochemical process allows to access areas which are impossible to post-process with e.g. sandblasting or vibratory polishing. These conclusions make Hirtisation interesting for further investigations for future applications, however the deepest valley depth could be reduced more by increasing the material removal in the process to improve the fatigue life more.