Selective Laser Melting of AlSi10Mg : influence of the thermal history during the process on the microstructure and mechanical properties

AlSi10Mg processed by Selective Laser Melting exhibits a very fine microstructure due to the high cooling rate undergone by the part during the process (10^3-10^7 K/s). However, the cooling rate is lower at the melt pool boundary than in the melt pool core resulting in a coarser microstructure at the boundary. Furthermore, an heat affected zone (HAZ) is also present around the melt pool due to the heat input accompanying the deposition of a new layer. Those microstructural inhomogeneities act like a weak zone where tensile failure occurs. The present study aims at investing which specific zone i.e, the coarse zone or the HAZ, is the most detrimental with respect to tensile failure. To do so, the microstructures are analyzed with a particular attention for the melt pool boundary and the HAZ. The Si phase fraction and the Si precipitate spacing are measured by image analysis. Fracture features are observed on broken samples. Hardness tests are performed by nanoindentation. The HAZ exhibits low hardness due to coarse non-coherent Si precipitates. This low hardness indicates that the HAZ is the preferential region where fracture is likely to occur. This is confirmed by Si precipitate spacing within the HAZ matching with dimple spacing observed in fracture surfaces.