Fracture mechanisms of selective laser sintered parts manufactured in build direction

Selective laser sintering (SLS) is an additive manufacturing process that, in addition to rapid prototyping, is becoming increasingly popular to produce end-use parts. Predictable mechanical properties of the produced parts through this process is a desired primary goal. It has long been known that the mechanical properties especially the elongation at break of SLS components decrease in the build direction. In various test series with tensile specimen fabricated in build direction by SLS, it was found that, on the one hand, these have a very large scatter of the elongation at break, and, on the other hand, it was noticed that these frequently break outside the test range in the upskin radius. The upskin area describes regions that point in a positive build direction relative to the building plate. The fracture occurs at a point where, due to the specimen cross-section, a lower fracture stress occurs than in the test area of the specimen. This fracture leads to a much lower elongation at break than in the case of specimens that fracture in the test area. This study investigates which mechanism triggers this behavior. It turns out that different roughnesses can be determined in the upskin and downskin radius and different defects can be seen in the fracture surfaces, especially in the edge areas. It cannot be predicted based on surface roughness measurements or surface profiling exactly where, or in which layer the fracture occurs. However, the defects lead to a local stress spike, at which the failure of the specimen begins.