01.11.2018 Additive manufacturing

Factors affecting printability of 316L powders using the DMLS process

The layer by layer build-up of metal parts by selective laser melting puts specific requirements on the powder quality. The powder should be able to spread homogeneously throughout the layer, to melt and fuse without forming detrimental defects. Consequently, the clean, highly spherical gas atomized powders with excellent flowability are used in this process. However, little work has been published on water atomized powder since higher oxygen levels and lower apparent density can be expected and are believed to be detrimental to build quality. Several variants of water atomized powder was used to investigate the effects of higher levels of impurities and cohesivity on the properties of the built part. Powder properties were analyzed using traditional methods such as Hall flow, particle size distributions, and various density measurements in combination with a modern powder rheometer for characterization. The behavior during building was evaluated through metallographic investigations, mechanical properties and the chemistry of the printed parts. All of which were compared to the properties of built specimens with industry standard gas atomized powder and forged material. Results indicate that the experimental powders with poor flowability can be successfully spread out onto a bed and consolidated to full density when building with a small layer thickness. The resulting mechanical properties match closely the reference, except for significantly lower impact toughness. This is probably caused by increased numbers of spherical nano-sized oxides homogenously distributed in the structure. This is in turn caused by the higher oxygen contents in the experimental powder. It was also found that part integrity went down when increasing the build layer thickness for the experimental powders.