Flow properties of additive manufacturing metal powders: influencing parameters and wall friction reduction by diamond-like carbon coating

Powder flowability plays a key role in additive manufacturing processes. It significantly affects the spreading step and, thus, the properties of the produced parts. Powder flowability and powder wall friction are also important in the conveying of the powder from storage to the layer formation step in production. In this study, the influence powder properties and of environmental condition (relative humidity) on the flowability of various additive manufacturing metal powders was investigated using an annular shear cell. Since the reduction of powder wall friction by diamond-like carbon (DLC) surface coating has been shown recently, DLC coating was included in powder wall friction tests. The results showed that increasing median particle size of the powders improves flowability, while increasing width of the particle size distribution decreases flowability. With increasing relative humidity, the flowability decreases gradually. The wall friction angle of the powders with stainless steel can be reduced by several degrees applying DLC coating on the steel. In contrast, with an ultra-high molecular weight polyethylene wall material, the wall friction angles were generally higher in comparison with stainless steel.