Is high-speed powder spreading really unfavourable for the part quality of laser powder bed fusion additive manufacturing?

Although high-speed powder spreading can efficiently enhance the productivity of laser powder bed fusion (LPBF) additive manufacturing, it is rarely used because it is generally believed to be unfavourable for the part quality. However, there is no systematic investigation to confirm this “common sense”. In this work, a series of powder spreading and melting experiments are carried out to investigate the role of the spreading speed in LPBF. In the single-layer experiments, the high-speed powder spreading indeed reduces the packing density of the powder layer and seems unfavourable as expected. However, the multilayer LPBF processes of cubic samples with various high powder-spreading speeds are successful, and the samples possess even fewer defects and thus better mechanical properties particularly fatigue life, which is counterintuitive and has never been reported before. To understand the physical mechanisms, we fabricate staircase samples under different powder spreading speeds, revealing the layer-by-layer evolution of the powder bed and deposited dense region. It is found that regardless of the powder spreading speed, the actual powder layer thickness gradually increases due to the shrinkage during powder melting, but always reaches the steady state in ∼10 layers, where the deposited dense layer thickness is equal to the nominal powder layer thickness, thereby achieving similar melting condition and quality. Furthermore, LPBF experiments with intended operational delays in the powder spreading procedure are conducted and prove our speculation that the slightly reduced pores and cracks in the samples fabricated with high-speed powder spreading are mainly attribute to the reduced cooling time between layers and consequently the higher temperature before next-layer melting as well as lower temperature gradients. The major drawback with the higher powder spreading speed is also discussed, which is the reduction of the dimensional accuracy of the fabricated sample along the building direction. This study provides unprecedented insight into the role of powder spreading speed in LPBF and corrects the inaccurate intuition that high-speed powder spreading is always unfavourable, which provides more potential solutions to enhance the productivity and part quality of LPBF.