Powder spreading and spreadability in powder-based additive manufacturing: State of the art and perspectives

Powder-based additive manufacturing (AM) technology has been widely used in various industries. The powder spreading process and its spreadability play a crucial role in ensuring the quality of the final product and the overall production system. This review aims to provide a comprehensive understanding of the issues related to powder spreading and spreadability in AM, as they significantly impact production consistency, process optimisation, and manufacturing cost reduction. A clear definition of spreadability and its corresponding metrics are presented, and the difference between the spreadability and flowability is also clarified. Meanwhile, the factors influencing the spreadability and spreading process, including the powder mixture and gas atmosphere, are thoroughly reviewed. The underlying mechanisms of these factors are discussed and summarised, particularly the critical spreading speed and the shear band developed in front of the spreader. Furthermore, the defects within the spread layer are carefully classified with a summary of the corresponding causes and mechanisms, in which the importance of particle jamming is clarified. The detection of defects using machine learning and the optimisation of spreadability are also reviewed. Finally, future trends and research opportunities, such as the integration of artificial intelligence into in-situ defect detection and subsequent adjustment of spreading conditions, are highlighted.