Mechanisms and influencing factors of defect formations during laser-based directed energy deposition with coaxial powder feeding: a review

During the multi-layer forming process of laser-based directed energy deposition (L-DED) with coaxial powder feeding, the cladding layer is prone to defects, such as the rough surface, pores, and cracks. Moreover, more potential defects, e.g. humps and depressions on the surface of the cladding track, can occur in special processing attitudes such as tilted substrate. Investigating the mechanisms of defect formations is a prerequisite for realising defect suppression. Exploring the factors influencing the defect formations, e.g. the temperature characteristics and flow characteristics of the melt pool, is a must to realise the defect suppression. Moreover, the characteristics of the influencing factors during the multi-layer cladding process are not invariable. To provide a reference for the study of effective defect suppression under different processing scenarios, based on the main defects in the cladding layer when the substrate is in the horizontal processing attitude, the formation mechanisms of potential defects in special processing attitudes are supplemented and analyzed in depth in this paper. Then, the characteristics of the influencing factors and their changes with the progress of the multi-layer cladding process are further discussed based on a summary of the influential relationship between several influencing factors and defect formation.