Electron beam metal additive manufacturing: Defects formation and in-process control

Electron beam heat source has certain advantages in processing some special high-performance materials because of its high energy efficiency and the ability to use its own high scanning rates to maintain a high temperature environment in a vacuum environment. However, defects formation hinders further application of Electron Beam Additive Manufacturing (EBAM). Unstable melt pools will lead to some processing stability related defects, such as smoking, spatter, lower adhesion, high surface roughness, high residual stress and deformation, and element evaporation, which in turn cause defects at the microstructural level such as porosity, anisotropy and heterogeneity of microstructure, cracks, and dislocations. Moreover, the defect formation process of EBAM in high preheating temperature and high vacuum environment is more complex and difficult to fully understand. Therefore, establishing the relationship between defects formation mechanisms and processing stability is of great significance to control defects and improve properties. Based on electron beam-material interaction factors, this paper systematically described these defects formation mechanisms in Selective Electron Beam Melting (SEBM) and Electron Beam Freeform Fabrication (EBF³) technology. The underlying causes of the effect of defects on the anisotropy of tensile and fatigue properties and the heterogeneity of microhardness were discussed. In addition, some in-process control methods were summarised to control defects in terms of the factors affecting the melt pool dynamics, including control of thermal process conditions during deposition, powder modification and in-situ wire alloying, and in-process online assistance. It can be stressed that these methods will be further research points for EBAM in the future and can drive the need for further applications of EBAM technology to refine additive manufacturing in the special high performance materials field.