Thermal Simulations for Cooling Rate Mapping in Electron Beam Additive Manufacturing

The powder-bed electron beam additive manufacturing (EBAM) process is a relatively new AM technology that utilizes a high-energy heat source to fabricate metallic parts in a layer by layer fashion by melting metal powder in selected regions. EBAM can be able to produce full density part and complicated components such as near-net-shape parts for medical implants and internal channels. However, the large variation in mechanical properties of AM build parts is an important issue that impedes the mass production ability of AM technology. It is known that the cooling rate in the melt pool directly related to the build part microstructure, which greatly influences the mechanical properties such as strength and hardness. And the cooling rate is correlated to the basic heat transport process physics in EBAM, which includes a moving heat source and rapid self-cooling process. Therefore, a better understanding of the thermal process of the EBAM process is necessary. In this study, a 3D thermal model, using a finite element method (FEM), was utilized for EBAM heat transport process simulations. The process temperature prediction offers information of the cooling rate during the heating-cooling cycle. The thermal model is applied to evaluate, for the case of Ti-6Al-4V in EBAM, the process parameter effects, such as the beam speed and power, on the temperature profile along the melt scan and the corresponding cooling rate characteristics. The relationship between cooling rates and process parameters is systematically investigated, through multiple simulations, by incorporating different combinations of process parameters into the thermal model. The beam scanning speed vs. beam power curves of constant cooling rates can be obtained from 3D surface plots (cooling rate vs. different process parameters), which may facilitate the process parameters selections and achieve consistent build part quality through controlling the cooling rate.