H.L. Wei

Heat transfer and fluid flow in additive manufacturing

In laser-based direct energy deposition additive manufacturing, process control can be achieved through a closed loop control system in which thermal sensing of the melt pool surface is used to adjust laser processing parameters to maintain a constant surface geometry. Although this process control technique takes advantage of important in-process information, the conclusions drawn about the final solidification structure and mechanical properties of the deposited material are limited. In this study, a validated heat transfer and fluid flow laser welding model are used to examine how changes in processing parameters similar to those used in direct energy deposition processes affect the relationships between top surface and subsurface temperatures and solidification parameters in Ti-6Al-4V. The similarities between the physical processes governing laser welding and laser-based additive manufacturing make the use of a laser welding model appropriate. Numerical simulations show that liquid pools with similar top surface geometries can have substantially different penetration depths and volumes. Furthermore, molten pool surface area is found to be a poor indicator of the cooling rate at different locations in the melt pool and, therefore, cannot be relied upon to achieve targeted microstructures and mechanical properties. It is also demonstrated that as the build temperature increases and the power level is changed to maintain a constant surface geometry, variations in important solidification parameters are observed, which are expected to significantly impact the final microstructure. Based on the results, it is suggested that the conclusions drawn from current experimental thermography control systems can be strengthened by incorporating analysis through mathematical modeling.

Grain Growth Modeling for Additive Manufacturing of Nickel Based Superalloys

In a laser based additive manufacturing process, the alloy powders undergo a rapid heating, melting, solidification and cooling process. The morphology and the scale of the solidification structure depend on the temperature gradient and the growth rate during the additive manufacturing process. A comprehensive three dimensional transient heat transfer and fluid flow model has been used to calculate the temperature distribution, thermal cycles and local solidification parameters during laser based additive manufacturing process for nickel based super alloys. The growth direction of columnar dendrites and the solidification texture are estimated based on the computed temperature field. The effects of the process parameters on the growth directions, morphologies and scale of the solidification structures are discussed.