A Novel Microstructure Simulation Model for Direct Energy Deposition Process

The microstructure of additive manufacturing parts has unique characteristics. In this paper, a novel microstructure simulation model is proposed in grain level, called "Invasion Model", to simulate the transient competitive grain growth behaviour in direct energy deposition (DED) process. Different from the phase field method, the invasion model does not focus on the exact dendrite geometry immersed in the liquid phase which will usually lead into high computational cost. The key point of the invasion model is its capability to reflect the transient competitive grain growth behavior under the triensent thermal gradient located at the bottom of the melt pool. An ‘invasion factor' is proposed to represent the possibility that one single crystal will invade its adjacent crystals in each time step. This ‘invasion factor' is calculated by the relation between crystallographic orientation and a changing thermal gradient under the help of the grain boundary criterions in competitive grain growth. Furthermore, the shape and the cooling condition of the melt pool are vital to the final microstructure. The geometry of the melt pool is extracted from the temperature history data of finite element and CFD simulation. The melt pool geometry is analyzed quantitively from interpolated temperature history data and inputted into the invasion model. A vertical cross section of DED fabricated Ti-6Al-4V in multiple layers was investigated by the invasion model, and the competitive grain growth behavior is simulated between the crystals with random crystallographic orientations.