Thermal Process Maps for Controlling Microstructure in Laser-Based Solid Freeform Fabrication

The ability to predict and control microstructure in laser deposited materials requires an understanding of the thermal conditions at the onset of solidification. The focus of this work is the development of thermal process maps relating solidification cooling rate and thermal gradient (the key parameters controlling microstructure) to laser deposition process variables (laser power and velocity). The approach employs the well-known Rosenthal solution for a moving point heat source traversing an infinite substrate. Cooling rates and thermal gradients at the onset of solidification are numerically extracted from the Rosenthal solution throughout the depth of the melt pool, and dimensionless process maps are presented for both thin-wall (2-D) and bulky (3D) geometries. In addition, results for both small-scale (LENS) and large-scale (higher power) processes are plotted on solidification maps for predicting grain morphology in Ti-6Al-4V. Although the Rosenthal results neglect temperature-dependent properties and latent heat effects, a comparison with 2-D FEM results over a range of LENS process variables suggests that they can provide reasonable estimates of trends in solidification microstructure. The results of this work suggest that changes in process variables could potentially result in a grading of the microstructure (both grain size and morphology) throughout the depth of the deposit, and that the size-scale of the laser deposition process is important. Introduction Laser deposition of titanium alloys and other metallic materials is currently under consideration for application to aerospace components, and offers significant increases in efficiency and flexibility compared to conventional manufacturing methods [1]. However, the widespread use of this promising technology will ultimately depend on the ability to predict and control the microstructure and resulting mechanical properties of the deposit [2]. To date, only limited experimental data exists to link deposition process variables (e.g., laser power and velocity) to resulting microstructure (e.g., grain size and morphology) in laser deposited titanium alloys [3-5], and suitable microstructures have typically been obtained only by trial and error. The ability to predict and control microstructure in laser deposition processes requires an understanding of the thermal conditions at the onset of solidification, which is the focus of this work. Recent studies in the literature [6-10] have employed the Rosenthal solution [11] for a moving point heat source on an infinite substrate to identify the dimensionless process variables governing thermal conditions in laser deposition processes. In conjunction with thermal finite element modeling, this has enabled the development of "process maps" relating deposition process variables to melt pool size and residual stress in both thin-wall (2-D) and bulky (3-D) geometries. In the current study, a similar approach is used to investigate solidification cooling rates and thermal gradients (the key parameters controlling microstructure) in laser deposition processes. Cooling rates and thermal gradients at the onset of solidification are numerically