Thermal process maps for predicting solidification microstructure in laser fabrication of thin-wall structures

The ability to predict and control microstructure in laser-deposited materials requires an understanding of the thermal conditions at the onset of solidification. To this end, 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). Attention is restricted to thin-wall deposits, which are commonly manufactured using Laser Engineered Net Shaping (LENSimage) and other small-scale laser deposition techniques. The approach employs the 2D Rosenthal solution for a moving point heat source traversing a semi-infinite substrate, which has been previously used in the literature to guide the development of process maps for controlling melt pool size and residual stress. In the current study, cooling rates and thermal gradients at the onset of solidification are numerically extracted from the 2D Rosenthal solution throughout the depth of the melt pool, and results are plotted on dimensionless process maps for predicting solidification microstructure. Results suggest that changes in laser power and velocity can have a substantial effect on solidification cooling rate and thermal gradient, which depending on the material system could have a significant effect on the resulting microstructure. Results are further plotted on solidification maps for predicting grain morphology specifically in Ti–6Al–4V, and the effects of laser power and velocity on trends in grain morphology are discussed. Although the Rosenthal predictions neglect the nonlinear effects of temperature-dependent properties and latent heat of transformation, a comparison with 2D nonlinear thermal finite element (FEM) results suggests that they can provide reasonable estimates of trends in grain morphology. In particular, both the Rosenthal and FEM results indicate a trend from columnar toward mixed/equiaxed microstructure with increasing laser incident energy, which is in keeping with recent experimental observations of thin-wall Ti–6Al–4V deposits.