Effect of a Distributed Heat Source on Melt Pool Geometry and Microstructure in Beam-Based Solid Freeform Fabrication

The ability to control geometric and mechanical properties of parts fabricated using laser-based manufacturing processes requires an understanding and control of melt pool geometry and microstructure. With the development of electron beam manufacturing or future beam-based deposition processes, the user may have more control over the distribution of incident energy, so that beam width becomes a potential process variable. As such, the focus of this work is the effect of a distributed heat source on melt pool geometry (length and depth) and the thermal conditions controlling microstructure (cooling rates and thermal gradients) in beam-based solid freeform fabrication. Previous work by the authors has employed the Rosenthal solution for a moving point heat source to determine the effects of process variables (laser power and velocity) on solidification cooling rates and thermal gradients controlling microstructure (grain size and morphology) in laser-deposited materials. Through numerical superposition of the Rosenthal solution, the current work extends the approach to include the effects of a distributed heat source for both 2-D thinwall and bulky 3-D geometries. Results suggest that intentional variations in beam width could potentially enable significant changes in melt pool geometry without affecting microstructure.