A Theoretical Investigation of Residual Stress Evolution During Selective Laser Melting of Nigerian Titanium Alloy Feedstocks for Biomedical Implant Applications

The theoretical understanding of residual stress evolution during selective laser melting additive manufacturing of titanium alloy implant components from Nigerian-processed feedstocks is incomplete, as the rapid thermal cycling, complex scan strategy-dependent heat flow, and alloy microstructure interactions unique to additive manufacturing conditions have not been incorporated into a unified predictive theory that accounts for the specific alloy composition variations expected from domestic Nigerian titanium feedstock processing chains. This research presents a theoretical investigation of residual stress evolution in Ti-6Al-4V components produced by selective laser melting, focusing on the foundational thermomechanical theory governing residual stress development as a function of scan strategy, layer thickness, build orientation, and substrate preheating, with application to feedstock compositions representative of domestically processed Nigerian titanite ore-derived titanium. A high-fidelity multi-scale thermomechanical model is developed, coupling a moving heat source thermal model for individual melt pool dynamics with a mesoscale layer-by-layer stress accumulation model and a macro-scale part-level residual stress distribution computation implemented in ABAQUS. The model incorporates temperature-dependent material properties and phase transformation kinetics for the alpha-beta transformation in Ti-6Al-4V during the thermal history of additive manufacturing. Theoretical analysis of the competing stress generation mechanisms, specifically the temperature gradient mechanism and the cool-down mechanism, produces dimensionless criteria identifying the parameter regimes in which each mechanism dominates. Experimental validation using synchrotron X-ray diffraction residual stress measurements on additively manufactured specimens confirms theoretical predictions within 14 percent. Keywords: selective laser melting, residual stress, titanium alloy, additive manufacturing, biomedical implant.