Comparative Analysis of Microstructure Evolution in Additively Manufactured vs. Conventional Metals

Thesis Abstract

This study addresses the persistent knowledge gap in how microstructure evolution differs between additively manufactured (AM) metals and conventionally processed metals under identical service-relevant thermal histories, and how these differences influence mechanical performance and reliability. The aim is to systematically compare, under controlled conditions, the evolution of phase content, grain morphology, and defect populations in AM and wrought or cast metals subjected to matched thermal cycles representative of aerospace and automotive applications. Specific objectives are (1) to characterize initial as-built microstructures in selected alloys—Ti-6Al-4V, IN718, and 304 stainless steel—generated by selective laser melting (SLM) and electron beam melting (EBM) alongside conventional forgings and castings; (2) to apply identical post-processing heat-treatment schedules (solution treatment, aging, and hot isostatic pressing) and monitor their effects on phase fractions (ferrite, austenite, martensite, and secondary phases) and grain size distributions; (3) to quantify defect populations (porosity, microcracks, and dislocation densities) and their evolution during heat treatment using high-resolution X-ray diffraction (HRXRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and high-resolution synchrotron diffraction where available; (4) to model microstructure–property relationships through regression and multivariate analysis to relate microstructural metrics to tensile properties, fatigue life, and fracture resistance; and (5) to synthesize findings within theoretical frameworks of phase transformation kinetics and dislocation theory to propose guidelines for selecting AM vs. conventional processing routes for target performance. The study adopts a comparative, cross-sectional research design conducted in two parallel streams AM metals processed by SLM and EBM, and conventionally processed metals produced by forging/ccasting. The population comprises three representative alloy systems (Ti-6Al-4V, IN718, and 304 stainless steel) with reproducible processing routes. Sample sizes include n = 30 specimens per alloy per processing route, with sub-samples allocated for different heat-treatment conditions, yielding a total of approximately 180 specimens for microstructural analysis and 90 for mechanical testing. Data collection instruments encompass (i) EBSD for grain size, grain boundary character distribution, and texture; (ii) TEM for dislocation substructure and nanoscale precipitates; (iii) HRXRD and synchrotron diffraction for phase quantification and residual stress; (iv) optical and scanning electron microscopy for porosity and defect mapping; (v) nanoindentation and microtensile testing for local mechanical responses; and (vi) standard tensile, fatigue (R = 0.1, 10^7 cycles), and fracture toughness tests in accordance with ASTM standards. Validity and reliability are ensured through calibration with certified reference materials, inter-laboratory cross-validation for diffraction patterns, and repeat measurements on at least 10% of specimens. Data analysis employs descriptive statistics to summarize microstructural features, inferential statistics including three-way ANOVA to assess the effects of material system, processing route (AM vs. conventional), and heat-treatment on grain size, phase fractions, and defect density. Regression analyses (multiple and hierarchical) will relate microstructural indicators to mechanical responses, while survival analysis will model fatigue life differences. A Bayesian updating framework will quantify uncertainty in predictions. A theoretical framework integrating phase transformation kinetics (Schwetz–Khachaturian models) and dislocation theory will guide interpretation of the results, with particular attention to non-equilibrium solidification features and residual stress evolution in AM parts. Expected findings anticipate that AM metals will exhibit refined as-built grain structures with texture and residual stress patterns distinct from conventional counterparts, leading to divergent phase transformation pathways during heat treatment and resulting in divergent precipitate distributions, dislocation densities, and defect populations. It is anticipated that post-process heat treatments will partially mitigate residual stresses but will have alloy-specific effects on phase fractions and mechanical performance, with Ti-6Al-4V showing pronounced sensitivity to cooling rates and IN718 displaying notable precipitation strengthening differences between routes. The study contributes to knowledge by providing a rigorous, quantitatively backed comparison of microstructure evolution under matched thermal histories, informing predictive models and enabling more informed decisions on material selection and processing for high-reliability applications. Recommendations include optimized heat-treatment schedules tailored to AM-produced microstructures, guidelines for defect mitigation in AM parts, and a framework for extending the comparative model to additional alloys and processing modalities. The study concludes that while AM and conventional metals can achieve comparable bulk properties under suitable processing, the microstructural pathways are fundamentally different, necessitating route-specific design criteria to ensure reliability and performance.