Thermal treatment and surface engineering of powder bed fusion alloys: microstructure and property recovery

Laser powder bed fusion (LPBF) and related powder bed processes impose solidification cooling rates of 104–107 K/s that arrest phase transformation sequences, generate non-equilibrium microstructures, and build residual stress fields approaching 80% of yield strength – conditions that render conventional thermal treatment specifications inadequate. This review examines, with critical and mechanistic depth, the thermal treatment responses, phase transformation pathways, pressure-assisted densification behavior, and surface engineering outcomes for four commercially important alloy systems – Ti–6Al–4V, IN718, 316L stainless steel, and AlSi10Mg – produced by LPBF. The scope is restricted to powder bed fusion processes; the literature base consists of 103 peer-reviewed studies (2000–2022) drawn from Web of Science and Scopus. Technical microstructure schematics (Figures 2–5) are included for all four alloy families to establish the mechanistic basis for divergence between as-built and conventional starting conditions. For Ti–6Al–4V, stress relief annealing at 650°C reduces peak surface tensile residual stress by 60–80%, raising elongation from 4–8% to 8–12%; beta-transus annealing converts the columnar prior-beta grain structure to equiaxed, mechanically isotropic microstructure. For IN718, the AMS 5662 solution treatment and double-aging sequence precipitates the suppressed γ′′ and γ′ strengthening phases, recovering yield strengths of 1100–1200 MPa. Hot isostatic pressing reduces internal void content by 96–99% and shifts fatigue fracture from internal-defect-controlled to surface-initiated failure. Six surface engineering routes are evaluated, and a novel comparison table distinguishes them by mechanism, achievable roughness, residual stress effect, geometric reach, and principal limitation. An original adaptive sensor-feedback framework connects in-situ acoustic emission, infrared thermography, and optical coherence tomography signals to individualized thermal treatment prescriptions through four explicit decision rules, replacing fixed population-average schedules with microstructure-aware, risk-proportionate, part-specific protocols. All threshold parameters are indicative and require experimental validation prior to production deployment. Six research gaps with concrete validation agendas are identified and discussed.