Guian Qian

High residual stresses (RS) in laser powder bed fusion (PBF-LB) components present significant structural challenges, yet traditional characterization techniques are cost- and time-intensive. This study introduces three rapid mechanical testing methodologies to estimate longitudinal RS in PBF-LB 316 L stainless steel by using the premature micro-yielding of pre-tensioned near-surface regions in cylindrical geometries. The proposed methods evaluate RS through: (I) plastic work density calculated from cyclic loading-unloading, (II) virtual unloading mathematically projected from standard monotonic tensile data, which also allows the analysis of existing tensile test results, and (III) the detection of a macroscopic micro-yielding threshold (0.01% strain). The estimates were benchmarked against neutron diffraction (ND) measurements across multiple heat-treatment conditions (as-built to 1000 °C). The energy-based approaches (Proposed Methods I and II) demonstrated linear correlations with relative ND stress reductions (R ² = 0.75 and 0.71, respectively). Furthermore, by subtracting the empirical micro-yielding threshold from condition-specific macroscopic yield strengths, Proposed Method III captured the relative RS relaxation trend. Electron backscatter diffraction (EBSD) analysis supported the proposed mechanistic interpretation by showing localized geometrically necessary dislocation (GND) accumulation prior to bulk yielding. These findings show that the proposed mechanical methods can serve as accessible tools for evaluating stress mitigation strategies in additive manufacturing.

A finite element framework is developed to investigate the temperature-dependent pitting corrosion–fatigue behavior of austenitic stainless steels. The framework couples Lemaitre’s fatigue damage evolution model with Gutman’s mechanoelectrochemical theory and employs an adaptive cycle block method to efficiently update corrosion-pit geometry and fatigue damage throughout the degradation process. Temperature-dependent electrochemical parameters of 316L and 304 stainless steels in pressurized water reactor (PWR) environments are obtained from potentiodynamic polarization data using Tafel extrapolation and incorporated into the corrosion kinetics. Numerical simulations reveal a successive corrosion–fatigue failure process involving corrosion-driven pit growth and increasing stress concentration, accelerated damage accumulation, crack initiation and rapid propagation, and eventual structural failure. Increasing temperature significantly accelerates pit growth and stress-concentration development, advances the pit-to-crack transition, and reduces corrosion–fatigue life. Using the proposed framework, corrosion-fatigue simulations of 316L and 304 austenitic stainless steels are conducted to quantitatively assess their behavior at different temperatures. The framework is further validated against corrosion–fatigue experiments on strain-hardened austenitic (SHA) stainless steel under different temperatures and stress amplitudes. The predicted fatigue lives generally fall within the 95% prediction bands of the experimental data and reproduce the observed effects of both temperature and stress amplitude. The proposed framework thus provides a quantitative approach for predicting the coupled evolution of localized corrosion, mechanical degradation, and fatigue life of austenitic stainless steels.

The very-high-cycle fatigue (VHCF) failure behavior of laser powder bed fusion (LPBF) GH4169 superalloy at elevated temperatures remains incompletely understood, thus delaying its practical application in industrial fields. To address this critical gap, we apply a multiscale characterization framework integrating ultrasonic fatigue testing at 650 ℃, fracture analysis, and high-resolution transmission electron microscopy to directly uncover the elevated temperature fatigue failure mechanism. Notably, we demonstrate a fundamental shift from an internal-defect dominant mode to a highly competitive dual-channel crack initiation mechanism involving both surface indentations and internal pores. Furthermore, a critical oxidation-fatigue interaction is clarified: high-temperature oxidation alters the early damage evolution process of surface crack initiation, enabling surface-governed failures to reach the VHCF regime. Furthermore, this study presents a unified mechanistic model describing the microstructural evolution governing VHCF crack initiation in LPBF GH4169 at both room and elevated temperatures for the first time. Driven by highly constrained cyclic plasticity associated with internal pore defects, dislocation proliferation occurs; subsequently, the shearing of precipitate phases by mobile dislocations leads to their dissolution, triggering continuous dynamic recrystallization (CDRX) and the formation of fine grains, within whose boundaries microcracks ultimately nucleate. Crucially, this localized behavior accelerates nanoscale (Ti, Nb)C precipitation, as well as the detrimental transformation of the strengthening γ′′ phase into the brittle δ phase. Collectively, these findings establish a unified mechanistic understanding of elevated temperature VHCF failure in LPBF GH4169, directly informing synergistic processing strategies, specifically eliminating internal pores and enhancing surface oxidation resistance to mitigate fatigue in extreme environments.

High residual stresses (RS) in laser powder bed fusion (PBF-LB) components present significant structural challenges, yet traditional characterization techniques are cost- and time-intensive. This study introduces three rapid mechanical testing methodologies to estimate longitudinal RS in PBF-LB 316L stainless steel by using the premature micro-yielding of pre-tensioned near-surface regions in cylindrical geometries. The proposed methods evaluate RS through: (I) plastic work density calculated from cyclic loading-unloading, (II) virtual unloading mathematically projected from standard monotonic tensile data, and (III) the detection of a macroscopic micro-yielding threshold (0.01% strain). Estimations were quantitatively validated against neutron diffraction (ND) measurements across multiple heat treatment conditions (as-built to 1000 °C). The energy-based approaches (Proposed Methods I and II) demonstrated linear proportionality with relative ND stress reductions (R2= 0.82 and 0.77, respectively). Furthermore, by subtracting the empirical micro-yielding threshold from condition-specific macroscopic yield strengths, Proposed Method III tracked absolute RS relaxation. Electron backscatter diffraction (EBSD) analysis physically validated these techniques, confirming that the initial non-linear macroscopic mechanical response is governed by localized geometrically necessary dislocation (GND) accumulation prior to bulk yielding. These findings establish the proposed mechanical methods as highly accessible alternatives for evaluating stress mitigation strategies in additive manufacturing.

Laser powder bed fusion (L‐PBF) technology has been commonly used in various industries to manufacture small‐lot or complex parts. However, process‐induced defects are inevitable, which limits the adoption of L‐PBF alloys for load‐bearing components. This work studied the fatigue behavior of L‐PBF AlSi10Mg subjected to rotating bending loading up to 10⁸ cycles, and compared two batches of specimens with different layer thicknesses of 50 and 80 μm. The results indicate that the specimens with a layer thickness of 50 μm exhibit better fatigue strength compared to those with a layer thickness of 80 μm. Fracture analysis shows that lack of fusion defects are the crack initiation location, and the characteristics of these defects have a significant effect on the fatigue behavior. Furthermore, the Kitagawa–Takahashi diagram was used to determine the safe life region in terms of the El‐Haddad formula. Finally, by taking into account the size, location, and shape of the defects, a modified model was proposed to predict the fatigue life. The predicted results are in good agreement with the experimental results.