Jerard V. Gordon

Additive manufacturing plays an essential role in fabricating complex geometries and provides unlimited potential. While some defects may occur during any manufacturing technology, the frequency and the size of the defects produced in metal AM can negatively influence the integrity of load-bearing components. Since this manufacturing method inherently introduces defects such as pores, inclusions, and unfused powder particles, inspection methods must achieve resolutions tailored to the target usage and application. In this study, 3D printed metal calibration samples for evaluating bulk wave ultrasonics were developed. Two methodologies were used: (a) printing a fully dense stainless-steel plate and subsequently introducing defects as small as 150μm, and (b) printing with embedded, subsurface defects as small as 100μm at varying depths. Samples were manufactured with simulated voids by selectively leaving regions of unmelted powder within the print representative of common additive defects, such as lack of fusion porosity. Before primary ultrasonic testing, we evaluated how surface roughness, from an unpolished surface to a polished surface at a range of grit sizes, affects the reflectivity of ultrasonic signals. Samples were prepared accordingly and scanned using immersion ultrasonics with a 10MHz transducer, clarifying the optimal surface preparation for further tests. Results from the ultrasonic test show that the subsurface features of the 3D-printed part microstructure act as a scattering surface for frequencies above 10MHz. A resolution of 10MHz with a 259μm spot size enabled the detection of artificially introduced defects down to 150μm. These findings show the detection limits of ultrasonic testing for qualifying additive-manufactured metal components.

Laser powder bed fusion (L-PBF) additive manufacturing offers a remarkable balance of strength and ductility across many structural alloys. However, L-PBF alloys often display much lower fracture toughness, in some cases up to 70% below conventionally wrought counterparts. The reasons for this toughness paradox have remained elusive, since conventional tools cannot directly visualize sub-surface microscale deformation processes that govern crack growth. Here we apply scanning 3D X-ray diffraction and phase contrast tomography to simultaneously capture microstructural evolution with 1 micron resolution near an advancing crack tip, utilizing 316L stainless steel as a model system. We demonstrate that the toughness paradox is not solely a consequence of extrinsic processing defects or residual stresses, but rather an intrinsic failure to relax crack-tip stresses via plasticity. While wrought material facilitates stable crack-tip blunting through localized dislocation accumulation, the L-PBF material undergoes premature work-hardening saturation that triggers extreme stress partitioning and high stress triaxiality. This results in a transition from ductile blunting to a sharp, unstable fracture mode. These findings identify work-hardening exhaustion as a systemic vulnerability inherent to L-PBF microstructures, where the exceptional initial dislocation density required for high yield strength acts as a saturation ceiling for damage tolerance. This work provides a physical basis for adapting damage models to L-PBF metals and challenges the assumption that high tensile ductility guarantees fracture resistance in rapidly solidified components.

The proposed research aims to characterize the process of plastic deformation and crack tip blunting in polycrystalline alloys made by laser-based additive manufacturing (LAM) using a combination of 3D imaging techniques. Fracture toughness in LAM alloys can be as much as 70% lower than conventional metals, however a mechanistic basis for these behaviors remains unclear. Both LAM and wrought stainless steel 316L (SS316L) polycrystals will be studied by in-situ scanning X-ray diffraction (s3DXRD) and phase contrast tomography (PCT) using the nanoscope station at ID11. The experimental data will give access to the temporal and spatial evolution of crack tip geometry and orientation-strain states to assess the influence of local grain morphology on plastic deformation and crack tip blunting throughout the 3D microstructure during in-situ crack growth experiments of notched samples.

Laser powder bed fusion (L-PBF) additive manufacturing offers a remarkable balance of strength and ductility across many structural alloys. However, L-PBF alloys often display much lower fracture toughness, in some cases up to 70% below conventionally wrought counterparts. The reasons for this toughness paradox have remained elusive, since conventional tools cannot directly visualize sub-surface microscale deformation processes that govern crack growth. Here we apply scanning 3D X-ray diffraction and phase contrast tomography to simultaneously capture microstructural evolution with 1 micron resolution near an advancing crack tip, utilizing 316L stainless steel as a model system. We demonstrate that the toughness paradox is not solely a consequence of extrinsic processing defects or residual stresses, but rather an intrinsic failure to relax crack-tip stresses via plasticity. While wrought material facilitates stable crack-tip blunting through localized dislocation accumulation, the L-PBF material undergoes premature work-hardening saturation that triggers extreme stress partitioning and high stress triaxiality. This results in a transition from ductile blunting to a sharp, unstable fracture mode. These findings identify work-hardening exhaustion as a systemic vulnerability inherent to L-PBF microstructures, where the exceptional initial dislocation density required for high yield strength acts as a saturation ceiling for damage tolerance. This work provides a physical basis for adapting damage models to L-PBF metals and challenges the assumption that high tensile ductility guarantees fracture resistance in rapidly solidified components.

This study explores the fatigue performance of novel metal material jetting (MMJ), a process that uses sub-micron powders to induce significantly refined microstructures compared with other sinter-based additive manufacturing (AM) technologies such as metal binder jetting (MBJ). Stainless steel 316L (SS316L) samples fabricated via MMJ were subjected to fully reversed uniaxial cyclic loading to generate a stress-life (S/N) curve, which was compared to literature data for MBJ SS316L. The fatigue performance of MMJ SS316L was markedly superior to MBJ, with as much as 16× improvement in the number of cycles to failure in the low cycle fatigue regime and 14× in the high cycle fatigue regime. This enhancement was attributed to the inherent smaller grains, leading to a high density of high-angle grain boundaries and annealing twins that improved resistance to crack initiation and propagation. A mechanics-based model incorporating hardness and defect size was used to estimate the fatigue limit, yielding a ∼5 % error compared to experimental values. These results underscore the influence of finer microstructural features and defect distribution on fatigue performance in sinter-based AM and highlight MMJ’s potential for structural applications requiring high fatigue resistance

Abstract Laser-powder bed fusion (L-PBF)-based additive manufacturing (AM) of pure Fe and two alloys in the Fe-Cu system (FeCu2.5 and FeCu5 wt.%) was used to understand the role of Cu in the microstructure and resulting mechanical properties at strain rates between 10 −3 s −1 and 10 3 s −1 . Small amounts of Cu were found to significantly increase yield strength at all strain rates because of a combination of grain refinement and the presence of nanoscale Cu precipitates within the Fe grains. The strengthening increments are interpreted in terms of Hall–Petch strengthening from Fe grain boundaries, strengthening from dislocations introduced via processing, and precipitation hardening from Cu precipitates. Enhancements in yield strength were accompanied by slight reductions in strain rate sensitivity and tensile ductility. Fracture surface analysis revealed that Fe and FeCu2.5 showed similar ductile fracture features at all strain rates, whereas FeCu5 exhibited a shear-dominated slanted fracture surface. The absence of solidification defects in these alloys can be rationalized in terms of CALPHAD-based Scheil and Clyne–Davies solidification simulations. The simulations show that the propensity for solidification cracking is expected to increase rapidly for Cu contents exceeding ~ 8%. This demonstrates the potential of rapid solidification simulations in aiding alloy design.

Non-destructive mapping of 3D microstructures in metal additive manufacturing (AM) remains challenging due to large orientation gradients and residual stress (RS) magnitudes. We leveraged scanning 3D X-ray diffraction (s3DXRD) to non-destructively resolve intragranular orientation-strain states and RS in laser powder bed fusion (L-PBF) SS316L for the first time. Type III von Mises RS displayed significant variation with proximity to grain boundaries, and in some locations, exceeded the macroscopic yield strength of L-PBF SS316L. Interestingly the spatial distribution of RS showed no correlation with orientation gradients, suggesting that stress relaxation during solidification is accommodated by lattice rotation.

• PCA of X-ray spectral images enhances the ability to detect trace element features thus enhancing the ability to detect chemical heterogeneities in small particles as compared to conventional elemental detection from individual pixels. • For features larger than 200 nm the TSEM-EDS method provides qualitatively similar detection results to STEM-EDS. • Matching the interaction volume size to the pixel resolution of the TSEM scan is necessary if researchers want to move from qualitative to quantitative analysis of localized chemistry. The objective of this work was to explore the capabilities of a field emission gun scanning electron microscope (FEG-SEM) equipped with a transmission scanning electron detector (TSEM) and energy dispersive spectroscopy (EDS) to identify nanoscale chemical heterogeneities in a gas atomization reaction synthesis (GARS) steel sample. The results of this analysis were compared to the same study conducted with scanning transmission electron microscopy (STEM) with EDS mapping. TSEM-EDS was performed using the standard spectral analysis approach, i.e., pixel-by-pixel identification of elements from the spectra, and a new principal component analysis approach to detect regions of similar spectra before identifying elemental contributions to each spectrum. It was determined that features over 200 nm were detectable with the TSEM-EDS standard spectra analysis technique but the PCA analysis approach was necessary for observing smaller features that contained trace elements. Monte Carlo simulations indicated that the spatial resolution expected from a 150 nm thick foil was consistent with those observed in experimental analysis. Simulations also confirm that thinner samples enable higher spatial resolution scans although smaller interaction volumes may require longer acquisition times.

Abstract Refractory complex concentrated alloys (RCCAs) show potential as the next-generation structural materials due to their superior strength in extreme environments. However, RCCAs processed by metal additive manufacturing (AM) typically suffer from process-related challenges surrounding laser material interaction defects and microstructure control. Multimodal in situ techniques (synchrotron X-ray imaging and diffraction and infrared imaging) and melt pool-level simulations were employed to understand rapid solidification pathways in two representative RCCAs: (i) multi-phase BCC + HCP Ti 0.4 Zr 0.4 Nb 0.1 Ta 0.1 and (ii) single-phase BCC Ti 0.486 V 0.375 Cr 0.111 Ta 0.028 . As expected, laser material interaction defects followed similar systematic trends in process parameter space for both alloys. Additionally, both alloys formed a single-phase (BCC) microstructure after rapid solidification processing. However, significant differences in microstructure selection between these alloys were discovered, where Ti 0.4 Zr 0.4 Nb 0.1 Ta 0.1 showed a mixture of equiaxed and columnar grains, while Ti 0.486 V 0.375 Cr 0.111 Ta 0.028 was dominated by columnar growth. These behaviors were well described by the influence of undercooling effects on columnar-to-equiaxed transition (CET). Distinct microstructure formation in each alloy was verified through CET predictions via analytical melt pool simulations, which showed a ~ 5 × increase degrees in undercooling for Ti 0.4 Zr 0.4 Nb 0.1 Ta 0.1 compared to Ti 0.486 V 0.375 Cr 0.111 Ta 0.028 . Overall, these results show that microstructure control based on modulating the freezing range must be balanced with process considerations which resist defect formation, such as solidification crack formation in RCCAs. Graphical abstract

The three-dimensional grain-averaged response of solid bar samples under non-proportional (NP) elastoplastic axial-torsional loading was investigated using in situ high energy diffraction microscopy (HEDM) and companion crystal plasticity finite element (CPFE) modeling. Important stress metrics including applied shear (σθZ) and axial (σZZ) stress tensor components, stress and stress deviator tensor invariants (I1, J2, and J3), von Mises equivalent stress (), maximum resolved shear stress (mRSS), stress triaxiality (η), and lode angle parameter () values were tracked for ∼ 300 grains under two different loading conditions: (1) Torsion-dominated loading (low NP) and (2) Tension-torsion loading (high NP) in equiatomic NiCoCr, a representative multicomponent face-centered cubic (FCC) superalloy. Overall, significant stress localizations existed within both samples as evidenced by the radial dependence of grain-resolved σθZ, , and J2; by comparison, I1, J3, η, and metrics did not show discernable trends within the volume. These stress localizations reveal a complex interplay between axial and shear stress components (e.g., stress coupling) resulting in grain yielding near the sample surface largely driven by shear stress, whereas internal grain yielding was largely accommodated by axial stress. Grain-resolved stress localization trends were described well by the CPFE model, although some discrepancies in magnitude occurred, particularly for volumetric stress metrics (I1 and η) due to initial type II residual stress distributions. The superposition of initial residual stress states onto CPFE grain-resolved data significantly improved model accuracy for η. This suggests that residual stresses more strongly influence the simulation of volumetric rather than deviatoric (yield) stress metrics.