In this study, laser-material interactions during laser-powder bed fusion of WE43 magnesium alloy were characterized through numerical and experimental analyses. Various melting modes (i.e., conduction, transition, and keyhole) were induced through deposition of laser tracks at powers ranging from 80 to 130 W, and used as input parameters for a thermo-fluid model. Results of microscopy demonstrated good agreement between numerical and experimental measurements of melt pool depth, as well as a strong correlation between melt pool microstructure and the thermo-fluid conditions predicted by the model. Specifically, for conduction mode at 80 W, a predominance of cellular subgrains within the melt pool was consistent with the predicted steep thermal gradients, while for keyhole mode at 130 W, low thermal gradients correlated with high presence of equiaxed dendrites. Moreover, convection currents attributed to high recoil pressure in keyhole melt pools, were in agreement with locations of numerous subgrain boundaries having non-uniform morphologies, while under conduction, outward Marangoni flow led to a unique alignment of cellular subgrains and fewer subgrain boundaries. This study demonstrates the interplay among processing, thermal history, fluid flow and microstructure in WE43, and provides a basis for future design of microstructures for improved material properties.

This study compares the role of microstructure on post-punching fatigue properties in three advanced high- strength steels (AHSSs) with a high-strength low-alloy (HSLA) steel commonly used in heavy-duty truck chassis. Microstructure characterization, tensile testing, high cycle fatigue (HCF) testing, fatigue crack growth rate (FCGR) testing, and neutron residual stress measurements are conducted. Punching significantly alters the microstructure, causing microstructure refinement, sub-grain formation, defect creation, tensile residual stresses, and a work-hardened shear-affected zone (SAZ) around, and a rough fracture zone, inside the punched hole. At 105 cycles, the HCF performance is primarily governed by the fatigue crack growth resistance of the as-rolled microstructure, with minimal sensitivity to punching. However, near the fatigue limit, post-punching fatigue failure is strongly related to strain localization when significant strength difference exists between micro- constituents (e.g., martensite and ferrite). Strain localization also promotes sub-grain formation, reducing the local threshold stress intensity factor range (Delta Kth), thus facilitating fatigue crack initiation. In microstructures with smaller strength differences (e.g., ferrite and bainite), sub-grains, together with surface roughness and residual stress, contribute significantly to the post-punching fatigue limit reduction. These findings provide insights into mechanisms of punching-induced fatigue performance degradation, offering potential strategies to optimize fatigue performance of AHSS for new applications.

In this study, we investigate the relation between microstructure and fatigue crack propagation mechanisms in three commercial hot-rolled thick-plate advanced high-strength steels (AHSSs), namely 800CP, 700MC, and 700MCPlus, and compare them with a conventional high-strength low-alloy (HSLA) steel, 500MC, commonly used in heavy-duty vehicle chassis production. Tensile testing, and fatigue crack growth rate (FCGR) assessments have been conducted, and mechanisms controlling the performance of these steels are comprehensively examined through microstructure characterization before and after fatigue testing. Notably, FCGR results reveal that, despite having the highest yield strength among the investigated steels, 700MCPlus exhibits the slowest FCGR in both near-threshold and stable crack growth regimes. This improved fatigue crack propagation resistance of 700MCPlus in terms of its threshold stress intensity factor range (Delta Kth) is attributed to its unique texture, which restricts slip activity, and the presence of martensite at grain boundaries, contributing to fatigue crack deflection. This martensite-induced crack deflection becomes more significant in the stable crack growth regime, where fatigue crack primarily propagates intergranularly as the stress intensity factor range (Delta K) increases. These mechanistic findings offer new design possibilities for AHSSs, combining excellent strength and fatigue performance.

Multi-physics thermo-fluid modeling has been extensively used as an approach to understand melt pool dynamics and defect formation as well as optimizing the process-related parameters of laser powder-bed fusion (L-PBF). However, its capabilities for being implemented as a reliable tool for material design, where minor changes in material-related parameters must be accurately captured, is still in question. In the present research, first, a thermo-fluid computational fluid dynamics (CFD) model is developed and validated against experimental data. Considering the predicted material properties of the pure Mg and commercial ZK60 and WE43 Mg alloys, parametric studies are done attempting to elucidate how the difference in some of the material properties, i.e., saturated vapor pressure, viscosity, and solidification range, can influence the melt pool dynamics. It is found that a higher saturated vapor pressure, associated with the ZK60 alloy, leads to a deeper unstable keyhole, increasing the keyhole-induced porosity and evaporation mass loss. Higher viscosity and wider solidification range can increase the non-uniformity of temperature and velocity distribution on the keyhole walls, resulting in increased keyhole instability and formation of defects. Finally, the WE43 alloy showed the best behavior in terms of defect formation and evaporation mass loss, providing theoretical support to the extensive use of this alloy in L-PBF. In summary, this study suggests an approach to investigate the effect of materials-related parameters on L-PBF melting and solidification, which can be extremely helpful for future design of new alloys suitable for L-PBF.

Microstructure, crystallographic texture, and mechanical properties of pure Mg, as a model hcp metal, were compared after processing by the two severe plastic deformation (SPD) techniques of equal channel angular pressing (ECAP) and simple shear extrusion (SSE). Both processes were performed on extruded bars at 250 degrees C for up to four passes, where the minimum grain sizes of 13.7 and 9.8 mu m were achieved in the ECAP and SSE processes, respectively. The fraction of dynamically recrystallized (DRX) grains, high angle grain boundaries (HAGBs), and the evolution of dislocation density with equivalent strain experienced the same trend in both processes with higher values after ECAP. The textural evolutions were completely different during ECAP and SSE despite their similar deformation modes. A conventional shear texture was developed after ECAP, while after SSE basal planes were aligned parallel to the processing direction. In the ECAP-processed material, texture softening and lower dislocation density counterbalanced the strengthening effect of grain refinement, resulting in a decrease in shear yield stress (SYS) and ultimate shear strength (USS), while in SSE the shear strength increased constantly with increasing number of passes. It can be deduced from the experimental results that SSE was more effective in achieving a fine-grained homogenous microstructure with high shear strengths as compared to ECAP.

Microstructure, mechanical properties and superplastic behavior of Mg-2Gd-xZn (x = 0, 1, 2 and 3 wt%) alloys were investigated after extrusion and equal channel angular pressing (ECAP). After only 2 passes of ECAP, a homogenous fine-grained microstructure with a grain size of 2.33 mu m and a high fraction of high-angle grain boundaries of 84% were formed in the Mg-2Gd-3Zn (GZ23) alloy, while 4 ECAP passes were necessary to create such a structure in the other alloys. This was attributed to the higher solute drag effect in the other alloys, retarding dynamic recrystallization (DRX). Although DRX occurred more easily in the GZ23 alloy, the final DRX grain size was slightly coarser compared to the other alloys. Shear punch testing (SPT) showed that grain refinement during ECAP leads to a slight increase in the shear yield strength of all studied materials after 2 ECAP passes, which was mostly balanced by texture softening caused by the shear texture component and grain growth after 4 ECAP passes. Contrary to the other alloys, the GZ23 alloy exhibited superplastic behavior after a lower number of ECAP passes. In addition, the superplastic temperature for GZ23 was 648 K, which was lower than the 673 K observed for the other alloys. The m-values of similar to 0.45-0.5 and activation energies of 98-114 kJ/mol suggested grain boundary sliding (GBS) controlled by grain boundary diffusion as the dominant deformation mechanism in the superplastic regime. This was confirmed by microstructural observations.

Constrained groove pressing (CGP) was utilized to modify the texture and mechanical properties of extruded Mg-2Gd-3Zn sheet with typical TD (transverse direction)-split texture and pronounced mechanical anisotropy. The texture evolution sequence during CGP was studied and it was observed that a new (1211) component with basal poles rotated 15-30 toward the extrusion direction (ED) is introduced during CGP, as a result of simultaneous activation of basal and prismatic slip and shear deformation. Finally, a non-basal ED-TD double split texture was obtained after the CGP process, resulting in significantly reduced mechanical anisotropy.

This study examines how different punching parameters affect the fatigue properties of thick-plate high-strength low-alloy (HSLA) steel used in heavy-duty truck chassis. Microstructural characterization, tensile testing, high cycle fatigue (HCF) testing before and after punching, and neutron residual stress measurements are performed. Punching causes significant microstructural changes, including refinement, sub-grain formation, defect creation, tensile residual stresses, a work-hardened shear-affected zone, and a rough fracture zone inside the punched hole. At higher stresses and fewer load cycles (10⁵ cycles), the post-punching HCF performance is comparable to the smooth condition and less sensitive to surface roughness, residual stress, and microstructural changes from punching. However, at lower stresses and more load cycles (10⁶ cycles), the post-punching fatigue strength is significantly decreased compared to the smooth condition, mainly due to surface roughness and, more importantly, residual stress, as fatigue cracks initiate at mid-thickness, corresponding to the location of the maximum measured tensile residual stress. Moreover, microstructural refinement and defects from strain localization after punching significantly affect post-punching HCF performance. The optimization of punching parameters to balance the hardening benefits with minimal defect and sub-grain formation can improve fatigue performance. These insights offer strategies to enhance fatigue performance of HSLA steel in heavy-duty truck chassis components.

 This study compares the role of microstructure on post-punching fatigue properties in three advanced high-strength steels (AHSSs) with a high-strength low-alloy (HSLA) steel commonly used in heavy-duty truck chassis. Microstructure characterization, tensile testing, high cycle fatigue (HCF) testing, fatigue crack growth rate (FCGR) testing, and neutron residual stress measurements are conducted. Punching significantly alters the microstructure, causing refinement, sub-grain formation, defect creation, tensile residual stresses, and a work-hardened shear-affected zone around, and a rough fracture zone, inside the punched hole. At 10⁵ cycles, the HCF performance is primarily governed by the fatigue crack growth resistance of the as-rolled microstructure, with minimal sensitivity to punching. However, near the fatigue limit, post-punching fatigue failure is strongly related to strain localization when significant strength difference exists between microconstituents (e.g., martensite and ferrite). Strain localization also promotes sub-grain formation, reducing the local threshold stress intensity factor range (∆K_th), thus facilitating fatigue crack initiation. In microstructures with smaller strength differences (e.g., ferrite and bainite), sub-grains, together with surface roughness and residual stress, contribute significantly to the post-punching fatigue limit reduction. These findings provide insights into mechanisms of punching-induced fatigue performance degradation, offering potential strategies to optimize fatigue performance of AHSS for new applications.