Variations in the Mn / N ratio greatly affect the formation of reformed austenite and precipitation and two-phase ratio in duplex stainless steel (DSS) during the welding thermal cycle. Therefore, in vestigating the influence of the Mn / N ratio on the multipass welding heat-affected zone (HAZ) microstructure is beneficial for enhancing the comprehensive mechanical properties of the HAZ of low-nickel-type DSS thick plates. This study comparatively investigated the effect of Mn / N ratio on the microstructure evolution and mechanical properties of DSS in the solution-treated and multi-pass welded HAZ, with 2205 DSS as a reference. The higher nitrogen content (Mn / N ratio = 3.77) resulted in substantial partial transformed austenite formation with a high volume fraction of 58.7%, yielding a tensile strength of 836 MPa and an elongation of 39.5%, indicating its high strength and plasticity. As the Mn / N ratio increases to 17.80, an increase in the Widmanstatten austenite (WA) amount caused cellular Cr2N formation at the delta/gamma phase interfaces and dislocation walls in ferrite for HAZ, considerably decreasing elongation compared to the solid solution state. For DSS with a high Mn / N ratio of 65.91, the grain boundary austenite (GBA) amount initially increased and then decreased, while the intragranular austenite (IGA) and WA amounts increased in HAZ, inhibiting Cr2N precipitation and reducing ductility and strength. Compared with the 2205 DSS (Mn / N ratio = 3.06), the fracture surfaces of DSS with lower Mn / N ratios exhibited brittle fracture. The combined strengthening effect of the high-nitrogen solid solution and Cr2N precipitation on the austenite phase increased the hardness difference between the two phases in the HAZ of the DSS with Mn / N ratio of 3.77. In addition, the formation of some large (Cr, Mn)O inclusions promoted crack initiation to some extent, lowering the impact energy to 24.2 J. High Mn / N ratio led to more GBA and WA segmented ferrite refinement, as well as the formation of small amounts of IGA, which hindered crack propagation. Moreover, the precipitation of a small amount of the sigma phase increased the hardness of ferrite, reduced the hardness difference between the two phases, and increased the impact energy to 134.8 J.
In high-quality steel production, the final stage of secondary refining typically involves a ladle holding period to facilitate the flotation and removal of inclusions. This improves the cleanliness of the molten steel. This study investigated the evolution of inclusion number density and area fraction during the ladle holding period by conducting industrial trial on composition adjustment via the sealed argon bubbling refining process of steel plate hot rolled commercial (SPHC) low-carbon steel. The results indicated that inclusions would be removed by floating during the ladle holding period. The inclusion number density and area fraction decreased from 52.8 mm(-2) and 279 & times; 10(-6) at the start of the holding period to 22.1 mm(-2) and 148 & times; 10(-6) after 15 min, respectively. To clarify the movement and removal of inclusions, a numerical model was developed based on a discrete phase model, incorporating multiphase flow and heat transfer during the ladle holding period. The calculations revealed the following conclusions: (1) inclusions smaller than 10 mu m primarily followed the molten steel's fluid flow, with removal rates of 51.6% and 66.7% after 900 and 1800 s, respectively; (2) inclusions with a diameter of 100 mu m were influenced by the molten steel's fluid flow and their buoyancy, achieving removal rates of 70.2% and 89.2% after 900 and 1800 s, respectively; (3) inclusions with a diameter of 1000 mu m primarily underwent self-floating, reaching an approximately 100% removal rate after 120 s.
As a key structural material for aviation, aerospace, and major equipment, the lightweight and high-performance development of ultra-high strength (UHS) steel is essential for the advancement of related fields. However, traditional UHS steels have long faced critical challenges: the difficulty in achieving a synergistic combination of high strength, high toughness, good weldability, and corrosion resistance, along with the constraint of high alloy costs that limit their widespread application. To address these issues, we systematically review the development history, composition, microstructure, and performance characteristics of traditional UHS steels, including low-alloy UHS, secondary hardening, maraging, and precipitation-hardening stainless steels. We also review the research progress in developing novel UHS steels through two main pathways: the regulation of composite nanoprecipitates and the design of multiphase composite microstructures. Special attention is paid to exploring the potential and preliminary practices of the "hybrid"design concept, which breaks the boundaries of traditional steel classifications and integrates multiple strengthening mechanisms to develop cost-effective, high-performance, and easi-ly weldable "all-round"UHS steels. The current research status regarding the evolution laws of precipi-tates and strengthening mechanisms in UHS steels is detailed. Finally, the future development directions of UHS steels are proposed, particularly regarding the synergistic control of complex precipitates, high-temperature stability, and industrial application.
Nano-multilayers composed of immiscible metals have been widely investigated over the past decades due to their exceptional microstructural stability. The Cu/Ta system, which is also immisci ble, is particularly notable because Ta offers higher melting point, strength, and superior wear and corrosion resistance compared to metals like Nb. Cu/Ta nano-bilayer films are extensively used in the electronics industry for semiconductors, microelectronic devices, optical systems, and magnetic applications. However, the presence of interfaces and the distinct deformation responses of the Cu and Ta layers during processing, fabrication, and service conditions make mechanical deformation and subsequent failure inevitable, potentially compromising the performance of microscale devices. Therefore, understanding the deformation mechanisms and enhancing the mechanical strength of Cu/Ta nano-bilayer films at the microscopic scale is essential. In this study, molecular dynamics simulations were employed to investigate the tensile behavior and deformation mechanisms of Cu/Ta nano-bilayer films. In addition, the effects of Al and W doping in the Cu layer on the deformation behavior were analyzed. The results indicate that tensile loading direction significantly influences the plastic deformation mode. When the bilayer films were stretched parallel to the interface, both Cu and Ta layers exhibited sequential plastic deformation. In contrast, when the loading was applied perpendicular to the interface, only the Cu layer deformed plastically, while the Ta layer remained elastically constrained throughout. Doping with Al or W atoms enhanced the overall hardness and yield strength of the nano-bilayer films. Furthermore, W doping induced stacking faults in both loading directions when stretched parallel to the interface, and a martensitic transformation from fcc to bcc structure occurred in the Cu layer under perpendicular tension.
Laser powder bed fusion (LPBF) has emerged as a promising additive manufacturing technique for producing high-performance 304L austenitic stainless steel, which is widely used in the aerospace and automotive industries and in biomedical engineering because of its excellent mechanical properties, corrosion resistance, and high-temperature stability. However, the mechanical properties of LPBF-fabricated materials are influenced by the complex interplay of various process parameters, includ-ing laser power, scanning speed, layer thickness, and hatch spacing. Traditional trial-and-error meth-ods for process optimization are costly and time-consuming, and they often fail to precisely control the material's microstructure, resulting in suboptimal performance. Therefore, there is a pressing need to adopt advanced approaches to systematically optimize the LPBF process and ensure reliable, high-performance outcomes. This study proposes an innovative hybrid intelligent framework that integrates Stacking-ensemble learning, interpretable machine learning techniques (Shapley additive explanation (SHAP) method), and multiobjective optimization (nondominated sorting genetic algorithm II-technique for order preference by similarity to ideal solution (NSGA-II-TOPSIS)). The primary objective is to devel-op an all-encompassing framework for the simultaneous prediction and optimization of ultimate tensile strength, yield strength, and elongation in LPBF-fabricated 304L stainless steel. The framework is de-signed not only to improve the accuracy of predicting the mechanical properties but also to provide a clear understanding of the influence of process parameters on material behavior. The Stacking-ensemble model demonstrates superior performance in terms of accuracy, generalization, and stability compared to individual machine learning models, such as random forest (RF), gradient boosting decision tree (GB-DT), and extreme gradient boosting (XGBoost). SHAP analysis has revealed that laser power plays a critical role in determining the mechanical properties of the material, making it the most important factor to consider in the optimization process. The multiobjective optimization approach facilitates the identifica-tion of the optimal process parameters, resulting in a balanced strength-ductility trade-off that is crucial for practical applications. Experimental validation was conducted to confirm the effectiveness of the pro-posed framework. The optimized LPBF samples exhibited refined, uniform cellular substructures, in-creased dislocation density, and the formation of twin boundaries, which significantly improved the material's mechanical properties.
