Yanqing Su

Aluminum–copper alloys, typified by ZL205A (AlCu5MnTiCdV, GB/T 1173), pose severe casting challenges due to their wide solidification interval, poor fluidity, and susceptibility to gas porosity and shrinkage defects. This study introduces a negative-pressure frozen sand mold casting process as a solution to these challenges and systematically quantifies its thermal and microstructural advantages over conventional resin sand and atmospheric frozen sand casting. Using inverse heat conduction analysis of multi-point thermocouple data, the interfacial heat transfer coefficient (IHTC) was quantitatively determined for 14 experimental conditions varying in mold type, pressure level, moisture content (2–6 wt.%), and initial freezing temperature (−20 to −40 °C). The results demonstrate that negative pressure combined with frozen sand molds (6 wt.% moisture, −40 °C) produces the highest active-period average IHTC of 236 W/(m²·K), representing a 2.8-fold increase over atmospheric frozen sand casting and a 1.6-fold increase over negative-pressure resin sand casting. This enhanced thermal driving force promotes grain refinement and effective gas removal, yielding superior mechanical properties: tensile strength of 200.6 MPa and elongation of 9.3% in the as-cast state, and 478.5 MPa and 7.4% after T6 heat treatment. Using the optimized process parameters, a thin-walled cabin component (Ø180 mm × 300 mm, minimum wall thickness 3 mm) was successfully fabricated without defects. These findings establish quantitative process–thermal–microstructure–property relationships for negative-pressure frozen sand casting of wide-solidification-interval aluminum alloys.

The control of chemical composition remains the single most formidable challenge restricting the industrial adoption of Additive Manufacturing (AM) for high-volatility-contrast multicomponent alloys like TiAl. The extreme energy density utilized in powder bed fusion or directed energy deposition processes, exemplified here by the electron beam directed energy deposition (EB-DED) technique, triggers severe selective evaporation of volatile components, fundamentally compromising alloy design fidelity. This study presents a paradigmatic shift in AM process modeling by establishing a robust composition-prediction framework. We first analytically establish that Al selective evaporation is the dominant mechanism (Al vapor pressure is 1-2 orders of magnitude higher than Ti), and that the mass loss rate is kinetically controlled by the gas-liquid interface reaction ( K _m << β _m ). To resolve the non-uniformity inherent to the AM melt pool, we developed an advanced model that integrates fluid-dynamic predictions of melt flow with interface kinetics. This model uniquely accounts for the formation of distinct surface regimes: flow-induced solute-enriched regions (laminar flow) and a surrounding compositionally uniform region (vortex flow). The resulting model, incorporating both the non-uniform temperature field and the heterogeneous solute concentration, successfully forecasts the final alloy chemistry. The model demonstrates exceptional predictive robustness, achieving an experimental deviation of less than 2 at. % when compared to measurements. Based on this validated framework, an optimal processing window (20– 30 mA beam current; 1.5– 2.5 mm/s deposition speed) is defined. This work provides the necessary theoretical foundation and predictive toolset for achieving high compositional fidelity in the additive manufacturing of all alloys susceptible to severe elemental volatilization.

Titanium alloys are in great demand in the field of materials science and engineering due to their enhanced mechanical properties, high corrosion resistance and biocompatibility. Heat treatment of titanium alloys is necessary to achieve the maximum level of mechanical and other properties due to a certain effect on phase transformations. However, determining the optimal type of heat treatment for a particular alloy is not an easy task, requiring a lot of time and experiments. The generally accepted approach is based on the diagnosis of the material after its processing in order to establish the final microstructure and properties, which is a laborious process. Optimization of heat treatment of a metastable β-titanium alloy obtained by vacuum-arc remelting is considered. Using the in-situ observation of phase transformations in an alloy at high temperatures performed on a laser scanning confocal microscope, the optimal temperature of its heat treatment was determined to achieve maximum material strength, which was 800 оC. The hypothesis was confirmed in the course of studies using scanning electron and transmission electron microscopy, as well as in the process of tensile testing of alloys. The experimental results showed that the primary α-phase was released along several orientations during the solid solution treatment, and the nanoscale secondary α-phase, released during the aging treatment, was evenly distributed. The strength test results showed that after processing a solid β-solution at a temperature of 800 оC, compared with other temperatures, the alloy has the highest tensile strength, which confirms the reliability of using the in-situ observation. The practical significance of this study lies in the possibility of applying its results to optimize the heat treatment of β-titanium alloys.

As a candidate material for turbine blades in aerospace engines, Nb-Si-based alloys have attracted significant research attention due to their high melting point and low density. However, their poor high-temperature oxidation resistance limits practical applications. Different alloying elements, including Ti, Mo, and Hf, were added to Nb-Si-based alloys to study the microstructural evolution of alloys. Additionally, the oxidation behavior and the oxidation kinetics of different alloys, as well as the morphology and microstructure of oxide scale and interior alloys at 1523 K from 1 h to 20 h were analyzed systematically. The current findings indicated that the Mo element is more conducive to promoting the formation of high-temperature precipitates of β-Nb₅Si₃ than the Ti and Hf elements. Inversely, the Ti element tends to cause the transition from high-temperature-phase β-Nb₅Si₃ to low-temperature-phase α-Nb₅Si₃, while the Hf element improves the appearance of the γ-Nb₅Si₃ phase but inhibits the other phases and refines the primary Nbss effectively. Noteworthily, compared with the oxidation weight gain of different alloys, Nb-16Si-20Ti-5Mo-3Hf-2Al-2Cr alloy has excellent high-temperature oxidation resistance, in which the oxidation products are TiNb₂O₇, Nb₂O₅, SiO₂, TiO₂, and HfO₂. It can be determined that in the oxidation process, the Ti element will preferentially form an oxide film of TiO₂, thereby wrapping around the matrix phases, protecting the matrix, and improving the antioxidant capacity, while the Hf element can form an infinite solid solution with the matrix and consume the small number of oxygen atoms entering the matrix, so as to achieve the effect of improving the oxidation resistance.

The potential of hydrogen plasma arc technology for the efficient deoxygenation and recycling of titanium alloy scrap is explored. The results of thermodynamic analysis reveal that hydrogen plasma is suitable for oxygen removal. The intermediate stages of the deoxygenation process are sequentially analyzed, showing that the hydrogen plasma arc primarily facilitated the reduction and dissolution of oxides as well as eliminated interstitial oxygen. The experimental results demonstrate that the deoxygenation dynamics are governed by the interaction between the hydrogen partial pressure and the surface area of the hydrogen plasma arc irradiation range. Furthermore, the results of the detailed characterization of the recycled titanium alloys indicate that hydrogen plasma arc technology not only reduces the oxygen content to levels compliant with industrial standards for Ti‐6Al‐4 V but also substantially enhances the tensile strength and ductility of Ti‐6Al‐4 V compared with those of conventional cast titanium alloys. The results of microstructural analysis show that hydrogen tends to concentrate in the β phase, whereas oxygen is predominantly found in the α phase. These findings highlight the effectiveness of hydrogen plasma arc technology, which offers environmental benefits and improves the performance of the resulting material, indicating its promise as a solution for titanium recycling.

A high Fe content easily produces Fe-rich phases with a harmful morphology, resulting in a huge detrimental effect on the properties and recycling ability of Al-Si alloys. Therefore, finding ways to effectively transform Fe-rich phases to form a beneficial phase or shape is of great significance. Accordingly, Al-Si-based alloys with Fe contents ranging from 0.1 wt.% to 2.0 wt.% were modified by different Mn additions. Moreover, experiments combined with simulations were utilized to comprehensively analyze the mechanism of Mn on the morphology and microstructural evolution of Fe-rich phases from different perspectives. The current findings determine that adding different Fe contents changes the phase-transition reactions in alloys. Without Mn, and by increasing the Fe content from 0.1 wt.% to 2.0 wt.%, the Fe-rich phases gradually convert from a skeleton-shaped α-Al₈Fe₂Si (β-Al₉Fe₂Si₂ with a fibrous (0.5 wt.%), needle-like (1.0 wt.%) and plate-like shape without curvatures (2.0 wt.%). The maximum length and mean aspect ratio increase from 12.01 μm to 655.66 μm and from 1.96 to 84.05, while the mean curvature decreases from 8.66 × 10⁻² μm⁻¹ to 8.25 × 10⁻⁴ μm⁻¹. The addition of 0.35 wt.% Mn promotes a new Chinese-character and petal-shaped α-Al₁₅(FeMn)₃Si₂, with an atomic ratio of Fe and Mn of 1:1 when the Fe content is lower than 0.5 wt.%, while it transforms to β-Al₁₅(FeMn)₃Si₂ with an atomic ratio of 5:1, presenting as a refined plate-like shape with a certain curvature, as the Fe content increases to 2.0 wt.%. Mn alters the phase reactions and increases the threshold of the Fe content required for β-Al₁₅(FeMn)₃Si₂, limiting the formation and growth of them simultaneously in time and space. The enrichment of Mn atoms and solute diffusion at the growth front of β-Al₁₅(FeMn)₃Si₂, as well as the strong atomic-binding ability, can deflect the growth direction of β-Al₁₅(FeMn)₃Si₂ for it to have a certain curvature. Additionally, the enriched Mn atoms easily form α-Al₁₅(FeMn)₃Si₂ and cause the long β-Al₁₅(FeMn)₃Si₂ to be broken and refined to further reduce the damages caused to the alloy’s performance. Ultimately, the maximum length and mean aspect ratio can be effectively reduced to 46.2% and 42.0%, respectively, while the mean curvature can be noticeably increased by 3.27 times with the addition of Mn.

Nano‐lamellar Ti ₃ Al/TiAl (α ₂ /γ) alloy with significantly improved nanohardness was prepared using dual‐wire‐fed electron beam‐directed energy deposition (EB‐DED) in this study. This investigation focused on the evolution of the colony shape and lamellar thickness of the Ti‐43Al lamellar alloy at different heights. Nanoindentation tests were employed to evaluate deformation resistance, and numerical simulations provided deeper insights into the deposition process. The results indicate that the colonies are mostly columnar, except for a few equiaxed colonies at the top. Rapid cooling significantly refines the α ₂ lamellae, resulting in an average spacing of 218 nm and thickness of 41 nm. Additionally, substantial microstrain and a nonequilibrium Al distribution lead to a significant generation of γ variants, refining the γ lamellae to 57 nm. Abundant γ/γ’ and α ₂ /γ interfaces, along with fine α ₂ phases, contribute to improved deformation resistance. Consequently, the nano‐lamellar TiAl alloy exhibited a notable 32% increase in nanohardness (8.3 GPa) while maintaining a similar modulus (197 GPa) to conventionally prepared alloys. This study holds significant promise for advancing high‐performance TiAl alloys through the dual‐wire‐fed EB‐DED process.

In this study, Ti-6Al-4V matrix composites reinforced with TiB ceramic whiskers were in situ synthesized and hydrogenated using the melt hydrogenation technique (MHT). The effects of MHT on the microstructure evolution and hot compression behavior of the composites were investigated by optical microscopy (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Hot compression tests were performed at strain rates of 0.1/s, 0.01/s, and 0.001/s and temperatures of 800 °C, 850 °C, and 900 °C; the hot workability of composites significantly improved after hydrogenation, for example, the 900 °C peak flow stress of hydrogenated composites (43 MPa) decreased by 53.76% compared with that of unhydrogenated ones (93 MPa) at a strain rate of 0.01/s. Microstructural observations show that MHT can effectively facilitate the dispersion of TiB whiskers and induce the α/β lath refinement of the matrix in our as-cast hydrogenated composite. During hot compression, MHT effectively promoted the as-cast composite microstructure refinement, accelerated the dynamic recrystallization (DRX) generation, and reduced the stress concentration at the interface between the reinforcement and matrix; in turn, the hydrogenated composites presented low peak stress during hot compression.

Improving the hot workability and reducing the processing cost are critical steps to expanding the application of TiC/Ti-6Al-4V composites. This study employed melt hydrogenation to fabricate TiC/Ti-6Al-4V composites under a mixed atmosphere of hydrogen and argon. Experimental results indicated that hydrogen had an obvious influence on the growth and morphology of eutectic TiC particles, and the size of eutectic TiC and primary β grains was significantly increased. As a result, large-sized eutectic TiC was distributed along the grain boundaries of primary β grains. Hot compression results showed that the peak flowing stress of composites was reduced by hydrogen, which resulted in an improvement of hot workability, especially in the (α + β) phase region, and the best results were obtained at 900 °C/0.01 s⁻¹, at which the peak stress decreased from 241 ± 9 to 190 ± 8 MPa (a decrease of 21.2%). Inspection of the microstructure after hot compression showed that hydrogen improved the proportion of DRX grains from ~62.7% to ~83.2%, and hydrogen also decreased the density of dislocations, which were attributed to hydrogen accelerating atomic diffusion. Enhanced hot workability resulted from hydrogen atoms decreasing the atomic bonding force of the titanium matrix, hydrogen reducing the β/(α + β) transition temperature, the higher proportion of DRX, and the higher mobility of dislocations. It is expected that the findings of this study may support the development of a simple and efficient method to reduce the processing cost of TiC/Ti-6Al-4V composites.