Abstract:Wire arc additive manufacturing (WAAM) has emerged as a promising approach for fabricating large-scale components. However, conventional WAAM still faces challenges in optimizing microstructural evolution, minimizing additive-induced defects, and alleviating residual stress and deformation, all of which are critical for enhancing the mechanical performance of the manufactured parts. Integrating interlayer friction stir processing (FSP) into WAAM significantly enhances the quality of deposited materials. However, numerical simulation research focusing on elucidating the associated thermomechanical coupling mechanisms remains insufficient. A comprehensive numerical model was developed to simulate the thermomechanical coupling behavior in friction stir-assisted WAAM. The influence of post-deposition FSP on the coupled thermomechanical response of the WAAM process was analyzed quantitatively. Moreover, the residual stress distribution and deformation behavior under both single-layer and multi-layer deposition conditions were investigated. Thermal analysis of different deposition layers in WAAM and friction stir-assisted WAAM was conducted. Results show that subsequent layer deposition induces partial remelting of the previously solidified layer, whereas FSP does not cause such remelting. Furthermore, thermal stress and deformation analysis confirm that interlayer FSP effectively mitigates residual stresses and distortion in WAAM components, thereby improving their structural integrity and mechanical properties.

Abstract:A dual-phase synergistic enhancement method was adopted to strengthen the Al-Mn-Mg-Sc-Zr alloy fabricated by laser powder bed fusion (LPBF) by leveraging the unique advantages of Er and TiB₂. Spherical powders of 0.5wt% Er-1wt% TiB₂/Al-Mn-Mg-Sc-Zr nanocomposite were prepared using vacuum homogenization technique, and the density of samples prepared through the LPBF process reached 99.8%. The strengthening and toughening mechanisms of Er-TiB₂ were investigated. The results show that Al₃Er diffraction peaks are detected by X-ray diffraction analysis, and texture strength decreases according to electron backscatter diffraction results. The added Er and TiB₂ nano-reinforcing phases act as heterogeneous nucleation sites during the LPBF forming process, hindering grain growth and effectively refining the grains. After incorporating the Er-TiB₂ dual-phase nano-reinforcing phases, the tensile strength and elongation at break of the LPBF-deposited samples reach 550 MPa and 18.7%, which are 13.4% and 26.4% higher than those of the matrix material, respectively.

Abstract:Laser powder bed fusion (LPBF) is highly suitable for forming 18Ni300 mold steel, thanks to its excellent capability in manufacturing complex shapes and outstanding capacity for regulating microstructures. It is widely used in fields such as injection molding, die casting, and stamping dies. Adding reinforcing particles into steel is an effective means to improve its performance. Nb/18Ni300 composites were fabricated by LPBF using two kinds of Nb powders with different particle sizes, and their microstructures and properties were studied. The results show that the unmelted Nb particles are uniformly distributed in the 18Ni300 matrix and the grains are refined, which is particularly pronounced with fine Nb particles. In addition, element diffusion occurs between the particles and the matrix. The main phases of the base alloy are α-Fe and a small amount of γ-Fe. With the addition of Nb, part of the α-Fe is transformed into γ-Fe, and unmelted Nb phases appear. The addition of Nb also enhances the hardness and wear resistance of the composites but slightly reduces their tensile properties. After aging treatment, the molten pools and grain boundaries become blurred, grains are further refined, and the interfaces around the particles are thinned. The aging treatment also promotes the formation of reverted austenite. The hardness, ultimate tensile strength, and volumetric wear rate of the base alloy reach 51.9 HRC, 1704 MPa, and 17.8×10^-6 mm³/(N·m), respectively. In contrast, the sample added with fine Nb particles has the highest hardness (56.1 HRC), ultimate tensile strength (1892 MPa) and yield strength (1842 MPa), and the volume wear rate of the sample added with coarse Nb particles is reduced by 90% to 1.7×10^-6 mm³/(N·m).

Abstract:Copper manufactured by laser powder bed fusion (LPBF) process typically exhibits poor strength-ductility coordination, and the addition of strengthening phases is an effective way to address this issue. To explore the effects of strengthening phases on Cu, Cu-carbon nanotubes (CNTs) composites were prepared using LPBF technique with Cu-CNTs mixed powder as the matrix. The formability, microstructure, mechanical properties, electrical conductivity, and thermal properties were studied. The result shows that the prepared composites have high relative density. The addition of CNTs results in inhomogeneous equiaxed grains at the edges of the molten pool and columnar grains at the center. Compared with pure copper, the overall mechanical properties of the composite are improved: tensile strength increases by 52.8% and elongation increases by 146.4%; the electrical and thermal properties are also enhanced: thermal conductivity increases by 10.8% and electrical conductivity increases by 12.7%.

Abstract:To explore the formation mechanism of anisotropy in Ti-6Al-4V alloy fabricated by selective laser melting (SLM), the compressive mechanical properties, microhardness, microstructure, and crystallographic orientation of the alloy across different planes were investigated. The anisotropy of SLM-fabricated Ti-6Al-4V alloys was analyzed, and the electron backscatter diffraction technique was used to investigate the influence of different grain types and orientations on the stress-strain distribution at various scales. Results reveal that in room-temperature compression tests at a strain rate of 10^-3 s^-1, both the compressive yield strength and microhardness vary along the deposition direction, indicating a certain degree of mechanical property anisotropy. The alloy exhibits a columnar microstructure; along the deposition direction, the grains appear equiaxed, and they have internal hexagonal close-packed (hcp) α/α' martensitic structure. α' phase has a preferential orientation approximately along the <0001> direction. Anisotropy arises from the high aspect ratio of columnar grains, along with the weak texture of the microstructure and low symmetry of the hcp crystal structure.

Abstract:The key parameters that characterize the morphological quality of multi-layer and multi-pass metal laser deposited parts are the surface roughness and the error between the actual printing height and the theoretical model height. The Taguchi method was employed to establish the correlations between process parameter combinations and multi-objective characterization of metal deposition morphology (height error and roughness). Results show that using the signal-to-noise ratio and grey relational analysis, the optimal parameter combination for multi-layer and multi-pass deposition is determined as follows: laser power of 800 W, powder feeding rate of 0.3 r/min, step distance of 1.6 mm, and scanning speed of 20 mm/s. Subsequently, a Genetic Bayesian-back propagation (GB-BP) network is constructed to predict multi-objective responses. Compared with the traditional back propagation network, the GB-back propagation network improves the prediction accuracy of height error and surface roughness by 43.14% and 71.43%, respectively. This network can accurately predict the multi-objective characterization of morphological quality of multi-layer and multi-pass metal deposited parts.

Abstract:Wire arc additive manufacturing (WAAM) holds significant application value in the aerospace field, but the instability of heat input leads to prominent issues such as poor geometric conformity and numerous internal defects in aluminum alloy thin-walled components. To address the restrictions of traditional methods in multi-physics coupling optimization, this study proposed a data-driven solution by constructing a dataset of process parameters (current, scanning speed and wire feed rate) and forming quality (path/interlayer wall thickness consistency and porosity). A back propagation (BP) neural network model was established and optimized using the genetic algorithm (GA), combined with the non-dominated sorting genetic algorithm II (NSGA-II) for multi-objective optimization. The results show that the optimized GA-BP model significantly improves the prediction accuracy of path wall thickness consistency and porosity, but its optimization effect on interlayer wall thickness consistency prediction is restricted. Four types of optimization strategies are proposed based on the 50 Pareto solution sets obtained through NSGA-II, and validation tests indicate the model prediction error of 8.89%, accurately achieving the collaborative optimization of forming quality indicators.

Abstract:The binder jet 3D printing (BJ3DP) process is currently a research hotspot. Generally, the powder bed printing process requires spherical powders, which limits the preparation and fabrication of some high-entropy alloy (HEA) powders with significant melting point differences. This study mainly focuses on the BJ3DP-sintering behavior of non-spherical particles. The results show that the non-spherical AlTiCrNiCu low-density HEA powder with bcc structure was prepared via mechanical alloying, with a particle size distribution of 6.72–67.52 μm and an average particle size of 21.17 μm, which complies with the process requirements of BJ3DP. The results of orthogonal experiment indicate that under the condition of ensuring green part morphology integrity, the higher green density (approximately 44.4%) is achieved in BJ3DP with a binder saturation of 70%, a layer thickness of 120 μm, and a powder spreading speed of 5 mm/s. After sintering at 1190 ℃ for 4 h, the relative density of the sintered sample reaches 91.6%. The AlTiCrNiCu low-density HEA exhibits a multiphase structure consisting of B2 phase as matrix, along with bcc, fcc, and a small amount of L2₁ phase. The AlTiCrNiCu low-density HEA exhibits high compressive properties, with a yield strength and compressive strength of approximately 840 and 960 MPa, respectively.

Abstract:Due to the excellent mechanical properties and outstanding biocompatibility, TC4 titanium alloy has been widely used in the aerospace and medical device field. Laser additive manufacturing (LAM) is an important technique for fabricating titanium alloys. The presence of large numbers of columnar crystals and acicular martensite in additive manufactured TC4 titanium alloy results in anisotropy and reduced plasticity. In this work, molybdenum (Mo) was added to tailor the microstructure and properties of additive manufactured TC4 titanium alloy, with a specific focus on the effect of Mo content. The results show that an appropriate Mo content can effectively refine the grains. Furthermore, with the addition of Mo element, the TiAl₃ phase is gradually precipitated from the alloy matrix, and its content is increased with the increase in Mo content. When the Mo content reaches 8wt%, a fine and dispersed lamellar structure is distributed in the alloy, and the β-phase content increases sharply. In addition, the maximum degrees of grain refinement and dislocation density are obtained. As Mo content increase from 0 to 10wt%, the tensile strength, hardness and corrosion resistance of the alloy increase first and then decrease, whereas the elongation shows the opposite trend. Concurrently, the Young's modulus gradually decreases. When Mo content is 8wt%, the alloy achieves the best mechanical properties: a tensile strength of 1065.6 MPa, an elongation of 11.5% and Young's modulus of 55.4 GPa. Additionally, its corrosion resistance is improved. Overall, TC4-8Mo sample has excellent mechanical properties and superior corrosion resistance, demonstrating high potential for use in human medical implant.

Abstract:Laser directed energy deposition technique was used to prepare 1vol% B₄C/Ti6Al4V composites with varying process parameters to investigate the effects of process parameters (laser power and scanning speed) on the microstructure and mechanical properties of laser melting deposited B₄C/Ti6Al4V composites and to uncover the relationship between the microstructure and mechanical properties of the composites under different process parameters. The results show that the reinforcement of the composites consists of undissolved B₄C, TiB, and TiC since the additional B₄C particles are partially dissolved. As the laser power rises or the scanning speed drops, the quantity of undissolved B₄C diminishes. Simultaneously, the quantities of in-situ formed TiB and TiC increase, and the grain size of β-Ti gradually grows. As laser power increases, the hardness of composites rises from 382.6 HV_0.5 to 406.5 HV_0.5. As scanning speed decreases, the hardness of composites likewise rises. The wear rate achieves a minimum value of 8.517×10^–4 mm³·N^–1·m^–1 when the laser power is 1200 W and the scanning speed is 500 mm/min. When the laser power drops or the scanning speed rises, the tensile strength rises, reaching a maximum value of 1180.4±6.2 MPa. The effect of grain refinement is responsible for the increase in tensile strength.

Abstract:Copper and its alloys exhibit great application potential in the field of thermal management due to their high thermal conductivity, while 3D printing technique serves as the key to expand their applications. Debinding technique is one of the main factors restricting the development of 3D printing technique. Quantitative indicators for the thermal debinding quality of binder jetting (BJ) printed green parts are established. The binder distribution inside the BJ-printed green parts is optimized, and the optimization results of different strategies are compared and verified through experiments. The results show that the optimal parameters for the optimized area of the binder distribution inside the green part are S₁/S₀=0.6, R₁/R₀=0.4, and R₂/R₀=0.8. The shape-optimized result of the binder distribution also shows a flared shape, which is narrower at the top and wider at the bottom. After printing verification, it is found that there is no significant difference in the printing effect. Analysis of the compressive strength of different binder distribution strategies with the same overall binder content reveals that the shape optimization improves the performance by 10% compared with that of the non-optimized case.

Abstract:Based on the pressure-controlled solid-state friction extrusion additive equipment, additive friction stir deposition (AFSD) process tests of AA7075-T6 were carried out to explore the effect of travel speed on the microstructure and mechanical properties of the deposited layer. The results show that well-formed and defect-free AA7075-T6 deposited specimens were fabricated at a spindle rotational speed of 300 r/min and travel speeds of 100 and 150 mm/min. The deposited area exhibits a fully dense and fine equiaxed grain microstructure. The average grain size is significantly reduced compared with that of the original feed rod (36.33±1.99 μm). Due to strong thermal-force coupling friction and extrusion, the recrystallization area fraction of the deposited area is more than 80%, while the hardness and tensile properties exhibit uniform distribution. As the travel speed increases from 100 mm/min to 150 mm/min, the hardness of the top of deposited layers increases from 120 HV to 141 HV, corresponding to 69.8% and 82.1% of that of the feed rod, respectively. The corresponding tensile strength along transverse direction increases from 416.5 MPa to 477.5 MPa, and the average elongation improves from 6.25% to 9.25%. However, the tensile strength of bottom deposited layers increases from 397.0 MPa to 414.0 MPa, and the average elongation increases from 11.5% to 12.25%. The largest tensile strength and elongation of deposited layers can reach 78.1% and 80.6% of those of the feed rod, respectively. The fracture mode of the deposited specimens is characterized by ductile fracture. This indicates that the increase in travel speed is favorable to the improvement of strength and plasticity of the AFSD-treated layers.

Abstract:The influence of aging temperature on the microstructure and mechanical properties of TB6 titanium alloy fabricated via selective laser melting (SLM) was investigated. The results show that the microstructure of SLMed TB6 titanium alloy predominantly consists of columnar β phase and equiaxed α phase, and the precipitation of lamellar or granular α_P phase and grain boundary α_GB phase occurs after solution treatment and aging. With the increase in aging temperature, the α phase is gradually diminished in size, the volume fraction of lamellar α phase is decreased, and the amount of granular α_P phase is increased. The tensile test results indicate that as the aging temperature increases, the tensile strength of the material decreases while its plasticity is significantly improved. The tensile strength of samples decreases from 1386.22 MPa (aged at 520 ℃) to 1129.00 MPa (aged at 560 ℃), whereas its elongation increases from 7.1% to 10.5%. It is concluded that changes in the morphology and size of the α phase significantly impact the strength and plasticity of the alloy. Reducing the volume fraction of lamellar α_L phase and increasing that of granular α_P phase improve the strong-plasticity matching. Elevating the aging temperature can decrease the volume fraction of lamellar α_L phase and enhance the plasticity of the material.

Abstract:The additive manufacturing of titanium/aluminum dissimilar metals can achieve the integration of lightweight and high strength. However, joining these two metals poses significant challenges. In this study, 6061 aluminum alloy was used as the substrate, and TC4 welding wire was employed as the cladding material for the laser additive manufacturing of titanium-aluminum dissimilar metals. With the aim of controlling the interfacial fusion ratio and minimizing intermetallic compound formation to enhance the interface performance, the process parameters were optimized. The optimal parameters were determined through single-factor experiments and nonlinear regression analysis. Under these parameters, the interfacial tensile strength of the aluminum/titanium dissimilar metal structure reaches to 60.67 MPa. The microstructural evolution of the dissimilar metal interface under different additive manufacturing layer numbers was analyzed. The results show that as the number of titanium alloy additive manufacturing layers increases, the brittle acicular compounds at the titanium-aluminum interface gradually diffuse, transform into granular particles and eventually aggregate into a continuous layer. The evolution of interfacial residual stress was analyzed through finite element simulation. The results indicate that the interfacial residual stress initially increases, then decreases, and finally rises sharply, with severe stress concentration observed at both ends of the weld bead.

Abstract:Due to their low density, good room-temperature plasticity, and excellent high-temperature toughness, refractory niobium alloys have been used in hot-end components in the aerospace field. However, niobium alloys prepared by traditional casting methods are difficult to process parts with complex geometries, and face problems such as long production time, high cost and high buy-to-fly ratio. The rapid development of additive manufacturing technique in recent years not only shortens production time and lowers cost but also achieves superior mechanical properties, presenting new opportunities for the further application of niobium alloys. To this end, this paper reviews the current state-of-the-art research on additive manufactured niobium alloys focusing on the laser and electron-beam additive manufacturing of two generations of typical niobium alloys (namely C-103 and Nb521), with specific attention to the control of their mechanical properties and microstructure. In addition, common types of niobium alloys and additive manufacturing techniques are briefly introduced. Finally, the review outlines future research directions and identifies remaining challenges for this field. By reviewing the current state of additive manufactured niobium alloys, this paper provides a reference for the further application of niobium alloys in the aerospace field for hot-end components with complex structures.

Abstract:The influence of Hf on the precipitation behavior of γ' phase and the subsequent tensile properties of a Ni-Cr-Mo alloy after long-term thermal exposure was investigated. The results reveal that the addition of Hf increases the average diameter of γ' phases after thermal exposure at 700 ℃ for 5000 h, which enhances the critical resolved shear stress required for dislocations to shear the γ' phases in the Ni-Cr-Mo alloy. Simultaneously, element Hf incorporated into the γ' phases increases the lattice mismatch between the γ' and γ phase, thereby strengthening the coherency strengthening effect. These two factors collectively contribute to the enhanced strength of the alloy. Thus, Hf alloying effectively improves the yield strength of the Ni-Cr-Mo alloy after thermal exposure at 700 ℃.

Abstract:Dissimilar AZ31B magnesium alloy and DC56D steel were welded via AA1060 aluminum alloy by magnetic pulse welding. The effects of primary and secondary welding processes on the welded interface were comparatively investigated. Macroscopic morphology, microstructure, and interfacial structure of the joints were analyzed using scanning electron microscope, energy dispersive spectrometer, and X-ray diffractometer (XRD). The results show that magnetic pulse welding of dissimilar Mg/Fe metals is achieved using an Al interlayer, which acts as a bridge for deformation and diffusion. Specifically, the AZ31B/AA1060 interface exhibits a typical wavy morphology, and a transition zone exists at the joint interface, which may result in an extremely complex microstructure. The microstructure of this transition zone differs from that of AZ31B magnesium and 1060 Al alloys, and it is identified as brittle intermetallic compounds (IMCs) Al₃Mg₂ and Al_l2Mg₁₇. The transition zone is mainly distributed on the Al side, with the maximum thickness of Al-side transition layer reaching approximately 13.53 μm. Incomplete melting layers with varying thicknesses are observed at the primary weld interface, while micron-sized hole defects appear in the transition zone of the secondary weld interface. The AA1060/DC56D interface is mainly straight, with only a small number of discontinuous transition zones distributed intermittently along the interface. These transition zones are characterized by the presence of the brittle IMC FeAl₃, with a maximum thickness of about 4 μm.

Abstract:Low-density short-duration pulsed current-assisted aging treatment was applied to the Ti-6Al-4V-0.5Mo-0.5Zr alloy subjected to different solution treatments. The results show that numerous α_p phases redissolve into the new β phase during the pulsed current-assisted aging process, and then the newly formed β phase is mainly transformed into the β_t phase, with occasional transition to new α_p phase, leading to a remarkable grain refinement, especially for the lamellar α_s phases. In comparison to conventional aging treatment, the pulsed current-assisted aging approach achieves a significant enhancement in strength without degrading ductility, yielding an excellent mechanical property combination: a yield strength of 932 MPa, a tensile strength of 1042 MPa, and an elongation of 12.2%. It is primarily ascribed to the increased fraction of β_t phases, the obvious grain refinement effect, and the slip block effect induced by the multiple-variant α_s colonies distributed within β_t phases.

Abstract:To mitigate the impact of interdiffusion reactions between the silicide slurry and Ta12W alloy substrate during vacuum sintering process on the oxidation resistance of the silicide coating, a micro-arc oxidation pretreatment was employed to construct a Ta₂O₅ ceramic layer on the Ta12W alloy surface. Subsequently, a slurry spraying-vacuum sintering method was used to prepare a Si-Cr-Ti-Zr coating on the pretreated substrate. Comparative studies were conducted on the microstructure, phase composition, and isothermal oxidation resistance (at 1600 ℃) of the as-prepared coatings with and without the micro-arc oxidation ceramic layer. The results show that the Ta₂O₅ layer prepared at 400 V is more continuous and has smaller pores than that prepared at 350 V. After micro-arc oxidation pretreatment, the Si-Cr-Ti-Zr coating on Ta12W alloy consists of three distinct layers: an upper layer dominated by Ti₅Si₃, Ta₅Si₃, and ZrSi; a middle layer dominated by TaSi₂; a coating/substrate interfacial reaction layer dominated by Ta₅Si₃. Both the Si-Cr-Ti-Zr coatings with and without the Ta₂O₅ ceramic layer do not fail after isothermal oxidation at 1600 ℃ for 5 h. Notably, the addition of the Ta₂O₅ ceramic layer reduces the high-temperature oxidation rate of the coating.

Abstract:The effect of microstructure with different twin boundary fraction on the corrosion behavior of GH3625 alloy tube in high-temperature (600–800 ℃) KCl-MgCl₂ molten salt was investigated using EBSD, XRD, SEM, and EDS. The results show that with the increase in annealing temperature, the proportion of annealing twin boundaries in the equiaxed grains of GH3625 alloy tube is increased. Consequently, the higher the proportion of twin boundaries in the alloy at the same corrosion temperature, the better its high-temperature resistance to KCl-MgCl₂ molten salt corrosion. Furthermore, as the temperature increases, the corrosion resistance of a given set of samples to KCl-MgCl₂ molten salt deteriorates. In addition, at a constant grain size, an increased fraction of annealing twin boundaries correlates with enhanced corrosion resistance of GH3625 alloy tubes in KCl-MgCl₂ molten salt at high temperature. This is mainly attributed to the excellent intrinsic corrosion resistance of high-density stable annealing twin boundaries, coupled with the fact that the triple junction containing twin boundaries breaks the connectivity of the original high-angle grain boundary network, thereby suppressing the corrosion of the grain boundaries.

Abstract:The combined effects of voltage, pulse frequency, duty cycle and processing time on the corrosion resistance of micro-arc oxidation coatings on TC4 titanium alloy were investigated using range analysis, with a subsequent objective of exploring the significance relationship of electrical parameters and their optimal combination. The influence mechanism of electrical parameters on the corrosion resistance of the coatings was explored by combining coating morphology and phase composition. A regression equation was established to facilitate regulation of the corrosion resistance of micro-arc oxidation coatings through manipulation of electrical parameters. The results show that the duty cycle exhibits the most significant influence on the electrochemical corrosion resistance of the coating, followed by pulse frequency and voltage, while the processing time shows a comparatively lesser effect. The duty cycle and pulse frequency influence both structure and performance characteristics of the coating by modulating arc ignition discharge duration and arc quenching cooling time. Increasing voltage, duty cycle and processing time, or decreasing pulse frequency can elevate the power supply output, which leads to an increase in coating thickness and pore size in microporous structures while reducing coating densification. Furthermore, this process promotes more efficient generation of Al₂TiO₅ in the coating; however, it ultimately results in diminished electrochemical corrosion resistance. The results of correlation coefficient testing indicate a strong relationship between the dependent and independent variables within the established regression equation. This finding provides theoretical support and predicting methods to regulate performance characteristics of titanium alloy micro-arc oxidation coatings.

Abstract:According to the varying load in different directions, the anisotropy control of porous materials can significantly enhance the load-bearing efficiency of materials, thus better addressing the need for lightweight designs. In this research, a modified Gibson-Ashby model for strut-based porous materials accounting for geometric parameters was established by taking G7 and bccz types of TC4 porous materials as examples. This model can serve as a guide for the precise control of anisotropy for strut-based porous materials. Based on this model, a series of anisotropic porous materials with similar configurations but distinct properties were created by adjusting geometric parameters of common unit cells. The effects of unit cell geometric parameters on the anisotropic compressive strength and failure modes of porous materials were investigated through vertical and lateral compressive tests, thereby validating the efficacy of the modified model. The results show that the compressive strength of strut-based porous materials is primarily determined by the aspect ratio and the inclination angle of their struts. By fine-tuning the inclination angle of struts, the anisotropic mechanical properties of porous materials can be effectively modulated. Under the same density, increasing the inclination angle of the diagonal struts from 35° to 55° can significantly increase the vertical compressive strength of G7 and bccz types of TC4 porous materials by 105% and 45%, respectively, with only a minor reduction in lateral compressive strength of 16% and 13%, respectively.

Abstract:To study the hot cracking sensitivity of high alloying difficult-to-deform superalloy GH4975, the crack morphology and microstructure characteristics of the GH4975 ingot were observed, and the causes of hot cracking were analyzed through solidification behavior and thermodynamic calculation. The results show that cracks propagate along grain boundaries and dendrites, and the equiaxed region has a greater cracking tendency than the columnar region. Shrinkage holes are easy to appear in the center of the ingot. The formation of continuous shrinkage holes leads to insufficient overlap between dendrites, which can be easily pulled apart under stress to form a crack source. At the same time, the segregation of Al, Ti and Nb elements between dendrites is severe. Complex precipitates, especially numerous (γ+γ′) eutectic phases, promote the nucleation and propagation of cracks. JMatPro calculation shows that GH4975 alloy has a high shrinkage rate and a wide temperature range for poor feeding during solidification. It facilitates the formation of shrinkage hole that can act as a crack source. Meanwhile, the linear expansion coefficient of the alloy changes significantly in the temperature range for poor feeding, thereby promoting crack propagation.

Abstract:By conducting adiabatic cyclic loading tests on three types of NiTi alloys with different martensite contents, dislocation densities, and grain sizes, the intrinsic influence mechanisms of different microstructures on the superelasticity, deformation modes, and elastocaloric cooling effect during the deformation process of NiTi alloys were investigated. The results show that the presence of a high dislocation density, high martensite content, and small grain size can reduce the degree of superelastic functional degradation and the possibility of local uneven deformation in NiTi alloys. However, the elastocaloric cooling ability is weak. A smaller strain value results in superior superelasticity (minimum ε_residual=0.23%), but inferior elastocaloric cooling ability (maximum ?T_cooling=0.63 K). Completely eliminating dislocations and martensite, as well as increasing grain size, can achieve a significant elastocaloric cooling capacity (?T_cooling=25 K), but induces severe functional degradation (a drop from 25 K to 9.6 K, a decrease of 61.6%). Annealing at 400 ℃ for 15 min to tailor the dislocation density, martensite content and grain size results in good superelasticity, uniform deformation ability and a considerable elastocaloric cooling ability (?T_cooling=7.2 K), along with improved resistance to functional degradation.

Abstract:The non-uniform microstructure and high temperature coordinated deformation behavior of friction stir butt-welded 2A97/5A06 dissimilar aluminum alloy plate were studied. The microstructure of each zone in the welded joint was observed, and the high temperature mechanical properties of both individual zones and the whole joint were studied. Results show that grains in the weld nugget zone of 2A97 and 5A06 are fine. The grain sizes of various zones on the 2A97 side are small and uniform, whereas those on the 5A06 side are slightly larger and exhibits a more significant variation. Under the condition of 430 °C and 10^–3 s^–1, the 2A97 and 5A06 base metals exhibit good high temperature properties, with elongations reaching 278.8% and 120.6%, respectively. The strength and elongation of the nugget zone in the joint are 18.4 MPa and 176.1%, respectively, which are between those of 2A97 and 5A06 base metals. The strength is about 2 times higher than that of the 2A97 base metal, and the elongation is about 1.5 times higher than that of the 5A06 base metal. The overall performance follows superposition principle. Due to the varying deformation resistance of each region, fracture occurs in the thermo-mechanically affected zone on the 2A97 side after concentrated deformation during transverse tensile testing. After correction, the flow stress is slightly higher than that of the base metal and the elongation is close to that of the base metal. The grain size and flow stress of each region after welding follow the creep equation. The smaller the grain size, the lower the flow stress.

Abstract:The extrusion deformation behavior of Mg-3Bi and Mg-6Bi alloys at 400 ℃ and a strain rate of 0.001 s^-1 was investigated, and the microstructure characteristics during extrusion were analyzed. Results show that as the Bi content increases from 3wt% to 6wt%, the plasticity of the alloy significantly improves, with the elongation increasing from 2.1% to 5.6%, and the tensile strength increasing from 188.2 MPa to 209.2 MPa. During high temperature extrusion deformation, the dominant softening mechanism of the Mg-3Bi alloy is discontinuous dynamic recrystallization (DDRX), whereas that of the Mg-6Bi alloy involves both DDRX and particle-stimulated nucleation (PSN). In this process, the grain size resulting from DDRX is larger than that produced by PSN. When Mg-Bi alloys are extruded through the die, the strain in the edge region of the die is significantly greater than that in the central region, consequently forming a deformation texture similar to that produced by equal-channel angular pressing (ECAP). Once the accumulated strain reaches a critical value, PSN is initiated. After extrusion deformation, the grains in both edge and central regions of the die exhibit a high degree of recrystallization, with their c-axes perpendicular to the extrusion direction, forming a typical extrusion fiber texture. The ranked contributions to the improvement of mechanical properties of Mg-Bi alloys, in descending order, are grain refinement, dislocation strengthening and nano-phase strengthening mechanisms. Compared with the Mg-3Bi alloy, the dimple density of Mg-6Bi alloy is higher, which disperses stress concentration and consequently enhances plasticity.

Abstract:Cathodic arc ablation restricts the stable operating time of arc plasma devices. Developing cathodes with long lifespan and service stability is essential for improving the operating capability of current equipment. Understanding cathodic arc ablation behavior and failure mechanisms is key to developing high-performance cathodes. This article firstly analyses the intricate arc ablation process of metallic cathodes and introduces failure mechanisms of sputtering, oxidation, and inhomogeneous ablation resulting from cathode spots. Furthermore, it reviews the recent advancements in improving cathode ablation resistance, including grain refinement, low work function addition, and gradient functionalization. In the final section, the future development of metallic cathodes is prospectively discussed based on in-situ observation of cathode spots, the construction of multi-field cathodic arc ablation model, and the establishment of a comprehensive cathode developing regime encompassing design, manufacturing, and testing processes.

Abstract:In the field of superconducting materials research, from the discovery of elemental mercury as a superconductor to the preparation of nickel-based superconductors, the study on physical properties and microscopic mechanisms of superconducting materials has greatly promoted the development of condensed matter physics. The development of practical high-temperature superconductors based on new preparation techniques plays an extremely important role in the fields of strong and weak electricity. As a new means, the high-pressure experimental technique has become one of the powerful tools for exploring novel superconductors and increasing their superconducting transition temperature (T_c). Focusing on three high-temperature superconductors with relatively high superconducting transition temperatures (exceeding 150 K), including H₃S, LaH₁₀ and HgBaCaCuO, this paper summarizes the research progress in their preparation techniques and clarifies the preparation strategies of practical high-temperature superconductors. It is concluded that high pressure facilitates the preparation of LaH₁₀, a hydrogen-rich compound superconductor with a special crystal structure, so that it can obtain a higher superconducting transition temperature. At the same time, high pressure can also affect copper oxide superconductors in a similar way of changing doping, thereby changing their superconductivity. High pressure technique is an effective way to fabricate high-temperature superconductors with special crystal structures (layered and caged).