The aerospace and automotive sectors have seen a remarkable increase in demand for high-end light alloy castings, driven by the ongoing pursuit of lightweight. The ability to regulate the solidification and cooling processes of high-end castings is of paramount importance, as it has been demonstrated to enhance the mechanical properties of castings and reduce the occurrence of defects. To explore the efficacy of engineered cooling in optimizing the solidification and cooling process of low-pressure aluminum alloy casting, this study investigates the impact of various engineered cooling strategies on the solidification time, microstructure and mechanical properties of the fabricated castings. The findings indicate that the implementation of adequate engineered cooling strategies can effectively regulate the cooling rate and solidification time of castings. Local heat insulation based on hollow sand mold can delay the solidification and cooling process of castings, while local enhanced cooling utilizing water-saturated resin can moderately accelerate the solidification and cooling process. The combination of these two cooling strategies are demonstrated to facilitate the achievement of sequential solidification in castings. Furthermore, a rational cooling strategy enhances the solidification shrinkage feeding, thereby leading to an improvement in microstructure and mechanical properties of castings. In low-pressure sand casting process, the presence and distribution of pore defects within castings usually exert a more substantial influence on mechanical properties than the grain size. This study provides a theoretical foundation and practical guidance for the optimization of cooling strategies in context of low-pressure sand casting, with the objective of improving the quality and performance of castings.
The electromagnetic stirring method has become one of the important methods for producing semi-solid slurries. Researchers typically use numerical models based on constant thermophysical parameters to predict the desired metrics. However, due to the fact that thermophysical parameters vary with temperature, this model leads to deviations in simulation results. Consequently, in this study, a three-dimensional model based on dynamic thermophysical parameters was constructed to simulate the electromagnetic field-velocity field-temperature field during the electromagnetic stirring of semi-solid aluminum alloy melt. Furthermore, the results were compared with those a constant-parameter model. The simulation model was verified through experiments such as magnetic induction strength measurement, temperature measurement, and metallurgical morphology analysis. The results reveal that the fluctuation trend of velocity at monitoring points obtained from the dynamic thermophysical parameters model exhibits better consistency with the actual situation during three-phase alternating-current electromagnetic stirring. Meanwhile, the temperature distribution trend predicted by the dynamic parameter model aligns more closely with the measured data. In addition, at the monitoring points, the simulated curve based on the dynamic thermophysical parameter model demonstrates superior agreement with the experimental measured values.
To study how the key parameters in the centrifugal spray forming process affect the thickness of a liquid film, a two-dimensional numerical model was constructed in Fluent to dynamically track the interface between the liquid film and air. The effects of forming process conditions and physical properties of molten metal materials on the thickness distribution of liquid film were studied. The thickness of the liquid film generally decreases with the increase of radial distance. However, when a hydraulic jump occurs, the thickness of the liquid film increases sharply. Under a given flow rate, whether a hydraulic jump occurs depends on the inlet diameter. The inlet diameter has a great influence on the liquid film thickness in the central area of the centrifugal disc but not in the edge area. The influences of the rotational speed and flow rate on the thickness of the liquid film are opposite. An increase in rotational speed leads to thinning of the liquid film at the edge, whereas an increase in flow rate increases the thickness of the liquid film at the same location. The influence of the temperature on the thickness of the liquid film is relatively small, indicating that temperature is not a decisive factor in this process. Through analysis, a model for predicting the thickness of the liquid film at the edge of the centrifugal disc was established on the basis of parameters such as the Weber number (We) and Onezog number (Oh). The numerical simulation results show that this prediction model can provide more accurate prediction results.
The reaction between Mn and Al in molten steel and SiO2 in the protective slag generates MnO and Al2O3, which can cause drastic changes in the composition of the protective slag, deteriorate the quality of the casting billet, and hinder the continuous casting process. The present work proposed a new idea of adding low-reactive liquid protective slag to respond to this bottleneck. Both water model experiment and numerical simulation were carried out to investigate the influence of height and position of the slag nozzle on the flow behavior of liquid slag in the initial stage of continuous casting using a slab mold (1,050 mm×135 mm). The results show that the turbulence kinetic at the steel-slag interface is the largest when the slag nozzle height is set to 150 mm and its position is 131.25 mm away from the narrow face of the mold, and the variance value of liquid level height is 3.27×10−4. This enhanced agitation is contributed to the flotation of inclusions and removal of bubbles in molten steel. At this time, the liquid slag spreading area is the largest, about 45.72
A novel semi-continuous induction melting technology for the production of large-volume titanium alloy ingots was introduced. The effects of the melting power and ingot drawing speed on the production process of titanium alloy ingots via this technology were systematically investigated. The results show that insufficient melting power or excessive drawing speed leads to failure in alloy melting, whereas excessively high melting power or overly slow drawing speed causes overheating of the cooling circulation system, leading to equipment failure. Optimal conditions are achieved at a melting power of 750 kW and an ingot drawing speed of 4 kg·min−1, resulting in efficient production of titanium alloy ingots with an energy utilization rate of 8.3
ATI 718Plus alloy powders were prepared by vacuum induction melting gas atomization (VIGA) and plasma rotating electrode preparation (PREP) methods. Effects of direct aging (DA) and homogenization-solution-aging (HSA) heat treatment processes on microstructure and mechanical properties of the alloy prepared by laser additive manufacturing (LAM) utilizing various powders mentioned above were systematically studied. The LAM process parameters such as laser power and scanning speed were optimized by orthogonal experiment. The results indicate that the porosity of the samples fabricated with PREP powder is controlled below 0.05
The NiTi alloy has great application potential in the engineering fields involving cavitation. The present study aims to reveal variations in surface and structure characteristics of the NiTi alloy subjected to submerged waterjet. Effects of the standoff distance and cavitation treatment duration were investigated. The volume loss, microstructure, phase transition temperature, and cross-sectional hardness were comprehensive analyzed. The results indicate that reduced standoff distance and prolonged waterjet exposure time result in increased cumulative volume loss and surface roughness of the NiTi alloy specimens. Cavitation imposes a significant thermal effect on the NiTi alloy. A 120-min cavitation treatment induces a hardened layer of approximately 200 µm in depth. The increase in microhardness of surface layer is closely related to dislocation strengthening and grain refinement. Compared to 316L stainless steel, the NiTi alloy exhibits higher resistance to cavitation, which is attributed to the existence of a large amount of twinning and high reversibility of phase transformation. The obtained conclusions are expected to shed light on the response of NiTi alloy to cavitation treatment and the mechanism of waterjet cavitation, providing a reference for application of NiTi alloys in engineering fields involving cavitation.
High-temperature creep resistance is a key requirement for structural materials used in aerospace applications. However, this property remains poorly understood for the EV31 alloy. This study investigates the creep mechanisms of EV31 alloy in temperature range from 200 °C to 300 °C through experimental characterization of creep curves and microstructures after creep deformation, combined with extracting the stress exponents and creep activation energies. The results show that the stress exponent (n) is primarily in a range of 4–6, but decreases to approximately 2.5 during creep deformation at 300 °C. The measured activation energy exceeds the lattice self-diffusion activation energy of α-Mg (135 kJ·mol−1). Microstructural observations reveal that the creep samples contain precipitate phases, mainly β′ and β1 within grains and β phase at grain boundaries, with precipitate-free zones forming on one side of the grain boundaries. Dislocation pile-up and tangle are observed around grain boundaries during creep at 250 °C. At 300 °C, dislocations are present within the grains and are hindered by the β′ phase. Based on these findings, it is concluded that the creep deformation of the EV31 alloy is dominated by mixed mechanisms. The grain-boundary gliding mechanism operates throughout the entire temperature range and dominates creep deformation at temperatures near 200 °C by mixing with dislocation creep. Dislocation climbing is the dominant mechanism at around 250 °C. At 300 °C, creep proceeds via precipitate-induced drag on dislocation motion, accompanied by grain-boundary gliding. This work casts new light on the development of high creep resistant magnesium alloys based on EV31 alloys and also provides critical information for EV31 alloys using at elevated temperatures.
To reveal the microstructural characteristics at specific locations of a TiAl turbocharger turbine under gravity casting conditions, metallographic observations were conducted on characteristic regions of its longitudinal section. The turbine hub exhibits primarily a columnar microstructure, with an average grain size significantly larger than that of the blade, which exhibits an equiaxed microstructure. The cooling rate varies significantly across the turbine, decreasing from the blade tip to the hub bottom, resulting in a negative correlation with grain size. The center of the turbine hub and the blade tips show higher γ phase volume fractions (14.028
Film-like MgAl2O4 spinel inclusions are among the most harmful oxide defects in Ni-based superalloys because their high aspect ratio exacerbates local stress concentration. In this work, K492M Ni-based superalloy was remelted and cast in a MgO-containing crucible using a vacuum induction furnace to clarify the origin and formation pathway of Mg-, Al-, and O-bearing inclusions. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and selected-area electron diffraction were used to characterize the morphology, composition, and crystal structure of the inclusions. The results show that the inclusions consist mainly of particulate MgO cores directly coated by MgAl2O4 shells, together with flocculent film-like MgAl2O4 products extending outward from the cores. Only local residual α-Al2O3 is detected, indicating that Al2O3 is a transient intermediate rather than a stable final product. Based on these observations, a three-stage mechanism is proposed: mechanical spallation of MgO particles from the crucible wall, rapid interfacial reduction coupled with solid-state transformation to form MgAl2O4, and stress-induced rupture and exfoliation of the spinel shell into thin films. This mechanism explains the coexistence of MgO-MgAl2O4 core-shell particles and film-like spinel inclusions, and provides guidance for controlling crucible-derived oxide contamination in Ni-based superalloy castings.
By adjusting the dosages of the inoculant and the vermiculizer respectively, the graphite morphology, vermicularity, thermal conductivity, and internal friction parameters of the vermicular graphite cast iron (VGI) were systematically studied. The results show that there is a synergistic regulatory relationship between the graphite vermicularity in the cast iron and the dosages of the inoculant and the vermiculizer. For different vermicularity ranges, there are different dosage windows for the inoculant and the vermiculizer. Among them, the window for the vermiculizer is relatively narrow, while that for the inoculant is relatively wide. In addition, there exists a critical graphite vermicularity window of 80
The TC4-4.34Cu titanium alloy was prepared by water-cooled copper crucible vacuum suspension melting technology, followed by hot isostatic pressing at 920 °C under 130 MPa. Subsequently, heat treatments were conducted at 720 °C and 800 °C, respectively. The microstructures of the alloy were analyzed using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy equipped with an energy dispersive spectrometer, and its mechanical properties, antibacterial performance, and corrosion resistance were evaluated. Results show that cast TC4-4.34Cu alloy matrix consists of α-phase, β-phase, and Ti2Cu phase. However, numerous porosity defects lead to a low plasticity of the alloy. Hot isostatic pressing effectively eliminates porosities and improves the densification of the alloy. Heat treatment at 720 °C promotes a more uniform and denser alloy matrix microstructure, yielding optimal mechanical properties, specifically, a tensile strength of 944 MPa and an elongation of 7
To improve mechanical properties of high-Nb TiAl alloys, boron nitride (BN) reinforcement was introduced, and an optimum Nb content was determined. Ti46Al2Cr0.5BN-xNb (at.
To gain a deeper understanding of how diverse microstructures influence the thermal conductivity of Al-Si alloys, the mechanism governing thermal conductivity variations in ADC12 alloy was elucidated through directional solidification and subsequent heat treatment processes. The contributions of grain boundaries, second phases, and solid-solution atoms to thermal conductivity were qualitatively evaluated. Results show that directional solidification substantially eliminates transverse grain boundaries, and the thermal conductivity reaches 189.59 W·m−1·K−1. After 24 h solid solution treatment, the average grain size decreases from 57.10 µm (as-cast) to 21.65 µm and the aspect ratio of eutectic Si reduces from 8.94 to 4.52. However, the extensive solid solution of Cu in the α-Al matrix induces severe lattice distortion, significantly reducing the thermal conductivity to 135.40 W·m−1·K−1. The achievement of high thermal conductivity is attributed to the substantial elimination of grain boundaries, which serves as a critical mechanism for thermal conductivity enhancement of ADC12 alloy. These findings provide novel strategies and theoretical insights for achieving excellent thermal conductivity in aluminum alloys by customizing their microstructures.
Diamond/aluminum composites with excellent performance have broad application prospects in electronic packaging and heat dissipation. In order to improve the interfacial wettability between diamond and aluminum and to prevent excessive formation of Al4C3, silicon addition to the aluminum was used as the matrix. The composites with varying silicon content in the matrix were fabricated using the vacuum-assisted pressure infiltration method, and the effects of silicon content on microstructure and properties of the composites were systematically studied. Results show that the diamond/Al-Si composites exhibit a uniform and dense microstructure with good interfacial bonding. As the silicon content increases, both the thermal conductivity and bending strength of the composites first increase and then decrease, while the coefficient of thermal expansion decreases monotonically. When the silicon content in the aluminum matrix is 12wt.
8Cr4Mo4V bearing steel has been widely used in the industrial applications due to its exceptional mechanical properties. To further improve the tribological properties, the microstructure of 8Cr4Mo4V bearing steel was controled through varying the austempering time in this work. The microstructure, hardness, and tribological behaviors were systematically investigated. The results show that 8Cr4Mo4V steel is mainly composed of martensite, bainite, and retained austenite after quenching and austempering treatment. With increasing the austempering time from 1 h to 8 h, the proportion of bainite increases gradually while martensite decreases correspondingly. The hardness decreases from 881 HV to 737 HV with the decrement of 16.34
Al-Cu alloys exhibit promising industrial applications owing to their low density and superior mechanical properties. However, their widespread adoption is severely limited by high hot tearing susceptibility. Hot tearing is closely related to the stress change and fluidity of the intergranular liquid films during the final stage of solidification. Due to the influences of grain size, liquid film thickness, and various defects, conventional polycrystalline samples cannot accurately reflect the stress-strain characteristics of the liquid films. To circumvent these inherent limitations in polycrystalline samples, a custom-designed device was developed to investigate the stress-strain behavior of monocrystalline intergranular liquid films. The effect of Sc on stress-strain of liquid films at the end of Al-Cu solidification was investigated using the self-made device. Meanwhile, combining the T-shaped mold experiment and differential thermal analysis, the relationship between the stress-strain of the liquid films and hot tearing susceptibility was analyzed. The temperature and stress fields during solidification were simulated using ProCAST. The results indicate that the addition of Sc refines the dendrites, modifies the morphology of θ-Al2Cu, alleviates the stress concentration during the solidification process, and prolongs the liquid feeding duration, thereby reducing the hot tearing susceptibility of the alloys. Numerical simulation results of temperature field, stress field, and hot tearing indicator of T-shaped mold are in good agreement with the experimental observations.
The 7xxx series aluminum alloys have emerged as a particularly promising class of lightweight structural materials. However, the inherent strength of these materials is primarily influenced by the content and type of alloying elements added during the manufacturing process, as well as casting defects. The present study investigated the effects of eutectics formed by solute atoms (Zn, Mg, and Cu), with equal mass ratios (Zn/Mg=2, Mg/Cu=3) but varying overall contents, on the liquid film thickness, crack propagation depth, and the mechanical properties of the Al-Zn-Mg-Cu alloy after heat treatment. The results from gravity casting indicate that the intergranular liquid film thickness increases with the increase of eutectic content. A thick intergranular liquid film in the casting can accommodate greater strain during grain contraction, thereby preventing liquid film rupture and subsequent hot tearing. Concurrently, during the solution treatment at 475 °C, the residual eutectic fraction in the Al-7Zn-3.5Mg-1.18Cu alloy diminishes from 9.1
A novel cast stainless steel featuring a multiphase microstructure and a nominal composition of Fe-13.5Cr-2.6Si-6.9Ni-1.1Cu-1.1Mn-1.0Mo-0.35Al-0.025C (wt.
