Journal of Materials Engineering and Performance最新文献

The microstructure evolution and mechanism of the heat-affected layer of nickel-titanium shape memory alloy (NiTi SMA) during low-speed wire electrical discharge machining (WEDM-LS) are investigated by integrating the finite element method and the cellular automata (CA) method. First, an accurate single-pulse discharge heat source model is established to simulate the distribution of temperature, stress and strain fields on the workpiece surface under different electrical parameters. Then, the dynamic recrystallization (DRX) and thermoelastic martensitic transformation of NiTi SMA during machining are simulated using the CA method. Furthermore, WEDM-LS machining experiments and metallographic analysis are conducted to validate the reliability of the simulation results. The findings reveal that DRX of austenite grains in the heat-affected layer contributes to grain refinement on the workpiece surface. The result shows that increasing the peak current from 8A to 12A enhances the DRX fraction from 9.3% to 20.9% while reducing the average grain size from 47.2 μm to 44.5 μm. In contrast, extending the pulse width from 9 μs to 15 μs decreases the DRX fraction from 9.3% to 6.3% and increases the grain size from 47.2 μm to 48.2 μm, indicating that peak current has a more pronounced effect on DRX than pulse width. Additionally, under the cooling effect of deionized water, when the workpiece surface temperature falls below the martensitic transformation start temperature (M_s), martensitic nucleation and growth occur at the austenite grain boundaries. The proposed prediction model accurately characterizes the influence of machining parameters on microstructural evolution, providing theoretical guidance for optimizing WEDM-LS processing of NiTi SMA.

The demand for advanced engineering materials tailored to specific industrial needs has become increasingly critical in today’s manufacturing industries. This necessity has driven the development of functionally graded materials (FGM), which offer unique and highly desirable characteristics compared to conventional materials like steels and cast irons. This study focuses on the creation of Al-Mn FGMs through a combination of powder metallurgy and sintering techniques. The development of Al-Mn FGMs is a noteworthy endeavor, aiming to harness the properties of aluminum (Al) and manganese (Mn) for enhanced material performance. The starting materials, fine Al powder with 99.99% purity and Mn powder with the same level of purity, were meticulously chosen to ensure the desired material quality. The novelty of this research lies in the varying composition of Mn in the multilayered composite, with weight percentages of 60%, 45%, 30%, and 15% in different layers. This approach offers the potential for a material with graded properties, tailored to specific applications. To evaluate the quality and effectiveness of the developed FGM, morphological aspects of each sample were examined through optical microscopy. This analysis helped in understanding the microstructure and layer-by-layer variations. Further, the study involved the calculation of critical properties such as layer wise density, thermal conductivity, and specific heat. The characteristic composition gradient, ranging from 15 to 60% Mn, was made possible by stacking alternating layers of Al and Mn powders. Microscopic analysis revealed the successful formation of intermetallic phases and a well-defined layered structure. The high thermal conductivity of layer 1 (233.12 W/mK) and the density of layer 6 (7403.5 kg/m³) were particularly notable. These findings confirmed that Al-Mn FGMs are promising materials for engineering applications as evidenced by the significant variations in mechanical and thermal characteristics across the graded-layers.

At present, the application of ultra-thick super-bainitic steel has attracted wide attention. In this paper, the effect of undercooled austenite cooling rate on the microstructure and wear properties of bainitic steel was investigated by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and x-ray diffraction using medium carbon carbide-free bainitic steel as the research object. The results show that the samples with faster cooling rates have lower wear loss and better wear properties. A deformation zone exists in the subsurface, and the greater the cooling rate, the smaller the thickness of the deformation zone. A significant retained austenite transformation was observed in the deformed layer.

Reinforced thermoplastic pipes (RTPs) are extensively utilized in oil and gas transportation systems. The stability of the polymer structure and performance during service is critical to ensuring the safe operation of the pipes. In this study, visual inspection, thermogravimetric analysis, tensile testing, Vicat softening temperature measurements, pressure testing, differential scanning calorimetry, x-ray diffraction, and gel permeation chromatography were employed to evaluate the structural and performance stability of polymers in RTP after actual service. After four years in operation, the hardness and VST of the liner pipe decreased by 17.0% and 4.2%, respectively. Mechanical properties, including elongation at break, yield strength, and modulus, declined by 50.1%, 12.1%, and 41.2%, respectively, indicating a reduction in both stiffness and toughness. The performance degradation is primarily attributed to swelling initiated by the transported medium, leading to a noticeable reduction in crystallinity (19.3%) and orientation (26.1%) in the liner pipe. The swelling had a minimal effect on the condensed state structure of the polyester fibers, preserving their crystallinity and orientation, which resulted in only minor degradation of the tensile strength of the reinforcement fibers. The outer protective layer, which had little to no contact with the transport medium, maintained its structural integrity and performance over time. For oil transmission conditions, the liner material should be made of polymer materials with strong molecular structure polarity, strong intermolecular forces or cross-linked structure, in order to reduce the effect of medium swelling on the mechanical properties of the lining material.

A bright anodic oxide layer with (NH₄)₂TiF₆ and polysilazane sealing was fabricated to improve the sweat corrosion resistance of 7075 aluminum alloy. The effects of sealing treatment on the reflectivity and corrosion behavior of anodic aluminum oxide were investigated by artificial sweat corrosion test and electrochemical measurement. The surface reflectivity of the one-step sealed sample with (NH₄)₂TiF₆ was slightly increased by the precipitation of the γ-AlOOH, the titanium dioxide and titanium hydroxide which weaken reflections of light in the pores. The uniform and flat surface of the two-step sealed sample with (NH₄)₂TiF₆ and polysilazane reduced the light scattering, which improved the surface reflectivity. After 48 h of artificial sweat corrosion, the surface of two-step sealing sample did not change color and corrode, and the surface reflectivity did not decrease significantly. The significant enhancement in sweat corrosion property of bright anodized aluminum after the two-step sealing treatment was mainly due to the volume expansion effect of γ-AlOOH, the deposition effect of the reaction products (titanium dioxide and titanium hydroxide) and the barrier property of polysilazane, which led to the densification of the sealed anodic oxide layer.

This study examines the impact of heat treatment on the microstructure, pore distribution, and electrical and thermal properties of pure copper prepared by selective laser melting (SLM). SLM copper was subjected to various heat treatment temperatures ranging from 200 to 1000 °C. The findings reveal that increasing heat treatment temperatures result in relative density decreasing from 99.8% in the as-built state to 99.4% due to small increase in porosity. Heat treatments in the range of 200 to 400 °C lead to recovery in microstructure reducing low angle boundaries among grains. The electrical conductivity improves from 90.26 to 92.8% IACS, and thermal conductivity increases from 374 to 389 W/m·K along the building directions after treatment at 400 °C. This work uniquely demonstrates the anisotropic thermal expansion behavior of SLM copper and reveals the role of high-temperature pore migration in causing a sharp decline in thermal conductivity at temperatures above 800 °C. Additionally, a detailed correlation is established between recrystallization, the formation of high-angle grain boundaries, and the enhancement of electrical conductivity. By annealing at temperatures above 600 °C, recrystallization and the formation of high-angle grain boundaries lead to the diffusion of pores to grain boundaries, increasing coefficient of thermal expansion. Pores migrating to grain boundaries result in the dropping of thermal conductivity to 342 W/m·K, while electrical conductivity increases to 99.8%% IACS due to a decrease in dislocation density and grain boundaries. The slight increase in pores results in adverse effect on thermal conductivity in annealing after overly high temperature treatments. Thermal expansion resulting from redistribution of pores further leads to change in the dimension of SLM Cu parts. These results indicate that heat treatment can markedly alter the properties of SLM-manufactured copper compared to traditionally fabricated copper. This study provides in-depth observations on dimensional stability and physical properties of the additive manufactured pure copper for industrial applications.

In this study, the tensile and fracture behavior of AA7075-T651 was investigated under different temperatures (room temperature to 300 °C) and strain rates (10⁻¹ to 10⁻⁴ s⁻¹). Tensile tests were done to obtain the ultimate tensile strength, yield strength, modulus of elasticity, % elongation, strain to failure, and plastic anisotropy ratio (r-value). The strain rate sensitivity of the alloy was also investigated. The fractographic study on the fracture surfaces of the tested samples was done using FESEM and optical microscopy to analyze the fracture behavior. It was found that with an increase in temperature, there was a gradual decrease in yield and ultimate strength values, while % elongation increases, and this effect became more pronounced at higher temperatures. It was observed that with a decrease in strain rate, the yield and ultimate strength values decrease while % elongation increases. The strain rate sensitivity reveals that the material exhibits increased sensitivity at higher temperatures. The Johnson Cook model was used to predict the stress–strain response, and it was found that the model fit showed good agreement with test conditions, validating its applicability for predicting the flow behavior of AA7075-T651. A difference in fracture mode was noticed for different strain rates in fractographic studies. At room temperatures, the brittle failure behavior of the alloy was more dominant, while at higher temperatures, a completely ductile fracture was observed.

After the combined treatment of pulsed magnetic field and deep cryogenic conditions (MDC), the microstructure and property evolution of Cr4Mo4V bearing steel following additional tempering treatment (MDCT) was investigated, and compared with samples that were treated only with deep cryogenic conditions (DC) followed by tempering (DCT). The results showed that the retained austenite content in DCT samples decreased from 23.6 ± 0.7% to 13.6 ± 0.5%, whereas the MDCT samples exhibited a decrease in retained austenite content from 23.3 ± 0.4% to 15.4 ± 1.1%. In MDCT samples, the stabilization of retained austenite correlated with the change in saturation magnetization and the reduction of carbide precipitation. Furthermore, electron backscatter diffraction (EBSD) analysis revealed that although the phase transformation in MDCT samples was somewhat hindered, leading to a higher proportion of low-angle grain boundaries, the dislocation density in MDCT samples (1.14 × 10¹⁵ m^-2) decreased compared to DCT samples (1.26 × 10¹⁵ m^-2). The microstructural evolution during the deep cryogenic treatment and tempering process indicated that the magnetic field hindered the formation of carbon clusters under deep cryogenic conditions, delayed the precipitation of carbides during tempering and influenced the decomposition of retained austenite. In terms of mechanical properties, compared to the yield strength of 1636 MPa for DCT samples, the MDCT samples decreased to 1587 MPa. The theoretical calculations of strength matched well with the actual results, and the decline in strength of MDCT specimens was ascribed to impeded austenite transformation, diminished dislocation density and reduced precipitation of fine carbides.

Investigating ductility and fracture load is crucial for enhancing the performance of joined materials. Different machine learning models, namely XGBoost, Decision Tree, Random Forest, Linear Regression, and support vector machine, have been utilized to investigate the friction stir spot welding (FSSW) process performances of AA2018-H2 and C10200 bi-metallic joints. Depending on the design of the experiment outlines, the experimental process was executed considering three key parameters: rotational speed (1500–2100 rpm), plunge depth (0.1–0.3 mm), and dwelling time (2–6 S). The regression equations created the correlations between FSSW controlling parameters with the fracture load and elongation. The significant results have been observed in the XGBoost model, with the coefficient of determination (R²) for fracture load at 0.9762, elongation at 0.2965, and mean squared error for fracture load at 7750.23 (N²), while elongation is at 0.0227 (sq.%) following the optimization. The FE-SEM fractographs simultaneously revealed dimple structures, which confirmed the occurrence of ductile fracture in the nugget area of the welded zone, across all specimens. The maximum fracture load and the corresponding elongation have been obtained as 4125 N and 1.2%, respectively, in the set of 1500 rpm, 0.2 mm, 6 s. This attempt highlights the importance of a methodological approach in addressing the direct control of the FSSW industrial challenges.

Energy transition has brought tighter requirements to high-performance gears, especially the demand for increased power density. Usually applied after grinding, isotropic superfinishing stands for a solution to reduce flank roughness and consequently the contact stresses. The objective of this study is comprehending how the residual stresses induced by the grinding process influence the superfinished surface integrity. Specimens were pointedly ground to induce distinct residual stress states in terms of maximum intensity, surface heterogeneity, and in-depth profile. They were then subjected to isotropic superfinishing in a single condition. The investigation showed that, after the isotropic superfinishing, the ground residual stress state is preserved. The results of both intensity and heterogeneity of residual stresses demonstrate that the superfinished surface is strongly influenced by the previous manufacturing stage, to which the proposed mechanism of interaction is verified.