Journal of Materials Engineering and Performance最新文献

Sc is commonly used as a strengthening element in Al-Si casting alloys, but its influence mechanism on hot tearing is still unclear. In this work, the effect of minor addition of Sc on the hot tearing performance of alloys was studied through a custom-made experimental device, and after the initial hot tearing point temperature and load were compared, the hot tearing sensitivity of alloys with different Sc contents was evaluated. The results were consistent with the visual observation of crack severity. The results show that minor Sc addition can reduce the hot tearing sensitivity of Al-Si-Cu alloy, but with a further increase in the amount of added Sc, the hot tearing sensitivity of the alloy increases. The microstructure morphology shows that the addition of Sc can refine the α-Al grain size and generate new W-Al₈-xCu₄ + xSc and Al₃Sc. Al₃Sc changes the precipitation position of the θ-Al₂Cu phase, which is not conducive to avoiding the occurrence of hot tearing. Furthermore, when Sc is used to refine the grains but fails to refine them into equiaxed grains, Al₃Sc acts as a heterogeneous nucleation site for α-Al, which leads to an increase in the formation time of the dendritic skeleton and reduces the liquid phase shrinkage time. On the other hand, Sc has a strengthening effect on the alloy skeleton, enabling it to resist higher shrinkage loads and helping to avoid the formation of hot tearing.

The addition of copper into electroless Ni-P plating (Ni-Cu-P) has shown enhanced thermal stability, deposition rate, and corrosion resistance compared to electroless Ni-P plating. Herein, the present work explores the effect of annealing temperatures (room temperature, 100, 120, 200 °C) on the characteristics of Ni-0.1Cu-P coating applied on stir-cast Al-Cu-Mg alloy, mitigating the saltwater corrosion in 0.1 molar NaCl solution. The microstructural evolution, adhesive strength, and scratch resistance, of Ni-0.1Cu-P coatings were thoroughly studied employing x-ray diffraction, field emission scanning electron microscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, and optical surface profilometry, revealing the formation of few microns thick, homogeneous, amorphous coatings that was bright, dense, and compact, featuring a smooth grain structure. Importantly, no morphological changes were observed after the heat treatment (100, 120 and 200 °C). Potentiodynamic polarization by Tafel extrapolation illustrates that Ni-0.1Cu-P specimen annealed at 120 °C (HT-120 °C) showed improved corrosion resistance, as evidenced by lowest corrosion current density (i_corr ~ 0.27 μA cm⁻²) and highest polarization resistance (R_p ~ 111370 Ω.cm²) among entire specimens. Mott-Schottky analysis confirms that the formation of p-n junction bipolar passive film in HT-120 °C specimen was the dominant factor for improved corrosion resistance. The passive film lowers the donor carrier density (N_d: ~ 4.99 × 10¹⁸), acting as a barrier, effectively inhibiting the ingress of chloride ions and outward diffusion of cations formed during anodic dissolution, improving corrosion resistance in saline water.

The deployment of 9-12% Cr steels for elevated temperature applications up to 650 °C presents a cost-effective alternative to more expensive nickel-based alloys in steam turbine power generation. To enhance creep resistance at this temperature range, a novel ferritic-martensitic steel, designated CPJ7, was developed and fabricated at the National Energy Technology Laboratory. The alloy design aimed to mitigate the transformation of strengthening carbides into deleterious phases that degrade creep performance. Results have demonstrated that CPJ7 exhibits favorable creep and oxidation resistance at 650 °C. However, its fatigue performance remains unexplored. This study builds upon prior research by evaluating the low cycle fatigue behavior of CPJ7 and verifying that modifications beneficial to creep performance were not detrimental to the fatigue performance. The alloy was tested at both 650 °C and ambient temperature under fully reversed bending conditions (R = − 1) and a load ratio of 0.05. The alloy exhibits cyclic softening, a behavior consistent with other 9-10 wt.% Cr steels. Analysis of the microstructure and hysteresis loops further corroborate cyclic softening mechanisms typical of ferritic-martensitic steels. Overall, the fatigue performance of CPJ7 meets or exceeds that of P91 steel, demonstrating its potential for high-temperature structural applications.

This study investigates the microstructural evolution and mechanical performance of austenitic stainless steel 316L components fabricated using wire arc additive manufacturing with and without the application of friction stir processing as a post-deposition grain refinement technique. As-deposited wire arc additive manufacturing samples exhibited coarse columnar austenitic grains aligned along the build direction, accompanied by interlayer porosity and tensile residual stresses, resulting in anisotropic mechanical properties. Friction stir processing effectively refined the microstructure by transforming columnar grains into fine grains through dynamic recrystallization. This refinement improved the overall microstructural uniformity and enhanced interlayer bonding. Mechanical testing revealed an increase in average microhardness from 195 HV (as-deposited) to 227 HV (FS processed), attributed to grain refinement and defect elimination. Tensile strength improved significantly, with yield strength increasing from 239 to 307 MPa, ultimate tensile strength from 343 to 593 MPa, and elongation from 32 to 54%. This approach not only enhances mechanical properties but also allows for more efficient material utilization. This innovative technique positions itself as a promising solution for the future of advanced manufacturing in the aerospace and automotive industries.

This study investigates the impact of laser shock peening (LSP) on the corrosion behavior of Wire Arc Additive Manufactured (WAAM) Monel-400 in a 3.5% NaCl environment. Monel-400 (Ni-Cu alloy) was selected for its exceptional corrosion resistance and high strength. The LSP process, applied at pulse energies of 2.5 J and 3.5 J, improves surface properties by inducing compressive residual stresses, enhancing microstructure, and refining grain boundaries. Electrochemical analysis revealed significant improvements in corrosion resistance after LSP treatment, impedance measurements confirmed enhanced corrosion resistance, with charge transfer resistance increasing from 2.740 kΩ cm² (untreated) to 8.200 kΩ cm² (LSP 3.5 J). Microstructural analysis showed that LSP treatment led to grain refinement, higher dislocation density, and improved surface hardness. Surface roughness was reduced from 10.2 µm (untreated) to 2.10 µm (LSP 3.5 J), and residual compressive stresses contributed to a more stable passive film and reduced corrosion product formation. These results revealed that LSP significantly enhances the corrosion resistance and mechanical properties of WAAM Monel-400, making it suitable for harsh marine environments.

In the present work, 12% tin bronze (CuSn12) was subjected to solutioning, artificial aging, and cryogenic treatment. The as-received CuSn12 material contained the α-solid solution and the δ-eutectoid (a mixture of the α- and δ-phases). Solutioning induced complete dissolution of the δ-phase. Subsequent artificial aging evoked discontinuous precipitation of the ε-phase at the grain boundaries. Cryogenic treatment accelerated discontinuous precipitation. Moreover, these treatments resulted in a substantial increase in the number of fine deformation twins inside the α-solid solution grains, which may act as preferential sites of continuous precipitation. Because grain boundary diffusion is much faster than grain interior diffusion, precipitation at the grain boundaries was much more pronounced. Consistently, microhardness increased substantially in the nearby grain boundary regions. All treatments reduced hardness and the yield strength but increased ductility and impact toughness. Aging had a slight but indisputable effect on all mechanical properties (an increase in hardness and yield strength but a decrease in ductility and toughness).

This study examines the response of 6014-T4P aluminum alloy to tensile loading under various strain rates at room temperature, focusing on its mechanical behavior and associated microstructural changes. High strain rate behavior was characterized using a split-Hopkinson tensile bar (SHTB) at strain rates between 2280 and 4150 s⁻¹, while the quasi-static response was evaluated at 0.001 s⁻¹. The experimental results demonstrate a clear enhancement in both flow stress and plastic deformation capacity with increasing strain rate. Under dynamic conditions, AA 6014-T4P achieved a 36.7% rise in ultimate tensile strength and a 55.2% gain in elongation compared to quasi-static loading. Furthermore, a modified Johnson–Cook (MJC) constitutive model was formulated, showing strong correlation with the experimental data. Additionally, fracture surface analysis indicated that high strain rate deformation produced a greater number of deeper dimples compared to the quasi-static case, suggesting enhanced ductility. Microstructural characterization further showed that elevated strain rates promoted an increased proportion of low-angle grain boundaries, and a higher density of geometrically necessary dislocations. The elevated intensified dislocation entanglement and precipitation hardening contribute to the superior plasticity and strength of AA 6014-T4P compared to those observed under high strain rate loading.

The present study explores fine-scale microstructure, chemistry, and dislocation substructure formation, and their effect on phase decomposition and transformation during and after additive manufacturing (DED-LENS^TM) of Ti-6Al-4V alloy. Optimized processing parameters were used to produce bulk alloy specimen. Electron microscopy was carried out to obtain details about microstructure and dislocation sub-structure formation as well as to analyze elemental distribution within the microstructural features. High temperature x-ray diffraction (XRD) and differential scanning calorimetry characterizations were conducted to study the phase transformation in as-deposited specimen. The prior β grain boundary regions either remain free from grain boundary (GB) α phase or contains smaller GB α variants. Lamellar α phase appears beside prior β grain boundaries while basket-weave structure with acicular α lamellae is present inside prior β grains. Larger primary and secondary acicular α lamellae evolve as part of basket-weave structure during deposition of a new layer, whereas refined tertiary and quaternary (alpha ) lamellae form on reheating of previously deposited layers. Reheating also results in rearrangement of dislocations and α/α interface formation that assists β penetration and promotes spheroidization of α lamellae. β nucleation from retained dislocations inside (alpha^{prime }) martensites further accelerates spheroidization. Morphology of different α/α′ phase along with substructure and nonequilibrium elemental distribution effects α → β phase transformation temperature post-deposition. The retained compressive residual stress also decreases with increase in temperature heat treatment and cause peak shifting and peak broadening in XRD pattern.

High-strength low-alloy (HSLA) steels are valued for their exceptional mechanical properties, combining high strength with excellent formability and weldability. The strength of HSLA steels arises from specific alloying elements that refine microstructural characteristics, minimizing the reliance on carbon content. This study utilized tungsten inert gas (TIG) welding to join 2-mm-thick HSLA steel sheets, aiming to evaluate the mechanical properties, formability, and microstructural attributes of the weldments. Various trials were conducted to select the optimal process parameters, which included welding speeds of 180 and 200 mm/min and welding currents of 180, 200, and 220A, based on visual inspections of the welds. Tensile tests, conducted according to ASTM E8 standards, which revealed that the base material exhibited a peak tensile strength of 814 MPa and 28% ductility, indicating superior mechanical properties. Conversely, the weldments showed reduced strength (657-693 MPa) and ductility (10-14%) compared to the base material. It was noted that with constant currents of 180A and 200A, when the welding speed increased the ductility got reduced, while at 220A, an opposite trend was observed. To investigate, weld macrostructures were analyzed using optical metallography, and fractography studies on tensile fracture surfaces were performed using scanning electron microscopy (SEM) to understand the failure modes. The heat input at each process parameter combination was expressed quantitatively by (welding current/welding speed) ratio and was correlated with the corresponding properties. Excessive heat generation (current/speed = 1.2) at 220A and 180 mm/min resulted in non-uniform melting and notch formation around the weld bead, with microvoid coalescence contributing to early failure. Formability assessment via Erichsen cupping test indicated failures primarily in the fusion zone, with the base material achieving the maximum dome height (10.50 mm), followed by specimens welded at 220A-200 mm/min (10.10 mm). Increased welding speed at constant current reduced formability, likely due to decreased heat input, except for the 220A condition. Electron Backscatter Diffraction (EBSD) analysis showed that specimens welded at 220A-180 mm/min exhibited low grain orientation spread (GOS: 1.22), because most of the as-welded grains being oriented toward <001>orientation. However, the 220A-200 mm/min combination displayed maximum grain stretching from their isotropic as-welded condition and higher average GOS (2.00), reflecting good formability. Microtextural analysis revealed the highest intensity of γ fiber components (114.926) in the 220A-200 mm/min welded specimen, achieving superior formability, whereas the 220A-180 mm/min specimen showed reduced γ fiber intensity (8.382), correlating with decreased formability.

Adding refiners to the melts is one of the most efficient refining methods for magnesium alloys. How to improve the effectiveness of refiners was a significant issue in the Mg alloy foundry industry. Commonly used methods (e.g., electromagnetic and ultrasonic techniques) required sophisticated equipment and involve high costs. In this study, the improvement in refining effect of Al-NbB₂ refiner on AZ91 alloy was realized by the addition of Gd elements. The Al-NbB₂-xGd (x = 0, 0.3, 0.5, 0.7) refiners for AZ91 alloy were prepared by solid-liquid reaction method. Adding 0.5 wt.% Gd to the Al-NbB₂ refiner reduced NbB₂ particle size, altered its morphology (hexagonal prism → spherical polyhedron), and improved its distribution and dispersion, significantly enhancing refining performance. The Al-NbB₂ refiner showed better refining effects than other commonly used refiners. Compared with the refining effect of Al-NbB₂ refiner on AZ91, the grain refining effect of Al-NbB₂-0.5Gd refiner on AZ91 alloy was enhanced by 30% (78–55 μm). The yield strength, tensile strength and elongation were increased by 8% (74–80 MPa), 12% (151–169 MPa) and 12% (5.1–5.7%), respectively. The existences of NbB₂ and Al₃Gd particles in AZ91 alloy was demonstrated using high-resolution transmission electron microscopy (HRTEM). The matching relationships between particles and α-Mg were determined using the edge-to-edge model (E2EM). The refinement mechanism of refiners in AZ91 alloy was analyzed.