Journal of Materials Engineering and Performance最新文献

Ti₂AlNb/TiAl composite columnar specimens without defects were fabricated by using direct laser deposition (DLD) process. In this study, the microstructure and mechanical properties were comprehensively investigated. Results revealed that each deposited layer contains two microstructure bands: the equiaxed grain band and the lamellar colony band. These two bands appeared as γ/α₂ lamellar with random lath orientation under high magnification, with different lamellar spacing of 210 and 380 μm, respectively. Unmelted Ti₂AlNb particles could be found in the lamellar colony band, which could decrease the dislocation density further to increase the strength. The tensile strength at room temperature reaches 541-543 MPa, with a corresponding elongation of 0.6-0.8%, while the fracture occurs mainly in the equiaxed grain bands with a translocation fracture mechanism. Hardness tests also showed higher hardness values in the lamellar colony bands than in the equiaxial grain bands. CT characterization tests did not reveal that fracture cracks started or passed through the Ti₂AlNb grains. These results demonstrate an in-depth understanding of the microstructure and properties of Ti₂AlNb/TiAl composites prepared by DLD, and provide a pioneering reference for further investigations of the strengthening effect of Ti₂AlNb on TiAl-based alloys.

This research seeks to create tube-based aluminum foam using friction stir tube deposition (FSTD) process. In this process, AA6063 consumable rods, pre-filled with a mixture of titanium hydride and aluminum powder, are deposited into a hollow mild steel tube using a conventional vertical milling machine. The results indicate that consumable rods with 12 pre-drilled holes ensure a more uniform distribution of the foaming agent. Furthermore, the study shows that increasing the tool’s rotational speed and the weight percentage of titanium hydride results in larger pore sizes and greater porosity. Specifically, for the same TiH₂ composition and rpm levels, the 12-hole filling strategy enhances porosity by 42.62 and 10.12% compared to the 8-hole and 10-hole methods. The optimal process parameters for developing aluminum foam are identified as using consumable rods with 12 holes containing 60% TiH₂ and a rotational speed of 1400 rpm.

In this study, 12Cr12Mo martensitic stainless steel was manufactured using selective laser melting (SLM). The resulting microstructures and mechanical properties were analyzed under optimal process parameters to understand the correlation between the process, microstructure, and properties. The results showed that only a single martensitic phase is present in the SLM samples, attributed to the exceptionally rapid solidification rate and high density of dislocations. It was observed that the 12Cr12Mo microstructure consists of interspersed columnar and equiaxed grains at the microscale, while fine body-centered cubic (bcc) lath martensite with high dislocation density is observed at the submicron scale. SLM-prepared 12Cr12Mo stainless steel exhibits impressive mechanical properties due to its hierarchical microstructure. Under optimal process parameters, the fabricated samples achieved a microhardness of 544.91 HV, with yield and ultimate tensile strengths of 729 ± 24 and 842 ± 19 MPa, respectively, but elongation is limited to 7 ± 0.6%. The cellular and martensitic structures with high dislocation density along grain boundaries are the mean factor for the increased strength but reduced ductility. Observations of a disintegrated surface and river-like patterns suggest a brittle fracture mode in 12Cr12Mo stainless steel prepared by SLM.

This study aims to elucidate the influence of varying TiC particle additions on the mechanical properties of Al-10Si-3.5Cu-2.5Ni-0.3Mg alloys. The alloys were fabricated using the gravity casting technique, with TiC additions of 0, 0.5, 0.75, and 1%. Following T6 heat treatment, the microstructure, tensile strength, and fracture mechanisms of the alloys were comprehensively analyzed. The research findings indicate that the microstructure is primarily composed of α-Al, eutectic Si, Al₃Ni, Al₃CuNi, (Al, Si)₂(Zr, Ti), and (Al, Si)₃(Zr, Ti) phases. Image J quantitative analysis indicated that increasing TiC content resulted in the refinement of both the eutectic silicon and the grains. Additionally, the Al₃Ni and Al₃CuNi phases gradually became spheroidized and had a homogeneous distribution. The 350 °C tensile strength of the alloy increased from 93.7 to 137.8 Mpa with increasing TiC content, an increase of 44.1 MPa (47%). This is mainly attributed to the stability of the (Al, Si)₂(Zr, Ti) phases at high temperatures and the refinement of the grains, eutectic silicon, and intermetallic second phases.

Application of nickel-based superalloys is gradually rising in many sectors such as aerospace, automotive and marine industries due to their exceptional thermo-mechanical properties. Superalloys are predominantly fabricated by casting process. The pursuit of customized materials possessing with exceptional properties has driven the scholars to investigate the possibility of powder metallurgy for preparing them and assess their appropriateness to produce gas turbine components. This study focuses on the use of powder metallurgy approach for synthesizing Nimonic 90 superalloy. Physical properties such as density, microhardness and macrohardness are determined to validate with the standard sample. Mechanical properties like tensile strength and compressive strength are measured and analyzed. Dry sliding wear test is done to study the wear characteristics. Density and hardness of the material are close to that of standard alloy, which is due to the appropriate selection of sintering temperature and stay time. Measured values for the ultimate tensile strength, 0.2% offset yield strength and percentage of elongation are approximately 900 MPa, 386 MPa and 52%, respectively. In microscopic study, it is observed that the material has γ/γ’ phase because of precipitation hardening and solid solution strengthening. Findings establish the fundamental basis for near-net-shape manufacturing by powder metallurgy.

NiMnCrMoW_x (x = 0.2, 0.4, 0.6, 0.8, 1.0 atomic fraction) high-entropy alloys are synthesized by mechanical alloying and conventional sintering techniques. Both alloy powders and the sintered pellets are characterized for microstructural, chemical, and mechanical properties. The phase analysis by x-ray diffraction (XRD) and high-resolution transmission electron microscopy of 70 h milled powder confirmed the dual phase of BCC as a major phase and FCC as a minor phase. Scanning electron microscopy is used for microstructural study of the milled powders and sintered pellets. The differential scanning calorimetry analysis of milled powders confirmed that these are thermally stable below 1000 °C. The XRD of annealed powders didn’t show new phases below 1000 °C, whereas 1000 °C annealed powders showed the presence of σ-phase; the XRD of the sintered pellets confirmed different volume fractions of σ-phase, MoNi₃, and MnNi phase. Results of Vickers hardness and wear studies indicated that the alloy containing 0.6 atomic fraction of tungsten possessed a maximum hardness of 644 HV₅ and maximum wear resistance. This might be attributed to the maximum extent of the σ-phase, MoNi₃, and MnNi phases in the W_0.6 alloy.

Although the self-piercing riveting (SPR) process is widely used in the automotive industry, it faces challenges in achieving mechanical interlock when joining high-strength steel. In this paper, the pre-holed self-piercing riveting (PH-SPR) process is adopted to join high-strength steel to aluminum alloy. This paper aims to investigate SPR joinability and select suitable rivets and dies for different steel–aluminum combinations. A 2D axisymmetric numerical model is developed using LS-DYNA commercial software to simulate the PH-SPR process with varying process parameters (e.g., rivet hardness, geometric dimensions of rivet and die, hole size, and material and thickness of sheet). The accuracy of the FE model is verified by comparing the forming quality parameters between the experimental test and the simulation result. The results show that (i) the rivet with strength of 0.9 GPa is suitable for the bottom sheet with yield stress of 89 MPa, and the rivet with strength of 1.34 GPa is appropriate for the bottom sheet with yield stress greater than 165 MPa. (ii) The increasing rivet diameter, rivet length, and hole size can improve forming quality, and the decreasing die depth and top sheet thickness can enhance the undercut. (iii) The undercut of the joint with 1.2 mm top sheet increases with the increase in yield stress of the bottom sheet, while a decreasing tendency is found for the joint with 1.6 mm top sheet. (iv) The minimum rivet length required for a successful joining increases with the increase in thickness ratio, while the opposite trend is observed for maximum rivet length.

Considering the microstructure grain heterogeneity of welded material can effectively reveal the mechanism during welding and improve welding quality. In this paper, a random microcrystalline model during laser-MIG hybrid welding was established based on the Voronoi method. The grain heterogeneity coefficient was determined, and the grain types were divided by nanoindentation experiments. The material properties were randomly assigned to Voronoi cells with a certain probability by writing a Python script program to introduce the grain heterogeneity structure. A moving heat source model of laser-MIG hybrid welding was established by programming a Fortran subroutine to couple Gaussian cone heat source and double ellipsoid heat source. The influence of the angle for the MIG welding gun on the heat input to weld pool was considered in the modeling, and the double ellipsoid heat source model was modified. Finally, the dendrite growth process of pure material was established by the phase field method, and the effects of anisotropy and flow velocity on dendrite growth were considered. The calculation shows that, compared with the conventional finite element model, considering the grain heterogeneity, the temperature field and stress field during laser-MIG hybrid welding show different changes. Among them, the temperature field difference is not significant, but the stress field shows an obvious uneven distribution. The stress at the adjacent grain boundary within the model abruptly changes, and the greater the difference in mechanical properties between grains, the more significant the mutation phenomenon. The phase field results reveal that the dendrite morphology is obviously asymmetrical when considering the flow velocity during welding solidification. This study provides an effective method to reveal the micro-evolution mechanism during laser-MIG hybrid welding and provides a reliable theoretical basis for improving the quality of hybrid welding and optimizing the hybrid welding process.

The degradation of microstructure and mechanical properties caused by overheating service of nickel-based single-crystal superalloys poses a serious threat to the safe use of turbine blades. In this work, the overheating treatment was conducted for a second-generation Ni-based single-crystal superalloy in the temperature range of 1100-1260 °C for 10 min. The effects of overheating temperature on microstructure and stress rupture properties (760 °C/800 MPa and 1050 °C/190 MPa) were studied. With the increase in the overheating temperature, the primary γ' phase progressively evolves in shape from cuboidal to spherical and then to petal shape. Meanwhile, the rate of precipitation and growth of the secondary γ′ phase increases with the rising temperature. In addition, the number and depth of interfacial grooves of experimental alloy increase with decreasing cooling rate. At 760 °C/800 MPa, the stress rupture life of the alloy after overheating at 1180 °C/10 min is abnormally increased to 228 h, which is higher than that of the standard heat treatment alloy. At 1050 °C/190 MPa, the stress rupture life of the alloy descends tardily with the increase in the overheating temperature.

Semiconductor, pharmaceutical, and food processing industries require highly clean and corrosion-resistant 316L stainless steel pipelines to prevent product contamination and ensure durability. This study employs an optimized electropolishing technique for small-diameter 316L stainless steel tubes, aimed at significantly reducing surface roughness and enhancing corrosion resistance to meet stringent industry standards. A comprehensive examination of the effects of electropolishing parameters—polishing time, current density, and temperature—on surface roughness and morphology was conducted. Optimal conditions were determined to be 105 seconds of polishing time, a current density of 50 A·dm^-2, and a temperature of 60 °C. Under these conditions, the tube achieves a mirror-like finish with a surface roughness of 0.063 μm. Corrosion resistance was characterized using electrochemical testing and x-ray photoelectron spectroscopy. The electropolished tube exhibits superior corrosion resistance compared to the mechanically polished tube, which is attributed to its increased thickness and chromium-rich passive film.