Key Engineering Materials Vol. 904

Abstract: The influence of W addition on microstructure and mechanical properties of Ti-Al-Sn-Zr-Mo-Nb-W-Si high temperature titanium alloys are investigated by optical microscope (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), tensile tests and large stress endurance tests at 650 °C. The results show that W is mainly solubilized in β phase. Microstructure observations indicate an obvious reduction in the size of transformed β structure (β_t), primary α phase (α_p) and the thickness of secondary lamellar α phase (α_L), with the increase of W content. It is also observed that adding more W could improve the elongation, tensile strength and large stress rupture properties at 650 °C. However, combined with previous research, adding more β stabilizing elements could refine the size of each phase, which will be detrimental to the high temperature yield strength of the alloy. Therefore, in order to reasonably utilize the strengthening effect of W and make the alloy have high yield strength and tensile strength at 650 °C, its content should be controlled between 1 ~ 2 wt%

Abstract: The development of modern high-tech industries of industrial production is impossible without the development of new methods for processing materials with high mechanical characteristics. There is a growing need for an increase in the proportion of parts made of aluminum alloys, a more complex configuration of cast parts, an increase in their reliability and durability in operation, etc. All this poses for metallurgists and foundry workers the task of creating new technologies for producing alloys, improving the technical and economic characteristics of structural materials, improving the quality and reducing the cost of castings.Hypoeutectic silumins have good casting properties, good weldability, machinability and corrosion resistance. However, they are prone to the formation of a coarse needle-like state, which reduces their useful characteristics. To eliminate this phenomenon, it is necessary to apply special technologies and the most common is their modification, which provides grain refinement. This makes it possible to use silumins for the manufacture of castings of complex shapes with increased density and low shrinkage porosity. Such parts can withstand average loads in critical units. Aluminum-silicon alloy AK7 or (ASi7Mg0.3) is a typical silumin, which is in demand in the automotive industry, construction, aircraft construction, machine, automobile and tractor production. It is appreciated for its good casting properties, weldability, machinability and corrosion resistance.

Abstract: ECAP is a continuous multi-pass extrusion process that enables the specimen to obtain considerable cumulative deformation to refine the grain. In this paper, ECAP was used to deform AS41 magnesium alloy at 350°C, and the microstructure was observed and analyzed. The results show that the ECAP process has excellent effect on grain refinement and uniform microstructure. The grain size of AS41 decreases from 200μm to 20μm, and the microstructure is more uniform than that of as-cast sample. The reason is that the original grain is broken and refined under the action of shear force, and dynamic recrystallization occurs at the same time, resulting in small recrystallized grains. The Mg₂Si particles were redistributed during ECAP and uniformly distributed in the crystal in rod shape.

Abstract: In this work, temperature distribution in semi-solid cast 6063 aluminum alloy workpieces during friction stir welding (FSW) was determined by finite element analysis (FEA). The FEA results were validated by comparing them with the measurement results from thermocouples. The maximum temperature of 534.2oC was predicted at the workpiece surface contacted with the tool shoulder. The temperature profiles obtained from FEA were used to explain microstructural changes during FSW. It was observed that relatively high temperature made α-Al grains became elongated and Mg₂Si intermatalics turned into a rod-like morphology with round edges.

Abstract: The effect of pre-aging on properties of Cu0.24Cr0.20Sn alloy before rolling and aging was studied in details. The results displayed the pre-aging was useful to improve the microhardness and conductivity of Cu0.24Cr0.20Sn alloy before cold rolling and aging, and the effect increased with the extension of pre-aging time. The microhardness and electrical conductivity of Cu0.24Cr0.20Sn alloy by first pre-aging at 400 °C for 2 h, second 85% rolling and then aging at 300 °C for 1 h can reach 189 HV and 85.4 %IACS, respectively. The TEM results indicated the density of precipitates increased with the increase of pre-aging time, and the interaction between precipitates and dislocations was gradually strengthened in the subsequent room-temperature rolling. The increase caused by pre-aging treatment before rolling and aging was mainly due to dislocation density strengthening.

Abstract: In this study, the mechanical properties and intergranular corrosion properties of 347H austenitic stainless steel were studied by tensile test, impact test, double-ring electrochemical potentiodynamic reactivation test (DL-EPR) and microstructure observation in three states of solution, primary and twice stabilized state. Results showed that the key mechanical properties of 347H stainless steel under three different conditions had little change, and the mechanical properties at room temperature were not affected by the stabilizing heat treatment. After 12h of sensitization, the solution material showed obvious sensitization behavior, and the Ir/Ia index exceeded 0.3, indicating that the material entered the range of complete sensitization. Both primary and twice stabilizing heat treatment can significantly reduce the occurrence time of sensitization and prevent the sensitization process. However, the stabilizing heat treatment cannot completely prevent the material sensitization, and it must be combined with other methods .

Abstract: In this paper, two kinds of heat affected zone (HAZ) simulation structures of 347H stainless steel, which are coarse grain zone (CGZ) and unmixed zone (UZ), were prepared by thermal simulator. The material properties of toughness, reheat crack susceptibility and intergranular corrosion susceptibility of the two kinds of HAZ simulation structures were studied by impact test, high temperature tensile test, electrochemical potentiodynamic reactivation (EPR) test and micro morphology test. The result shows that CGHAZ had better toughness. But after the stabilizing heat treatment, it was weakened while that of the UZ was enhanced. The reheat crack susceptibility of the CGZ and UZ both increases after stabilization heat treatment, and the tendency of the UZ are more obvious. Stabilizing heat treatment has a certain effect on the prevention of sensitization process, which can improve the intergranular corrosion resistance of the material. Stabilizing heat treatment is double-edged to 347H HAZ, and it needs combined with the specific situation to used.

Abstract: Since the 21st century, the development of automotive and home appliance industries has greatly contributed to the prosperity of the plastics industry, which has led to an increasing demand for molding molds. In the production process, the molds will be impacted, worn and corroded, especially in the production of plastic products made of PVC, fluoroplastics and flame retardant ABS, etc. The molds will be corroded by the corrosive gases generated such as hydrogen chloride, hydrogen fluoride and sulfur dioxide, which requires plastic molds to have a certain degree of corrosion resistance while ensuring strong toughness.S136 SUP is a modified martensitic stainless steel based on S136 from ASSAB, Sweden, which has fine-tuned the content ratio of some elements to ensure a certain strength and high toughness at the same time. The chemical composition, heat treatment and microstructure of the steel have a certain influence on its corrosion resistance, and the tempering temperature of the heat treatment has a greater influence on the corrosion resistance of martensitic stainless steel. Therefore, in this paper, the microstructure and corrosion resistance of S136 SUP at different tempering temperatures are explored and studied.

Abstract: For obtaining a unique microstructure in Ti-6Al-4V, hydrogen is utilized as a temporary alloying element; therefore, the mechanism of hydrogen diffusion in α and β phases should be understood. In this study, the electrochemical hydrogenation was applied to the half-length of thin titanium rods, and the diffusion annealing heat treatment was implemented at different temperatures. The hydrogen diffusion coefficient of α phase (D_α) and the hydrogen diffusion coefficient of β phase (D_β) was determined by employing Abaqus software and C^# program for three different homogeneous microstructures. The obtained results showed that D_β increases, and D_α decreases when the hydrogen concentration in β phase increases. Furthermore, it was observed that each microstructure has a specific temperature in which the maximum hydrogen amount is absorbed. The hydrogen uptake depends more on the volume fraction of β phase than the volume fraction of α phase, which is considered an obstacle to hydrogen diffusion in this alloy.

Abstract: We investigated the kinematic viscosity and electrical resistivity of the multicomponent Fe₇₄Cu₁Nb_1.5Mo_1.5B_8.5Si_13.5 melt during three heating–cooling cycles. The temperature dependence of kinematic viscosity and electrical resistivity have the anomalous zones in the same temperature range and they are associated with the liquid–liquid structure transition (LLST). The anomalies were explained by changes in the activation energy and the cluster size. As the cluster size decreases, the activation energy decreases, but the viscosity and electrical resistance increase. LLST begins with the cluster dissolution, and as a result, the Arrhenius plot becomes nonlinear in the transition temperature range. After three cycles of heating–cooling, the temperature dependences of the kinematic viscosity and electrical resistance did not qualitatively change, and this allows us to conclude that LLST is thermoreversible. With an increase in the number of thermal cycles, the activation energy of viscous flow decreases, as well as the onset temperature and temperature range of LLST.