Materials Science Forum Vol. 749

Abstract: Cu/MgB₂ composites with different content of MgB₂ (10, 20, 30 vol.%) were fabricated by vacuum sintering of copper and MgB₂ powders. The effects of MgB₂ content and sintering process on the properties of Cu/MgB₂ composites including the microstructures, relative density, electrical conductivity and hardness were then investigated. The results showed that the distribution uniformity of MgB₂ on copper matrix decreased obviously with the increase of MgB₂ content, the hardness of Cu/MgB₂ composites increased and the relative density decreased slightly though the electrical conductivity decreased greatly. The relative density and hardness of Cu/MgB₂ composites increased after the repressing-resintering process, the electrical conductivity was also improved slightly.

Abstract: Working conditions, failure forms and property requirements of cold working die steels were presented. This review attempted to introduce the development of cold working die steels, the ways to improve its quality, heat treatment and surface treatment technology at home and abroad. Meanwhile, the characteristic and application prospects of spray-formed cold working die steels were analyzed.

Abstract: t is well known that excellent soft magnetic properties are obtained in electrical steel with Si content of 6.5wt.%, such as high permeability, low coercive force, near zero magnetostriction, etc. However due to its poor ambient temperature ductility it is very hard to fabricate this material by conventional hot-cold rolling process. Meanwhile, high-density electropulsing treatment (EPT) attracts widespread applications as a new heat treatment approach. In this paper, heat treatment effect of high-density pulsing current on microstructure, micro-hardness and mechanical properties of as rolled Fe-6.5wt.%Si alloy sheet were investigated. The results showed that the microstructure and mechanical properties were closely related to the parameters of the pulses. Refined and uniform recrystallized grains were obtained by optimizing parameters of the high density pulses. Compared with the conventional heat treatment method, the time and temperature for heat treatment can be reduced dramatically. Therefore, it is considered that EPT is an efficient heat treatment approach for this rolled sheet. Mechanism of the effect of high-density pulsing current has been discussed also in this investigation.

Abstract: The fine and homogenous distribution of the SiC particles in Al metal matrix is basic prerequisite for improving the properties of the SiCp/Al composites. In this paper, the effects of high energy ball milling and ordinary ball milling on the spatial distribution of reinforcement of the SiCp/Al composites have been investigated. The result showed that high energy ball milling is the most effective method to get homogeneous distribution of SiC particles in Al matrix. There were many clusters of SiC particles in the composites fabricated by ordinary ball milling.

Abstract: According to the metal powder injection molding process, the main influence factors of injection molding billet density distribution (such as: injection velocity, injection temperature, injection pressure, etc) was analyzed and a multiple input & multiple output BP neural network model for injection molding was build up to predict the density distribution of the billet intelligently based on ANN and GA. In addition, in light of the requirements for the density distribution of the metal injection molding billet, the influence factors were controlled intelligently. Applying this model in the metal injecting process, the density distribution of billet was predicted according to the injection parameters and the injection parameters was optimized according to the required density distribution of the billet. As the result, the error was less than 5% between the prediction values and the actual values of the density distribution of billet. With the optimized injection parameters to the injection process, the density distribution of billet closed to the requirements was achieved.

Abstract: The copper alloy/30CrMnSi steel bi-metal composite materials were prepared by the interface diffusion bonding method. The diffusion of elements close to the bonding interface was studied and the formation and growth mechanism of dissolution layer were discussed as well. The results showed that a diffusion transition layer could be formed with the different widths for copper alloy/30CrMnSi steel integrated material. A diffusion transition layer was formed close to 30CrMnSi steel side due to the inter-diffusion of the alloy elements. The microstructure characterization showed that no harmful brittle phase presented around the interface, and two heterogeneous materials had a good metallurgical bonding.

Abstract: In this paper, the effects of different TIG welding parameters on corrosion resistance of 321austenitic stainless steel welding joint were studied. Intergranular corrosion, stress-strain curves of SSRT and the fracture surface of the welding joint were investigated by means of intergranular corrosion test, SCC test and SEM. The results showed that the intergranular corrosion cracks did not occur for all joints welded in the range of TIG welding current of 130A~190A. In pure water at room temperature, strain rate at the range of 1×10^-5 s^-1~1×10^-6 s^-1 had little effect on the maximum tensile strength of the material under the same welding conditions and the time of joint fracture increased as the strain rate decreased. At strain rate of 1×10^-6 s^-1, the maximum tensile strength of the joint welded in the current of 130A and 150A decreased significantly in the dyeing assistant and presented stress corrosion sensitivity. Joints welded in the current of 170A and 190A, showed excellent resistance to stress corrosion. Through the analysis of the fracture surface of joints, the joint welded in current of 130A presented a tendency to brittle fracture, while the joints in the current of 170 A and190 A were characterized with ductile fracture.

Abstract: Friction stir welding (FSW) experiments were carried out on AZ31 magnesium alloy under the optimized welding parameters. The temperature distribution, macrostructure and microstructure, mechanical properties of the joint were studied along three dimensions (transverse direction, longitudinal direction, thickness direction). Temperature distribution, joint appearance and microstructure demonstrate asymmetric gradient in three dimensions. The peak temperature of featured points at the welding beginning stage was lower, and the one in the ending stage was higher. The temperature of advancing side was higher than that of the retreating side. More in detail, the dynamically recrystallized microstructure in weld nugget zone (WNZ) was uniform and small. The thermomechanically affected zones (TMAZ) closed to the WNZ, which were characterized with bended and elongated grains. At advancing side, the interface between TMAZ and WNZ was very distinct. The microhardness distribution showed a typical W shape. The profile showed a slightly lower hardness in the WNZ than in the base metal,HAZ, and TMAZ correspond to the lowest one. The FSW joints were observed to fail mostly at the boundary between WNZ and TMAZ at the advancing side.

Abstract: The effect of cryogenic treatment on mechanical properties and wear resistance at room temperature of 06Cr19Ni10 austenitic stainless steel was studied in the present study. The cryogenic treatment process was carried out at-160 and different time in program-controlled cryogenic container. The results showed that, after cryogenic treatment, the room-temperature mechanical properties of the 06Cr19Ni10 steel remained stable without embrittlement, friction coefficient of the 06Cr19Ni10 steel decreased and the wear resistance improved after cryogenic treatment. When cryogenic treatment was treated at-160 for two hours and thirty minutes, the friction coefficient of the 06Cr19Ni10 steel was the smallest and wear resistance was the best.

Abstract: The two kinds of flower-like ultrafine cobalt particles were prepared by reducing cobalt chloride (CoCl₂·6H₂O) with hydrazine hydrate (N₂H₄·H₂O) under ultrasonic and microwave radiation, in which ethanol-water or ethylene glycol-water mixture was used as solvent. The morphology, crystal structure and magnetic properties of the as-prepared particles were characterized by scanning electron microscope (SEM), x-ray diffraction pattern (XRD) and vibrating sample magnetometer (VSM). The results show that the petals of the flower-like cobalt particles prepared in the ethanol-water system were dendritic, while the petals of the flower-like cobalt particles prepared in the ethylene glycol-water system were sword-like. The crystal structure of cobalt particles prepared in the two kinds of systems both consisted of hexagonal close-packed cubic phase and face-centered cubic phase, but the relative content was different. The saturation magnetization of the cobalt particles with dendritic petals and the cobalt particles with sword-like petals was the same approximately, but their coercivity was greatly different (the difference in value about 7184.14Am^-1), which might be attributed to the magnetocrystalline anisotropy and shape anisotropy.