Materials Science Forum Vol. 749

Abstract: In this study, MoCu gradient materials were prepared by the combination methods of liquid phase sintering and permeability copper. The effect of pressure and sintering temperature on the properties of MoCu gradient materials was studied. The physical and mechanical properties of MoCu20/MoCu40 and MoCu20/MoCu30/MoCu40 gradient materials were tested respectively. The results showed that the relative density of green compact and sintered gradient materials increased with the increase of pressing force from 10 tons to 30 tons. The electrical conductivity and hardness of sintered compact achieved the maximum value by the 20 tons. Within the sintering temperature range of 1100 to 1400, the relative density, electrical conductivity and hardness of sintered gradient materials increased with the increase of sintering temperature. The overall properties of sintered materials were obtained at 1350. For two-layer and three-layer MoCu gradient materials, their microstructures and chemical compositions showed a continuously and gradient change. The bending strength and the thermal conductivity of three-layer MoCu gradient materials were better than that of two-layer gradient materials.

Abstract: A study on grain evolution character of discal parts roll forming was carried out by means of experiment and numerical simulation. The discal part material is aluminum alloy 6061 and titanium alloy TC4. The roll forming temperature is 480-500 and 930-1020 for aluminum alloy 6061 and titanium alloy TC4, respectively. A digital double-sided roll forming machine was used for roll forming experiment of discal parts. The high frequency induction heating was used for disk heating, where the deformation zone is in the state of constant temperature. A numerical simulation of roll forming of the 6061 aluminum alloy disk was carried out. The simulated results include macroscopic deformation and various grain size evolution variables. The dynamic recrystallization, the dynamic and static grain growth were considered in the numerical simulation. The experimental and simulated results showed that there may appear not only the grain refinement due to dynamic recrystallization but also dynamic and static grain growth. It is different that from discal parts integrally forging there are the largest deformation, the highest strain rate and the best recrystallization and grain refinement in the zones contacted with the roll tools. In the zones far from the rollers, the strain rate is very low and average grain size increases until once again contact to the rollers and start new grain refinement.

Abstract: In the this paper, the compositions design with low carbon and addition of sulfur and manganese which can enhance fish-scale resistance by MnS as hydrogen trap was adopted to produce DC03EK cold-rolled enamel steel. The effect of sulfur and manganese contents on formability and fish-scale resistance of DC03EK cold-rolled enamel steel was investigated. Results show that yield strength, ultimate tensile strength, elongation, work hardening exponent and average plastic strain ratio of the steel with sulfur content of 0.024wt% and manganese content of 0.22wt% is 133 MPa, 310 MPa, 36.8%, 0.22 and 1.46, respectively, showing the well formability. And its hydrogen penetration time and diffusion coefficient is higher than 12 min and lower than 9.26×10^-7 cm²/s, respectively, showing excellent fish-scale resistance. However, for the steel with sulfur content of 0.012wt% and manganese content of 0.12wt%, even though better formability is obtained, the fish-scale resistance is poor with hydrogen penetration time of 4.7 min and hydrogen diffusion coefficient of 2.36×10^-6 cm²/s which is far from the expectation of steel for enamel.

Abstract: The microstructure and mechanical properties of AZ81magnesium alloys after unequal channel angular pressing (UCAP) was investigated using self-made 90° die under different temperature. The results showed that under the same pressing speed, there was a better grain refinement and mechanical property improvement with the decreasing of pressing temperature, however, the tiny cracks on the surface of the processed samples increased. When squeezed at 250 , the average grain was refined from the initial 150um to10um. The tensile strength was changed to 350Mpa, and the elongation was of 10. Tensile fractures presented the increment of dimples structure, which was the result of refined α-Mg matrix and intermetallic compound β-Mg₁₇Al₁₂ phase by severe shear and plastic deformation at high temperature press.

Abstract: Upon non-equilibrium solidification, the intrinsic parameters, such as moving velocity, temperature, solute partition coefficient, and liquid and solid concentrations at the interface, deviate from their equilibrium characteristics, and the morphology of the as-solidified structure and the grain size are influenced by the non-equilibrium liqulid/solid transformation, which further influences the subquent solidstate transformation. Adopting molten glass purification technology combined with cycle superheating method, the microstructure evolution of Ni-11at.%Si alloy in different undercooling was investigated. It was found that, with the increase of the initial undercooling, grain refinement occurred in microstructures of undercooled Ni-11at.%Si alloy. Meanwhile, the NL model was used to discuss the two different dendrite morphologies. According to Karmas model for dendrite fragmentation, the grain refinement of undercooled Ni-11at.%Si alloy was in good agreement with the experimental data, and the grain size was reduced with the increasing ΔT. The energy-dispersive spectroscopy (EDS) measurement was applied to analyze the solid solubility of Si atom in α-Ni matrix. It was found that the solid solubility of Si atom in α-Ni matrix increased with undercooling. At the undercooling of T>220K , a complete solute trapping occurred.

Abstract: As a novel material, Mg-based foam material not only has fantastic physical characteristics, such as low density, high specific surface area, high specific strength and stiffness, and good biocompatibility, but also has special functional properties, for example, electromagnetic wave shielding, vibration reduction, sound absorption, and so on. It can be widely applied to aeronautical and aerospace, military, shipbuilding, transportation, automotive and medical industries. Mg-based foam material was prepared by direct foaming in magnesium alloy liquid in this study, and the effect of technological parameters on the products, such as addition amount of and granularity of SiC particles and MgCO₃ particles, stirring temperature, stirring time, stirring velocity, foaming temperature and foaming time, was investigated. The aim of this research was to develop a new technology which could fabricate large scale Mg-based foam material in air condition. The results showed that variations of technological parameters may affect preparation of the foam materials in some extent and resulted in the changes of the products in apparent density, porosity and structural uniformity. The light weight Mg-based foam with homogeneous pores could be obtained by suitable combination of the technological parameters.

Abstract: Heat-resistant steel (12%wt. Cr) used in the ultra supercritical set blade was quenched and tempered by vacuum heat treatment furnace, the mechanical properties and metallographic structures at room temperature can meet the specific requirements. However, there are some problems existed on high-temperature short-time lasting performance. To improve the high temperature endurance performance of materials, the experimental testing and analysis were carried out fro different types of specimen. The research showed that short-time rupture is a kind of creep fracture, and short lasting strength was closely related to test temperatures between laths and grain boundary and carbide coarsening. Energy spectrum analysis revealed that cast in the hole had high B element content (19.98%wt.), which meant grain boundaries of the B at high temperature began to migrate and form in the crystal B in that second phase particles are contained, and grain boundary strength was obviously decreased. The enduring high temperature experiment indicated that the strength were declined rapidly and fractured suddenly at high temperature.

Abstract: In present work, bimetallic composite materials (BCMs) were prepared by one-step sintering in box-furnace followed by quenching in salt furnace. The morphology and mechanical properties of the fabricated BCMs were characterized systematically. The results showed that Ni60-WC/Cr12MoV BCMs with a typical transition zone of ~480 μm could be successfully prepared by sintering at 1120 °C for 15min followed by furnace cooling to 400 °C and air cooling to ambient temperature. The resultant samples were heat treated by preheating at 860 °C, heating at 1020 °C, quenching at 520 °C, and tempering for two times at 200 °C for 2h. The average micro-hardness of the BCMs was measured to be of 869.34, 755.44~413.68, and 758.02 HV, which corresponded to the parts of Ni60-WC, transition zone and Cr12MoV, respectively. The interface binding strength of the BCMs was 234.22 MPa.

Abstract: High-temperature shock consolidation and under-water shock wave are two effective methods to eliminate cracks generated when shock wave propagating the powder bed. In this work, a novel assembly consists of a chemical furnace and a water column was used to fabricate tungsten-copper composites. The heat released from the reaction of a SHS reaction mixture was used as chemical furnace to preheat the precursor powder. The water column as well as the explosive attached was detached from the furnace by a solenoid valve fixed on the slide guide. So the explosive and water column was kept cooling during the preheating process. The W-Cu powders with the grain size of 2μm were first blended with mass ratio of 9:1 by mechanically alloying in a planetary ball mill. Prior to application of shock wave, the elemental powders were preheated at different temperatures, i.e. the highest temperature up to 1000°C. The intensity of the shock wave loading was under 10GPa. The consolidated specimens were then characterized by microstructure analysis and micro-hardness testing. The different micromechanical behaviors of W and Cu phase in the consolidated sample were studied by using in situ high-energy X-ray diffraction technique. The result showed that a fine-grained 90W/10Cu composite with no cracks could be compacted to a density of 16.44g/cm3 by hot-shock consolidation at a preheating temperature of 970°C.

Abstract: The rapid development of micro-system and its related products has caused an upsurge of micro components, and the micro powder injection molding is potential for its mass production. In this work, the solvent debinding mechanism of the in-house feedstock was studied. The influence of solvent debinding, thermal debinding and sintering on the micro gears was investigated. The weight loss of 3Y-ZrO2 micro gears ranging from 200 to 900μm were compared by using six types of solvent including ether mixture withanhydrous alcohol, petroleum ether, chloroform and are hexane etc. The thermal debinding scheme was determined based on the DSC and TGA results of the binder system. The effect of the heating rate on the micro gears was investigated. The experimental results showed that the petroleum ether mixture with anhydrous alcohol yielded the best debinding effect for the micro gears, which was controlled by diffusion and dissolution. Using the established debinding and sintering schedule, the defect free micro gears were successfully fabricated.