Key Engineering Materials Vols. 353-358

Abstract: The group velocity of the S0 mode of a Lamb wave under the first cut-off frequency is measured during the tensile loading for the unidirectional composite materials. The group velocity of the S0 mode has the characteristic of increasing with the strain. The propagation velocity of a wave is generally proportional to the square root of the ratio of the modulus to the density. The elastic modulus is considered as a constant as the stress linearly increases with the strain. The increase of the group velocity with the strain is caused by a decrease in the density. It is proposed that the measurement of the group velocity of the S0 mode is useful for making an evaluation of the tensile strain and of any degradation in the unidirectional composite materials.

Abstract: Mechanical properties of the TiAl alloy produced by centrifugal spray deposition (CSD), compared to that produced by ingot metallurgy (IM), were investigated at different temperatures from 293 to 973K. The result shows that the ultimate strength, yield strength and plasticity of the CSD TiAl alloys, with excellent compression properties and plasticity, are higher than those of as-cast TiAl alloys at room temperature as well as at high temperature. There exists a critical temperature of 873K in the relationship between strength and temperature, in which strength increases with increasing temperature above 873K. The effects of CSD on mechanical properties of the TiAl alloy are discussed, and the higher strength with moderate ductility achieved is because of the finer lamellar structure got in the CSD processing, and this structure is also believed to be beneficial to ductility.

Abstract: Dual-phase steel is a mixture of ferrite matrix and martensitic islands distributed at grain boundaries with the possible addition of bainite. Its formability, capacity to absorb crash energy, and ability to resist fatigue make it a desirable material for use in the automotive industry to enhance automobile’s strength and reduce its weight. However, the resistance spot weldability of dual-phase steel is sometimes limited by a propensity for weld metal failure due to the welding process. To improve its weldability, a servo gun spot welding system is introduced to control electrode force during the welding process. The paper take 1.5mm DP600 steels as an example, experimental data shows that by optimizing electrode force during welding process, the width of weld lobe can be enlarged from 0.9kA to 2.1kA and the weldability of dual phase steel is improved greatly.

Abstract: Nanostructure surface layer was fabricated on a low carbon steel cylinder specimen by means of circulation rolling plastic deformation (CRPD), and the effect of annealing temperature on microstructure and properties of surface nanocrystalline structure was studied. The microstructure of the surface layer on the samples was observed by transmission electron microscopy and the microhardness variation along the depth was measured on the cross-sectional samples by using microhardness instrument. After CRPD treatment for 250min, the average grain size was about 10nm in the top surface layer and increased with an increment of the distance from the top surface. The surface nanocrystallization samples were annealed at 200°C, 300, 400°C and 500°C for 30min respectively. The nanocrystallization grain of surface layer did not grow for samples after annealed at 200°C and 300°C. After surface nanocrystallization by CRPD treatment the microhadness of top surface obviously increase from 220HV0.1 to 520HV0.1.

Abstract: 1.1%Li was added to 7075 alloys to obtain the Li-containing Al-Zn-Mg-Cu alloys. The microstructure and hardness of the alloys are investigated by transmission electron microscopy (TEM) and Vickers hardness. The hardness of the single-aged alloys is low. When the alloys were double-aged or multi-aged, the hardness is comparable to that of Al-Zn-Mg-Cu alloys at peak ageing. Two peaks were present in the hardness curves of the multi-aged Li-containing Al-Zn-Mg-Cu alloys. With the last-step temperature increases, two-peak phenomenon becomes prominent. The density and size of precipitates are influenced remarkedly by the ageing processes. Coarse grain boundary precipitates and PFZ (precipitate free zone)can be observed when the Li-containing Al-Zn-Mg-Cu alloys were multi-aged, and the higher the last-step ageing temperature, the wider the PFZ is.

Abstract: Inconel 690 alloy is a Ni-Cr-Fe austenite solid solution alloy with a wide range of applications such as steam generator tubes in nuclear power plant due to its good mechanical properties and corrosion resistance. In this study, the effects of heat treatment on the microstructural properties of Inconel 690 alloy were investigated. Inconel 690 alloy was annealed at solid solution temperature of 1150°C for 20 min, and cooled down to 800°C with different cooling rates, and then aged at 800°C for 100 min in vacuum, and then cooled down to room temperature by water quenching. The optical microscopy, scanning electron microscopy, and transmission electron microscopy results indicated that in case of the cooling rate of 0.5°C/min, discontinuous carbides along the grain boundaries were formed and when the cooling rate was 10°C/min, continuous carbides were formed. In both cases, many annealing twins were also formed. These results can help to improve the understanding of microstructural properties of Inconel 690 alloy.

Abstract: High pressure die casting is the most common method in making magnesium alloys for both auto parts and 3C products. Pressure variations in the mold during mold filling and solidification process have direct influences on the quality and properties of die castings. In this paper, a cylinder head cover was produced to experimentally study pressure variations in the mold during magnesium alloy die-casting process in real time for the first time. Pressure varies at different positions in the mold during die casting process. This study indicates that mold filling and solidification process of magnesium alloy die castings can be described by pressure curves obtained by pressure measurement at different test positions in the cavity in real time.

Abstract: Laser shock processing (LSP) employs high-energy laser pulses from a solid-state laser system to create intense shock waves into a material, which can induce compressive residual stresses in the target surface and improve its mechanical property efficiency. Residual stress of Ti6Al4V alloy both before and after LSP with multishocks was analysised. The depth of compressive residual stress was found to have a dependence on the number of shocking layers and a slight dependence on the level of irradiance. Surface stress improvements of more than 50% increases are possible after laser shock processing with either large spot or small spot patterns. The large spot gave a surface stress of 432MPa and a depth of over 1mm. The low intensity small spot gave a surface stress of 285MPa with a depth comparable to the large spot. Laser shock processing induces a compressive residual stress field, which increases fatigue crack initiation life and reduces fatigue crack growth rate.

Abstract: Warm compacting and sintering behaviors of pre-mixed Fe-2Ni-2Cu-1Mo-1C powders were studied. Results showed that green density increased with compacting temperature and then fell slightly; the maximum green density was obtained at about 120°C. Green compact density and spring back effect of the pre-mixed powder increased gradually as the compacting pressure increased. Sintered density first increased and then fell as the temperatures went up under different sintering temperatures for 50 minutes, but the trends of sintering dilatation were first reduced and then increased. Sintered density first reduced and then increased with the prolonged sintering time at 1300°C, but the trends of dimension change after sintering were the very reverse. Tensile strength, elongation and hardness of the warm compacted Fe-2Ni-2Cu-1Mo-1C materials generally increased as sintering temperature and sintering time went up.