Solid State Phenomena Vols. 116-117

Abstract: Engineering safety diagnosis of crashed subway electric multiple units (EMUs) was conducted for safety assessment. Several advanced engineering analysis techniques including nondestructive evaluation (NDE) techniques and stress and structural analyses programs, were performed for better understandings and exploration of failure analysis and safety concerns. NDE techniques such as ultrasonic testing and magnetic particle testing, were used to detect manufacture-induced and/or in-service defects and collision-induced flaws, and measure the dimensions of deformed and non-deformed parts on damaged EMUs due to a rear-end collision. Moreover, stress and structural analyses using commercial I-DEAS software provided important information on stress distribution and load transfer mechanisms as well as the amount of damages during the crash. A good agreement has been found between structural analysis results and the results of actual damages in EMUs during crash. In this investigation, various advanced engineering analysis techniques for the safety analysis of subway EMUs have been introduced and the analysis results have been used to provide the critical information for the safety assessment of crashed EMUs.

Abstract: The aim of this study is to design flash geometry of bearing hub to apply the automatic transfer system in hot forging process. The flash geometry is very important in hot forging process because the flash geometry effects on the metal flow, material losses, forging load, die pressure and so on. In this study, the problem of designing the flash geometry is studied with flash thickness and width considering the maximum die pressure to apply an automatic transfer system in hot forging process for bearing hub. The numerical analysis was conducted by means of the commercial S/W DEFORM. On the basis of numerical analysis the flash geometry of hot forging die was redesigned, and experiment was conducted. From the experimental results, it was possible to produce bearing hub with an automatic transfer system without any deterioration of die lifetime.

Abstract: In order to investigate the effect of PVD coatings on the performance of proto-type high temperature mold and to identify the best candidate thin films for Cu semi-solid processing, TiAlN/CrN multilayer coatings with various superlattice periods was synthesized using a closed field unbalanced magnetron sputtering method with separate Cr and TiAl and characteristics of these films were compared with those from TiN/CrN and AlN/CrN films in terms of microstructure, nanoindentation hardness, and oxidation test at 900
in air. TiN/CrN film showed the best mechanical properties but their oxidation resistance, the most critical property for high temperature semi-solid processing was the poorest. Therefore TiAlN films with a superlattice period of 6.1nm which has intermediate plastic deformation resistance with the best oxidation resistance were considered to be the best candidate out of three different films studied in this work.

Abstract: The effects of die temperature and plunger velocity on the soundness of semisolid cast products were investigated in regards to the flow behavior of the slurry of A356 alloy in the H-NCM rheo-diecasting process. The sound cast was obtained with the die-temperatures of above 220
. The sudden changes of casting velocity resulted in irregular microstructures with micro-porosity and shrinkage defects due to turbulent filling pattern. The high casting velocity at the end of filling has no effect on the castability and the formation of casting defects in H-NCM(Hong-NanoCasting method). The Sr modification provides much finer and spheriodized eutectic silicon particles.

Abstract: By using equal channel angular extrusion (ECAE) as strain induced step in strain induced melt activated (SIMA) and completing melt activated step by using semi-solid isothermal treatment, a new SIMA method is introduced firstly. The results show that semi-solid billet with highly spheroidal and homogeneous grains with the average grain size of 20μm can be prepared by new SIMA method. High mechanical properties, such as ultimate tensile strength of 321.8MPa and elongation of 15.2% are obtained in magazine plate components thixoforged using semi-solid billet prepared by new SIMA.

Abstract: Transmission electron microscopy (TEM) was used to assess the microstructural changes accompanying semisolid molding of a creep resistant Mg-5%Al-2%Sr alloy. It was revealed that the processing created thixotropic morphologies with globular solids of α-Mg, having well-developed sub-structures. The quickly-solidified liquid fraction, enriched in Al and Sr, led to the formation of increased volumes of a lamellae type Al4Sr phase, surrounding the secondary α-Mg. Traces of Mg17Al12 still existed, predominantly as irregular shape compounds located mainly in grain interiors. Both the primary solid and high melting point precipitates of Mn-Al modified with Sr, acted as nucleation substrates for secondary α-Mg grains.

Abstract: Transmission electron microscopy (TEM) was used to assess the microstructural changes accompanying semisolid molding of a creep resistant Mg-5%Al-2%Sr alloy. It was revealed that the processing created thixotropic morphologies with globular solids of α-Mg, having well-developed sub-structures. The quickly-solidified liquid fraction, enriched in Al and Sr, led to the formation of increased volumes of a lamellae type Al4Sr phase, surrounding the secondary α-Mg. Traces of Mg17Al12 still existed, predominantly as irregular shape compounds located mainly in grain interiors. Both the primary solid and high melting point precipitates of Mn-Al modified with Sr, acted as nucleation substrates for secondary α-Mg grains.

Abstract: An investigative study was conducted on how two different dies designed to produce sound specimens for evaluating mechanical properties of semi-solid alloys could be filled optimally. The first die contains one cavity that produces a wedge with a sectional thickness varying from 19.05 mm (0.75 in) to 9.52 mm (0.375 in). The second die contains four cavities fed by a single runner, producing ASTM-type round bars with a diameter of 9.52 mm (0.375 in). Experiments were undertaken to cast partial and complete shots of A356 alloy test specimens under controlled injection conditions. Flow patterns developed from the moment the alloy entered the cavity were noted, including difficulties experienced in balancing flow in the four-cavity die. Microstructural abnormalities were observed in the bar specimens examined using optical and electron microscopy. It was found that control of filling and solidification in the wedge die was much easier and specimens produced did not result in any significant alloy segregation.

Abstract: There have been five academic conferences organized by the chairman of semi-solid processing academic committee, Prof. Shoujing Luo, in China since 2000. Based on the achievements of the five academic conferences, six aspects of research progress of semi-solid processing in China will be discussed here by the authors, which includes the research on fundamental theories of semi-solid processing, that is, theories of high-pressure solidification, plastic deformation densification and rheological filling; the research on technological theories of semi-solid processing, that is, theories of preparation of semi-solid slurries or billets and their spheroidization, theories of thixoforging and controlling of technological parameters, theories of double controlling of semi-solid die casting and semi-solid die forging and their numerical simulation; the research on characterization of semi-solid metal and their design; the application present status and potential prediction of semi-solid processing; present status of academic organization, academic communication and cultivating persons and the differences between the other countries and China. The research results show that China is a large country for metal processing, but China isn’t a strong in metal processing. As with other processing technologies, under the support the government foundations and other funds, more research work should be done in semi-solid processing and originality innovation and key technologies on preparation semi-solid slurries or billets and forming must be completely successfully in order to improve the competitive powder of semi-solid processing. It is should be noted that semi-solid processing will become a important forming method of simple, high efficient, low cost and good quality in the fields of materials processing.

Abstract: The solidification behavior of the semi-solid slurry with rapid shear-rate changes was investigated by using succinonitrile-5at.%water transparent alloy and Sn-15wt.%Pb alloy on a self-made apparatus. The results showed that the rapid changes of shear-rate improved the nucleation and promoted the globular growth of solidification microstructure in the stirred melt. The increase of shear-rate was in favor of promoting the grain growth with globular shape to a certain extent, however, when the shear-rate was beyond a critical value, the further increase of the shear-rate will lead the disappearance of the globular crystals. Thus, the microstructure of semi-solid processing could be refined by a controlled stirring process.