| NiTi alloy possesses a good shape memory effect and an excellent pseudoelasticity. It is applied widely in aeronautics and astronautics, electromechanics, chemical industry, and bio-medical industry. Since the NiTi alloy has a property of high strength, and its plasticity at low temperature is relatively poor, an excellent comprehensive property can not be achieved merely through regular processing. Recently, relative good results had been achieved applying equal channel angular extrusion method in the processing of the NiTi alloy at a high temperature. However, the deformation process is complex and hard to be observed from experiment, which leads to little improvement made in its application. For those reasons, the crafts are needed to be improved for its popular industry application.Based on the studying of the ECAE experiments of the NiTi alloy, this paper applied finite element analysis to the whole ECAE process from pre-heating to extrusion process. And a series of important results were achieved.First, the heating procedure of the NiTi alloy specimen was analyzed by the finite element analysis (FEA) and theoretic calculation was completed. The study showed that: the internal temperature of the 10mmX10mm specimen rose up rapidly at a 770℃furnace temperature at the beginning of heating. Then 1200s later, the rise speed of temperature was not significant; When the heating time lasted to 800s, distribution of temperature in the specimen became uniform, which provided gist for the latter FEA for ECAE. The FEA result of heating procedure is in accordance with theoretic calculation.Secondly, the stopping period of the specimen on the pre-extrusion stage has a significant impact on the ECAE process. Generally speaking, the overall process has better not last for more than 6s at a 750℃preheating temperature and a 500℃mold temperature. However, when the stopping period was less than 2s, the ECAE process will almostly not be influenced.Thirdly, a three dimensional model of the NiTi specimen was established based on the large deformation thermoplastic coupled finite element analysis. The ECAE process with different fiction coefficients at a 90ocircumscribed arc angularΨand a 90o mold internal angularΦ. The results indicated that the distributions of the equivalent strain along all three crosses sections were not even no matter there exists friction influence or not. Meanwhile, as the friction factor increased, the uneven level of the internal equivalent strain distribution of the specimen reduced, but the maximum extrusion force increased sharply.Fourthly, the extrusion heat has great influence on the final extrusion results. It can be seen clearly through the analysis that the high temperature area due to the plastic deformation heat was in accordance with the oxidized area on the surface of the specimen. This also indicates that when apply ECAE, the heat factor should be taken account in.Fifthly, a new near equal channel angular extrusion method (NECAE) was first established, and simulated with the finite element method. The results indicated that the equivalent strain distribution of the specimen processed by NECAE method was much even than that used the ECAE method. And the amplitude of the extrusion force was much smaller. This means that this new method can reduce the impaction on the mold.Through the computer aided simulation and analysis, this paper studied the preheating process and stopping time process by FEA; Thermoplastic coupled FEA for ECAE was built, distributions of equivalent strain under different friction condition were analized; a new NECAE model was created and was introduced into the NiTi alloy thermoplastic coupled FEA extrusion process. The study improved the research for high temperature of ECAE, supplied theoretical proofs to the related experiments, and on the other hand provided optimal solutions to the extrusion molding processes of the NiTi alloy and other hard to machine alloys (e.g. aluminum bronze alloy). And what's more, this paper can give guidance to the similar thermoplastic coupled finite element analysis. |
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