اثر ارتعاشات آلتراسونيك بر فرايند ECAP و مواد ريز ساختار حاصل از آن

چكيده انگليسي :

Effect of Ultrasonic Vibrations on ECAP Process and Its Produced Ultrafine Grain Materials Farshid Ahmadi fahmadi@me iut ac ir Date of Submission 2014 11 20 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree Ph D Language Farsi Supervisors Mahmoud Farzin farzin@cc iut ac ir Mohammad Reza Forouzan forouzan@cc iut ac ir Abstract Equal channel angular pressing ECAP is one of the most prominent procedures for fabrication of ultra ne grained structures among various severe plastic deformation techniques The objective of this research is to investigate the in uence of longitudinal ultrasonic vibrations on ECAP process A robust experimental ECAP system was designed and manufactured in which the punch can be excited by ultrasonic vibration ECAP experiments were carried out under ultrasonic vibrations as well as conventional procedure on pure Al The microstructure of the specimens processed with ultrasonic assisted ECAP and conventional ECAP were characterized The results of this study shows that superimposing ultrasonic vibration on the ECAP process can improve the grain re nement e ciency Homogeneity of the microstructure was also improved by using ultrasonic vibrations Higher vibration amplitudes caused more homogenous structure The yield strength and ultimate tensile strength of the specimens after one pass of ECAP were higher in comparison with the conventional ECAP It was also observed that a reduction in the forming force was obtained while using ultrasonic vibration In order to reveal the main cause of volume effects of ultrasonic vibrations on the plastic behavior of pure aluminum specimens an experimental tensile test equipment was designed and made in which the specimens could be excited by ultrasonic vibrations with a frequency of 20 kHz Five specimens with different grain sizes were prepared by ECAP Tensile tests of the specimens were performed at room temperature and at constant speed of 0 2 mm min under static load and superimposed ultrasonic excitations It was found that ultrasonic vibration had a remarkable influence on the plastic behavior of pure aluminum and after applying ultrasonic vibrations flow stress of the all specimens reduced Reduction of flow stress was dependent on grain size Larger grain sizes had more flow stress reductions This observation proved that softening due to ultrasonic vibrations is caused by movement of dislocations within the grains and grain boundary mechanisms are less effective In other words the volume softening caused by ultrasonic vibrations known as Blaha effect is attributed to absorption of ultrasonic energy by dislocations and consequently their easier movement within the grains 2D and 3D Finite element analyses were performed to simulate a conventional and ultrasonic assisted ECAP Two models were considered in the simulations In the first model the die and the punch were considered as rigid bodies while in the second model the punch was considered as an elastic body The proposed 3D model of the ultrasonic assisted ECAP based on the elastic punch was validated with experimental data A new route for ECAP process was also proposed and was validated by FEM and experimental studies The results showed that the strain homogeneity achieved by the proposed route was better than the other conventional routes Keywords ECAP Ultrasonic vibrations Grain size Force Homogeneity ultrasonic softening Route FEM

احمدي، فرشيد

اثر ارتعاشات آلتراسونيك بر فرايند ECAP و مواد ريز ساختار حاصل از آن

نانو ساختار , اندازه دانه , نيرو , همگني ساختار , نرم شدگي آكوستيكي