Research And Expriment On Eliminating Residual Stress By Ultrasonic Vibratory Aging

In practice, different degrees of residual stress generally exist in workpiecesduring forging, machining, welding, heat treatment, assembling assembly andother processes. Residual stress may reduce the intensity, the fatigue limit andthe structural stability of workpieces, and even results in cracks or deformation.Therefore, eliminating the residual stress within the workpieces is of greatimportance in the production process. In general, there are three methods toeliminate the residual stress, including natural aging, thermal aging andvibration aging. Owing to the advantages of low energy consumption, low cost,high efficiency, and high environmental protection, the vibration aging hasgradually replaced the natural aging and heat aging process.The current domestic research on vibration aging mainly concentrates onthe low frequency vibration aging, based on which many equipments have beenwidely used in production and achieved good performances. However, theexisting method is difficult to eliminate the residual stress in small parts, sincetheir first-order natural frequency is beyond the low frequency range. On theother hand, with the development of products miniaturization and precision, thedemand for dimensional stability of small parts becomes higher and higher. Forthe purpose, in this study, ultrasonic vibration aging is proposed to eliminate the residual stress in small parts. Its main features are low amplitude but highfrequency, large energy input per time unit, short time for the elimination ofresidual stress. Up to the present, few work has been done in the mechanismstudy, equipment development and applications of ultrasonic vibration aging.The main content of this study is as follows:(1) Built up an ultrasonic vibration device and press platform, designultrasonic amplitude and bar used in the experiments, and calculate the residualstress in the bar. In addition, ANSYS is used to simulate the residual stressinside the bar, the simulation error of maximum residual stress is small. Theresults indicate that simulation helps to obtain the distribution of internalresidual stress in bar accurately.(2) Using the X-ray diffraction method to accurately measure the surfaceresidual stress before and after ultrasonic vibration aging treatment of metal rod.The experimental results show that the peak value of residual stress decreasesobviously, and the overall distribution of residual stress is even. It proves theeffectiveness of the ultrasonic vibration aging in residual stress elimination forsmall parts.(3) To explore the new method to assess the residual stress elimination,scanning electron microscope is introduced to observe the dislocation densitybefore and after ultrasonic vibration of workpieces. The effect of residual stressreduction can be directly assessed by comparing the distribution of thedislocation density. (4) This study discusses the micro mechanism of ultrasonic vibrationaging on residual stress reduction. For the first time, the differences ofmechanisms between ultrasonic vibration aging and low frequency vibrationaging is proposed. The effect of ultrasonic vibration in residual stresselimination depends on the vibration energy gained by parts of the internal grain.Even the sum of residual stress and dynamic stress is less than the yield limit ofmaterial, the ultrasonic vibration aging can also reduce the residual stress inworkpieces, which has been validated by the experiments.