| The main failure forms of metal materials are stress fatigue and surface corrosion.Ultrasonic impact treatment(UIT),which is based on ultrasonic vibration as the power source,eliminate harmful stress and introduce of high pressure stress by pins impact surface of metal materials.Compared with the traditional shot peening process,UIT can significantly improve the metal surface fatigue limit and anticorrosion capability.It has the advantages of small equipment,flexible use,energy saving,high efficiency and easy control,so it is widely used in industrial applications.But the existing ultrasonic impact equipment for processing with single pin or multi pins array,need longer time for achieving high coverage.As one of the lightest metal structure materials,AZ31 B wrought magnesium alloy has the characteristics of high specific strength,high specific stiffness and excellent machinability.It can meet the application requirements of diversified engineering structures.Magnesium alloy oxide film is loosely porous,and its structure is unstable.Corrosion problems of magnesium alloys limit its application in most corrosive environment.In this paper,the ultrasonic impact process is controlled digitally through the additional rotating system and servo system.Achieving the high coverage rate of ultrasonic impact strengthening the surface of AZ31 B magnesium alloy,is in order to improve the surface fatigue limit and corrosion resistance.The main work content as follows:(1)Analyzing the process of single pin impact plate according to the theory of plastic dynamics,elastoplastic mechanics and contact mechanics.Solving stress distribution and pit parameters about the impact pit.Consequently,the obtained results of the theoretical value are close to the simulation results.The impact coverage function is designed by MATLAB software.The error of the theoretical value and the simulation result is within 10%.The theoretical impact force is both close to the simulation results and the measured values in the impact distance of 1mm.(2)Based on the wave equation and the traditional composite horn design theory,a conical transitional stepped composite horn with larger magnifying coefficient and better shape factor is designed.By means of ANSYS platform,modal analysis and harmonic response analysis of horn are carried out,and its structure is optimized.The optimized result is that the amplification coefficient of horn is increased by 70% compared with the original one.Through impedance analysis and laser vibration analysis,the results show that the optimized horn has better mechanical vibration performance.The error range of the resonant frequency and output amplitude compared with the simulation results is less than 3%.The optimized horn accords with the requirements of ultrasonic impact test.(3)A rotary ultrasonic impacting testing device designed and manufactured.Single pin ultrasonic impact forces are measured by dynamometer in different impact distances.The results show that the smaller impact distance,the greater impact force.Rotary assisted ultrasonic impact experiments on AZ31 B magnesium alloy was carried out.The strengthening effect of magnesium alloy was evaluated by surface hardness test,surface residual stress test and electrochemical corrosion test.The results show that with the decreasing both of workpiece moving speed and array rotation speed,the surface pressure and surface hardness increase,and surface corrosion resistance is slightly enhanced.The maximum stress on the surface is-76.01 MPa,and the surface hardness is about 25.1% higher than that of the substrate.(4)The ABAQUS/Explicit dynamic module is used to simulate the different impact models.Using different diameters' pins impact magnesium plate one or more times.Then,the distribution of residual stress and the change of the parameters of the impact pits are analyzed with the above models.Different horizontal velocities are set up for single pin to simulate the impact effect of different rotation speeds.Using the model of multi pins array,magnesium alloy strengthened by rotating auxiliary ultrasonic impact is simulated and analyzed.The simulation strengthening effect is evaluated with the residual stress and the impact coverage. |
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