Grain Refinement Mechanism And Experimental Study Of Ultrasonic Assisted Extrusion Cutting Chip

Ultrafine grained(UFG)materials have attracted extensive attention from scholars because of their excellent properties such as high toughness,high strength,high strain rate and low-temperature superplasticity.If UFG materials are applied to micro part,its overall performance will be significantly improved.Due to the large shear strain,the UFG materials are prepared by refining the microstructure inside the material,and its grain size can even be refined to nano scale.A large number of literatures show that if ultrasonic vibration is introduced into SPD in the same conditions,the grain refinement degree of the material surface will increase and a thicker nanostructure layer will be formed.At the same time,the mechanical properties of materials have been significantly improved.The combination of ultrasonic vibration and SPD can obtain better machining results than the traditional machining methods.In order to improve the efficiency of preparing UFG materials by traditional SPD,ultrasonic assisted extrusion cutting(UAEC)is proposed in this paper as a new machining method.In this paper,the ultrasonic assisted extrusion cutting pure copper is taken as the object.Firstly,the motion trajectory of the cutting edge is analyzed,and the theoretical models of strain,strain rate,cutting force and cutting heat are established.The finite element model of ultrasonic assisted extrusion cutting is established,and the effects of machining parameters(ultrasonic vibration mode,amplitude and tool rake angle)on the parameters of cutting deformation zone are studied in detail.The experimental study of ultrasonic assisted extrusion cutting pure copper is carried out based on the theory and simulation study.The changes of cutting temperature,microstructure and mechanical properties of pure copper chips in different tool rake angles and ultrasonic amplitudes are studied.The main strengthening mechanisms of improving the pure copper chip mechanical properties are discussed,and the feasibility and superiority of ultrasonic vibration assisted technology to improve the machining efficiency of traditional SPD are verified.The research shows that internal structure of pure copper chips is greatly refined after ultrasonic assisted extrusion cutting.When the tool rake angle is 30°,the grain shape is mainly elongated grain,and it will be gradually refined to equiaxed grain with the decrease of tool rake angle.At the same time,increasing the ultrasonic amplitude can improve the grain refinement of the material.The XRD analysis shows that higher dislocation density will be produced in the chip due to the impact of ultrasonic vibration.The chip hardness is directly proportional to the ultrasonic amplitude,and the hardness of pure copper chip is about 8.1%higher than EM chip when A_Y=5μm,and the hardness of chip meets the Hall-Petch relationship.