Study On The Mechanism Of Ultra-fine Grain Strips Produced By Ultrasonic Vibration-assisted Large Strain Extrusion Machining

Large Strain Extrusion Machining(LSEM)is a new type of ultra-fine-grained strip preparation technology with large plastic deformation.The processed material undergoes the dual action of cutting and extrusion to form a regular strip with a set thickness.In the traditional large strain extrusion machining process,due to the friction between the tool and the chip,the surface quality of the strip is poor,and there are many defects.Secondary processing is required to meet the requirements of use.Ultrasonic vibration cutting can improve the surface quality of the processed workpiece,widely studied and has been applied in practice.To overcome the shortcomings of the traditional large strain extrusion machining process,this paper combined the advantages of ultrasonic vibration cutting,and proposed a new type of approach,namely Ultrasonic Vibration assisted-Large Strain Extrusion Machining(UV-LSEM).In this paper,AUV-LSEM processing and results are the research objects,and simulations and experiments are carried out to analyze and study the forming mechanism of chips,the change law of material microstructure,and the optimization design processing parameters.The main tool and the constraint tool blade's cutting motion trajectory equations in the process of axial and tangential ultrasonic vibration-assisted large strain extrusion machining are established,and the theoretical cut marks on the chip surface are predicted.The optimal ultrasonic vibration direction is obtained by finite element simulation technology.The results show that under reasonable cutting and ultrasonic vibration parameters,Axial Ultrasonic Vibration assisted-Large Strain Extrusion Machining(AUV-LSEM)has a more significant equivalent strain,and equivalent strain rate is more conducive to improving chip microstructure and mechanical properties.The AUV-LSEM finite element model and the LSEM grain size prediction model were established.The influence of tool rake angle,chip thickness compression ratio,and ultrasonic amplitude on the cutting deformation zone's deformation parameters,such as equivalent strain and equivalent strain rate during AUV-LSEM and LSEM processing,are compared and analyzed.The LSEM chip grain size distribution is obtained,and it is found that the simulation results are in good agreement with the experimental results.Through the AUV-LSEM experiment,this paper analyzed the actual cut marks on the chip surface and verified the correctness of the theoretical cut marks.Put forward the quality evaluation index of the chip surface,compare and analyze the surface quality of AUV-LSEM chips and LSEM chips,compare and analyze the microstructure and Vickers hardness of AUV-LSEM chips and LSEM chips.Design a multi-factor orthogonal experiment,comprehensively apply the ANOVA method to analyze the average hardness of the chip surface statistically,and compare the results of AUV-LSEM and LSEM optimization design experiments.The dynamometer installation table was designed,and the highest temperature and average cutting force generated during the processing of AUV-LSEM and LSEM were compared and analyzed.Through comparative analysis experiments,it is found that the surface quality of AUV-LSEM chips is better,and there are fewer defects,while the surface of LSEM chips has a large number of defects such as scales,grooves,and cracks.The axial ultrasonic vibration can effectively reduce the friction coefficient between the tool and the chip,thereby reducing the friction force.The high-frequency vibration in the extrusion channel reciprocates "ironing" the chip surface,and the chip's surface quality can be significantly improved.The average grain size of AUV-LSEM chips is smaller,the dislocation density is greater,and the Vickers hardness is greater.That is because the axial ultrasonic vibration accelerates the accumulation of the dislocation density and causes the acoustic residual hardening effect,which promotes the deepening of the grain refinement in the chip and the increase of the average surface hardness.The optimized design experiment result of AUV-LSEM is similar to that of LSEM.The maximum temperature and average cutting force during AUV-LSEM processing are less than LSEM.That is because the axial ultrasonic vibration reduces the generation of cutting heat and reduces the frictional force,thereby reducing The cutting temperature and average cutting force are calculated.