High Speed Deformation Behavior And Machinability Of Pure Iron In High Speed Machining

As a kind of metal material with high plasticity,low hardness and superconducting magnetism,pure iron is widely used in military equipment,aerospace,power electronics and other industrial fields.In the manufacturing process of high-speed machining,dynamic punching and stamping of pure iron parts,the deformation speed has a great influence on the plastic deformation process and fracture behavior of workpiece materials.In this paper,the plastic deformation process and fracture behavior of pure iron are studied by changing the strain rate,and the processing technology is developed according to the plastic deformation behavior under different strain rates,which provides a craft reference for the efficient and high-quality processing of pure iron.The main contents and conclusions are as follows:Firstly,the double notch shear test of pure iron with impact speed of 16-64 m/s is carried out by improved SHPB technology in the room temperature.The dynamic deformation behavior of the shear deformation zone is collected and observed by high-speed camera and metallographic microscope,respectively.According to the obtained video pictures and metallographic micrographs,the dynamic deformation process is analyzed.The variation of the true shear band width,the relationship between the nominal strain?strain rate?and the true strain?strain rate?,and the distribution of the fracture strain in shear deformation zone are explored.The results show that the double notch shear specimen undergoes four deformation stages:uniform deformation,localized shear,crack initiation and macroscopic fracture.The localized shear band formed in the stage of localized shear deformation results in the decrease of the true shear band width,and the difference forms between the real strain?strain rate?and the nominal strain?strain rate?.The fracture strain of the specimen is composed of the strain at the stage of uniform deformation and localized shear deformation.Secondly,the double notch shear test of pure iron in the large range of strain rate(3.3×10^-3-2.5×10⁵ s^-1)is carried out by MST universal testing machine and improved SHPB technology in the room temperature.The deformation and fracture behavior of the shear deformation zone are analyzed and observed by metallographic microscope and SEM,respectively.The deformation and fracture mode of pure iron at the different strain rate are explored.Fracture strain in the range of large strain rate is calculated by metallographic micrograph of shear deformation zone,and the plastic-brittle transition point of pour iron is obtained.The results show that there are three different deformation?fracture?modes for pure iron in the large range of strain rate:isothermal deformation?fracture?,adiabatic deformation?fracture?and sound velocity deformation?fracture?.When the strain rate reaches 1.74×10⁴ s^-1,the material embrittles.The fracture strain decreases rapidly with the increase of strain rate.Finally,according to the deformation characteristics of pure iron under different deformation speeds,the cutting technology based on the thermal softening phenomenon of pure iron is put forward.The cutting experiment of pure iron is carried out by changing the cutting edge type of PCBN tool to obtain different deformation states,and the high-speed cutting experiment of pure iron is carried out to explore the influence of cutting speed on chip shape and verify the plastic-brittle transition of pure iron material.A reasonable cutting edge type is obtained through the analysis of chip shape,tool wear,machined surface roughness,machined surface morphology and surface microhardness.Thus,on the premise of ensuring the tool life,the machining accuracy and quality of pure iron can be improved.The results show that the negative chamfering angle of PCBN tool is 30°,the width is 0.05mm and 0.2mm respectively,and the tool with the angle of 10°and the width of 0.1mm is more suitable for machining of pure iron.At the cutting speed of 9000m/min,the spallation phenomenon of pure iron chips has taken place,forming typical brittle intergranular and transgranular fracture morphologies.