Application Of Brittle Fracture Characteristics Of Materials To Chip Formation Control At High Strain Rate

As a typical material of aviation light alloy,aviation aluminum alloy 7050-T7451 has low density,high specific strength,high specific stiffness,excellent high temperature resistance,corrosion resistance and fatigue resistance.It can meet the lightweight requirements of aerospace equipment and is widely used in the manufacture of aircraft structural parts.High-speed cutting technology is often used due to the large amount of removal during processing and the length of cutting.During the high-speed machining process,the local high strain rate and high temperature of the material in the deformation zone will cause the dynamic mechanical properties of the plastic material to exhibit brittle material characteristics,and plastic-brittle transformation will occur to form saw-toothed chips.At present,the high-speed cutting damage model is not used to consider the plastic-brittle transition effect of chip formation under high strain rate.Therefore,it is an important theoretical basis and premise to realize the active control of chip formation by establishing the damage model of plastic-brittle transition characteristics.Firstly,the material fracture failure process under high strain rate was studied,and the pre-fabrication of the sample was carried out by considering the texture difference generated in the forming process of the pre-stretching and rolling.The dynamic mechanical properties and fracture characteristics were analyzed by Hopkinson rod,scanning electron microscopy（SEM）,ultra-deep-field microscopy,metallographic microscope and electron backscatter diffraction.The dynamics under high strain rate loading and the causes of the fracture characteristics were analyzed.The critical conditions for plastic-brittle conversion were revealed.Secondly,quasi-static tensile test simulation of different notch radii was conducted with the help of finite element method.Meanwhile,quasi-static(0.001-0.1s^-1)and dynamic(1000s^-1,2000s^-1)tensile tests were conducted to establish the initial j-c damage model by comparison.The dynamic elastic modulus and strain at break of the material under the plastic-brittle transition strain rate(4000s^-1)during the tensile process were analyzed.At the same time,combining the elastic modulus and strain relationship of J-C constitutive,the dynamic brittleness function is introduced to characterize the brittleness of plastic materials.A J-C damage model considering plastic-brittle transition was established and applied to finite element simulation to simulate the chip formation process under different parameters.Finally,the cutting simulation results with different cutting speeds were verified experimentally to verify the validity and reliability of the simulation and experimental results.The process simulation under different process parameters（cutting speed,feed and cutting depth）was compared to analyze the chip morphological evolution law under different cutting speeds,The chip formation mechanism under the influence of high-speed cutting plastic-brittle transformation was revealed,and the critical cutting speed of sawtooth-shaped chip transformation was obtained.The degree of sawtoothing and the frequency of sawtoothing were used as evaluation indexes to compare and verify the accuracy of the verification model.The ultra-high speed（4000-6000m/min）cutting simulation was carried out to characterize the chip morphology at higher speeds and predict the deformation behavior of machining chips,providing data and application support for the active control of chip morphology.