Constitutive Modeling And Experimental Study Of The Sawtooth Chips In High Speed Cutting

The cutting workability of difficult-to-machining materials is poor, its application further to be restricted in the aerospace industry as well as other industrial field, how to realize the high efficiency and low consumption machining of difficult-to-machining materials, is an embarrassed question which in the difficult-to-machining materials promotion application urgently awaited to be solved. The high speed cutting processing is one of the most effective process technologies to machine difficult-to-machining materials. The high speed cutting mechanism and the numerical simulation technology of difficult-to-machining materials are holding the important status in the material high-speed cutting domain. By the theoretic analysis, the experimental study and the numerical simulation, the physical modeling and the formation mechanism of sawtooth chip during orthogonal cutting of H13 hardened steel and Ti6Al4V titanium alloy are investigated in this dissertation. The main original fundamental research works are listed as following.The orthogonal cutting experiment of high-strength hardened steel material H13 was investigated, and then, the behavior of adiabatic shear and the law of chip morphology change are revealed. As the cutting speed increases, the chip morphology has in turn experienced four types which are the ribbon chips, the deformation sawtooth chips, the transformation sawtooth chips and the fracture sawtooth chips. There are two forms of adiabatic shear bands in the sawtooth chips, which are deformation bands with no organizational change and transformation bands with obvious organizational refinement. Deformation bands are caused by plastic formation, and transformation bands generation mainly come from re-crystallization, phase transition and the diffusion of carbon. Based on observation and analysis of microstructure,and measurement of microhardness in the sawtooth chips,it is pointed out that the deformation mechanism of saw-tooth chips is distinctly different from ribbon chips, and the microhardness of deformation bands results from work hardening, but the microhardness in the transformation bands is affected by phase transition hardening.The composition analysis near the adiabatic shear bands was performed by electron probe, and it is discovered that carbon content is lower. It can be explained that the short distance diffusion of carbon and the precipitation of carbide occur within the adiabatic shear bands according to the adiabatic shear theory in the high strain rate.A split Hopkinson pressure bar apparatus was used to measure curve of the true flow stress and true strain for Ti-6Al-4V and H13 alloys at the wide scope of temperature and high rate of strain. Based on the genetic algorithm and the BP neural network, effective methods of constitutive modeling in high speed cutting were developed. The material constitutive models with the physical conception have been established, which have high precision in forecasting material flow stress.The finite element analysis (FEA) was used to study the high speed orthogonal cutting process of the Ti6Al4V titanium alloy. The analytic results of FEA show that the first softening of workpiece material close to the edge due to high temperature and subsequent thermoplastic instability make the first distortion zone centralized slip and distortion which induce adiabatic shearing, and then the dentate chips appear. Meanwhile, experimental research of the high speed orthogonal cutting process for the Ti6Al4V titanium alloy was carried out. The results of experimental study and FEA show the similar trend, and it is proved that deformation mechanism of sawtooth chips is caused by thermoplastic instability.