The Study On The Application Of The Improved Constitutive Equation For Titanium Alloy In High Speed Milling

Ti-6A1-4V titanium alloy is often used in the aircraft industry due to the good compromise. However, this material is known to be difficult to machine. The high speed machining is one of the most effective process technologies for the contradiction between increasing machining demand and poor machining performance. The high speed machining is one of the most important approaches to the difficult-to-cut material, and the numerical simulation technology is the most effective way for the high speed machining.While the constitutive models study is the key technology in the high speed machining mechanism research. The widely used Johnson-Cook constitutive model is not suitable for Ti-6A1-4V alloy in the high-speed cutting finite element simulation; it has not response material dynamic recrystallization softening effect under high pressure and high temperature, and thus can not correctly reflect the stress state, and deformation characteristics of adiabatic shear band and serrated chips, and so on during high-speed cutting. In view of the complexity in nature, a method combining theoretical analysis with experiment and finite element simulation is proposed in this dissertation. Starting from the microscopic structure of titanium alloy Ti-6A1-4V, a new modified constitutive model of Ti-6A1-4V titanium alloy based on recrystallization is proposed considering the phenomenon during machining. The main investigation contents are listed as following:(1)The dynamic recrystallization condition and law of Ti-6A1-4V alloy at all strain rates are investigated in the paper. The isothermal constant strain rate experiments are performed on Ti-6A1-4V alloy, which discoveried that the deformation temperature is proportional to the frequency of dynamic recrystallization, and a Arrhenius equation of Ti-6A1-4V alloy is established. It is systematically researched from the microstructure of chips, surface roughness and cutting force during the high-speed cutting. The relationship between the cutting factors, chip formation, cutting force and surface roughness is built. The research shows that it has the possibility of dynamic recrystallization at all strain rates. The plastic deformation resistance decline and the thermoplastic instabilitydue occur due to the dynamic recrystallization softening effect, and the adiabatic shearing band and serrated chip is found in the Ti-6A1-4V alloy.(2)J-C constitutive model is inaccurate for machining simulation of titanium alloy with high temperature and heavy impact.In the paper, a new modified J-C constitutive model of Ti-6A1-4V alloy based on recrystallization softening effect is established. High-temperature SHPB (Split Hopkinson Pressure Bar) test was conducted to determine the true flow stress-strain relationship of Ti-6A1-4V alloy. With the SHPB test data, the modified constitutive model was developed by the variable separation method. The modified J-C constitutive model expresses as two expressions according to the different characteristics of material constitutive during critical strain values. The modified J-C constitutive model is proved correct and more suitable for the SHPB test curve than the J-C constitutive model, especially in the high-temperature range.(3)A subroutine is coded and embedded in the finite element simulation software AdvantEdge FEM with the return mapping stress integration algorithm. The comparison is carried between the experimental data of milling and the simulation, and it shows that the modified J-C constitutive model is more suitable for the practical milling process. The new modified J-C constitutive model is built by the incremental method, and the the comparison is carried on of stress characteristics and chip formation in high-speed milling. It shows that the modified J-C constitutive model is more suitable than J-C constitutive when it express formation process of adiabatic shear band, chip morphology, stress and shear characteristics in the high-speed milling.(4) A new high-speed cutting database system is established by the network database technology, DNC technology, genetic algorithms and other technical. The system combines high-speed cutting parameter optimization and recommendation, the material constitutive generated automatically, finite element simulation, as well as tool database management, which realizes the recommendation and management of the HSM parameter in its full life cycle.