Surface Residual Stress Prediction And Experimental Research For High-speed Milling Of Aerospace Aluminum Alloy

With significant advantages of light weight, less assembly work, better anti-corrosion performance and higher fatigue strength, large aerospace thin-walled monolithic structures have been widely used as load-bearing structural components in the field of aviation. Due to its high cutting efficiency, high surface quality of the workpiece and less cutting force and heat, high-speed milling is widely used in aerospace industry, especially for large aluminum thin-walled monolithic components manufacturing. Surface residual stress is inevitably generated during the interaction between the tool and workpiece in milling process. The nature and magnitude of surface residual stress have great effects on the fatigue life of components, which has a direct impact on aircraft reliability and security. So it’s urgent to study the internal relations between the high-speed milling process and surface residual stress.Metal cutting is a very complex process involving material deformation and dynamic fracture. The mechanism of chip formation and machined surface evolution is very complicated. Surface residual stress is closely related to the strong non-uniform stress field formed by coupling between mechanical stress and thermal stress during machining process. Nowadays constitutive models of numerical analysis mostly use empirical fitting equations which have poor prediction accuracy under large strain and high strain rate. Also they can’t consider the adiabatic softening effect of the material during high-speed milling process, which reduces the prediction accuracy for surface residual stress. This paper carries out systematic study for high-speed milling of aerospace aluminum 7075-T651 on surface residual stress prediction and process control method by means of numerical simulation and experimentsFirstly, dynamic compression tests have been conducted on Split Hopkinson Pressure Bar device with temperature range from 25℃ to 400℃ and strain rate range from 600s-1 to 12000s-1. Flow stress data was obtained under different temperatures and strain rates. The adiabatic shear effect at high strain rate was observed by means of metallographic experiments. Using experimental data, a physically based constitutive model, which reflects the characteristics of large strain, high strain rate, high temperature and adiabatic shear effect for high-speed milling condition was built for aluminum alloy 7075-T651.Based on existing research, the relations between equivalent failure strain and stress triaxiality in three different regions(compression, shear, tensile & shear) were obtained. Using physically based constitutive equation as material model and fracture criterion as chip separation criteria, a two-dimensional variable thickness continuous milling simulation model considering tool rotation and feeding motion was created. The simulation model was validated based on experimental data.Because of numerical simulation limitations, high-speed milling experiments were carried out to study the influences of cutting parameters, cutting tool coating, tool geometry parameters and coolant on surface residual stress. The mechanism of surface residual stress formation was analyzed from the perspective of coupled mechanical-thermal effect.This paper is aiming to solve the problem of surface residual stress prediction difficulty and accuracy for large aviation monolithic thin-walled structure components. A constitutive model of aluminum 7075-T651 reflecting the characteristics of large strain, high strain rate and adiabatic softening effect was built. The relationships of fracture strain and triaxiality in three regions(tensile, shear, shear & compression) were obtained. A two-dimension simulation model considering tool rotation and feed movement was created in order to get the non-uniform stress field and temperature field distribution of high-speed milling. Surface residual stress prediction accuracy of high-speed milling was improved. Simulation results can be used as initial conditions for followed-up assembly simulation. At the mean time high-speed experiments were carried out to study the factors that influence the surface residual stress. From the perspective of aerospace structures service performance, process control method of surface residual stress for high-speed milling was obtained, which can give process guidance for large aviation monolithic thin-walled structures for high-speed milling.