Researches On Residual Stress And Work Hardening On Micro-milling Nickel-base Superalloy Inconel718

In recent years, there is an increasing demand of micro parts in aerospace, electronics and optics fields. These micro parts are required to have high strength and high corrosion resistance under high temperature working environment such as micro turbine blades. The nickel-base superalloy can meet the requirements under such situations for its high strength and high corrosion resistance under high temperature. However, it's a typical difficult-to-machine material due to large plastic, low thermal conductivity and being easy to work harden. Micro-milling is a useful technology in machining micro parts with high accuracy and complicated topographies and it's suitable for machining micro parts of nickel-base super alloys. Mechanical properties of micro-milled parts determine their service performance and life. Residual stresses will cause severe deformation for thin wall parts, which influences the machining accuracy and assembling accuracy. Compressive residual stresses can improve the fatigue resistance while tensile residual stresses weaken it. Excessive work hardening can damage fatigue property and corrosion resistance of micro parts and accelerate tool wear. Therefore, the research of residual stresses and work hardening caused by micro-milling is of great importance for improving the service performance and life of micro parts working under high temperature.This paper aims to study the residual stresses and work hardening caused by micro-milling. A three dimensional (3D) finite element simulation model was set up to simulate micro-milling nickle-base superalloy Inconel718 process. Stresses results of the model were used to predict the residual stresses in experiments. Strain results combined with a mathematical connection between residual strain and microhardness were used to predict the microhardness. Micro-milling experiments were conducted to validate the predicted results. The single factor influences of feed rate per tooth on residual stress were studied. The single factor influences of spindle speed, feed rate per tooth and axial cutting depth on microhardness were studied and analyzed based on experimental results and their interactions were studied based on experimental results with the Response Surface Method. The main researches in this paper are as follows:Firstly, a 3D finite element simulation model based on ABAQUS software to simulate the micro-milling Inconel718 process was set up. Clamping, milling, tool retracting and constraints translation were simulated in the model. Residual stresses and residual strain field were obtained.Secondly, residual stresses under different feed rate per tooth were simulated in both the feed direction and the direction vertical to feed in the finite element simulation model. Micro-milling experiments were conducted and residual stresses of the micro-milled surface were measured with X ray stress analysis to validate the simulated results. Influences of feed rate per tooth on residual stress were studied.Finally, residual strains under different spindle speed, feed rate per tooth and axial cutting depth were simulated in the finite element simulation model. A mathematical model between residual strain and microhardness was set up. Combine the mathematical model and residual strain results to predict the microhardness of micro-milled parts. Micro-milling experiments were conducted and microhardness of the micro-milled surface were measured to validate the predicted results. The single factor influences of spindle speed, feed rate per tooth and axial cutting depth on microhardness were studied based on experimental results and their interactions were studied with the Response Surface Method.