A finite element analysis based compliance method coupled with wet etching to determine residual stress in high speed milling

High speed milling (HSM) is widely used in automotive and aerospace industries in fabricating mechanical components from high strength aluminum and other alloys due to high productivity and good surface finish. HSM induced residual stresses may significantly impact the fatigue life and corrosion resistance of the machined components. Traditional methods of residual stress (RS) measurement, such as hole drilling, X-ray diffraction, and neutron diffraction, are very time consuming and expensive, especially for the shallow subsurface (usually <100 μm) of a machined component. The compliance method provides a convenient alternative to these approaches to determine the residual stress distributions in the subsurface. However, the compliance method using wire EDM is prone to experimental errors. In addition, the traditional approach to calculate compliance function is very complex. This paper presents a new wet etching approach to obtain strains as a function of slot depth introduced in the subsurface. The strain readings were collected from a strain gauge mounted on the specimen surface near the slot edge. The compliance function can be conveniently calculated by simulating slot cutting using the finite element method via a Legendre polynomial subroutine as the applied load. These calculated compliance functions and measured strain values at different depths were used as inputs into a program to calculate residual stress. This leads to much a faster and less expensive method of determining residual stresses when compared with the traditional methods of residual stress determination. The capability of this new approach was demonstrated by high speed milling 6061-T651 and 7050-T7451 aluminum alloys. A design of experiment (DOE) method was adopted to conduct fifty-four cutting conditions with three levels of cutting speed, feed rate, and depth of cut. Residual stress profiles with twelve data points with spatial resolution as small as 1 μm in the subsurface were then obtained using this new approach. Residual stress sensitivity to cutting conditions was investigated. In addition, subsurface microstructure and microhardness were characterized.