Damage of the machined surface and subsurface in orthogonal milling of FGH95 superalloy

FGH95 is one kind of powder metallurgy (PM) superalloy which has excellent mechanical properties at high temperature. It has been developed for turbine disc applications to improve aeroengine efficiency under higher operating temperatures. However, this kind of superalloy is very difficult to machine because of its poor thermal diffusivity and work-hardening properties during the machining process. The machining process can lead to damage of the machined surface and subsurface. Thus, the purpose of this paper was to investigate damage of the machined surface and subsurface in hard machining of FGH95 PM superalloy. Orthogonal milling experiments using coated carbide inserts were carried out on a CNC machining center. The machined surface was observed and recorded using an optical microscope, white light interferometer, and scanning electron microscope. Machined surface defects were recorded and analyzed. The effects of cutting speed on machined surface roughness, white layer thickness, plastic deformation, and microhardness were investigated. The research results show that better surface roughness can be generated at higher cutting speeds, while several defects appeared on the FGH95 machined surface. White layer thickness and machined surface microhardness increase with the cutting speed. Also, plastic shear strain in the machined surface layer increases with cutting speed. The depth of plastic deformation decreases with the increase of cutting speed. These investigation results are essential for the evaluation of PM superalloy surface integrity and are significant for the prediction of PM superalloy service life.