Transformation of Microstructure and Mechanical Properties in the Machined Surface of Powder Metallurgy Superalloy

Abstract

During the machining of powder metallurgy (PM) superalloy parts, the machined surface bears severe plastic deformation which leads to the transformation of microstructure and mechanical properties. White layer which reflects the transformation of microstructure and mechanical properties, often appears on the top of machined surface during hard machining of PM superalloy. White layer has significant effect on the machined surface integrity. The effects of cutting speed on the white layer formation have been investigated in order to reveal the transformation of the microstructure and mechanical properties in the machined surface of PM superalloy FGH95. Results show that white layer thickness increases with the increasing of cutting speed. The machined surface exhibits densification with no obvious structural characteristics. FGH95 superalloy bulk material exists in the form of Ni-based solid solution, while the microstructure of white layer is significantly different from that of bulk materials. It's because of the microstructure of Ni-based solid solution which has transformed during the cutting of FGH95. The higher the cutting speed is, the more obvious the grain refinement is. A higher cutting speed could also lead to higher values of hardness in white layer. Residual stresses in the machined surface of FGH95 are tensile in all cutting conditions, which show an increasing trend with the increasing of cutting speed. This research can provide the theoretical basis for the investigation and controlling of machined surface quality.