Surface Integrity For High Speed Machining Of Powder Metallurgy Superalloy FGH95

PM superalloy with high tensile properties has been developed for turbine disk applications in advanced turbo-engines due to its homogeneous and fine grains. Although machined surface integrity of PM superalloy has significant impacts on the parts service performance and fatigue life, there is lack of sysmatic research done in this area. Thus, the aim of this dissertation is to investigate the surface integrity in machining of PM nickel-based superalloy FGH95. In this study, the machinability of FGH95has been investigated according to its material properties. The machined surface integrity such as surface roughness, work-hardening, residual stress, white layer, surface plastic deformation and its influencing factors in the machining of FGH95are analyzed. The outputs of this research can provide theoretical base and technical guidance for the control of cutting parameters to improve the PM superalloy parts machined surafce properties in the actual production.The investigations on the microstructure, phase, non-metallic inclusions inside the material and material mechanical properties can provide references in the machined surface microstructure and mechanical properties changes in the machining of FGH95. Contrast to the machinability of GH4169superalloy, the effects of cutting parameters on the cutting forces and cutting temperature are revealed. The effects of cutting speed on chips morphology are investigated. In order to study the chip roots fracture properties in the cutting process, the orthogonal milling experiments are carried out and SEM observation is used to analyze the chip root fracture properties. The periodical fracture mechanism in FGH95chip formation is proposed, the nature in the machining of FGH95is revealed, which lays the foundation for the study of the machined surface integrity.Taking into account the influence of elastic recovery on machined surface, the surface roughness theoretical model in orthogonal milling of FGH95is established. The influences of depth of cut and cutting speed on the surface roughness are investigated in orthogonal milling of FGH95. The results show that the machined surface roughness increases with the increasing of depth of cut. The roughness values generated in the conventional cutting speed range are higher than roughness values generated in the high speed machining. The effects of tool flank wear on the machined surface roughness are investigated through the cutting tests. The blunt standard of coated carbide cements in the cutting of FGH95PM superalloy is proposed. The depth of work-hardening layer model is established according to the relationship of cutting forces and stress in the machined surface, the reliability of this model is verified through the cutting tests. The residual stress in the FGH95machined surface is analyzed through the cutting tests and finite element simulation, and the distribution of residual stress along the depth of cut direction is revealed.Computational model of the machined surface plastic deformation is developed based on the the fourth deformation zone formation theory. The surface plastic deformation and its distribution are predicted by the developed computational model. The cutting tests and finite element simulation for the machining of FGH95are emloyed to verify the validity of computational model for machined surface plastic deformation. The results show that in the conventional cutting speeds, with the increasing of cutting speed the depth of machined surface plastic deformation decreased, the depth of plastic deformation is in0.02-0.04mm. However, the machined surface plastic shear strain increases with the increasing of cutting speed, the machined surface plastic shear strain is in1.2-4.0. In high speed machiing, smaller depth of machined surface plastic deformation and plastic shear strain is generated in the cutting speeds range of400-2400m/min. The machined surface plastic deformation has maximum value on the machined surface and decreases rapidly along perpendicular to the machined surface direction.Microstrctures of white layer and bulk material have significantly differences under the EDS, XRD analysis. Strengthening phase γ' contents in white layer is higher than that in the bulk material, and the strengthening phase refinement is appeared. Ni-based solid solution phase of FGH95superalloy changes in the cutting process and the lattice mismatch of strengthening phase γ' and the matrix phase γ is increased. White layer has poor crystallinity and occurrs grain refinement, with the increasing of cutting speed, the grain refinement is more serious. With the increasing of cutting speed, the number of large-sized and medium-sized grains continus to reduce but the number of small-sized grains increases. The deformation phase transformation mechanism in PM superalloy machined surface white layer formation is explored. The effects of cutting speed and tool flank wear on FGH95machined surface white layer thickness are investigated. The influences of cutting stress, strain rate and cutting temperature on white layer thickness are analyzed. The results show that with the increase of cutting stress and strain rate, white layer thickness increases. The increasing of cutting temperature also induces to increase of white layer thickness, but white layer is still formed when the cutting temperature is lower than material phase transition temperature, which indicates that severe plastic deformation plays an integral role in the formation of white layer.This work is sponsored by National Basic Research Program of China (973)(2009CB724401) and Foundation of Shandong Province of China for Distinguished Young Scholars (JQ200918) for financial supports.