Comparative study of surface integrity aspects of Incoloy 825 during machining with uncoated and CVD multilayer coated inserts

Abstract

One of the major concerns related to machining of Ni-based super alloy is surface integrity since it directly affects the performance of the machined component during its intended application. In the current study, the influence of cutting speed (51, 84 and 124m/min) and CVD multilayer tool coating (TiN/TiCN/Al2O3/ZrCN) on various aspects of surface integrity such as surface roughness, crystal structure and microstructure of the surface and sub-surface region, thickness of white layer and work hardening tendency have been investigated during dry turning of Incoloy 825. Particular emphasis has been given to understand the mechanism of formation of modified surface layer and the associated process of dynamic recrystallization. The study indicated coated tool resulted in better surface finish compared to that obtained with uncoated tool only at high cutting speed. Various macro features of machined surface included feed mark, material smearing, surface ploughing, re-deposited materials and chip debris. Transformation of crystallographic phase of the machined surface could not be detected compared to that of bulk material. Increase in cutting speed caused gradual refinement of grains, and increased white layer thickness. Coated tool, on the other hand, resulted in the generation of large number of nucleation sites and consequently finer grains at high cutting speed, whereas the uncoated tool promoted growth of sharply defined recrystallized grains. The coated tool prevented the formation of white layer at low and medium cutting speed and also decreased work hardening tendency of Incoloy 825 when compared with that of uncoated tool.