Investigation of surface integrity in grinding of nickel based superalloy under different cooling conditions

While grinding has been employed as an important machining method to meet the workpiece quality requirement, the machined surface integrity of nickel based superalloy with/without coolants applied and under different cooling pressure has not been studied in detail regarding the subsurface microstructure and mechanical properties variation. This study gives a comprehensive investigation on the surface and subsurface characterization of workpieces machined with selected grinding conditions (dry, flood cooling and high pressure cooling), focusing on the in-depth exploration of the thermal influence on surface morphology generation, subsurface microstructure alteration, crystal orientation variation, and mechanical properties formation. The dry grinding process leads to significant subsurface material alternation in respect of both microstructure and mechanical properties. An obvious recrystallization and white layer are observed at the same time, which is an uncommon phenomenon in the former investigation of the ground surface. In addition, evident material soft is identified in the subsurface area of the workpiece acquired from the dry grinding scenario (hardness reduced to only half of the bulk materials in a large depth location). Interestingly, a plate-like η phase is observed inside the grains, which is influenced by the extreme thermal load. As a comparison, the introduced coolant not only improves the surface morphology (Ra value reduced 3.8 % and 13.2 % with flood cooling and high pressure cooling, respectively) but also alters the subsurface properties to a more industrial and machining preferable state. Smooth surface, no recrystallization, less hardness variation (microhardness variation depths are 1400 μm, 800 μm, and 600 μm under dry, flood cooling, high pressure cooling conditions, respectively), and compressive residual stress are obviously found from the present study. It is found that high pressure cooling can sufficiently carry away the grinding generated heat in time and deliver the coolant into the grinding region, which improves the ground workpiece quality. These findings could be of help for choosing suitable cooling methods in the industry and help understand the obtained part/components quality.