Study on surface quality and subsurface damage mechanism of nickel-based single-crystal superalloy in precision turning

Abstract

Nickel-based single-crystal superalloy has no grain boundary, which eliminates the weak link of thermal-force coupling deformation at high temperature. Therefore, it has excellent thermal fatigue resistance to become an ideal material for the manufacture of turbine disks in aeroengines using the turning process. However, most of the current research results on turning nickel-based superalloy focus on the field of polycrystalline alloys, from which nickel-based single-crystal superalloy have great structural differences. Hence, it is impossible to directly guide the turning of nickel-based single-crystal superalloy through existing theoretical results of turning nickel-based polycrystalline superalloy to achieve high-efficiency and low-damage turning. In this paper, the turning surface quality and subsurface damage mechanism of single-crystal nickel-based superalloy were studied. Firstly, the effects of cutting velocity vs, feeding rate vw, and turning depth ap on surface roughness and surface topography were investigated separately, and the mechanisms underlying the different laws were analyzed. Secondly, the influence of tool wear on the turning surface morphology was investigated, and the measure to improve the turning surface quality of nickel-based single-crystal superalloy from the aspect of tool wear was put forward. Finally, a transmission electron microscope (TEM) was used to observe and analyze the subsurface microstructure of the subsurface layer to investigate the plastic deformation pattern and microstructure evolution of the subsurface after turning the nickel-based single-crystal superalloy. The results show that the surface roughness decreases and the surface morphology becomes smooth with the increase of vs. With the increase of vw, the surface roughness becomes larger and the surface morphology is poor. The effect of ap on surface roughness and surface morphology is roughly the same as that of vw. As the cutting distance increases, the tool goes through three different wear stages, and in the third stage, the surface quality is the worst. The degree of plastic deformation in the subsurface presents a gradient-decreasing trend in the direction of cutting depth. The grain size in the subsurface microstructure increases in the direction of cutting depth, and recrystallization of grain refinement occurs in the high stress-strain zone of the subsurface. The research results of this paper can provide proper guidance and reference for turning parts of nickel-based single-crystal superalloy.