Nickel superalloys are widely used in the aeronautical engineering field due to their high thermal stability, fatigue resistance, high-temperature strength, and corrosion resistance. Nickel superalloys are applied in aero-engine components such as turbine discs, turbine blades, and the engine cabins of aero-engines [1–3]. However, due to their high strengths at elevated temperatures, high work hardening, and low thermal diffusivity, nickel-based superalloys are categorized as difficult-to-machine materials . The root of the NDG matching of a turbine blade is characterized by the depth-to-width ratio exceeding 2 and the groove width being smaller than 4 mm. The methods for machining a component of NDG, in the order of the traditional processes, include milling a groove, quenching, and grinding with a normal abrasive wheel. In particular, it is inevitable for the wheel to experience extreme wear and larger roughness of the ground surface, and thermal damage often occurs in the profile grinding process, with its industrial application being constrained. As a super abrasive machining tool, a single-layer cBN grinding wheel provides a potential way to fabricate this type of material with the structure of the NDG contributing to its high hardness, high thermal and chemical stability, and excellent wear resistance. A single-layer cBN wheel usually exhibits higher bonding strength, larger grit protrusion, and acceptable grit distribution, which is suitable for grinding a groove with a larger depth of cut [5, 6]. In addition to the creep feed in the grinding process, the grinding force can be decreased, for which an NDG with a high-quality surface is easily implemented. Thermal damage can be hardly induced due to the reduced thickness of the chip formation. Therefore, a single-layer cBN grinding tool is widely employed in machining parts with nickel-based superalloys [7–9]. Generally, single-layer cBN wheels containing a metal bond layer are fabricated by brazing and electroplating. The fabricated temperatures of electroplated cBN grinding wheels are lower than those of brazed cBN grinding wheels, and small amounts of residual stress and deformation tolerance can be introduced in the preparation . Thus, creep feed grinding with a single-layer electroplated cBN grinding wheel is considered to be one of the most suitable technologies for machining a NDG with nickel superalloy. However, the difficult-to-machine behavior of nickel superalloys coupled with the large rate of the depth-to-width leads to cBN grit wear occurring in the grinding process. The precision of the parts is decreased due to the grit wear and when the grit wear of the grinding wheel reaches a certain level, the topography of the wheel experiences progressive alterations, and the poor grinding performance becomes a major challenge. In recent years, significant research attention has been drawn towards the wheel wear mechanism and the grinding behavior with respect to controlling the wear behavior and effective grinding. The wear characteristics of the grit at the cutting edges during grinding are the main factor dominating the service life of grinding wheels. Recent studies have indicated that the wheel wear progression consists of an initial transient period and a steady-state period. At the initial transient period, the radial wheel wear rate progressively decreases, and it is mainly dominated by the pullout of weakly held grits. The wear characteristic is kept at a steady-state period when the radial wear rate remains constant, and the main wear mode is grit fracture. Attrition and fracture lead to the grit edges being dull, which increases the grinding power . Similar results of cBN wear progression were drawn in the latest publication, in which the effects of the grit roughness and the nickel plating thickness on the wear behavior were investigated. Further experiment results indicated that an accelerating wear regime occurred towards the end of the wheel life from the wheel tested up to failure. Also, the wear progression of the wheel was classified as the initial high wear rate regime, steady-state regime, and accelerating wear regime . The wear behavior of a randomly selected cBN grit was observed in grinding process. It was claimed that the wear progression of the cBN grit was micro attrition wear, extreme attrition wear, and a micro fracture. The main grit wear patterns generated in grinding were attrition wear, grit fracture, and chip adhesion. The wear of the cBN wheel was quantitatively evaluated by using the percentage of the wear flat area. The area percentage of the wear flat on the grinding wheel could be kept to an acceptable level. High wear resistance has been presented in the high-speed grinding of an Inconel 718 . The protrusion height of cBN grit in terms of the wheel failure was studied. The evolution of the grit protrusion height had a distribution that changed from unimodal to bimodal. The wheel worn modes were the grit pullout, attrition, and fracture. The grits with the highest protrusion were pulled out rapidly, which resulted in the redistribution of the grinding force around their surroundings. The attrition and fracture wear were triggered as the load arose on the grits. It could be said that the grit wear rate primarily depended on the workpiece feed rate rather than a grinding wheel rotation . During the grinding of Inconel 718 nickel superalloy with an electroplated cBN wheel, the radial wheel wear and the wear modes were evaluated with the advance of the accumulated material removal. The cBN grit wear progression was divided into the initial transient region, the steady-state region, and the failure region. With the increase in accumulated material removal, the wear behavior of the grinding wheel was displayed with the sequence of the grit pullout, small attrition wear, micro fracture, large attrition wear, and macro fracture. The grinding wheel was dull due to the work material intensely adhering among the gap of the grits and the occurrence of critical attrition wear . Mechanical stress-induced fracturing and thermal stress-induced fracturing were found in cBN grits after grinding Ti-6Al-4V titanium alloys . The mechanisms of the mechanical stress fracturing and the thermal stress fracturing were analyzed.
According to the results drawn in the literature mentioned above, the analysis of wheel wear is based on the process of surface grinding. Only the grits located on the circular surface are involved in the machining process, with the radial wear is uniformly distributed on the wheel surface. In the profile grinding process, the grinding loads are different due to the varied distribution of the stock allowance along the workpiece profile along the normal direction, which leads to the variations of the grinding force, grinding temperature, and wheel wear [17, 18]. The blade root-mounting slots and fir tree blade root forms are the typical profile grinding components. Researchers have conducted profile grinding experiments by using diamond and cBN grinding wheels, as well as investigating the wheel wear mechanism, wheel dimensional accuracy, surface roughness, specific grinding energy, grinding temperature. The results showed that the specific grinding energy could stay at a low level by adjusting the grinding parameters . The dimensional accuracy of the groove could meet the requirements of tolerance, and a fine surface integrity was obtained [16, 19]. Wear flats, workpiece material adhesion, and micro fractures were induced in the profile grinding wheel. The wear modes varied from one location to another in the profile grinding wheels . A finite element method was also applied to investigate the grinding temperature, and the grinding temperature was less than 120°C without grinding burn [17, 20]. When grinding an NDG with the creep feed parameter, the whole volume of the groove material was removed. Compared with surface grinding or profile grinding, the magnitude of material removal was much larger for one grinding pass.
Previous studies clarified that the material remove methods of cBN grits varied in different sections of the wheel edge during the profile grinding of an NDG. The different material removal methods resulted in the uneven distribution of wheel wear, which had negative effects on machining accuracy. Nevertheless, few studies have focused on the wheel wear in the grinding of an NDG. The investigations of the wear mechanisms occurring in a wheel based on the theory of grinding edge division have not yet been carried out sufficiently.
In this research, a single-layer electroplated cBN grinding wheel was employed to machine nickel superalloy NDG in creep feed grinding. Multiple experiments were carried out to investigate the wheel wear. Firstly, the grit protrusion height of a randomly selected area on a cBN grinding wheel was tested after each grinding pass, and the protrusion height distribution of the cBN grit was studied. The wear revolution of the grinding wheel was categorized in terms of the average grit protrusion height. Then the cBN grit wear modes at different grinding edges (top edge, side edge, and transition edge) were evaluated. The fracture mechanism of the cBN grits on the side edge was clarified. Finally, the wear behavior of the nickel bonding layer on the grinding wheel was analyzed. This study will help to provide a deep understanding of the mechanism of wheel wear and provide a way to machine an NDG.