Contemporary Understanding of Abrasive Machining in Single-Crystal Superalloys
This paper examines the evolution of the removal mechanism in the grinding of nickel-based superconductors (Ni-SCs). It provides an in-depth analysis of how various factors—such as material composition, process parameters, and grinding tool properties—interact with one another. The paper also reviews advancements in process optimization and strategies for controlling damage based on the removal mechanism. Additionally, it highlights potential future developments in this area of research.
01 Introduction
Nickel-based single-crystal superalloys (Ni-SCs) have gained prominence as essential materials for critical high-temperature applications, such as turbine blades in aircraft engines and gas turbine hot-end components. Their unique single-crystal structure provides remarkable high-temperature mechanical properties, including exceptional creep strength, extended fatigue life, and excellent resistance to oxidative corrosion.
Unlike polycrystalline alloys, single-crystal materials completely eliminate grain boundary defects, giving them superior long-term stability under extreme operating conditions. This quality places stringent demands on their machining processes, necessitating submicron precision control to ensure surface integrity.
Precision grinding technology has emerged as a key process for manufacturing Ni-SC components due to its significant advantages in achieving complex surface forms and maintaining high surface quality. However, the inherent low thermal conductivity, high work-hardening tendency, and strong abrasive adhesion of Ni-SCs present considerable challenges during grinding. These challenges can lead to issues such as thermal damage, subsurface dislocation accumulation, and compromised surface integrity. Therefore, systematic research on the grinding properties and material removal behavior of these superalloys is urgently needed.
Nickel-based superalloys (Ni-SCs) demonstrate exceptional mechanical properties at high temperatures, including resistance to tensile stress, creep, and fatigue. Their advantage stems from their single-crystal structure, which lacks grain boundaries. This absence of grain boundaries allows these alloys to outperform polycrystalline materials in high-temperature environments. Without grain boundaries, the risk of intergranular sliding, corrosion, and cracking is significantly reduced, resulting in superior mechanical properties and long-term stability under extreme conditions.
However, the very properties that make Ni-SCs desirable also present challenges in machining, particularly during grinding. Their low thermal conductivity combined with high strength at elevated temperatures causes heat to accumulate in the cutting area, which necessitates overcoming greater cutting forces. This results in a limited material removal rate. Additionally, the oxygen-affinity elements (such as aluminum and chromium) present in Ni-SCs are prone to reacting with oxygen, leading to the adhesion of abrasive grains and diminishing their effectiveness.
Furthermore, the grinding process can induce severe work hardening, which increases the hardness of the machined surface and complicates the material removal process. As depicted in Figure 1, research into the material removal mechanisms of Ni-SCs has advanced alongside developments in materials science and processing technology.
Given the stringent requirements for high precision and complex shapes in nickel-based superalloy (Ni-SC) products, grinding technology plays a crucial role in their processing. To better understand the material removal process of Ni-SCs, CHEN et al. developed an innovative 3D simulation model using Abaqus to study the micro-grinding removal mechanism of second-generation Ni-SCs. Their findings indicate that increasing the feed rate tends to produce serrated wear chips.
MIAO et al. concentrated on the creep feed grinding practices for turbine blade tenons made from Ni-SCs. They discovered that material removal under burn-free conditions can lead to efficient processing and high-quality surface finishes. However, when burning occurs, tool wear becomes more intense, causing the removal mechanism to shift to a slip-plow-dominated mode, which results in decreased efficiency and reduced surface quality. This finding emphasizes the strong relationship between the material removal mechanism and the processing state.
The studies mentioned above illustrate that the material removal mechanism during the grinding process not only impacts the integrity of the machined surface but is also directly related to processing costs and efficiency. Therefore, this article reviews the latest research on the grinding removal mechanism of Ni-SCs, along with mechanism-driven optimization of the grinding process and damage control, aiming to provide valuable insights for the research and development of Ni-SC grinding processing technology.
02 Current Status of Research on the Grinding Removal Mechanism of Nickel-Based Single Crystal Superalloys
Grinding removes material from a workpiece through the combined action of tens of thousands of tiny abrasive grains on the grinding wheel. Ding Wenfeng et al. discovered that within the grinding arc zone, the high-speed rotation of the grinding wheel, along with the continuous feed motion of the workpiece, alters the interaction between individual abrasive grains and the workpiece material. Based on this observation, the grinding process can be divided into three distinct phases: the rubbing phase, the plowing phase, and the chip formation phase, as illustrated in Figure 2.
As an abrasive grain enters the cutting zone, the cutting depth between the abrasive edge and the workpiece initially increases before rapidly decreasing. At very small cutting depths, the interaction force between the abrasive and the workpiece is minimal, resulting in elastic deformation of the workpiece material, which allows the abrasive to scratch the surface without generating grinding chips.
Once the cutting depth reaches a certain threshold, the force exerted by the abrasive surpasses the material’s elastic limit, causing the workpiece material to undergo plastic flow. This lead to the accumulation of material on either side of the abrasive and at the front cutting edge, creating a bulging effect. If the grinding depth continues to increase, the force exceeds the fracture strength of the material, causing it to be torn from the workpiece surface by the abrasive, resulting in the formation of grinding chips.
The microscopic characteristics of the grinding chips produced during this process can provide valuable insights into the material removal mechanism in grinding. This is because the shape and size of the grinding chips reflect the plastic deformation of the material and the processing method used during the grinding process.
In summary, studying material removal mechanisms is closely tied to factors like workpiece composition, grinding parameters, and abrasive grain characteristics. Many researchers have performed detailed analyses on these elements.
2.1 Influence of Workpiece Composition
Researchers have optimized the physical properties of Ni-SCs by adding rare earth elements and alloying elements.
As shown in Figure 3, CAI et al. conducted a systematic analysis of how composition affects material removal during grinding, focusing on three alloys: the nickel-based polycrystalline superalloy GH4169, the rhenium-containing single crystal alloy DD5, and the rhenium-free single crystal alloy DD98. The experiments revealed that the chip morphology and serration characteristics of DD5 and DD98 were quite similar. This similarity arises because, although rhenium significantly enhances the high-temperature durability of the material, it does not change the chip formation mechanism of the single crystal alloy.
In contrast, the chip morphology of GH4169, which contains grain boundaries, is notably different from that of DD5. This difference is attributed to the distinct grinding removal mechanisms of single crystal and polycrystalline alloys. The plastic deformation of GH4169 primarily occurs through grain boundary migration or fracture. Since DD5 lacks grain boundaries, plastic deformation in this single crystal alloy happens mainly through dislocation slip within the crystal lattice. When the stress applied externally exceeds the material’s elastic limit, dislocations in the crystal lattice begin to move, causing relative sliding between the lattice planes, which ultimately leads to material removal.
Miao et al. conducted a comparison of the grinding efficiency among four alloys: DZ408, K403, DD6, and GH4169. They discovered that the grinding ratio, which is the material removal rate divided by the grinding wheel wear, was significantly higher for the single-crystal alloy DD6 compared to the polycrystalline alloys. Additionally, they noted that as the material removal rate increased, the surface roughness values of all alloys also increased. Among them, the DD6 alloy exhibited the lowest surface roughness, while the GH4169 alloy had the highest. This suggests that a thorough analysis of the grinding removal mechanism can help optimize the grinding process, thereby improving both machining efficiency and surface quality.
2.2 Effect of Grinding Parameters
Grinding parameters have a direct impact on the grinding stage and the overall material removal process during grinding.
Based on the Hill model, a 3D single-grain grinding model was established using Abaqus, which showed that the grinding process progresses sequentially through three stages: scratching, plowing, and chip formation, as illustrated in Figure 4. When the grinding speed increased from 15 m/s to 100 m/s, the proportion of the chip formation stage rose from 85% to 89.5%, while the scratching stage remained stable, fluctuating between 2.5% and 3%. This indicates that increasing the grinding speed significantly shortens the plowing stage and speeds up the transition to a more efficient cutting state.
Experiments confirmed the influence of speed on chip morphology. When the grinding speed was adjusted to a range between 20 m/s to 165 m/s, the impact of alloy type on parameter sensitivity was further investigated. By comparing GH4169 (a polycrystalline alloy), DD5 (a single crystal containing rhenium), and DD98 (a single crystal without rhenium), it was observed that the removal process for DD5 became more challenging at lower grinding speeds, high feed rates, and large grinding depths compared to DD98. This suggests that the grinding parameters for single crystal alloys need to be precisely controlled based on their compositional characteristics.
Research has demonstrated that high-speed and high-efficiency grinding can easily result in microcracks and increased plastic deformation on the surface and subsurface of alloys. To mitigate this issue, it is essential to take measures to reduce the thickness of the plastic deformation layer. These measures include:
1. Increasing the spindle speed while simultaneously reducing the grinding depth and feed rate.
2. Adjusting the production process to minimize operations that could lead to plastic deformation prior to the solution treatment of single-crystal parts.
3. Reducing the surface stress induced by the CNC precision milling process.
Implementing annealing processes can help eliminate residual stresses generated during grinding and control the wear morphology of different alumina grinding wheels, thereby reducing the plastic deformation energy of the ground micro-part surface.
2.3 Influence of Grinding Wheel Characteristics
The interaction between abrasive grains and the workpiece surface—encompassing their shape, size, arrangement, and behavior during engagement, sliding, and cutting—significantly influences the material removal mechanism.
To enhance the performance of abrasive grains, simulations conducted with Abaqus highlighted the impact of grain size on the grinding behavior of single-crystal alloy DD5. Utilizing finer abrasives can decrease the size of grinding marks and improve surface quality. Additionally, by focusing on the geometry of the abrasives, femtosecond pulsed laser technology was employed to convert traditional negative rake angle (NRA) grinding into positive rake angle (PRA) grinding. This transition considerably enhanced the surface quality of materials that are difficult to machine by optimizing the cutting force state. Both studies demonstrated that optimizing abrasive parameters can effectively control the material removal process.
The properties of the grinding wheel material also play a crucial role in the removal mechanism. Due to the high plasticity of the Ni-SCs material removal surface, abrasive chips tend to accumulate on the abrasive rake face. This accumulation leads to continuous wear of the grinding wheel during the grinding process, resulting in an upward trend in grinding force and temperature. Moreover, increased grinding force and temperature influence the morphology of chips during material removal. A comparison of the grinding behavior between brown corundum (BA) and microcrystalline corundum (MA) wheels for Ni-SCs revealed that the MA wheel, with its high toughness and hardness, allows abrasive particles to break more easily, forming new cutting edges and thus achieving efficient material removal. In contrast, the BA wheel relies on the shedding of abrasive particles for material removal.
A direct comparison of the wear characteristics of MA and BA grinding wheels found that the MA grinding wheel exhibited lower grinding forces and radial wear compared to the BA wheel. Additionally, the specific grinding energy of the MA grinding wheel was lower than that of the BA wheel, indicating that the MA grinding wheel has higher material removal efficiency.
2.4 Effect of Cooling and Lubrication Conditions
Cooling and lubrication are essential for optimizing the material removal process in grinding. They help reduce grinding temperatures, enhance interfacial lubrication, remove wear debris, and decrease grinding forces.
Figure 5 illustrates a single-factor experimental system used to compare the effects of dry grinding, conventional wet grinding, and minimum quantity lubrication (MQL) on grinding forces and chip morphology. The results showed that both conventional wet grinding and MQL effectively reduced material microhardness and improved removal rates. In the MQL environment, the chip serration of single crystal alloy DD5 was stable, exhibiting minimal fluctuations in formation frequency. In contrast, in the dry environment, the chip morphology was irregular, resulting in greater fluctuations in formation frequency and unstable chip formation.
Furthermore, the research expanded to include comparisons between various cooling and lubrication modes: dry, flow, palm oil (MQL-PO), multilayer graphene (MQL-MG), and MQL with aluminum oxide (MQL-Al2O3) nanoparticles. These comparisons aimed to assess their effects on the removal mechanism of nickel-based alloys.
Research has shown that dry grinding can reduce the sharpness of the cutting edge due to chip retention. While conventional Minimum Quantity Lubrication (MQL) reduces friction through oil film lubrication, high-viscosity oil films can easily lead to surface scratches. However, the “ball bearing” effect of nanoparticles in MQL-Al2O3 changes sliding friction into a mixed sliding-rolling friction, which significantly enhances material removal efficiency. The grinding interface under the MQL-Al2O3 method is illustrated in Figure 6.
This finding advances our understanding of the cooling and lubrication mechanisms from a thermomechanical coupling perspective, highlighting the critical role of grinding fluid in controlling the quality of the CNC machined parts surface and the wear of the tool during micro-grinding. The cooling effect of the fluid aids in heat dissipation within the arc zone. Its lubrication minimizes friction between the grinding wheel and the workpiece, while its flushing action helps remove chips from the grinding arc zone, preventing chip accumulation on the machined surface and tool clogging in the micro-grinding tool. This conclusion clearly demonstrates the beneficial regulatory role of cooling and lubrication in the material removal process.
2.5 Impact of Ultrasonic Processing
A study was conducted to explore the effects of ultrasonic vibration on the cutting behavior of abrasive particles. The results revealed that these particles exhibit intermittent cutting behavior when subjected to ultrasonic vibration. The wear pattern transitions from block-like fragmentation to micro-fragmentation, indicating a shift in the material removal mechanism from traditional plowing-type extrusion to an intermittent shearing mode.
Furthermore, a comprehensive analysis of the surface generation and material removal characteristics associated with ultrasonic vibration-assisted grinding was performed. The findings show that ultrasonic vibration increases the number of dynamically active abrasive particles, creates a more uniform thickness of undeformed chips, and reduces surface roughness. Additionally, it promotes the initiation and propagation of cracks, enhancing material removal and improving grinding efficiency.
Another advantage of ultrasonic vibration is its ability to induce self-sharpening of abrasive particles, which helps maintain their sharpness and further boosts grinding efficiency. The ultrasonic vibration also has a reconstructive effect on the properties of the material surface. During ultrasonic-assisted high-speed grinding, the workpiece surface experiences not only the intense friction from high-speed rotating abrasive particles but also the significant impact generated by high-frequency ultrasonic vibrations.
This combination helps to lower grinding temperatures and forces while inducing surface plastic deformation due to high shear strains, high strain rates, and large strain gradients.
03 Current Research Status of Mechanism-Driven Grinding Process Optimization and Damage Control
3.1 Damage Forms
Nickel-based superalloys are characterized by low thermal conductivity and thermal strength. As a result, grinding difficult-to-machine materials, such as nickel-based superalloys (Ni-SCs), generates high temperatures and grinding forces. This can lead to poor surface quality, excessive wheel wear, and grinding burns.
As illustrated in Figure 7, the significant heat produced during the grinding of Ni-SCs can cause substantial thermal softening of the material, which, in turn, can result in grinding burns. These conditions contribute to the formation of defects such as protrusions, grooves, microgrooves, and microcracks.
The mechanism behind the formation of workpiece surface topography during the grinding of Ni-SCs is demonstrated in Figure 8. When the grinding wheel speed decreases, the contact time between the abrasive and the workpiece increases. This prolongation exacerbates surface plastic deformation and elastic recovery. Consequently, the effective density of abrasives per unit area decreases, leading to an uneven distribution of grinding scratches. This unevenness results in deeper grooves and the accumulation of wear debris, creating surface microstructural defects.
Additionally, increasing the grinding depth can degrade surface quality and intensify localized chip adhesion. Although higher feed speeds can enhance efficiency, they may impede chip discharge due to the increase in grinding forces.
To better understand the subsurface characteristics of nickel single crystals (Ni-SCs) under specific grinding conditions, we analyzed their subsurface microstructure during creep feed grinding. The results indicate that surface plastic deformation is primarily influenced by strain and strain rate, with the material removal process being regulated by grinding force and temperature.
To quantify the differences in material removal between single-crystal and polycrystalline materials, we developed a model for the maximum undeformed chip thickness during micro-grinding, which accounts for size effects. This model highlights a fundamental distinction between single-crystal fracture, which occurs within the crystal lattice, and polycrystalline fracture, which depends on grain boundary sliding.
Comparative studies also reveal that the tribological properties of single-crystal materials differ from those of polycrystalline materials, with material removal being influenced by crystal orientation. Ni-SCs are more easily removed along the {111} planes because, during grinding, the material removal process is primarily driven by the abrasive cutting action and the plastic deformation of the material. Furthermore, the shear slip surfaces of Ni-SCs are most prevalent on the {111} planes, making them more susceptible to plastic deformation and thus easier to remove through abrasive cutting.
As the feed rate and grinding depth increase, and spindle speed decreases, both the micro-grinding force and the thickness of subsurface plastic deformation increase.
3.2 Damage Control
Researchers have proposed targeted optimization strategies based on the damage mechanisms they studied. They examined how different crystal planes, such as the {111} plane, and crystal orientations affect the surface quality during the micro-grinding of DD98 nickel-based single crystal alloy. Their analysis, which included grinding parameters and tool wear, revealed that grinding the {111} plane resulted in the lowest surface roughness (Ra) and the thinnest recrystallized layer, as shown in Figure 9. Additionally, they found that increasing the spindle speed while reducing the feed rate and grinding depth can enhance the surface quality of micro-grinding.
To effectively optimize the grinding process parameters for Ni-SCs, Cai Ming et al. [33] assessed the influence of various parameters on the grinding of DD5 alloy through orthogonal experiments. They found that the linear speed of the grinding wheel has the greatest impact on surface roughness, followed by the feed rate, while the grinding depth has a comparatively minor effect. Based on these findings, they proposed an optimal combination of process parameters for the surface grinding of DD5: a grinding wheel linear speed of 30 m/s, a grinding depth of 20 μm, and a feed rate of 0.4 m/min. This combination successfully balances efficiency and surface quality requirements. Additionally, the MA grinding wheel exhibits superior self-sharpening properties, resulting in sharper cutting edges during the grinding process.
04 Conclusion
This paper provides an overview of nickel-based superalloys (Ni-SCs) and their material properties, summarizing key research findings by domestic and international scholars regarding the grinding removal mechanism, process characteristics, and surface integrity of Ni-SCs. From the analysis of existing research, several conclusions can be drawn:
1. Utilizing finer abrasives or those with a negative rake angle can enhance material removal effectiveness, improve stress conditions during the grinding process, and elevate the surface quality of difficult-to-process materials.
2. The application of ultrasonic vibration increases the number of dynamically active abrasive particles, leading to a more uniform undeformed chip thickness and reduced surface roughness. This technique also promotes the initiation and propagation of cracks, facilitating material removal and improving grinding efficiency.
3. The cooling effect of grinding fluid aids in heat dissipation within the arc zone, while its lubrication properties reduce friction between the grinding wheel and the workpiece. Additionally, the flushing action of the grinding fluid helps remove chips from the grinding area, preventing chip accumulation on the machined surface and clogging of the micro-abrasive tool, which positively impacts material removal.
While significant advancements have been made in the grinding technology for nickel-based superalloys, the continuous improvement in their high-temperature and creep resistance presents considerable challenges. Current processes do not fully meet the specific requirements for these materials. Therefore, future research should focus on the following areas:
1. Enhancing the temperature tolerance of Ni-SCs by incorporating rare earth elements.
2. Integrating grinding with multi-energy fields, such as laser assistance and ultrasonic vibration, to study their effects on the material removal mechanisms of Ni-SCs.
3. Developing adaptive control systems that can automatically adjust process parameters, such as grinding force, temperature, and wheel wear, in real time to maintain optimal conditions and improve both grinding efficiency and quality consistency.
4. Optimizing abrasive materials and structures to enhance the hardness, wear resistance, heat resistance, and self-sharpening properties of abrasive tools to extend tool life, refine material removal mechanisms, and boost grinding efficiency and quality.
If you want to know more or inquiry, please feel free to contact info@anebon.com
Anebon’s goal is to understand the excellent craftsmanship in manufacturing and to provide top-notch support to domestic and international clients wholeheartedly for 2024 in high-quality stainless steel, aluminum, and high-precision custom-made CNC turned components, milling, and machining spare parts for aerospace; in order to expand our international market, Anebon mainly supplies our oversea customers Top quality performance mechanical parts, precision metal stamping parts and CNC turning service.