Nickel-based superalloys are widely used in manufacturing impellers of high-performance machinery such as aero-engines, gas turbines, and nuclear power machinery due to their high strength, corrosion resistance, and elevated temperature properties. The materials have common characteristics such as high machining hardness, low thermal conductivity, and low grinding ratio, which present many challenges in accurate fabrication of impellers.
Process Difficulties in Grinding Nickel-based Alloy Impellers
Nickel-base superalloys like Inconel 718 and GH4169 are typically used in manufacturing aero-engine impellers due to their (excellent) high-temperature strength properties and corrosion behavior. However, machinability of the material is relatively low, mainly reflected in the following aspects:
Material Hardness and Cutting Force
Nickel-based alloys are of high hardness, so there is enormous cutting force created during grinding and compound grinding wheel wear occurs. Therefore, how to control the grinding force properly and extend tool life became a severe problem in the machining process.
Low Thermal Conductivity and Thermal Damage
Since the nickel-based alloys are of poor thermal conductivity, localized high temperatures are most likely to be produced during grinding not only hindering machining precision but also possible burning or micro-cracks on the workpiece that will have negative impact on the service performance of the impeller.
Complexity of Workpiece Shape
The workpieces of the impeller have complex 3D curved surfaces, especially in the free-form surface areas, where interference is most likely to occur during grinding. Therefore, how to precisely control the tool path and ensure stability and accuracy during milling is a technical problem.
High Precision Requirements
In aero-engines, dimensional accuracy and form-position of impellers is critical, especially in surface roughness and blade contours that need to meet very stringent standards. Impeller blades possess a thin-walled structure, which becomes prone to vibration or deformation; hence, utmost care should be exercised specially to avoid vibration or deformation affecting the quality of machining during the machining operation.
To overcome these difficulties, accurate grinding processes must be coupled with high-response adaptive control systems and implemented in a multi-variable, multi-objective collaborative control machining approach.
Optimization of Grinding Process Parameters and Grinding Wheel Selection
Grinding, being a key procedure following processing in high-precision grinding of nickel-base alloy impellers, reasonable adjustment of its process parameters and structure of grinding wheels directly and also significantly affects the quality of the finished surface, dimensional accuracy, and performance of the workpiece. Especially in conventional nickel-based superalloys such as Inconel 718, due to the high strength, low thermal conductivity, and severe work hardening behavior of the material, special grinding methods and optimized combined processes need to be employed for obtaining machining stability along with high-quality forming of complex blade geometries.
Setting of Grinding Parameters: Balance between Thermal Management and Machining Stability
Linear speed (Vs) is also one of the principal parameters in controlling the generation and conduction efficiency of grinding heat in impeller grinding. It is suggested to maintain the linear speed between 25–35 m/s. Based on ensuring the safe operating state of the grinding wheel, increased linear speed disperses single abrasive grain cutting-generated heat and lessens the heat concentration tendency. The feed rate (Vf) needs to be regulated in 200–600 mm/min based on the workpiece stiffness and hardness of the grinding wheel.
Too high a feed rate will contribute to a higher grinding force per unit time and consequently higher frictional heat between workpiece and tool, which tends to deteriorate the machining quality. In order to cope with the good cutting resistance characteristics of nickel alloys, a light cutting depth of 0.005–0.03 mm/layer is recommended. The multi-layer light feed machining mode has an excellent effect of decreasing the depth of the thermal influence layer and of suppressing the formation of micro-cracks and white layers. In addition to this, a high-pressure cooling system must be installed, utilizing oil mist or high-pressure coolant (such as emulsified oil or composite cutting fluid) to provide a proper heat conduction condition, which can actually alleviate the temperature increase at the grinding point and avoid annealing or scorching of the surface of the workpiece.
Selection of Grinding Wheel Structure: Composite Wheel Type Facilitates Complex Contour Grinding
Focusing on nickel-based alloy materials’ grinding properties, the grinding wheel type, bond, and structural form should exactly match the process demand. Ceramic-bonded CBN grinding wheels with very high hardness and heat stability are best suited for high-precision surface grinding of nickel-based impellers. They have high wear resistance to significantly enhance the grinding cycle and reduce dressing frequency.
Open-porous resin-bonded grinding wheels are widely used to grind impeller components with complex contours or channel flow structures due to the good chip removal ability and thermal diffusion characteristics and can therefore significantly reduce the risk of grinding wheel clogging and lower the grinding temperature. In applications requiring ultra-high shape accuracy and edge integrity, such as blade roots and tips, electroplated diamond grinding wheels must be utilized. With the use of their fixed abrasive structure and sharp edges, high-precision small-amplitude grinding can be achieved. They have high workpiece shape retention ability during machining and are insensitive to slight (jitter) or edge chipping.
Grinding Path Planning: Five-axis Simultaneous Algorithm Optimizes Thermal Load Distribution
The complex impeller geometry requires the grinding path to be extremely flexible and adaptive. Implementation of a five-axis simultaneous path planning algorithm with adaptive interpolation policy to real-time coordinate tool attitude and grinding contact angle variations, thereby avoiding concentration of heat loads at blade root or corner due to deleterious paths, is recommended.
In real path programming, path refinement processing has to be carried out for curvature mutation areas (e.g., blade transition curves or root fillets), and tool feed speed has to be appropriately decreased to stabilize the cutting force and heat distribution. Furthermore, preset logic of path speed variation should be introduced by simulation software (e.g., UG CAM, PowerMill), and offline simulation should be adopted in order to simulate the grinding point movement trajectory and workpiece deformation response in order to ensure practical thermal stress continuity, security, and maneuverability. Reasonable path optimization not only improves grinding accuracy but also extends the life of the grinding wheel and stabilizes the machine tool system’s operating status.
Functions and Integration of Adaptive Control System in Grinding Process
Adaptive Control System (ACS) is an important new development trend of modern grinding technology. It can monitor and control the necessary parameters in real time during the grinding process to make the grinding process stable and precise. The main tasks of the adaptive control system are:
Real-time Monitoring of Grinding Load
By placing spindle current sensors and force sensors, the system is able to real-time measure the grinding load fluctuation. This information may be used to determine the degree of tool wear and grinding wheel (passivation) state, and automatically adjust the feed speed or trigger grinding wheel dressing according to the situation in an effort to maintain the grinding process under the optimal condition.
Thermal Compensation Control
Temperature fluctuation during grinding has a strong impact on the workpiece’s dimensional accuracy. With the measurement of thermal distribution on the workpiece surface by temperature sensors, the system can make real-time compensation of dimension according to the thermal expansion and contraction effect, thus not allowing the machining accuracy of the workpiece to be affected by thermal deformation.
Dynamic Feed Rate Control
The adaptive system also has the capability to dynamically adjust the feed speed according to the actual grinding load change in real time. Especially in grinding complex curved surfaces, the system is capable of slowing down automatically in order to maintain constant cutting force, improve surface quality, and extend tool life.
Integration with CNC System
The adaptive control system is coupled with the PLC control system in CNC machine tools (for example, FANUC, Siemens, etc.) to execute the coupling with measurement modules, cooling circuits, and dressing mechanisms. On the basis of data bus implementation, real-time process monitoring and quality checking can be achieved to ensure the high efficiency and high accuracy of machining process.
Analysis of Application Examples
In an aero-engine impeller producing project, excellent results were achieved by using an adaptive control system to enhance the grinding process. The concrete application program plan is as follows:
- Material: Inconel 718 forging
- Equipment: Five-axis high-precision CNC grinding center (equipped with ACS system)
- Grinding wheel: Ceramic-bonded CBN grinding wheel (Φ150 mm)
- Detection system: Integrated laser probe and contour scanning module
The data comparison before and after optimization is as follows:
| Item | Before Process Optimization | After Adaptive System Optimization |
| Average Surface Roughness (Ra) | 0.55 μm | 0.28 μm |
| Processing Cycle Time (Single Piece) | 42 min | 33 min |
| Grinding Burning Rate | 3.2% | <0.5% |
| Workpiece Consistency Qualification Rate | 94.6% | 99.3% |
| Grinding Wheel Dressing Frequency | 1 time per 5 pieces | 1 time per 8 pieces |
Through optimizing the improvement of the adaptive control system, the grinding process’ stability and accuracy are further guaranteed, and the surface quality and tool life have been effectively guaranteed as well.
Conclusion
By adopting extensive application of complicated impeller structure and heat-resisting alloy materials, the common (extensive) grinding has proved difficult to meet the requirement of contemporary manufacturing for high precision, high consistency, and low cost. Through the introduction of an adaptive control system for grinding and combination with rational process parameters, grinding wheel configurations, and path planning, not only can thermal impact and wear processes be effectively controlled, but the intelligence, stability, and visualization of grinding processing can also be achieved.
