As a key power component in the aviation, automotive, and energy industries, the surface quality of aluminum alloy impellers will have a direct impact on the aerodynamic performance and service life of the impellers. As one of the main final processes in machining impellers, precision milling has the ability to significantly improve the surface finish of the impeller and ensure geometric accuracy.
Introduction
Aluminum alloy impellers have been widely used in high-quality industries such as aviation, automobile, and energy. As main power components, their surface quality directly affects the aerodynamic performance and service life of impellers. Owing to their such properties as low density, high strength, and high thermal conductivity, aluminum alloy materials have become typical material for fabricating impellers. However, aluminum alloys are prone to causing machining defects such as built-up edges (BUE), burrs, and surface waviness during machining, which significantly reduce impeller surface quality. Being a most crucial process in impeller machining, precision milling can eliminate these surface defects and improve the surface finish and machining accuracy of impellers through precise tool selection and parameter optimization.
Focusing on the intricate geometry of aluminum alloy impellers and vibration and thermal issues caused during machining, precision milling process optimization not only comprises strict machine control of cutting parameters but also sensible design in tool choice, path planning for machining, damping of vibrations, etc. This study will expound on the ways precision milling processes will improve the quality of aluminum alloy impeller surfaces with precise control mechanisms to meet technical requirements of high-performance engines for impellers.
Characteristics and Challenges of Precision Milling of Aluminum Alloy Impellers
Being a key component in high-grade equipment such as aerospace, vehicle supercharging systems, and high-speed compressors, precision milling of aluminum alloy impellers requires ultra-high geometric accuracy and surface quality and faces several technical difficulties imposed by material properties and structural complexity. This paper is concerned with the most serious issues faced in precision milling of aluminum alloy impellers from three aspects: material properties, structural characteristics, and high-speed cutting.
Influence of Material Properties on the Machining Process
Aluminum alloy materials are widely used in the manufacture of complex rotary impellers due to their specific strength, heat conductivity, and corrosion resistance. However, they also bring in typical machining challenges with their low hardness and high plasticity. Tools easily form built-up edges (BUE) on the machined surface of aluminum alloys during high-speed cutting, not only changing the actual cutting edge’s geometry but also degrading the machined surface roughness directly, and even initiating micro-tears and secondary cutting. Besides, while the thermal conductivity of aluminum alloys is good, the temperature propagates quickly in high-temperature conditions, which tends to easily lead to local overheating of the tool and consequently accelerate edge wear and peeling of the coating, particularly during the machining of thin-walled regions. Accordingly, while precision milling aluminum alloys, one should carefully control the rise in cutting temperature and select tool coatings with excellent anti-adhesion properties as well as effective lubrication and cooling controls.
Requirements of Complex Blade Structure for Tool Path and Interference Control
Aluminum alloy impeller blades usually have a complex free-form surface structure. The deformation of the edge of the leading edge of the blade, back bend surface, and intersection area of the channel occurs violently, and the irregular change in the thickness of the wall is prone to local over-cutting or excessive residue. To avert interference or scratching of the tool during reversal of curvature zone, it needs to be integrated with CAM software in order to carry out refined planning of five-axis (simultaneous) path and ball-end tools of small diameters or special variable helix angle milling cutters are utilized to insert into the variable curvature structure. In these constant multi-surface machining zones, tool attitude real-time adjustment and path adaptive control must be achieved so that the tool is always kept in a reasonable contact angle with the blade surface during machining. Meanwhile, for some multi-channel narrow channel or closed ring impellers, the tool accessivity is best, which must determine the potential interference zones in advance during the process planning and tool structure design stage and conduct simulation and proof machining.
Challenges to System Stability Brought by High-Speed Precision Milling
For mirror-grade surface quality, high spindle speed (>20,000 rpm) combined with low feed rate was often the adopted approach for precision milling of aluminum alloy impellers. Under such high-speed milling, however, dynamic rigidity, thermal stability, and the ability of vibration control of the system became the determining factors in machining quality. The spindle system must have sufficient dynamic balance performance and axial stability, otherwise it will cause tool swing, which will lead to corrugated texture, micro-vibration marks, or dimension deviations. In addition, the minor imbalance during high-speed rotation will be enlarged to an noticeable scale, affecting the machine’s overall vibration level and the life of the tool. Therefore, in precise milling of aluminum alloy impellers, dynamic balance tool holders, flexible vibration-isolating tool rods, and active vibration suppression technologies, accompanied by a solid gas-liquid cooling system for reducing spindle thermal drift, need to be employed. At the same time, the tool path planning must also comply with the equal load cutting principle to avoid machine tool vibration mark or resonance due to local loading fluctuation.
Analysis of Influencing Factors of Precision Milling Process on Surface Quality
In the precision machining of aluminum alloy impellers, not only does the surface quality directly determine the aerodynamic performance and fatigue life of the product but also influence the subsequent assembly matching and overall operation stability. Being a crucial process within the manufacturing process, the control effect of precision milling is entirely affected by various factors including tool selection, cutting parameters, path planning, and system stability. The following provides a detailed explanation of the major factors that play a crucial role in the precision milling process on the surface quality from four respects.
Tool Selection
Tool material and geometric shape selection is an important factor that has a significant impact on surface quality machining. Cemented carbide tools are usually selected for machining soft materials such as aluminum alloys, which can provide excellent wear and heat resistance and maintain the cutting edge sharp. The surface coating of the tool is also one of the most significant parameters determining the milling effect. The DLC or TiAlN coated tools can effectively reduce the tool sticking phenomenon and decrease surface defects.
Additionally, to meet the intricate curved surface curvature of the impeller, it is more appropriate to select ball-end tools or circular arc tools. These tools can reduce the tool tip wear and ensure the stability of the machining process. Low-tooth tools (e.g., 2-3 edge tools) can effectively reduce cutting vibration and promote chip removal capacity, and therefore improve the surface quality of the machining.
Optimization of Cutting Parameters
Reasonable optimization of cutting parameters is important to enhance surface quality. High spindle speed and low feed speed are combined to effectively decrease the cutting force fluctuation, minimize the tool-workpiece friction, and subsequently minimize the depth of tool marks. In addition, an appropriate small cutting depth (commonly 0.05-0.2 mm) and optimized feed rate (0.01-0.05 mm/z) help control the cutting load and avoid plastic deformation of the material.
Tests have demonstrated that optimal cutting parameters not only can greatly reduce the surface roughness efficiently but also improve the machining efficiency of the impeller. For example, in some cases, with optimal cutting parameters, the surface roughness of the impeller reduces over 60%, and the precision of machining is significantly improved.
Machining Path Planning
The path planning also has a major influence on improving surface quality. Spiral or contour cutting paths can impart continuity and smoothness of the machining process and remove tool mark textures. In addition, following alternating cutting directions, the surface directional lines can be minimized effectively, and obvious tool marks in the machining can be avoided.
Multi-axis dynamic inclined tool entry technology is used, which can spread the load of the tool, reduce the tool tip load, and thus impart evenness to the surface of the impeller. In the precision milling process, efficient planning of the machining path can drastically reduce surface defects and improve the surface quality.
Vibration and Temperature Control
Machining vibration is a significant problem to be prevented in the precision milling process of high-speed cutting. Vibration will cause surface waviness and loss of precision. Therefore, optimization of machine tool rigidity and tool balance to reduce vibration is critical to improving the machining quality. An efficient cooling and lubrication system, such as minimum quantity lubrication (MQL) technology, can effectively prevent the phenomenon of chip accumulating and reduce the cutting temperature, thereby ensuring the surface quality of the machining.
Case Analysis
When the traditional tools and parameters were used in the machining process of an aviation impeller, the surface roughness Ra was approximately 1.6 μm, and obvious tool marks and chip sticking phenomena existed. For optimal surface finish, designers used a Φ6 mm ball-end tool with DLC coating, increased spindle speed to 22,000 rpm, maintained cutting depth at 0.08 mm, controlled the feed rate at 0.02 mm/z, and utilized a minimum quantity lubrication system. After optimization, the roughness of the impeller surface was reduced to 0.5 μm, the surface was smooth and free from chip sticking, and the quality of machining was significantly improved.
Conclusion
Reasonable design of the high-precision milling process is very important to improving the surface quality of impellers made of aluminum alloys. Based on scientific tool selection, control of cutting parameters, reasonable path planning, and vibration control, the surface quality of the impeller can be significantly improved to meet the technical demands of high-performance engines.
