As an indispensable part of aero-engines and high-speed fluid machinery, aluminum alloy impellers typically have intricate 3D curved surface topologies, large-depth cavities, narrow walls, and other typical geometric shapes. Meanwhile, the surface roughness Ra should be usually maintained lower than 0.4 μm, while ultra-precision machining levels lower than 0.2 μm are even required in local areas.
Their production process is based mainly on high-speed milling (HSM) with five-axis simultaneous machining paths for controlling and continuity of intricate contours. However, regarding the real machining process, aluminum alloy high plasticity and low hardness are likely to lead to serious built-up edge (BUE) problems, making tool wear and edge deformation faster and leading to more contour errors and poorer surface quality later.
Introduction
Mist cooling, MQL, and emulsion cooling are conventional lubrication and cooling technologies that, although still effective in some working conditions, are apt to have problems in high-rotation and high-speed machining processes, for instance, lubricant material being blown out by centrifugal force, uneven lubrication distribution, difficult cleaning, and potential harm to the environment and operators’ health. Therefore, an alternative technology with increased environmental flexibility, lubricant stability, and convenience is particularly needed.
Based on this, the technology of solid-oil cutting, by virtue of its unique release mechanism of the lubricant molecules of solid lubricants, significantly improves the lubrication condition at the cutting interface without the inconvenience of complex liquid supply systems and hence is a useful technical measure to avoid adhesive wear and thermal crack failure in high-speed cutting of aluminum alloy impellers. In my multiple rejoining in the process of developing aluminum alloy precision impellers’ development projects, I’ve also tried to bring solid-oil cutting into the processing process multiple times, and the result is remarkable. Not only can it significantly prolong tool life, but also it may considerably improve the machining stability in free-form surfaces’ transition areas.
Principles and Key Characteristics of Solid-Oil Cutting Technology
Solid-oil cutting is an eco-friendly machining process using solid lubricants releasing lubricating agents for continuous lubrication and cooling control. Its basic principle is pre-installing graphite, molybdenum disulfide (MoS₂), fluoropolymers (like PTFE), or new-type composite solid oil agents in the tool, workpiece, or clamping system. During cutting, the lubricating oil film is washed away in the shape of high pressure, high temperature, and shear stress, covering the tool-chip surface as well as the surface of the cutting area to form a stable low-friction lubricant film.
The technology has the following major technical characteristics:
- Strong lubrication continuity: The lubricant slowly releases when heated to form a micro-nano lubricating film which is hard to be worn off by high-speed rotation, and the lubricating effect is more persistent and resilient;
- Environmentally friendly: No liquid oil is expelled to avoid the environmental pressure of machining waste liquid treatment;
- Excellent anti-adhesion performance: The film solid structure of oil is capable of effectively separating the bonding tendency between aluminum alloy chips and tools, thus avoiding the phenomenon of built-up edges;
- Strong adaptability to high speed and high temperature: It is still maintained with excellent lubricity in the machining temperature range of 180–250°C, especially for high-speed and high heat load cutting of aluminum alloys;
- High adaptability: It can be installed in the existing CNC system for application, with minimal process modification and easy maintenance.
In the solid-oil cutting projects, I have found that, particularly in the process of multi-axis continuous machining, solid-oil technology has taken a positive technical compensation effect on the spindle operation stability, vibration control, and temperature rise prevention.
Key Technical Challenges in High-Speed Machining of Aluminum Alloy Impellers
Aluminum alloys have both advantages and disadvantages in high-speed machining. Although aluminum alloys are low hardness and tiny cutting resistance, they are high ductility and strong adhesion. In most real machining, the following problems exist:
Frequent Built-Up Edge (BUE)
At medium-high feed and high rotational speed (>12000 rpm), cutting temperature rises rapidly, and tool surface is extremely susceptible to sticking with softened aluminum chips, resulting in abrupt edge changes, micro-grooves, or periodical scratches.
Rapid Tool Wear
The tool edge is extremely hot. With low lubrication, thermal fatigue or micro-welding may occur, resulting in nonlinear tool wear development.
Poor Lubrication Stability
Traditional mist lubrication or emulsion cooling are easily swept away by centrifugal force in high-speed cutting, and lubricating components are difficult to provide continuous action on the cutting zone, resulting in local heat accumulation.
Difficult Surface Roughness Control
Surface micro-defects are prone to be caused by tool micro-jump or unequal lubrication in the transition areas of free-form surfaces or small curvatures, affecting conformity to Ra gauges.
Application Strategies of Solid-Oil Cutting in Aluminum Alloy Impeller Machining
During aluminum alloy impeller machining by high-speed CNC, because aluminum materials have a soft texture, high thermal conductivity, and tend to bond easily with tools, poor surface finish, rapid tool wear, and hard chip processing are usually encountered. The solid-oil cutting technology forms a stable lubrication film on the cutting interface, significantly reducing the friction coefficient and the possibility of tool-chip adhesion, ensuring a stable foundation for improving the machining quality and efficiency of aluminum alloy impellers. With engineering practical applications, this paper analyzes its technical means from three aspects: tool design, machining parameters, and solid-oil loading methods.
Tool Design and Application of Solid-Oil Coatings
Tool structure is the fundamental foundation of solid-oil cutting effect. Due to the high viscosity and low hardness of aluminum alloy materials, low-friction composite coated tools need to be used, such as TiN+graphite, AlCrN+MoS₂ and other multi-layer composite structures. These coatings not only have good anti-adhesion but also can withstand stable thermal-mechanical properties at high-speed cutting conditions, inhibiting chip adhesion and thermal accumulation phenomena.
Meanwhile, edge micro-rounding treatment (0.01–0.03 mm) cannot be neglected either. Micro-edge circle can efficiently distribute the first contact stress, postpone the damage of the lubricating film, and assist the persistence of the lubrication effect and the stability of the cutting process, which is especially adaptive for the dynamic cutting condition of aluminum alloy free-form surface structures.
Matching and Optimization of Process Parameters
In solid-oil cutting, process parameters are closely related to lubrication performance, and matching reasonably can significantly improve the forming and retaining effect of the lubricating film. According to practical verification, the following parameter ranges are suggested:
- Spindle speed: 12000–20000 rpm. High speed can reduce cutting force and likelihood of chip adhesion;
- Feed per tooth: 0.05–0.2 mm/tooth, suitable for maintaining stable cutting temperature and preventing thermal softening;
- Radial cutting depth control: ae ≤ 0.5 mm, axial cutting depth control: ap ≤ 0.3 mm, to avoid excessive single cutting load, good for multi-track light cutting and continuous lubrication.
In path strategy, recommending a composite of equal residual thickness strategies and contour paths is advisable. The composite can maintain the constancy of cutting load and reduce cutting fluctuations, thus making the formation and stability of the lubricating film better, serving to deal with complicated contours and space transition areas.
Development and Practice of Solid-Oil Loading Technology
For the sake of fulfilling the sustained release and stable supply of lubricants, there is a need to employ scientific loading technology. The following three ways are presently more mature application modes:
- Pre-impregnated tool method: Spinning coat the tool with solid lubricant in order to pre-load a layer of solid-oil film on its surface to provide initial lubrication protection in the first cutting;
- Fixture embedded oil design: Place controllable release solid-oil sheets within the tool fixture or workpiece location system, and apply mechanical motion or heat to initiate the release of lubricant, suitable for long-cycle machining processes;
- Microcapsule composite lubrication system: Applied with the MQL (minimum quantity lubrication) system, atomize a microcapsule blend of solid-oil particles onto the cutting zone, and release solid lubricants under the power of high-temperature shearing to generate the “secondary lubrication release” effect, which can be employed for continuous multi-track machining conditions.
Verification of Practical Application Effects
During a high-speed milling test of an aviation turbocharging system 7075 aluminum alloy impeller, a solid-oil cutting ball-end mill with a graphite composite coating was utilized in combination with a five-axis machine to finish the surface of complex blades, and the results were as follows:
- The surface roughness from Ra 0.65 μm dropped to Ra 0.28 μm, and the finishing surface greatly enhanced;
- The edge build-up issue was significantly reduced, and microscopic examination of the edge revealed that the adhesion area reduced by more than 70%;
- Workpieces machined in one tool were raised from the original 18 to more than 42, doubling the working life;
- The traditional emulsion cooling system was (stopped), saving about 30 liters of coolant per day, and both environmental protection and cost were improved.
The results verify the enhanced cutting performance advantage of solid-oil lubrication, especially exhibiting enhanced stability over normal MQL or dry cutting in free-form surface areas.
Conclusion
Given that solid-oil cutting technology is the convergence of green manufacturing and processing efficiency concepts, it exhibits sound technical worth in high-speed machining for aluminum alloy impellers. Not only does it have significant advantages in tool life and surface quality control, but it also presents a new path to future intelligent manufacturing to reduce environmental pollution and simplify the system structure of refrigeration systems. With the gradual improvement of solid-oil material systems and intelligent control techniques, it is expected that solid-oil cutting will become more and more an important technological means for processing complex structural components, precision processing, and aeronautical parts and become a main technical support for promoting the progress of green high-end manufacturing.
