Machining of aluminum and its alloys
Aluminum and its alloys are widely used in aerospace, automotive, and electronic equipment due to their low density, high strength, corrosion resistance, and excellent thermal conductivity. Compared to materials like steel and cast iron, turning aluminum and its alloys present unique characteristics and requirements. Aluminum and its alloys have relatively low strength and hardness (typical aluminum alloys have a hardness of 50-150 HBW), but exhibit good plasticity, making them prone to tool sticking during cutting. Their high thermal conductivity (approximately three times that of steel) facilitates heat transfer to the workpiece and tool, impacting machining accuracy and surface quality. Therefore, turning aluminum and its alloys requires the selection of appropriate tool materials, geometry, and cutting parameters to achieve optimal results.
The choice of tool material significantly impacts the turning quality and efficiency of aluminum and its alloys. Tool material must meet the requirements of good wear resistance, strong adhesion resistance, and high thermal conductivity. High-speed steel tools, due to their toughness and sharp cutting edge, are suitable for low-speed precision turning of aluminum and its alloys. This is especially true when machining complex workpieces, where high-speed steel tools offer better precision. However, high-speed steel’s poor wear resistance makes it unsuitable for high-speed cutting. Carbide tools are the preferred choice for turning aluminum and its alloys. Tungsten-cobalt carbides (such as YG6 and YG8) offer excellent adhesion and wear resistance, making them suitable for machining general aluminum alloys. For high-strength aluminum alloys or high-speed cutting, ultrafine-grain carbides (such as YG6X) offer even higher hardness and wear resistance. Coated carbide tools (such as TiN coatings) offer increased hardness, but are generally not recommended due to the coating’s strong affinity with aluminum, which can cause tool sticking. Diamond tools (natural diamond or artificial polycrystalline diamond) have extremely high hardness and wear resistance, low friction coefficient with aluminum, and good anti-adhesion. They are suitable for high-precision and high-finish aluminum alloy turning, such as processing workpieces with mirror surfaces (Ra≤0.02μm).
Optimizing tool geometry is key to reducing tool sticking and improving surface quality when turning aluminum and its alloys. A large rake angle (15°-30°) is recommended. This sharpens the cutting edge, reduces cutting deformation and friction, and lowers cutting forces and temperatures, thereby reducing tool sticking. However, an excessively large rake angle can weaken the tool head, so the optimal tool geometry should be determined based on the tool material and machining conditions. High-speed steel tools can use a larger rake angle (25°-30°), while carbide tools should use a 15°-20° angle. A large clearance angle (10°-15°) is also recommended to reduce friction between the tool flank and the workpiece surface, improving surface quality. The tool nose radius should be kept small (typically 0.2-0.5mm). This increases cutting forces and heat, potentially leading to workpiece deformation and tool sticking. A positive rake angle (5°-10°) ensures chip flow toward the surface being machined, preventing scratches on the machined surface and facilitating chip evacuation. In addition, the rake and flank faces of the tool should be finely ground, and the surface roughness should be controlled below Ra0.1μm to reduce the friction and adhesion between the chips and the tool.
The selection of cutting parameters must consider the machining characteristics of aluminum and its alloys to maximize efficiency and avoid tool sticking and surface damage. The cutting speed should be as high as possible. High-speed cutting allows chips to quickly exit the cutting zone, reducing chip-tool contact time and thus reducing tool sticking. Furthermore, most of the heat generated by high-speed cutting is carried away by the chips, helping to keep both the workpiece and tool cool. Generally speaking, cutting speeds for high-speed steel tools range from 100-300 m/min, for carbide tools from 300-1000 m/min, and for diamond tools from 1000-5000 m/min. The feed rate should be determined based on the surface quality requirements. A lower feed rate ( 0.05-0.1 mm/r) is recommended for fine finishing to achieve a higher surface finish. A higher feed rate (0.1-0.3 mm/r) can be used for roughing to improve efficiency. The cutting depth is determined according to the machining allowance. It can be 2-5mm for rough machining and 0.1-0.5mm for fine machining. Due to the good plasticity of aluminum and its alloys, too small a cutting depth can easily cause extrusion deformation and affect the surface quality.
Cooling, lubrication, and clamping methods also significantly impact the turning quality of aluminum and its alloys. The primary purpose of cooling and lubrication is to reduce friction, lower cutting temperatures, prevent tool sticking, and improve surface quality. Cutting fluids with excellent lubrication and cooling properties should be selected, such as kerosene, diesel, or specialized aluminum alloy cutting fluids. Kerosene has excellent penetrability and lubricity, effectively reducing tool sticking and making it suitable for finishing. Diesel oil has a better cooling effect and is suitable for roughing. It should be noted that while water-soluble cutting fluids offer excellent cooling effects, they can easily cause oxidation and discoloration on the aluminum alloy surface. Therefore, the concentration and pH value must be controlled during use. Workpiece clamping should be avoided to prevent plastic deformation. For thin-walled aluminum alloy workpieces, soft jaws or specialized clamps can be used to increase contact area and reduce clamping force. For long aluminum alloy workpieces, a steady rest or center rest is required to prevent bending during machining. After machining, the cutting fluid and chips on the workpiece surface must be cleaned promptly to prevent residual cutting fluid from corroding the aluminum alloy. Through reasonable tool selection, parameter optimization and process control, efficient and high-quality turning of aluminum and its alloys can be achieved.
