Machinability of aluminum and its alloys
Aluminum and its alloys are widely used in aerospace, automotive, telecommunications, and medical devices due to their low density (approximately 2.7g/cm³), high specific strength, excellent thermal conductivity, superior corrosion resistance, and ease of machining. Applications include aircraft fuselages, automobile engine blocks, and mobile phone casings. While the machinability of aluminum and its alloys is generally considered good, significant differences exist between different grades of aluminum alloy due to their composition and heat treatment. Machinability is generally evaluated based on cutting force, cutting temperature, tool wear, surface quality, and chip control. Understanding the machinability characteristics of aluminum and its alloys is crucial for selecting appropriate tools, cutting parameters, and machining methods.
The machinability of pure aluminum has unique characteristics. Pure aluminum (such as 1050 and 1060 ) has high purity (aluminum content ≥ 99.5% ), soft texture (hardness 25-35HB ), good plasticity (elongation 30%-40% ), and is prone to tool sticking and built-up edge during cutting, resulting in increased surface roughness ( Ra can reach 5-10μm ). Pure aluminum has a high thermal conductivity (about 237W/(m・K) ), and the heat generated during cutting is easily carried away by the workpiece and chips. The cutting temperature is low (generally ≤ 200 ℃), and the heat resistance requirements for the tool are not high. However, due to the good plasticity of pure aluminum, the chips are in the form of continuous strips, not easy to break, and easily entangled on the tool and workpiece, affecting the smooth progress of the processing process. The cutting force of pure aluminum is relatively small, about that of 45 steel. 1/3-1/2, so a higher cutting speed (such as 1000-3000m/min) can be used to improve processing efficiency. To improve the machinability of pure aluminum, pure aluminum with fine grain structure can be used, or cooling lubricants (such as kerosene or special aluminum alloy cutting fluids) can be used during processing to reduce tool sticking and built-up edge and improve surface quality.
The machinability of wrought aluminum alloys varies significantly depending on alloy composition and heat treatment. These alloys primarily include rust-resistant aluminum alloys (such as 5052 and 5A02), hard aluminum alloys (such as 2024 and 2A12), superhard aluminum alloys (such as 7075 and 7A04), and wrought aluminum alloys (such as 6061 and 6A02). Rust-resistant aluminum alloys contain elements such as magnesium and manganese, offering excellent corrosion resistance and ductility. They have a lower hardness (50-80 HB) and good machinability, but can still suffer from tool sticking and built-up edge. They are suitable for high-speed cutting (cutting speeds of 800-2000 m/min) and can achieve surface roughnesses of Ra 1.6-3.2 μm. Hard aluminum alloys, containing elements such as copper and magnesium, achieve higher hardness (120-160 HB) after quenching and aging, exhibit high strength, moderate machinability, and possess cutting forces 20%-30% greater than pure aluminum. Chips are short, easily controlled, and can achieve surface roughnesses of Ra 0.8-1.6 μm. Superhard aluminum alloys contain elements such as zinc, magnesium, and copper. They are the strongest aluminum alloys (tensile strength can reach over 600 MPa) and have a hardness of 150-180 HB. However, they have poor machinability, high cutting forces, and rapid tool wear, requiring the use of wear-resistant tools and appropriate cutting parameters. Wrought aluminum alloys contain elements such as silicon and magnesium. After heat treatment, they exhibit excellent overall properties and machinability, making them one of the most widely used wrought aluminum alloys.
The machinability of cast aluminum alloys is closely related to the silicon content of the alloy. Cast aluminum alloys primarily include aluminum-silicon alloys (such as ZL101 and ZL104), aluminum-copper alloys (such as ZL201 and ZL202), aluminum-magnesium alloys (such as ZL301 and ZL303), and aluminum-zinc alloys (such as ZL401 and ZL402). Aluminum-silicon alloys (also known as silicon-aluminum alloys) are the most widely used cast aluminum alloys. Silicon content generally ranges from 5% to 12%, with silicon present in a free state, giving the alloy excellent fluidity and casting properties. At low silicon contents (5% to 8%), the alloy is softer, with machinability similar to that of pure aluminum, but prone to tool sticking. At higher silicon contents (10% to 12%), the free silicon particles have a high hardness (HV800-1100), which increases tool wear, but also results in chip breakage and excellent surface quality (Ra0.8-1.6μm). Aluminum-copper alloys offer high strength but poor corrosion resistance and moderate machinability. Cooling during cutting is crucial to avoid thermal deformation. Aluminum-magnesium alloys offer excellent corrosion resistance and mechanical properties, but poor casting properties. They offer good machinability and are suitable for high-speed cutting. Aluminum-zinc alloys offer good casting properties but poor corrosion resistance, and their machinability is similar to that of aluminum-silicon alloys.
The choice of tool material significantly impacts the quality and efficiency of machining aluminum and its alloys. Because aluminum and its alloys have low hardness, good plasticity, and are prone to tool sticking, tool materials must possess excellent wear resistance, adhesion resistance, and thermal conductivity. High-speed steel tools (such as W18Cr4V and W6Mo5Cr4V2) are suitable for low-speed cutting (cutting speeds ≤ 300 m/min) and complex tools (such as taps and broaches). They are low-cost, but they offer poor wear resistance and are prone to built-up edge. Carbide tools are the preferred choice for machining aluminum alloys. Commonly used grades include YG (such as YG6 and YG8) and ultrafine-grained carbides (such as WC-Co alloys). YG carbides have low affinity for aluminum, good adhesion resistance, and moderate wear resistance, making them suitable for machining general aluminum alloys. Ultrafine-grained carbides have higher hardness (HV1800-2000) and better wear resistance, making them suitable for machining high-silicon aluminum alloys, as they reduce tool wear caused by silicon particles. Diamond tools (natural diamond and artificial polycrystalline diamond PCD) have extremely high hardness (HV8000-10000) and wear resistance, have very low affinity with aluminum, do not produce built-up edge, are suitable for high-speed precision cutting (cutting speed 1000-5000m/min), and have a surface roughness of up to Ra0.02-0.1μm. They are suitable for mirror processing and processing of high-silicon aluminum alloys, but they are relatively expensive and brittle, making them unsuitable for intermittent cutting.
Optimization of cutting parameters and cooling and lubrication methods can effectively improve the machinability of aluminum and its alloys. The cutting speed has a significant impact on the processing quality of aluminum alloys. Low-speed cutting (≤300m/min) is prone to built-up edge and poor surface quality. During high-speed cutting (≥1000m/min), the cutting temperature increases, the built-up edge disappears, and the surface quality improves. Therefore, high-speed cutting should be used as much as possible. The feed rate is determined according to the surface quality requirements. The feed rate is 0.1-0.3mm/r during rough machining and 0.05-0.1mm/r during fine machining. Excessive feed rate will increase the surface roughness, and too small feed rate will increase the friction between the tool and the workpiece and cause work hardening. The cutting depth is generally 1-5mm, which can be appropriately increased during rough machining to improve efficiency. The selection of cooling lubricant requires