Machining of lead alloys
Lead alloys are alloys based on lead with alloying elements such as tin, antimony, and copper. They feature high density (approximately 11.34 g/cm³), low melting point (pure lead melts at 327°C, while alloys melt even lower), good plasticity, and strong corrosion resistance (especially sulfuric acid). They are widely used in chemical equipment, batteries, radiation protection, and precision counterweights. While lead alloys have low hardness (pure lead has a hardness of 5-10 HB, while alloys have a hardness of 15-30 HB) and low strength (tensile strength of 10-30 MPa), they are free-cutting materials. However, turning can lead to problems such as tool sticking, chip entanglement, and poor surface quality. Therefore, it is crucial to select the appropriate tool, cutting parameters, and cooling method based on the material’s properties to ensure machining quality and efficiency.
Tool selection for lead alloy turning requires careful consideration of tool material and geometry to minimize tool sticking and improve surface quality. Because lead alloys are soft and plastic, tool materials with low affinity for lead and moderate wear resistance should be selected. High-speed steel tools (such as W18Cr4V) are preferred. Their sharp cutting edges minimize cutting distortion and are inexpensive, making them suitable for low-speed finishing. Carbide tools (such as YG) are suitable for high-speed rough turning. YG8 carbide offers high toughness and can withstand high feed rates, but care must be taken to avoid tool sticking to the lead alloy. Tool geometry should be designed to reduce cutting forces and inhibit built-up edge (BUE). The following parameters should be considered: A rake angle of 15°-25° is recommended. Increasing the rake angle reduces cutting deformation and friction, lowering the risk of tool sticking. A clearance angle of 8°-12° ensures sufficient clearance between the tool and the workpiece, minimizing flank wear. A medium lead angle of 45°-75° balances radial and axial cutting forces, preventing workpiece deformation. A nose radius of 0.2-0.5mm is recommended. A smaller nose radius reduces surface residual area and improves surface quality. For example, when fine-turning lead-antimony alloy, a W18Cr4V high-speed steel turning tool with a 20° rake angle, a 10° clearance angle, a 60° lead angle, and a 0.3mm nose radius can effectively reduce tool sticking and achieve a surface roughness of up to Ra 1.6μm.
The cutting parameters for lead alloy turning need to be reasonably determined according to the processing stage and surface quality requirements, taking into account both efficiency and quality. The cutting speed has a significant impact on the processing quality of lead alloys. When cutting at low speeds (≤100m/min), built-up edge and tool sticking are easily generated, resulting in increased surface roughness (Ra≥6.3μm); when cutting at medium and high speeds (100-300m/min), the cutting temperature increases, the built-up edge disappears, and the surface quality improves (Ra≤3.2μm). When rough turning, to improve efficiency, the cutting speed is 150-200m/min, the feed rate is 0.2-0.3mm/r, and the cutting depth is 2-5mm; when finishing turning, to ensure surface quality, the cutting speed is 200-300m/min, the feed rate is 0.05-0.1mm/r, and the cutting depth is 0.1-0.5mm. It’s important to note that lead alloys have a low melting point. Excessively high cutting speeds (>300 m/min) can cause localized overheating and melting of the workpiece, affecting dimensional accuracy. Therefore, the maximum cutting speed must be controlled. For example, when turning a 10mm thick lead-tin alloy plate, a rough turning speed of 180 m/min, a feed of 0.25 mm/r, and a depth of cut of 3 mm are sufficient. Fine turning with a cutting speed of 250 m/min, a feed of 0.08 mm/r, and a depth of cut of 0.3 mm yields excellent results.
Cooling, lubrication, and chip control are crucial for smooth machining during lead alloy turning. Because lead alloys are prone to tool sticking, the cooling lubricant must possess excellent lubricity and chip removal properties. Commonly used cooling lubricants include kerosene, diesel, or specialized nonferrous metal cutting fluids. Kerosene offers superior lubricity, effectively reducing tool-workpiece adhesion and facilitating chip separation. Low-pressure jets (0.5-1 MPa) are used for cooling to avoid high-pressure conditions that could cause surface dents or deformation. Chip control measures include milling chip grooves into the tool, adjusting their width and depth based on the feed rate (greater feed rates increase the groove width and depth). Chip breakers are used to mechanically separate the chips. Regular chip removal during machining prevents excessive accumulation and scratches on the machined surface. For lead alloys with high antimony content (antimony content > 5%), due to their increased brittleness and resulting chip fragmentation, enhanced operator protection is required to prevent the spread of lead dust, which could pose a health risk.
Quality control in lead alloy turning requires a focus on dimensional accuracy, surface quality, and safety. Regarding dimensional accuracy, lead alloys have a low elastic modulus (approximately 18 GPa), making them susceptible to springback deformation after cutting. Therefore, a correction allowance of 0.05-0.1 mm is required during finish turning, and dimensional tolerances (generally controlled within IT10-IT12 levels) are maintained through secondary finish turning. Regarding surface quality, in addition to controlling surface roughness (Ra ≤ 3.2 μm), tool sticking, scratches, and dents must be avoided. This can be achieved by increasing cutting speeds, optimizing tool angles, and enhancing cooling. Regarding safety, lead is a toxic heavy metal, and the lead dust and chips generated during turning pose a health hazard. Operators must wear protective equipment such as dust masks and gloves. Ventilation equipment must be installed in the workplace, and air lead concentrations must be regularly monitored (the national standard limit is 0.05 mg/m³). Chips must be collected and treated as hazardous waste to prevent environmental pollution. In addition, the machinability of lead alloy is greatly affected by temperature. When the ambient temperature is lower than 10°C, the brittleness of the material increases and it is easy to crack. When it is higher than 30°C, the plasticity of the material increases and the sticking phenomenon intensifies. Therefore, the processing environment temperature needs to be controlled within the range of 15-25°C to ensure stable processing performance.
