Machining of difficult-to-cut materials
Difficult-to-cut materials generally refer to materials with high strength, high hardness, high toughness, or high wear resistance, such as high-temperature alloys, titanium alloys, stainless steel, high-strength steel, and composite materials. These materials present problems such as high cutting resistance, high cutting temperatures, rapid tool wear, and poor surface quality during turning, placing extremely high demands on machining processes and tool performance. Difficult-to-cut materials are widely used in high-end fields such as aerospace, energy, and military. For example, turbine blades for aircraft engines are made of high-temperature alloys, and their turning must not only ensure dimensional accuracy but also avoid residual stress generated during machining that affects the performance of the parts. Therefore, in-depth research on the turning characteristics of difficult-to-cut materials and the development of reasonable machining plans are key to ensuring product quality and production efficiency.
The selection of tool materials is crucial for turning difficult-to-cut materials. Tool material selection requires high hardness, wear resistance, heat resistance, and good impact resistance. For turning high-temperature alloys (such as GH4169), ceramic or cubic boron nitride (CBN) tools are recommended. Ceramic tools can reach hardnesses of 90-95 HRA and heat resistances of up to 1200-1400°C, maintaining excellent cutting performance even at high temperatures. CBN tools are suitable for machining high-hardness materials (hardness greater than 45 HRC) and offer wear resistance 10-50 times that of cemented carbide, making them particularly well-suited for interrupted cutting. When machining titanium alloys (such as TC4), tool sticking is a common problem due to their strong affinity with tool materials. Therefore, tungsten-cobalt cemented carbides (such as YG8) or ultrafine-grained carbides should be selected. These tools offer excellent anti-sticking properties and thermal conductivity, reducing chip-to-tool adhesion. For high-strength stainless steel (such as 316L), coated carbide tools (such as TiAlN coating) can be used. The coating can effectively reduce friction and wear and increase tool life.
The optimization of tool geometry significantly impacts the turning performance of difficult-to-cut materials. Parameters such as the rake angle, clearance angle, and tool tip radius need to be adjusted according to the material properties. When machining high-strength steel, a smaller rake angle (-5°-5°) and a larger clearance angle (8°-12°) should be used. A smaller rake angle can increase tool head strength and resist greater cutting forces, while a larger clearance angle can reduce friction between the back face and the workpiece. The tool tip radius should be 0.8-1.2mm to enhance the tool tip’s impact resistance. When turning titanium alloys, the rake angle should be positive (5°-10°) to reduce cutting deformation and tool sticking, and the clearance angle should be 10°-15° to avoid intense friction between the back face and the workpiece surface. Due to the low elastic modulus of titanium alloys, springback is prone to occur, so the tool tip radius should not be too large (0.4-0.8mm) to prevent tearing of the machined surface. When processing high-temperature alloys, the rake angle is usually 0°-5°, the back angle is 8°-10°, and a negative chamfer (width 0.1-0.3mm, angle -10°–5°) is used to improve the blade strength and reduce tool chipping.
Cutting parameters should be selected based on the principle of “low cutting speed, small feed rate, and reasonable depth of cut” to reduce cutting temperature and tool wear. When turning high-temperature alloys, the cutting speed is typically controlled between 10-50 m/min (carbide tools) or 50-100 m/min (ceramic tools). Excessive cutting speeds can lead to a sharp increase in cutting temperature and increased tool wear. The feed rate should be 0.1-0.2 mm/min; excessive feed rates can increase cutting forces and vibration. The depth of cut is determined by the machining allowance, with a range of 2-5 mm for roughing and 0.5-1 mm for finishing. When machining titanium alloys, the cutting speed is generally 30-100 m/min. Due to the low thermal conductivity of titanium alloys, cutting heat is concentrated in the cutting area, requiring strict control of the cutting speed to prevent tool overheating. The feed rate should be 0.05-0.15 mm/min to reduce chip-tool contact time. The depth of cut should be 1-3 mm to avoid cutting into the hardened layer due to a too small depth of cut. For high-strength steel, the cutting speed is 50-100m/min, the feed rate is 0.1-0.3mm/r, and the cutting depth is 2-6mm. At the same time, sufficient cutting fluid supply must be ensured to reduce the cutting temperature.
Cooling, lubrication, and process-aiding measures are crucial for improving turning conditions for difficult-to-cut materials. Due to the high cutting temperatures encountered during turning, conventional emulsions have limited cooling effectiveness. Therefore, extreme-pressure cutting fluids or oil-based cutting fluids should be used. Extreme-pressure additives such as sulfur, phosphorus, and chlorine in extreme-pressure cutting fluids form a lubricating film at high temperatures, reducing friction and wear. Oil-based cutting fluids have poor thermal conductivity but excellent lubrication properties, making them suitable for high-speed and heavy-load cutting. For deep-hole turning or enclosed cutting areas, a high-pressure cooling system (10-30 MPa) can be employed to spray the cutting fluid directly onto the cutting edge, enhancing cooling and chip evacuation. Furthermore, proper workpiece clamping and tool rigidity are crucial. When machining slender, difficult-to-cut workpieces such as shafts, a steady rest or steady rest is required to enhance workpiece rigidity and prevent bending and deformation. Short toolholders should be used to increase tool system rigidity and reduce vibration. A comprehensive approach to tool optimization, parameter adjustment, and auxiliary measures can effectively improve turning performance in difficult-to-cut materials, ensuring both machining quality and efficiency.
