Practical process parameters for turning sprayed materials
Spray coatings are coatings formed on substrate surfaces through techniques such as thermal spraying and cold spraying. They exhibit excellent properties such as wear resistance, corrosion resistance, and high temperature resistance, and are widely used in the surface strengthening and repair of mechanical parts such as engine crankshafts, rolls, and molds. There are many different types of spray coatings, including metal-based spray coatings (such as nickel-based alloys and cobalt-based alloys), ceramic-based spray coatings (such as Al₂O₃ and Cr₂O₃ ), and metal-ceramic spray coatings (such as WC-Co and Cr₃C₂-NiCr ). When turning spray coatings, traditional cutting parameters and tool materials are difficult to meet due to the coating’s high hardness (up to 50-70 HRC ), brittleness, and limited bonding strength with the substrate. Therefore, it is necessary to select appropriate tool materials, geometric parameters, and cutting parameters based on the characteristics of the spray coating to ensure machining quality and efficiency.
Tool material selection is crucial for turning sprayed materials and should be carefully considered based on the material’s hardness and wear resistance. For metal-based sprayed materials (hardness 30-50 HRC ), such as nickel-based alloy coatings, carbide tools such as YT726 (hardness 92.5 HRA ) or YW2 (hardness 91 HRA ) are recommended. These tools offer high bending strength and wear resistance, and can withstand certain impact loads. For ceramic-based sprayed materials (hardness 60-70 HRC ), such as Al₂O₃ coatings, ceramic or cubic boron nitride ( CBN ) tools are recommended. Al₂O₃ -based ceramic tools (such as AG2 ) have a hardness of up to 93 HRA and are suitable for continuous cutting. Si₃N₄ -based ceramic tools (such as SNMN ) offer superior toughness and are suitable for intermittent cutting. For metal-ceramic spray materials (hardness 55-65 HRC), such as WC-Co coatings, CBN tools are the best choice. For example, the solid CBN tool BN-S20 boasts a hardness exceeding 3500 HV, excellent wear resistance, and the ability to withstand cutting temperatures exceeding 1200°C. Diamond tools, due to their strong affinity for iron-based elements, are only suitable for machining nonferrous metal-based spray materials, not iron-based or ceramic-based ones.
Optimization of tool geometry parameters requires adjustment based on the characteristics of the sprayed material to reduce cutting forces and tool wear. Regarding the rake angle, when machining brittle ceramic-based sprayed materials, a negative rake angle (-5°-0°) should be used to enhance blade strength and prevent chipping. When machining metal-based sprayed materials, a smaller positive rake angle (5°-10°) can be used to reduce cutting deformation. A clearance angle of 6°-10° ensures sufficient clearance between the tool and the workpiece to reduce friction and wear. A lead angle of 45°-75° reduces radial cutting forces and prevents coating flaking. When machining thin-walled parts or coatings with low bond strength, a larger lead angle (e.g., 75°) should be used to minimize the impact of radial forces on the coating. The tool tip radius is 0.4-0.8mm. A larger tip radius improves tool tip strength but increases cutting forces, so the tool tip radius should be selected based on coating thickness and hardness. For example, when turning WC-Co coating (thickness 2-3mm, hardness 60HRC), the tool uses a -5° rake angle, 8° clearance angle, 60° main deflection angle and 0.6mm tool nose radius, which can effectively reduce tool wear and coating peeling.
The selection of cutting parameters requires a balance between machining efficiency and tool life. Different parameters are required for different types of spray materials. Regarding cutting speed, when turning metal-based spray materials, the cutting speed of carbide tools is 80-120 m/min; when turning ceramic-based spray materials, the cutting speed of ceramic tools is 100-150 m/min, and CBN tools can reach 150-250 m/min; when turning metal-ceramic spray materials, the cutting speed of CBN tools is 120-200 m/min. The feed rate should be determined based on the surface quality requirements. For rough turning, use 0.1-0.2 mm/r, and for finish turning, use 0.05-0.1 mm/r. Excessive feed rates can lead to coating flaking and increased surface roughness, while too low feed rates can increase tool wear due to increased friction between the tool and the workpiece. The cutting depth should be selected based on the coating thickness, generally 0.1-0.5mm . The initial cutting depth should be 0.1-0.2mm greater than the coating thickness to avoid cutting at the interface between the coating and the substrate (weak interface strength can easily cause coating peeling). For example, when turning a 2mm thick Cr₃C₂-NiCr coating, a cutting speed of 150m/min , a feed rate of 0.1mm/r , and a cutting depth of 0.3mm can achieve good machining results.
Cooling, lubrication, and machining processes significantly impact the turning quality of sprayed materials. Due to the poor thermal conductivity of sprayed materials (for example, the thermal conductivity of ceramic-based coatings is only 1/10-1/20 of that of metals), high cutting temperatures are likely to occur during cutting, necessitating effective cooling. When turning metal-based and metal-ceramic sprayed materials, an extreme pressure emulsion (10%-15%) can be used, sprayed directly into the cutting zone at high pressure (2-3 MPa), to reduce temperatures and improve lubrication. When turning ceramic-based sprayed materials, thermal cracking can easily occur due to improper cooling due to the brittleness of the ceramic. Therefore, oil mist or air cooling can be used to reduce thermal shock. Regarding machining techniques, a “layered cutting” approach should be employed to avoid cutting to final dimensions in a single pass. Each layer should be cut to a depth of 0.1-0.3 mm, gradually removing the coating and reducing cutting force fluctuations. For coatings with weak bonding strength, pre-grooving the coating edges is necessary to prevent overall flaking during cutting. The surface roughness after turning is generally required to be Ra≤3.2μm. For high-precision parts, grinding can be used for finishing, but attention should be paid to the selection of grinding parameters to avoid overheating of the coating and cracks.
