Explore the key parameters of CNC turning of titanium alloy blades

When CNC turning titanium alloy blades, the key parameters involve tool materials and geometric parameters, cutting parameters, cooling and lubrication, etc. The following is a specific analysis:

Tool materials and geometric parameters

Tool material:

Hard alloy cutting tools are the mainstream choice. They have good toughness and low cost, and are suitable for intermittent cutting. Ultrafine grain matrix can be adopted, combined with high cobalt content to enhance impact resistance.

Physically coated cemented carbide cutting tools have balanced performance. Coating technologies such as TiAlN/AlCrN multi-layer composite coatings have high hardness and good heat resistance. Compared with uncoated tools, the tool adhesion rate is reduced and the surface roughness of the machined surface is better.

Ceramic cutting tools are suitable for continuous cutting and semi-finishing stages, but they have relatively low bending strength and need to avoid vibration and impact loads. The fracture toughness can be improved by adding tungsten carbide particles to the silicon nitride ceramic matrix.

Tool geometric parameters:

The rake Angle and main deflection Angle of the cutting tool should be small, and an appropriate arc of the tool tip must be ground. The rough turning rake Angle can be taken as a negative rake Angle to enhance the strength of the cutting edge, while the fine turning rake Angle is taken as a positive rake Angle. The relief Angle should be relatively large. Generally, for hard alloy turning tools, it can be 5° to 8°. If the relief Angle is less than 5°, friction will occur; if it exceeds 10°, the cutting edge will be worn due to its low strength.

The roughness values at the front and back of the cutting tool should be small, generally less than 0.2μm.

Cutting parameters

Cutting speed: Cutting speed has the greatest impact on tool durability. It is best to operate the tool at the optimal cutting speed with the least relative wear. Due to the low thermal conductivity of titanium alloys, the cutting heat is concentrated near the cutting edge, with a high temperature in the edge zone, resulting in severe tool wear. Moreover, they have a strong chemical affinity and are severely bonded to hard alloys containing Ti. Therefore, a lower cutting speed should be adopted. Generally speaking, the cutting speed should be 50% lower than that of steel parts with the same hardness. For instance, when processing Ti6Al4V, the cutting speed of cemented carbide tools can reach 60-120m /min. The optimal cutting temperature for high-speed steel tools cutting titanium alloys is approximately 480℃ - 540℃, while for cemented carbide tools, it is about 650℃ - 750℃.

Feed rate: The feed rate has a relatively small impact on the durability of the cutting tool. Under the condition of ensuring the surface roughness of the machined surface, a larger feed rate can be selected, generally ranging from 0.1 to 0.3mm/r. If the feed rate is too small, the tool will cut within the hardened layer, increasing tool wear. Meanwhile, extremely thin chips are prone to spontaneous combustion at high cutting temperatures. Therefore, it is not allowed to be less than 0.05mm/r.

Cutting depth: The cutting depth has the least impact on the durability of the tool. Generally, a larger cutting depth is selected. This not only prevents the tool tip from cutting within the hardened layer, reducing tool wear, but also increases the working length of the cutting edge, which is conducive to heat dissipation. It is usually taken as 1-5mm. During rough machining, the cutting depth can be taken as 2-5mm, and during finish machining, it can be taken as 0.1-0.5mm.

Cooling and Lubrication

Coolant selection: Generally, extreme pressure emulsion should be used for cooling, and the flow rate should be sufficient. However, when the fatigue strength of the parts is required to be relatively high, the cutting fluid should not contain sulfur or chlorine. In such cases, ordinary emulsions should be selected. If chlorine-containing cutting fluid is used, hydrogen gas will be released during the cutting process at high temperatures and absorbed by titanium, causing hydrogen embrittlement. It may also lead to high-temperature stress corrosion cracking of titanium alloys.

Cooling strategy: High-pressure internal cooling (pressure ≥7MPa) combined with oil-based cutting fluid can reduce the cutting temperature by 40%.