Why do we think that titanium alloy is a difficult-to-machine material? Because of the lack of deep understanding of its processing mechanism and phenomenon.
The cutting force of titanium alloy is only slightly higher than that of steel with the same hardness, but the physical performance of machining titanium alloy is much more complex than that of machining steel, which makes the machining of titanium alloy face enormous difficulties.
The thermal conductivity of most titanium alloys is very low, only 1/7 of that of steel and 1/16 of that of aluminum. Therefore, the heat generated in the process of cutting titanium alloy will not be quickly transferred to the workpiece or taken away by the chips, but will be gathered in the cutting area, and the temperature generated can be as high as more than 1 000 ℃, so that the cutting edge of the tool is rapidly worn, cracked, and built-up edge is generated, and the rapidly worn cutting edge generates more heat in the cutting area, which further shortens the life of the tool.
The high temperature generated in the cutting process also destroys the surface integrity of titanium alloy parts, resulting in the decline of geometric accuracy of parts and the appearance of work hardening which seriously reduces their fatigue strength.
The elasticity of titanium alloys may be beneficial to the performance of parts, but in the cutting process, the elastic deformation of the workpiece is an important cause of vibration. The cutting pressure causes the “elastic” workpiece to move away from the tool and rebound, so that the friction between the tool and the workpiece is greater than the cutting action. The friction process also generates heat, which aggravates the problem of poor thermal conductivity of titanium alloys.
This problem is more serious when machining thin-walled or ring parts, which is easy to deform. It is not easy to machine titanium alloy thin-walled parts to the desired dimensional accuracy. As the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range and produced plastic deformation, and the material strength and hardness at the cutting point has increased significantly. At this point, machining at the originally determined cutting speed becomes too high, further resulting in rapid tool wear.
“Heat” is the “culprit” of difficult processing of titanium alloy!
Based on the understanding of the processing mechanism of titanium alloys and previous experience, the main process know-how for processing titanium alloys is as follows:
(1) Inserts with positive angle geometry are used to reduce cutting forces, cutting heat, and deformation of the workpiece.
(2) Maintain a constant feed to avoid hardening of the workpiece, the tool should always be in the feed state during the cutting process, and the radial engagement ae should be 30% of the radius during milling.
(3) High-pressure and large flow cutting fluid is used to ensure the thermal stability of the processing process and prevent the surface denaturation of the workpiece and the damage of the tool due to excessive temperature.
(4) Keep the blade edge sharp. Dull tools are the cause of heat build-up and wear, which can easily lead to tool failure.
(5) Work in the softest state of the titanium alloy as much as possible, because the material becomes more difficult to work after hardening, and heat treatment improves the strength of the material and increases the wear of the blade.
(6) Use a large nose radius or chamber to cut as much as possible into the cutting edge. This reduces cutting forces and heat at every point, preventing local breakage. In the milling of titanium alloy, the cutting speed has the greatest influence on the tool life VC, and the radial cutting depth (milling depth) AE takes the second place.
Groove wears of the blade in titanium alloy machining is the local wear of the back and front along the cutting depth direction, which is often caused by the hardened layer left by the previous machining. The chemical reaction and diffusion between the tool and the workpiece material at the processing temperature of more than 800 ℃ is also one of the reasons for the formation of groove wear. Because in the process of machining, titanium molecules of the workpiece accumulate in front of the blade and “weld” to the blade edge under high pressure and high temperature, forming a built-up edge. As the built-up edge peels away from the cutting edge, it carries the carbide coating of the insert with it. Therefore, machining of titanium alloys requires special insert materials and geometries.
The focus of titanium alloy processing is heat, and a large amount of high-pressure cutting fluid should be sprayed on the cutting edge in time and accurately, so that the heat can be removed quickly. There is a unique structure of milling cutter specially used for titanium alloy processing on the market.