Why do we think titanium alloy is a kind of difficult material? Due to lack of understanding of its processing mechanism
Most titanium alloys have low thermal conductivity, only 1/7 of steel and 1/16 of aluminum. Therefore, the heat generated in the process of cutting titanium alloy will not be rapidly transferred to the workpiece or taken away by the chips, but will be concentrated in the cutting area, and the resulting temperature can be as high as 1000 ℃ or more, which will make the cutting edge of the tool rapidly wear, crack and generate chip accumulation, and the rapidly worn cutting edge will also make the cutting area generate more heat, further shortening the tool life.
The high temperature generated in the cutting process also destroys the surface integrity of the titanium alloy parts, resulting in the reduction of the geometric accuracy of the parts and the work-hardening phenomenon that seriously reduces the fatigue strength of the parts.
The elasticity of titanium alloy may be beneficial to the performance of parts, but in the process of cutting, the elastic deformation of the workpiece is an important cause of vibration. The cutting pressure causes the "elastic" workpiece to leave the tool and rebound, so that the friction between the tool and the workpiece is greater than the cutting effect. The friction process also produces heat, which aggravates the poor thermal conductivity of titanium alloy.
This problem is even more serious when machining thin-walled or annular parts that are easily deformed. It is not easy to process titanium alloy thin-walled parts to the expected dimensional accuracy. Because when the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range, resulting in plastic deformation, and the strength and hardness of the material at the cutting point are significantly increased. At this time, the machining speed determined previously becomes too high, further leading to sharp tool wear.
"Heat" is the "culprit" of titanium alloy difficult processing!
2. Technological know-how for processing titanium alloy
On the basis of understanding the mechanism of titanium alloy processing, combined with previous experience, the main technological know-how for processing titanium parts are as follows:
(1) The blade with positive angle geometry is used to reduce cutting force, cutting heat and workpiece deformation.
(2) Keep constant feed to avoid hardening of the workpiece. The tool should always be in the feed state during the cutting process. During milling, the radial feed a e should be 30% of the radius.
(3) High-pressure and large-flow cutting fluid is used to ensure the thermal stability of the processing process and prevent the workpiece surface from denaturing and tool damage due to high temperature.
(4) Keep the blade edge sharp. Blunt tools are the cause of heat accumulation and wear, which can easily lead to tool failure.
(5) Try to machine in the soft state of titanium alloy Z, because the material becomes more difficult to machine after hardening, and the heat treatment improves the strength of the material and increases the wear of the blade.
(6) Use a large tip arc radius or chamfer to cut, and put more blades into the cutting as far as possible. This can reduce the cutting force and heat at each point and prevent local damage. When milling titanium alloy, the cutting speed has a great influence on the tool life vc, and the radial feed (milling depth) takes the second place.
3. Solve the titanium processing problem from the blade
The blade groove wear during titanium alloy machining is the local wear in the direction of cutting depth at the back and front, which is often caused by the hardened layer left by the previous machining. The chemical reaction and diffusion of tool and workpiece material at processing temperature of more than 800 ℃ is also one of the reasons for groove wear. Because in the process of machining, titanium molecules of the workpiece accumulate in the front of the blade and "weld" to the blade under high pressure and high temperature, forming a chip accumulation tumor. When the chips are peeled off from the blade, the hard alloy coating of the blade will be taken away. Therefore, titanium alloy processing requires special blade material and geometry.
4. Tool structure suitable for titanium processing
The focus of titanium alloy machining is heat. A large amount of high-pressure cutting fluid must be sprayed onto the cutting edge in time and accurately to quickly remove heat. There is a unique structure of milling cutter specially used for titanium alloy processing on the market.
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