How difficult is titanium alloy processing?
Why do we think titanium alloy is a difficult material to machine? Because of the lack of deep understanding of its processing mechanism and phenomenon.
1. Physical phenomena of titanium processing
The cutting force of titanium alloy is only slightly higher than that of steel with the same hardness, but the physical phenomenon of titanium alloy processing is much more complex than that of steel processing, which makes titanium alloy processing face great difficulties.
The thermal conductivity of most titanium alloys is very low, only 1/7 of steel and 1/16 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 concentrated in the cutting area. The temperature generated can be as high as 1000 ℃ or above, which makes the cutting edge of the tool rapidly wear, crack and generate chip accretion. The rapidly worn cutting edge will also generate more heat in the cutting area, further shortening the tool life.
The high temperature produced in the cutting process has destroyed the surface integrity of titanium alloy parts at the same time, leading to the decline of the geometric accuracy of parts and the appearance of work hardening that seriously reduces their fatigue strength.
The elasticity of titanium alloy may be beneficial to the performance of parts, but in the cutting process, the elastic deformation of the workpiece is an important reason for vibration. The cutting pressure makes the "elastic" workpiece 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 generates heat, which aggravates the poor thermal conductivity of titanium alloys.
This problem is even more serious when machining thin-walled or annular parts that are easily deformed. It is not easy to process thin-walled titanium alloy 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 have increased significantly. At this time, the cutting speed originally determined will become too high, further causing sharp tool wear.
"Heat" is the "culprit" of titanium alloy difficult to process!
2. Know how to process titanium alloy
On the basis of understanding the processing mechanism of titanium alloy, combined with previous experience, the main technological know-how for processing titanium alloy is as follows:
(1) The blade with positive angle geometry is used to reduce cutting force, cutting heat and workpiece deformation.
(2) Maintain constant feed to avoid hardening of the workpiece. The tool shall always be in the feed state during the cutting process. During milling, the radial feed ae shall be 30% of the radius.
(3) High pressure and large flow cutting fluid is used to ensure the thermal stability of the machining process and prevent the workpiece surface from denaturation and tool damage due to excessive temperature.
(4) Keep the blade sharp. The blunt tool is the cause of heat accumulation and wear, which is easy to lead to tool failure.
(5) As far as possible, it should be processed in the soft state of titanium alloy, because the material becomes more difficult to process after hardening. Heat treatment improves the strength of the material and increases the wear of the blade.
(6) Use a large tool tip arc radius or chamfer to cut in, and put as many blades into the cutting 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, followed by the radial cut (milling depth) ae.
3. Solve titanium processing problems from the blade
The groove wear of the blade during titanium alloy processing is the local wear of the back and front along the cutting depth, which is often caused by the hardening layer left by the previous processing. The chemical reaction and diffusion of tool and 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 processing, titanium molecules of the workpiece accumulate in front of the blade and are "welded" to the blade under high pressure and high temperature, forming a chip buildup tumor. When the built-up chip is stripped from the blade, the cemented carbide coating of the blade is taken away. Therefore, titanium alloy processing requires special blade materials and geometric shapes.
4. Tool structure suitable for titanium processing
The focus of titanium alloy processing is heat. A large amount of high-pressure cutting fluid should be sprayed on the cutting edge in time and accurately to remove the heat quickly. There are special milling cutter structures for titanium alloy processing on the market.
