Why do we think titanium alloy is a difficult-to-process material? Because of the lack of deep understanding of its processing mechanism and phenomenon.
1. Physical phenomena of titanium processing
The cutting force during processing of titanium alloy is only slightly higher than that of steel of the same hardness, but the physical phenomenon of processing titanium alloy is much more complicated than that of processing steel, which makes titanium alloy processing face huge difficulties.
The thermal conductivity of most titanium alloys is very low, only 1/7 that of steel and 1/16 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 concentrated in the cutting area. The temperature generated can be as high as 1,000 ℃, causing the cutting edge of the tool to quickly wear, crack and Generate built-up edge, wear out the cutting edge quickly, and generate more heat in the cutting area, further shortening the life of the tool.
The high temperature generated during the cutting process also destroys the surface integrity of the titanium alloy parts, resulting in a decrease in the geometric accuracy of the parts and a work hardening phenomenon that severely reduces its fatigue strength.
The elasticity of titanium alloy may be beneficial to the performance of parts, but during the cutting process, 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 action. The friction process also generates heat, which aggravates the problem of poor thermal conductivity of titanium alloys.
This problem is even more serious when processing thin-walled or ring-shaped parts that are easily deformed. It is not an easy task 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 and plastic deformation occurs, and the material strength and hardness of the cutting point increase significantly. At this time, machining according to the originally determined cutting speed becomes too high, which further leads to sharp tool wear.
“Hot” is the “culprit” that is difficult to process titanium alloys!
2. Process know-how for processing titanium alloys
On the basis of understanding the processing mechanism of titanium alloy, coupled with past experience, the main process know-how for processing titanium alloy is as follows:
(1) Use positive-angle geometry inserts to reduce cutting force, cutting heat and workpiece deformation.
(2) Maintain a constant feed to avoid hardening of the workpiece. The tool must always be in the feed state during the cutting process, and the radial tool engagement ae during milling should be 30% of the radius.
(3) Use high-pressure and large-flow cutting fluid to ensure the thermal stability of the machining process and prevent surface degeneration and tool damage caused by excessive temperature.
(4) Keep the cutting edge of the blade sharp. Blunt 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. Heat treatment increases the strength of the material and increases the wear of the blade.
(6) Use a large arc radius or chamfer to cut in, and put as many cutting edges as possible into the cutting. This can reduce the cutting force and heat at every point and prevent local damage. When milling titanium alloys, among the cutting parameters, the cutting speed has the greatest influence on the tool life vc, followed by the radial tool engagement (milling depth) ae.
3. Start with the blade to solve the problem of titanium processing
The blade groove wear that occurs during titanium alloy processing is the local wear along the cutting depth direction at the back and the front, and it is often caused by the hardened layer left by the previous processing. The chemical reaction and diffusion of the tool and the workpiece material at a processing temperature of more than 800°C are also one of the reasons for the formation of groove wear. Because during the machining process, the titanium molecules of the workpiece accumulate in the front area of the blade and are “welded” to the blade under high pressure and high temperature, forming a built-up edge. When the built-up edge is peeled from the blade, it will take away the carbide coating of the blade. 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 must be sprayed on the cutting edge in time and accurately to quickly remove the heat. There is a unique structure of milling cutter specially used for titanium alloy processing on the market.