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 during titanium alloy processing 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, so titanium alloy processing faces 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 concentrated in the cutting area, the resulting temperature can be as high as 1 000 °C or more. The edge of the tool is rapidly worn, cracked, and built-up edge is formed, and the worn edge appears quickly, and more heat is generated 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 part and a work hardening phenomenon that seriously reduces its fatigue strength.
The elasticity of titanium alloys may be beneficial to the performance of the part, 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 bounce 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 problem of poor thermal conductivity of titanium alloys.
This problem is exacerbated when machining deformable parts such as thin-walled or annular parts, and machining titanium thin-walled parts to the expected dimensional accuracy is not an easy task.
Because as 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 point, machining at the previously determined cutting speed becomes too high, further leading to sharp tool wear.
"Hot" is the "culprit" of titanium alloy difficult processing!
2. Process know-how for processing titanium alloys
On the basis of understanding the processing mechanism of titanium alloy, coupled with previous experience, the main process know-how for processing titanium alloy is as follows:
(1) Adopt a blade with positive angle geometry to reduce the cutting force, cutting heat, and workpiece deformation.
(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 eating amount ae during milling should be 30% of the radius.
(3) High-pressure and high-flow cutting fluid is used to ensure the thermal stability of the machining process and prevent the surface denaturation of the workpiece and tool damage caused by excessive temperature.
(4) Keep the blade edge, blunt tools are the cause of heat accumulation and wear, which can easily lead to tool failure.
(5) As far as possible, the softest state of the titanium alloy is processed, because the material becomes more difficult to process 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 as many cutting edges as possible. This reduces cutting force and heat at every point and prevents localized breakage. When milling titanium alloys, the cutting speed has the greatest influence on the tool life Vc among the cutting parameters, followed by the radial eating amount (milling depth) ae.
3. Start with the blade to solve the titanium processing problem
The wear of the blade groove during 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 preliminary machining.
The chemical reaction and diffusion between 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 processing process, the titanium molecules of the workpiece accumulate in the front area of the blade and are "welded" to the cutting edge under high pressure and high temperature, forming a built-up edge.
When the built-up edge peels off the cutting edge, the carbide coating of the blade is taken away, therefore, titanium alloy processing requires special insert materials and geometries.
4. Tool structure suitable for titanium machining
The focus of titanium alloy processing is heat, and a large amount of high-pressure cutting fluid must be sprayed onto the cutting edge in time and accurately to quickly remove the heat.
At present, there are milling cutter structures specially designed for machining titanium alloys on the market.
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