Why do we think titanium alloy is a difficult material to process? Because we lack a deep understanding of its processing mechanism and phenomena.
1. Physical phenomena of titanium processing
The cutting force during titanium alloy processing is only slightly higher than that of steel with the same hardness, but the physical phenomenon of titanium alloy processing is much more complicated than that of steel processing, which makes titanium alloy processing face great difficulties.
Why do we think titanium alloy is a difficult material to process? Because of the lack of understanding of its processing mechanism
The thermal conductivity of most titanium alloys is very low, only 1/7 of steel and 1/16 of aluminum. Therefore, the heat generated during the cutting of titanium alloy will not be quickly transferred to the workpiece or carried away by the chips, but will accumulate in the cutting area. The temperature generated can be as high as 1,000°C or more, causing the cutting edge of the tool to wear, crack and generate built-up edge quickly, and the worn blade will quickly 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 titanium alloy parts, resulting in a decrease in the geometric accuracy of the parts and a work hardening phenomenon that seriously reduces their fatigue strength.
The elasticity of titanium alloys 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 poor thermal conductivity of titanium alloys.
This problem is more serious when processing thin-walled or ring-shaped parts that are easy to deform. It is not easy to process titanium alloy thin-walled parts to the expected dimensional accuracy. Because as the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range and produces plastic deformation, and the material strength and hardness of the cutting point are significantly increased. At this time, the processing according to the originally determined cutting speed becomes too high, further causing rapid wear of the tool.
"Heat" is the "culprit" of the difficulty in processing titanium alloys!
2. Processing tips for titanium alloy processing
Based on the understanding of the titanium alloy processing mechanism and previous experience, the main process tips for processing titanium alloys are as follows:
(1) Use a blade with a positive angle geometry to reduce 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. The radial cutting depth ae during milling should be 30% of the radius.
(3) Use high-pressure and high-flow cutting fluid to ensure the thermal stability of the processing process and prevent surface degeneration and tool damage due to excessive 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) Process titanium alloy in the softest state as much as possible, because the material becomes more difficult to process after hardening. Heat treatment increases the strength of the material and increases blade wear.
(6) Use a large tool tip radius or chamfer to cut in and put as much of the blade 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 the greatest impact on tool life vc among all cutting parameters, followed by radial cutting depth ae.
3. Solve the titanium processing problem from the blade
The blade groove wear that occurs during titanium alloy processing is the local wear of the back and front along the cutting depth direction. It is often caused by the hardened layer left by the previous processing. The chemical reaction and diffusion between the tool and the workpiece material at a processing temperature exceeding 800°C is also one of the reasons for the formation of groove wear. Because during the processing, the titanium molecules of the workpiece accumulate in front of the blade and "weld" to the blade under high pressure and high temperature to form a built-up edge. When the built-up edge is peeled off from the blade, the carbide coating of the blade is taken away. Therefore, titanium alloy processing requires special blade materials and geometries.
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 onto the cutting edge in a timely and accurate manner to quickly remove the heat. There are unique structures of milling cutters on the market specifically for titanium alloy processing.
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Company: Baoji Jimiyun Dynamic Co., Ltd
Country:China
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