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Why is titanium alloy difficult to process?

Why do we consider titanium alloy to be a difficult to machine material? Because there is a lack of profound understanding of its processing mechanism and phenomena.

1. Physical phenomena in 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 processing titanium alloy is much more complex than that of processing steel, which makes titanium alloy processing face huge difficulties.

Most titanium alloys have low thermal conductivity, 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, resulting in a temperature of over 1000 ℃. This causes the cutting edge of the tool to rapidly wear, crack, and form chip deposits, resulting in a rapidly worn cutting edge and generating more heat in the cutting area, further shortening the tool's lifespan.

The high temperature generated during the cutting process simultaneously damages the surface integrity of titanium alloy parts, leading to a decrease in geometric accuracy of the parts and the occurrence of work hardening phenomena that seriously reduce their fatigue strength.

The elasticity of titanium alloy may be beneficial for 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 detach from the tool and rebound, resulting in greater friction between the tool and the workpiece than the cutting effect. The friction process also generates heat, exacerbating the problem of poor thermal conductivity of titanium alloys.

When processing thin-walled or ring-shaped parts that are prone to deformation, this problem becomes even more serious. It is not an easy task to machine titanium alloy thin-walled parts to the expected dimensional accuracy. Because as the workpiece material is pushed away by the cutting tool, the local deformation of the thin-wall has exceeded the elastic range and plastic deformation occurs, resulting in a significant increase in the material strength and hardness at the cutting point. At this point, machining at the originally determined cutting speed becomes too high, further leading to sharp tool wear.

"Heat" is the culprit behind the difficulty in processing titanium alloys!

2. Process tips for processing titanium alloys

On the basis of understanding the processing mechanism of titanium alloy and adding previous experience, the main process tips for processing titanium alloy are as follows:

(1) Adopting a blade with a regular geometric shape to reduce cutting force, cutting heat, and workpiece deformation.

(2) Maintain a constant feed rate to avoid hardening of the workpiece. During the cutting process, the tool should always be in the feed state. During milling, the radial feed rate ae 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 surface denaturation of the workpiece and tool damage caused by high temperature.

(4) Keep the blade edge sharp. Blunt cutting tools are the cause of thermal accumulation and wear, which can easily lead to tool failure.

(5) Try to process titanium alloy in its softest state as much as possible, as the material becomes more difficult to process after quenching. Heat treatment improves the strength of the material and increases blade wear.

(6) Use a larger tip radius or chamfer to cut in, and try to insert as many cutting edges as possible. This can reduce the cutting force and heat at each point, preventing local damage. When milling titanium alloys, the cutting speed has the greatest impact on the tool life VC among various cutting parameters, followed by the radial feed (milling depth) ae.

3. Starting from the blade to solve the problem of titanium processing

The blade groove wear that occurs during titanium alloy processing is local wear along the cutting depth direction in the back and front, often caused by the hardened layer left by the previous processing. The chemical reaction and diffusion between cutting tools and workpiece materials at processing temperatures exceeding 800 ℃ are also one of the reasons for the formation of groove wear. Because during the machining process, titanium molecules in the workpiece accumulate in front of the blade and are "welded" to the blade under high pressure and high temperature, forming chip deposits. When the chips are peeled off from the blade, the hard alloy coating on 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, and a large amount of high-pressure cutting fluid needs to be sprayed onto the cutting edge in a timely and accurate manner in order to quickly remove heat. There are unique structures of milling cutters specifically designed for titanium alloy processing on the market.

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