How hard it is to work titanium alloys

Why does the titanium tube manufacturer think that titanium alloy is a difficult material to manufacture? Because of its processing mechanism and phenomenon lack of deep understanding.

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, which makes the processing of titanium alloy face great difficulties.

Most titanium alloys have very low thermal conductivity, 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 be chip away, and gathered in the cutting area, the temperature produced can be as high as 1 000℃ above, so that the cutting edge of the tool rapidly wear, crack and chip tumor, rapid wear of the blade, and make the cutting area produce more heat, further shorten the life of the tool.

The high temperature generated during the cutting process also damages the surface integrity of titanium alloy parts, leading to the reduction of geometric accuracy and the occurrence of work hardening which severely reduces the fatigue strength of the parts.

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 cause of vibration. Cutting pressure causes the “elastic” workpiece to move away from 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 alloy.

This problem is more serious when processing thin-wall or ring and other easily deformed parts. It is not an easy thing to process thin-wall parts of titanium alloy to the expected dimensional accuracy. Because when the workpiece material is pushed by the tool, the local deformation of the thin wall has exceeded the elastic range and produced plastic deformation, the strength and hardness of the material at the cutting point increased significantly. At this point, machining at the previously determined cutting speed becomes too high, further leading to sharp tool wear.

“Heat” is difficult to process titanium alloy “culprit”!

2. Processing know-how of titanium alloy

On the basis of understanding the processing mechanism of titanium alloy and previous experience, the main technology know-how of processing titanium alloy is as follows:

(1) Adopt the blade with positive Angle geometry to reduce cutting force, cutting heat and workpiece deformation.

(2) Maintain a constant feed to avoid hardening of the workpiece. In the cutting process, the tool should always be in the feed state. During milling, the radial cutting amount ae should be 30% of the radius.

(3) The high pressure and large flow cutting fluid is used to ensure the thermal stability of the processing process and prevent the workpiece surface deformation and tool damage due to high temperature.

(4) Keep the blade edge sharp, blunt tool is the cause of heat accumulation and wear, easy to lead to tool failure.

(5) Machining the titanium alloy in the softest state possible, because the material becomes more difficult to work after hardening. Heat treatment increases the strength of the material and increases blade wear.

(6) Use large tip radius or chamfering to cut in, as much as possible into the cutting edge. This reduces cutting force and heat at every point, preventing local breakage. In milling titanium alloy, the cutting speed has the greatest effect on the tool life vc, followed by the radial cutting amount (milling depth) ae.

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

The blade groove wear in titanium alloy processing is the local wear in the back and front along the cutting depth direction, which is often caused by the hardening layer left by the early processing. The chemical reaction and diffusion between the tool and the workpiece material at the processing temperature of more than 800℃ is also one of the reasons for the formation of groove wear. Because during processing, titanium molecules of the workpiece accumulate in front of the blade and are “welded” to the blade at high pressure and temperature, forming chip nodules. As chip nodules peel away from the blade, they take the hard alloy coating away from the blade, so special blade materials and geometry are required for titanium machining.

4. Tool structure suitable for titanium processing

The focus of titanium alloy processing is heat, a large number of high pressure cutting fluid to timely and accurate injection to the cutting edge, to quickly remove the heat. There are special milling cutter structures on the market for titanium alloy processing.

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