Why titanium alloy is difficult to proceed
Why do we think titanium alloy is a difficult material to work? Let’s have one quick understanding of titanium process characters.
1.Physical phenomena of titanium processing
The cutting force of 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 complex than that of steel, which makes the processing of titanium alloy facing great difficulties.
The thermal conductivity of most titanium alloys is very low, only 1/7 of that of steel and 1/16 of that of aluminum. Therefore, in the process of cutting of titanium alloy heat will not quickly passed to the workpiece or taken away by chip, and agglomeration in cutting area, the temperature can be as high as 1 000 ℃ above, make the cutter blade rapid wear, cracks, and generate the devolop tumor, rapid wear of the blade, and make the cutting area produces more heat and further shorten the life of cutting tools.
The high temperature in the cutting process also destroys the surface integrity of the titanium alloy parts, which leads to the decrease of the precision of the parts and the work hardening phenomenon which severely reduces the fatigue strength.
The elasticity of titanium alloys may be beneficial to the performance of the parts, but the elastic deformation of the workpiece is an important cause of vibration during cutting. Cutting pressure causes the "elastic" workpiece to leave the tool and rebound, so that the friction phenomenon between the tool and the workpiece is greater than the cutting action. Friction process also produces heat, which aggravates the poor thermal conductivity of titanium alloy.
This problem is more prominent when machining thin-walled or annular parts which are easy to deform. It is not easy to machining thin-walled titanium alloy parts to the desired dimensional accuracy. As the workpiece is pushed by the tool, the local deformation of the thin wall exceeds the elastic range and produces plastic deformation, the 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.
"Heat" is the most important reason for difficult machining of titanium alloy.
2. Some methods of machining titanium alloy
On the basis of understanding the processing mechanism of titanium alloy, plus previous experience, the main processing technology of titanium alloy is as follows:
(1) the use of positive Angle shape of the blade, in order to reduce the cutting force, cutting heat and workpiece deformation.
(2) adhere to the constant feed to avoid hardening of the workpiece, in the cutting process, the tool should always be in the feed state, the radial cutting amount of milling A E should be 30% of the radius.
(3) the use of high pressure and large flow cutting fluid, in order to ensure the thermal stability of the processing process, to prevent the workpiece surface degeneration and tool damage caused by high temperature.
(4) adhere to the sharp edge of the blade, blunt tool is the cause of heat accumulation and wear, easy to lead to tool failure.
(5) Machined in the softest state of titanium alloy as far as possible. Because the data becomes more difficult to process after hardening, heat treatment improves the strength of the data and adds the wear of the blade.
(6) Use a large tip arc radius or chamfer cutting to put as much edge into the cutting as possible. This reduces cutting force and heat at each point and prevents local breakage. In the milling process of titanium alloy, the cutting speed has the greatest influence on the tool life VC, and the radial cutting depth (milling depth) AE is the second.
3.Tackle the titanium processing problem from the blade
The groove wear of the blade in titanium alloy processing is the local wear in the back and front along the cutting depth direction, which is often formed by the hardening layer left by the previous processing. The chemical reaction and diffusion between the tool and the workpiece data in the processing temperature over 800℃ is also one of the reasons for the formation of groove wear. In the process of processing, the titanium molecules of the workpiece accumulate in front of the blade, and "weld" to the blade under high pressure and high temperature, forming the accumulation of debris tumors. The carbide coating of the blade is taken away when the chip is stripped from the blade. Therefore, titanium alloy processing requires special blade information and shape.
4. Suitable tool structure for titanium processing
The focus of titanium alloy processing is heat, a lot of high pressure cutting fluid to timely and accurate injection to the cutting edge, in order to quickly remove the heat. There are special milling cutters on the market for titanium alloy processing unique structure.
