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Why are titanium alloys difficult to work?
Why do we think titanium alloy is a difficult material to work? Because of the lack of deep understanding of its processing mechanism and phenomenon.
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 artifact or taken away by chip, and agglomeration in cutting area, the temperature can be as high as 1 000 ℃ above, make the cutter blade quickly 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 geometric accuracy of the parts and the work hardening phenomenon which seriously 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. Heat is also generated during the friction process, which aggravates the poor thermal conductivity of titanium alloy.
This problem is more serious 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. 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 material strength and hardness of 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. Technical know-how for processing titanium alloys
On the basis of understanding the processing mechanism of titanium alloy, plus previous experience, the main processing techniques of titanium alloy are as follows:
(1) the use of positive Angle geometric shape of the blade, to reduce the cutting force, cutting heat and workpiece deformation.
(2) maintain a constant feed to avoid hardening of the workpiece, in the cutting process to always be in the feed state, milling radial cutting amount AE 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 denaturation and tool damage caused by 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) Machine titanium alloy in the softest state as far as possible, because the material becomes more difficult to process after hardening, heat treatment improves the strength of the material and increases 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. Solve 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 caused by the hardening layer left by the previous processing. The chemical reaction and diffusion between the tool and the workpiece material in the processing temperature over 800℃ is also one of the reasons for the formation of groove wear. Because in the processing process, the titanium molecules of the workpiece in front of the blade accumulation, under high pressure and high temperature "welding" to the blade, the formation of the accumulation of debris tumor. Special blade materials and geometry are required for titanium machining because the carbide coating of the blade is removed when the chip is removed from the blade.
4 suitable for titanium processing tool structure
The focus of titanium alloy processing is heat, a large amount of high pressure cutting fluid to timely and accurate injection to the cutting edge, to be able to quickly remove heat. There are special milling cutters on the market for titanium alloy processing unique structure.
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 artifact or taken away by chip, and agglomeration in cutting area, the temperature can be as high as 1 000 ℃ above, make the cutter blade quickly 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 geometric accuracy of the parts and the work hardening phenomenon which seriously 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. Heat is also generated during the friction process, which aggravates the poor thermal conductivity of titanium alloy.
This problem is more serious 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. 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 material strength and hardness of 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. Technical know-how for processing titanium alloys
On the basis of understanding the processing mechanism of titanium alloy, plus previous experience, the main processing techniques of titanium alloy are as follows:
(1) the use of positive Angle geometric shape of the blade, to reduce the cutting force, cutting heat and workpiece deformation.
(2) maintain a constant feed to avoid hardening of the workpiece, in the cutting process to always be in the feed state, milling radial cutting amount AE 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 denaturation and tool damage caused by 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) Machine titanium alloy in the softest state as far as possible, because the material becomes more difficult to process after hardening, heat treatment improves the strength of the material and increases 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. Solve 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 caused by the hardening layer left by the previous processing. The chemical reaction and diffusion between the tool and the workpiece material in the processing temperature over 800℃ is also one of the reasons for the formation of groove wear. Because in the processing process, the titanium molecules of the workpiece in front of the blade accumulation, under high pressure and high temperature "welding" to the blade, the formation of the accumulation of debris tumor. Special blade materials and geometry are required for titanium machining because the carbide coating of the blade is removed when the chip is removed from the blade.
4 suitable for titanium processing tool structure
The focus of titanium alloy processing is heat, a large amount of high pressure cutting fluid to timely and accurate injection to the cutting edge, to be able to quickly remove heat. There are special milling cutters on the market for titanium alloy processing unique structure.
