In milling, an important characteristic of titanium alloys is extremely poor thermal conductivity. Due to the high strength and low thermal conductivity of titanium alloy materials, extremely high cutting heat (up to 1200°C if not controlled) is generated during processing. The heat is not discharged with the chips or absorbed by the workpiece, but is concentrated on the CNC cutting edge. Such high heat will greatly shorten tool life.
Turning and milling are particularly difficult when the hardness of titanium alloy is greater than HB350. When the hardness of titanium is less than HB300, the sticking phenomenon is likely to occur, and it is also difficult to cnc cutting. But the hardness of titanium alloy is only one aspect that is difficult to cut.
CNC machining of titanium alloys should start from two aspects: reducing cutting temperature and reducing adhesion. Select tool materials with high thermal hardness, high bending strength, good thermal conductivity, and poor affinity with titanium alloys. YG cemented carbide is more suitable. Due to the poor heat resistance of high-speed steel, tools made of cemented carbide should be used as much as possible. Commonly used cemented carbide tool materials include YG8, YG3, YG6X, YG6A, 813, 643, YS2T and YD15.
Set the geometric parameters of turning and milling titanium tools to improve the product quality of titanium alloy parts. Products are delivered quickly and on time.
(1) The rake angle of the tool γ0: The contact length between titanium alloy chips and the rake face is short. When the rake angle is small, the contact area of the chip can be increased, so that the cutting heat and cutting force are not excessively concentrated near the cutting edge. Improve the heat dissipation conditions, and can strengthen the cutting edge and reduce the possibility of chipping. Turning titanium generally takes γ0=5°～15°.
In the process of CNC turning and milling titanium alloy, the matters that should be paid attention to are:
(1) Due to the small modulus of elasticity of titanium alloy, the clamping deformation and force deformation of the workpiece during machining are large, which will reduce the processing accuracy of the workpiece; The clamping force should not be too large when the workpiece is installed, and auxiliary support can be added when necessary.
When titanium alloy is milled at low speed in an inert gas medium, the milling deformation coefficient is greater than 1.0; But in the atmosphere, when the milling speed Vc=30 m/min, the chip deformation coefficient is less than 1.0. This is because titanium alloys have a great affinity for oxygen and nitrogen in the atmosphere during high-temperature milling.
The drilling is semi-closed CNC cutting. The cutting temperature is very high in the process of drilling titanium alloy, the rebound after drilling is large, the drill chips are long and thin, easy to stick and not easy to discharge. Drilling of titanium often causes the bit to be bitten, twisted, and so on. Therefore, the drill bit is required to have high strength and good rigidity, and the chemical affinity between the drill bit and the titanium alloy is small. It is best to use cemented carbide drills, but the most commonly used at present is still twist drills, after taking some measures to improve, better results can also be achieved.
The special properties of titanium alloys make it more and more widely used. High strength/weight ratio, excellent toughness and excellent corrosion resistance. The titanium alloy can be used to make medical human implants, racing parts, ship parts, aircraft parts, underwater breathing devices, golf club heads, and military armor.
When high-speed milling of TC4, Gc.2, and pure titanium integral structures, down-milling is generally used. The tool slowly cuts into the titanium workpiece to reduce the heat generated and reduce the radial force.
When milling titanium alloy TC4 (Ti-6Al-4V), asymmetric down milling is often used, so that the front tip of the cutter tooth first contacts the workpiece.
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