Titanium alloy has the advantages of small density, high strength, corrosion resistance and so on, so it has been widely used in aviation, aerospace, power generation equipment, nuclear energy, ships, chemicals, medical devices and other fields. Why is titanium alloy a difficult material to work with? Today we analyze titanium alloys in terms of their processing mechanisms and physical phenomena.
Titanium alloys are machined with cutting forces that are only slightly higher than steel of equivalent hardness, but the physical phenomena of machining titanium alloys are much more complex than machining steel, which makes machining titanium alloys extremely difficult.
Most titanium alloys have a very low thermal conductivity, only 1/7th that of steel and 1/16th that of aluminum, so the heat generated during the cutting of titanium alloys is not quickly transferred to the workpiece or carried away by the chips, but is concentrated in the cutting area, where the temperature generated can be up to 1,000°C or more, causing rapid wear, cracking and accumulation of chip tumors at the cutting edge of the tool, and the rapid appearance of a worn cutting edge, which in turn generates more heat in the cutting area, further reducing the life of the tool.
The high temperature generated during the cutting process also destroys the surface integrity of the titanium alloy parts, leading to a decrease in the geometric accuracy of the parts and the occurrence of machining hardening that severely reduces their fatigue strength.
The elasticity of titanium alloys may be beneficial for part performance, but the elastic deformation of the workpiece during the cutting process is an important cause of vibration. The cutting pressure causes the "elastic" workpiece to leave the tool and bounce back, so that the friction between the tool and the workpiece is greater than the cutting action. The frictional process also generates heat, exacerbating the problem of poor thermal conductivity of titanium alloys.
This problem is exacerbated when machining deformable parts such as thin-walled or ring-shaped parts, and machining thin-walled titanium parts to the desired dimensional accuracy is not an easy task. Because the local deformation of the thin wall is already out of elasticity as the workpiece material is pushed away by the tool, the material strength and hardness of the cutting point increases significantly. At this point, machining at the originally determined cutting speed becomes too high, further leading to sharp tool wear.
Based on an understanding of the mechanism of titanium alloy processing, together with past experience, the main process know-how for titanium alloy processing is as follows.
(1) Inserts with a positive angle geometry to reduce cutting forces, cutting heat and workpiece deformation.
(2) Maintain a constant feed to avoid workpiece hardening, the tool should always be in the feed state during the cutting process, the radial feed AE should be 30% of the radius during milling.
(3) High-pressure high-flow cutting fluid is used to ensure the thermal stability of the machining process and to prevent surface degradation and tool damage caused by high temperature.
(4) Keeping the blade sharp and dull is the cause of heat build-up and wear, which can easily lead to tool failure.
(5) Machining in the softest state possible for titanium alloys, since hardened material becomes more difficult to work, heat treatment increases the strength of the material and increases wear on the inserts.
(6) Use a large tip radius or chamfer to bring as much of the blade into the cut as possible. This reduces the cutting force and heat at each point and prevents localized breakage. When milling titanium alloys, the cutting speed of each cutting parameter has the greatest effect on tool life VC, with radial tool eating (milling depth) AE second.
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