The thermal conductivity of titanium alloy is small, about 1/3 of iron, and the heat generated during machining is difficult to be released through the workpiece. At the same time, due to the small specific heat of titanium alloy, the local temperature rises quickly during machining. Therefore, it is easy to cause the tool temperature to be very high, which causes sharp wear of the tool tip and reduces the service life. Experiments show that the temperature of the tip of the cutting tool for cutting titanium alloy is 2-3 times higher than that of cutting steel. Thermal conductivity, elastic modulus, chemical activity, alloy type and microstructure are the main factors that affect the machining performance of titanium alloys. The low elastic modulus of titanium alloy makes the processed surface prone to springback, especially the thin-walled parts. The machining springback is more serious, and it is easy to cause strong friction between the flank surface and the processed surface, thereby wearing out the tool and chipping. Titanium alloys are very chemically active, and easily react with oxygen, hydrogen, and nitrogen at high temperatures, increasing their hardness and decreasing their plasticity. It is difficult to machine the oxygen-rich layer formed during heating and forging. Titanium alloys have different alloy compositions and different machining properties. In the annealed state, the a-type titanium alloy has better mechanical machining performance; the a + β-type titanium alloy is second; the β-type titanium alloy has high strength and good hardenability. But the machining performance is the worst.
There are many methods for machining titanium alloys, including: turning, milling, boring, drilling, grinding, tapping, sawing, EDM, etc.
1.Turning and boring of titanium alloy
The main problems of turning titanium alloys are: high cutting temperature; severe tool wear; large cutting springback. Under suitable machining conditions. Turning and boring are not particularly difficult processes. For continuous cutting, mass production, or cutting with a large amount of metal removal, cemented carbide tools are generally used. When forming cutting, grooving or cutting, suitable steel tools are used, and cermet tools are also used. As with other machining operations, a constant forced feed is always used to avoid cutting interruptions. Do not stop or slow down during cutting. Generally do not cut, but should be fully cooled; the coolant can use 5% sodium nitrate aqueous solution or 1/20 soluble oil emulsion solution. Before forging, the cemented carbide tool is used for turning the oxygen-rich layer on the surface of the original bar. The cutting depth should be greater than the thickness of the oxygen-rich layer. The cutting speed is 20-30m / min and the feed rate is 0.1-0.2mm / r. Boring is a finishing process, especially for thin-walled titanium alloy products in the boring process, to prevent burns and deformation of parts clamping.
2.Drilling and machining of titanium alloy
When drilling titanium alloy, it is easy to generate long and thin curled chips. At the same time, the drilling heat is large, and the chips are easy to accumulate or adhere to the drilling edge. This is the main reason for the difficulty of drilling titanium alloy. Drilling should use short and sharp bits and low-speed forced feed, the support bracket should be fastened, and repeated and sufficient cooling should be given, especially for deep hole drilling. During the drilling process, the drill bit should maintain the drilling state in the hole without allowing idling in the hole, and should maintain a low and constant speed of drilling. Drill through holes carefully. When drilling soon, in order to clean the drill bit and the drill hole, and to remove the drill cuttings, it is best to return the drill bit. When the hole is finally broken, forced feed is used to obtain a smooth hole.
3.Tapping of titanium alloy
Tapping of titanium alloys is probably the most difficult machining process. When tapping, the restriction of titanium chips and the severe tendency to bite will result in poor thread fit, causing the tap to jam or break. When the tapping is completed, the titanium alloy tends to dry and tighten on the tap. Therefore, it is necessary to avoid machining blind holes or through holes that are too long to prevent the surface roughness of the internal thread from becoming large or the phenomenon of broken cones. At the same time, the tapping method should be continuously improved, for example, the trailing edge of the tap can be ground off. Along the length of the tooth edge, the axial chip removal groove is ground on the tooth tip. On the other hand, taps with oxidized, oxidized or chrome-plated surfaces are used to reduce bite and wear.
4.Sawing machining of titanium alloy
When sawing a titanium alloy, a low surface speed and continuous forced feeding are used. The experiment proves that the coarse-toothed high-speed steel saw blade with a tooth pitch of 4.2 mm to 8.5 mm is suitable for sawing titanium alloy. If a band saw is used to saw a titanium alloy, the blade pitch of the saw blade is determined by the thickness of the workpiece, which is generally 2.5 mm to 25.4 mm. The thicker the material, the larger the pitch. At the same time, the mandatory feed capacity and the required coolant must be maintained.
5.EDM machining of titanium alloy
EDM of titanium alloy requires an operating gap between the tool and the workpiece. The range of the gap is preferably 0.005mm0.4mm. The smaller gap is often used for finishing with a smooth surface, and the larger gap is used for roughing that requires rapid metal removal. The electrode material is preferably copper and zinc.