Found in the production of titanium alloy machining surface quality common failures over corrosion, hanging grey, oxide skin is not removed and streak-like splotch several kinds.

1, over corrosion

Excessive corrosion refers to the surface of titanium alloy after pickling pits or unevenness and other defects, and the material organisation reveals a difference, generally leading to excessive corrosion defects is the ratio of hydrofluoric acid and nitric acid is out of proportion, too high a concentration of hydrofluoric acid or nitric acid concentration is insufficient can lead to the defects, the other reason is that the pickling time is too long, the general pickling t is 1mm ~ 4min, according to the operation of the scene to adjust the technology parameters, shorten the pickling time appropriately.

2, hanging grey

Hanging ash refers to the oxide attached to the surface of the titanium alloy after pickling, pickling by dry titanium alloy and acid chemical reaction, resulting in the accumulation of oxides on the surface, preventing the reaction from occurring further, the defects of hanging ash is generally too much ash deposition by pickling and not enough rinsing after pickling. Pickling should constantly shake the parts, so that the reaction products from the surface of the titanium alloy off, pickling should be strengthened after spraying or rinsing method to remove hanging ash. Domestic generally take compressed air and tap water mixed with high-speed water rinsing parts, the effect is good.

3, the oxide skin is not removed

The causes of this defect are more, each process is possible. There may be poor oil removal, or molten salt treatment time is not enough, or pickling solution failure. When the defect occurs, it should be one by one to exclude all possible factors, when necessary, can be added in the pre-treatment of sand blasting process.

4, streak-like splotch

Thermal conductivity, modulus of elasticity, chemical activity and alloy type and microstructure are the main factors affecting the machining performance of titanium alloy. Titanium alloy thermal conductivity is small, about 1/3 of iron, the heat generated during machining is difficult to release through the workpiece; at the same time, due to the small specific heat of titanium alloy, the local temperature rises quickly during processing, therefore. It is easy to cause the tool temperature is very high, so that the tip of the tool sharp wear, service life is reduced. Experiments have proved that the cutting tool tip temperature of titanium alloy is 2-3 times higher than that of cutting steel. Titanium alloy low modulus of elasticity, so that the processed surface is prone to rebound, especially the processing of thin-walled parts rebound is more serious, easy to cause strong friction between the back face and the processed surface, thereby wearing the tool and chipping. Titanium alloy chemical activity is very strong, high temperature is very easy with oxygen, hydrogen, nitrogen role, so that its hardness increases, plasticity decreases, in the heating and forging process of the formation of oxygen-rich layer of machining difficulties. Titanium alloys with different alloy compositions have different machining properties. In the annealed state, a-type titanium alloy has better machining performance; a + -type titanium alloy is second; -type titanium alloy has high strength and good hardenability, but the worst machining performance.

1, titanium alloy turning and boring

The main problems of turning titanium alloy are: high cutting temperature; more serious tool wear; large cutting rebound. Under suitable machining conditions. Turning and boring is not a particularly difficult process. For continuous cutting, mass production or large metal removal cutting, generally use carbide tools, when forming cutting, turning groove or cut off, suitable for adjusting steel tooling, metal ceramic tools are also used. As with other machining operations, cutting interruptions can be avoided by always using a constant forced feed. Do not stop or slow down during cutting. Generally do not cut but cool sufficiently; the coolant may be a 5 per cent aqueous sodium nitrate solution or a 1/20 soluble oil emulsion solution in water. Before forging, turning the original bar surface oxygen-rich layer using carbide tools, the cutting depth should be greater than the thickness of the oxygen-rich layer, cutting speed of 20 ~ 30m/min, feed 0.1 ~ 0.2mm / r. Boring is finishing, especially for thin-walled titanium alloy products in the boring process, should be prevented from burns and deformation of the parts clamping.

2, titanium alloy drilling

Titanium alloy drilling is easy to grow and thin curly chips, while drilling heat is large, easy to make excessive accumulation of chips or adhesion in the drilling edge, which is the main reason for the difficulty of drilling titanium alloy. Drilling should use a short and sharp drill bit and low-speed forced feed, the support bracket should be tight, and should be given to repeat adequate cooling, especially deep hole drilling. During drilling the drill bit should be kept in the hole and not allowed to idle in the hole, and a low and constant drilling speed should be maintained. Drilling through the hole should be careful, when about to drill through, in order to clean up the drill bit and drill hole, and to remove drilling debris, it is best to return the drill bit, and finally break the hole using forced feed, so that you can get a smooth hole.

3, Tapping of titanium alloy

Tapping of titanium alloy is probably the most difficult machining process. When tapping, the titanium chip exclusion is restricted and severe galling tendency will lead to poor thread fit, resulting in tap jamming or breakage. At the completion of tapping, the titanium tends to dry and tighten on the tap. Therefore, should try to avoid the processing of blind holes or too long through-hole, in order to prevent the internal thread surface roughness becomes large or broken cone phenomenon. At the same time, the tapping method should be improved continuously, such as the back edge of the tap can be ground away. Along the length of the tooth edge at the top of the tooth grinding axial chip removal groove and so on. On the other hand, the tap with oxidised, oxidised or chrome-plated surface is used to reduce the bite and wear.

4,Sawing of titanium alloy

When sawing titanium alloy, low surface speed and continuous forced feed should be used. Experiments have proved that the tooth spacing of 4.2mm ~ 8.5mm coarse teeth of high-speed steel saw blade is suitable for sawing titanium alloy. If the band saw saw titanium alloy, the saw blade tooth distance from the thickness of the workpiece, generally 2.5mm ~ 25.4mm, the thicker the material thickness, the greater the tooth distance. At the same time must maintain the forced feed capacity and the required coolant.

5, titanium table gold electric discharge machining

Titanium alloy electric discharge machining requires tools and workpieces between - an operating gap. Gap range is best taken in 0.005mm 0.4mm, smaller gap is commonly used in the requirements of smooth surface finishing, larger gap is used in the requirements of rapid removal of metal roughing. Copper and zinc are preferred as electrode materials.