Titanium Alloy Machining Surface Analysis

Found in production, titanium alloy machining surface quality of common failures over corrosion, hanging gray, oxide skin is not removed and streak-like splotch several kinds.
1. Overcorrosion
Excessive corrosion refers to the surface of titanium alloy after pickling pits or unevenness and other defects, and the material organization 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, another reason is that the pickling time is too long, the general pickling t is 1mm ~ 4min, according to the operation of the site to adjust the process Another reason is that the pickling time is too long.
2. Hanging ash
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 the hanging ash. Domestic generally take compressed air and tap water mixed with high-speed water rinsing parts, the effect is good.
3. Oxidized 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, should be eliminated one by one various possible factors, when necessary, can be added in the pre-treatment of sand blasting process.
4. Streak pattern
The cause of this defect is generally due to uneven reaction. It can be eliminated by shaking the parts during pickling and reducing the temperature of the pickling solution. In addition to the above defects, sometimes also found after pickling inspection of qualified products, after a period of time, the surface of the phenomenon of spots. For this phenomenon, now less research, may be due to the surface of the residual acid after pickling or subsequent production of corrosive media brought into the presence of the joint action of stress, in the microscopic detection with the general corrosion pattern is different, generally speaking does not affect its use of performance, can be removed by the method of pickling again, but the stressed parts to strengthen the second pickling after the dehydrogenation process.
I. Factors affecting the machining performance of titanium alloy
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 the temperature 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 machining performance is better; a + β-type titanium alloy is second; β-type titanium alloy has high strength, good hardenability, but the worst machining performance.
In view of the above, in order to carry out high-efficiency and high-precision machining of titanium alloys, corresponding measures should be taken to avoid the generation of defects in machining.
Second, the study of various machining of titanium alloys
There are many methods of titanium alloy machining, mainly including: turning, milling, boring, drilling, grinding, tapping, sawing, EDM and so on.
1. Turning and boring of titanium alloy

The main problems of turning titanium alloys are: high cutting temperature; more severe tool wear; and high cutting kickback. Under suitable machining conditions. Turning and boring are not particularly difficult processes. For continuous cutting, mass production or large metal removal cutting, generally use carbide tools, when the molding 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 can be 5% sodium nitrate aqueous solution or 1/20 soluble oil emulsion aqueous solution. Before forging, turning the original bar surface oxygen-rich layer using carbide tools, the depth of cut 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 process
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 drilling titanium alloy difficulties. 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 holes should be done carefully, and when about to drill through, it is advisable to return the drill bit in order to clean the bit and the hole, and to remove drill cuttings, and to use a forced feed when the hole is finally broken, so that a smooth hole can be obtained.
3.Tapping of titanium alloy
Tapping of titanium alloy is probably the most difficult machining process. When tapping, limited titanium chip exclusion and severe galling tendencies will result in a poor thread fit, causing the tap to jam or break. 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 continuously improved, 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, taps with oxidized, oxidized or chrome-plated surfaces are used to reduce galling and wear.
4. Sawing processing of titanium alloy
When sawing titanium alloy, low surface speed and continuous forced feeding should be used. Experiments have proved that the tooth spacing of 4.2mm ~ 8.5mm coarse tooth high-speed steel saw blade is suitable for sawing titanium alloy. If the band saw saw titanium alloy, the saw blade tooth pitch is determined by the thickness of the workpiece, generally 2.5mm ~ 25.4mm, the thicker the material thickness, the larger the tooth pitch. 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 requirements of tools and workpieces between - an operating gap. The 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 electrode materials are preferred.
Through the above analysis and research, the failure causes of titanium alloy machining surface quality are derived, and a variety of methods in the machining process are analyzed, so as to find out practical ways to solve the problems of titanium alloy machining surface quality.