A common failure analysis
Found in production, Titanium Alloy machining surface quality common failures over corrosion, hanging gray, oxide skin is not removed and streak-like splotch several kinds.
1, over corrosion
Overcorrosion refers to the surface of the titanium alloy after pickling pits or unevenness and other defects, and material organization reveals a difference, generally leading to overcorrosion defects is due to 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 parameters can be adjusted according to the operation site, and the pickling time can be shortened appropriately.
2, hanging gray
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, 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 the various possible factors, when necessary, can be added in the pre-treatment of sand blasting process.
4, streak-like pattern
The cause of this defect is generally due to uneven reaction. Can be excluded 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 the stress, in the microscopic detection of the general corrosion pattern with the difference, in general, does not affect the performance of its use, can be removed by the method of pickling again, but the stressed parts to strengthen the second pickling after the dehydrogenation process.
Second, the factors affecting the performance of titanium alloy processing parts
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 temperature of the front end of the cutting tool of cutting 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 has better machining performance; a + β-type titanium alloy is second; β-type titanium alloy has high strength and 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.
