TC4 titanium alloy has a series of advantages such as excellent corrosion resistance, low density, high specific strength, better toughness and weldability, etc., and has been successfully applied in the aerospace, petrochemical, shipbuilding, automotive and pharmaceutical sectors. During the hot forming process of TC4 titanium alloy, a layer of oxide scale will be formed on the surface, and the color of the obtained oxide scale will be different at different hot forming temperatures. Among them, when the thermoforming temperature is about 600°C, the oxide scale is blue; when the thermoforming temperature is about 850°C, the oxide scale is gray to reddish; and when the thermoforming temperature is 900°C, the oxide scale is gray. With the increase of temperature, the difficulty of removing oxide scale gradually increases. The removal of titanium alloy scale at home and abroad mostly adopts a two-step method, that is, first molten alkali washing and then acid washing; however, the temperature of molten alkali washing medium is higher, which can reach about 450℃, and it is easy to cause semi-finished products of α+β and β titanium alloy The premature aging strengthening of sulphate brings difficulties to further processing. In addition, washing with nitric acid and hydrofluoric acid after hot alkaline washing may also cause hydrogen embrittlement corrosion on the surface of the titanium alloy.
The researchers studied the process of removing the oxide layer on the surface of the oxide scale obtained at the three temperatures of 590, 850, and 920°C by first washing with a low-temperature alkaline solution and then pickling.
The lye used in the test is a mixed supersaturated solution of industrial pure NaOH and industrial pure NaNO2, and a mixed solution of hydrofluoric acid and nitric acid (volume ratio HF:HNO3:H2O=1:1:3) is used for pickling. The TC4 titanium alloy that has been washed with hot alkali and washed with water is completely immersed in the acid solution for 20 seconds and then taken out, and then washed with tap water. Finally, the aforesaid sample was cleaned by acetone or alcohol ultrasonic wave and then dried and weighed. The mass of the sample before and after hot alkaline washing and acid washing was measured with an electronic balance, with an accuracy of 0.001g. The analysis shows that the oxide scale on the surface of the TC4 titanium alloy can be completely removed by the process of hot alkaline washing and then acid washing. conclusion as below:
(1) The thickness of TC4 titanium alloy hot forming surface oxide scale is less than 2μm.
(2) Through the hot alkaline washing and pickling process, the oxide scale on the surface of the TC4 titanium alloy can be completely removed. The lye ratio is 700g/L NaOH+250g/L NaNO2, and the boiling temperature is about 160℃. Soaking for 6h can achieve the removal effect.
(3) The hydrogen mass fraction of TC4 titanium alloy with an oxide scale does not change much through the hot alkaline washing and pickling process, which meets the production needs.
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