Titanium and its alloys react with oxygen when heated in air or an oxygen-containing atmosphere. When heated below 428°C, a protective oxide film is formed. As the temperature increases, the thickness of the oxide film increases. When the temperature rises above 538°C, the oxide film begins to lose its protective effect. Oxygen diffuses through the film into the interior of the metal, forming obvious outgassing. layer. If it rises above 815°C, a layer of loose oxide scale will form on the surface of the titanium alloy.
The interaction between hydrogen and titanium alloy is related to heating temperature and time. When it is lower than 427°C, if there is an oxide film on the surface of the alloy, it can prevent the inhalation of hydrogen. When it is higher than 427°C, hydrogen begins to penetrate the oxide layer and enter the interior of the alloy structure. The extent of the impact of hydrogen inhalation on the properties of titanium alloys is also directly related to the structural state of the alloy. Since the solubility of hydrogen atoms in the β phase is much greater than that in the α phase, the amount and shape of the β phase of the alloy determines hydrogen contamination. one of the main factors.
In order to prevent titanium alloy from oxidizing, absorbing hydrogen and contaminating other trace elements during superplastic forming, technical measures need to be taken to ensure that the formed titanium alloy parts have excellent performance.
At present, the main measures are: coating protection, vacuum heating and inert gas (argon) protection.
1. Coating protection method
After the surface of the formed blank is cleaned, a protective coating of a certain thickness is applied. After the parts are demoulded, the coating is removed by alkali washing, pickling or sand blowing.
Coatings should have the following main properties:
a. High temperature resistance, can be used at high temperatures of 750-1050℃;
b. It should have a certain lubrication effect to prevent the blank from being scratched during forming;
c. The coating can firmly adhere to the surface of the blank at the working temperature;
d. Easy to remove after heating;
e. No harmful substances, no pollution to the environment and no harm to human health.
2. Vacuum forming
For β-type titanium alloys with thin wall thickness, high surface brightness requirements and strong susceptibility to hydrogen embrittlement, vacuum forming is the most ideal.
Vacuum forming does not necessarily require expensive vacuum heating equipment. As long as a sealed space is created between the blank and the upper and lower cavities of the mold, a vacuum unit is used to gradually remove the air in the upper and lower cavities during the heating process, especially during the period from above 400°C to the forming temperature, the degree of contact between the upper and lower cavities of the mold must be maintained To achieve a vacuum degree of 10^(-3) Torr or above, the forming purpose can be achieved by switching the pipeline valve during forming and filling with argon gas. This method is used in titanium foil corrugated plate forming and satisfactory results are achieved. When the vacuum degree is controlled at 10^(-3)Torr, the hydrogen content is lower than the standard requirement. When the vacuum degree reaches 10^(-5)Torr, parts with bright surfaces can be obtained.
