Titanium is a highly active metal and its activity increases with temperature.
Titanium and its alloys react with oxygen when heated in air or in an oxygenated atmosphere.When heated at less than 428℃, a protective layer of oxide film was formed, and the thickness of the oxide film increased with the increase of temperature, and the film began to lose its protective effect when heated at more than 538℃, and oxygen diffused into the metal through the film, forming an obvious permeability layer.If the temperature rises above 815℃, a loose oxide coating forms on the titanium composite surface.
The action of hydrogen and titanium alloy depends on heating temperature and time.When the temperature is lower than 427℃, if there is an oxide film on the surface of the alloy, the inhalation of hydrogen can be prevented, while when the temperature is higher than 427℃, hydrogen begins to enter the alloy through the oxide layer.The degree of influence of hydrogen inhalation on the properties of titanium alloy is also directly related to the microstructure of the alloy. Since the solubility of hydrogen in the pattern phase is much greater than that in the hydrogen phase, the number and shape of the pattern phase of the alloy is one of the main factors determining the degree of hydrogen pollution.
In order to prevent the pollution of oxidation, hydrogen absorption and other trace elements in the superplastic titanium alloy, technical measures should be taken to make the formed parts of titanium alloy have excellent properties.
The main measures are;Coating protection, vacuum heating and inert gas (argon) protection.
1. Coating protection law
After cleaning the surface of the forming billet, apply a protective coating with a certain thickness, and remove the coating by alkali washing, acid washing or sand blowing after the parts are de-moulded.
The coating shall have the following main properties:
A. High temperature resistance and can be used at 750 -- 1050℃;
B. It shall have a certain lubricating effect to prevent the blank from being scraped in the forming process;
C. The coating can be firmly attached to the surface of the billet at operating temperature;
D. easy to remove after heating;
E. No harmful substances, no pollution to the environment and no harm to human health.
The coatings that have been identified as suitable for supershaping titanium tubes and titanium alloys include: Ti -- 2 alcohol soluble preparation can be used together with Ti -- 3 graphite lubricant, suitable for hot forming at 750 -- 1050℃;KBC - 12 water soluble preparation, can be used with graphite water agent.
2. Vacuum forming
Vacuum forming is ideal for type ii titanium alloys with thin wall thickness, high surface brightness and high sensitivity to hydrogen embrittlement.
Vacuum forming does not necessarily require expensive vacuum heating equipment.As long as it will cause sealed space between billet and mold on inferior vena, on the heating process with vacuum unit will gradually take off the air in the inferior vena, especially during the forming temperature above 400 ℃ to make the mold of the inferior vena bing/degrees up to 10 ^ (3) torr vacuum, forming conversion line cut the door, filling the argon gas can accomplish forming, in titanium foil corrugated plate forming using this method, satisfactory effect.When the vacuum is controlled at 10^(-3) pumps, the hydrogen content is lower than the standard requirement, and when the vacuum is 10^(-5) pumps, parts with a bright surface can be obtained.
In addition, for the parts with medium thickness and no higher requirements for surface concave brightness, it is also possible to use the vacuum argon filled protection method in the forming of spherical gas cylinder, and the effect is also better.