Classification and Heat Treatment of Titanium Alloy
Pure titanium is a kind of silver white metal with the phenomenon of isomeric transformation; below 882 ℃, it is a close packed hexagonal lattice, called α - titanium (α - Ti), above 882 ℃, it is a body centered cubic lattice, called β - titanium (β - Ti). Pure titanium has low density (4.58g / cm3), high melting point (1677 ℃), small coefficient of thermal expansion, good plasticity and easy processing; it has good corrosion resistance and is not easy to be oxidized below 550 ℃. Its corrosion resistance in sea water and steam is better than aluminum alloy, stainless steel and nickel alloy.
Titanium alloy classification
Titanium alloy is based on titanium and other elements. There are two kinds of homomorphic and heterocrystalline titanium: α - titanium with close packed hexagonal structure below 882 ℃ and β - titanium with body centered cubic structure above 882 ℃.
1. α - titanium alloy
The main alloying elements are al and Sn. At room temperature or higher temperature, it is single-phase α solid solution structure, which can not be strengthened by heat treatment. α - titanium alloy has stable structure and good weldability.
2. β - titanium alloy
Copper, chromium, aluminum, vanadium and iron are mainly added to the β - titanium alloy to promote the stability of β - phase. When they are normalized or quenched, they are easy to retain the high temperature β - phase to the room temperature structure and obtain a relatively stable β - phase structure.
It has good plasticity and high alloy density, so it is not widely used in production.
3. α + β - titanium alloy
In addition to the stable elements of β - phase such as chromium, vanadium and key, there are also stable elements of α - phase such as tin and aluminum. When cooling to a certain temperature, β → α phase transition occurs, which is α + β two-phase structure at room temperature.
Heat treatment of titanium alloy
Annealing of metallic materials is to eliminate internal stress, improve plasticity and structure stability, and obtain better comprehensive properties. The commonly used heat treatment methods of titanium alloy are annealing, solid solution and aging treatment.
(1) Stress relief annealing: the purpose is to eliminate or reduce the residual stress produced in the processing. Prevent chemical erosion and reduce deformation in some corrosive environments.
(2) Complete annealing: the purpose is to obtain good toughness, improve processing performance, facilitate reprocessing and improve the stability of size and structure.
(3) Solution treatment and aging: the purpose is to improve its strength, α titanium alloy and stable β titanium alloy can not be strengthened by heat treatment, only annealing in production.
In order to meet the special requirements of the workpiece, dual annealing, isothermal annealing, β heat treatment, deformation heat treatment and other metal heat treatment processes are also used in industry.
