Titanium is very stable in the air at room temperature. […]
Titanium is very stable in the air at room temperature. When heated to 400-550°C, a firm oxide film is formed on the surface to protect it from further oxidation. Titanium has a strong ability to absorb oxygen, nitrogen, and hydrogen. This type of gas is a very harmful impurity to metallic titanium. Even a small amount (0.01% to 0.005%) can seriously affect its mechanical properties. Among titanium compounds, titanium dioxide (TiO2) has the most practical value. Ti02 is inert to the human body, non-toxic, and it has a series of excellent optical properties. Ti02 is opaque, has high gloss and whiteness, high refractive index and scattering power, strong hiding power, and good dispersibility. The pigment made is white powder, commonly known as titanium dioxide, and is widely used. The appearance of titanium rods is very similar to steel, with a density of 4.51 g/cm3, which is less than 60% of steel. It is the lowest density metal element among refractory metals. The mechanical properties of titanium, which are generally called mechanical properties, are closely related to purity. High-purity titanium has excellent machining properties, good elongation and reduction of area, but low strength, which is not suitable for structural materials. Industrial pure titanium contains an appropriate amount of impurities, has high strength and plasticity, and is suitable for making structural materials.
Titanium alloy has low strength and high plasticity, medium strength and high strength, ranging from 200 (low strength) to 1300 (high strength) MPa, but in general, titanium alloys can be regarded as high strength alloys. They are stronger than aluminum alloys that are considered to be medium-strength, and can completely replace certain types of steel in terms of strength. Compared with the rapid decline in strength of aluminum alloys at temperatures above 150°C, some titanium alloys can still maintain good strength at 600°C. The dense metal titanium is highly valued by the aviation industry because of its lighter weight, higher strength than aluminum alloy, and its ability to maintain higher strength than aluminum at high temperatures. In view of the density of titanium being 57% of steel, its specific strength (strength/weight ratio or strength/density ratio is called specific strength) is high, corrosion resistance, oxidation resistance, and fatigue resistance are all strong. 3/4 of titanium alloy is used as Structural materials represented by aviation structural alloys, one-fourth of them are mainly used as corrosion-resistant alloys. Titanium alloy has high strength, low density, good mechanical properties, toughness and corrosion resistance. In addition, titanium alloys have poor process performance and are difficult to cut. In hot processing, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen, and carbon. There is also poor abrasion resistance and complex production processes. Industrial production of titanium began in 1948. The development of the aviation industry requires the titanium industry to develop at an average annual growth rate of about 8%. At present, the world's annual output of titanium alloy processing materials has reached more than 40,000 tons. There are nearly 30 types of titanium alloy grades. The most widely used titanium alloys are Ti-6Al-4V (TC4)'Ti-5Al-2.5Sn (TA7) and industrial pure titanium (TA1, TA2 and TA3).
There are three kinds of heat treatment processes for titanium rods and titanium alloy rods:
1) Solution treatment and aging: The purpose is to improve its strength. α titanium alloy and stable β titanium alloy cannot be strengthened by heat treatment, and only annealing is carried out in production. α+β titanium alloy and metastable β titanium alloy containing a small amount of α phase can further strengthen the alloy through solution treatment and aging.
2) Stress relief annealing: the purpose is to eliminate or reduce the residual stress generated during the processing. Prevent chemical attack and reduce deformation in some corrosive environments.
3) Complete annealing: The purpose is to obtain good toughness, improve processing performance, facilitate reprocessing and increase the stability of size and structure.