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 against further oxidation. Titanium has a strong ability to absorb oxygen, nitrogen, and hydrogen. Such gases are very harmful impurities to metal titanium, and 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 has a series of excellent optical properties. Ti02 is opaque, with high gloss and whiteness, high refractive index and scattering power, strong hiding power, and good dispersibility. The pigment made is a white powder, commonly known as titanium dioxide, which is widely used. The appearance of a titanium rod is very similar to steel, with a density of 4.51 g/cm3, which is less than 60% of steel, and is the metal element with the lowest density among refractory metals. The mechanical properties of titanium, commonly known as mechanical properties, are closely related to purity. High-purity titanium has excellent machinability, good elongation, and reduction of area, but low strength and is not suitable for structural materials. Industrial pure titanium contains a moderate amount of impurities, has high strength and plasticity, and is suitable for making structural materials.
Titanium alloys are divided into low-strength and high-plasticity, medium-strength and high-strength, ranging from 200 (low strength) to 1300 (high strength) MPa, but titanium alloys can generally be regarded as high-strength alloys. They are stronger than aluminum alloys that are considered medium strength, and can completely replace some types of steel in terms of strength. Compared with aluminum alloys whose strength decreases rapidly at temperatures above 150°C, some titanium alloys can still maintain good strength at 600°C.
Dense metal titanium is highly valued by the aviation industry because of its lightweight, higher strength than aluminum alloy, and ability to maintain higher strength than aluminum at high temperatures. In view of the fact that the density of titanium is 57% of that of steel, its specific strength (strength/weight ratio or strength/density can be called specific strength) is high, and its corrosion resistance, oxidation resistance, and fatigue resistance are strong, and 3/4 of titanium alloy is used as Of the structural materials represented by aerospace structural alloys, 1/4 are mainly used as corrosion-resistant alloys. Titanium alloy has high strength and low density, good mechanical properties, good toughness, and corrosion resistance. In addition, the process performance of titanium alloy is poor, and it is difficult to cut and process. During thermal processing, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen, and carbon. Also have poor wear resistance and a complex production process. The industrial production of titanium began in 1948. The needs of the development of the aviation industry make the titanium industry 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, and there are nearly 30 kinds of titanium alloy grades. The most widely used titanium alloys are Ti-6Al-4V(GR5)'Ti-5Al-2.5Sn(GR6) and industrial pure titanium (GR1, GR2 and GR3).
There are three 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 subjected to strengthening heat treatment, and only annealing is performed in production. α+β titanium alloys and metastable β titanium alloys containing a small amount of α phase can be further strengthened by solution treatment and aging.
2. Stress relief annealing: the purpose is to eliminate or reduce the residual stress generated during processing. Prevents chemical attacks and reduces deformation in some corrosive environments.
3. Complete annealing: the purpose is to obtain good toughness, improve processing performance, facilitate reprocessing and improve dimensional and organizational stability.
