Titanium is very stable in the air at normal temperature. When heated to 400 ~ 550 ℃, a strong 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 for metallic titanium. Even a very small amount (0.01% to 0.005%) can seriously affect its mechanical properties. Among the titanium compounds, titanium dioxide (TiO2) has the most practical value. Ti02 is inert to human body, non-toxic, it 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 dispersion. The pigment is white powder, commonly known as titanium white, which 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, commonly known as mechanical properties, are closely related to purity. High-purity titanium has excellent machinability, good elongation and shrinkage, but low strength and is not suitable for structural materials. Industrial pure titanium contains appropriate impurities, has high strength and plasticity, and is suitable for making structural materials.

Titanium alloys are divided into low-strength high-plasticity, medium-strength and high-strength, but generally titanium alloys can be regarded as high-strength alloys. They are stronger than aluminum alloys, which are considered to be medium-strength, and can completely replace certain types of steel in 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 compact metal titanium is highly valued by the aviation industry because of its light weight and higher strength than aluminum alloys, which can maintain higher strength than aluminum at high temperatures. Given that the density of titanium is 57% of steel, its specific strength (strength / weight ratio or strength / density ratio is called specific strength) is high, and its corrosion resistance, oxidation resistance, and fatigue resistance are strong. 3/4 of titanium alloy is used as Structural materials represented by aviation structural alloys 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, titanium alloys have poor process performance and are difficult to cut. In hot working, it is very easy to absorb impurities such as hydrogen, oxynitride and carbon. There is also poor abrasion resistance and complicated production process.

Titanium rods and titanium alloy rods have the following three heat treatment processes:

• Solution treatment and aging: The purpose is to improve its strength. Alpha titanium alloy and stable beta titanium alloy cannot be strengthened and heat treated, and only annealed in production. The α + β titanium alloy and the metastable β titanium alloy containing a small amount of α phase can be further strengthened by solution treatment and aging.
•  Stress relief annealing: The purpose is to eliminate or reduce the residual stress generated during processing. Prevent chemical attack and reduce deformation in some corrosive environments.
• Complete annealing: The purpose is to obtain good toughness, improve processing performance, facilitate reprocessing and improve the stability of size and structure.