The mechanical properties of titanium, commonly known as mechanical properties, are closely related to purity. High-purity titanium has excellent machining properties, with good elongation and area reduction, but its strength is low and it is not suitable for use as structural materials. Industrial pure titanium contains a moderate amount of impurities, has high strength and plasticity, and is suitable for making structural materials.
The thermal conductivity of titanium and titanium alloy billets is low, which will cause a huge temperature difference between the surface layer and the inner layer during hot extrusion. When the temperature of the extrusion barrel is 400 degrees, the temperature difference can reach 200~250 degrees. Under the combined influence of the inhalation strengthening and the large temperature difference in the cross section of the billet, the metal on the surface and center of the billet produce very different strength properties and plastic properties, which will cause very uneven deformation during the extrusion process. A large additional tensile stress is generated in the extruded product, which becomes the source of cracks and cracks on the surface of the extruded product. The hot extrusion process of titanium and titanium alloy products is more complicated than the extrusion process of aluminum alloy, copper alloy, and even steel. This is determined by the special physical and chemical properties of titanium and titanium alloys.
Until now, lubricants have been necessary for the extrusion process of titanium rods. The main reason is that titanium will form a fusible eutectic with iron-based or nickel-based alloy mold materials at temperatures of 980 degrees and 1030 degrees, causing strong wear of the mold. When graphite lubricant is used, deep longitudinal scratches can be formed on the surface of the product. This is a consequence of the titanium rod and titanium alloy rod sticking to the mold. When glass lubricants are used to extrusion profiles, a new type of defect is caused called "pockmarks", i.e. cracks in the surface layer of the product. Research shows that the appearance of "pockmarks" is due to the low thermal conductivity of titanium and titanium alloys, which causes the surface layer of the billet to cool violently and the plasticity to drop dramatically.
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 generally titanium alloys can be regarded as high-strength alloys. They are stronger than aluminum alloys, which are considered medium-strength, and can completely replace some types of steel in terms of strength. Compared with the rapid decrease in strength of aluminum alloys at temperatures above 150°C, some titanium alloys can still maintain good strength at 600°C. Dense metallic titanium is highly valued by the aviation industry because of its light weight, 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 steel, its specific strength (strength/weight ratio or strength/density ratio is called specific strength) is high, and its anti-corrosion, anti-oxidation, and anti-fatigue capabilities are strong. 3/4 of titanium alloys are used as 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, titanium alloys have poor process performance and are difficult to process. During thermal processing, they easily absorb impurities such as hydrogen, oxygen, nitrogen, and carbon. It also has poor wear resistance and complex production process. Industrial production of titanium began in 1948. The needs of the development of the aviation industry have caused 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, with 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).
