Structural changes in TA1 titanium rods during hot extrusion processing

The appearance of TA1 titanium rods is very similar to steel, with a density of 4.51 g/cm3, less than 60% of steel, and is the lowest density metal element in refractory metals. The mechanical properties of titanium, commonly known as mechanical properties and purity is very relevant. High purity titanium has excellent machinability, elongation, section shrinkage are good, but low strength, not suitable for structural materials. Industrial pure titanium contains a moderate amount of impurities, with high strength and plasticity, suitable for the production of structural materials. Titanium and titanium alloy billet thermal conductivity is low, in the hot extrusion will make the surface layer and the inner layer produces a great temperature difference, when the temperature of the extrusion cylinder is 400 degrees, the temperature difference can reach 200 ~ 250 degrees. In the suction strengthening and billet section has a large temperature difference under the joint influence of the billet surface and the center of the metal to produce very different strength properties and plastic properties, in the extrusion process will cause very uneven deformation, in the surface layer of the large additional tensile stresses, to become in the extrusion of the surface of the product to form cracks and cracks in the root cause. Titanium and titanium alloy products hot extrusion process than aluminum alloys, copper alloys, and even steel extrusion process is more complex, this is by the titanium and titanium alloy special physical and chemical properties of the decision.

So far, the extrusion process of titanium rods must use lubricants. The main reason is: titanium at 980 degrees and 1030 degrees temperature, will be formed with iron-based or nickel-based alloy die material fusible eutectic, so that the die strong wear. When using graphite lubricants, deep longitudinal scratch lines can be formed on the surface of the product, which is a consequence of titanium rods and titanium alloy rods adhering to the work on the mold. Extrusion of profiles with glass lubricants leads to a new type of defect "pockmarks", i.e. cracks in the surface layer of the product. It has been shown that "pockmarks" appear due to the low thermal conductivity of titanium and titanium alloys, which results in a sharp cooling of the surface layer of the billet and a sharp decrease in plasticity.

Titanium alloys have low-strength high-plasticity, medium-strength and high-strength, for 200 (low-strength) ~ 1300 (high-strength) MPa, but by and large, titanium alloys can be regarded as high-strength alloys. They have a higher strength than aluminum alloys, which are considered medium-strength, and can completely replace certain types of steel in terms of strength. Compared with aluminum alloys, which lose strength rapidly at temperatures above 150°C, some titanium alloys maintain good strength at 600°C. Dense metal titanium due to light weight, higher strength than aluminum alloys, can be maintained at high temperatures than aluminum for high strength and by the aviation industry attaches great importance to. In view of the density of titanium for steel 57%, its specific strength (strength / weight ratio or strength / density ratio are called specific strength) high, corrosion, oxidation, fatigue resistance are strong, titanium alloys, 3 / 4 used for aerospace structural alloys as a representative of the structural materials, 1/4 is mainly used as corrosion-resistant alloys. Titanium alloy has high strength and small density, good mechanical properties, toughness and corrosion resistance is very good. In addition, titanium alloy process performance is poor, cutting and processing difficulties, in the thermal processing, very easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon. There is also poor abrasion resistance, the production process is complex. The industrialized production of titanium was started in 1948. The needs of the aviation industry development, so that the titanium industry to an average annual growth rate of about 8% development. At present, the world's annual output of titanium alloy processing materials has reached more than 40,000 tons of titanium alloy grade nearly 30 kinds. The most widely used titanium alloy is Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (TA7) and industrial pure titanium (TA1, TA2 and TA3).