Titanium and titanium alloy billet thermal conductivity is low, in the hot extrusion will make the surface layer and the inner layer of the 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 centre 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 at the root cause. Titanium and titanium alloy products hot extrusion process than aluminium alloys, copper alloys, and even steel extrusion process is more complex, which is determined by the special physical and chemical properties of titanium and titanium alloys.
The main factors affecting metal flow during extrusion.
(1) Extrusion method. Reverse extrusion than forward extrusion metal flow uniformity, cold extrusion than hot extrusion metal flow uniformity, lubricated extrusion than non-lubricated extrusion metal flow uniformity. The effect of extrusion method is realised by changing the friction conditions.
(2) Extrusion temperature. The uneven flow of metal increases when the extrusion temperature increases and the deformation resistance of the billet decreases. During extrusion, if the heating temperature of the extrusion barrel and die is too low, and the temperature difference between the metal in the outer layer and the centre layer is large, the unevenness of metal flow increases. The better the thermal conductivity of the metal, the more uniform the temperature distribution on the end face of the ingot billet.
(3) Metal strength. All other conditions being equal, the higher the metal strength, the more uniform the metal flow.
(4) Die angle. The larger the die angle (i.e. the angle between the end face of the die and the central axis), the more uneven the metal flow is. When extruding with a porous die, the die holes are arranged reasonably, and the metal flow tends to be uniform.
(5) Deformation degree. If the degree of deformation is too large or too small, the metal flow is not uniform.
(6) Extrusion speed. When the extrusion speed increases, the inhomogeneity of metal flow increases.
Industrial titanium alloy metal flow dynamics research shows that in the temperature zone corresponding to the different phase states of each alloy, the flow behaviour of the metal appears to be very different. Therefore, one of the main factors influencing the characteristics of extruded flow of titanium and titanium alloys is the billet heating temperature, which determines the state of the metal phase transition.
Extrusion at a or a+P phase zone temperature results in more uniform metal flow compared to extrusion at p phase zone temperature. It is very difficult to obtain high surface quality of extruded products. Until now, the extrusion of titanium alloys has required the use of lubricants. The main reason for this is that titanium forms fusible eutectic crystals with iron-based or nickel-based alloy die materials at temperatures of 980 and 1030 degrees Celsius, which results in strong wear of the die. When using graphite lubricants, deep longitudinal scratch lines can be formed on the surface of the product, which is a consequence of titanium and titanium alloys adhering to the work on the die. 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.
