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.


The main factors affecting metal flow during extrusion:
(1) Extrusion method. The metal flows more evenly in reverse extrusion than in forward extrusion, the metal flows more evenly in cold extrusion than in hot extrusion, and the metal flows more evenly in lubricated extrusion than in unlubricated extrusion. The influence of the extrusion method is achieved through changes in friction conditions.
(2) Extrusion temperature. The uneven flow of metal intensifies when the extrusion temperature increases and the deformation resistance of the blank decreases. During the extrusion process, if the heating temperature of the extrusion barrel and the die is too low and the temperature difference between the metal in the outer layer and the center layer is large, the unevenness of the metal flow will increase. The better the thermal conductivity of the metal, the more uniform the temperature distribution will be on the end face of the ingot.
(3) Metal strength. When other conditions are equal, the higher the strength of the metal, the more uniform the metal flow will be.
(4) Die angle. The larger the die angle (that is, the angle between the end face of the die and the central axis), the more uneven the metal fluidity will be. When a porous die is used for extrusion and the die holes are arranged reasonably, the metal flow tends to be uniform.
(5) Degree of deformation. If the degree of deformation is too large or too small, the metal flow will be uneven.
(6) Extrusion speed. As the extrusion speed increases, the unevenness of metal flow intensifies.
Research on the metal flow dynamics of industrial titanium alloys shows that in the temperature zones corresponding to the different phase states of each alloy, the flow behavior of the metal is greatly different. Therefore, one of the main factors affecting the extrusion flow characteristics of titanium and titanium alloys is the billet heating temperature that determines the phase transformation state of the metal.


Compared with temperature extrusion in the a or a+P phase zone, the metal flow is more uniform than temperature extrusion in the p phase zone. It is very difficult to obtain high surface quality of extruded products. Until now, lubricants have been necessary for the extrusion process of titanium alloys. 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 titanium and titanium alloys adhering 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.