The thermal conductivity of titanium rods is low, and hot extrusion will cause a large temperature difference between the surface and the inner layer. When the temperature of the extrusion barrel is 400, the temperature difference can reach 200~250 degrees. Under the combined influence of suction enhancement and large temperature difference in the cross section of the billet, the metal on the surface and the center of the billet produces completely different strength and plasticity, which will cause very uneven deformation during the extrusion process and produce a large additional tensile stress on the surface, becoming the source of cracks on the surface of the extruded product. The hot extrusion process of titanium and titanium alloy products is more complicated than that of aluminum alloys, copper alloys and even steel, which is determined by the special physical and chemical properties of titanium and titanium alloys.
Research on the flow dynamics of industrial titanium alloy metals shows that there are great differences in the metal flow behavior in different corresponding temperature zones of each alloy. Therefore, one of the main factors affecting the extrusion flow characteristics of Baoji titanium rods and titanium alloy rods is the billet heating temperature that determines the metal phase change state. Compared with the temperature extrusion in the P phase zone, the metal flow in the a phase zone or the P phase zone is more uniform. It is difficult to obtain products with high surface quality. So far, lubricants must be used in the extrusion processing of titanium alloy bars. The main reason is that titanium forms eutectic with iron-based or nickel-based alloy mold materials at 980 degrees and 1030 degrees, causing severe mold wear.
Main factors affecting metal flow during extrusion:
(1) Extrusion method. The metal flow of reverse extrusion is more uniform than that of forward extrusion, cold extrusion is more uniform than that of hot extrusion, and lubricated extrusion is more uniform than that of non-lubricated extrusion. The influence of extrusion method is achieved by changing the friction conditions.
(2) Extrusion speed. As the extrusion speed increases, the unevenness of metal flow increases.
(3) Extrusion temperature. When the extrusion temperature increases and the deformation resistance of the billet decreases, the uneven flow of metal intensifies. During the extrusion process, if the heating temperature of the extrusion barrel and the mold is too low, the temperature difference between the outer layer and the center layer is large, and the unevenness of metal flow will increase. The better the thermal conductivity of the metal, the more uniform the temperature distribution on the end face of the ingot.
(4) Metal strength. When other conditions are the same, the stronger the metal, the more uniform the metal flow.
(5) Die angle. The larger the die angle (i.e. the angle between the die end face and the center axis), the more uneven the metal flowability. When using a multi-hole die for extrusion, the die holes are arranged reasonably and the metal flow tends to be uniform.
(6) Deformation degree. If the deformation degree is too large or too small, the metal flow is uneven.
