Titanium rod and
titanium alloy rod billet low thermal conductivity, hot extrusion will cause a large temperature difference between the surface layer and the inner layer. When the temperature of the extrusion cylinder is 400, the temperature difference can reach 200~250 degrees. Under the joint influence of suction strengthening and large temperature difference of billet section, the strength and plasticity of the billet surface and the metal in the center are completely different, which will cause very uneven deformation and produce a large stress on the surface, which is the source of cracks and cracks on the surface of the extruded products. The hot extrusion process of titanium rod and titanium alloy rod is more complicated than that of aluminum alloy, copper alloy and even steel, which is determined by the special physical and chemical properties of titanium rod and titanium alloy rod.
The study on the metal flow dynamics of industrial titanium alloys shows that the metal flow behavior is very different in different temperature regions. Therefore, one of the main factors affecting the extrusion flow characteristics of titanium rod and titanium alloy rod is the heating temperature of billet, which determines the phase transition state of the metal. Compared with the temperature extrusion in the P phase zone, the metal flow in the A phase zone or P phase zone is more uniform. It is difficult to obtain high surface quality extruded products. Until now, titanium alloy bars have had to be extruded using lubricants. The main reason is that titanium at 980 degrees and 1030 degrees will form eutectic with iron or nickel base alloy mold materials, so that the mold wear is strong.
Main factors affecting metal flow during extrusion:
(1) Extrusion method. The metal flow in reverse extrusion is more uniform than that in forward extrusion, cold extrusion is more uniform than that in hot extrusion, and lubricated extrusion is more uniform than that in non-lubricated extrusion. The effect of the extrusion method is achieved by changing the friction conditions.
(2) extrusion speed. With the increase of extrusion speed, the non-uniformity of metal flow becomes worse.
(3) extrusion temperature. When the extrusion temperature increases, the deformation resistance of billet decreases, and the uneven metal flow increases. In the extrusion process, if the heating temperature of the extrusion cylinder and the mold is too low, the temperature difference between the outer layer and the center layer is large, the uneven metal flow will increase. The better the thermal conductivity of the metal, the more uniform the temperature distribution of the ingot face.
(4) Metal strength. Other things being equal, the stronger the metal, the more evenly the metal flows.
(5) Mold Angle. The larger the die Angle (that is, the Angle between the die end face and the central axis), the more uneven the metal flow. When the porous die is used for extrusion, the die holes are arranged reasonably and the metal flow tends to be uniform.
(6) Degree of deformation. If the deformation is too large or too small, the metal flows unevenly.
