The thermal conductivity of titanium bar and titanium alloy bar billet is low and the temperature difference between surface and inner layer will be great during hot extrusion. When the temperature of the extrusion cylinder is 400 degrees, the temperature difference can reach 200~250 degrees. In inspiratory reinforcement and billet section under the common effect of large temperature difference, the center of the billet surface and the strength of the metal to produce vastly different performance and the plastic performance, which can cause very uneven in the extrusion process. The deformation of the big gallas attached stress in the surface layer, by forming a crack in the surface of extrusion products and the root cause of the crack. The hot extrusion process of titanium rod and titanium alloy rod products 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.
Studies on the flow dynamics of industrial titanium alloys show that the flow behavior of metals varies greatly in the temperature region corresponding to different phase states of each alloy. Therefore, one of the main factors affecting the extrusion flow characteristics of titanium rods and titanium alloy rods is the billet heating temperature which determines the metal phase transition state. Metal flows more uniformly in a or a +P phase than in P phase. It is very difficult to obtain high surface quality for extruded products. Until now, the extrusion process of titanium alloy bars had to use lubricants. The main reason is that titanium will form fusible eutectic with iron or nickel base alloy die material at 980 degrees and 1030 degrees, which makes the die wear strongly.
Main factors affecting metal flow during extrusion:
1) Extrusion method. The metal flow is uniform in reverse extrusion ratio and better in cold extrusion ratio than in hot extrusion ratio and in lubricated extrusion ratio. The influence of the extrusion method is realized by changing the friction conditions.
2) Extrusion speed. With the increase of extrusion speed, the non - uniformity of metal flow is intensified.
3) Extrusion temperature. When the extrusion temperature increases and the deformation resistance of the blank decreases, the non-uniform flow of the metal intensifies. In the process of extrusion, if the heating temperature of the extrusion cylinder and the mold is too low, and the metal temperature difference between the outer layer and the central layer is large, the metal flow non-uniformity will increase. The better the thermal conductivity of the metal, the more uniform the temperature distribution on the end face of the billet.
4) Metal strength. When the other conditions being equal, the stronger the metal, the more uniform its flow.
5) Modulus Angle. The larger the die Angle (i.e. the angle between the end face of the mold and the central axis), the less uniform the metal flow will be. When using a porous die extrusion, the die hole arrangement will be reasonable, and the metal flow will tend to be uniform.
6) Degree of deformation. Too much or too little deformation, metal flow is not uniform.