The six major factors influencing the metal flow during the extrusion of titanium alloy materials

The thermal conductivity of titanium rods and titanium alloy rod billets is low. During hot extrusion, a significant temperature difference will occur between the surface and the inner layer. When the temperature of the extrusion cylinder is 400 degrees, the temperature difference can reach 200 to 250 degrees. Under the combined influence of the vacuum strengthening and the large temperature difference in the cross-section of the billet, the metal at the surface and the center of the billet exhibits very different strength and plastic properties. During the extrusion process, very uneven deformation occurs, and large additional tensile stress is generated in the surface layer, which becomes the root cause of cracks and fissures forming on the surface of the extruded products. The hot extrusion process of titanium rods and titanium alloy rods is more complex than that of aluminum alloys, copper alloys, and even steel. This is determined by the special physical and chemical properties of titanium rods and titanium alloy rods.

The research on the metal flow dynamics of industrial titanium alloys indicates that in the temperature ranges corresponding to different phase states of each alloy, the metal flow behavior shows significant differences. Therefore, one of the main factors affecting the extrusion flow characteristics of titanium rods and titanium alloy rods is the heating temperature of the billet that determines the metal phase transformation state. Compared with extrusion at a temperature in the a or a + P phase region, the metal flow is more uniform during extrusion at a temperature in the p phase region. It is very difficult to obtain high surface quality for the extruded products. So far, lubricants must be used in the extrusion process of titanium alloy rods. The main reason is that at temperatures of 980 degrees and 1030 degrees, titanium will form easily fusible eutectic crystals with iron-based or nickel-based alloy mold materials, causing severe wear of the mold.

The main factors affecting the flow of metal during compression:
1) Compression method. Reverse compression results in more uniform metal flow compared to forward compression. Cold compression leads to more uniform metal flow than hot compression, and lubricated compression results in more uniform metal flow than non-lubricated compression. The influence of the compression method is achieved by altering the friction conditions.
2) Extrusion speed. An increase in the extrusion speed leads to a greater degree of unevenness in the metal flow.
3) Extrusion temperature. When the extrusion temperature rises and the deformation resistance of the billet decreases, the uneven flow of the metal becomes more pronounced. During the extrusion process, if the heating temperatures of the extrusion barrel and the mold are too low, and the temperature difference between the outer layer and the center layer of the metal is large, the unevenness of the 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 will be.
4) Metal strength. Under the same conditions, the higher the metal strength, the more uniform the metal flow will be.
5) Modulus Angle. The larger the modulus angle (the angle between the end face of the mold and the central axis), the more uneven the metal flow will be. When using a porous mold for extrusion, if the mold holes are arranged reasonably, the metal flow will tend to be more uniform.
6) Degree of deformation. If the deformation is too large or too small, the metal flow will not be uniform.