The thermal conductivity of titanium rods and titanium alloy rod blanks is relatively low, and this characteristic is particularly significant during the hot extrusion process. Due to insufficient thermal conductivity, a huge temperature difference will occur between the surface layer and the inner layer of the billet. Especially when the extrusion barrel temperature is set to 400 degrees, this temperature difference may even reach 200~250 degrees. The inhalation strengthening effect and the cross-section temperature difference of the billet work together to cause significant differences in the strength and plasticity properties of the metal on the surface and center of the billet. This difference will cause uneven deformation during the extrusion process, thereby generating large additional tensile stress in the surface layer, which is the main reason for the formation of cracks and cracks on the surface of extruded products.
The hot extrusion process of titanium and titanium alloy rods is more complicated than that of aluminum alloys, copper alloys, and even steel. This complexity mainly stems from titanium and titanium alloys's special physical and chemical properties. Research on metal flow dynamics of industrial titanium alloys shows that the flow behavior of metals will change significantly in the temperature range of different phase states. Therefore, the heating temperature of the billet, a key factor that determines the phase transformation state of the metal, has an important impact on the extrusion flow characteristics of titanium rods and titanium alloy rods. Extrusion at the temperature in the α or α+β phase zone results in a more uniform metal flow than extrusion at the temperature in the β phase zone.
However, obtaining extruded products with high surface quality is a difficult task for titanium alloy rods. Until now, the extrusion process of titanium alloy rods had to rely on the use of lubricants. This is mainly because titanium will form a fusible eutectic with iron-based or nickel-based alloy mold materials at high temperatures of 980 degrees and 1030 degrees, causing the mold to suffer severe wear.
During the hot extrusion process of titanium alloy rods, the main factors affecting metal flow include:
1. Extrusion method: Compared with forward extrusion, reverse extrusion has a more uniform metal flow; cold extrusion has a more uniform metal flow than hot extrusion; lubricated extrusion has a more uniform flow than non-lubricated extrusion. , the metal flow is also more uniform. These effects are mainly achieved by changing friction conditions.
2. Extrusion speed: As the extrusion speed increases, the unevenness of metal flow will intensify.
3. Extrusion temperature: As the extrusion temperature increases, the deformation resistance of the billet decreases, but the uneven flow of metal will also intensify. If the heating temperature of the extrusion barrel and the mold is too low, the temperature difference between the metal in the outer layer and the center layer will increase, and the unevenness of the metal flow will also increase. The better the thermal conductivity of the metal, the more uniform the temperature distribution will be on the end face of the ingot.
4. Metal strength When other conditions are the same, the higher the metal strength, the more uniform the metal flow will be.
5. 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. However, when a multi-hole die is used for extrusion and the die holes are arranged properly, the metal flow tends to be uniform.
6. Degree of deformation If the degree of deformation is too large or too small, it will cause uneven metal flow.
In summary, the hot extrusion process characteristics of titanium rods and titanium alloy rods are complex and changeable, and multiple factors need to be considered comprehensively to ensure the quality and performance of extruded products.
