Characteristics Of Hot Extrusion Process Of Titanium And Titanium Alloy Materials
Aug 13, 2025
The thermal conductivity of titanium and titanium alloy billets is low, which will cause a large temperature difference between the surface and the inner layer during hot extrusion. When the temperature of the extrusion barrel is 400 degrees, the temperature difference can reach 200~250 degrees. Under the combined influence of suction strengthening and the large temperature difference of the billet cross section, the metal on the surface and in the center of the billet produces extremely different strength and plastic properties, which will cause very uneven deformation during the extrusion process, and generate large additional tensile stress in the surface layer, which becomes the root cause of cracks and cracks on the surface of the extruded product. The hot extrusion process of titanium and titanium alloy products is more complicated than the extrusion process of aluminum alloy, copper alloy, and even steel. This is determined by the special physical and chemical properties of titanium and titanium alloys.
The main factors affecting metal flow during extrusion:
(1) Extrusion method. The metal flow is more uniform in reverse extrusion than in forward extrusion, the metal flow is more uniform in cold extrusion than in hot extrusion, and the metal flow is more uniform in lubricated extrusion than in non-lubricated extrusion. The influence of the extrusion method is achieved through the change of friction conditions.
(2) Extrusion temperature. When the extrusion temperature increases and the deformation resistance of the billet decreases, the uneven flow of the metal intensifies. During the extrusion process, if the heating temperature of the extrusion barrel and the die is too low, the temperature difference between the outer layer and the center layer of the metal is large, and 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. (3) Metal strength. When other conditions are the same, the higher the metal strength, the more uniform the metal flow. (4) 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 flow. When a multi-hole die is used for extrusion, the die holes are arranged reasonably, and the metal flow tends to be uniform. (5) Deformation degree. If the deformation degree is too large or too small, the metal flow is uneven. (6) Extrusion speed. As the extrusion speed increases, the unevenness of the metal flow intensifies. Research on the flow dynamics of industrial titanium alloy metals shows that in the temperature zones corresponding to the different phase states of each alloy, the flow behavior of the metal varies greatly. Therefore, one of the primary factors influencing the extrusion flow characteristics of titanium and titanium alloys is the billet heating temperature, which determines the metal's phase transformation state.
Extrusion in the a or a+P phase region results in more uniform metal flow than extrusion in the p phase region. Achieving high surface quality in extruded products is extremely challenging. Until now, the extrusion of titanium alloys has required the use of lubricants. The primary reason for this is that at temperatures between 980°C and 1030°C, titanium forms a fusible eutectic with iron- or nickel-based alloy die materials, resulting in severe die wear. Using graphite lubricants can create deep longitudinal scratches on the product surface, a consequence of titanium and titanium alloys adhering to the die. Using glass lubricants for extrusion can lead to a new defect, pitting, which is a crack in the product's surface layer. Research indicates that pitting is caused by the low thermal conductivity of titanium and titanium alloys, which leads to rapid cooling of the billet's surface layer and a sharp decrease in plasticity.
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