The thermal conductivity of the titanium rod and titanium alloy rod blanks is low, which will cause a great temperature difference between the surface layer 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 large temperature difference of the blank section, the metal on the surface and the center of the blank produce very different strength and plastic properties, which will cause very uneven deformation during the extrusion process. It produces large additional tensile stress, which becomes the source of cracks and cracks on the surface of extruded products. The hot extrusion process of titanium rods and titanium alloy rods is more complicated 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.
Industrial titanium alloy metal flow dynamics research shows that in the temperature zone corresponding to the different phase states of the alloys, the metal flow behavior is extremely different. Therefore, one of the main factors affecting the extrusion flow characteristics of titanium rods and titanium alloy rods is the heating temperature of the blank that determines the phase transformation state of the metal. Compared with the temperature extrusion in the a or a + P phase region, the metal flow is more uniform than the temperature extrusion in the p phase region. Extruded products have great difficulty in obtaining high surface quality. So far, lubricants have to be used in the extrusion process of titanium alloy rods. The main reason is that titanium will form fusible eutectic with iron-based or nickel-based alloy mold materials at temperatures of 980 degrees and 1030 degrees, thereby causing the mold to wear out strongly.
The main factors affecting the metal flow during extrusion:
- (1) Extrusion method. Reverse extrusion has a more uniform metal flow than forward extrusion, cold extrusion has a more uniform flow than hot extrusion, and lubricated extrusion has a more uniform flow than non-lubricated extrusion. The impact of the extrusion method is achieved by changing the friction conditions.
- (2) Squeeze speed. As the extrusion speed increases, the unevenness of the metal flow increases.
- (3) Extrusion temperature. As the extrusion temperature increases and the deformation resistance of the billet decreases, the uneven flow of the metal increases. During the extrusion process, if the heating temperature of the extrusion cylinder and the mold is too low, and the temperature difference between the outer layer and the center layer 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 surface of the ingot.
- (4) Metal strength. When other conditions are the same, the higher the metal strength, the more uniform the metal flow.
- (5) The die angle. The larger the die angle (ie, the angle between the end face of the die and the central axis), the more uneven the fluidity of the metal. When the porous die is used for extrusion, the arrangement of die holes is reasonable, and the metal flow tends to be uniform.
- (6) The degree of deformation. If the degree of deformation is too large or too small, the metal flow is uneven.