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Titanium rods and titanium alloy rod blanks have low thermal conductivity, which will cause a great temperature difference between the surface layer and the inner layer during hot extrusion. When the temperature of the extrusion cylinder is 400 degrees, the temperature difference can reach 200 to 250 degrees. Under the combined influence of suction strengthening and the large temperature difference in the section of the blank, the metal on the surface of the blank and the center of the blank have very different strength and plastic properties, which will cause very uneven deformation during the extrusion process. Large additional tensile stress is generated in the extruded product, which becomes the source of cracks and cracks on the surface of the extruded product. The hot extrusion process of titanium rods and titanium alloy rod products is more complicated than the extrusion process of aluminum alloy, copper alloy, and even steel, which is determined by the special physical and chemical properties of titanium rods and titanium alloy rods.
Research on the metal flow kinetics of industrial titanium alloys shows that in the temperature zone corresponding to the different phase states of each alloy, the flow behavior of the metal is greatly 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 billet that determines the phase transition state of the metal. Extrusion at the temperature of a or a+P phase zone compared with extrusion at the temperature of p phase zone, the metal flow is more uniform. It is very difficult to obtain high surface quality for extruded products. So far, the extrusion process of titanium alloy rods must use lubricants. The main reason is that titanium will form a fusible eutectic with iron-based or nickel-based alloy mold materials at 980 degrees and 1030 degrees, which will cause the mold to wear strongly.
The main factors affecting metal flow during extrusion:
- 1) Extrusion method. The reverse extrusion is more uniform than the forward extrusion, the cold extrusion is more uniform than the hot extrusion, and the lubricated extrusion is more uniform than the unlubricated extrusion. The influence of the extrusion method is realized by changing the friction conditions.
- 2) Extrusion speed. As the extrusion speed increases, the unevenness of the metal flow increases.
- 3) Extrusion temperature. When the extrusion temperature increases and the deformation resistance of the blank 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 metal temperature difference between the outer layer and the center layer is large, the unevenness of the metal flow will increase. 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) Die angle. The larger the mold angle (that is, the angle between the end face of the mold and the central axis), the more uneven the metal fluidity. When using porous die extrusion, the die hole arrangement is reasonable, and the metal flow tends to be uniform.
- 6) Degree of deformation. If the degree of deformation is too large or too small, the metal will flow unevenly.
Link to this article：Six factors affecting metal flow during extrusion of titanium alloy materials
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