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

The thermal conductivity of titanium rods and titanium alloy rods is low. During hot extrusion, a significant temperature difference will occur between the surface and the inner layer. When the extrusion cylinder temperature 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, becoming the root cause of cracks and cracks 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.

Industrial titanium alloy metal flow dynamics research shows that in the temperature zones corresponding to different phase states of each alloy, the metal flow behavior shows great 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 temperature extrusion in the a or a + P phase zone, the metal flow is more uniform in temperature extrusion in the p phase zone. It is very difficult to obtain high surface quality in the extruded products. So far, lubricants must be used in the extrusion process of titanium alloy rods. The main reason is that titanium will form easily fusible eutectics with iron-based or nickel-based alloy mold materials at temperatures of 980 degrees and 1030 degrees, causing severe wear of the mold.

The main factors affecting metal flow during extrusion:
1) Extrusion method. Reverse extrusion has more uniform metal flow than forward extrusion. Cold extrusion has more uniform metal flow than hot extrusion. Lubricated extrusion has more uniform metal flow than non-lubricated extrusion. The influence of the extrusion method is achieved through changes in friction conditions.

2) Extrusion speed. The increase in extrusion speed intensifies the non-uniformity of metal flow.

3) Extrusion temperature. When the extrusion temperature increases and the deformation resistance of the billet decreases, the non-uniform flow of the metal intensifies. During the extrusion process, if the heating temperature of the extrusion cylinder and the mold is too low, the temperature difference between the outer layer and the center layer of the metal is large, and the non-uniformity of metal flow increases. The better the metal thermal conductivity, the more uniform the temperature distribution on the end face of the ingot billet.

4) Metal strength. Under the same other conditions, the higher the metal strength, the more uniform the metal flow.

5) Mold angle. The larger the mold angle (i.e., the angle between the mold end face and the central axis), the more uneven the metal flow. When using a porous mold for extrusion, if the mold hole arrangement is reasonable, the metal flow tends to be uniform.

6) Deformation degree. If the deformation degree is too large or too small, the metal flow is not uniform.