When the ratio of the original length and diameter of the ingot is less than 4:1, that is, when the upsetting ratio is within a reasonable range, the flow process of the ingot during the filling stage first forms a single drum shape under the action of axial pressure, and the metal is extruded toward the ingot and When the gap in the inner wall of the pressure cylinder flows, a small part of the metal flows to the die hole. As the filling process proceeds, the metal on the front end surface circumference that is in contact with the mold gradually transfers to the surface in contact with the work belt, forming a rollover phenomenon on the front end surface. After entering the advection extrusion stage, the overturned metal becomes the side surface of the product head. Due to the flow characteristics of the outer circumferential metal of the head end in the initial filling stage, the metal on the end surface of the ingot head is subjected to radial additional tensile stress at this stage. When the extrusion ratio is small, the front end of the product is prone to cracking under the action of additional tensile stress.
Due to the constraints of the extrusion tool, when the extrusion filling process reaches the stage of contact with the inner wall of the extrusion barrel (upsetting), this process is more complicated than the free upsetting process of the cylinder, so the metal flow situation is different from that of the free upsetting process. The upsetting ones are also different. When the filling stage proceeds to the point where the drum-shaped portion of the side surface contacts the inner wall of the extrusion cylinder, due to the open state of the extrusion die, its stress situation is also different from that of the closed state of the extrusion pad.
After the filling reaches a certain stage, due to the influence of the open state of the die hole, some metal flows to the die hole. As the filling stage continues, the metal flowing to the gap of the extrusion barrel and the metal flowing to the die hole are affected by the flow respectively. The friction force is in the opposite direction, and the extrusion gasket is only subject to friction resistance. In order to prevent the wall of the metal filling cylinder from being subject to frictional resistance, as the filling process proceeds, the gap between the ingot and the extrusion cylinder becomes smaller and smaller until the blank is completely filled with the extrusion cylinder. The filling and extrusion phase ends. Due to the open extrusion die The influence is that the axial stress in the annular area from the periphery is large, and the axial stress of the metal facing the die hole is reduced.
The diameter of the ingot gradually increases during the filling process, so that the contact surface gradually increases, and the unit pressure required for deformation gradually rises. When the ingot is completely filled with the extrusion barrel, the extrusion force required for filling deformation reaches the maximum, as shown in Figure 2-27 In the I area of the extrusion pressure curve, the extrusion force rises approximately in a straight line.
When using a split die for extrusion, the first extrusion of a new die or the first extrusion after the die has been repaired and nitrided, the filling stage includes two processes: the first process and the extrusion filling process described above. Basically the same; the second process is the filling process of metal in the welding cavity.
