Defects in the filling and extrusion stage can also easily cause cracks at the head end of the extruded bar, as shown in Figure 2-33. This is due to the flow and stress characteristics of the metal head end in the filling and extrusion stage. As mentioned earlier, due to the filling During extrusion, the outer circumferential metal of the front end surface flows to the surface of the mold working belt, forming the side surface of the head end of the product, thereby forming an additional tensile stress in the center of the front end surface. When the tensile stress exceeds the fracture strength of the metal at this temperature, head cracking occurs. .
The stress and strain experienced by the metal in the extrusion barrel during the extrusion of aluminum profiles are very complex and vary with the extrusion method and process conditions. Studying the flow behavior of metal during the extrusion deformation process is of extremely important practical significance. The structure, performance, surface quality, outer dimensions, dimensional accuracy, yield, design of extrusion tools and molds, tool life and production efficiency of extruded products are all closely related to metal flow.
The flow behavior characteristics of metal during extrusion deformation can be studied by two major methods: analytical method and experimental method. The two methods are suitable for studying different objects.
On the ingot there is the positive pressure (reaction force) on the surface of the mold and the mold working belt and the friction on the contact surface. Under the action of these external forces, the ingot forms a strong three-dimensional compressive stress state in the deformation zone. That is, as shown in Figure 2-26, the center line in the deformation zone is selected for analysis. The stresses here are radial compressive stress a, circumferential tangential compressive stress σQ and axial compressive stress a = deformation in this deformation zone. The states are radial compression deformation δ and tangential compression deformation δ. and axial extension deformation8.
According to the theory of plastic deformation, in an axially symmetrical state, such as an extrusion process, the stress and deformation in the circumferential direction and the radial direction can be theoretically considered equal to each other. According to the flow characteristics of metal during the deformation process, in order to facilitate the study of the problem , usually the metal deformation process during extrusion can be roughly divided into three stages: filling extrusion stage, advection extrusion stage, and turbulent flow extrusion stage.
These three stages correspond to areas I, II, and III on the extrusion stroke curve respectively, as shown in Figure 2-27.
