If industrial aluminum profiles do not undergo external action after the end of the extrusion process, then the non-uniformity of the deformation state is the basic cause of residual stress. Due to the uneven temperature in each part of the plastic deformation zone, residual stress may also occur in industrial aluminum extrusion products. However, since the non-uniformity of the temperature is relatively small, the resulting residual stress value is not large. Therefore, it can be considered that the instantaneous stress state of the extruded aluminum extrusion product from the plastic deformation zone mainly depends on the residual stress caused by the uneven deformation.

In most extrusion processes, the main tensile deformation of the peripheral layer is greater than in the central layer. Therefore, the tensile elastic deformation of the peripheral layer is greater than the tensile elastic deformation of the central layer. In accordance with the conditions for the balance of internal forces, this will lead to less shrinkage in the surrounding layer to completely eliminate elastic deformation. The central layer has a larger shrinkage to eliminate elastic deformation completely. As a result, in the industrial aluminum extrusion product flowing out from the die hole, the center layer generates a longitudinal compressive stress, and the peripheral layer thereof generates a residual tensile stress. In extruded (without subsequent processing) round bars.

It should be noted that in the extrusion of industrial aluminum profiles, the non-uniformity of the flow speeds of the various parts is another cause of residual stress. These residual stresses are balanced by the interaction forces of the various sections of the profile and thus may alter the above-described distribution pattern of longitudinal residual forces. Practical experience shows that after the bars flow out from the plastic deformation zone, their radial dimensions increase slightly due to elastic deformation. Due to this increase in radial dimension, a radial compressive stress is bound to be created between the concentric ring layers. This stress state is just like the stress state in the same group subjected to a concentric tube in which the internal and external pressure gradually decreases to zero. . Figure 2-17b shows the distribution of radial stress. Due to the symmetrical distribution of the stresses, although they are all negative, they can still be balanced with each other. The two longitudinal and radial residual stress distribution patterns that have been shown determine the distribution of the circumferential stress pattern. In fact, due to the radial compressive stress, the transverse stress in the peripheral annular layer can only be tensile. Therefore, in order to balance the stress between the layers, the transverse stress in the center of the inner annular layer must be the compressive stress.

When hot extrusion is performed, the above-mentioned stress state is often changed due to the subsequent cooling of the industrial aluminum extrusion product, and this change is sometimes quite obvious. For example, when slowly cooled, it can often result in a result similar to when performing a low temperature annealing, ie, the residual stress may be almost completely eliminated. In this profile with small surface area, the possibility of this slow cooling pattern is greater due to the large thermal inertia.

In the extrusion of large-diameter bars and thick-walled industrial aluminum profiles, in addition to changes in the stress state caused by the transformation of the structure, new residual stresses may also be generated due to the uneven cooling of the surrounding layers and the center layer.

The rapid cooling of the surrounding layers will initially lead to shrinkage of the surrounding layers and increase of the longitudinal tensile stress, which may then disappear due to the influence of the inner layer heat. However, the inner layer metal shrinks due to cooling, so that the longitudinal layer compresses stress in the peripheral layer, and the inner layer induces tensile stress. At the same time, hoop stress may also occur.

Here, you should notice the effect of the diameter (or wall thickness). Obviously, the smaller the diameter (or wall thickness), the smaller the thermal inertia, the faster the temperature of the longitudinal layer is balanced, and the less likely there is a temperature residual stress. Under the condition of asymmetric deformation, due to the appearance of an asymmetrical residual task, the total residual stress value is increased.

The direct result of asymmetrical residual stresses is the warpage of industrial aluminum extrusions. Under conditions of non-uniform temperature, asymmetrical residual tasks may also occur. When extruding a tube that has become cold, more stress is generated because the tube encloses the hot needle. This stress can cause longitudinal cracks in the pipe. Therefore, it is of great significance to reduce the deformation and temperature field inhomogeneity in the deformation zone.