Pressing aluminum profiles may become impossible, if the required pressing force exceeds the maximum pressing force or if the pressing temperature, for example, exceeds the solidus temperature aluminum alloy. For the correct and economical operation of expensive pressing equipment and tools, it is necessary to clearly correlate the pressing parameters for a specific aluminum profile from a specific aluminum alloy: first of all, initial billet temperature, pressing speed, material flow stresses and pressing ratio (figure 1).
The ratio of the pressing R matrices with n points is given by the formula R = AC/nAE, Where n – number of symmetrical matrix points, AC- square cross-section of the container cavity, AE - cross-sectional area of the aluminum profile. The level of mechanical work depends on the pressing ratio, which must be spent when pressing through this matrix. When the ratio of pressing the die (or the pressed profile) is small, degree of plastic deformation of the material, that is, its so-called elaboration, will also be insignificant. The structure of such a material will be similar to a coarse-grained cast structure and will have a reduced strength.. It is known, what aluminum profiles, which have been pressed with a pressing ratio of less 10, cannot guarantee mechanical and physical properties, given by standards. Normal pressing ratio, for example, alloy 6060 (AD31) - from 10 to 100.
Hot metal pressing relate to the processes of hot metal forming. Hot working is defined as deformation under conditions, when the temperature and rate of deformation are, that the process of recovery of strain hardening takes place almost instantly and simultaneously with deformation. Aluminum extrusion is carried out at elevated temperatures, since it does not have sufficient ductility at room temperature and, Moreover, this ensures a reduction in pressing forces. Temperature is one of the most important pressing parameters. With increasing temperature, the flow stress decreases, and pressing requires less effort and can be pressed at a higher speed. However, an increase in the pressing speed leads to, in turn, to increase the pressing temperature. This temperature should not exceed the solidus temperature of the alloy - the temperature, at which local melting of low-melting components of an aluminum alloy begins.
The degree of resistance of aluminum alloy to pressing depends, including, on strain rate. Therefore, an increase in the speed of movement of the ram leads to an increase in the pressing pressure. With an increase in the pressing speed, the rate of increase in the profile temperature also increases.. The deformation rate is directly proportional to the speed of movement of the ram, and the amount of heat generated in proportion to the deformation speed. The slower the stamp moves, the more time it takes to dissipate the generated heat. The relationship between the speed of the ram and the speed of the aluminum profile at the exit from the die (pressing speed) is easily calculated from the condition of constant volume at the inlet and outlet from the press: VE = VRR, Where VE and VR – pressing speed and ram travel speed, respectively, a R - the ratio of pressing.
Metal flow stress σ – this is instantaneous voltage value, which is required to continue plastic deformation of the material. Roughly speaking, yield stress is the yield stress of a material as a function of strain rate, temperature. Flow stress is an important parameter, since in the process of plastic deformation the forming force or stress is a function of the geometry of the cross section of the aluminum profile, frictional conditions and flow stress of a deformable material. The flow stress is influenced by the following factors: 1) the chemical composition and metallurgical structure of the aluminum alloy; 2) deformation temperature, strain rate and strain rate. Uniaxial tensile tests are used to determine the flow stress of aluminum alloys., uniform compression and torsion test. The relationship between the flow stress of aluminum alloys and the rate of their pressing is shown in the figure 2 .