Pressing and friction
Understanding friction processes, which occur during pressing aluminum and aluminum alloy, very important to ensure dimensional and shape accuracy aluminum profiles, as well as high quality of their surface. The degree of participation of friction in pressing process strongly depends on, which pressing method is used - direct or reverse. The figure 1 friction forces are shown, which arise during direct pressing, and Figure 2 - with back pressing. In both cases, flat dies are used - dies without any fillets at the metal entrance. Note, that during back pressing the dead zone of the metal has an angle αi a lot more, than the angle αd in direct pressing. This results in a thinner press cake with the same profile.. The nature of the metal flow during reverse pressing with a flat die is very similar to that, which could be with lubricated direct pressing.
Physics of friction
Friction is resistance to relative motion, which arises, when two solids are in contact with each other. Strength, which is necessary, to overcome this resistance, and which is directed in the opposite direction of movement, and there is friction force. Known from the school desk, Coulomb's law is as follows: Ftr = μN, where μ is the coefficient of friction, N - the projection of the acting force on the normal to the plane of friction and Ftr – friction force. This friction pattern works very well, while the contact surface of solids is relatively lightly loaded, and their surfaces do not contact each other over the entire surface, but only along the protruding "hills" (figure 3(and)). This model is for the pressing process, where the contacts between bodies are much denser, and the pressures are much higher, no longer fits.
Friction in workpiece-container contact
The area of contact between two bodies increases with increasing contact pressure as shown in the figure 3(and). The frictional force is directly proportional to the actual contact area. During the crimping of the workpiece, the initial incomplete contact area AR gradually becomes equal to the full area AA, as shown in Figure 3(b). With direct pressing, frictional forces arise between the workpiece and the container, and also between the moving metal and the stationary metal of the conical dead zone.
Two conditions of friction
Two friction conditions are possible: 1) adhesion friction: there is no relative movement between the workpiece material and the container and 2) sliding friction: the flow of metal through the lubricant in contact with the container. When aluminum is pressed, the first variant of the friction condition is realized - adhesion friction. The model of adhesion friction during pressing gives an expression for the friction force Ftr : formula Ftr = MKAA, Where m - coefficient of friction, k - material shear strength, AA - full contact area. For sticking friction, m = 1, whereas with good lubrication m tends to zero. Friction stress τtr in the case of sticking friction has the form: τtr = k = σt/√3, Where k is equal to σt/√3 according to von Mises plasticity criterion, a pt - material flow stress.
Friction of metal movement on “dead zone”
The dead zone of the metal is shown in the figure 1 for material, which is pressed through a square die - die, in which the girdle surface is perpendicular to the matrix mirror. In this case material in the corners between the matrix and the container does not participate in the flow, but sticks to the matrix mirror, forming a tapered channel, through which the workpiece material flows to the die. The friction between the metal of the dead zone and the flowing metal cannot be greater, than the shear stress of the material and is given by the expression τtr = k = σt/√3.
Friction of metal movement through the belt
INThe friction conditions of the metal on the girdle of the matrix change during pressing. Due to the strong adhesion of aluminum and the matrix material - heat-resistant tool steel - aluminum adhesion occurs. Matrix surface treatment, for example, nitriding, increases the hardness of the belt and reduces the intensity of adhesion of aluminum to it. The figure 4 schematically shows a typical morphology of aluminum adhesion on a matrix shoulder in a section, perpendicular to the pressing direction. At the beginning of pressing, there are zones and adhesions on the die belt, and sliding (figure 4(and)), but after several pressing cycles, aluminum adheres to the entire surface of the belt (figure 4(b)).
A source: Saha P.