Aluminum extrusion: a little theory
The most important operation in the production of aluminum profiles is pressing. The pressing of aluminum and its alloys is a process of plastic deformation, in which the workpiece under pressure is forced to flow through one or more points of the matrix with a cross-sectional area much smaller, than the original workpiece. Press Temperature Range, the magnitude of the flow stress and the friction conditions of the metal on the tool determine the characteristics of pressing aluminum alloys. The temperature range for pressing aluminum profiles is from 350 to 550 ° C and depends on the type of alloy, requirements for the quality of the surface of the profile and its mechanical properties.
Aluminum and aluminum alloy profiles are almost always extruded in direct contact with the container and die, which are made of heat resistant steels. Aluminum has a significant chemical affinity and adhesion to iron: even in a solid state, it tends to "stick" to the surface of a steel tool. A feature of aluminum pressing is, that when the pressure in the container increases, the frictional force between the steel wall of the container and aluminum is so great, that shear plastic deformation of aluminum in the inner layers of the workpiece begins earlier, than its slipping on the container wall.
To understand the patterns metal pressing – its course – apply the theory of plasticity. With simple uniaxial compression or tension, the metal begins to flow plastically when the stress exceeds the yield strength. The plastic flow of aluminum during pressing is a complex three-dimensional shear flow. The peculiarity of the shear flow of aluminum and its alloys from other metals is that, that the inner layers of the aluminum billet begin to deform plastically first, and its peripheral layers - later. The flow of metal during pressing depends on many factors, such as:
- material properties of the workpiece;
- friction conditions between the container and the workpiece, metal and matrix;
- compaction ratio.
The classical types of flow during the pressing of a homogeneous material with different friction conditions in the container and matrix are shown in the figure. 1.
Types of aluminum flow
Flow type S occurs in the absence of friction in the container and matrix. Such a flow would ensure uniform properties of aluminum profiles. This type of flow can be regarded as purely theoretical., as aluminum is almost never pressed with grease. In this case, dead zones of the metal flow do not occur..
Matrix Friction Flow
Type A flow occurs when pressing with friction on the matrix mirror and in the absence of friction in the container. This is typical for back pressing.. In the center of the workpiece, metal moves faster, than on the periphery. In the corners of the front end of the workpiece, dead zones of the "fixed" metal are formed between the matrix mirror and the container wall. This dead zone forms immediately after being pressed.. The workpiece material flows along the conical border of this zone as a result of shear plastic deformation and moves “diagonally” to the hole of the matrix to form the outer layers of the aluminum profile.
Current with “a shirt”
In a type B flow, metal friction occurs as a container, so about the matrix. This happens with direct compression.. This case is characterized by a more intense shear strain., than with type A flow. The surface of the workpiece is stationary relative to the wall of the container, and shear deformation reaches a maximum immediately below the surface layer. A dead zone is formed in front of the matrix mirror.. The surface of the aluminum profile is not formed from the surface layer of the workpiece, and the inner layers of the workpiece, which move along a dead zone boundary by a shear flow. The final profile has significantly more heterogeneous properties compared to the profile, obtained by type A flow. The surface layer of the workpiece, which first “sticks” to the container wall, and then “shaves off” with a suitable press washer. Material, which is assembled and compressed before the press washer, contains material from the so-called inverse layer of the ingot with a high oxide content, alloying elements and impurities. This surface shell is sometimes called “a shirt”, and the pressing method itself – shirt pressing.
Aluminum Flow Factors
Thus, pressed properties aluminum profiles to a large extent depend on the characteristics of the metal flow during pressing. The flow of metal is influenced by many factors:
- Pressing method, direct or reverse.
- The maximum press force, container size and shape.
- Friction processes on the matrix and container.
- A type, matrix dimensions and design.
- Workpiece length and type of alloy.
- Compression ratio (stretch ratio).
- Temperature of die and press tool.
- Extrusion speed.
The degree of deformation of the metal depends on type, matrix sizes and designs. for instance, in hollow matrices, much more energy is spent on material deformation, than in solid. As I mentioned, in the corners between the container and the matrix a cone-shaped "dead zone" is formed. The material in its movement along the container to the matrix is deformed by a shift along the boundary of this zone. This dead zone plays the role of the conical surface of the matrix. The dimensions of this dead zone are determined by the angle a this conical surface in relation to the container axis (figure 2). This angle depends on the pressing ratio., material flow stress, coefficient of friction between the workpiece and the container, as well as between the current metal and the matrix mirror. With the same coefficient of friction between the container and the workpiece, the conical half-angle of the dead zone depends on the pressing ratio: with an increase in the pressing ratio, this angle increases and the length of the shear line decreases.
At the end of the pressing cycle of each workpiece, the remainder of the workpiece is left – press residue - thicker than the length of the pressing dead zone. This prevents the formation of surface and subsurface defects in the aluminum profile due to the ingress of material from blank surface layer. In industry, the press balance is usually “held” in the range from 5 to 15 % from the original length of the workpiece. Since the thickness of the press residue is related to the dead zone of pressing, it also depends on the pressing ratio, type of matrix, workpiece temperature, friction conditions between the container and the workpiece, as well as the flow stress of the workpiece material. The figure 2 shows the relationship between the thickness of the press residue and the conical surface of the dead zone of pressing. Stopping the pressing at a safe distance from the dead zone prevents unwanted metal and non-metallic inclusions from entering the pressed product, which accumulate in it.
Usually, at each press production, its own experience is developed on the permissible minimum thickness of the press residue under various pressing conditions, which is confirmed by the results of etching the sections of press residues on the macrostructure. As a result, there is an understanding of the change in the conical angle of the dead zone (and the permissible thickness of the press cake) with a change in the pressing parameters., shape and size of the matrix point (number of points) and the type of matrix (solid or hollow).
1. Aluminum Extrusion Technology / Saha P.
2. Extrusion, еd. M.Bauser et al, 2006.