Aluminum extrusion: principles
Metal extrusion
metal extrusion, in particular, aluminum and its alloys, called plastic deformation process, wherein the cold or heated preform, usually part of a round ingot ("pillar"), is pushed through a special tool with one or more holes of a given shape - a matrix (figure 1).
Extrusion of aluminum and aluminum alloys
For aluminum extrusion used special equipment - extrusion presses, usually, hydraulic, which provide on the rod (ram), which directly "pushes" the workpiece, effort by 500 to 4000 tonnes, and sometimes more, depending on the application and performance of the press.
See. also Aluminum extrusion: from billet to finished profile
The English term «extrusion» just indicates the term "ejection, squeezing ". Extrusion can be cold or hot depending on alloy and method used. In hot extrusion, the billet is preheated to facilitate plastic deformation..
profiles, tubes, rods, stripes
Aluminum profiles, which receive compression, but not, let us say, rolling or flexible, and called – extruded aluminum profiles. Profiles are often understood as all types of extruded products., Although standards differ "profiles", "Pipe", "rods" and "strips" and even panels, for example, for rolling stock- and aircraft. profiles, in turn, may be, solid and hollow, standard and special, pipes - general purpose and seamless, and bars - round, square, rectangular, hexagon. And there are also extruded wire and extruded semi-finished products for further processing: rolling, drawing, bending, etc..
The main methods of aluminum extrusion and aluminum profiles are two – direct and back. Most industrial presses work by direct extrusion.
Direct extrusion
Figure 1
The principle of direct extrusion is clearly visible from the figure. 1. The heated preform is placed in a container and by a so-called ram, "Pushed" through the matrix. Direct extrusion is used in the production of solid profiles (simple bars and complex special profiles), tubes and hollow profiles. With direct extrusion, the direction of metal flow coincides with the direction of movement of the press ram. During this process the preform slides relative to the container walls. To overcome arises when this friction force spent a considerable part of the efforts of the press. A plot of the load or pressure on the displacement of the slide type generally has a, shown in Figure 2. There are three stages of the process: 1 - unpressurization (upsetting) of the workpiece and the beginning of pressing, 2 - the pressure drop and the, what is called a steady state, 3 - the pressure is at its minimum, followed by its sharp rise in the formation of the discard.
Figure 2
Reverse extrusion
During the reverse extrusion of aluminum, the matrix is located at the front end of the hollow press ram and moves with it inside the container (Figure 3) . In this case there is no relative movement between the workpiece and the container. Therefore no frictional forces between the workpiece surface and the container. There is also no movement of the center of the workpiece relative to peripheral portions of the workpiece. The change in load and pressure with the advance of the reverse extrusion ram is shown in the figure. 2. Reverse extrusion is usually used for the extrusion of hard-to-extrude alloys and with stringent requirements for cross-sectional dimensions and uniformity of properties over the section and length of the profile..
Figure 3
Direct extrusion “joint to joint”
For aluminum alloy extrusion, which are easily welded at extrusion temperature and pressure, use the butt-to-butt method: each subsequent billet is loaded into a container at the joint of the previous. The length of the extruded profile in this process is limited by the length of the exit table and can reach 50 and more meters. Final welding workpiece in the container with the next workpiece takes place, when the joint passes through the matrix deformation zone. The principle of the method of pressing "joint to joint," with the removal of the discard shown in Figure 4: the new workpiece is welded to the remaining metal in the die or the so-called feeder, remaining from the previous workpiece.
Figure 4
Direct extrusion “joint to joint” without removal of the discard
Much less often, the “butt-to-butt” extrusion option is used without removing the press residue: the end of the next billet is moved in the container close to the end of the residue of the previous billet, still in the container (figure 5). Extrusion without removing the press residue requires additional costs, including, to ensure a clean surface of the preform and especially their ends. Therefore, such a molding is mainly used only in special cases, for example, in the manufacture of blank products (rolling, semi-finished product) in coils for further processing, for example, drawing.
Figure 5
Easily extrudable aluminum alloys
Butt-to-butt extrusion requires the ability of aluminum alloys to be welded at extrusion temperature and pressure.. These qualities are possessed by the so-called easily pressed ("soft") alloys: all grades of aluminum, alloys of Al-Mn (series 3xxx) and alloys of Al-Mg-Si (6xxx series), including the most popular alloy AD31, as well as "normally" extruded ("medium-strength") alloys: Al-Mg (5xxx series) with magnesium content up to 3 %, and Al-Zn-Mg alloys (7xxx series) without copper alloying, for example, 1915 GOST 4784 (7005 according to the international classification).
Difficult-to-extrude aluminum alloys
So-called hard-to-extrude alloys, which include all the alloys Al-Cin–Mg (series 2xxx), alloys Al–Mg (series 5xxx) with a magnesium content of more than 3 % and alloys Al–Zn–Mg-Cu (series 7xxx) is usually not extruded by the butt-to-butt method. For them, extrusion of each workpiece is used separately..
Source: Aluminum Extrusion Technology / Saha P.
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