metal pressing, 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).
Pressing of aluminum and aluminum alloy
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.
Pressing = extrusion
The English term «extrusion» just indicates the term "ejection, squeezing ". The compression can be hot or cold depending on the alloy and the method used. When hot-pressing the preform 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 – aluminum extrusions. Under profiles often understand all kinds of molded products, Although standards differ "profiles", "Pipe", "Rods" and "strip" and even chipboard 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 molded and extruded wire semi-finished products for further processing: rolling, drawing, bending, etc..
The main methods of pressing aluminum and aluminum profiles are two – direct and back. Most industrial presses operate direct compression method.
The principle of direct compression 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 rods and complex special profiles), tubes and hollow profiles. When direct compression direction of metal flow coincides with the direction of movement of the slide. 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.
When back pressing aluminum, the matrix is located at the front end of the hollow ram and moves with it into 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. load and pressure changes with the advancement of the ram for pressing the reverse shown in Figure 2. Reverse compression is usually used for pressing the pressed hard alloys and rigid requirements on the cross-sectional size and homogeneity properties over the cross section and length of the profile.
compression molding “joint to joint”
For the extrusion of aluminum alloys, which are easily welded at the temperature and pressure of pressing, they use the "joint-to-joint" method: each subsequent workpiece is loaded into the container at the joint of the previous. The length of the extruded profile in the process is limited by the length of the output section 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.
compression molding “joint to joint” without removal of the discard
Much less often the "joint-to-joint" pressing option is used without removing the press residue: the end of the next workpiece moves in the container close to the end of the remainder of the previous workpiece, still in the container (figure 5). Compression without removal of the discard 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.
Legkopressuemye aluminum alloys
For pressing "butt joint" requires the ability of aluminum alloys welded at a temperature and pressure compaction. 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).
Difficulty pressed aluminum alloys
So-called hard alloys compressible, 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 pressed using the joint-to-joint method. To apply them separately compressing each blank.
Source: Saha K. P.