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Aluminum Extrusion: Introduction

Aluminum Extrusion

By admin

31 Aug 2018

Extrusion of aluminum (or aluminum extrusion) is an industrial molding process for the production of long aluminum profiles of constant cross-section. Aluminum refers to both unalloyed aluminum grades and aluminum alloys.

The following is a brief overview of the aluminum pressing process. It is intended for the initial familiarization of the technical personnel of the press production with this technology, and can also be useful as information for management and support staff.

Extrusion press

Aluminum cylindrical rods - billets - are heated and pressed through a matrix with one or more holes to obtain a profile with a given cross section. The most common type of pressing is direct pressing, when the press stamp pushes the aluminum billet through a stationary (fixed) die. Most presses for this technology are horizontal hydraulic presses. The performance of the press depends on the size of the used dies, which can have a diameter of 100 to 1000 mm. For most extruded aluminum products, dies with a diameter of 175 to 250 mm are used. For dies of this diameter, extrusion presses with a force of 1,500 to 2,000 tons are required.

Direct pressing

A diagram of a typical extrusion press for direct aluminum pressing is shown in Figure 1.

1 - a stamp; 2 - a press washer; 3 - container; 4 - feeder; 5 - matrix; 6 - lining; 7 - bolster; 8 - matrix holder; 9 - matrix kit holder

The stamp (1) transfers the force from the main cylinder of the extrusion press to the aluminum billet, which is loaded into the container (3). A steel press washer (2) is mounted on the press stamp. The press washer enters the container with a very small gap, which prevents extrusion of aluminum (shown in black) into the container. The matrix (5) is part of the matrix kit, which is made of heat-resistant tool steels. The lining (6), the bolster (support ring) (7) and the matrix holder (8) and the matrix kit holder (9) support the matrix under pressure. A feeder (4) can be installed in front of the matrix in order to expand the flow of aluminum from the container to a wider section of the matrix.

Non-lubricated pressing

Since grease is not used in direct aluminum pressing, the outer layer of the workpiece is cut off by friction against the container wall. This favorably affects the quality of extruded aluminum products. The fact is that the outer layer of cast billets has a high content of iron-containing intermetallic particles and Mg2Si particles [1]. These contaminants can be sources of surface defects on the finished extruded aluminum product. With direct pressing without lubrication, they accumulate at the end of the workpiece.

Press residue and transverse weld

This part of the workpiece, which is called the press residue, is cut off before the new workpiece is loaded into the press. During the next working stroke of the press, aluminum from a new billet under the influence of high pressure and elevated temperature is welded to aluminum from the previous billet, which remained in the matrix. This creates a continuous extruded aluminum product. The weld point of two adjacent workpieces is called a transverse weld. Since the profile has reduced mechanical properties near this seam, as well as an unsatisfactory appearance, it is usually cut out and sent for re-melting.

Heating the workpiece and pressing tool

Before loading into the press, the aluminum billet is heated to 400-500 ºС, depending on the alloy. This facilitates the deformation process and minimizes strain hardening of aluminum. The heated preform is in a plastic (but not liquid) state. The container and die are also heated to prevent the workpiece from being chilled. The dimensions of the holes of the matrix do somewhat larger than the dimensions of the cross-section of the finished extruded profile since during cooling, aluminum is reduced more than the tool steel of which the matrix is ​​made.

 

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