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TEKCT4A CONVENTIONAL HOT, WARM AND COLD EXTRUSION

1. Extrusion is one of the youngest of the metal-forming processes and was probably first developed at the end of the eighteenth century for the manufacture of lead pipes. (The process is one whereby slug or billet of metal is forced to flow, under high pressure, through a die shapadVJ^ -give the required cross-section to the prod­uct. An extraordinary diversity of special sections can be produced by extrusion in a rapid and economical man­ner with high dimensional accuracy upon which finishing and machining operations are reduced to a minimum or dispensed with them altogether. The_r versatility of the method has also been extended by combined forging and extrusion processes.

2. There are two common methods of working, known as direct (forward) and reverse (indirect, inverted, back-' ward) extrusion. In direct extrusion, the punch and prod­uct travel in the same direction, in the reverse method, they travel in opposite directions. The form of the die to be most frequently adopted is the square or 90° die. Until recent years, extrusion was almost exclusively a hot-working operation, though collapsible tubes and other such products were worked cold from the softer metals by impact extrusion.

3. In conventional hot extrusion, the billet is generally extruded above its recrystallization temperature. The biggest advantages of this process are the ability to achieve a large billet reduction, produce a long product from a relatively short billet, and extrude at relatively low pres­sures (generally less than 180,000 psi).¹ The high reduction is possible because of the billet's low flow stress and the absence of strain-hardening effects.

Conventional hot extrusion is usually conducted in a horizontal hydraulic extrusion press. In some cases, ver­tical hydraulic presses are used when sufficient 'day­light' is available above or below the press² to accommo­date the extrusion products. The hot extrusion process is usually selected in order to make long products (prod­ucts about 15 ft in length and greater).

4. The lubrication system used in conventional hot extrusion depends largely on the billet material. The glass lubrication process is used frequently for the extru­sion of high-strength materials such as steels, titanium, superalloys. Lubricants with solid-film additives are often used for moderately high-strength materials³ such as copper, brasses and others. In most cases, low-strength materials such as aluminium alloys are extruded without any billet lubrication. However, there is a serious problem in conventional hot extrusion of high-strength aluminium alloys. These alloys are'sensitive to surface cracking when the extrusions reach,excessive temperatures at high exit speeds. To avoid this, it is necessary to extrude extremely slowly — at exit speeds in the order of 1-5 fpm. This means that it could take one or more hours just to extrude, one billet. Consequently, every specialist knows the con­version cost for these materials to be quite high.

5/Warm extrusion is considered to be a relatively new approach. It is defined as a process in which the billet is heated to some point below its recrystallization

temperature with the primary aim of reducing its flow strength and, in some cases, improving its ductility. 6. Cold extrusion proved to be an established, pro­duction process widely used in industry. It is defined as an extrusion process in which the billet and tooling are initially at room temperature.

Conventional cold extrusion has demonstrated itself^ to be a precision metalworking technique. Close-tolerance^ parts with excellent surface finishes are produced in steels and other high-strength materials. In spite of the high pressures used for extruding steels (typically 250,000— 300,000 psi, and occasionally higher) the good lubrication ensures minimal tooling wear even over thousands of extrusions.⁴

Cold extruded steel parts are produced in solid and tubular form and in a variety of shapes. With the develop­ ment of hydrostatic extrusion technology, it now appears that the capabilities of the cold extrusion process could be extended even further.