Machinery's Handbook, 31st Edition
1520 EXTRUSION OF METALS process can be performed at either room or high temperature, depending on the alloy and method. Cross sections of varying complexity can also be produced, depending on the materials and dies used. In the specially constructed presses used for extrusion, the load is transmitted by a ram through an intermediate dummy block to the stock. The press container is usually fitted with a wear-resistant liner and is constructed to withstand high radial loads. The die stack consists of the die, die holder, and die backer, all of which are supported in the press end housing or platen, which resists the axial loads. The following are characteristics of different extrusion methods and presses: 1) The movement of the extrusion relative to the ram. In “direct extrusion,” the ram is advanced toward the die stack; in “indirect extrusion,” the die moves down the con tainer bore 2) The position of the press axis, which is either horizontal or vertical 3) The type of drive, which is either hydraulic or mechanical 4) The method of load application, which is either conventional or hydrostatic In forming a hollow extrusion, such as a tube, a mandrel integral with the ram is pushed through the previously pierced raw billet. Cold Extrusion: Cold extrusion has often been considered a separate process from hot extrusion; however, the only real difference is that cold or only slightly warm billets are used as starting stock. Cold extrusion is not limited to certain materials; the only limiting factor is the stresses in the tooling. In addition to the soft metals such as lead and tin, aluminum alloys, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel can be extruded cold or at low deformation temperatures. Cold extrusion has many advantages, such as no oxidation or gas/metal reactions; high mechanical properties due to cold-working if the heat of deformation does not initiate recrystallization; narrow tolerances; good surface finish if optimum lubrication is used; fast extrusion speeds can be used with alloys subject to hot shortness. Examples of cold extruded parts are collapsible tubes, aluminum cans, fire extinguisher cases, shock absorber cylinders, automotive pistons, and gear blanks. Hot Extrusion: Most hot extrusion is performed in horizontal hydraulic presses rated in size from 250 to 12,000 tons. The extrusions are long pieces of uniform cross sections, but complex cross sections are also produced. Most types of alloys can be hot extruded. Owing to the temperatures and pressures encountered in hot extrusion, the major problems are the construction and the preservation of the equipment. The following are approximate temperature ranges used to extrude various types of alloys: magnesium, 650–850 ° F (343–454 ° C) aluminum, 650–900 ° F (343–482 ° C) copper, 1200–2000 ° F (649–1093 ° C) steel, 2200–2400 ° F (1204–1316 ° C) titanium, 1300–2100 ° F (704–1149 ° C) refractory alloys, up to 4000 ° F (2204 ° C). nickel 1900–2200 ° F (1038–1204 ° C) In addition, pressures range from as low as 5000 psi (34.5 MPa) to over 100,000 psi (690 MPa). Therefore, lubrication and protection of the chamber, ram, and die are generally required. The use of oil and graphite mixtures is often sufficient at the lower temperatures; while at higher temperatures, glass powder, which becomes a molten lubricant, is used. Extrusion Applications: The stress conditions in extrusion make it possible to work materials that are brittle and tend to crack when deformed by other primary metalworking processes. The most outstanding feature of the extrusion process, however, is its ability to produce a wide variety of cross-sectional configurations; shapes can be extruded that have complex,
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