Plastic working in which a billet in a container is pushed by a ram through a die opening to make long products of constant cross section. Aluminum profiles are a typical example.
Japanese name: 押出し
Extrusion appears in 1 practice question across 1 topic, including Materials processing and joining.
Extrusion is a process in which a heated billet placed in a container is pushed by a ram and forced out through a die opening. Besides direct extrusion, in which the billet slides forward within the container, and indirect extrusion, in which the die side moves to eliminate friction between billet and container, there is also hydrostatic extrusion, in which pressure is applied through a liquid surrounding the billet. The amount of working is expressed by the extrusion ratio R = A₀/A, the ratio of the original cross-sectional area to that of the product, and for aluminum alloys ratios of more than several tens are possible. Because the material is under compressive stress, a large deformation can be given in one pass.
Drawing also reduces the cross-section by passing material through a die, but because it pulls on the product, the product tears if the drawing stress exceeds the material's strength. Even in the ideal case, ignoring friction and other effects, the drawing stress is Y ln R (with Y the flow stress), so ln R cannot exceed 1 and the reduction in area in one pass is limited to at most about 63%. Extrusion pushes the billet from behind and has no such limit; for R = 40 the ideal pressure is Y ln R ≈ 3.7Y. The required pressure is higher, but a large change in cross-section can be given at once.
Hollow sections such as aluminum window frames and heat-exchanger tubes are made with porthole dies (bridge dies). Inside the die the material is first split into several streams, which rejoin around a mandrel and are pressure-welded together, so the product retains longitudinal weld seams. For 6000-series aluminum alloys the billet is heated to about 400 to 500°C, and the section leaves the die even hotter because of deformation heating; if it is cooled by air or water right after leaving the die and then aged, it gains strength without a separate solution treatment (T5 temper). If the extrusion speed or temperature is too high, heat from friction and deformation causes fine surface cracks, so conditions are controlled for each alloy.
Pushing the billet out to the very end creates a defect in which surface oxides and dirt are drawn into the center, so the end portion is discarded rather than used as product. Under conditions of low extrusion ratio and large die angle, arrowhead-shaped internal cracks (chevron cracks) can also form. Besides aluminum alloy sections, extrusion is used for copper alloy rods and tubes, seamless steel tubes and titanium alloy members. For materials extruded at high temperature such as steel, glass powder can be used as a lubricant. Extrusion molding, which feeds polymers or foods continuously with a screw, follows the same idea of making products of constant cross-section.
Answer: B. About 139 MPa
Cross-sectional area is proportional to the square of the diameter, so the extrusion ratio is R = (100/25)² = 16. Then p = Y ln R = 50 × ln 16 ≈ 50 × 2.77 ≈ 139 MPa. About 69 MPa takes the logarithm of the diameter ratio of 4, and about 800 MPa uses Y × R without the logarithm. The actual extrusion pressure is considerably higher than this ideal value because of friction with the container wall and die and the redundant shear deformation inside the material. Indirect extrusion, in which the die moves and there is no sliding between billet and container, requires less pressure than direct extrusion, in which the billet slides through the container.
It is the cross-sectional area of the billet before extrusion divided by that of the extruded product. For round bar it is the square of the ratio of diameters. The larger the extrusion ratio, the larger the deformation and the required extrusion pressure, which in the ideal case is proportional to the natural logarithm of the extrusion ratio.
Direct extrusion uses simpler equipment and suits products of many shapes, but needs extra force to overcome friction between billet and container. Indirect extrusion has little friction, so less force is needed, the billet heats up less and quality is more uniform, but because the product is taken out through a hollow ram, its size and shape are restricted.
Because the material passes straight through the shape of the die opening. Changing the shape of the opening allows L shapes, H shapes and complex hollow sections. If material flows at different speeds through thick and thin parts of the section, however, bending and twisting result, so the die is designed to even out the flow.
Cross-sectional area is proportional to the square of the diameter, so the extrusion ratio is R = (100/25)² = 16.
An aluminum alloy billet 100 mm in diameter is hot extruded…Carbon equivalent Heat-affected zone Hot isostatic pressing Investment casting Sensitization Shrinkage cavity Springback