What Is Metal Extrusion - Its Introduction, Production Process, Types, Applications, Advantages, Defects 

Introduction :-

Extrusion is a compression process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. The process can be likened to squeezing toothpaste out of a toothpaste tube. Extrusion dates from around 1800. A wide variety of solid or hollow cross sections may be produced by extrusion, which essentially are semifinished parts. Because a chamber is involved, each billet is extruded individually; thus, extrusion is a batch, or semicontinuous, process. Extrusion can be economical for large as well as short production runs. Tool costs generally are low, particularly for producing simple, solid cross sections.

Commonly extruded materials are aluminum, copper, steel, magnesium, and lead; other metals and alloys also can be extruded, with various levels of difficulty. Depending on the ductility of the material, extrusion is carried out at room or elevated temperatures. Extrusion at room temperature often is combined with forging operations, in which case it generally is known as cold extrusion.

Applications :-

Typical products made by extrusion are railings for sliding doors, Window frames, tubing Die having Various cross sections, aluminum ladder frames, and numerous structural and architectural shapes. Extrusions can be cut into desired lengths, which then become discrete parts, such as brackets, gears, and coat hangers. It has numerous important applications, including fasteners and components for automobiles, bicycles, motorcycles, heavy machinery, and transportation equipment.

Advantages of metal extrusion :- 

1. A variety of shapes are possible, especially with hot extrusion;
2. Grain structure and strength properties are enhanced in cold and warm extrusion;
3. Fairly close tolerances are possible, especially in cold extrusion; and
4. A characteristic of extrusion is that large deformations can take place without fracture. because the material is under high triaxial compression.
5. Since the die geometry remains unchanged throughout the operation, extruded products typically have a constant cross section.
6. In some extrusion operations, little or no wasted material is created. However, a limitation is that the cross section of the extruded part must be uniform throughout its length.

Types of Metal Extrusion :-

1. Hot Extrusion :-

For metals and alloys that do not have sufficient ductility at room temperature, or in order to reduce the forces required, extrusion is carried out at elevated temperatures. Hot extrusion involves prior heating of the billet to a temperature above its recrystallization temperature. This reduces strength and increases ductility of the metal, permitting more extreme size reductions and more complex shapes to be achieved in the process. Die wear can be excessive, and cooling of the surfaces of the hot billet and the die can result in highly nonuniform deformation. To reduce cooling of the billet and to prolong die life, extrusion dies may be preheated. Lubrication is critical in hot extrusion for certain metals and special lubricants have been developed that are effective under the harsh conditions in hot extrusion. Glass is sometimes used as a lubricant in hot extrusion; in addition to reducing friction, it also provides effective thermal insulation between the billet and the extrusion container.

Because the billet is hot, it develops an oxide film, Oxide films can be abrasive and can affect the flow pattern of the material. Their presence also results in an extruded product that may be unacceptable when good surface finish is important. In order to avoid the formation of oxide films on the hot extruded product, the dummy block placed ahead of the ram is made a little smaller in diameter than the container. As a result, a thin shell consisting mainly of the outer oxidized layer of the billet is left in the container. The skull is removed later from the chamber.

2. Cold Extrusion :

Developed in the 1940s, cold extrusion is a general term that often denotes a combination of operations, such as direct and indirect extrusion and forging. Cold extrusion and warm extrusion are generally used to produce discrete parts, often in finished form. Some important advantages of cold extrusion include increased strength due to strain hardening, close tolerances, improved surface finish, absence of oxide layers and high production rates. Cold extrusion at room temperature also eliminates the need for heating the starting billet.

Cold extrusion is used Widely for components in automobiles, motorcycles, bicycles, and appliances and in transportation and farm equipment. Cold-extruded parts weighing as much as 45 kg and having lengths of up to 2 In can be made, although most parts weigh much less.

DEFECTS IN EXTRUDED PRODUCTS :-

1. Surface Cracking :-

If extrusion temperature, friction, or speed is too high, surface temperatures can rise significantly, which may cause surface cracking and tearing. These defects occur especially in aluminum, magnesium, and zinc alloys, although they may also occur in high temperature alloys. They often occur when extrusion speed is too high, leading to high strain rates and associated heat generation. Other factors contributing to surface cracking are high friction and surface chilling of high temperature billets in hot extrusion. This situation can be avoided by lowering the billet temperature and the extrusion speed.

2. Centerburst :-

This defect is an internal crack that develops as a result of tensile stresses along the centerline of the work part during extrusion. Although tensile stresses may seem unlikely in a compression process such as extrusion, they tend to occur under conditions that cause large deformation in the regions of the work away from the central axis. The signifi cant material movement in these outer regions stretches the material along the center of the work. If stresses are great enough, bursting occurs. Conditions that promote centerburst are high die angles, low extrusion ratios, and impurities in the work metal that serve as starting points for crack defects. The diffi cult aspect of centerburst is its detection. It is an internal defect that is usually not noticeable by visual observation. Other names sometimes used for this defect include arrowhead fracture, center cracking, and chevron cracking.

3. Piping :-

It is the formation of a sink hole in the end of the billet. The type of metal-flow pattern in extrusion tends to draw surface oxides and impurities toward the center of the billet-much like a funnel. This defect is known as pipe defect, tailpipe, or fishtailing. As much as one-third of the length of the extruded product may contain this type of defect and thus has to be cut off as scrap. Piping can be minimized by modifying the flow pattern to be more uniform, such as by controlling friction and minimizing temperature gradients. The use of a dummy block whose diameter is slightly less than that of the billet helps to avoid piping. Another method is to machine the billet’s surface prior to extrusion, so that scale and surface impurities are removed. These impurities also can be removed by the chemical etching of the surface oxides prior to extrusion.