Metal extrusion is a metal forming process where a metal billet is forced through a die chamber in a manner similar to squeezing toothpaste from a tube. A wide variety of solid or hollow cross-section profiles may be produced by extrusion. (Refer picture as reference)
Typical products made by extrusion are railings for sliding doors, window frames, and numerous structural and architectural shapes. Commonly extruded materials are aluminum, copper, and low carbon steel.
Depending on the natural ductility of the material, extrusion may be carried out at room or elevated temperatures (in most cases). Extrusion at room temperature is often combined with forging operations, in which case it generally is called cold extrusion. In the lighting industry, the cold extrusion process is widely used in producing pin fin heat sink that is a key component of the LED light.
The most common process is called direct or forward extrusion, in which a billet is placed in a chamber and forced through a die opening by a hydraulically driven ram as the above figure indicated.
In indirect extrusion, also called backward extrusion, the die moves toward the billet, forcing material flow backward through the die in the hollow plunger or ram. Indirect extrusion has the advantage of having no billet-container friction, hence there has no extrusion pressure affected by the length of the billet
In hydrostatic extrusion, as the right figure showed, the billet is smaller in diameter than the chamber that is filled with fluid. The pressure is transmitted to the fluid by the plunger or ram. There has no friction between material and container surface, the fluid pressure is enforced around all the way on the billet material and thus improves material formability.
Cold extrusion is a general term that often denotes a combination of direct or indirect extrusion and forging as the picture indicated.
Cold extrusion has the below advantages over hot extrusion:
- The cold work hardening improves mechanical properties
- Precise dimension tolerance may be obtained, reducing the needs for further machining or finishing operation
- Improved surface finish due to the absence of surface oxidation film and effective lubrication
- Production rate would be much high and cost-competitive if produced through the machine forging process introduced in the article “Metal forging: the process to forge your strength and toughness”.
Lubrication is critical, especially with steels, because of the possibility of sticking (seizure) between workpiece and tooling. The most effective lubrication is a phosphate-conversion coating, followed by a coating of soap or wax.
As figured per the below picture, extrusion ratios, A0/Af, is the ratio of the cross sectional area of the billet to that of the extruded part.
The ratios, R, usually range from 10 to 100, and maybe higher for softer nonferrous metals or lower for less ductile materials, but has to be at least 4 to deform material plastically through the bulk of the workpiece. The ratio is a fundamental geometric parameter, once it is determined, the size of billet and extrusion die profile may be determined.
The extrusion force may be calculated as well based on selected process parameters, such as billet temperature, machine plunger travel speed, and type of lubricant used for cold extrusion. The extrusion force, F, can be estimated by the formula:
Where k is the extrusion constant determined by experiment and can be figured out from the correlation picture.
Extrusion and Die Design
Die materials for hot extrusion usually are hot-worked die steel. Coatings, such as partially stabilized zirconia, may be applied to the dies to extend their life.
Die design requires considerable experience. Hollow cross-sections can be extruded by welding-chamber methods and using dies knew as spider die as the picture indicated.
During extrusion, the metal divides around the spider legs on the internal mandrel, and flows into strands, these material strands then become rewelded under the high pressure in the welding chamber before they exit out the die.
The rewelded surface has good strength because they have not been exposed to the environment. The welding-chamber process is suitable only for aluminum alloy because of its capability to develop strong weld under pressure.
Design is for manufacturing. Extrusion profile design shall be beneficial for die manufacturing and ensure extrusion quality, the design shall avoid sharp corner and wall thickness extreme change, keep extruded profile symmetry to eliminate unbalanced extrusion pressure and profile deformation problem.
Extruded products may have several types of defects that are caused by the material condition and process variables. There have two principle extrusion defects: surface cracking and internal burst or porosity.
If extrusion temperature, friction, or speed is too high, the surface temperatures may rise significantly, which may cause surface cracking or tearing. The defect occurs, especially in aluminum and zinc alloys.
The center of extruded product may develop cracks, called center burst. These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die, a situation similar to the necked region in a tensile-test specimen.
The defect may be eliminated or minimized through decreasing die angle or increasing extrusion ratio and friction.
In sum, metal extrusion has a very broad application in many different industry sectors. It is the nearly net shape and high production rate manufacturing process, so it is efficient and cost-competitive.