Cold Extrusion Or Hot Extrusion

Metal extrusion is a forming process, like other metal forming processes, it can be performed either hot or cold. The characteristics of hot forming and cold forming were discussed in detail in the fundamentals of metal forming section.

Hot forming, or hot working, involves working a metal above its recrystallization temperature. Hot working has many advantages in the improvement of the mechanical properties of the part's material. Cast metal contains pores and vacancies throughout the material. Hot working will push and redistribute material, closing up these vacancies. Impurities in molten metal usually combine together in masses upon hardening, forming solid inclusions within the metal. These inclusions cause weakness in the surrounding material. Hot working causes these inclusions to break up and distributes them throughout the mass of metal. Large, irregular, columnar grain structures are usually present in cast parts. Hot working a metal will break up irregular structures and recrystallize the mass of material into a finer wrought grain structure. Mechanical properties of the part, such as impact resistance, ductility and strength characteristics, are improved. If a hot extrusion is performed on a cast work piece, then the advantages of hot working will be imparted to the work. However, most metal extrusions in manufacturing industry are performed on billets that have already been hot formed, thus the mechanical advantages of hot forming have already been imparted to the material.

In addition to the improved physical characteristics of the metal, hot working does offer other advantages in a manufacturing process. A metal above its recrystallization temperature is more easily manipulated than a cold metal. An increase in temperature results in a corresponding decrease in strength and an increase in ductility, factors more advantageous in the forming of the metal.

When metals are worked above their recrystallization temperature strain hardening does not occur, thus hot working allows for a large amount of shape change. One of the major disadvantages of hot working of metals is the oxidation that occurs over the surface of the hot work. This results in a layer of oxide scale build up on the external surfaces of the work piece. Scale can affect surface finish and accuracy of the part as well as increasing friction and wear at die metal interfaces. Heating to, and maintenance of, high working temperatures, decreased tolerances, and increased die wear, are all disadvantages of a hot forming manufacturing process over a cold one.

Choosing between hot extrusion or cold extrusion will depend on the specific details of the manufacturing process. Some of the more difficult to form materials may have to be worked hot. Some easy to extrude metals, such as aluminum, can be worked either hot or cold depending upon other factors in the process. Hot extrusion of metal is generally preferred for larger parts, more extreme changes in shape and extrusions with more complex geometry. Cold extrusion of metal is usually used for smaller parts, less complex shapes, more workable materials and the manufacture of discrete extrusions that create a single part with each operation. Impact extrusion, a discrete manufacturing process, is most often performed cold.

Advantages of cold extrusion over hot extrusion include, not having to heat the work, higher production rate, no oxidation and scale form on surfaces, greater geometric accuracy, better surface finish and the ability to strengthen the part by way of strain hardening. In hot extrusion, like other hot forming processes, the heat transfer between the work piece and the cooler surfaces of the die presents a problem during the manufacturing operation. In order to mitigate this issue, die used for extruding can be preheated to lessen the temperature gradient. Lubricants also help in the reduction of heat transfer between the part and the mold. With some particularly difficult to extrude materials isothermal extrusion may be employed, this is similar in concept to isothermal forging. In these instances, the mold is maintained at, or slightly below, the temperature of the work during the entire process.