The cold extrusion of metal powder
The hot extrusion of a "canned" refractory metal powder
- Powders can be consolidated by cold or hot extrusion.
- Cold extrusion of powders requires a considerable amount of binder (up to 50%) such as paraffin waxes, starches, benzol, resins or shellac. This has to be removed during sintering of the cold extruded material, giving a large amount of porosity. The process is used for the production of carbide twist drills and cutters.
- Hot extrusion of powders is usually carried out by “canning” the powders in a mild steel can, which is evacuated and sealed at its end. This prevents oxidation of the powders at the high temperatures (which can be as high as 1400˚C for some of the refractory metals). High extrusion ratios can be achieved, leading to good densification to near theoretical density.
- Process is a batch type process and in some instances has been superseded by the hot isostatic and Conform techniques.
- Cemented carbides are cold extruded with the addition of a suitable binder.
- The materials that are hot extruded in powder form include the following:
- SAP (sintered aluminium powder)
- refractory metals such as tungsten and molybdenum
- cermet fuel elements (UO2 in a stainless steel matrix)
- magnesium alloys
- nickel-based superalloys
- High temperature refractory metals have to be protected against oxidation by the canning technique.
- High strengths and good ductility can be achieved in hot extruded powder extrudates.
- Cold extrusion is mainly used to produce components with uniform cross sections, such as twist drills or milling cutter shapes.
- Hot extrusion is useful for the production of long lengths of rods, bars, tubes and other shapes.
- Canning technique necessitates removal of mild steel skin by machining or chemical techniques, and allowance should be made for the reduction in component dimensions.
This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.