Friction extrusion was developed at TWI in the early 1990s, and employs frictional heat to generate the necessary heat required for extrusion. It has been demonstrated successfully with several aluminium alloys.
The required temperature is generated and maintained by frictional contact between the material to be extruded and the extrusion die. A high shear strain rate is developed at the boundary between the die and the extruded material,and within the plastic zones of the extruded material. Figure 1 shows the basic principle. A material to be extruded in the form of round bar is rotated and pressed against a die which generates frictional heating, allowing softenedmaterial to be extruded through the die orifice.
The ensuing temperature rise can lead to deformation at localised regions. Since the material softens locally, further deformation is concentrated within the region and under an axial load it is continuously extruded through theopen ended die.
The process offers the potential to process unusual alloys, for example MMCs. Here any discontinuities or inhomogeneities in the material will be redistributed as it passes through the plasticised zone. Friction extrusion is not acommercial process yet and is confined to R&D activities,
Friction co-extrusion (cladding) is a variation of this technique where material, under a radially compressive load, forms a sheath of uniform thickness around the core of the material (Fig. 2). A relatively thick clad layer can beproduced in one application.
In co-extrusion cladding, the sheath material is extruded at a different rate from the core material. This axial differential in feed rate between sheath and core material assists in rupture of oxide layers, which in turn aidsformation of the bond between the co-extrusion materials.
Further information
Friction extrusion technology file
