W M Thomas, E D Nicholas, and S W Kallee
Paper presented at TMS Friction Stir Welding and Processing Conference, November 2001, Indianapolis
Friction, which requires relative motion, pressure and time, is an efficient thermal energy source for the welding and reprocessing of materials. Friction based technology is now extensively used in industries as divergent as sub-sea and aerospace. This paper will describe some of the variants of friction welding and material processing techniques, with particular reference to their relationship to Friction Stir Welding and Friction Stir Processing.
In friction welding and processing technologies, the parts to be joined or processed are subjected to relative motion and pressure so that frictional heat is developed at the interface between the faying surfaces. Typically, the parts are subjected to dry friction during the initial contact, but quite rapidly, as a result of microscopic local seizures and subsequent rupture, a 'third-body' layer of finite thickness is formed. This transient 'third-body', effectively providing a quasi-hydrodynamic bearing [1-4]
, is illustrated in Fig.1a
Fig.1. Friction welding variants a) rotary friction welding; b) friction surfacing; c) friction stir welding
Rotary friction welding
Two variants of the rotary friction welding process have been developed. These are known as conventional 'continuous drive friction welding' and stored energy friction welding where the most widely adopted is inertia friction welding.
In both these methods, friction welds are made by holding a rotating component in contact with a non-rotating component while under a constant or increasing axial load. The interface reaches the appropriate welding temperature, at which point rotation is stopped and the weld completed.
The essential difference between the two rotary friction welding methods is that continuous drive welding is carried out at a constant rotational speed (that may be changed to higher or lower constant rotational speeds at different stages of the weld cycle), while inertia welding starts at a relatively high rotational speed and progressively reduces to zero. The concept underlying inertia friction welding is that a predetermined amount of kinetic energy can be stored in a flywheel and converted into heat at the weld interface.
On an historic note, the use of friction technology to reprocess and reform materials goes back 110 years to Bevington  , whereby friction heating was used to friction form and join tubes.
Friction surfacing uses a consumable in the form of a solid bar, or tube, and is now well established as a surfacing technique [6-9] . The basic principle of the friction surfacing process is illustrated in Fig.1b. Among the materials which can be deposited by this process are tool steels, aluminium alloys, stainless steels, nickel super alloys, hardfacing materials and metal matrix composites.
The deposit, a product of a hot forging action, is inherently homogeneous and of good mechanical strength. The solid phase nature of the process ensures that there is negligible dilution of the surfaced layer together with excellent adhesion. The interface region usually remains intact, even after resisting loads equal to the ultimate tensile strength of the weaker material. Figure 2 shows a 6082 aluminium friction surfaced deposit.
Fig.2. Aluminium alloy 6082 friction surfaced deposit on 6082 substrate with side bend tested specimen
Friction surfacing has the advantage of allowing material to be used which would otherwise be metallurgically incompatible with the substrate, whilst still providing a layer of deposited material with a high degree of adherence. Special combinations of material properties can be provided by this technique that usually cannot be realised in monolithic materials. This reduces usage of the more expensive or strategic materials. Although still in the development stage, the process has been used to manufacture and process in-situ metal matrix composite (MMC) clad layers in both ferrous and non-ferrous materials.
Friction surfacing - extrusion variant
The friction surfacing technique can be adapted to extrude essentially flat bar aluminium products. An oxidised steel substrate can be employed to prevent significant adhesion of the surface layer from occurring. A non-stick roller and a wedge action support plate are shown in Fig.3
. Circumferential movement of the substrate, relative to the rotating consumable, enables the plasticised material to be deposited temporarily onto the roller, before being separated. Limited trials have been carried out on the extrusion variant of the friction surfacing process, but it is not fully developed.
Fig.3. Material processing by friction surfacing
Friction transformation processing
Friction transformation processing is a surface processing technique whereby a rotating wheel in contact with the workpiece surface is used to plasticise and re-process the surface and sub-surface layers of alloy steels [10,11] . This friction wheel based technique dates back 35 years.
The friction heat is mainly generated by hysteresis heating caused by a rapidly pulsing force applied by the dynamic orbit of an 'out of balance' rotating wheel; normal contact friction also contributes to the heating of the surface layers. Figure 4 shows the basic principle.
Fig.4. Basic principle of friction transformation processing
Fig.5. Transverse macrosection of friction transformation processed heat treatable alloy steel, showing a white layer heat affected zone
Friction extrusion processing
The salient features of the friction extrusion process for recycling are illustrated in Fig.6
. Essentially a rotating cartridge is filled with swarf for recycling or metallic powder for processing and while subjected to relative motion an axial load is applied to extruded plasticised material through a die [12,13]
. Friction extrusion is a solid-phase technique whereby the frictional heat and pressure caused by the relative motion allows a plasticised third-body layer to form, without the need for any external heat being applied. The plasticised layer is of limited thickness and forms in close contact to the plunger. The orifice within the plunger provides the necessary restriction in the extrusion flow to consolidate the extruded product.
Fig.6. Friction extrusion, batch technique
Fig.7. Longitudinal section of dispersed silicon carbide (SiC) particles in 2818 aluminium alloy matrix produced by friction extrusion, using recovered overspray material (pre-alloyed spray deposit aluminium 218, nominally 40%SiC)
Friction extrusion investigations using 6082 aluminium over a range of parameters have shown that machine swarf can be extruded to give up to 99.8% compaction and good ductility on 180° bending.
Because of detrimental interactions between metals and ceramics that can occur in the liquid state, it is difficult to reclaim overspray from spray manufacturing processes for metal matrix composite materials by conventional techniques. The result of extruding pre-alloyed silicon carbide (SiC 40%) particulate in a 2618 aluminium matrix material is shown in Fig.7. The extrusion of the metal matrix composite pre-alloyed powder material proved more difficult than aluminium swarf and caused much more wear on the die orifice. However, the friction extrusion technique does offer promise for the in-situ manufacture and processing of certain composite materials.
During the manufacture of aluminium components, much of the original material is discarded as machine waste, e.g. swarf, metal chips, etc. Techniques that can compact waste products thereby allow more material to be stored. In addition, recycling and re-processing techniques that help recover some of the material costs are being encouraged. For aluminium and its alloys, the energy required to extract the metal from the primary ore is substantial. The recycling and re-processing of aluminium waste products is, therefore, becoming commercially more attractive.
Friction hydro pillar processing
Friction Hydro Pillar Processing (FHPP) involves rotating a consumable rod co-axially in an essentially circular split mould whilst under an applied load so as to generate a plasticised layer 
. The salient features of this thermomechanical material processing technique are illustrated in Fig.8
Fig.8. Basic principle of friction hydro pillar processing
Fig.9. Concept for large scale friction hydro pillar processing
During FHPP the consumable member is fully plasticised across the bore of the mould and through the depth of the mould. The plasticised material develops at a rate faster than the feed rate of the consumable bar, which means that the frictional rubbing surface rises up the bar to form a dynamically reformed microstructure. The plasticised material at the interface is maintained in a sufficiently viscous condition for the hydrostatic forces to be transmitted, thereby consolidating the material. The extensive working of the softened material during the processing operation ensures homogenisation of the microstructure, a refinement of the grain structure, removal of casting defects (such as porosity or cracks - if a cast bar is processed), and an improvement in mechanical properties. Using the FHPP technique, trials with cast nickel-aluminium-bronze alloys demonstrated that the original cast microstructure could be fully hot forged and refined. Examples of the microstructures of this alloy in the as-cast condition and after reprocessing are shown in Fig.10. The FHPP process has far reaching possibilities for certain primary metal processing industries. The process is practical and can be used for reprocessing and refining both monolithic and particulate reinforced composite materials.
a) cast material before processing by FHPP
b) material after processing by FHPP
Fig.10. Nickel aluminium bronze processed by FHPP
Friction stir welding (FSW)
Friction stir welding (FSW), Fig.1c
, is still regarded as a new process, invented and developed at TWI. FSW is a technique which allows aluminium, lead, magnesium, steel and copper to be welded, continuously, with a non-consumable tool [14,15,16]
. The technique brings the benefits of solid-phase friction welding to certain materials regarded as difficult to weld by fusion processes. The FSW technique like most of the friction welding variants described in this paper can also be used to process material [17,18]
. Figure 11
shows details of macrostructural features within a 25mm thick 6082-T6 condition friction stir welded test weld.
Fig.11. Macrostructural features in nugget region. Weld produced in 25mm thick 6082-T6 aluminium alloy using a Whorl TM tool, at a weld travel rate of 4mm/sec (240mm/min)
The skew-stir TM
variant of FSW differs from the conventional method in that the axis of the tool is given a slight inclination (skew) to that of the machine spindle 
, as shown in Fig.12
Fig.12. Basic principle of skew-stir TM showing different focal points
The skew-stir TM technique enables the ratio between the 'dynamic' (swept) volume and the static volume to be increased in addition to that provided by the use of re-entrant features machined into the probe. It is this ratio that is a significant factor in enabling a reduction or elimination of void formation.
The arrangement shown in Fig.12 results in the shoulder face being oblique to the axis of the skew tool and square to the axis of the machine spindle. This shoulder face remains in a fixed relationship with respect to the plate top surface. Tilting the plate workpiece or the machine spindle will produce a plate to tool tilt that can be varied to suit conditions.
The focal length of a skewed tool affects the amplitude of the orbit of the tool shoulder and probe. With the focal length at the shoulder position i.e. the top of the workpiece, the shoulder essentially provides a rotary motion i.e. at with no orbit. When the focal length is positioned slightly above the top surface of the workpiece, at any position through the thickness of the workpiece, or slightly below the workpiece, the shoulder contact face makes a nominal orbital movement. In addition, the orbital motion of the shoulder is dependent on the angle of skew and the distance that the intersection (focal point) is away from the top of the plate. The greater the skew angle and the greater the distance that the focal point is away from the top workpiece surface the greater is the amplitude of the shoulder orbit.
The skew action results in that the same region of the probe making contact with the extremities of the weld region. The FSW tool does not rotate on its own axis, and therefore only a specific part of the face of the probe surface is directly involved in working the substrate component material. Consequently, the inner part of the tool can be cut away to improve the flow path of material during welding. This results in an asymmetric shaped probe.
The skew-stir TM technique provides a flow path and a weld nugget region of width greater than the diameter of the probe. This feature is ideally suited for lap and 'T' welds and similar welds where the interface is 90° to the machine axis, i.e. parallel with the workpiece surface.
When FSW is used for material processing the increase in weld width (and hence the volume of material processed) is particularly advantageous  . The skew-stir TM technique provides a method of increasing the width of the material that can be processed.
Investigations at TWI are continuing to explore and develop a number of solid-phase welding and processing techniques one of these will be the use of FSW to process composite materials as shown in Fig.13.
Fig.13. Friction stir welding being used to process composite materials
Con-stir TM (continuous manufacture of composite materials, MMCs and intermetallics by hot wire friction stir)
This paper has described some of the friction welding and material processing techniques, which are currently in varying stages of technical development. These offer potential technical and commercial advantage for a number of industries. Some of these solid-phase processing techniques are able to achieve what was previously considered impossible by conventional welding and processing technology.
The authors wish to thank K I Johnson, R E Dolby, A B M Braithwaite, D G Staines, P B Fielding, P L Threadgill, P Temple-Smith and E R Watts.
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