WO2005065856A1

WO2005065856A1 - Method and apparatus for extrusion

Info

Publication number: WO2005065856A1
Application number: PCT/GB2005/000009
Authority: WO
Inventors: Yan Huang
Original assignee: Yan Huang
Priority date: 2004-01-06
Filing date: 2005-01-05
Publication date: 2005-07-21
Also published as: CN100431728C; CN1898041A; GB2409997A; GB0400142D0; GB2409997B

Abstract

A method and apparatus for extrusion of a workpiece (10) includes providing an extrusion die assembly (40; 50) and a driving means (20) for forcing the work-piece (10) through the extrusion die assembly (40, 50), then causing a surface of the workpiece (10) and a surface of driving means (20) to contact to form an interface therebetween. Application of a force to the workpiece (10) directed towards the interface secures a frictional force at the interface, and the workpiece (10) is forced through the die assembly (40; 50) using the frictional force to plastically deform the material of the workpiece (10) during extrusion through the extrusion die assembly (40; 50) without changing the dimensions of the workpiece (10).

Description

METHOD AND APPARATUS FOR EXTRUSION

The present invention relates to methods and apparatus for deformation processing of metals, alloys and of other crystalline materials, in order to control material microstructure, texture and physical and mechanical properties. More specifically, though not exclusively, the present invention relates to methods and apparatus for continuous frictional angular extrusion, in particular to achieve intensive plastic deformation of crystalline materials.

The microstructure, particularly, grain size of a crystalline material has a large effect on its properties, and refinement of the grain size has many technological benefits. For example, at low temperatures, a small grain size may increase the strength and toughness of the material, and, at high temperatures, fine-grained alloys may become superplastic.

The as-cast grain size of most industrial metals and alloys is generally large (>1 OOμm), and further grain refinement is achieved by thermomechanical processing, including various combinations of thermal and mechanical action upon the material to be worked. Processes of plastic deformation are very important in a general cycle of thermomechanical treatment and, in fact, it has been shown that sub-micron grain structures may be formed directly by very large strain deformation, with little or no subsequent annealing. [see: F. J. Humphreys, P. B. Prangnell, J. R. Bowen, A. Gholinia and C. Harris, Developing stable fine-grain microstructures by large strain deformation, Phil Trans R Soc Lond, A357(1999), 1663-1681].

The strain required to achieve a fine-grained microstructure is large. For example, a mean grain size of less than 10μm obtained in an aluminium alloy AA7075 requires a true strain of about 2.3 under warm rolling conditions [see: J. A. Wert, N. E. Paton, C. H. Hamilton and M. W. Mahoney, Grain refinement in 7075 Aluminium by thermo-mechanical processing, Metallurgical Transactions 12A (1981 ), 1267-1276.]. A sub-micron microstructure requires higher strains, depending on individual materials and deformation modes. Many industrially important metal-forming methods, such as rolling and extrusion, impart large strains and in certain cases fine-grained microstructures may be formed. However, during such processing, one or more dimensions of the workpiece are continuously reduced and, eventually, foil or filamentary materials, having limited use for structural applications, are produced.

Recently, several severe plastic deformation methods have been developed to overcome difficulties in achieving high strains necessary for fine-grained microstructure and texture formation in conventional forming operations, including cyclic extrusion-compression (CEC) [see: J. Richert and M. Richert, A new method for unlimited deformation of metals and alloys, Aluminium, 62 (1986), 604-607], high-pressure torsion (HPT) [see: I. Saunders and J. Nutting, Deformation of metals to high strains using a combination of torsion and compression. Metals Science, 18(1984), 571-575], equal channel angular extrusion (ECAE), accumulative roll-bonding (ARB) [see: H. Saito, N. Utsunomiya and T. Tsuji, Acta Mater 47 (1999), 579-583] and continuous confined strip straining (C2S2) [J-C Lee et al., Controlling the textures of the metal strips via the continuous confined strip shearing (C2S2) process, Materials Research Bulletin 36(2001) 997-1004]. Some of the methods such as ECAE have shown exciting features of being able to apply ultrahigh strains and promising potential for large-scale application. However, these severe plastic deformation methods for microstructure refinement in their present formation are all limited in eithertheir ability to produce the type, size and quantities of material for commercial use or their viability and productivity for industrial application.

Equal channel angular extrusion (ECAE) was initially invented in the former Soviet Union by Segal [see: V. M. Segal, Invention Certificate of the USSR No.575892, 22 Oct 1974] and further developed in the USA [see: US patent 5,400,633 (3 Septl 993); V, M. Segal, Plastic deformation of crystalline materials, US patent 5,513,512 (17June 1994); V. M. Segal, Method and apparatus for intensive plastic deformation of flat billets, US patent 5, 850, 755 (8 Feb1995)]. During ECAE, the billet is extruded in a closed die that has two intersecting channels of equal size offset at a predetermined angle. The deformation in ECAE, under ideal conditions, is simple shear in a narrow region along the intersecting plane. As the billet dimensions are unchanged after extrusion, the process can be repeated to generate ultra-high strains. The problems of the ECAE process include that the billet size is limited by the length of the first die channel because material outside of the die channel will suffer upsetting and that the extrusion force rises rapidly with the increase of the size of die channels. There are also some other factors that restrict the application of ECAE at a commercial scale including difficulties in operation, high scrap rate and low productivity.

A continuous extrusion process, generally known in the art as Conform, was originally developed by the United Kingdom Atomic Energy Authority in early 1970s [see: D. Green, Atomic Energy Authority UK, UK patent GB1370894, Oct.1973], and has recently been used in combination with an ECAE process [see: G.J. Raab et al., Materials Science and Engineering A 382 (2004) 30-34] to apply severe plastic deformation. The ECAE-Conform methodwith its existing designs, however, can only process samples with small size in cross section such as wires, having limited applications in the deformation processing of materials.

The present invention provides methods and apparatus that overcome the above- described problems. The method and apparatus of the present invention inter alia are based on the deformation of bulk materials and are applicable to large quantities of conventional structural metals and alloys and other crystalline materials to achieve large plastic strains and eventually fine or ultra-fine grained microstructure with enhanced physical and mechanical properties.

In a first of its aspects the present invention provides a method of extrusion of a workpiece including: providing an extrusion die assembly and a driving means for forcing the workpiece through the extrusion die assembly; causing a surface of the workpiece and a surface of driving means to contact to form an interface therebetween; applying a force to the workpiece directed towards the interface to secure a frictional force at the interface; forcing the workpiece through the die assembly using the frictional force; plastically deforming the material of the workpiece during extrusion through the extrusion die assembly without changing the dimensions of the workpiece.

The applied force results in the generation of an applied pressure directed towards the interface. Preferably, mainly or substantially only frictional forces are used to force (e.g. push) the workpiece into the die assembly.

Thus, with a driving surface of the driving means and the surface of the workpiece in pressed contact at an interface therebetween, moving the driving surface at the interface applies the frictional force.

For example, the present invention preferably provides a method of refining the microstructure of a crystalline material workpiece comprising a continuous frictional angular extrusion process in which a crystalline material workpiece is extruded continuously through a die assembly by frictional force between the workpiece and a driving instrument which moves or rotates by power means and plastic deformation occurs as the workpiece passes through the angular die assembly which is configured so as to enable plastic deformation of the crystalline material to take place without changing the dimensions of the workpiece. The frictional force is most preferably secured by the application of a pressure normal to the interface between the workpiece and the driving instrument.

A significantly important advantage of the present invention is that the extrusion of a single workpiece of unlimited length may be repeated as many times as needed through the die assembly without changing the dimensions of the workpiece and as a result intensive plastic strain may be achieved with substantially refined microstructure and enhanced physical and mechanical properties.

Another significant advantage of the present invention is that large quantities of metals and alloys and other crystalline materials may be processed in various forms, such as strip, sheet, plate, bar and rod, which are the main forms of metals and alloys for structural, automobile and aircraft manufacturing applications, with high efficiency and low scrap rate due to the continuous nature of the process.

Another significant advantage of the present invention is that the present continuous frictional angular extrusion process is capable of commercial exploitation and can be easily incorporated into existing production lines. For example, the extrusion process can be used immediately after continuous strip casting or after hot rolling and so on.

Preferably, the workpiece has a predetermined direction of travel into the die assembly which is configured so that the workpiece is extruded from the die assembly at an angle (the extrusion angle) with respect to the predetermined direction of travel of the workpiece into the die assembly. The extrusion angle may be greater than or equal to 90 degrees, preferably in the range 90 degrees to 135 degrees.

The method may include applying a force in a direction substantially normal to the interface between the driving surface and said surface of the workpiece thereby to secure said frictional force. The method may include providing a first extrusion channel and a second extrusion channel contiguous with the first extrusion channel and inclined to the first channel at an extrusion angle, forcing the workpiece into the die assembly via the first extrusion channel to exit the die assembly via the second extrusion channel, wherein the first and second extrusion channels have the same dimension normal to the direction of travel of the workpiece through the respective extrusion channel in use corresponding to the thickness of the workpiece.

The die assembly preferably comprises a first extrusion channel formed between the driving surface provided by the driving instrument and a surface of a first die member with the driving surface having substantially a greater dimension in the direction in which the workpiece travels for engaging the material than the first die member and a contiguous second extrusion channel formed between another surface of said first die member and a surface of a second die member, said first and second extrusion channels have substantially identical cross-sections corresponding to a cross-section of the workpiece or have identical open cross sections with an equal dimension corresponding to the thickness of the workpiece, wherein the second channel is inclined to the first channel at an angle which corresponds to the extrusion angle, and the workpiece enters the die assembly through the first extrusion channel and exits the die assembly through the second extrusion channel. The provision of the first and second extrusion channels of substantially small dimension in the direction in which the workpiece travels is advantageous since it minimises the negative friction between said die members and the workpiece and reduces the tendency of upset to the workpiece upstream of the region in which plastic deformation takes place.

Preferably, the method includes providing the first extrusion channel as a spacing between the driving surface and an opposed surface of the extrusion die assembly, and moving the driving surface relative to the opposed surface of the die assembly thereby to translate the workpiece through into the first extrusion channel and through the die assembly using said frictional force.

The continuous frictional angular extrusion process preferably includes the step of translating the workpiece through the die assembly by moving the first extrusion channel defining surface or driving surface of the driving instrument relative to the first extrusion channel defining surface of the first die member using frictional drag developed between the workpiece and the first extrusion channel defining surface of the driving means to allow plastic deformation to occur in the die assembly. The driving instrument may comprise a roll member (driving roll), which rotates by power means. The employment of a driving roll provides means for continuously feeding the material to be extruded into the die assembly and continuously extruding the material through the die assembly.

The method preferably includes pressing the driving surface against the workpiece to secure said frictional force. The method may include pressing the workpiece against the driving surface to secure said frictional force.

The driving roll may be a plain roll having an ungrooved driving surface - the first extrusion channel defining surface thereof, and the first and second die members may be plain plates or bars or of any particular shapes as needed. The workpiece may be in the form of a sheet including strip and plate. The ratio of the width to the thickness of the sheet workpiece may be greater than 5, preferably greater than 10, which is ideal for producing a plane strain deformation mode.

The present invention provides further numerous technical advantages and benefits. For example, the provision of friction as the driving means of the process substantially reduces the negative effects of friction upon the workpiece, which occur in an ECAE process, as it passes through the first extrusion channel. A plane strain extrusion mode can be readily obtained in the present process with deformation occurring in simple shear in a narrow region and the material flowing through the die assembly may be under substantially same conditions and undergoing same plastic strain. A predetermined amount of strain may be obtained accurately due to the well-defined deformation mode. Therefore, substantially uniformly distributed intensive strain throughout the entire workpiece and consequently a uniformly refined microstructure with enhanced mechanical and physical properties may be achieved using the present invention.

Alternatively, a driving roll having an endless circumferential groove therein may be employed combined with a first die member comprising a shoe member which covers part of the length of the groove in the driving roll and a second die member comprising an abutment member which projects into the groove in the driving roll. A grooved driving roll is particularly useful in applications in which bar- and rod-shaped workpieces are extruded.

The driving roll which is either grooved or ungrooved may have a modified surface, the surface being modified to enhance its lubricating properties and heat and/or wear resistance. This surface modification is particularly important for deformation carried out at an elevated temperature.

The continuous frictional angular extrusion process preferably comprises the step of applying a normal pressure to the workpiece so that a sufficiently high frictional force acts on the workpiece under the drive of the driving instrument to enable the extrusion of the workpiece through the die assembly to take place. The step of applying a normal pressure to the workpiece may be performed by the driving instrument, in which case a workpiece supporter may be required to support the workpiece to balance the normal pressure from the driving instrument. A supporter with a belt arrangement may be used, which lessens largely the negative friction between the workpiece and the supporter and reduces the normal pressure to the workpiece required for actuating the extrusion process. A normal pressure to the workpiece may also be applied using a separate press against the driving surface or the first extrusion channel defining surface of the driving instrument. The press may also be a belt-press arrangement.

Preferably, the method includes providing a reactive pressure to the workpiece to balance the pressure applied thereto by the driving surface. The method may also include providing a workpiece support means such that the pressing of the driving surface against the workpiece results in the pressing of the workpiece against the workpiece support means thereby with the workpiece support means opposing the pressure from the driving surface.

The workpiece support may be provided with a support conveyor means, and the method may include conveying the workpiece over the workpiece support means using the support conveyor means as the workpiece moves in response to said frictional force.

Preferably, the press means is provided with a press conveyor means for pressing the workpiece against the driving surface, and the method includes conveying the workpiece over the press means using the press conveyor means as the workpiece moves in response to said frictional force.

Preferably, the conveyor means includes rollers and a belt supported by, and arranged to revolve around, the rollers.

Preferably, the coefficient of friction between the drive surface and the workpiece which is controlled to be greater than the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the press means.

The method may include determining the pressure with which the pressed contact is made according to the difference between the coefficient of friction between the drive surface and the workpiece and the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the press means.

The continuous frictional angular extrusion process preferably comprises the step of applying a backpressure to the workpiece being extruded. By doing so it is possible to increase the hydrostatic pressure on the material in the region in which plastic deformation takes place and consequently to minimise or eliminate any defects which might be created in brittle materials during deformation. The step of applying a backpressure to the workpiece being extruded may be preformed using a frictional means at the outlet of the die assembly.

Preferably, a variable reverse frictional force is applied to a surface of the workpiece as it exits the extrusion die assembly thereby to generate a variable back pressure upon the workpiece to apply a pressure (e.g. a hydrostatic pressure) to the workpiece within the extrusion die assembly. The controllable backpressure is preferably applied to the workpiece at the outlet of the second extrusion channel.

The continuous frictional angular extrusion process may comprise the step of heating the workpiece before, and/or during the extrusion process in the case when deformation is performed at an elevated temperature.

The method may further comprise the step of heat-treating the workpiece after extrusion thereof through the die assembly. Such treatment can be effective in ensuring that advantageous microstructures are achieved. Continuous recrystallisation annealing is a preferred type of heat treatment. In general, heat treatments at relatively low temperatures are advantageous. A plurality of continuous frictional angular extrusion processes through the die assembly may be performed. It is an advantage of the present invention that, subject to the material having sufficient ductility, deformation to an essentially unlimited strain can be achieved by repeating the continuous frictional angular extrusion process.

The microstructure of the crystalline material may be refined to produce a fine-grained, preferably an ultra-fine grained microstructure. Fine grain is understood to refer to a material having an average grain size in the range 1 to 10μm, whilst ultra-fine grain is understood to refer to a material having an average grain size of less than 1 μm.

In another of its aspects the present invention may provide a method of refining the microstructure of the crystalline material to produce a fine grained or an ultra-fine grained microstructure using the method of extrusion according to the invention in its first aspect.

In a second of its aspects the present invention provides an apparatus for extrusion of a workpiece including: an extrusion die assembly and a driving means for forcing the workpiece through the extrusion die assembly, the driving means having a driving surface being arranged to contact a surface of the workpiece to form an interface therewith; pressing means for applying a force to the workpiece directed towards the interface to secure a frictional force at the interface; wherein the driving means is arranged to force the workpiece through the die assembly using the applied frictional force thereby to plastically deform the material of the workpiece during extrusion through the extrusion die assembly, the die assembly being configured to enable said extrusion to occur without changing the dimensions of the workpiece.

With the driving surface and the surface of the workpiece in pressed contact at an interface therebetween, the driving means is arranged to move the driving surface at the interface to apply the frictional force.

For example, the invention may provide an apparatus for refining the microstructure of a crystalline material workpiece comprising continuous frictional angular extrusion means including a driving instrument which moves or rotates by power means and drives a crystalline material workpiece, by virtue of friction between the driving instrument and the workpiece, through a die assembly which is configured so as to enable plastic deformation of the crystalline material to take place without changing the dimensions of the workpiece. The pressing means is preferably arranged to apply a force in a direction substantially normal to the interface between the driving surface and said surface of the workpiece thereby to secure said frictional force. Preferably the pressing means applies a pressure normal to the interface between the workpiece and the driving instrument for generating sufficient frictional force to actuate and carry out the deformation process.

The apparatus preferably includes a first extrusion channel and a second extrusion channel contiguous with the first extrusion channel and inclined to the first channel at an extrusion angle, wherein the driving means is arranged to force the workpiece into the die assembly via the first extrusion channel to exit the die assembly via the second extrusion channel, wherein the first and second extrusion channels have the same dimension normal to the direction of travel of the workpiece through the respective extrusion channel in use corresponding to the thickness of the workpiece.

The continuous frictional angular extrusion apparatus preferably, in use, translates the workpiece into the die assembly along a predetermined direction of travel using the driving instrument provided, and the die assembly is preferably configured so that the workpiece is extruded from the die assembly at an angle (the extrusion angle) with respect to the predetermined direction of travel. The extrusion angle may be greater than or equal to 90 degrees, preferably in the range 90 degrees to 135 degrees.

The first extrusion channel is preferably defined by a spacing between the driving surface and an opposed surface of the extrusion die assembly, the driving means being arranged to move the driving surface relative to the opposed surface of the die assembly thereby to translate the workpiece through into the first extrusion channel and through the die assembly using said frictional force. The pressing means may be arranged to press the driving surface against the workpiece to secure said frictional force. The pressing means may be arranged to press the workpiece against the driving surface secure said frictional force.

The die assembly may comprise a first extrusion channel formed between the driving surface provided by the driving instrument and a surface of a first die member with driving surface having substantially a greater dimension in the direction in which the workpiece travels for engaging the material than the first die member and a contiguous second extrusion channel formed between another surface of said first die member and a surface of a second die member. Preferably, the first and second extrusion channels have substantially identical cross-sections corresponding to a cross-section of the workpiece or have identical open cross sections with an equal dimension corresponding to the thickness of the workpiece, wherein the second channel is inclined to the first channel at an angle which corresponds to the extrusion angle, and the workpiece enters the die assembly through the first extrusion channel and exits the die assembly through the second extrusion channel.

The driving instrument may comprise a roll member (driving roll), which rotates by power means in the predetermined direction to force the workpiece through the die assembly using frictional drag developed between the workpiece and the driving roll. The employment of a driving roll provides means for continuously feeding the material to be extruded into the die assembly and continuously extruding the material through the die assembly. The driving roll may be either a plain roll for deformation processing of sheet workpieces or a plain roll having a groove therein for processing bar- and rod- shaped workpieces.

The apparatus preferably includes balance means arranged to provide a reactive pressure to the workpiece to balance the pressure applied thereto by the driving surface. The balance means may include a workpiece support means arranged such that the pressing of the driving surface against the workpiece results in the pressing of the workpiece against the workpiece support means such that the workpiece support means opposes the pressure from the driving surface.

The workpiece support may have a support conveyor means arranged to convey the workpiece over the workpiece support means as the workpiece moves in response to said frictional force. The pressing means preferably includes a press conveyor means for pressing the workpiece against the driving surface, and for conveying the workpiece over the press means as the workpiece moves in response to said frictional force.

The conveyor means preferably includes rollers and a belt supported by, and arranged to revolve around, the rollers.

The continuous frictional angular extrusion means may comprise means for applying a normal pressure to the workpiece so that a sufficiently high frictional force acts on the workpiece to enable the extrusion of the workpiece through the die to take place. The means of applying a normal pressure to the workpiece may be incorporated into the driving instrument, in which case a workpeice supporter may be required to support the workpiece to balance the normal pressure from the driving instrument. A supporter with a belt arrangement may be used, which lessens largely the negative friction between the workpiece and the supporter and reduces the normal pressure to the workpiece required for actuating the extrusion process. The means of applying a normal pressure to the workpiece may also be a press. The press may also be a belt-press arrangement. Preferably the coefficient of friction between the drive surface and the workpiece is greater than the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the pressing means. Most preferably, the pressure with which the pressed contact is made is determined according to the difference between the coefficient of friction between the drive surface and the workpiece and the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the pressing means.

The apparatus may include backpressure means arranged to apply a variable reverse frictional force to a surface of the workpiece as it exits the extrusion die assembly thereby to generate a variable back pressure upon the workpiece to urge the workpiece back into the die assembly after it has been plastically deformed. The backpressure means is preferably arranged to apply a controllable backpressure to the workpiece at the outlet of the second extrusion channel.

The continuous frictional angular extrusion means may comprise further the step of applying a backpressure to the workpiece being extruded to enhance the ductility of the material in the deformation region. A frictional means at the outlet of the die assembly may be used to apply the backpressure in a way in which the backpressure may be maintained constant or adjustable during the extrusion process.

The apparatus may include heating means for heating the workpiece before and/or during the extrusion process. It may include heating means for heat-treating the workpiece after extrusion through the extrusion die assembly. The heat treating means is preferably arranged to apply continuous re-crystallisation annealing to the workpiece. The heat-treating means may be arranged to apply recovery annealing in between any two said extrusions.

In another of its aspects, the present invention may provide an apparatus for refining the microstructure of the crystalline material to produce a fine grained or an ultra-fine grained microstructure using the apparatus according to the invention in its second aspect.

Embodiments of methods and apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which: -

Fig. 1 a is a diagrammatic drawing in section through the central line of a first embodiment of the invention, depicting the arrangement of main parts and the operational principle of the method and the apparatus of the present embodiment;

Fig. 1 b is a part-sectional view along line A-A in Fig. 1 a, showing the configuration of die members in the outlet of the die assembly relating to a sheet workpiece in process and to the housing of the apparatus.

Fig. 2 is schematic drawing of part-sectional view through the central line of the first embodiment, showing the configuration of workpiece, driving instrument, die members, supporters of the workpeice and die members and the definition of operational surfaces and process parameters as well;

Fig. 3 shows part-sectional view of a modified first embodiment of the invention in association with the use of a workpiece supporter with a belt-on-roller arrangement;

Fig. 4 shows part-sectional view of a modified first embodiment of the invention in association with the use of a workpiece supporter with a simple belt arrangement;

Fig. 5(a) shows an enlarged partial sectional view of a second embodiment of the invention, depicting the arrangement of driving means and die assembly for processing bar-shaped workpieces;

Fig. 5(b) is a part-sectional view along line B-B of Fig. 5(a), showing the configuration of a bar-shaped workpiece in between a grooved driving roll and a workpiece supporter;

Figures 5(c), 5(d) and 5(e) show aspects of the die assembly of Fig. 5(a);

Fig. 6(a) is a diagrammatic drawing in section through the central line of a third embodiment of the invention, depicting the formation and main parts of the apparatus with horizontal outlet arrangement in association with the application of a separate press assembly and a back pressure assembly;

Fig. 6(b) is an enlarged part-sectional view through the central line of the third embodiment, showing the arrangement of press assembly, die assembly and back pressure assembly, and the definition of the operational surfaces of die members in relation to driving roll and workpiece; Fig. 7(a) is a part-sectional view of a modified third embodiment of the invention, depicting a press assembly of a belt arrangement together with driving roll, workpiece and die assembly;

Fig. 7(b) is a sectional view along line C-C in Fig. 7 (a), showing the belt arrangement within the press assembly.

Preferred but non-limiting embodiments of the present invention are shown in Fig. 1 a to Fig. 7b in which like numerals are used to describe features that are common to various of the figures. A first embodiment of the present invention is shown in Fig.1 a through Fig. 4, a second embodiment of the present invention is shown in Fig. 5a through Fig. 5e and a third embodiment is given in Fig. 6a through Fig. 7b. The drawings are diagrammatic, showing the operational principles of the method and the key parts of the apparatus and their approximately relative positions and dimensions.

As shown in Fig.1 a, the first embodiment of the present invention includes mainly a feeding assembly 1 , a driving roll 20 which is the driving means of the process, die members 40 and 50, workpiece supporter 30 and die member supporters 43 and 53 and a pair of delivery rolls 2. Workpiece 10 from a coiler or a roller table (not shown) is fed by feeding assembly 1 onto driving 20 and is supported by workpiece supporter 30 against driving roll 20. Driving roll 20, which rotates by power means as indicated by arrow A1 , translates workpiece 10 forward with the help of the friction between the driving roll and the workpiece, towards a die assembly consisted of die members 40 and 50 and supporters 43 and 53 as described above and forces it through the die assembly. Driving roll 20 also acts as a pressing means to apply a normal pressure as indicated by arrow A2 to workpiece 10 in order to generate sufficient frictional force therebetween to actuate and carry out the process. Workpiece 10 undergoes plastic deformation as it is extruded through the die assembly and is sent to a coiler or a roller table (not shown) after extrusion via delivery rolls 2. Delivery rolls 2 may apply a moderate pull force to workpiece 10 to reduce extrusion force if necessary. At the end of the extrusion process, a large force may be required from delivery rolls 2 or from a coiler in use to pull workpiece 10 out of the die assembly because at this stage of processing the frictional force provided by driving roll 20 may not be large enough to perform this task.

Supporters 30, 43 and 53 may be made as an integrated body or separate parts in the apparatus depending on applications. Detailed structures of individual assemblies with regard to their installation in the housing (100 as indicated in Fig. 1 b) are not shown for simplicity of presentation.

Generating a frictional force necessary for continuous extrusion process by applying a normal pressure to the workpiece to be processed is unique and is an advantageous feature of the present invention. It is highly preferable, however, that the friction coefficient between workpiece 10 and its supporter 30 (f_r), which exerts a resistance to the process, must be significantly smaller than the friction coefficient between driving roll 20 and workpiece 10 (fd), which provides the driving force for the process, to allow driving roll 20, while it rotates by power means and applies a normal pressure to workpiece 10, to generate a net frictional force to the workpiece in the direction tangential to the surface of driving roll 20 at each corresponding point towards the die assembly. If the normal pressure on workpiece 10 is sufficient, the resultant frictional force will be able to force workpiece 10 to flow through the die assembly with plastic deformation taking place in the workpiece.

Fig.2 gives an enlarged view of relationships between the workpiece, driving roll, die assembly and the supporting means and also shows major process parameters. It may be seen from the figure that the working surface of workpiece supporter 31 is made in curved shape corresponding to the radius of driving roll 20 and thus allows the normal force applied to workpiece 10 to be evenly distributed on the workpiece over the contact length (L), which is the product of the contact arc-angle (φ_p) and the radius of driving roll 20 (R).

It may also be seen from Fig. 2 that driving roll 20, die member 40 and die member 50 define a passageway for workpiece 10 to pass through. Driving surface 21 of driving 20 and surface 41 of die member 40 define therebetween the first part of the passageway named as first extrusion channel and surface 42 of die member 40 and surface 52 of die member 50 define therebetween the second part of the passageway named as second extrusion channel. The die assembly is configured such that the cross-sections of the first and second extrusion channels are identical corresponding to a cross-section of workpiece 10. The two extrusion channels are contiguous and disposed at an angle (2x9) named as extrusion angle. It should be noted that a sliding fit is required between driving surface 21 of driving roll 20 and surface 51 of die member 50 to ensure a completely closed contiguous passageway comprised of a first extrusion channel and a second extrusion channel. The dimension of the first and second extrusion channels in the directions in which workpiece 10 travels is substantially smaller than the circumferential length of the driving roll. During extrusion processing, workpiece 10 under frictional drag enters the die assembly at the first extrusion channel and exits through the second extrusion channel with plastic deformation taking place in a narrow region along the intersectional plane of the two channels.

The intersectional plane of the first and second extrusion channels is indicated by a dotted line OC in Fig. 2, whereas O represents the meeting point (line) at surface 21 of driving roll 20 which defines in part the first extrusion channel and surface 51 of die member 50 which defines in part the second extrusion channel and C is the point on die member 40 that joints surface 41 which defines in part the first extrusion channel and surface 42 which defines in part the second extrusion channel. AO, tangential to point O on driving roll 20, is the principle direction of travel of workpiece 10 in the first extrusion channel and OB is the direction in which workpiece 10 is extruded through the second extrusion channel, i.e., the extrusion direction. The angle between OA and OB defines the extrusion angle (2 ), which is equal to or larger than 90° and in general ranges between 90° to 135°. Since the cross-sections of the two extrusion channels are identical, the intersectional plane OC divides equally the extrusion angle (2 ) and the dimensions of workpiece 10 remain unchanged after deformation. The repeat of the extrusion process with identical geometrical conditions can be performed and eventually intensive plastic strain can be achieved with refined microstructure and enhanced properties of the crystalline material workpiece.

Preferably, the first embodiment of the present invention is employed for the deformation processing of sheet (including strip and plate) workpieces and Fig. 1 b shows the configuration of die members in the outlet of the die assembly relating to a sheet workpiece. The workpiece may have a width smaller than the corresponding dimension of the extrusion channel as shown in Fig. 1 b, which means that the extrusion channels can be made open in the workpiece width direction or strictly in the direction perpendicular to the longitudinal plane in which the workpiece travels. If the ratio of workpiece width to its thickness is greater than 5 or preferably greater than 10, material flow during deformation will take place exclusively in the longitudinal plane of the workpiece, i. e., the deformation takes place under plane strain mode. Under ideal conditions, i.e., the friction in the deformation region is negligible, every increment of plastic strain is shear, and the whole deformation is then a simple shear in the plane OC. The plastic strain that the workpiece undergoes through the die assembly is dependent only on the extrusion angle: 7 = 2cot# (1 )

where ;κis the shear strain produced in each pass, and the corresponding equivalent strain per pass (ε) is then

The total equivalent strain produced after n passes can be added as ε_n = nε (3) For an extrusion angle of 120°, £- = 0.664 and after 10 passes ε_l0 = 6.64. It is an advantage of simple shear deformation mode that strain at each pass is addible. The extrusion pressure (p) is a function of the yield stress of the material (σ_s) and the extrusion angle (2x9):

or p = 2kcotθ (4')

where σ_s is the yield strength of the material to be extruded and k is the shear strength of the material, k = σ 1^ according to Levy-Mises criterion.

The present continuous extrusion process is a frictionally actuated process. The effective friction coefficient (/_e) defined as the difference in friction coefficient f_θ = (fd- f_r), where fd is the friction coefficient in workpiece 10-driving roll 20 interface and f_r is the friction coefficient in workpiece 10-workpeice supporter 30 interface, is one of a few key factors that determine the magnitude of the required normal pressure to the workpiece to establish and carry on the extrusion process. Other factors include the strength of the material (σ_s), the thickness of the workpiece (f), and the extrusion angle (2 ). Higher allows harder materials and thicker workpieces to be processed or, for processing a workpiece with certain thickness and strength, higher f_e requires a lower normal pressure to the workpiece. Generally, higher f_θ enhances the capacity of the apparatus on one hand and reduces energy requirements on the other hand.

The direction of the frictional force on workpiece 10 is tangential to the corresponding surface point of driving roll 20 towards the first extrusion channel as indicated by arrow F in Fig 2 and only the resolved frictional force parallel to AO direction will contribute to the extrusion process as indicated by arrow P in Fig. 2. The total frictional force attributable to the extrusion (F_θ) is a function of the normal pressure (p_n) on workpiece 10, the arc length of the surface with the normal pressure applied ( ) corresponding the angle (φ_p) as defined in Fig. 2, and friction coefficient f_d and f_r, the width (w) and thickness (t) of workpiece 10 and the radius of driving roll 20 (R):

F. = w £ (f_dp„Rcos φdφ -f_rPn (R + 1) cos φdφ) (5) If the normal pressure p_n and friction coefficient f_d and f_r are constant, and t«R, then

E, « w/ NRsin(-) (6)

This frictional force F_e must produce a load on the material in the deformation zone along the intersectional plane OC of P = 2wtσ cotθlJX* for establishing and carrying out a continuous extrusion process with the present invention, where P is the required extrusion force, and the normal pressure on the workpiece required to generate such a load is

It may be seen that the normal pressure p_π on the workpiece required for performing the extrusion process is dependent on the strength of the material to be deformed, the thickness of the workpiece, the effective friction coefficient, the extrusion angle, the arc length with the normal pressure applied and the radius of the driving roll. If R = 500mm, L = 200mm, 2(9=120°, t = 4mm, = 0.25 and σ_s = 750MPa, we get a normal pressure p_π = 40MPa. For a sheet workpiece of a width w = 500mm, the force required to produce such a normal pressure is about 400 tons, which is smallerthan the separation force required for cold-rolling a sheet of similar strength and dimensions to the same strain.

There are several ways of increasing the effective friction coefficient (f_e) including: (a) increase the roughness of driving surface 21 ; (b) decrease the roughness of workpiece supporting surface 31 ; and (c) lubricate surface 31. Increasing or decreasing the surface roughness of workpiece 10 is of limited assistance because this will affect both sides and the final effect will be cancelled out. Increasing the roughness of the workpiece surface in contact with driving roll 30 and decreasing the roughness of the other side may be useful in terms of raising the effective friction coefficient (f_e) but practically expensive.

The lubrication of workpiece supporting surface 31 using solid lubricants is preferable because it may provide improvements to the extrusion process without troubling the performance of the process. Application of liquid lubricants directly to the workpiece is not preferred as they can cause problems such as to require a cleaning treatment of the lubricated surface before the next round of processing and to create temperature gradient across the thickness of the workpiece due to their cooling effect, which can be critical when deformation is carried out at elevated temperatures. Alternatively, workpiece supporting surface 31 may be made from a self-lubricating material or lined with a wear resistant material with low friction coefficient. Furthermore, surface 31 may be modified to enhance surface smoothness and wear resistance.

In addition to the means mentioned above, a structural modification to the workpiece supporter has been made to reduce the negative friction from the supporter and to increase significantly the effective friction coefficient (f_e). Fig. 3 is a part-sectional view through the central line of a modified first embodiment of the present invention, showing a modified workpiece supporting assembly together with driving roll 20, workpiece 10 and the die assembly. As shown in Fig.3, a belt 131 is used in contact with workpiece 10, instead of using directly the top surface of the supporting block 130. Belt 131 is supported by a group of rollers 132 which are mounted in supporter 130. Thus the belt revolves on rollers 132 around supporter 130 while workpiece is translated on top of the belt by driving roll 20 towards the die assembly. With the application of this belt-on- roller arrangement, there will be no relative movement between workpiece 10 and its supporting mechanism and the resistance to the workpiece is only the rolling friction between belt 131 and rollers 132 and 133, which is practically negligible. A couple of rollersl 33 are used at the bottom of supporter 130 as shown in Fig.3, allowing belt 131 to travel in cycle continuously and smoothly during processing. The material and structure of belt 131 should be chosen such that the pressure from rollers 132 is spread and distributed over the whole surface of the belt. The workpiece supporting assembly may be located below driving roll 20 symmetrically about its vertical central line or may be arranged in a position biased from the vertical central line of driving roll 20 if it is beneficial for mechanical considerations in design.

In this modified first embodiment of the present invention, a separate die supporting mechanism may be necessary to give a gap between the workpiece supporting assembly and the die assembly for belt 131 to travel without interfering the arrangement of the die assembly as shown in Fig. 3. The gap should be at its minimum to restrict bulging of workpiece 10 into it.

Fig. 3 also shows a way of assembling die members 140 and 150 in their supporters 143 and 153 using screw sets 144 and 155 for this modified embodiment, although formations and dimensions of the die members and their supporters should be in practice designed in considerations of the material to be processed, process parameters and the scale of the apparatus and so on.

It is an advantageous feature of this modified embodiment of the present invention to translate the workpiece to be processed using a belt arrangement, allowing resisting friction to be at its minimum.

There might be a possibility of concentrated forces on workpiece 10 due to limited contact areas between rollers 132 and belt 131 and due to the flexible nature of the belt, which may not be able to fully spread the force acting on it into evenly distributed pressure on workpiece 10, in the modified embodiment as shown in Fig. 3. The localised forces, if they happen, may cause unwanted localized plastic deformation in the workpiece before extrusion through the die assembly. To overcome this problem, a well-lubricated top surface 234 of the supporting block as shown in Fig. 4 may be employed to support the belt directly to replace the belt-on-roller arrangement. Rollers 232 and 233 are used for reducing travelling resistance to belt 231.

Although the present continuous frictional angular extrusion process is preferably used for the deformation of strip, sheet and plate crystalline materials, it is possible to apply the process to crystalline materials of other shapes such as bars and rods. Since the cross sectional dimensions of bars or rods are roughly equal in either width or height directions, the material under pressure tends to flow in the width direction at the same time of flowing in the longitudinal extrusion direction. If this occurs, the dimensions of the workpiece cannot be maintained after extrusion and the repeat of deformation in the manner designed in this process will be impossible. Therefore, side restrictions in the width direction need to be applied for processing bar- and rod-shaped workpieces. This may be achieved using extrusion channels which are closed in the width direction with a width identical to that of the workpiece to be processed. Furthermore, a groove in the outer edges of the driving roll may be provided to restrict material flow in the width direction during feeding and extrusion. Fig. 5(a) shows a second embodiment of the present invention for processing a bar-shaped workpiece. A groove 321 with a bottom surface 322 and side surfaces 323 is provided in driving roll 320. The bar-shaped workpiece 310 is guided via a feeding assembly similar to that in the first embodiment of the present invention (not shown) into grove 321 , in which workpiece 310 is pressed by driving roll 320 against workpiece supporter 330 and translated towards and forced through a die assembly for extrusion, by virtue of friction between workpiece 310 and driving surfaces 322 and 323 of groove 321. The die assembly consists of mainly die members 340 and 350 and their supporters 343 and 353. Fig. 5(b) shows the configuration of workpiece 310 in between grooved driving roll 320 and supporting block 330. Fig. 5(c), 5(d) and 5(e) show a configuration of the die assembly for extruding a bar-shaped workpiece. The bottom surface 322 and side surfaces 323 of groove 321 , top surface 341 and side surfaces 343 of die member 340 define a first extrusion channel, and surface 342 and 343 of die member 340 and surface 352 of die member 350 together with side surfaces 323 of groove 321 define second extrusion channel. Again, the above-defined first and second extrusion channels are contiguous, having identical cross-sections corresponding to a cross-section of workpiece 310.The second extrusion channel is inclined to the first extrusion channel at an angle named as extrusion angle similar to the definition in the first embodiment of the present invention. The two channels preferably intersect at an angle between 90 degrees and 135 degrees, allowing plastic deformation to occur in a narrow region along the intersectional plane in a mode of simple shear. The present process has some unique features including continuity in operation and applicability to bulky materials of various forms. As an example of application, an integrated die supporter 360 is used as shown in Fig. 5a through 5e. A tapered plate 361 is used to fasten die member 350 onto die holder 360 with the help of a group of socket countersunk head screw sets 362. Surface 351 of die member 350 should be made to have a sliding fit with surface 322 of groove 321. Die member 340 is secured onto die holder 360 by a group of screw sets 363 and 364. The shoulders 360' of die block holder 360 are inserted into the housing of the apparatus (not shown).

The working length of the driving roll is short for extruding a bar-shaped workpiece and in practice more than one bar-shaped workpieces may be processed simultaneously by providing a number of grooves in the driving roll and corresponding multiple sets of the die assembly, although a single supporting assembly may be used to support a plurality of bar-shaped workpieces. The dimensions of bars and rods that can be processed using this approach may be very large as it is seen that significant limitations to the size of the workpiece may come only from the capacity of the apparatus adopted.

A benefit of using a bar- or rod-shaped workpiece is that it allows better control of texture by feeding the workpiece at altered orientations in different extrusion passes, i. e., rotating the workpiece at an angle, preferably 90 degrees, about its longitudinal axis in between extrusion passes.

Fig. 6(a) is a diagrammatic drawing in section through the central line of a third embodiment of the present invention, which is preferably used for processing sheet workpieces. The special feature of this third embodiment is the use of a separate press assembly for applying a normal pressure to the workpiece for processing to generate frictional force for the extrusion. As shown in Fig.6a, the embodiment comprises mainly a feeding assembly from 401 to 405, a driving roll 420 with a couple of auxiliary rolls 406 and 407, a press assembly of 430 and 435, a die assembly with die members 440 and 450 and their holders 443 and 453, a backpressure assembly 460 and a delivery assembly from 411 to 415. In the process, feeding rolls 405 send workpiece 10 from a coiler or a roller table (not shown) onto driving roll 420, which rotates by power means as indicated by arrow A3 in Fig. 6a and is the drive of the process. Driving roll 420 brings workpiece 10 forward and eventually forces the workpiece, by virtue of friction between the driving roll and the workpiece, through the die assembly that is configured so as to enable plastic deformation of the worlpiece to take place without changing its dimensions.

Bending occurs when workpiece 10 is translated from feeding assembly to the die assembly. However, detailed calculations have shown that the bending moment or force required is negligible compared with the extrusion force needed and that the total bending strain and spring back force due to the bending that the workpiece undergoes during the process are also negligible if the diameter of driving 420 is reasonably large. Rotating-free auxiliary rolls 406 and 407 are employed to apply a moderate force to workpiece 10 and to ensure the workpiece to be in solid contact with driving roll 420. A pull from feeding assembly can be helpful in this regard, and it is then preferable to set feeding rolls 405 idle as soon as workpiece 10 is fed under auxiliary rolls 406 and 407.

The use of a separate press assembly provides alternatives for the design of apparatus and the control of the process. It may be an advantage in practice to have a simple driving roll as that shown in Fig. 6a, which may facilitate manufacturing and maintenance of apparatus. A servo controlled hydraulic press system is used in this design as shown in Fig. 6a although other types of press mechanisms are applicable. The main parts are simply a press head 430 and a hydraulic press 435. Arrow A4 shows the force from the hydraulic press.

In order to enhance the ductility of the material to be deformed, a mechanism to apply a backpressure to the material in deformation is optionally adopted. As shown in Fig. 6a a press beam 460 is employed to apply a pressure, as indicated by arrow A5, to workpiece 10 in the outlet of the die assembly against die supporter 443. As a result, a force to the material in the deformation zone is produced due to the friction between workpiece 10 and press beam 460. It is an important and advantageous feature of the present invention to utilize a backpressure applying means to reduce internal defects and to enhance the ductility of the material in deformation.

Workpiece 10 after extrusion is guided to a pair of delivery rolls 411 via a transition guide 412 and is then delivered into a coiler or onto a roller table (not shown) through a delivery guide 413. Delivery rolls 411 may apply a slight pulling force on workpiece 10 to reduce extrusion force or to ensure that the workpiece 10 is fully stretched to avoid unnecessary contact with die supporters. Again, in a similar way to that in the first embodiment, delivery rolls 411 may need to pull the workpiece out of the die assembly at the end of the extrusion process. A special feature of the third embodiment as shown in Fig. 6a is that it is suitable for horizontal operation, which may be an advantage in practice. ι

Fig. 6b is a partly enlarged view of the third embodiment, showing in detail the mechanical and operational relationships between workpiece, driving roll, press assembly, die assembly and backpressure assembly. Press head 430 is designed to transmit and distribute concentrated force provided by hydraulic press 435 into distributed pressure on workpiece 10. Its surface 431 in contact with workpiece 10 is curved corresponding to the radius of driving roll 420. The press assembly should be arranged as closely as possible to the die assembly next to it to make the frictional force generated more effective and to avoid as well upset of the workpiece upstream of the region in which plastic deformation takes place.

Press head 430 can have structures and thus allow a better control of normal pressure distribution on the workpiece than the first embodiment. This is particularly important when the width of workpiece to be processed is large.

The configuration of the die assembly in this third embodiment is identical to that in the first embodiment as the relationship between driving means, workpiece and die assembly and deformation mode are concerned. As shown in Fig. 6(b), driving surface 421 of driving roll 420 and surface 441 of die member 440 define a first extrusion channel therebetween and surface 442 of die member 440 and surface 452 of die member 450 define a second extrusion channel therebetween. The die assembly is configured such that the cross-sections of the first and second extrusion channels are identical corresponding to a cross-section of workpiece 10. The two extrusion channels are contiguous and disposed at an angle named as extrusion angle, which is 90° in Fig. 6a and Fig. 6b. A sliding fit is required between surface 421 and surface 451 to maintain the designed configuration of the die assembly. During extrusion, workpiece 10 under frictional drag enters the die assembly at the first extrusion channel and exits through the second extrusion channel with plastic deformation occurring in a narrow region along the intersectional plane of the two channels. All the principles described above in respect of Fig. 2 apply here.

Similar to the first embodiment and as may be seen from Fig.7b workpiece 10 in its width direction in the extrusion channels is free, allowing sheet workpieces of various widths to be processed.

The bottom surface 444 of die holder 443 needs to be levelled with the bottom surface 442 of die member 440 and thus the frictional force generated between surface 461 and workpiece 10 will act on the material in the deformation zone against driving roll 420, applying effectively a hydrostatic pressure to the material in the deformation zone. The magnitude of backpressure that can be applied depends on the distance between the deformation zone and backpressure assembly 460: the smaller the distance, the higher backpressure applicable. The maximum pressure in the present embodiment should be below the yield stress of the material in processing. The backpressure applied by the present method can be maintained constant readily and can also be altered easily if required. It should be noted that the application of a backpressure would increase the extrusion force accordingly for the process.

Again, the friction between workpiece 10 and press head 430 should be at its minimum and a belt-on-roller or a simple belt arrangement can be utilized to fulfil this requirement. Fig. 7a and 7b show a simple belt arrangement in replacement of press head 430 in Fig. 6a and Fig. 6b. A slot 533 is made in belt supporter 530, allowing belt 531 to travel around smoothly and continuously with the help of supporting rollers 532 and 533. Fig.7a and Fig.7b show a two-hydraulic presses arrangement although different arrangements are applicable. Belt supporter 530 and driving roll neck 420 are installed in housing 400 in their necks 530' and 420'. Again detailed housing structures are not shown for simplicity of presentation.

It is an aim of the present invention to produce a fine-grained microstructure for metals and alloys and other crystalline materials. The achievement of large plastic deformation by the present continuous frictional angular extrusion process may give rise directly to a required fine-grained microstructure. However, heat treatments may be required, depending on individual materials and processing parameters. It has been shown that annealing at a relatively low temperature can provide conditions for recovery of substructures and continuous recrystallization to occur, producing a stable and equiaxed fine-grained structure. Continuous recrystallization annealing is preferred as a final heat treatment after deformation although the exact annealing temperatures are dependent on materials and processing parameters.

Work hardening and microstructure refinement after several passes of extrusion may cause a substantial increase in the strength of material and a loss of its ductility at the same time. Again a relative low temperature annealing treatment to soften the material without loosing high angle grain boundaries is preferable before the next round of extrusion. This annealing treatment may be performed at any stages of the present deformation processing procedure. The present method and apparatus can be used at room temperature and elevated temperatures as well to produce intensive plastic deformation. In the practice of processing at elevated temperatures, the workpiece needs to be heated either before extrusion using a separate furnace or on the extrusion line during extrusion (detailed arrangements for on-line heating are not shown). A problem for processing at elevated temperatures is the possibility of the presence of a temperature gradient across the thickness of the workpiece. This may be caused by the difference in cooling rate at the two sides of the workpiece. The side of the workpiece in contact with driving roll 20 may have a lower temperature than the other side because of the higher heat capacity of driving roll 20, which produces a higher cooling rate for the heated workpiece. To overcome this problem, a preheated driving roll may be useful. Modifications to the driving surface of the driving roll may be an alternative choice to enhance its heat resistance and wear resistance as well.

Modifications and variants, such as would be readily apparent to the skilled addressee, may be made to any of the embodiments described above without departing from the scope of the present invention.

Claims

CLAIMS:

1. A method of extrusion of a workpiece including: providing an extrusion die assembly and a driving means for forcing the workpiece through the extrusion die assembly; causing a surface of the workpiece and a surface of driving means to contact to form an interface therebetween; applying a force to the workpiece directed towards the interface to secure a frictional force at the interface; forcing the workpiece through the die assembly using the frictional force; plastically deforming the material of the workpiece during extrusion through the extrusion die assembly without changing the dimensions of the workpiece.

2. A method according to claim 1 including applying said force in a direction substantially normal to the interface between the driving surface and said surface of the workpiece thereby to secure said frictional force.

3. A method according to any preceding claim including pressing the driving surface against the workpiece to secure said frictional force.

4. A method according to any preceding claim including pressing the workpiece against the driving surface to secure said frictional force.

5. A method according to claim 3 which includes providing a reactive pressure at the workpiece to balance the pressure applied thereto by the driving surface.

6. A method according to claim 5 including providing a workpiece support means such that the pressing of the driving surface against the workpiece results in the pressing of the workpiece against the workpiece support means thereby with the workpiece support means opposing the pressure from the driving surface.

7. A method according to claim 6 including providing the workpiece support with a support conveyor means, and conveying the workpiece over the workpiece support means using the support conveyor means as the workpiece moves in response to said frictional force.

8. A method according to claim 4 including providing a press means with press conveyor means for pressing the workpiece against the driving surface, and conveying the workpiece over the press means using the press conveyor means as the workpiece moves in response to said frictional force.

9. A method according to any of claims 7 and 8 wherein the conveyor means includes rollers and a belt supported by, and arranged to revolve around, the rollers.

10. A method according to any of claims 3 to 9 including providing a coefficient of friction between the drive surface and the workpiece which is greater than the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the press means.

11.A method according to claim 10 including determining the pressure with which the pressed contact is made according to the difference between the coefficient of friction between the drive surface and the workpiece and the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the press means.

12. A method according to any preceding claim including applying a variable reverse frictional force to a surface of the workpiece as it exits the extrusion die assembly thereby to generate a variable back pressure upon the workpiece thereby to apply a hydrostatic pressure to the workpiece within the extrusion die assembly.

13. A method according to any preceding claim including providing a first extrusion channel and a second extrusion channel contiguous with the first extrusion channel and inclined to the first channel at an extrusion angle, forcing the workpiece into the die assembly via the first extrusion channel to exit the die assembly via the second extrusion channel, wherein the first and second extrusion channels have the same dimension normal to the direction of travel of the workpiece through the respective extrusion channel in use corresponding to the thickness of the workpiece.

14. A method according to claim 13 including providing the first extrusion channel as a spacing between the driving surface and an opposed surface of the extrusion die assembly, and moving the driving surface relative to the opposed surface of the die assembly thereby to translate the workpiece through into the first extrusion channel and through the die assembly using said frictional force.

15. A method according to claim 14 including applying a controllable backpressure to the workpiece at the outlet of the second extrusion channel.

16. A method according to any preceding claim in which the driving surface is provided by a surface of a rotatable drive roll member, and the method includes rotating the drive roll member to drag the workpiece by friction therewith through the extrusion die assembly.

17. A method according to claim 16 in which the driving surface is provided by a substantially ungrooved surface of the drive roll member.

18. A method according to claim 16 or 17 in which the driving surface forms in part the first extrusion channel and drives the workpiece by friction developed therewith through the die assembly.

19. A method according to any previous claim in which the workpiece is in the form of a sheet including strip and plate, the ratio of the width to the thickness of the sheet workpiece is greater than 5, or greater than 10.

20. A method according to any of claims 16 to 19 in which the driving roll has an endless circumferential groove therein, a first die member of the die assembly comprises a shoe member which covers part of the length of the groove, forming the first extrusion channel therewith and a second die member of the die assembly comprises an abutment member which projects into the groove in the driving roll and forms with the first die member the second extrusion channel.

21. A method according to any previous claim comprising the step of heating the workpiece before and/or during the extrusion process.

22. A method according to any previous claim comprising the step of heat-treating the workpiece after extrusion through the extrusion die assembly.

23. A method according to claim 22 in which the step of heat-treating comprises continuous re-crystallisation annealing

24. A method according to any previous claim in which a plurality of continuous frictional angular extrusion processes through the die assembly is performed.

25. A method according to claim 24 in which a step of heat-treating comprising recovery annealing is performed in between any two said extrusion processes.

26. A method of refining the microstructure of the crystalline material to produce a fine grained or an ultra-fine grained microstructure using the method of extrusion according to any preceding claim.

27. An apparatus for extrusion of a workpiece including: an extrusion die assembly and a driving means for forcing the workpiece through the extrusion die assembly, the driving means having a driving surface being arranged to contact a surface of the workpiece to form an interface therewith; pressing means for applying a force to the workpiece directed towards the interface to secure a frictional force at the interface; wherein the driving means is arranged to force the workpiece through the die assembly using the applied frictional force thereby to plastically deform the material of the workpiece during extrusion through the extrusion die assembly, the die assembly being configured to enable said extrusion to occur without changing the dimensions of the workpiece.

28. An apparatus according to claim 27 in which the pressing means is arranged to apply said force in a direction substantially normal to the interface between the driving surface and said surface of the workpiece thereby to secure said frictional force.

29. An apparatus according to any of claims 27 to 28 in which the pressing means is arranged to press the driving surface against the to secure said frictional force.

30. An apparatus according to any of preceding claims 27 to 29 in which the pressing means is arranged to press the workpiece against the driving surface to secure said frictional force.

31. An apparatus according to claim 29 including balance means arranged to provide a reactive pressure at the workpiece to balance the pressure applied thereto by the driving surface.

32. An apparatus according to claim 31 in which the balance means includes a workpiece support means arranged such that the pressing of the driving surface against the workpiece results in the pressing of the workpiece against the workpiece support means such that the workpiece support means opposes the pressure from the driving surface.

33. An apparatus according to claim 32 in which the workpiece support has a support conveyor means arranged to convey the workpiece over the workpiece support means as the workpiece moves in response to said frictional force.

34. An apparatus method according to claim 30 in which the pressing means includes a press conveyor means for pressing the workpiece against the driving surface, and for conveying the workpiece over the press means as the workpiece moves in response to said frictional force.

35. An apparatus according to any of claims 33 and 34 wherein the conveyor means includes rollers and a belt supported by, and arranged to revolve around, the rollers.

36. An apparatus according to any of claims 32 to 35 in which the coefficient of friction between the drive surface and the workpiece is greater than the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the pressing means.

37. An apparatus according to claim 36 in which the pressure with which the pressed contact is made is determined according to the difference between the coefficient of friction between the drive surface and the workpiece and the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the pressing means.

38. An apparatus according to any of preceding claims 27 to 37 including backpressure means arranged to apply a variable reverse frictional force to a surface of the workpiece as it exits the extrusion die assembly thereby to generate a variable back pressure upon the workpiece to urge the workpiece back into the die assembly after it has been plastically deformed.

39. An apparatus according to any of preceding claims 27 to 38 including a first extrusion channel and a second extrusion channel contiguous with the first extrusion channel and inclined to the first channel at an extrusion angle, wherein the driving means is arranged to force the workpiece into the die assembly via the first extrusion channel to exit the die assembly via the second extrusion channel, wherein the first and second extrusion channels have the same dimension normal to the direction of travel of the workpiece through the respective extrusion channel in use corresponding to the thickness of the workpiece.

40. An apparatus according to claim 39 in which the first extrusion channel is defined by a spacing between the driving surface and an opposed surface of the extrusion die assembly, the driving means being arranged to move the driving surface relative to the opposed surface of the die assembly thereby to translate the workpiece through into the first extrusion channel and through the die assembly using said frictional force.

41. An apparatus according to claim 40 in which the backpressure means is arranged to apply a controllable backpressure to the workpiece at the outlet of the second extrusion channel.

42. Apparatus according to any of preceding claims 27 to 41 including a rotatable drive roll member, wherein the driving surface is a surface of a rotatable drive roll member arranged such that rotation of the drive roll member drags the workpiece by friction therewith through the extrusion die assembly.

43. Apparatus according to claim 42 in which the driving surface is a substantially ungrooved surface of the drive roll member.

44. Appararus according to claim 42 or 43 in which the driving surface is arranged to form, in part, the first extrusion channel and is arranged to drive the workpiece by friction developed therewith through the die assembly.

45. Apparatus according to any of claism 27 to 44 in which the workpiece is in the form of a sheet including strip and plate, the ratio of the width to the thickness of the sheet workpiece is greater than 5, or greater than 10.

46. Apparatus according to any of claims 42 to 45 in which the driving roll has an endless circumferential groove therein, and a first die member of the die assembly comprises a shoe member which covers part of the length of the groove thereby forming the first extrusion channel therewith and a second die member of the die assembly comprises an abutment member which projects into the groove in the driving roll and forms with the first die member the second extrusion channel.

47. An apparatus according to any of previous claims 27 to 46 including heating means for heating the workpiece before and/or during the extrusion process.

48. An apparatus according to any of previous claims 27 to 47 including heating means for heat-treating the workpiece after extrusion through the extrusion die assembly.

49. An apparatus according to claim 48 in which the heat treating means is arranged to apply continuous recrystallisation annealing to the workpiece.

50. An apparatus according to claim 48 or 49 in which the heat-treating means is arranged to apply recovery annealing in between any two said extrusions.

51. An apparatus for refining the microstructure of the crystalline material to produce a fine grained or an ultra-fine grained microstructure using the apparatus according to any preceding claim.