GB2409997A

GB2409997A - Microstructure refinement by continuous frictional extrusion

Info

Publication number: GB2409997A
Application number: GB0400142A
Authority: GB
Inventors: Yan Huang
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-01-06
Filing date: 2004-01-06
Publication date: 2005-07-20
Anticipated expiration: 2024-01-06
Also published as: CN100431728C; WO2005065856A1; GB2409997B; GB0400142D0; CN1898041A

Abstract

A method and apparatus for extrusion of a workpiece includes providing an extrusion die assembly 60,70 and a driving means 20 for forcing the workpiece through the extrusion die assembly, then causing a surface of the workpiece and a surface of driving means to contact to form an interface therebetween. Application of a force to the workpiece directed towards the interface secures a frictional force at the interface, and the workpiece is forced through the die assembly using the frictional force to plastically deform the material of the workpiece during extrusion through the extrusion die assembly without changing the dimensions of the workpiece.

Description

MICROSTRUCTURE REFINEMENT BY CONTINUOUS

FRICTIONAL 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, the present invention relates to methods and apparatus for continuous frictional 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.

There are a number of methods of producing crystalline materials with submicron grain sizes, including rapid solidification, powder metallurgy, and vapour condensation methods. However, most of these methods are only applicable to the production of very small quantities of material, often of unusual compositions.

The as-cast grain size of most industrial metals and alloys is general large (> 1 00pm), 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]. In alloys that undergo massive solid- state phase transformations, such as steels and titanium alloys, grain refinement may be obtained via such transformations, and for steels, controlled rolling during the phase transformation (I - a) may result in ferrite grain sizes of less than Aim. However, with the employment of intensive

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plastic deformation, microstructures of reduced grain size (< tam) and improved homogeneity and properties as well can be achieved in steels and titanium alloys [see: R. Priestner and A. K. Ibraheem, Processing of steel for ultrafine ferrite grain structures, Mater Sci & Technol., 16(2000), 1267-1272; A. Yu. Vinogradov, V. V. Stolyarov, S. Hashimoto and R. Z. Valiev, Cyclic behaviour of ultrafine-grain titanium produced by severe plastic deformation, Materials Science and Engineering, A318 (2001) , 163-173].

The strain required to achieve a fine-grained microstructure is large. For example, a mean grain size of less than lOpm 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 submicron 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.

In the case offorging, in order to develop cumulative strain sufficient to provide grain refinement by recrystallization during subsequent annealing, it is necessary to apply a number of successive forging stages along the three perpendicular axes of a billet. However, such a forging operation may be used only with billets having approximately equal dimension along their perpendicular axes. The treatment of plates by such a process results in a marked change of billet dimensions from a plate to a bar-shape.

Recently, several high-strain deformation methods have been developed to overcome difficulties in achieving high strains necessary for finegrained microstructure and texture formation in conventional forming operations. The typical methods include cyclic extrusion-compression (CEC), high-pressure torsion (HPT), equal channel angular extrusion (ECAE) and accumulative roll-bonding (ARB).

Cyclic extrusion-compression (CEC) is a combination of extrusion and compression, which alternate, allowing arbitrarily large strain deformation of a sample with the preservation of the original sample shape [see: J. Richert and M. Richert, A new method for unlimited deformation of metals and alloys, Aluminium, 62 (1986), 604-607; M. Richert, Q. Liu and N. Hansen, Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion-compression, Mater Sci & Eng, A260 (1999) 275-283]. CEC has an advantage that the sample is fully constrained, thereby allowing processing of less ductile materials. This method has been used to achieve redundant strains as high as 90 and grain sizes ofthe order of 2 urn have been achieved in pure aluminium. A disadvantage of this method is the involvement of reversible strain path, which is less efficient for storing dislocations in the material and for breaking up the original grain structure than a directional deformation process. Large-scale application of this method is almost impossible due to the requirement of extremely high deformation forces.

High-pressure torsion (HPT), adapted from the Bridgeman anvil [see: P. W. Bridgeman, Studies in large plastic flow and fracture, McGraw-Hill, New York, 1952], has been used with a wide range of materials [see: I. Saunders and J. Nutting, Deformation of metals to high strains using a combination oftorsion and compression. Metals Science, 18(1984), 571-575; N. A. Smimova, V. I. Levit and V. P. Pilyugin, Evolution of structure of foe single crystals during strong plastic deformation, Phys Met Metal, 61(1986), 127-134]. In this technique, a thin disc is deformed in torsion using friction provided by the application of a large hydrostatic pressure. This method is quite effective in microstructure refining. The equivalent strains that have been induced with this method are typically of the order of 7, and grain sizes as fine as 0.2um have been produced by deformation at room temperature. However, the microstructure obtained by this method is not uniform through the thickness and radius of the specimen and, more importantly, this method cannot be readily scaled up and is only suitable for small-scale laboratory investigations.

An alternative method is equal channel angular extrusion (ECAE) developed in the former Soviet Union by Segal [see: V. M. Segal, Invention Certificate ofthe USSR No. 575892, 22 Oct 1974; V. M. Segal, Working of metals by simple shear deformation process, In: proceedings of 5th / international Aluminium Extrusion Technology Seminar, Chicago, vol2, 1992, 403-406; V. M. Segal et al, Plastic working of metals by simple shear, Russian Metallurgy 1 (1981) 99- 105; R. Z. Valiev, A. V. Korznikov, R. R. Mulyukov, Structure and properties of ultrafine-grained materials produced by severe deformation, Mater Sci Eng, A168 (1993) 141-148; V. M. Segal, R. E. Goforth, and K. T. Hartwig, Apparatus and method for deformation processing of metals, ceramics, plastics and other materials. US patent 5,400,633 (3 Sept.1993); 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 Febl995); Y. Iwahashi et al., Principle of equal- channel angular pressing for the processing of ultra-fine "rained materials, Scripta mater, 35(1996) 143-146, the contents of all of which are herein incorporated by reference]. Recently, this method has attracted extensive interest and a lot of laboratory research has been carried out worldwide to develop this method for eventually being applied in commercial industry.

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 ultrahigh strains.

However, in ECAE process the billet size is limited by the size of the first die channel because material outside of the die channel will suffer upsetting and extrusion force rises rapidly with the increase ofthe 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 efficiency of the process.

Accumulative roll-bonding (ARB), invented by Saito et al. [see: H. Saito, N. Utsunomiya and T. Tsuji, Acta Mater 47 (1999), 579-583] in Japan, involves roll-bonding two sheets using a 50% reduction, to a thickness equivalent to that ofthe starting sheet. The roll-bonded sheet is then cut in half and the two halves are stacked on top of one another and the process repeated until the required strain is achieved. The true strain achieved per pass is approximately 0.7. In principle, the technique can be used to build up unlimited strains in a material, as there is no change in sheet dimensions. This is the only intense straining process which can produce large bulky materials till now, and it has been demonstrated that ultra-fine "rained structure (mean grain size smaller than 1 m) can be achieved in various kinds of steels and aluminium alloys by ARB [see: N. Tsuji, Y. Saito, H. Utsunomiya and T. Sakai In: Ultrafine Grained Materials, Editors: R.S. Mishra, S.L.

Semiatin, C. Suryanarayana, N.N. Thadhani and T.C. Lowe, TMS, Warrendale, PA(2000), 207].

However, in an ARB process half of the surface regions come to the centre in the next cycle and such a procedure is repeated and this would result in complicated distributions of the surface regions and in many cases, introduce inevitably interior contaminations by the enrolment of surface oxides and impurities, causing significant change of the purity of the material when a large amount of cycles are applied. The quality of interracial bonding is another problem for ARB process, especially when large dimensions of sheets are employed, leaving in doubt the reliability of the material produced by this method. Surface friction condition induced heterogeneity of deformation through the thickness of the sheet to be worked may make it difficult to control the final microstructure in ARB process. Furthermore, the method can only be applicable to ductile materials because it is very hard to apply an extra hydrostatic pressure to improve the ductility of the material to be processed. The size limit ofthe sheet to be processed by this method due to the re-cutting and stacking procedures, together with the requirement for surface treatments add yet more disadvantages to the practical application of this method.

Extrusion is a preferred deformation process for microstructure refinement of metals and alloys as the presence of a relatively high hydrostatic pressure under extrusion minimises or eliminates defects which might be created in the material during deformation and enhances effectively the ductility of the material and allows a relatively large strain in a single pass to be produced.

Particularly when extrusion is carried out under plane strain mode in which the stress state at every point throughout the plastically deforming region is characterized by the superposition of a hydrostatic stress on a pure shear stress, providing arguably the most efficient way of deformation in terms of energy utilization and microstructure refinement. In general, an extrusion process is less productive than rolling. However, the combination of a continuous extrusion process and the merits of extrusion as described above has the potential to provide a process for deformation processing of materials with productivity comparable to that of a rolling process. A continuous extrusion process, generally known in the art as Conform, was originally developed by the United Kingdom Atomic Energy Authority in early 1 970s [see: D. Green, Atomic Energy Authonty UK, UK patent GB 1370894, Oct. 1973, the contents of which are herein incorporated by reference], and since has been used for different purposes in many kinds of materials including metals and alloys, polymers and even food. With the existing designs of Conform, however, the products are generally small in cross section such as wires, rods and bars and the applications ofthe process are limited, exerting particularly difficulties for its application 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 alla 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 strain and eventually fine or ultra-fine "rained microstructure with enhanced physical and mechanical properties.

According to a first aspect of the present invention there is provided a method of refining the microstructure of a crystalline material workpiece comprising a continuous frictional extrusion process in which the friction-actuated extrusion takes place continuously 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.

A significantly important advantage of the present invention is that the extrusion of a single workpiece 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 ofthe 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 present invention.

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Another significant advantage of the present invention is that the present continuous frictional 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.

According to the present invention, the workpiece has a predetermined direction oftravel 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 oftravel ofthe workpiece into the die assembly. The extrusion angle may be greater than or equal to 90 , preferably in the range 90 to 135 .

The die assembly comprises a first extrusion channel formed between a moveable first die member and a fixed second die member with the first moveable die member having substantially a greater dimension in the direction in which the workpiece travels for engaging the material than the fixed second die member and a contiguous second extrusion channel formed between the fixed second die member and a fixed third 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 the fixed die members and the workpiece and reduces the tendency of upset to the workpiece upstream of the region in which plastic deformation takes place.

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

The driving roll may be a plain roll having an ungrooved workpiece driving surface - the first extrusion channel defining surface thereof, and the fixed second and third die members may be plain plates or bars. The dimensions of the cross-sections of the first and second extrusion channels in the directions normal to the extrusion channel defining surfaces may be substantially smaller than their dimension in the direction perpendicular to the planes defined by the directions in which the workpiece travels and the directions normal to the extrusion channel defining surfaces. 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 use of a driving roll as a member of the die assembly and of extrusion channels with significantly small dimensions in the directions in which the workpiece travels substantially reduces the negative effects of friction upon a workpiece 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 all ofthe 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 fixed second die member comprising a shoe member which covers part of the length of the groove in the driving roll and a fixed third die member comprising a 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 workplaces are extruded.

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

The continuous frictional extrusion process may utilise one or more bending rolls to effect roll bending of the workpiece. A bending roll may additionally act as an auxiliary driving roll.

The continuous frictional extrusion process 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 roll 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 using a press. The press may be a belt-press arrangement. The belt-press arrangement lessens largely the negative friction between the press and the workpiece and decreases the normal pressure applied to the workpiece required for actuating the extrusion process. The driving roll itself may be used to apply a normal pressure to the workpiece which may be supported by a beam arrangement.

The continuous frictional extrusion process 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.

The continuous frictional 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.

According to the present invention, 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

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ensuring that advantageous microstructures are achieved. Continuous recrystallization annealing is a preferred form of heat treatment. In general, heat treatments at relatively low temperatures are advantageous.

A plurality of continuous frictional 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 repeated continuous frictional extrusion processes.

The microstructure ofthe crystalline material may be refined to produce a fine "rained, preferably an ultra-fine "rained microstructure. Fine grain is understood to refer to a material having an average grain size in the range 1 to loom, whilst ultra-fine grain is understood to refer to a material having an average grain size of less than 1 m. The invention can be performed so as to achieve severe or intensive plastic deformation, the latter being particularly important in the production of ultra finegrained microstructures, and in the achievement of enhanced mechanical and physical properties.

According to a second aspect ofthe present invention there is provided apparatus for refining the microstructure of a crystalline material workpiece comprising continuous frictional extrusion means including 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 continuous frictional extrusion means translates the workpiece into the die assembly along a predetermined direction of travel, and the die assembly 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. The extrusion angle may be greater than or equal to 90 , preferably in the range 90 to 135 . The die assembly comprises a moveable first die member and a fixed second die member defining a first extrusion channel therebetween, and a fixed third die member defining a contiguous second extrusion channel with the fixed second die member, said contiguous first and second extrusion channels are of substantially identical cross-section corresponding to a cross- section of the workplace or of identical open cross sections with an equal dimension corresponding to the thickness ofthe workpiece, wherein the second extrusion channel is inclined to the first extrusion 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 moveable first die member 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 driving roll may be either a plain roll for deformation processing of sheet workplaces or a plain roll having a groove therein for processing bar- and rod-shaped workpieces.

The continuous frictional extrusion means may comprise one or more bending rolls, said bending rolls effecting roll bending ofthe workpiece. A bending roll may be adapted to act additionally as an auxiliary driving roll.

The continuous frictional extrusion means may further 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 ofthe workpiece through the die to take place. The pressure applying means may be a press. The press may have a press head having a modified surface which is modified to provide enhanced lubrication performance and wear resistance. For example, the surface may be lubricated, or be lined with a material having a low coefficient of friction. The press may be a beltpress arrangement or the driving roll itself, and in the latter case the apparatus is effectively compacted and the diversity of the application is increased.

The continuous frictional extrusion means comprises further the step of applying a backpressure to the workpiece being extruded to enhance the ductility ofthe 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.

Embodiments of methods and apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figurel is a diagrammatic drawing in section through the central line of a first embodiment ofthe invention, depicting the arrangement of main parts and the operational principle ofthe method and the apparatus of the present invention; Figure 2(a) (scale 3.5:1 to Fig 1) is an enlarged part-sectional view through the central line ofthe first embodiment, showing detailed structures of press and die assembly in relation to driving roll and workplace including the arrangement of parts in the housing; Figure 2(b) (scale 1.75:1 to Fig 1) is a sectional view along the line B-B in Figure 2(a); Figure 2(c) (scale 1.75:1 to Fig 1) is a sectional view along the line C-C in Figure 2(a); Figure 2(d) (scale 1. 75:1 to Fig 1) is a sectional view along the line D-D in Figure 2(a); Figure 3 is a schematic drawing of a more general configuration of workpiece, driving roll and die blocks, and shows the definition of extrusion parameters; Figure 4(a) is a front view of the first embodiment after removing delivery guides and rolls with half sectional view along the line A-A of Figure 1; Figure 4(b) (scale 3.5:1 to Fig 1) is a partial side view taken in the direction S in Fig.4a, showing the arrangement of parts in housing; Figure 5 is a plan view of the first embodiment, showing the arrangement of guides, rolls and hydraulic cylinders in relation to housing and workpiece; Figure 6 (scale 3.5:1 to Fig 1) is a part-sectional view of a second embodiment, depicting a modified press assembly together with driving roll, workpiece and die assembly; Figure 7(a) is a front view ofthe second embodiment after removing delivery guides and rolls with half of the hydraulic cylinder holder broken away; rid Figure 7(b) (scale 3.5:1to Fig 1) is a sectional view along the line E-E in Figure 7 (a); Figure 7(c) (scale 3.5:1 to Fig 1) is a part- sectional view along the line F-F in Figure 7(b); Figure 8 is a cross- sectional view of a third embodiment of the invention; Figure 9 (scale 1. 75:1 to Fig 8) shows cross-sectional views of (a) a workplace supporting assembly and die assembly and (b) a modified workplace supporting assembly and die assembly for the third embodiment of the invention; Figure 10 (scale 3.5:1 to Fig 8) shows (a) a cross-sectional view (b) a plane view and (c) a front view along the T direction as shown in (a) of a die assembly for the third embodiment of the invention; Figure ll(a) (scale 1.75:1 to Fig 8) shows an enlarged partial sectional view of a fourth embodiment ofthe invention, depicting the structures ofthe driving roll and the die assembly for the extrusion of bar-shaped workpieces; Figure 11(b) (scale 1.75:1 to Fig 8) is a part-sectional view along the line G-G of Figure 11(a), showing the configuration of a bar-shaped workpiece in between grooved driving roll and supporting assembly; and Figures 11(c), (d) and (e) (scale 3.5:1 to Fig 8) show, respectively, a cross sectional view, a plan view and a front view of a die assembly used with the fourth embodiment of the invention.

Preferred but non-limiting embodiments of the present invention are shown in Fig. 1 to 11 in which like numerals are used to describe features that are common to various of the Figures. A first embodiment ofthe present invention is shown in Fig. 1 through Fig. 5, a second embodiment ofthe invention is shown in Fig. 6 through Fig.7c, a third embodiment ofthe invention is given in Fig. 8 through Fig. 10 and Fig. 11 presents a forth embodiment ofthe invention. The drawings are diagrammatic, showing the operational principles ofthe method and the key parts ofthe apparatus and their approximately relative positions and dimensions.

As shown in Fig.1, the first embodiment of the present invention includes mainly a feeding assembly from 1 to 6, a driving roll 20 and die blocks 60 and 70, a group of bending rolls 22, 24 and 26, a hydraulic press assembly from 40 to 57, a backpressure assembly from 80 to 89 and a delivery assembly from 11-17 and other less important components and subassemblies. The feeding assembly consists of a pair of motor-driven feeding rolls 6, an entry guide 1 ahead of the feeding rolls 6 and a transition guide 4 between the feeding rolls 6 and the driving roll 20. The feeding rolls 6, which rotate by power means in the direction as indicated by the arrows therein in Fig. 1, drive a workpiece 10 to be extruded from a caller or a roller table (not shown) onto the driving roll 20. Entry guide 1 is mounted on its supporting beam 2 and transition guide 4 is mounted on its supporting beam 5. Both beams 2 and 5 are inserted and secured in the housing 100. Side guides 3 (see Fig. 5) may be used at either side ofthe workpiece 10 to restrict its lateral movement if there is any. Side guides 3 are also mounted on the supporting beams 2 and 5.

Detailed structures of the feeding and delivery assemblies are not shown for simplicity of presentation. However, these structures are standard and are readily available from professional reference books.

The workpiece 10 is then brought forward by the feeding rolls 6 and the driving roll 20, which rotates by power means in a clockwise direction as indicated by the arrow A1 in Fig. 1, into the gap between the driving roll 20 and the first bending roll 22 and the workpiece 10 bends therein as required. Further bending ofthe workpiece 10 occurs under the middle bending roll 24 and under the final bending roll 26 to ensure the workpiece 10 is in solid contact with the driving roll 20.

The driving roll 20 is the main drive of the extrusion process.

The first bending roll 22 is set on top of the driving roll 20, inclined with an angle (spa) to the vertical radial direction of the driving roll 20 towards the extrusion direction as shown in Fig. 1.

The magnitude of angle (pi should be chosen so as to ensure that the workpiece 10 can be readily fed by the feeding rolls 6 into the gap between the first bending roll 22 and the driving roll 20 without requiring any other means. Advantageously, the bending rolls 22, 24 and 26 are driven by a single motor to ensure that they all rotate at the same speed corresponding to the tangential speed of the outer surface of the bent workpiece 10. Preferably, the position of the bending rolls is adjustable in the radial direction ofthe driving roll 20 to fit workpieces of different thickness, and the bending rolls are evenly distributed over the bending arc corresponding to the bending angle (elf -I,), where At is the angle between the vertical radius of the driving roll 20 and the line through the centres of the driving roll 20 and the final bending roll 26 as shown in Fig. 1. The number of the bending rolls employed can vary depending on the requirement of the bending moment which is determined by the radius ofthe driving roll 20 and the thickness and strength of the workpiece 10. During bending, it is the driving roll 20 that drives the workpiece 10 by virtue of friction therebetween. In order for the friction to take effect firmly and steadily, the bending rolls 22,24 and 26 should apply a proper normal pressure on the workpiece 10. A pull from the entry side can be helpful in this regard, and it is then preferable to set the feeding rolls 6 to idle as soon as the workpiece 10 is fed into the gap between the driving roll 20 and the first bending roll 22 and to apply a slight normal pressure on the workpiece 10.

The working length ofthe driving roll 20, the feeding rolls 6, the bending rolls 22,24 and 26 and the delivery rolls 14 can be designed to be the same, as shown in Fig.5, and they can all be installed in the same housing 100 in their necks. An integrated bearing assembly 102 may be required for the driving roll 20 and the bending rolls 22,24 and 26, whereas separate bearing assemblies 101 and 103 may be used for the feeding rolls 6 and the delivery rolls 14 respectively.

The detailed structures ofthe bearing assemblies and other housing mechanisms are not shown in the drawings for simplicity of presentation. However, these structures are standard and available from professional reference books.

The last bending roll 26 can be used as an auxiliary driving roll, if necessary, by applying a larger normal pressure and moment to the workpiece than necessary for bending and consequently producing a frictional force on the workpiece 10 to be added to the total extrusion force. In this case, it may be preferable to drive the last bending roll 26 separately.

A servo controlled cylinder-piston hydraulic press system in between the last bending roll 26 and the die assembly is used to apply a normal pressure on the workpiece 10 against the driving roll 20. The purpose of applying this normal pressure is to actuate a sufficient frictional force between the driving roll 20 and the workpiece 10 to establish and carry out the extrusion process.

Generating a frictional force necessary for continuous extrusion process by applying a normal pressure to the workpiece is an advantageous feature of the present invention.

The principal components of the press assembly include one or more hydraulic cylinders 50 with pistons 52 and a pressing beam 45 and a press head 40. Fig. 2(a), 2(b), 2(c) and 2(d), Fig. 4(a) and 4(b) and Fig.5 show a three-cylinder press assembly. A cylinder 50 is held on its rim 51 in the hole 55 of a cylinder holder 54, which is inserted in the housing 100 by edges (not shown). The pressing beam 45 is designed to transmit and distribute relatively concentrated force provided by the pistons 52 into distributed pressure on the workpiece 10 through a plate press head 40. The surface 41 of the press head 40, which is in contact with the workpiece 10, is curved corresponding to the radius of the driving roll 20. In order to fit the workpiece 10 with variable thickness and to accommodate possible mismatch between the workpiece 10 and the surface 41, the plate press head 40 should be slightly bendable. As shown in Fig. 2(a) a cavity 49 is made in the pressing beam 45 to allow the bending of the plate press head 40 to occur to get a fully intimate contact with the workpiece 10. The hump 48 of the press head 40 embedded into the cavity 49 in half way, is used to restrain relative movement vertically between the press head 40 and the pressing beam 45 and to enhance the bending resistance ofthe press assembly vertically. A tapered key 56 is mounted in a keyway 57 to restrict relative movement between the pressing beam 45 and the press head 40 in the other directions. The shoulders 47 ofthe pressing beam 45 and the shoulders 42 of the press head 40 are mounted in the housing 100 on top of a T-shaped block 67, which is employed to secure and fasten the die assembly in the housing 100. A spring welded to a block 104, which is secured in the housing 100 by a tapered key 106, is employed to hold the press head 40 and the pressing beam 45 in position, giving enough space between the driving roll 20 and the press head 40 at the initial stage ofthe extrusion process for the workpiece to pass through before applying a required normal pressure thereto. The spring 105 is designed to hold the weight of the press head 40 and of the pressing beam 45 along the plane surface 68 and to be forced back when the press head 40 is pressed against the workpiece to apply a normal pressure for actuating the extrusion process. The plane surface 68 should be substantially parallel to both the axial direction of the hydraulic cylinders 50 and the top surface 66 of the upper die al block holder 63 to minimise or avoid forces from the press assembly on the die blocks 60 and 70.

One of the basic requirements for the press head 40 is that the friction coefficient between the press head 40 and the workpiece 10 (fir) must be significantly smaller than the friction coefficient between the driving roll 20 and the workpiece 10 (fa). Thus the driving roll 20, while rotates by power means, will be able to create a net frictional force to the workpiece 10, which is under a normal pressure from the hydraulic press assembly, in the direction tangential to the surface ofthe driving roll 20 at each corresponding point towards the die assembly. If the normal pressure on the workpiece 10 is sufficient, the resultant frictional force will be able to force the workpiece 10 to flow through the die assembly below the press assembly with plastic deformation taking place in the workpiece.

The vertical bending ofthe press head 40 and the beam presser 45 due to the friction between the workpiece 10 and the press head 40 may apply a force on the die assembly and result in an unwanted pressure on the material to be extruded and therefore should be strictly controlled. The press assembly should be made as closely as possible to the die assembly next to it to avoid upset of the workpiece upstream of the region in which plastic deformation takes place.

The principal members of the die assembly include the rotary driving roll 20 and two fixed die blocks - an upper die block 60 and a bottom die block 70. As shown in Fig. 2(a), the surface 21 of the driving 20 and the surface 61 of the upper die block 60 define a first extrusion channel therebetween and the surface 62 ofthe upper die block 60 and the surface 71 ofthe bottom die block 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 crosssection ofthe workpiece 10. The two extrusion channels are contiguous and disposed at an angle named as extrusion angle. The dimension of the first and second extrusion channels in the directions in which the workpiece 10 travels is substantially smaller than the circumferential length ofthe driving roll. During extrusion processing, the 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.

The intersectional plane ofthe first and second extrusion channels is indicated by a dotted line OB in Fig. 3, which shows a more general relationship, than in Fig. 1 and Fig. 2(a), (b), (c) and (d) in which the extrusion angle is 90 , between the driving roll 20 - a moveable die member, the workpiece 10, the press head 40 and the die blocks 60 and 70. The surface 21 of the driving roll which defines in part the first extrusion channel and the surface 71 of the bottom die block 70 which defines in part the second extrusion channel meet at the point O. FO, tangential to the meeting point O. is the direction of travel of the workpiece 10 in the first extrusion channel and OE is the direction in which the workpiece is extruded through the second extrusion channel, i.e. , the extrusion direction. The angle between OF and OE defines the extrusion angle (20) 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 OB divides equally the extrusion angle (20) and the dimensions of the 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.

Preferably, the first embodiment of the present invention is employed for the deformation processing of sheet (including strip and plate) workplaces of crystalline materials. If the ratio of the width and thickness ofthe sheet workpiece is greater than 5 or preferably greater than 10, the material flow during deformation will take place exclusively in the longitudinal planes defined by the longitudinal direction of the workpiece in which the workpiece travels and the thickness direction or normal direction of the workpiece, i. e., the deformation is carried out 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 OB. The plastic strain the workpiece undergoes through the die assembly is dependent only on the extrusion angle: y= 2 cot t) (1) where His the shear strain produced in each pass, and the corresponding equivalent strain per

-

pass (a) is then = cot R (2) The total equivalent strain produced after n passes can be added as In = ne (3) For an extrusion angle of 90 , = 1.15 and after 10 passes em = 11.5. The extrusion pressure O is a function ofthe yield stress of the material (firs) and the extrusion angle (20: p=cot (4) or p = 2k cot (4) where crS is the yield strength of the material to be extruded and k is the shear strength of the material, k = crs / according to Levy-Mises criterion.

In the die assembly employed in the first embodiment of the invention the first and second extrusion channels are open in the width dimension (in the direction perpendicular to the planes defined by the directions normal to the extrusion channel defining surfaces 61 and 62 or 72 and the directions in which the workpiece 10 travels), although extrusion channels of closed sections (not shown) are applicable, allowing sheet workplaces of various widths to be processed. Fig. 2(b), 2(c) and 2(d) show, together with Fig. 2(a), detailed structures of a die assembly with extrusion channels opening in the width direction and of the associated assemblies.

The upper die block 60 is mounted in its supporter 63 and the bottom die block 70 is mounted in its supporter 72 which is inserted in the housing 100 and also secured on its foot 77 to the base 108 using three set-bolts 76. The die blocks and their supporters are all inserted in the housing 100. Fig. 4(b) shows in the housing 100 the arrangement of die blocks 60 and 70, the shoulders 64 of the upper die block supporters 63 and the shoulder 73 s of the bottom die block supporter 72.

The extended part 65 of the upper die block supporter's shoulder 64 is used to set an accurate distance, corresponding to the thickness of the workpiece, between the surfaces 62 and 71. A T-shaped block 67 is used on top of the die assembly in the housing 100, with the help of a couple of set-bolts 107 in the sink 69, to fasten and secure the die blocks 60 and 70 and the shoulders of the die block supporters 64 and 73 in the housing 100 and at the same time to act as a supporter of the press head 40 (shoulder 42) and the pressing beam 45 (shoulder 47).

In order to enhance the ductility of the material to be deformed, a means of applying a backpressure to the material in the deformation region is optionally adopted. As shown in Fig. 2(a), 2(d) and Fig. 4(a), a couple of hydraulic cylinders are employed to apply the backpressureto the workplace 10 through a beam 80, which is mounted in the bottom die block supporter 72 at the outlet of the second extrusion channel. Each of two cylinders 85 with their piston 84, is held on its rim 86 in the cylinder holder 88. A part 87 represents the supplier of high-pressure fluid.

The pressure applied by the pressing beam 80 is balanced by the upper die block supporter 63 through a plate 82, which is inserted into the upper die block supporter 63 and secured therein by a pair of countersunk head slotted screws 89. The bottom surface 83 of the plate 82 is levelled with the bottom surface 62 ofthe upper die block 60 and the top surface 81 ofthe pressing beam is levelled with the top surface 71 of the bottom die block 70. Thus, when the hydraulic press system applies a pressure on the beam 80 a frictional force created at interfaces between the workpiece 10 and the beam 80 and the plate 82 will act on the material against the driving roll 20, applying effectively a hydrostatic pressure to the material in the deformation zone. The surface 81 and surface 83 should preferably have identical friction coefficients and thus the pressure applied to the material in the deformation region by this means will only increase the level of the hydrostatic pressure in the material without changing the deformation mode. The magnitude ofthe backpressure that can be applied depends on the distance between the deformation region and the pressing beam 80, and the maximum pressure in the present assembly is below the yield stress of the material to be deformed. The backpressure applied by the present method can be maintained constant readily and can also be altered easily if required. However, care must be taken when large backpressure is applied to avoid bending of the upper die block 60 vertically. The pressing beam 80 can be lowered to rest on the surface 75 if a backpressure is not required.

The workpiece 10 after extrusion is sent off to a pair of delivery rolls 14 via a transition guide consisted of a main guide 11, a supporting beam 12 and side guides 13, and then through a delivery guide 15 with a supporting beam 16 and side guides 17, delivered into a caller or onto a roller table (not shown). The delivery rolls 14 may apply a proper pull force on the workpiece 10 to reduce extrusion force or to ensure that the workpiece 10 is fully stretched to avoid asymmetrical friction between the workpiece 10 and the surface 81 and 83 if a backpressure is applied. At the end ofthe extrusion process, however, a large force is required from the delivery rolls 14 or from a coiler, if in use, to pull the workpiece 10 out ofthe die assembly because, at this stage of processing, the frictional force provided by the driving roll 20 may not be large enough to perform this task.

The present continuous extrusion process is a frictionally actuated process. The effective friction coefficient (fe) defined as the difference in friction coefficient fe = (fd-f,) with Id the friction coefficient in the workpiece 10-driving roll 20 interface end I, the friction coefficient in the workpiece 10-press head 40 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. The other factors include the strength ofthe material (its)' the thickness ofthe workpiece (t), and the extrusion angle (20). For a hydraulic press system of a limited capacity, higherfe allows harder materials and thicker workplaces to be processed or, for processing a workpiece with certain thickness and strength, higher Me requires a lower pressure. Generally, higher fe enhances the capacity of the apparatus on one hand and reduces energy requirements on the other hand.

The direction ofthe frictional force on the workpiece 10 is tangential to the corresponding surface point of the driving roll 20 towards the first extrusion channel and only the resolved frictional force parallel to FO direction will contribute to the extrusion process (see Fig. 3). The total frictional force attributable to the deformation (Fe) is a function ofthe normal pressure (pn) on the workpiece 10, the arc length of the surface with the normal pressure applied (L) corresponding the angle (up - d) as defined in Fig. 3, and friction coefficient Id end I,, the width (w) and thickness (I) of the workpiece 10 and the radius ofthe driving roll 20 (R): Fe = Wf (fpnR cos -frPn (R + I) cos Am) (5) If the normal pressured and the effective friction coefficients =fd-fr are constant, and f<<R, then Fe WfeNR sin( R) (6) This frictional force Fe must produce a load on the material in the deformation zone along the intersectional plane OB of P = 2wtaS cot/ for establishing and carrying out an continuous extrusion process with the present invention, and the normal pressure on the workpiece required to produce such a load is p 2tS cot R feRSin(R) It may be seen that the normal pressured 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 ofthe driving roll. If R = 500mm, L = 200mm, 2t7=90 , t = 4mm, fe = 0.25 and crS = 750MPa, we get a normal pressured = 71.2MPa. For a sheet workpiece of a width w = 1 000mm, the force required to produce such a normal pressure is 1,423 tons, which is close to the force required for the cold rolling of a sheet of similar strength and dimensions to those defined above.

The application of a backpressure may well increase the extrusion pressure (p) and correspondingly increases the required normal pressure on the workpiece. The bending of the workpiece along the surface of the driving roll before extrusion could be another factor which may affect the extrusion pressure and or apply limitations to the extrusion process. The maximum bending strain (8b) occurs, for an extrusion angle of 90 , in the outer layer of the workpiece in tension: (8) If the workpiece thickness (t) is 4mm and the radius of the driving roll (R) is 500mm, we get lab #0.004. The permissible bending strain is about three fourth ofthe rupture strain for engineering metals and alloys (see: W. Johnson and P. B. Mellor, Engineering Plasticity, 1973, Van Nostrand Reinhold Company Ltd., pl55). Since most engineering materials are of much higher rupture strain than the possible maximum bending strain given by equation 8, the bending procedure adopted in the present frictional continuous extrusion process does not give rise to a substantial limitation in this aspect.

The bending moment of a flat workpiece (M) is defined as M;'2 d where t is the thickness ofthe workpiece, lb is the bending length which is equal to the maximum free length of the workpiece and Uris the stress in the workpiece under pure bending condition, i.e., without shear deformation (tension in the outer fibres and compression in the inner fibres).

The maximum bending moment occurs when plastic bending takes place through the thickness of the workpiece, i. e., cr= crS everywhere and the corresponding bending moment is M= 'I (9) where the maximum bending length Lb is equivalent to the free arc length between two neighbor bending rolls.

The bending force (Fb) is then Fb = M = chit (10) If As = 750MPa, t = 4mm, we get Fb = 0.3 tons; if I = 1 Omm, As = 750MPa, Fb = 1. 87tons. These bending forces are negligible compared to the extrusion force and a spring back force due to the bending, which should be less than the bending force is also negligible. It seems then that bending, as a means of continuously feeding the workpiece, does not exert significant problems to the extrusion process.

There are several ways of increasing the effective friction coefficient '(bye) including: (a) increase the surface roughness ofthe driving roll 20; (b) decrease the roughness ofthe surface 41 ofthe press head 40; and (c) lubricate the surface 41. Increasing or decreasing the surface roughness of the workpiece is of somewhat limited assistance because this process tends to affect both sides and thus the final effects will be cancelled out. Increasing the roughness of the workplace surface in contact with the driving roll and decreasing the roughness of the other side may be useful in terms of raising the effective friction coefficient (fe) but practically expensive.

The lubrication of the surface 41 of the press head 40 using solid lubricants is preferable because it may provide excellent improvements to the extrusion process without troubling the performance ofthe process. Liquid lubricants are not preferred as they can cause problems such as: (a) creating a temperature gradient across the thickness of the workpiece due to the cooling effect of solid lubricants. This effect can be critical when deformation is carried out at elevated temperatures and the effect may be also considerable for room temperature deformation since it is inevitable that there is a deformation heat production and consequently some temperature rise in the workpiece during deformation. The main problem of the temperature gradient across the thickness of the workpiece is the variation of the strength of the material with temperature, causing instability to the extrusion process; (b) requiring additional systems to apply the lubricants; (c) eroding parts of the apparatus; and (d) requiring a cleaning treatment of the lubricated surface before the next round of processing.

Alternatively, the press head 40 may be made using a self-lubricating material or lined with a wear resistant material with low friction coefficient on top of the working surface 41 to enhance the performance. Furthermore, the working surface 41 of the press head 40 may be modified to enhance surface smoothness and wear resistance.

In addition to the means mentioned above, a structural modification to the press assembly is made to reduce the negative friction between the workpiece and the press head for applying a normal pressure to the workpiece and, as a result, to increase significantly the effective friction coefficient (fe). Fig. 6 is a part-sectional view of a second embodiment of the present invention, showing a modified press assembly together with the driving roll 20, the workpiece 10 and the die assembly.

Fig.7(a) shows the arrangement of a modified two-hydraulic cylinder press assembly and Fig. 7(b) is an enlarged sectional view along the line E-E in Fig. 7(a) and Fig.7(c) is an enlarged view along the line F-F in Fig. 7(b). Instead of using a plate press head 40, a belt 110 is used to press against the workpiece 10 to apply the normal pressure necessary for extruding the workpiece. The belt is supported by a group of rollers 112 with bearings! 13, which are installed in a modified pressing beam 115. Two rollers 114 are used at the back of the modified pressing beam 115 and thus the belt revolves on the rollers 112 around the modified pressing beam 115. With the application ofthis modified press system, there is no relative movement between the workpiece 10 and the pressing head - the belt 110 and the resistance to the workpiece is only the rolling frictionbetween the belt 110 and the rollers 112 and 114, which is practically negligible.

As shown in Fig.6 and Fig. 7(a), two hydraulic cylinders are placed at either end of the pressing beam 115 to allow the belt 110 to revolve around the pressing beam 115 continuously. The hydraulic system applies the required force on the pressing beam 115 which distributes the force on the workpiece through the belt 110 on the rollers 112. The width ofthe belt 110 is preferably a bit larger than the width ofthe workpiece 10. As shown in Figures 7 (a), 7(b), and 7(c), in either end ofthe pressing beam 115 rollers 112 and 114 are inserted with bearings 113. Bearing chocks and 121 are used at the side in contact with the workpiece 10 to fix and secure the rollers 112 using set-bolts 124, 130 and 131 and socket head screws 128 in the sink 117, and bearing chocks 122 and 123 are used at the side in contact with the hydraulic cylinders 50 to fix the rollers 114 using set bolts 125, 126, 127 and socket head screws 129 in the sink 117.

When two hydraulic cylinders are used they can be installed in either side of the housing 100 without the supporting beam 54 (not shown). Two sets of belts and a single central hydraulic cylinder in between the two belts may be used as an alternative arrangement ofthe modified press assembly (not shown). The material and structure of the belt 110 should be chosen such that the pressure from the rollers 112 is spread and distributed over the whole surface of the belt 110 when it is pressed against the workpiece 10. Calculation has suggested that the rollers 112 themselves cannot be used directly to apply a normal pressure on the workpiece high enough to actuate the continuous extrusion process before they deform the workpiece. The bending of the modified pressing beam 115 can be a problem if the width of the workpiece 10 is large. An advantage of the arrangement shown in Fig. 6 and Fig. 7(a), 7(b), and 7(c) over installing the hydraulic cylinders in the housing 100 is that a counterbalance mechanism can be employed such as the use ofthe spring 105 to cancel out partly or completely the bending ofthe pressing beam if there is any.

In a third embodiment ofthe present invention a more compact apparatus and enhanced versatility of the method is provided as shown in Fig. 8, which is a cross-sectional view of the third embodiment. The principal feature ofthe modified apparatus is that the driving roll 220 instead of using a separate press assembly is used to apply the normal pressure on the workpiece 10 required for generating frictional force for extrusion. In this compacted third embodiment the driving roll 220 applies a normal pressure to the workpiece 10, which is translated so as to be under the driving roll 220, and rotates in an anti-clockwise direction as indicated by the arrow A2 to force the workpiece 10 through a die assembly comprising the driving 220 and die blocks 250 and 270 using frictional drag developed between the workpiece 10 and the driving roll 220.

Another important feature ofthe third embodiment is that, as mentioned above, the workpiece 10 is placed under the driving roll 220, and as a result, the distance that the workpiece 10 needs to travel along the surface of the driving roll 220 before deformation is shortened and the corresponding bending ofthe workpiece is reduced, leading to an enhanced range of material and of workpiece thickness that can be processed by the present process. An entry assembly similar to red that of the first embodiment is used to feed the workpiece 10 to a single bending roll 210. The bending roll 210 actually backs up the workpiece 10 against the driving roll 220 so that, under the drive by the bending roll 210 and the driving roll 220, the workpiece is introduced into the gap between the driving roll 220 and the workpiece supporter 230. Bending about the bending roll 210 occurs when the workpiece 10 passes through it. Having passed through the bending roll 210, the workpiece 10 stays on the surface of the driving roll 220 due to a spring back force in the workpiece 10 as a result ofthe bending about the bending roll 210, and is readily fed into the gap between the driving roll 220 and the workpiece supporting assembly. The extruded workpiece 10 is sent to a caller or a roller table (not shown) via delivery rolls 280.

Fig. 9(a) is an enlarged partial central line sectional view, showing a preferred design of the workpiece supporting assembly and a die assembly for the third embodiment of the present invention. A belt 231 is used on top of a group of rollers 232 with bearings 233, which are installed in the supporting beam 230, to support the workpiece 10 and to balance the pressure from the driving roll 220 with a largely reduced frictional resistance to the workpiece 10. The supporting beam 230 is located below the driving roll 220 and symmetrical about its vertical central line. At the bottom ofthe beam 230, a couple of rollers 234 are used to allow the belt 231 to travel around the beam 230 continuously and smoothly. Bearings 233 and 235 are used for protecting the rollers 232 and 234 respectively and are applied through the length of the rollers.

The end bearing assemblies are inserted in the housing (not shown) in which the entry rolls 6, the transition guide supporting beams 2 and 5, the driving roll 220, the die block holders 260 and the delivery rolls 280 are also installed.

When the supporting rollers 232 are used there is a possibility of creating concentrated forces on the workpiece 10 due to limited contact areas between the rollers and the belt 231 and the flexible nature of the belt 231, which may not be able to fully spread the force acting on it into evenly distributed pressure on the workpiece 10. The localised forces may cause unwanted localized plastic deformation in the workpiece before extrusion through the die assembly. To overcome this problem, a well-lubricated surface 240 may be directly employed to support the workpiece 10 instead of using rollers 232 and an arrangement of such a workpiece supporting assembly is shown in Fig. 9(b). The rollers 236 and 234 are used for reducing the travelling resistance to the belt 231.

Fig. 9(a) and 9(b) shows a die assembly configuration for the third embodiment of the invention with a general extrusion angle, which is between 90 and 135 . The configuration of the die assembly is identical to that in the first embodiment, although the orientation ofthe die blocks250 and 270 and the structure of their supporting components are different. In Fig. 9(a) and Fig. lO(a), lO(b) and lO(c), the die block 250 is equivalent to the upper die block 60 in Fig. 1 and the die block 270 is equivalent to the bottom die block 70. Correspondingly, the surface 252 of the die block 250 and the surface 221 ofthe driving roll 220 define the first extrusion channel, and the surface 251 of the die block 250 and the surface 271 of the die block 270 define the contiguous second extrusion angle, wherein the second extrusion channel is disposed to the first extrusion channel at an angle which corresponds to the extrusion angle. The dimensions of the first and second extrusion channels in the directions normal to the channel defining surface 252, 251 and 271 are identical corresponding to the thickness of the workplace 10 and the channels are open in the direction corresponding to the width of the workpiece 10. The workplace 10 enters the die assembly through the first extrusion channel and exits the die assembly through the second extrusion channel. The surface 272 should be manufactured carefully so as to have a sliding fit with the driving roll 220. Fig. lO(a), lO(b) and lO(c) show the detailed structure of the die assembly for the third embodiment. Both die blocks 250 and 270 are mounted in the single die holder 260, in which a channel 262 is adopted for mounting, at the top thereof, the die blocks 250 and 270 and guiding and delivering the deformed workpiece 10 to delivery rolls 280. The rolls 280 drive the workplace 10 to a coiler or a roller table for further processing (not shown). The die block 250 is fastened using the socket countersunk head screw sets 254 and 255 and the die block 270 is fastened by the socket countersunk head screw sets 273. In this die assembly, the die blocks 250 and 270 are shorter in length than their holder 260 for ease of replacement in case for repair or for changing die blocks of different properties and dimensions. The shoulders 261 ofthe holder 260 are inserted in the housing (not shown) together with the shoulders ofthe supporting beam 230 and other assemblies and parts. A backpressure may also be applied with the third embodiment.

Although the present continuous frictional 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 will be impossible. Therefore, side restrictions in the width direction should be applied for processing bar- and rod-shaped workpieces. This may be achieved using extrusion channels which are closed in the width direction with the width of the channels corresponding to the width ofthe 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. 11 (a) shows a fourth embodiment of the invention for processing a bar-shaped workpiece. A groove 322 with a bottom surface 323 and side surfaces 324 is provided in the driving roll 320. The bar-shaped workpiece 310 is guided using a bending roll 315 into the grove 322, in which the workpiece 310 is pressed by the driving roll 320 against a belt 331. The belt 331 is placed on the top surface 340 of a supporting beam 330. The normal pressure created on the workpiece 310 must be high enough to force the workpiece, by virtue of friction between the workpiece 310 and the surfaces of the groove, through a die assembly for extrusion. The die assembly comprises a moveable member- the driving roll and fixed shoe member - die block 350 and a fixed abutment member - die block 370. Figure l l(b) shows the configuration of the workpiece 310 in between the grooved driving roll 320 and the supporting belt 331 and beam 330. Figures l l(a), l l(d) and l l(e) show a configuration ofthe die assembly for extruding the bar-shaped workpiece. The bottom surface 323 and side surfaces 324 of the groove 322, the entrance surface 352 of the die block 350 and the side surfaces 354 of the die block 350 define the first extrusion channel, and the surface 351 of the die block 350 and the surface 371 of the die block 370 together with the side surfaces 324 of the groove 322 and the side surfaces 354 ofthe die block 350 define the second extrusion channel. The above-defined first and second extrusion channels are contiguous, having identical cross-sections corresponding to a cross-section of the workpiece 310.The second extrusion channel is inclined to the first extrusion channel at an angle corresponding to the extrusion angle. The two channels preferably intersect at an angle between 90 and 135 , allowing plastic deformation to occur in a narrow region along the intersectional plane in a mode of simple shear. This configuration of the die assembly bears some similarity to that in an ECAE process. In one aspect, the present invention combines the die configuration adopted in the equal-channel angular extrusion (ECAE) process with the continuous extrusion process, and applies this approach to the deformation of bulky workplace of metals and alloys and other crystalline materials. The principles described above in respect of Fig.3 apply here. A tapered plate 363 is used to fasten the die block 370 using a group of socket countersunk head screw sets 364 onto the die holder 360. The surface 372 of the die block 370 should be made to have a sliding fit with the bottom surface 323 ofthe groove 322.

The die block 350 is secured onto the die holder 360 by a group of socket countersunk head screw sets 355 and 356. The shoulders 361 of the 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 barshaped workpiece and in practice more than one bar-shaped workplaces may be processed simultaneously by providing a number of grooves in the driving roll 320 and corresponding sets of the die assembly, although a single supporting assembly may be used to support a plurality of barshaped workpieces. The short travel distance and consequently largely reduced bending strain in the forth embodiment render it possible to deform bars and rods of very large size. Significant limitations to the size of bars or rods that can be processed by the present method may come only from the capacity ofthe 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 workplace at an angle, preferably 90 , about its longitudinal axis in between extrusion passes.

It is an essential purpose of the present invention to produce a finegrained microstructure for metals and alloys and other crystalline materials. The achievement of large plastic deformation by the present continuous frictional 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 after several passes of extrusion may cause a substantial increase ofthe strength of the material being processed 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 may be performed at any stages of the extrusion processing.

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 workplace needs to be heated either before extrusion using a separate furnace or on the extrusion line during extrusion (the detailed arrangements for online heating are not shown). A problem for processing at elevated temperatures is the possibility ofthe presence of a temperature gradient across the thickness ofthe 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 the driving roll 20 may have a lower temperature than the other side because of the higher heat capacity of the driving roll 20, which produces a higher cooling rate for the heated workpiece.

To overcome this problem, a preheated driving roll may be useful. The modification of the working surface 21 of the driving roll may be used as an alternative choice to enhance the heat resistance and wear resistance as well. Other solutions would readily suggest themselves to the skilled reader.

Claims

CLAIMS: 3= 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 ol 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.
1 1. 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 workpieee by friction developed therewith through the die assembly.
19. A method according to any previous claim in which the workpieee is in the form of a sheet including strip and plate, the ratio of the width to the thickness of the sheet workpieee 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 heattreating 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 "rained or an ultra-fine "rained 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 as described in any embodiment hereinbefore with reference to to the accompanying drawings. * .-- .. .

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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. Ad

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 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.

I

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 recrystallization 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 "rained or an ultra-fine "rained microstructure using the apparatus according to any preceding claim.

52. A method substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.

53. An apparatus substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of extrusion of a workpiece including: providing a die assembly comprising first and second channels that intersect at an angle and a driving means having a driving surface for forcing the workpiece through the die assembly via said first and second channels; said first channel being defined as a spacing between the driving surface and an opposed surface of said die assembly, with the driving surface having substantially a greater dimension in the workpiece traveling direction for engaging the material than said opposed surface; providing pressing means and therewith pressing against the workpiece thereby applying a pressure to the workpiece directed towards the interface between the driving surface and the workpiece to secure a frictional force at the interface; moving the driving surface relative to said opposed surface of the die assembly thereby to force the workpiece from said first channel into said second channel and through the die assembly using said frictional force; plastically deforming the material of the workpiece during extrusion through. . the die assembly. . 2. A method according to claim1 wherein the first and second channels have substantially identical cross-sections and the workpeice undergoes plastic deformation without substantial changes in the dimensions thereof.

3. A method according to any preceding claim wherein the pressure applied to the workpiece is in a direction substantially normal to the interface between the driving surface and the workpiece thereby to secure said frictional force at the interface.

4. A method according to any preceding claim wherein the pressing means includes said driving surface and the method includes pressing the driving surface against the workpiece to secure said frictional force. alto)

5. A method according to any preceding claim including providing a workpiece support means to support the workpiece and to balance the pressure applied by the driving surface to the workpiece.

6. A method according to any of claims 1 to 3 including pressing the workpiece against the driving surface with said pressing means to secure said frictional force.

7. A method according to claim 5 including providing said workpiece support means with a support conveyor means, and conveying the workpiece over said workpiece support means using the support conveyor means as the workpiece moves in response to said frictional force. . . . . - Is 8. A method according to claim 6 including providing said pressing means with a press conveyor means for pressing the workpiece against the driving surface, : and conveying the workpiece over said pressing 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 5 to 9 including providing a coefficient of friction between the driving surface and the workpiece greater than the coefficient of friction between the workpiece and the workpiece support means or between the workpiece and the pressing means.

11.A method according to any preceding claim including providing a backpressure means and therewith applying a controllable backpressure to the material of the workpiece in a deformation region at the outlet of the second extrusion channel.

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12.A method according to any preceding claim wherein the driving surface includes a surface of a rotary driving roll member, and the method includes rotating the drive roll member to force the workpiece by the frictional force S between the drive roll surface and the workpiece through the die assembly.

13. A method according to claim 12 wherein the driving surface includes a substantially ungrooved surface of said drive roll member.

14.A method according to any previous claim wherein said workpiece is in the form of a sheet e.g. including strip and plate.

15.A method according to claim 12 wherein said drive 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 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 channel. .

. ..- : :..CLME: 16.A method according to any of preceding claims 1 to 12 and 15 wherein said workpiece is in the form of a bar or a rod.

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

18.An apparatus for extrusion of a workpiece including: a die assembly comprising first and second channels that intersect at an angle and a driving means having a driving surface for forcing the workpiece through the die assembly via said first and second channels; said first channel being defined as a spacing between the driving surface and an opposed surface of said die assembly, with the driving surface having substantially a greater dimension in the workpiece traveling direction for engaging the LF3 material than said opposed surface; pressing means arranged to press against the workpiece thereby to apply a pressure to the workpiece directed towards the interface between the driving surface and the workpiece to secure a frictional force at the interface; wherein said driving means is arranged to move the driving surface relative to said opposed surface of the die assembly thereby to force the workpiece from the first channel into the second channel and through the die assembly using said frictional force, thereby to plastically deform the material of the workpiece during extrusion through the die assembly.

1 9.An apparatus according to claim 18 in which said first and second channels have substantially identical cross-sections such that during extrusion therethrough the workpiece undergoes plastic deformation without substantial changes in the dimensions thereof.

20.An apparatus according to any of claims 18 to 19 in which the pressing means..

is arranged to apply a pressure to the workpiece substantially normal to the. : interface between the driving surface and the workpiece thereby to secure said frictional force at the interface. I'

21.An apparatus according to any of claims 18 to 20 in which the pressing means includes said driving surface and is arranged to press the driving surface against the workpiece to secure said frictional force.

22.An apparatus according any of preceding claims 18 to 21 including a workpiece support means to support the workpiece and to balance the pressure applied by the driving surface to the workpiece.

23.An apparatus according to any of claims 18 to 20 wherein said pressing means is arranged to press the workpiece against the driving surface to secure said frictional force.

24.An apparatus according to claim 22 in which the workpiece support means 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.

25.An apparatus according to claim 23 in which said pressing means includes a press conveyor means for pressing the workpiece against the driving surface and for conveying the workpiece over the pressing means as the workpiece moves in response to said frictional force.

26.An apparatus according to any of claims 24 and 25 in which the conveyor means includes rollers and a belt supported by, and arranged to revolve around the rollers. '.

- -- - Is 27.An apparatus according to any of preceding claims 18 to 26 including a backpressure means arranged to apply a controllable backpressure to the: material of the workpiece in a deformation region at the outlet of the second: channel. . , . 28.Apparatus according to any of preceding claims 18 to 27 in which the driving means includes a rotary drive roll member and the driving surface is a surface of a rotary drive roll member arranged such that rotation of the drive roll member forces the workpiece by friction therewith through the die assembly.

29.Apparatus according to claim 28 in which the driving surface is a substantially ungrooved surface of the drive roll member.

30.Apparatus according to claim 28 in which said drive 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 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 channel.

31.An apparatus according to any of previous claims 18 to 30 including heating means for heating the workplace before and/or during the extrusion process.

32. A method substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.