WO2005039792A1

WO2005039792A1 - A method of treating a metal billet

Info

Publication number: WO2005039792A1
Application number: PCT/GB2004/003961
Authority: WO
Inventors: Andrzej Rosochowski
Original assignee: University of Strathclyde
Current assignee: University of Strathclyde
Priority date: 2003-10-08
Filing date: 2004-09-16
Publication date: 2005-05-06
Anticipated expiration: 2006-04-08
Also published as: GB0323541D0; WO2005039792A8

Abstract

A method of treating a metal billet of pre-determined dimensions to change said billet's mechanical and/or physical properties by reducing the billet's grain size, the method involving subjecting the billet to plastic strain in three dimensions in a single operation. More specifically, the method involves forcing the billet through a first passage (1); forcing the billet from the first passage (1) into a second passage (2), the second passage (2) being consecutive with and inclined to the first passage (1), whereby the first and second passages (1 and 2) together define a first plane (4), and forcing the billet from said second passage (2) into a third passage (3), the third passage (3) being consecutive with and inclined to the second passage (2), whereby the second and third passages (2 and 3) together define a second plane (5), the second plane (5) being different from the first plane (4). As the billet exits the third passage (3) its grain size is reduced due to the billet having been subjected to plastic strain in three dimensions. Ideally, the dimensions of the billet on exiting the third passage (3) are substantially the same as the dimensions of the billet on entry to the first passage (1).

Description

A METHOD OF TREATING A METAL BILLET

The present invention relates to a method of, and to apparatus for, treating a metal billet of pre-determined dimensions to change said billet's mechanical and physical properties by reducing the billet's grain size.

The grain size of a metal has an effect on its properties. Reduction 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, whilst at high temperatures, fine-grained alloys may become superplastic. The as-cast grain size of most industrial alloys is generally large (greater than 100 μm). Grain size reduction is generally achieved by thermo mechanical processing, for example, the controlled rolling of steel during phase transformation may result in ferrite grain sizes of less than 5 μm.

Smaller grains than those mentioned above, including sub-micron grains, can be produced by a number of non-conventional methods such as rapid solidification, ball milling and vapour condensation methods followed by compaction and sintering. However, most of these methods are only applicable to the production of small quantities of material, they are expensive, can be hazardous (eg. nanopowders) and can produce porous materials. A further group of methods, which enable the realisation of the benefits of very small grains on a greater scale, are based on large strain deformation of bulk metal. These further methods also overcome the problems, which hinder normal metal processing, of plastic flow localisation and ductile fracture, and have the additional advantage of enabling severe plastic strain to be achieved without changing the shape of the billet.

One of these methods, Equal Channel Angular Extrusion (ECAE), also known as Equal Channel Angular Pressing (ECAP), is described by N.M. Segal in Materials Science & Engineering A volume 197 (1995) pages 157-164. ECAE is based on simple shear taking place in a thin layer at the crossing plane of two passages inclined to each other at an angle greater than or equal to 90°. A metal billet is forced sequentially through both passages. On exiting the second passage the^' billet is rotated by a certain angle about its axis and the operation is repeated by re-entering the billet into the first passage. After several such cycles, large plastic strain and a fine grain structure can be achieved. ECAE as described above is a multi-stage operation and, as a result, is cumbersome and inefficient and not suited to industrial application.

Using more than one sequential turn has been investigated by Liu et al (Materials Science & Engineering A volume 242 (1998) pages 137-140), Nakashima et al (Materials Science & Engineering A volume 281 (2000) pages 82-87) and Nishida et al (United States patent no. 6209379). The common feature of the methods proposed in these articles is that the rotation practically realised between sequential turns is either 180° or 0°. Furukawa et al (Materials Science & Engineering A volume 257 (1998) pages 328-332) and Komura et al (Materials Science & Engineering A volume 297 (2001) pages 111-118) have shown that in terms of grain size morphology and associated superplastic properties, the best results are achieved when the billet is rotated 90° between sequential turns, and that rotations of 180° and 0° are respectively 40% and 50% less efficient than one of 90°.

According to the present invention there is provided a method of treating a metal billet of pre-determined dimensions to change said billet's mechanical and physical properties by reducing the billet's grain size, the method involving forcing said billet through a first passage; forcing the billet from said first passage into a second passage, said second passage being consecutive with and inclined to the first passage, whereby the first and second passages together define a first plane; forcing the billet from said second passage into a third passage, said third passage being consecutive with and inclined to the second passage, whereby the second and third passages together define a second plane, said second plane being different from said first plane.

By subjecting the billet to plastic strain in three dimensions in a single operation, the grain size thereof can be significantly reduced.

Ideally, the dimensions of the billet on exiting said third passage are substantially the same as the dimensions of said billet on entry to the first passage. Consecutive passages may be orthogonal. Additionally or alternatively, the first plane and the second plane may be orthogonal. The minimum length of each passage may be 50% of the passages' width. The cross-section of each passage may be square. Alternatively the cross-section of each passage may be circular.

The method may further involve applying a force to the billet to retard the billet's progress through the die.

According to another aspect of the present invention, there is provided an apparatus for treating a metal billet of pre-determined dimensions, the apparatus including a die having a plurality of connected consecutive passages through which a billet can be directed, the passages including a first passage; a second passage, said second passage being consecutive with and inclined to the first passage, whereby the first and second passages together define a first plane; a third passage, said third passage being consecutive with and inclined to the second passage, whereby the second and third passages together define a second plane, said second plane being different from said first plane, and means for forcing a metal billet sequentially through the first, second and third passages of the die.

The means for forcing a billet through the die is a ram associated only with the first passage and arranged to sequentially push a series of billets into and along the first passage. Alternatively each of the passages is provided with a ram, the rams being operable in sequence to force a single metal billet sequentially through each of the passages.

Consecutive passages may be orthogonal. Additionally or alternatively, the first plane and the second plane may be orthogonal. The minimum length of each passage may be 50%) of the passages' width. The cross-section of each passage may be square. Alternatively the cross-section of each passage may be circular.

At least one of the passages may include means for applying a force to the billet to retard the movement of the billet through the at least one passage to facilitate uniform deformation and suppress fracture of the billet. The retarding means may be at least one ram associated with at least one passage.

It will be understood that the billet can be shorter, longer or equal in length to any of the passages. Furthermore, the die can consist of more than three consecutive passages.

By virtue of the present invention the grain size of the billet is reduced due to said billet having been subjected to plastic strain in three dimensions in a single operation. Ideally, the dimensions of the third passage are such that the dimensions of the billet on exiting the third passage are substantially the same as the dimensions of said billet on entry to the first passage.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a three-passage die system; Figure 2a is a partial view of a three-passage die having rams to force a billet through the die. A billet is. shown at the entrance to the die; Figure 2b shows the billet of figure 2a in the first passage; Figure 2c shows the billet of figure 2a in the second passage; Figure 2d shows the billet of figure 2a in the third passage; Figure 2e shows the billet of figure 2a exiting the die; Figure 3a is a partial view of a three-passage die having a single ram to force a plurality of billets through the die. A first billet is shown at the entrance to the die; Figure 3b shows the first billet in the first passage; Figure 3 c shows the first billet in the second passage; Figure 3d shows a second billet at the entrance to the first passage; Figure 3e shows the second billet being placed in the first passage; Figure 3f shows the first billet in the third passage and the second billet in the second passage; Figure 3g shows a third billet at the entrance to the first passage; Figure 3h shows the third billet being placed in the first passage; Figure 3i shows the first billet exiting the die, the second billet in the third passage and the third billet in the second passage; Figure 4a shows a partial perspective view of a die utilising resistance rams to provide backpressure on a billet moving through the die. A billet is shown in the first passage; Figure 4b shows the billet being forced into the second passage; Figure 4c shows the billet in the second passage; Figure 4d shows the billet being forced into the third passage; Figure 5 a shows a side view of a partially assembled four passage die split into four parts; Figure 5b shows a perspective view of the partially assembled four passage die of Figure 5 a; Figure 5c shows a perspective view of the die of Figures 5a and 5b fully assembled; Figure 6 is a perspective view of a billet having undergone partial extrusion; Figure 7 is a photograph of the grain structure of the billet of figure 6 prior to processing, and Figure 8 is a photograph of the grain structure of the billet of figure 6 post processing; Figure 9a shows a top view of an assembled three piece, four passage die with the second and third passages shown in broken outline; Figure 9b is a perspective view of one of the pieces of the die of Figure 9a; Figure 9c is a perspective view of another of the pieces of the die of Figure 9a.

Figure 1 shows a schematic representation of a three-passage die system, of square cross-section, generally indicated by reference numeral 10 for treating a metal billet of pre-determined dimensions to change said billet's mechanical and physical properties by reducing the billet's grain size. The three passage die includes a first passage 1, an adjoining second passage 2, the first and second passages together defining a first plane 4, a third passage 3 adjoining the second passage 2, the second and third passages together defining a second plane 5, the second plane 5 being orthogonal to the first plane 4. Each of the passages 1, 2, and 3 has the same cross section, which is substantially the same as the cross section of the billet, so that the billet has a snug, sliding fit within each passage. Because of this the overall shape of the billet that is extruded from the die is substantially the same as the shape of the original billet.

A metal billet of pre-determined dimensions, not shown, enters the first passage 1 at entrance 6 and is forced through said first passage 1 into the adjoining second passage 2 passing through a first shear plane 7 at the intersection of the first passage 1 and said second passage 2. The billet is then forced through the second passage 2 into the adjoining third passage 3 passing through a second shear plane 8 at the intersection of the second passage 2 and said third passage 3. Then, the billet is forced through the third passage 3 before finally exiting said third passage 3 at exit 9. The position of shear plane 8 in relation to shear plane 7 is equivalent to a 90° rotation between subsequent turns of the billet. Upon exiting the third passage 3 at exit 9 the billet's mechanical and physical properties have changed due to said billet having been subjected to plastic strain in three dimensions in a single operation, the dimensions of the billet on exiting the third passage 3 being substantially the same as the dimensions of said billet on entry to the first passage 1.

Figure 2a to 2e show a partial view of a preferred embodiment describing the path of a billet moving through a three-passage die by the action of three rams. The die 20 is shown in Figure 2a including a first passage 1 , a second passage 2, and a third passage 3. Also shown is a billet 14 at the entrance 6 to the first passage 1, a first ram 11, a second ram 12 and a third ram 13. In Figure 2b the billet 14 has been inserted in the first passage 1 and the first ram 11, under the action of the first ram drive (not shown), has followed the billet 14 into the first passage 1. Referring now to Figure 2c the first ram 11 continues to act on the billet 14, forcing said billet 14 into an adjoining second passage 2 until the first ram 11 reaches the intersection of the first passage 1 and said second passage 2. The sequential controller that co-ordinates the actions of the rams (not shown) now prompts the second ram 12, under the action of the second ram drive (not shown), to act on the billet 14 and force it through the second passage 2 and into an adjoining third passage 3 until the second ram 12 reaches the intersection of said second passage 2 and said third passage 3 as indicated in Figure 2d. Referring now to Figure 2e the sequential controller that co-ordinates the actions of the rams (not shown) now prompts the third ram 13, under the action of the third ram drive (not shown), to act on the billet 14 and force it through the third passage 3 until the third ram 13 reaches the end of said third passage 3 and the billet 14 has exited the die.

Figures 3a to 3i show a partial view of an alternative embodiment describing the path of a plurality of billets moving through a three-passage die by the action of a single ram. The die 20 is shown in Figure 3a including a first passage 1, a second passage 2, and a third passage 3. Also shown is a first billet 22 at the entrance 6 to the first passage 1 and a ram 21. In Figure 3b the billet 22 has been inserted in the first passage 1 and the ram 21, under the action of the ram drive (not shown), has followed the first billet 22 into the first passage 1. Referring to Figure 3 c the ram 21 continues to act on the first billet 22, forcing the first billet 22 into the adjoining second passage 2 until the ram 21 reaches the intersection of the first passage 1 and the second passage 2. The ram 21 is then withdrawn from the first passage and a second billet 23 is placed over the entrance 6 of the first passage 1, as indicated in Figure 3d. In Figure 3e the second billet 23 has been inserted in the first passage 1 and the ram 21, under the action of the ram drive (not shown), has followed the second billet 23 into the first passage 1. As shown in Figure 3f the ram 21 continues to act on the second billet 23, which in turn acts on the first billet 22, forcing the second billet 23 into the adjoining second passage 2, which in turn forces the first billet 22 into the adjoining third passage 3. The ram 21 is then withdrawn from the first passage and a third billet 24 is placed over the entrance 6 of the first passage 1, as indicated in Figure 3g. In Figure 3h the third billet 24 has been inserted in the first passage 1 and the ram 21, under the action of the ram drive (not shown), has followed the third billet 24 into the first passage 1. As shown in Figure 3i, the ram 21 continues to act on the third billet 24, which in turn acts on the second billet 23, which in turn acts on the first billet 22, forcing the third billet 24 into the adjoining second passage 2, which in turn forces the second billet 23 into the adjoining third passage 3, which in turn forces the first billet to exit the die.

Figures 4a to d show a partial perspective view of an alternative embodiment showing the use of a resistance ram to exert a force on a billet moving through a three-passage die to facilitate uniform deformation and suppress fracture of the billet. The die 100 is shown with a first passage 102, a second passage 104 and a third passage 105. The first passage 102 includes a first driving ram 106 and the second passage 104 includes a second driving ram 110 and a resistance ram 112. In Figure 4a a billet 114 is shown in the first passage 102 after it has been inserted in the first passage 102. The first driving ram 106 follows the billet 114 along the first passage 102 until the first driving ram 106, reaches the rear edge 115 of the billet 114. In Figure 4b the first ram 106 forces the billet 114 to rum into the second passage 102 and into contact with the resistance ram 112 (Figure 4b). The resistance ram 112 will counteract the movement of the billet 114 through the second passage 104 by applying a back-pressure to the leading edge 118 of the billet 114. The first driving ram 106 continues to act on the billet 114 until it reaches the extent of its travel at 120, as shown in Figure 4c. At this stage the second driving ram 110 acts on the billet 114, forcing the billet 114 along the second passage 104 and into the third passage 105. The third passage 105 also has a driving ram and a resistance ram associated with it (not shown).

With a die such as shown in Figure 4, the billet can be returned through the die with the resistance rams acting as driving rams and vice versa. In this way a number of cycles can be performed on the billet without the billet leaving the die. In this scenario, however, an additional ram would need to be incorporated in the first passage to act as a driving ram on the billets return through the die. To achieve uniform deformation and to suppress fracture resistance rams are only necessary in the second and subsequent passages. Plastic deformation, except small upsetting, does not occur in the first passage.

An experimental apparatus was manufactured and experiments carried out using a metal billet manufactured from 99.96% pure aluminium. These experiments will be discussed with reference to Figures 5 to 8.

Figure 5 a shows a partially assembled four passage die, generally indicated by reference numeral 30, suitable for performing 3 dimensional equal channel angular extrusion. The die includes four sections 32, 34, 36 and 38. Sections 36 and 38 are identical, and section 36 is shown mounted on the inner surface 40 of section 32. The die 30 is held together by pin 42, which passes through sections 32 and 36, and is shown emerging from hole 44 in section 36.

Figure 5b shows the next step in the assembly of die 30. Section 38 has been positioned against face 40 of section 32. Section 38 is secured in position by a pin (not shown) passed through hole 46 in section 32 (shown on fig 4a) and hole 48 in section 38. To fully assemble the die 30, section 34 is rotated 180° so the face 54 of section 34 engages face 56 of section 38 and face 58 of section 34 engages face 60 of section 36. Pin 42 can then pass through hole 52 and the pin securing section 38 to section 32 (not shown) can pass through hole 50, thereby securing all four pieces together (as shown in Figure 5c).

With sections 36 and 38 mounted to face 40 of section 32, as shown in Figure 5b, the channel 62 through which the billet (not shown) will be passed, can be seen. This particular embodiment shows a three-turn channel. This particular configuration of die 30 produces a channel 62 in which each surface of the channel is separated from each adjacent surface by a split, i.e. surface 64 is part of section 38 and surface 66 is part of section 32, and are separated by split 68. The splits, such as split 68, assist in reducing the build-up of stress in the die. Stress concentrations occur at sharp corners in materials and having sharp comers manufactured from two separate pieces of materials reduces the stress build-up in the die.

Figure 5c shows the fully assembled die 30. Pin 42 can be pushed through the die to secure the sections. The four sections of the assembled die 30 define a channel through the die 30, the entrance 70 to which is shown. The die 30 will be further secured by prestressing rings (not shown) around the circumference of the die 30 to ensure the die 30 remains intact when exposed to the extensive mechanical pressure to which it will be subject in use.

Figure 6 shows a photograph of a partially processed billet, generally indicated by reference numeral 80. It was manufactured from commercial purity 1070 aluminium and had a square cross section of 8mm x 8mm. The billet 80 has been forced through the first and second channels of the die 30 shown in Figures 5a to 5c. The leading face 84 has been forced through two orthogonal turns, each turn subjecting the material to 1.15 units of plastic strain. The force to push the billet 80 through the die is applied on the trailing face 82 of the billet.

Figure 7 shows the grain size in the aluminium test billet 80 prior to processing. The grains are approximately 200 - 300μm in size. To test the method of the invention, the billet 80 was passed through the die 30 twice. This means that the billet 80 was subjected to three turns on each pass, six turns altogether. These six turns subjected the material to 6.90 units of plastic strain (six turns, 1.15 units per turn). Figure 8 shows the grain size of the aluminium billet 80 after processing, i.e. after being subjected to 6.9 units of plastic strain. In this photograph the grain size is now approximately lμm.

Figures 9a to c show a three piece, four passage die 200 that can be used to provide three-dimensional plastic strain of a billet that has a circular cross section. Figure 9a shows three component parts 202, 204 and 206. Also visible is the entrance 208 to the first passage. The second and third passages 210 and 212 are shown in broken outline. Figure 9b shows a perspective view of the first component part 202 with the first passage 209, second passage 210, third passage 212 and fourth passage 214 visible. Figure 9c shows a perspective view of the third component part 206 with the third passage 212 and fourth passage 214 visible. In this case, the die is not intended to be pre-stressed with outer rings, although this is possible. Instead, in a preferred embodiment, the three pieces of the die are held together by multiple rams or a releasable clamping device. An advantage of this is that it allows the die to be opened after each operation to remove the billet, apply lubrication and feed in a new billet.

Figure 10 shows schematic representation of another four-passage die that can be used to provide three-dimensional plastic strain of a billet. In this case, the die has four adjoining passages 220, 224, 228, 232, with the first two passages 220, 224 together defining a first plane 236, the second and third passages 224, 228 together defining a second plane 240 and the third and fourth passages 228, 232 together defining a third plane 244 where each passage lies at 120° to the adjoining passages and plane 236 is perpendicular to plane 240, which in turn is perpendicular to plane 244. As an alternative to the die of Figure 10, the channels can be arranged such that plane 244 lies parallel to the input passage 220, as shown in Figure 11. This shows a die configuration having four adjoining passages 220 - 232, with the first two passages 220, 224 together defining a first plane 236, the second and third passages 224, 228 together defining a second plane 240 and the third and fourth passages 228, 232 together defining a third plane 244, where each passage lies at 120° to the adjoining passages and with plane 244 being parallel to first passage 220. In this embodiment, an angle between the passage planes 236, 240 and 244 is approximately 109.5° (equivalent to 70.5°).

As an example, two dies were constructed, each having a round channel consisting of four adjoining passages and having three turns. One die (the 90° die) had an angle of 90° between adjoining passages, as shown in Figure 9, and the other (the 120° die) had an angle of 120° between adjoining passages, as shown in Figure 11. Both dies were used to process 10mm diameter, 47.5mm long, commercial purity aluminium (1070 grade) billets at room temperature and at a constant speed of lrnm/s. The total length of the channels was longer than the length of each billet, so two or three billets were in the die at any one time.

The reduction in grain size is dependent on the amount of equivalent plastic strain placed on the billet. The equivalent plastic strain generated in the material upon passing through one turn of an equal channel depends on the angle between the passages and is cumulative with each successive turn. For the 90° die, the equivalent plastic strain produced due to one turn is 1.15 while for the 120° die, the equivalent plastic strain produced due to one turn is 0.67. Similarly, after one complete pass through the 90° die, the total equivalent plastic strain generated is 3.45 but is only 2 for the 120° die. The effect of this can be seen in TEM micrographs of the aluminium 1070 billets after processing through the 90° die as shown in Figure 12a and the 120° die as shown in Figure 12b.

From Figures 12 a and 12b, it can be seen that the grain sizes after processing through the 90° die are smaller than those after processing through the 120° die. Another difference is the higher mis-orientation angles for the 90° die than for the 120° die as marked on the Figures 12a and 12b. The grain structure influences mechanical properties as shown in the graphs of Figures 13 - 18. From these it can be seen that although the hardness of the billets produced from either die is approximately similar for the same equivalent plastic strain, processing with the 90° die yields billets having a higher yield strength and tensile strength and a lower percentage elongation, area reduction and fracture strain at equivalent plastic strain than processing through the 120° die. Having said this, there may be circumstances in which using the 120° die would be preferable. This is because the force required to push billets through the 120° die is relatively low, being only approximately 50%> that of the force required to push a similar billet through the 90° die. Furthermore, as can be seen in Figure 19, the billets after extrusion from the 120° die preferentially have more rounded ends than the billets extruded from the 90° die. Thus the angle between the passages in the apparatus and method of the present invention may be varied depending on the application with consideration to the above factors.

It will be obvious to those of skill in the art that various modifications and improvements may be made to the embodiments hereinbefore described without departing from the scope of the invention. Those of skill in the art will also recognise that the embodiments described above provide a method and apparatus that may be utilised to change the mechanical and physical properties of a metal billet by subjecting the billet to plastic strain in three dimensions in a single operation. This is, of course, a considerable advantage to material processors as the billet may be processed without the need for handling between sequential turns, thereby reducing costs and maximising machine utilisation.

Claims

1. A method of treating a metal billet of pre-determined dimensions to change said billet's mechanical and/or physical properties by reducing the billet's grain size, the method involving the sequential steps of: forcing said billet through a first passage; forcing the billet from the first passage into a second passage consecutive with and inclined to the first passage, whereby the first and second passages together define a first plane, and forcing the billet from the second passage into a third passage consecutive with and inclined to the second passage, whereby the second and third passages together define a second plane, the second plane being different from the first plane.

2. The method according to claim 1 wherein consecutive passages are orthogonal.

3. The method according to claim 1 wherein consecutive passages are non-orthogonal.

4. The method according to any of the preceding claims wherein the first plane and the second plane are orthogonal.

5. The method according to any of claim 1 to 3 wherein the first and second planes are non-orthogonal.

6. The method according to any of the preceding claims wherein the minimum length of each of said passages is 50% of the passages' width.

7. The method according to any of the preceding claims further including the step of applying a force to the billet to hinder the billet's progress through the die.

8. The method according to any of the preceding claims wherein more than three consecutive passages are provided.

9. An apparatus for treating a metal billet of pre-determined dimensions to change said billet's mechanical and/or physical properties by reducing the billet's grain size, the apparatus comprising: a die having a plurality of connected consecutive passages through which a billet can be directed, the passages including: a first passage; a second passage consecutive with and inclined to the first passage, whereby the first and second passages together define a first plane, and a third passage, said third passage being consecutive with and inclined to the second passage, whereby the second and third passages together define a second plane, said second plane being different from said first plane, and means for forcing a metal billet sequentially through the first, second and third passages of the die.

10. The apparatus according to claim 9 wherein the means for forcing a billet through the die is a ram associated with each of the passages, the rams being operable in sequence to force a single metal billet sequentially through each of the passages.

11. The apparatus according to claim 9 wherein the means for forcing a billet through the die is a ram associated only with the first passage and arranged to sequentially push a series of billets along the first passage.

12. The apparatus according to any of claims 9 to 11, wherein consecutive passages are orthogonal.

13. The apparatus according to any of claims 9 to 11 wherein consecutive passages are non-orthogonal.

14. The apparatus according to any of claims 9 to 13 wherein the first plane and the second plane are orthogonal.

15. The apparatus according to any of claims 9 to 13 wherein the first and second planes are non-orthogonal.

16. The apparatus according to any one of claims 9 to 15 wherein the minimum length of each of said passages is 50% of the passages' width.

17. The apparatus according to any of claims 9 to 16 wherein at least one of the passages includes means for applying a force to the billet to retard the movement of the billet through the at least one passage to facilitate uniform deformation and prevent fracture of the billet.

18. The apparatus according to claim 17 wherein the retarding means is at least one ram associated with at least one passage.

19. The apparatus according to any of claims 9 to 18 wherein more than three consecutive passages are provided.

20. A method of treating a metal billet of pre-determined dimensions to change said billet's mechanical and/or physical properties by reducing the billet's grain size, the method involving subjecting the billet to plastic strain in three dimensions in a single operation.

21. A die for use in the method and/or apparatus of any of the preceding claims, the die having a plurality of connected consecutive passages through which a billet can be directed, the passages including: a first passage; a second passage consecutive with and inclined to the first passage, whereby the first and second passages together define a first plane, and a third passage, said third passage being consecutive with and inclined to the second passage, whereby the second and third passages together define a second plane, said second plane being different from said first plane.

22. The die according to claim 21, wherein consecutive passages are orthogonal.

23. The die according to claim 21 wherein consecutive passages are non- orthogonal.

24. The die according to any of claims 21 to 23 wherein the first plane and the second plane are orthogonal.

25. The die according to any of claims 21 to 23 wherein the first and second planes are non-orthogonal.

26. The die according to any of claims 21 to 23 wherein more than three consecutive passages are provided.