FIELD OF THE INVENTION

The present invention relates to a process of Severe Plastic Deformation (SPD) method by Equal Channel Angular Extrusion (ECAE) of magnesium and its alloys more particularly relates to a method to produce ultrafine grain size magnesium and its alloys.

BACKGROUND OF THE INVENTION AND PRIOR ART

Wrought magnesium and its alloys have high potential to be used in automobile, aerospace and electronic industries, due to superior specific stiffness and strength. However, these application prospects are impeded by poor formability and limited ductility at room temperature because of its hexagonal crystal structure and a limited operating slip system. Therefore strength and ductility needs to be improved to enable their widespread consumption in industries. Processing of magnesium by both conventional and non-conventional, which are mainly severe plastic deformation (SPD) techniques is possible at or above 200°C. However, effective reduction of grain size was not possible, due to the fact that the deformation temperature was quite above the minimum recrystallization temperature. Possible chances of recrystallization and grain growth during deformation, which impede the material from grain refinement and cannot achieve enhancement in properties. Hence, there is quest for room temperature processing of magnesium and its alloys. Magnesium have been processed by severe plastic deformation at room temperature, which provides a window of opportunity to see the onset of nucleation of recrystallized grains in magnesium and to generate sub micron grain size.

Unconventional Deformation processes such as severe plastic deformation (SPD) methods are used in order to generate specific texture and possible grain refinement in order to enhance to properties such as strength and ductility. Equal Channel Angular Extrusion (ECAE) is one of the SPD methods in which two equal channels are placed at some angles; in this case a 90° ECAE die is used. ECAE are carried out by A, Bc and C route in a 90° die. The route A involves no rotation around the billet axis, in Be the sample is rotated by 90° in one direction (clockwise or anti-clockwise) between each pass, and in route C the sample is rotated by 180° between the passes. ECAE is carried-out on wrought magnesium to produce bulk nanostructured material.

In recent years, a number of innovative severe plastic deformation techniques have been developed for deforming metals to a very high degree of plastic strains with the aim of producing greatly refined grain structures without entailing requirements of exotic alloy additions or costly thermo-mechanical treatments. These include severe plastic torsion straining (SPTS), multi-axial forging (MAF), accumulative roll bonding (ARB) and equal channel angular extrusions (ECAE). Severe plastic deformation processes produces materials with a grain size of the order of 100-1000 nanometers. A distinct advantage of this process is that it is scalable to produce large billets in industry and is relatively simple and cheap process. The novel feature of the process is that the net shape of the final product remains essentially the same as the starting material after any given number of passes, so there is no constraint on the strain that is build up in the material. In comparison to conventional metal working process, like rolling, extrusion, effective strains greater than 4 can only be obtained in foils and filaments which have fewer structural applications. ECAE involves abrupt changes in strain path. It has been shown that certain passes to be more favorable towards rapid refinement and grain size over others.

In the paper entitled 'Formation of sub-micron and nanocrystalline grain structure by severe plastic deformation' by P. B. Pragnell, J. R. Bowen and A. Gholina published in the proceedings of 22nd Risoe International Symposium on Materials Science, pp. 105-126 (2001), the definition of submicron or nanocrystalline grain size has been proposed as a structure where (a) average spacing of the high angle grain boundaries (HAGBs), having misorientation angle greater than 15°, must be less than 1 micron in all orientations, and (b) the proportions of HAGB area with respect to total boundary area in the material must be greater than 70%.

The ECAE die used for extruding the magnesium samples is constructed. The paper entitled 'A new design for equal channel angular extrusion' by J. P. Mathieu, S. Suwas, A. Eberhardt, L. S. Toth and P. Moll provides a detailed description of the same, is published in Journal of Materials Processing Technology 173, pp.29-33 (2006). The authors of this publication proposed a die design over a prior design of the ECAE die given by V. M. Segal, R. E. Goforth and K. T. Hartwig in the US patent No. 5,400,633
entitled 'Apparatus and method for deformation processing of metals, ceramics and other materials' in 1995 and a modified version invented by V. M. Segal in US patent No. 5,513,512 entitled 'Plastic deformation of crystalline materials' issued in 1996. In this new version of the die, an oblong section for the piston and the entrance channel where it glides in were suggested to reduce the stress-concentration to its minimum. Again, it was also proposed to separate the channel for maintenance as and when needed, which resulted into separation of die into three discrete parts, namely piston, die body and the drawer. Through these design optimizations there is a net reduction of load requirement as compared to the conventional die.

The theory of ECAE is described in a paper by V. M. Segal, V. I. Reznicov, A. E. Drobishevskiy and V. I. Dopylov entitled 'Plastic working of metals by simple shear' which was published in Russian metallurgy, vol.1, pp. 99-105, (1981) and other papers and patents to Segal, Dunlop, Semiatin and others describe the use of ECAE to process metals, alloys, plastics and other materials into rods and plates. Segal has shown the ways of processing involving multiple ECAE of a billet with or without rotation between subsequent extrusions, which may be followed by forging or cold rolling.

Severe Plastic Deformations such as ECAE is very effective in enhancing the workability and strength of Mg alloys due to grain refinement and preferred crystallographic texture for their extensive usage in industries. Since ECAE of Magnesium was possible up to a minimum temperature of about 200°C which is much higher compared to the minimum recrystallization temperature of magnesium, which is approximately 100°C Processing of magnesium and its alloys at and above 200°C has been reported by many researchers such as in the paper entitled 'Enhanced ductility in strongly textured magnesium produced by equal channel angular extrusion' by S. R. Agnew, J. A. Horton, T. M. Lillo, D. W. Brown published in Scripta materialia 20, pp. 377-381 (2004), 'Crystallographic texture evolution of three wrought magnesium alloys during equal channel angular extrusion' by S. R. Agnew, P. Mehrotra, T. M. Lillo, G. M. Stoica, P. K. Liaw published in Materials Science and Engineering A 408, pp 72-78 (2005), 'Texture evolution of five wrought alloys during route A equal channel angular extrusion: Experiments and simulations' by S. R. Agnew, P. Mehrotra, T. M. Lillo, G. M. Stoica, P.

K. Liaw published in Acta materialia 53, pp. 3135-3146 (2005), 'Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure' by Toshiji Mukai, Masashi Yamanoi, Hiroyuki Watanabe and Kenji Higashi published in Scripta materialia 45, pp. 89-94 (2001). In all these cases the average grain size is always greater than 2 microns. Possible chances of recrystallization and grain growth during deformation, which impede the material from grain refinement, and therefore cannot achieve much enhancement in properties.

In a recent paper entitled 'Evolution of crystallographic texture during equal channel angular extrusion (ECAE) and its effect on secondary processing of magnesium' by Satyam Suwas, G. Gottstein, R. Kumar published in Materials Science and Engineering A 471, pp 1-14 (2007) shows an extensive study on evolution of microstructure and texture during ECAE of pure Mg, and also reported subsequent workability of ECAE processed materials. It was shown that the ECAE processed magnesium can be rolled at room temperature for different percentage of up to 80% reduction with out any fracture in the material. Rolling of Magnesium is possible at room temperature because of the formation of non basal texture and the decrease in grain size during the ECAE process. However the grain size achieved by authors was not less than 2-3 microns.

In a paper entitled 'Deformation and fracture during equal channel angular pressing of AZ31 alloy' by Feng Kang, Jing Tao Wang, Yong Peng to be published in Materials Science and Engineering A doi:10.1016/j.msea.2007.09.063 (2007) showed that below 200°C, the AZ31 magnesium alloy exhibits fracture which is characterized by the formation of a series of segments along the direction of the extrusion at all strain rates.

In the papers 'Severe plastic deformation of magnesium alloy AZ31 at low temperatures' by X. Wu, P. Luo, J. T. Wang, M. Lang, S. Xie, K. Xia published in Materials Forum, Volume 29 (2005) and Equal channel angular pressing of magnesium alloy AZ31 by K, Xia, J. T. Wang, X. Xu, G. Chen, M. Gurvan published in Materials Science and Engineering A 410-411 (2005) 324-327, ECAE was done at 150°C for the first 4 pass and 100°C from the 5th up to 8 passes. Starting ECAE at 150°C was possible only because of a very low pressing speed of 0.2 mm/min. No Evidence of grain size less than 1 urn for I the 8 pass 100° C ECAE process sample is presented. The pressing speed of ECAE was 0.2 mm/min, which is very less to be incorporated for industrial applications, whereas in the present study the pressing speed was about lmm/sec. A back pressure of 50MPa was used so that the material does not fracture, which cannot be extendable for extrusion of large samples of industrial scale i.e., cannot be implemented for industrial application, because the load requirements will then be tremendous. In the present study no back pressure is required.

On way to produce finer grains is provided in a Korean patent application bearing publication number KR20050024736. It provides that ECAE of magnesium and its alloys can be done at a minimum temperature of 200°C to produce finer grains. The proposed method in the present application provides for deformation of magnesium at room temperature to produce ultrafine grains of the order of nanometers.

In the paper 'Mechanical properties and microstructure of AZ31 Mg alloy processed by two-step equal channel angular extrusion' by Li Jin, Dongliang Lina,T, Dali Maoa, Xiaoqing Zenga, Wenjiang Ding published in Materials Letters 59 (2005) 2267- 2270, Magnesium alloy was processed at 225°C for the first 4 pass and the 5th pass was done at 180°C, the grain size reported was but optical microscope was not able to resolve the microstructure and TEM image shows incompletely developed subgrains and cell structure formed by the tangled dislocations. The sampled area for TEM was too small to be a true representative of grain size. They did not mention the methodology used for calculating the grain size.

Exotic alloying additions like Zr, Y and Li may lead to lower temperature of deformations and higher grain refinement. In the paper 'Compressive deformation of Mg-Zn-Y-Zr alloy processed by equal channel angular pressing' by M.Y. Zheng , S.W. Xu, X.G. Qiao, K. Wu, S. Kamado, Y. Kojima published in Materials Science and Engineering A (2007) (doi:10.1016/j.msea.2006.09.160). The authors reported a grain size of about 0.8 (a m was produced by subjecting the extruded alloy to equal channel angular pressing (ECAP) for eight passes at 200°C.

The solid solution of magnesium can be deformed at room temperature because of the formation of body centered cubic (bcc) magnesium-lithium lattice to get a grain size of about 200nm was reported in the paper 'Microstructure evolution of Mg-14% Li-1% Al alloy during the process of equal channel angular pressing' by T. Liu, S.D. Wu, S.X. Li, P.J. Li published in Materials Science and Engineering A 460-^61 (2007) 499-503. But these alloying additions will definitely increases the overall cost of the material. It is always a quest to deform magnesium and its single phase (HCP) alloys at room temperature. In this method the room temperature deformation of magnesium was possible and there is no requirement of any alloying addition to deform magnesium at room temperature to get a submicron grain size.

In the present process it is shown that to obtain the room temperature deformation and preferred grain refinement to submicron level, it is not necessary to add Zr, Y, Li etc.

OBJECTS OF THE INVENTION

The principal object of the present invention is to develop a method to produce ultrafine grain size magnesium and its alloys.

Yet another object of the present invention is to produce a magnesium billet of average grain size of about 250 nanometers with 90% of the grains below 200 nm.

STATEMENT OF INVENTION
Accordingly, the invention provides for a method to produce ultrafine grain size magnesium and its alloys, said method comprising acts of placing preheated magnesium and its alloys in the form of billet inside preheated die; extruding the placed billet to refine grain size of the billet; and cooling the extruded billet at room temperature to produce the ultrafine grains of the magnesium and its alloys billet., a magnesium billet has an average grain size of about 250 nanometers with 90% of the grains below 200 nm and Industrial appliances made of magnesium having ultrafine grains of order of nanometers produced at room temperature, wherein said industrial appliances are selected from a group comprising automobiles and aerospace body parts and electronic components.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

Fig.1 shows Inverse Pole Figure (IPF) Map of the initial material.

Fig. 2 shows a schematic representation of the billet and ECAE die; and the reference direction it was put in the die inlet.

Fig. 3a shows IPF Map of ECAE Route A for 4th Pass sample.

Fig. 3b shows grain size distribution and misorientation angle map of ECAE Route A for 4l Pass sample.

Fig. 4a shows IPF Map of ECAE Route A for 6th Pass sample.

Fig. 4b shows grain size distribution and misorientation angle map of ECAE Route A for 61 Pass sample.

Fig. 5a shows IPF Map of ECAE Route A for 7th Pass sample.

Fig. 5b shows grain size distribution and misorientation angle map of ECAE Route A for 7th Pass sample.

Fig. 6a shows IPF Map of ECAE Route A for 8th Pass sample.
Fig. 6b shows grain size distribution and misorientation angle map of ECAE Route A for 8th Pass sample.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is in relation to a method to produce ultrafine grain size magnesium and its alloy comprising acts of; placing preheated magnesium and its alloys in the form of billet inside preheated die; extruding the placed billet to refine grain size of the billet; and cooling the extruded billet at room temperature to produce the ultrafine grains of the magnesium and its alloys billet.

In yet another embodiment of the present invention the preheating and extrusion of said billet is carried out at a temperature ranging from about 245°C to about 255°C for about 10 minutes each for first four extrusions.

In still another embodiment of the present invention the preheating and extrusion of said billet is carried out at a temperature ranging from about 195°C to about 205°C, from 145°C to about 155°C, and from about 95°C to about 105°C for about 10 minutes each for fifth, sixth and seventh extrusions respectively.

In still another embodiment of the present invention the preheating and extrusion of said billet is carried out at room temperature ranging from about 25°C to27°C to produce the ultrafine grains of the billet.

In still another embodiment of the present invention the extrusion of billet is carried out with pressing speed of about lmm/s.

In still another embodiment of the present invention the extruded billet has grain size ranging from about 50 nanometers to about 1800 nanometers, and has an average grain size of about 250 nanometers.

In still another embodiment of the present invention said die is an equal channel angular extrusion (ECAE) die comprising two channels of equal size placed at 90° to each other.

The present invention is in relation to a magnesium billet comprising an average grain size of about 250 nanometers with 90% of the grains below 200 nanometers.

The present invention is in relation to industrial appliances made of magnesium having ultrafine grains of order of nanometers produced at room temperature, wherein said industrial appliances are selected from a group comprising automobiles and aerospace body parts and electronic components.

A combination of severe plastic deformation and ambient temperature of deformation will definitely generate grains of the order of nanometers. Equal channel angular extrusion is used to deform magnesium at room temperature. This is carried out by slowly reducing the temperature from 250°C at 4th pass by 50° after each pass using route A. In this manner room temperature deformation of magnesium is possible at 8th pass to generate an average grain size of 250 nanometers.

There is a possibility to deform magnesium at room temperature by ECAE to get ultra-fine grain size. An ECAE die with 90° inter-channel angle with an outer arc angle of 0° is used, which resulted in an effective strain of 1.15 on each separate pass of the billet. The ECAE apparatus made up of die body, piston and drawer are made up of Inconel 718 super-alloy. Molybdenum sulphide (M0S2) mixed with grease was used as a lubricant was applied to both the billet and the die interior. The preheated billet enters the first channel of the die through inlet. Heating elements heat the whole die; piston applies force against billet and extrudes it through the second channel and out of the die through exit. As the billet is extruded, it experiences shearing force along the plane of intersection of the two channels and undergoes severe plastic deformation. The dimensions of the billets are largely unaffected after extrusion since the two inlet and outlet channels are almost of the same cross section. The pressing speed was about 0.5-lmm/sec.
The initial material is commercially pure magnesium warm rolled to have a non-axisymmetric basal texture, the microstructure in the form of Inverse pole figure map is shown in Fig .I. Keeping the RD, ND and TD into account the samples are cut into l0mmXI0mmX 100mm for ECAE experiments. The billets are then placed in the ECAE die with the ND plane in the front and the RD direction down along the 100mm dimensions as shown in Fig.2. The ECAE is carried out by route A. The first 4 passes were done at 250°C±5°C to get a grain size of 6.3um; Fig. 3a shows the inverse pole figure map and Fig. 3b shows the grain size distribution and misorientation angle map of the 4th pass ECAE sample. From 5th pass onward the temperature of the ECAE is reduced slowly. The 5th pass is carried out at 200°C±5°C, the 6th pass at 150°C±5°C, the 7th at 100°C±5°C and the 8th pass at room temperature that is 27°C. Prior to each pass the billets are preheated to their respective temperature of extrusion for 10 minutes to warm the billet. The average grain size reduced to 690nm after 6th pass (Fig.4a), to 280nm after f pass (Fig.5a), and 250nm after 8th pass (fig.6a). The grain size distributions and misorientation angle for these samples are also shown in Fig.3b, Fig 4b, Fig 5b and Fig 6b. The grain size has been measured by linear intercept method. Grains become more and more homogenous after each pass. There is a considerable amount of dynamic recrystallization present, which reduces as the temperature of deformation is reduced. The texture after each pass remains almost same and was measured by electron back scattered diffraction (EBSD) using field emission gun-scanning electron microscope (FEG-SEM).

The invention has been described in connection with its preferred embodiments. However, it is not limited thereto. Changes, variations and modifications to the basic design may be made without departing from the inventive concepts in this invention. In addition, these changes, variations and modifications would be obvious to those skilled in the art having the benefit of the foregoing teachings. All such changes, variations and modifications are intended to be within the scope of this invention. The technology of the instant Application explained with the examples should not be construed to limit the scope of the invention.

References

1) Formation of sub-micron and nanocrustalline grain structure by severe plastic deformation P. B. Pragnell, J. R. Bowen and A. Gholina, Proceedings of 22" Risoe International Symposium on Materials Science, 2001, pp. 105-126.

2) A new design for equal channel angular extrusion J. P. Mathiew, S. Suwas, A. Eberhardt, L. S. Toth and P. Moll, Journal of materials processing technology 173 (2006) 29-33.

3) US patent No. 5,400,633; Apparatus and method for deformation processing of metals, ceramics and other materials; V. M. Segal, R. E. Goforth and K. T. Hartwig (1995)

4) US patent No. 5,513,512; Plastic deformation of crystalline materials; V. M. Segal 1996.

5) Plastic working of metals by simple shear; V. M. Segal, V. I. Reznicov, A. E.

Drobishevskiy and V. I. Dopylov, Russian metallurgy vol.1, pp. 99-105, (1981).

6) Enhanced ductility in strongly textured magnesium produced by equal channel angular extrusion; S. R. Agnew, J. A. Horton, T. M. Lillo, D. W. Brown, Scripta materialia, 20, pp. 377-381 (2004).

7) Crystallographic texture evolution of three wrought magnesium alloys during equal channel angular extrusion; S. R. Agnew, P. Mehrotra, T. M. Lillo, G. M. Stoica, P. K. Liaw, Materials Science and Engineering A 408, pp 72-78 (2005).

8) Texture evolution of five wrought alloys during route A equal channel angular extrusion: Experiments and simulations; S. R. Agnew, P. Mehrotra, T. M. Lillo, G. M. Stoica, P. K. Liaw, Acta materialia 53, pp. 3135-3146 (2005).

9) Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure Toshiji Mukai, Masashi Yamanoi, Hiroyuki Watanabe and Kenji Higashi, Scripta materialia 45, pp. 89-94 (2001).

10) Evolution of crystallographic texture during equal channel angular extrusion (ECAE) and its effect on secondary processing of magnesium; Satyam Suwas, G. Gottstein, R. Kumar, Materials Science and Engineering A 471, pp 1-14 (2007).

11) Deformation and fracture during equal channel angular pressing of AZ31 alloy, Feng Kang, Jing Tao Wang, Yong Peng, Materials Science and Engineering, doi: 10.1016/j.msea.2007.09.063 (2007).

12) Severe plastic deformation of magnesium alloy AZ31 at low temperatures; X. Wu, P. Luo, J. T. Wang, M. Lang, S. Xie, K. Xia, Materials Forum Volume 29 (2005)
13) Equal channel angular pressing of magnesium alloy AZ31; K, Xia, J. T. Wang, X. Xu, G. Chen, M. Gurvan, Materials Science and Engineering A 410-411 (2005) 324-327.

14) Mechanical properties and microstructure of AZ31 Mg alloy processed by two-step equal channel angular extrusion; Li Jin, Dongliang Lina,T, Dali Maoa, Xiaoqing Zenga, Wenjiang Ding, Materials Letters 59 (2005) 2267- 2270.

15) Compressive deformation of Mg-Zn-Y-Zr alloy processed by equal channel angular pressing; M.Y. Zheng , S.W. Xu, X.G. Qiao, K. Wu, S. Kamado, Y. Kojima, Materials Science and Engineering A (2007) (doi:10.1016/j.msea.2006.09.160)

16) Microstructure evolution of Mg-14% Li-1% Al alloy during the process of equal channel angular pressing; T. Liu, S.D. Wu, S.X. Li, P.J. Li, Materials Science and Engineering A 460161 (2007) 499-503

We Claim:

1. A method to produce ultrafine grain size magnesium and its alloys, said method comprising acts of:

a. placing preheated billets of magnesium and its alloys inside preheated die;

b. extruding the placed billet to refine grain size of the billet; and

c. cooling the extruded billet at room temperature to produce the ultrafine grains of the magnesium and its alloys billet.
2. The method as claimed in claim 1, wherein the preheating and extrusion of said billet is carried out at a temperature of ranging from about 245°C to 255°C for about 10 minutes each for first four extrusions.

3. The method as claimed in claims 1 and 2, wherein the preheating and extrusion of said billet is carried out at a temperature ranging from about 195°C to about 205°C, from about 145°C to about 155°C, and from about 95°C to about 105°C, each for about 10 minutes for fifth, sixth and seventh extrusions respectively.

4. The method as claimed in claim 1, wherein the preheating and extrusion of said billet is carried out at room temperature ranging from about 25°C to 27°C to produce the ultrafine grains of the billet.

5. The method as claimed in claim 1, wherein said extrusion of the billet is carried out with pressing speed of about lmm/s.

6. The method as claimed in claims 1 and 4, wherein said extruded billet after eight pass has grain size ranging from about 50 nanometers to about 1800 nanometers, and has an average grain size of about 250 nanometers.

7. The method as claimed in claim 1, wherein said die is an equal channel angular extrusion (ECAE) comprising two channels of equal size placed at 90 to each other.

8. Magnesium billet comprising an average grain size of about 250 nanometers with 90% of the grains below 200 nanometers.

9. Industrial appliances made of magnesium having ultrafine grains of order of nanometers produced at room temperature, wherein said industrial appliances are selected from a group comprising automobiles and aerospace body parts, and electronic components.