Description:FIELD OF THE INVENTION:

The present invention generally relates to severe plastic deformation (SPD) processes. More particularly, it relates to an improved SPD process for producing ultra-fine grains structured sheets from metals or powder samples.

BACKGROUND:

Materials are processed for various applications in which it primarily involves plastic deformation. The selection of a specific processing technology is guided by the mechanical properties inherent to the material being processed. Introducing substantial strains into metals is recognized as an effective means of enhancing mechanical properties, particularly strength. In the context of metals, the overall strength is primarily determined by the size of their grains.

Severe plastic deformation (abbreviated herein as SPD) processes are widely acknowledged for their ability to generate ultra-fine grains in materials, characterized by a high density of lattice defects. These processes have garnered significant attention and undergone substantial development, yielding noteworthy results. The manipulation of a material’s "grain size" is a recognized pivotal factor in achieving desired properties. However, it is crucial to acknowledge that a material's properties are intricately linked to its microstructure. Consequently, various SPD processes have been innovatively devised and refined over the past three to four decades to exert control over the microstructure and enhance material properties.

The well-known developed SPD processes are High-Pressure torsion (HPT), Equal Channel Angular Pressing (ECAP), and Accumulative Roll Bonding (ARB). Each of these processes has its own merits and shortcomings. High-Pressure torsion (HPT) processes can achieve exceptionally fine grains or sub-grains in a single shot. However, the sample size is generally quite small, and most importantly, there is a strain gradient in the material. ECAP-processed samples are large but multiple passes are required to reach the desired fine grains. Accumulative Roll Bonding (ARB) is only the SPD process that can be used for batch production, nevertheless, the process is quite cumbersome and needs at least ten (10) ARB cycles to approach the steady state grain size regime. Moreover, the debonding of sheets and reduction in the ductility of the materials have also been observed as the major drawbacks of this process.

When developing a new or advanced Severe Plastic Deformation (SPD) process, it is always essential to consider several key factors including determining the extent of strain imparted to the material, assessing the magnitude of hydrostatic compressive stress, ensuring stress and strain continuity and homogeneity, defining the maximum size of the processed sample, and evaluating the overall complexity of the process. Addressing these factors is crucial for the effective design and successful implementation of any novel SPD technique.

Recently, another developed SPD process, named High-Pressure Compressive Shearing (abbreviated herein as HPCS) was proposed by Toth, et al, entitled “The mechanics of High-Pressure Compressive Shearing with application to ARMCO® steel”, where the mechanics of the process underwent scrutiny and the analytical expressions were formulated for all stress and strain components, with the exception of the directly measured applied compression and shear forces. High-Pressure Compression Shear (HPCS) was employed on ARMCO® steel using an apparatus that facilitated real-time monitoring of shear force, normal force, and shearing distance. This setup enabled the measurement of the stress-strain response in situ, reaching the steady-state work hardening regime.

However, there is a need for an advanced solution to overcome the limitations of the existing SPD processes described earlier i.e., a) to determine the extent of strain imparted to the material, b) to assess the magnitude of hydrostatic compressive stress, ensuring stress and strain continuity and homogeneity, c) to define the maximum size of the processed sample, d) to reduce the overall complexity of the process and e) to achieve an effective design for the process.

Thus, the present invention provides with a new and improved method which is unique for fabricating a metal sheet and that is entirely different from any other existing techniques in the field of Severe Plastic Deformation (SPD) processes.

The present invention provides an advanced and improved High-Pressure Compressive Reverse Shearing (abbreviated herein as HPCRS) process that imparts a large shear strain along with compressive strain into the metallic workpiece, in a single-step deformation operation for producing ultra-fine grain structured sheets from metal samples.

OBJECTIVE OF THE INVENTION:

The primary objective of the present invention is to provide an improved severe plastic deformation (abbreviated herein as SPD) process for producing ultra-fine grains structured sheets from metals or powder samples.

Another objective of the present invention is to provide an improved SPD process for achieving a large shear strain along with compressive strain into a material, in a single-step deformation operation.

SUMMARY:

To achieve the above-mentioned objectives, the present invention provides an improved SPD process for producing ultra-fine grain structured sheets from metals or powder samples.

According to the present invention, the improved SPD process is referred to as High-Pressure Compressive Reverse Shearing, (abbreviated herein as HPCRS), which is an advanced version of the High-Pressure Compressive Shearing (abbreviated herein as HPCS) process.

The improved process of the present invention comprises the steps of: a) providing a secure die assembly which comprises of two dies, namely a (i) stationary forging die and (ii) an oscillating / shearing die, where (i) stationary forging die is provided to apply compressive force and (ii) an oscillating / shearing die is provided to apply bi-directional movement; b) positioning one metallic workpiece within the cavity of the die assembly, where the workpiece is made of compatible material and dimensions; c) applying controlled compressive force to the workpiece with the forging die and simultaneously oscillating the shearing die at a predetermined frequency and amplitude, further inducing controlled shear strain on the workpiece; d) obtaining a processed metal sheet of reduced thickness from the step (c); e) cutting the obtained metal sheet into multiple pieces and stacking together; f) repeating the steps of b), c) and d) for a predetermined number of cycles, until the workpiece reaches a desired final thickness of about 1 mm to 2 mm thickness.

In one embodiment, the improved process of the present invention involves positioning one or two metallic workpieces together, within the cavity of the die assembly.

Further, the present invention also repeats the ‘bonding’ technique with the processed metal sheets. The bonding of two or more bulk metals is carried out to produce an ultra-fine grain structured sheet. Here, the processed sheet is cut into five equal parts, cleaned, and the surfaces are stacked together again to repeat the process. This technique is called accumulative high-pressure compressive reverse shearing (abbreviated herein as AHPCRS). The AHPCRS technique can be repeated multiple times to impart enormous equivalent strain into the material and also to obtain nanolayered metal sheets.

Thus, the present invention employs an HPCRS process to produce ultra-fine grain structured sheets from metals or powder samples.

The improved process imposes a large homogeneous strain on the material and generates compressive hydrostatic stresses.

Further, the improved process is used to produce materials with the desired mechanical properties, like the strength of the material, ductility, toughness, and elastic modulus.

The advantages of the present invention are severe plastic deformation with large strain accumulation, processing-controlled properties (PCP), control of texture evolution, dynamic process, and metal forming speed.

Other objects, advantages, and features of the present invention will become more apparent from the following detailed description and claims, taken in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed for the purpose of illustration only and are not intended as a definition of the limits of the invention.

REFERENCE NUMERALS OF THE DRAWINGS:

10 - Oscillating die or shearing die
20 - Forging die
30 - Cavity/Channel
40 - Workpiece/Billet

BRIEF DESCRIPTION OF THE DRAWINGS:

The objective of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

FIGS. 1(a) and 1(b) illustrate the basic design of the oscillating die or shearing die and forging die, respectively.

FIG. 1(c) illustrates the side view of the oscillating die or shearing die and forging die assembly with billet in the cavity;

FIG. 2(a) illustrates the schematic representation of the High-Pressure Compressive Reverse Shearing (HPCRS) process for one cycle and the experimental shape change of the specimen;

FIG. 2(b) shows the stress-strain response of CP Al tensile samples after HPCRS, processed at different frequencies for the same total deformation;

FIG. 2(c) shows the geometry of the initial and final samples;

FIG. 3(a) shows the plot between ‘thickness reduction’ and the time required to get a processed sample having a 1 mm thickness;

FIG. 3(b) shows the obtained value of the accumulative von Mises equivalent strain;

FIG. 3(c) shows the plot between accumulative shear strain and thickness reduction;

FIG. 3(d) shows the value of accumulative compressive strain;

FIG. 4(a) shows the micrograph of the initial commercially pure Al;

FIG. 4(b) and 4(c) show the microstructures corresponding to the deformed state, processed at the shearing frequencies of 20 Hz, and 0.1, Hz, respectively;

FIG. 5(a) shows the tensile testing plot for magnesium metal processed by HPCRS;

FIG. 6(a), 6(b) and 6(c) show the measured average grain size (GS) and grains morphology for base metal (pure Mg), samples processed at 20Hz, and 1Hz, respectively;

FIG. 7(a) shows the green compacted aluminum powder sample and the processed powder sample;

FIG. 7(b) shows the green compacted magnesium powder sample and the processed sample;

FIG. 8(a) shows the capability of the process to obtain a sheet metal from the bonding of multiple bulk metals; and

FIG. 8(b) shows the EBSD micrograph of the bonded sample.

DETAILED DESCRIPTION OF THE INVENTION:

The present disclosure is best understood with reference to the detailed figures and description set forth herein. Various embodiments have been discussed with reference to the figures. However, those skilled in the art will readily appreciate that the detailed descriptions provided herein with respect to the figures are merely for explanatory purposes, as the methods and systems may extend beyond the described embodiments.

As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context dictates otherwise.

The term “process” refers to methods, manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those methods, manners, means, techniques, and procedures either known to, or readily developed from known methods, manners, means, techniques and procedures by practitioners of the art to which the invention belongs. The descriptions, examples, methods, and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only. Those skilled in the art will envision many other possible variations within the scope of the technology described herein.

The term “workpiece” refers also to a billet, material, metal, sample, specimen or any other possible variations within the scope of the present invention. Also, the workpiece or material or billet can be selected from one or more of magnesium, aluminum, iron, metals, and alloys.

The present invention describes herein is an improved SPD process for producing ultra-fine grains structured sheets from metals or powder samples.

The present invention relates to an improved SPD process for achieving ultra-fine grains by introducing high plastic strain into the material, in a single-step deformation operation.

According to the present invention, the improved SPD process is called HPCRS (High-Pressure Compressive Reverse Shearing), which is an advanced version of the HPCS (High-Pressure Compressive Shearing) process.

The HPCRS process of the present invention produces ultra-fine grain structured sheets from metal samples. The HPCRS process comprises the following steps of:
a) providing a secure die assembly which comprises of two dies, namely (i) a stationary forging die and (ii) an oscillating / shearing die, where the stationary forging die is provided to apply compressive force and the oscillating / shearing die is provided to apply bi-directional movement;
b) positioning one metallic workpiece within the cavity of the die assembly, where the workpiece is made of compatible material and dimensions;
c) applying controlled compressive force to the workpiece with the forging die and simultaneously oscillating the shearing die at a predetermined frequency and amplitude, further inducing controlled shear strain on the workpiece;
d) obtaining a processed metal sheet of reduced thickness from the step (c);
e) cutting the obtained metal sheet into multiple pieces and stacking together;
f) repeating the steps of b), c) and d) for a predetermined number of cycles, until the workpiece reaches a desired final thickness.

In one embodiment of the present invention, the desired final thickness corresponds to about 1 mm to 2 mm thickness.

Thus, the present invention employs an HPCRS process to produce ultra-fine grain structured sheets from metals or powder samples.

The HPCRS process of the present invention is a versatile SPD technique, that involves: a) making an ultra-fine grain structured metal sheet from a bulk material, b) bonding of two or more bulk metals into an ultra-fine grain structured sheet, c) producing nanolayered metal sheets by accumulative HPCRS (AHPCRS) and d) producing ultra-fine grain structured sheets from metal powders.

The present invention of producing ultra-fine grain structured sheets by employing HPCRS process has been carried with various materials. In one embodiment, the workpiece or material or billet can be selected from any one of magnesium, aluminum, iron, metals, and alloys. In another embodiment, the workpiece or material or billet can be selected from one or more of magnesium, aluminum, iron, metals, and alloys

In the present embodiment, the HPCRS process of the present invention comprises commercially pure (abbreviated herein as CP) Aluminum (Al) and pure Magnesium (Mg) and are subjected to 83% and 76% reduction in thickness respectively. The ultra-fine grain microstructures were obtained in the micron grain size range, together with a unique shear texture, which is very different from a rolling texture that is normally obtained in rolled sheets. A bulk of pre-compacted metal powder has been successfully converted into a metal sheet. Further, the improved process has also been exploited to process metal powders.

In accordance with the present embodiment, referring to the illustrated figures, FIGS. 1(a), and 1(b) show the basic design of the oscillating die or shearing die (10) and the forging die (20), respectively. The HPCRS process comprises of a die assembly, which has the two dies: 1) a forging die (20) and 2) an oscillating die or shearing die (10). The test setup consists of these two dies (10, 20), where the dimensions of the die cavity, in the forging die (20), are width = 25 mm, thickness = 15 mm, and length = 220 mm. An initial gap of 20 microns is left between the width/lateral dimension of the oscillating die (10) and the forging die (20). First, a billet (40) is placed in the middle of the channel (30) on the forging die (20) and afterward, the shearing die (10) is inserted into the cavity/channel (30) to make a contact with the billet (40).

The assembly of the oscillating die or shearing die (10) and forging die (20) along with the billet (40) placed in the cavity (30), is shown in FIG. 1(c) as side view. Here, the oscillating die (10) is not in proper contact with the billet (40), which is placed inside the cavity (30). This is not the running setup and is just an illustration to show how the shearing die is fitted into the forging die.

The schematic representation of the High-Pressure Compressive Reverse Shearing (HPCRS) process for one cycle is shown in FIG. 2(a). Also, it shows the change in the shape of the billet in each pass and illustrates the effect of shearing load and compressive load acting simultaneously with the change of strain path.

As shown in FIG. 2(a), the process has two punches: the driving punch (also called the oscillating die or the shearing die) (10), and the normal punch (also called the forging die) (20). The specimen (40) is placed in the middle of the cavity (30) in the forging die (20). Any cycle of the process comprises a shearing movement of the shear-die under the assistance of the compression force of the forging die (20). The punches are well equipped with hydraulic actuators so that the driving punch can oscillate at a controlled frequency and amplitude. The normal punch can apply sufficient load when the oscillating punch is moved, so the two forces deform the specimen plastically. The normal load is kept constant or varied linearly (or both) during the process. The oscillating shear-die is set for different frequencies. For the present results, the displacement was kept fixed i.e., 2 mm for all the processing frequencies. However, this displacement can be made higher or lower i.e., from 0.1 mm to 50 mm.

The stress-strain response of CP Al tensile samples after HPCRS, processed at different frequencies for the same total deformation is shown in FIG. 2(b). With the increase in frequency, there is a gradual drop in the strength of the material and an improvement in ductility. The graph represents the tensile results (Engineering. Stress Vs Engineering Strain) of CP Al processed sample at different shearing frequencies (BM: base metal).

First, the cuboidal sample/billet of CP aluminum with initial dimensions of 25 mm, 25 mm, and 6 mm for the length, width, and thickness, respectively, is placed into the die cavity, as shown in FIG. 1(b). Before the oscillation of the driving punch, a small compression load is applied on the sample by the normal punch to avoid free fall of the sample due to gravity. The channel is vertical, where the sample goes through the deformation process. The load is insufficient to cause any plastic deformation. For example, in case of Al and Mg, it can be 20 kN which causes a stress of 32 MPa (less than the yield value).

The geometry of the initial and final samples is shown in FIG. 2(c). It is known that in plastic deformation the volume of the material remains unchanged. Hence to confirm the homogeneous deformation of the sample the ‘volume constancy’ has been checked. It has been found that the thickness of the processed sample is constant throughout its length with minimal change of width w.r.t the unprocessed sample. Therefore, the ‘volume constancy’ of the material was maintained, i.e., Initial volume (25 mm x 25 mm x 6 mm) = Final volume (150 mm x 25 mm x 1 mm).

A single cycle of the HPCRS process consists of four steps. The driving punch oscillates from its mean position for the given frequency of oscillation and its amplitude/displacement is assisted by continuous pressing by the normal punch. In the results presented, the compression load was linearly increased with time. It was needed because of the increase of the sample surface area, for maintaining the necessary condition for plasticity.

With the increase in shearing frequency, the time required for the completion of the experiments is reducing drastically. The plot between ‘thickness reduction’ and the time required to get a processed sample having a 1 mm thickness is shown in FIG. 3(a). The high metal-forming speed of the developed invention should be noted. For the 20 Hz shearing frequency, one can achieve the desired processed sample in only 13 seconds.

The experiments were carried out under multi-loading condition for the forging punch the normal force was controlled, while for the shear-punch, its displacement. The loading parameters were optimized for each oscillation frequency to obtain a processed sheet with 1 mm thickness. So that the sole effect of the variation of oscillating frequency on the properties of the processed material could be studied.

Moreover, the improved process has also been tested for the processing of metal powders. A bulk of green compact aluminum powder of 6 mm thickness has been successfully converted into a metal sheet having a thickness of 1 mm.

The von Mises equivalent strain has been calculated from Equation. 6, accumulated for all cycles, and plotted in FIG. 3(b). In any SPD process, the mathematical value of the equivalent strain is considered an important parameter to compare the effectiveness of different SPD processes. For the current invention, this value is found to be extremely large. For a thickness reduction of 5 mm in CP Al material (i.e., from 6 mm to 1 mm), see FIG. 3(b), the obtained value of the accumulative von Mises equivalent strain was 44.

In the HPCRS process, it involves a single-step deformation operation, i.e., the process is capable of imparting very large strain into the material. For any combination of shearing frequency and amplitude of the oscillating die, the amount of strain accumulation mainly depends on the thickness-reduction of the sample. In the current investigation, for a thickness reduction of 83%, the values for von Mises accumulative equivalent strain, accumulative shear strain, and accumulative compressive strain are 44, 74, and 1.8, respectively. It is the shear strain that mainly increases the value of the equivalent strain. The plot between accumulative shear strain and thickness reduction is shown in FIG. 3(c). For a thickness reduction of 5 mm or 83%, the accumulative shear strain imparted into the processed material is 74, which is exceedingly high.

The value of accumulative compressive strain is shown in FIG. 3(d). For the mentioned thickness reduction, the value of the accumulative compressive strain is 1.8.

The micrograph of the initial commercially pure Al, which had an average grain size of 100 microns is shown in FIG. 4(a). The microstructures corresponding to the deformed state, processed at the shearing frequencies of 20 Hz, and 0.1, Hz, are displayed in FIGS. 4(b) and 4(c), respectively. Due to the severe deformation of the material, high grain fragmentation took place. The measured average grain sizes were found to be 1 µm, and 3.3 µm, respectively.

The tensile testing plot for Magnesium metal processed by HPCRS is shown in FIG. 5(a). It is interesting to observe that there is an increase in strength with no loss of ductility for both the samples, processed at shearing frequencies of 1 Hz and 20 Hz. The graph represents the tensile results (Engineering Stress Vs Engineering Strain) for Magnesium sample.

The sample processed at higher shearing frequencies has significant strength, large ductility, and a strong shear texture. There is no need for post-processing treatments like annealing. The processed samples can be directly put into their respective applications. Though the SPD processed samples have very high strength but, lack of ductility limits their use. Therefore, before use, the processed samples need to undergo heat treatment to enhance the ductility which may lead to loss of shear texture and thereby decrease in formability. Thus, the unique shear texture increases the formability of the metal or alloy.

The measured average grain size (GS) and grains morphology for base metal (pure Mg), samples processed at 20Hz, and 1Hz, are shown in FIGS. 6(a), 6(b), and 6(c) respectively. Thus, the grain size (GS) depends on the type of material and frequency.

The green compacted Aluminum powder sample and processed powder sample is shown in FIG. 7(a). The powder sample was processed at a frequency of 0.1Hz and the given shearing amplitude was 3 mm. The initial dimensions of the compacted powder sample were 20 mm, 15 mm, and 6 mm in length, width, and thickness respectively. A thickness reduction of 5 mm has been achieved.

The green compacted magnesium powder sample and the processed sample are shown in FIG. 7(b). The improved process succeeded in converting a bulk Mg metal powder into a sheet. The powder sample was processed at a much lower frequency of 0.01 Hz and the shearing displacement per pass was 5 mm. The initial dimensions of the compacted powder sample were 15 mm, 15 mm, and 5 mm in length, width, and thickness, respectively. The final thickness of the fabricated Mg sheet was 1.5 mm.

The capability of the improved process to obtain a sheet metal from the bonding of multiple bulk metals is shown in FIG. 8(a). Here, five sheets with 2 mm thickness each were stacked together and processed at a relatively higher frequency of 20 Hz and a shearing displacement of 1 mm to obtain a sheet having 2 mm final thickness. That means, a thickness reduction of 80% to get a good interface bonding strength. However, at higher frequencies, there is heat generation in the sample. Therefore, the combination of high pressure and heat results in the bonding of sheets.

The examples given in this present invention achieves the desired reduced thickness. However, various thickness reductions can be achieved.

The invention also succeeded in repeating the above-mentioned ‘bonding’ technique with the processed metal sheets. Here, the processed sheet was cut into five equal parts, cleaned the surfaces, and again stacked together to repeat the process. This technique can be called accumulative high-pressure compressive reverse shearing (AHPCRS). The AHPCRS technique can be repeated multiple times to impart enormous equivalent strain into the material and also to obtain nanolayered metal sheets.

The electron backscatter diffraction (EBSD) micrograph of the bonded sample is shown in FIG. 8(b). There is a huge fragmentation of grains and the average grain size is reduced from 70 microns to 1.5 microns. The color bands on the micrograph depict that the grains are heavily stretched in the shearing direction. Further, with the strain accumulation, the stretched grains form sub-grains having nearly similar orientations. This IQ+ IPF (image quality + inverse pole figure) EBSD micrograph showing ultra-fine grain microstructure in the final sheet was obtained by bonding of five initial bulk sheets.

Similar to the formula used in HPCS process, the same formula has been applied in HPCRS process for calculating the compression and shear strains, their corresponding deformation rates, as well as the values of the stress components and are presented here below.

Let the initial dimensions of the sample be l_i, t_(i,) and w_i for the length, thickness, and width, respectively. The width of the sample remains unchanged due to the constraint provided by the walls of the channel. The applied displacement in one movement to the right or left is denoted by ‘l'.

The true compression strain (e), and its rate (e ?) are given by:

e = ln l_i/(l_i+l ) , e ? = - l ?/(l_i+ l) (1, 2)

The shear strain (? ) and its strain rate (? ?) in one displacement are:

? = 1/(2l_i t_i )[(l_i+l)2 - ?l_i?^2] , ? ? = 1/(l_i t_i ) (l_i + l) l ? . (3, 4)

The von Mises equivalent strain rate of the deformation process can be calculated from:

(e ? ) ~_(eq.) = l ? v(4/(3?(l_i+l)?^2 )+ ?(l_i+l )?^2/(3 t_i^2 ?l_i?^2 )) . (5)

The total Von Mises equivalent strain in one displacement is:

e ~_(eq.) = 1/(v3 a_1 ) (a_2 - a_3) + 1/(2v3) (ln|(a_1- a_2 )/(a_1+ a_2 )| - ln|(a_1- a_3 )/(a_1+ a_3 )|) (6)

where a_1 = 2l_i t_i, ? a?_2 = v(a_1^2+?(l+ l_i )?^4 ), and ? a?_3 = v(?a_1?^2+ l_i^4 ) .

The total accumulative values of the compression, shear, and equivalent strains can be obtained by adding the contributions calculated in each cycle of the process.

The true compression (s) and shear stresses (t ) are obtained from:

s=F/(w·(l_i+l)^2 ) , t=S/(w·(l_i+l)^2 ) , (7, 8)

where F is the compression force and S is the shear force.

A work-conjugate equivalent stress (s_eq) can also be defined for the process:

s_eq= t·v(3+48·(e ?/? ? )^2 ) . (9)

Finally, the magnitude of the hydrostatic stress (s_h) is calculated from the formula:

s_h=s-4t e ?/? ? . (10)

The main advantage of the HPCRS process are as follows:

- the process imposes large homogeneous strain in the material and generate compressive hydrostatic stresses, similar to the HPCS process.

- the process is used to produce materials with the desired mechanical properties, like the strength of the material, ductility, toughness, elastic modulus, etc.

- the process able to control the properties of the material by varying the processing parameters. This is very much conducive to what is called, ‘processing-controlled properties (PCP)’.

Further, the HPCRS process of the present invention provides the advantages as follows:

Severe plastic deformation with large strain accumulation: The values obtained for the accumulative effective strain are extremely high: in CP Aluminum the von Mises equivalent strain of 44 could be readily reached in a single operation. Microstructural investigations show enormous stretching of the grains and the formation of sub-grains. The frequent change of strain path during deformation leads to a heavy fragmentation of the grains so that the steady-state stage of grain fragmentation can be reached with a grain size of about 1 micron.
Processing-controlled properties (PCP): The process is very much conducive to what is called, “processing-controlled properties (PCP)”. For CP Al, a three-fold increase in strength has been obtained for the sample processed at a lower shearing frequency, i.e., 0.1 Hz. With the increase of frequency (from 0.1 Hz to 20 Hz), for different samples for the same thickness reduction, an increase in ductility (from 20% to 33%) has been found with a gradual drop in strength (from 180 MPa to 100 MPa). Therefore, having large ductility, there is no need for post-processing treatment, like annealing of the processed samples. The samples can be directly put into their respective applications. Moreover, for Mg samples, it is interesting to note that for the shearing frequencies of 1 Hz, and 20 Hz there is an enhancement in strength without loss in ductility.
Control on texture evolution: Crystallographic texture gets evolved with the deformation of a material. If the deformation mechanism involves large shear along with compression, it may attribute to strong texture formation. In the current investigation, the shear component of the total deformation is huge; it can be 50 times larger than the compression strain. The consequence is that shear-type textures are obtained in the samples. For example, in Mg processed samples, a very high-intensity B- fiber appears. Also, in CP Al processed samples, at all shearing frequencies, a characteristic shear texture is seen. The shear texture improves the Lankford parameter (R-value) of an aluminum sheet metal thereby making it more suitable for deep drawing applications. Moreover, the intensity of the texture can be controlled by limiting the thickness-reduction value and the amplitude/displacement of the shearing component per pass.
Dynamic process: Deformation processing of the metals can be done both at room temperature and at elevated temperatures. Processing at higher shearing frequencies leads to significant heat generation in the sample. This can also be evidenced by the micrographs of the samples processed at 20 Hz having relatively larger grain sizes than those obtained at lower frequencies. Hence, the HPCRS process is a dynamic process.
Metal forming speed: The dimensions of the final processed sample and the forming speed are very important factors to be considered during the development of any new SPD process. The metal-forming speed of HPCRS is exceedingly high. One can convert a piece of a square block of thickness 6 mm into a metal sheet of thickness 1 mm in less than 13 seconds.

The HPCRS process has remarkable potential that needs to be exploited. Further, the design is its large range of frequency for the oscillating die, variation of loading rate, and versatility, where the high frequency of oscillation of the shearing die enables to study the deformation behavior of the workpiece at a higher strain rate. However, it also generates heat at high frequency. Therefore, the process is quite versatile to study the deformation behavior of the material at room temperature, elevated temperature, low strain rate, high strain rate, and many more. All the experiments have been done under ‘Load control’ option and to include load as a function of time i.e., the loading rate can be varied in a single experiment. Further, it is also possible to vary the loading rate, amplitude, frequency in a single experiment and thus, the present invention has a great control over the properties of the processed material. The process is very much conducive to what is previously referred to as ‘Processing Control Properties (PCP)’.

While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope of the invention as claimed.
, Claims:1. An improved severe plastic deformation process for producing ultra-fine grain structured sheets from a metal, comprising the steps of:
a. providing a secure die assembly, comprising a stationary forging die and an oscillating shearing die;
b. positioning a metallic workpiece within a cavity of the die assembly;
c. obtaining processed metal sheets by applying controlled compressive force to the workpiece with the forging die and simultaneously oscillating the shearing die at a predetermined frequency and amplitude, inducing controlled shear strain on the workpiece;
d. cutting the obtained metal sheet into multiple pieces and stacking together; and
e. repeating the steps b, c and d for a predetermined number of cycles or until the workpiece reaches a desired final thickness.

2. The process as claimed in claim 1, wherein the process imparts a large shear strain along with compressive strain into the workpiece, in a single-step deformation operation.

3. The process as claimed in claim 1, wherein the metallic workpieces are commercially pure Aluminum or pure Magnesium or metal or alloy.

4. The process as claimed in claim 1, wherein the frequency is about 20 Hz shearing frequency.

5. The process as claimed in claim 1, wherein the thickness of obtained ultra-fine grain structured sheet is about 1 mm to 2 mm.

6. The process as claimed in claim 1, wherein the process imparts very large strain into the material producing grain size of about 1 micron.

7. The process as claimed in claim 1, wherein the process is carried out to obtain the sheet metal in 13 seconds.