DESC:FIELD OF THE INVENTION
[001] The present invention relates to an aluminum alloy and a method to prepare the same, particularly an aluminum alloy which is homogenous and free from abnormal grain growth (AGG).

BACKGROUND OF THE INVENTION
[002] Grain size generally refers to crystallite sizes of a material and increase in size of these crystallites is termed as grain growth. There are two types of grain growth observed in materials, normal and abnormal. Abnormal grain growth is a process wherein the grain sizes are large, and a few select grains grow at the expense of other grains. In metals, alloys, etc. abnormal grain growth occurs usually at higher temperatures during or after extrusion process.
[003] Deformed metal when subjected to temperature >0.3 Tm (Tm is melting temperature in K) results in the commencement of softening processes due to onset of recovery, recrystallization, and grain growth. Aluminum having high stacking fault energy, provides faster recovery processes and when deformation occurs at high temperatures, dynamic recovery processes are activated. Additionally, when the dynamically recovered microstructure is subjected to higher temperatures without any further deformations, abnormal grain growth may proceed which leads to occurrence of very large grain sizes of the order of even few mm.
[004] Direct Hot Extrusion is a manufacturing process performed at >0.7 Tm in general and the microstructure generally conforms to the conditions which may lead to abnormal grain growth. Controlling abnormal grain growth is especially challenging when after extrusion, solutionising is done as a separate step. This is usually the case in commercially available alloys which are heavily used in defense and aerospace applications, which is governed by a stringent requirement of having a microstructure devoid of coarse grains.
[005] Coarse graining is peculiar in extrusions owing to non-uniform flow of metal leading to abnormal grain growth in specific location of profiles. Abnormal grain growth usually occurs when subjected to very high strains, strain rates, and temperatures. Abnormal grain growth (AGG) is usually at the periphery of extruded solid rods, tubes or sections and is therefore, sometimes referred to as Peripheral Coarse Grain or Peripheral Grain Growth. AGG is a defect which is highly undesirable in commercial alloys employed in defense applications where safety may be compromised owing to such defects. These defects are also not desirable for other applications in general.
[006] Conventionally, various alloys with controlled abnormal grain growth and methods to prepare the same were discovered. For instance, use of taper dies, alloying additions and homogenization practices have been disclsoed in the existing techniques. Further, T. Shepard et al, Journal of Materials Processing Technology 177 (2006) 26–35, describes the method of predicting extrusion microstructure after solutionising in terms of sub grain size and recrystallized grain size with the help of modelling. A homogenization practice to restrict recrystallization is also known in the art. X. Duan, T. Sheppard, Materials Science and Engineering A351 (2003) 282-292, discloses recrystallization control through die design.
[007] Conventional or commercially available alloys do not provide sufficient control on the alloy chemistry and process parameters for homogenization, extrusion and solutionising for achieving a homogenous AGG free microstructure. The available alloys are therefore prone to the aforementioned defects which is likely to cause safety concerns when employed for defense and aerospace applications. Moreover, the existing art does not disclose the size, number density and area fraction of stable intermetallics or dispersoids in the microstructure for obtaining the desired AGG free extrudates.
[008] In view of the above, there is a need for an aluminum alloy, particularly a method for preparing an aluminum alloy which addresses at least the aforementioned problems.

SUMMARY OF THE INVENTION
[009] In one aspect, the present invention relates to a method for preparing an aluminum alloy which is homogenous and free from abnormal grain growth (AGG). The method includes extruding billets of the aluminum alloy comprising: 0.6 wt% to 7.0 wt% of copper (Cu), 0.05 to 1.5 wt% of silicon (Si), 0.1 to 0.35 wt% of iron (Fe), 0.7 to 1.5 wt% of manganese (Mn), 0.1 to 2.0 wt% of magnesium (Mg), 0.01 to 0.40 wt% of chromium (Cr), 0.001 to 0.25 wt% of zinc (Zn), 0.001 wt% to 0.25 wt% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.% < Zr+Ti < 0.3 wt.%; and remaining being aluminum; wherein the wt.% is based on the total weight of the alloy. The method is carried out at an extrusion ratio ranging between 5 to 60.
[010] In an embodiment, the aluminum alloy comprises: 3.8 wt.% to 5.0 wt.% of copper (Cu); 0.05 wt.% to 1.2 wt.% of silicon (Si); 0.1 wt.% to 0.3 wt.% of iron (Fe); 0.7 wt.% to 1.2 wt.% of manganese (Mn); 0.2 wt.% to 1.8 wt.% of magnesium (Mg); 0.01 wt.% to 0.1 wt.% of chromium (Cr); 0.001 wt.% to 0.25 wt.% of zinc (Zn); 0.001 wt.% to 0.25 wt.% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.%< Zr+Ti < 0.25 wt.%; and remaining being aluminum.
[011] In another embodiment, prior to extruding, the billets are homogenized at a temperature ranging between 430°C to 480°C with a soaking time ranging between 5 h to 20 h followed by cooling to room temperature with heating rate of 20°C/ h to 120°C/h.
[012] In yet another embodiment, the billets are extruded at an initial billet temperature ranging between 390°C to 450°C and a container temperature ranging between 380°C to 420°C.
[013] In still another embodiment, an extrusion exit temperature ranging between 450°C to 490°C is maintained to obtain extruded profiles.
[014] In a further embodiment, the extruded profiles are solutionized at a temperature ranging between 450°C to 508°C with a soaking time ranging between 0.25 h to 2 h at a heating rate ranging between 100°C/h to 300°C.
[015] In another embodiment, the extruded profiles are subjected to ageing at a temperature ranging between 120°C to 200°C for a soaking time ranging between 0.5 h to 15 h.
[016] In still further embodiment, the number density of intermetallics in the aluminum alloy is greater than 1 /µm2 and area fraction is greater than 1%.
[017] In yet another embodiment, the aluminium alloy has a homogeneous microstructure with an intermetallic size in the range of 10 nm to 500 nm with region of abnormal grain growth (AGG) restricted to 2mm on a periphery of the extruded profile.
[018] In a still further embodiment, the extruded profile is a rod or tube having a diameter ranging between 30 mm to 100 mm or profiles with an equivalent surface area.

BRIEF DESCRIPTION OF THE DRAWINGS
[019] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
[020] Figure 1 shows a 52mm diameter macro-etched extruded rod prepared in accordance with comparative example 1.
[021] Figure 2 shows a 36mm diameter macro-etched extruded rod prepared in accordance with comparative example 2.
[022] Figure 3 shows a magnified image obtained through a polarized light optical microscope of a sample prepared in accordance with comparative example 1.
[023] Figure 4 shows a 52mm diameter macro-etched extruded rod prepared as per inventive example 1 in accordance with an embodiment of the present invention.
[024] Figure 5 shows magnified images of a 52mm diameter extruded rod prepared as per inventive examples 3 in accordance with an embodiment of the present invention.
[025] Figure 6 shows macro-etched image prepared as per inventive example 4 in accordance with an embodiment of the present invention.

BRIEF DESCRIPTION OF THE INVENTION
[026] Before the compositions and formulations of the present invention are described, it is to be understood that this invention is not limited to particular compositions and formulations described, since such compositions and formulations may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting since the scope of the present invention will be limited only by the appended claims.
[027] The terms “comprising”, “comprises” and “comprised of” as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms “comprising”, “comprises” and “comprised of” as used herein comprise the terms “consisting of”, “consists” and “consists of”.
[028] Furthermore, the terms “first”, “second”, “third” or “(a)”, “(b)”, “(c)”, “(d)” etc. and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. In case the terms “first”, “second”, “third” or “(A)”, “(B)” and “(C)” or “(a)”, “(b)”, “(c)”, “(d)”, “i”, “ii” etc. relate to steps of a method or use or assay there is no time or time interval coherence between the steps, that is, the steps may be carried out simultaneously or there may be time intervals of seconds, minutes, hours, days, weeks, months or even years between such steps unless otherwise indicated in the application as set forth herein above or below.
[029] In the following passages, different aspects of the present invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.
[030] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention and form different embodiments, as would be understood by those in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.
[031] Furthermore, the ranges defined throughout the specification include the end values as well, i.e., a range of 1 to 10 implies that both 1 and 10 are included in the range. For the avoidance of doubt, the applicant(s) shall be entitled to any equivalents according to applicable law.
[032] In the present context, an “aluminium alloy” refers to a chemical composition of aluminum with one or more alloying elements as discussed hereinbelow.
[033] Further, a “billet” refers to a solid, elongated metal structure, typically cylindrical or rectangular in shape. In the present context, the billet is made of the aluminum alloy, as described hereinbelow.
[034] Furthermore, an “extrusion ratio” refers to the ratio of cross-sectional area of the billet to the cross-sectional area of an extruded profile. Specifically, the extrusion ratio is calculated by dividing the initial cross-sectional area of the billet by the final cross-sectional area of the extruded profile.
[035] Herein, “intermetallics” refers to crystalline compounds formed by the combination of aluminum with the alloying elements and optionally auxiliary alloying elements, exhibiting distinct phases with ordered atomic structures. The formation and distribution of intermetallic phases within the aluminum alloy may be influenced by factors including alloy composition, processing conditions, cooling rates, and heat treatment methods, each of which has been discussed hereinbelow.
[036] Furthermore, “grain size” pertains to the average dimension of individual crystalline grains within the alloy matrix, typically measured along specific directions within the material's microstructure. The grain size of the aluminum alloy is influenced by various factors including alloy composition, processing conditions, cooling rates, and heat treatment procedures. Smaller grain sizes within the alloy matrix are generally associated with improved mechanical properties such as tensile strength, hardness, and fatigue resistance, as well as enhanced formability and weldability. Conversely, larger grain sizes may result in reduced mechanical performance and increased susceptibility to deformation and fracture.
[037] An aspect of the present invention relates to a method for preparing an aluminum alloy which is homogenous and free from abnormal grain growth (AGG).
[038] In an embodiment, the method comprises extruding billets of the aluminum alloy comprising: 0.6 wt% to 7.0 wt% of copper (Cu), 0.05 to 1.5 wt% of silicon (Si), 0.1 to 0.35 wt% of iron (Fe), 0.7 to 1.5 wt% of manganese (Mn), 0.1 to 2.0 wt% of magnesium (Mg), 0.01 to 0.40 wt% of chromium (Cr), 0.001 to 0.25 wt% of zinc (Zn), 0.001 wt% to 0.25 wt% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.% < Zr+Ti < 0.3 wt.%; and remaining being aluminum. Unless specified otherwise, the wt.% is based on the total weight of the alloy.
[039] In the present context, copper, silicon, iron, manganese, magnesium, chromium, zinc, zirconium, and titanium are collectively referred to as alloying elements.
[040] In another embodiment, the alloy further comprises up to 0.15 wt.% impurities. In fact, the impurities in the alloy may be up to 0.05 wt.% on an individual level with the total amount not exceeding beyond 0.15 wt.%. A person skilled in the art is aware of typical impurities that may be found in an aluminum alloy. In an embodiment, the impurities are different than the alloying elements and/or their compounds.
[041] In another embodiment, the alloy optionally comprises up to 1.5 wt.% of nickel (Ni); up to 1.0 wt.% of silver (Ag), and bismuth (Bi); up to 2.6 wt.% of lithium (Li); and up to 2.0 wt.% of tin (Sn) and lead (Pb). These elements are collectively referred to as auxiliary alloying elements. The auxiliary alloying elements might result in further reduction of AGG and/or retain the AGG in the aluminum alloy achieved upon addition of alloying additives.
[042] In yet another embodiment, the method is carried out at an extrusion ratio ranging between 5 to 60.
[043] In still another embodiment, the aluminum alloy comprises: 3.8 wt.% to 5.0 wt.% of copper (Cu); 0.05 wt.% to 1.2 wt.% of silicon (Si); 0.1 wt.% to 0.3 wt.% of iron (Fe); 0.7 wt.% to 1.2 wt.% of manganese (Mn); 0.2 wt.% to 1.8 wt.% of magnesium (Mg); 0.01 wt.% to 0.1 wt.% of chromium (Cr); 0.001 wt.% to 0.25 wt.% of zinc (Zn); 0.001 wt.% to 0.25 wt.% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.%< Zr+Ti < 0.25 wt.%; and remaining being aluminum.
[044] In an embodiment, the aluminum alloy consists of: 0.6 wt% to 7.0 wt% of copper (Cu), 0.05 to 1.5 wt% of silicon (Si), 0.1 to 0.35 wt% of iron (Fe), 0.7 to 1.5 wt% of manganese (Mn), 0.1 to 2.0 wt% of magnesium (Mg), 0.01 to 0.40 wt% of chromium (Cr), 0.001 to 0.25 wt% of zinc (Zn), 0.001 wt% to 0.25 wt% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.% < Zr+Ti < 0.3 wt.%; up to 0.15 wt.% impurities; and remaining being aluminum.
[045] The aluminum alloy has temperature stable intermetallics defined in terms of size, density, and area fraction, along with homogeneous and AGG free microstructure lacking any coarse grains.
[046] Further, the present invention has observed that at least one of Cr, Mn and Zr on the higher side, but within the aforesaid range, results in higher fraction of stable intermetallic in extruded profiles.
[047] In another embodiment, prior to extruding, the billets are homogenized at a temperature ranging between 430°C to 480°C with a soaking time ranging between 5 h to 20 h followed by cooling to room temperature with heating rate of 20°C/ h to 120°C/h.
[048] In an embodiment, the billets are subjected to an intermediate step of soaking during homogenization wherein the billets are heated at a temperature ranging between 350°C to 430°C for a duration ranging between 2h to 12h prior to heating them at homogenization conditions. Said otherwise, there may not be intermediate step for soaking, however a single intermediate step is preferred.
[049] In yet another embodiment, the billets are extruded at an initial billet temperature ranging between 390°C to 450°C and a container temperature ranging between 380°C to 420°C. Although, the present invention is not limited by the speed or rate of extrusion, it is preferred that the extrusion is carried out at maximum possible speed without leading to surface cracking or other issues in the profiles.
[050] In a further embodiment, an extrusion exit temperature ranging between 450°C to 490°C is maintained to obtain extruded profiles.
[051] Subsequently, the extruded profiles are solutionized at a temperature ranging between 450°C to 508°C with a soaking time ranging between 0.25 h to 2 h at a heating rate ranging between 100°C/h to 300°C. In fact, a temperature range close to solidus is maintained during solutionising. Herein, solidus refers to the temperature at which a liquid phase starts to form during heating of the metal.
[052] Thereafter, the extruded profiles are subjected to ageing at a temperature ranging between 120°C to 200°C for a soaking time ranging between 0.5 h to 15 h.
[053] Thus, the method of the present invention can be summarized to have the following steps: (i) providing billets, (ii) homogenizing the billets, (iii) extruding the billets to obtain extruded profiles, (iv) solutionising the extruded profiles, and (v) ageing the extruded profiles.
[054] Accordingly, in an embodiment, the method comprises the following steps:
(A) homogenizing billets comprising: 0.6 wt% to 7.0 wt% of copper (Cu), 0.05 to 1.5 wt% of silicon (Si), 0.1 to 0.35 wt% of iron (Fe), 0.7 to 1.5 wt% of manganese (Mn), 0.1 to 2.0 wt% of magnesium (Mg), 0.01 to 0.40 wt% of chromium (Cr), 0.001 to 0.25 wt% of zinc (Zn), 0.001 wt% to 0.25 wt% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.% < Zr+Ti < 0.3 wt.%; and remaining being aluminum,
at temperature ranging between 430°C to 480°C with soaking time ranging between 5 h to 20 h followed by cooling to room temperature with heating rate of 20°C/ h to 120°C/h,
(B) extruding the billets at the initial billet temperature ranging between 390°C to 450°C and the container temperature ranging between 380°C to 420°C to obtain the extruded profiles,
(C) solutionising the extruded profiles at temperature ranging between 450°C to 508°C with the soaking time ranging between 0.25 h to 2 h at the heating rate ranging between 100°C/h to 300°C, and
(D) ageing the extruded profiles at temperature ranging between 120°C to 200°C for the soaking time ranging between 0.5 h to 15 h.
[055] In another embodiment, the method comprises the following steps:
(A) homogenizing billets comprising: 0.6 wt% to 7.0 wt% of copper (Cu), 0.05 to 1.5 wt% of silicon (Si), 0.1 to 0.35 wt% of iron (Fe), 0.7 to 1.5 wt% of manganese (Mn), 0.1 to 2.0 wt% of magnesium (Mg), 0.01 to 0.40 wt% of chromium (Cr), 0.001 to 0.25 wt% of zinc (Zn), 0.001 wt% to 0.25 wt% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.% < Zr+Ti < 0.3 wt.%; and remaining being aluminum,
wherein the billets are heated at a temperature ranging between 350°C to 430°C for a duration ranging between 2h to 12h, prior to homogenizing them at temperature ranging between 430°C to 480°C with soaking time ranging between 5 h to 20 h followed by cooling to room temperature with heating rate of 20°C/ h to 120°C/h,
(B) extruding the billets at the initial billet temperature ranging between 390°C to 450°C and the container temperature ranging between 380°C to 420°C to obtain the extruded profiles,
(C) solutionising the extruded profiles at temperature ranging between 450°C to 508°C with the soaking time ranging between 0.25 h to 2 h at the heating rate ranging between 100°C/h to 300°C, and
[056] ageing the extruded profiles at temperature ranging between 120°C to 200°C for the soaking time ranging between 0.5 h to 15 h.
[057] In an embodiment, the number density of the intermetallics in the aluminum alloy is greater than 1 /µm2 and area fraction is greater than 1%. Further, the aluminium alloy has a homogeneous microstructure with an intermetallic size in the range of 10 nm to 500 nm with region of AGG restricted to 2mm on a periphery of the extruded profile. These parameters may be determined using suitable techniques such as Scanning Electron Microscope (SEM) and image analysis softwares.
[058] In an embodiment, the present invention is not limited by the shape, size, and dimension of the extruded profiles and the same is well known to the person skilled in the art. Preferably, the extruded profile is a rod or tube having a diameter ranging between 30 mm to 100 mm or profiles with an equivalent surface area.
[059] Advantageously, the extruded profiles obtained in accordance with the present invention are AGG free and have homogeneous microstructure with intermetallic size ranging between 10 nm to 500 nm covering approximately 10% of sample from periphery towards the center and less than 10 µm throughout the microstructure. Further, the aluminum alloy of the present invention finds application in defense and aerospace, however, not limited thereto.
[060] Moreover, as highlighted above, the present invention provides sufficient control on the alloy chemistry and process parameters for homogenization, extrusion and solutionising for achieving homogenous AGG free microstructure. Therefore, the present invention alloys are comparable or even better than the commercially available alloys.

EXAMPLES
[061] The following examples are illustrative of the present invention but not limitative of the scope thereof:
[062] Following methods were used for characterization purpose:
[063] Homogeneity of microstructure: Imaging through optical microscope imaging and analyzing the grain and subgrain structure or macro-etching the sample through Tuckers reagent.
[064] Intermetallics size and fraction: Imaging through Scanning Electron Microscope (SEM) and analyzing in ImageJ software using routine thresholding and image analysis.
[065] Subgrain size: Imaging in SEM or EBSD and analyzing in ImageJ software using routine thresholding and image analysis.
[066] Abnormal grain growth: Imaging through optical microscope imaging and analyzing the grain and subgrain structure or macro etching the sample through Tuckers reagent.
[067] Comparative Example-1
[068] Commercial aluminum alloy comprising: Cu at 4.2 wt%, Si at 0.74 wt%, 0.32 wt% of Fe, Mn at 0.63 wt%, Mg at 0.43 wt%, 0.03 wt% of Ti and other elements with content less than 0.15 wt%, was subjected to direct chill (DC) cast industrially and homogenized at heating rate of around 120 °C/h at 475°C to 495°C for soaking duration of 9 hours to 12 hours. Homogenized billets were cooled at a rate of > 200 °C/h and extruded in direct extrusion industrial press at billet temperature between 390°C to 410°C with container temperature between 390°C to 395°C at maximum ram speed possible (typically between 1.5-4mm/s). The samples were solutionised at 505°C for 120 minutes. The input billet was 230 mm diameter and was extruded to final rods of diameter 52mm. Following this process, the typical levels of AGG were around 4mm which can go up to 6mm as shown in Figures 1 and 3. In fact, Figure 3 shows abnormally large grains of upto the depth of around 6mm, extending non uniformly up to 9mm.
[069] Comparative Example-2
[070] Another commercial aluminum alloy comprising: Cu at 4.2 wt%, Si at 0.74 wt%, 0.32 wt% of Fe, Mn at 0.63 wt%, Mg at 0.43 wt%, 0.03 wt% of Ti and other elements with content less than 0.15 wt%, was subjected to direct chill (DC) cast and homogenized at heating rate of around 120 °C/h at 475 °C to 495 °C for soaking duration of 9 hours to 12 hours. Homogenized billets were cooled at a rate of > 200 °C/h and extruded in direct extrusion industrial press at billet temperature between 385°C to 400°C with container temperature ranging between 380°C to 385°C at maximum ram speed possible (typically between 1.5-4mm/s). The samples were solutionised at 505 °C for 120 minutes. The input billets were 230 mm diameter and were extruded to final rods of diameter 36 mm with two cavity die. Following this process, the AGG obtained was asymmetrical and the typical level of AGG was around 3.5mm as shown in Figure 2.
[071] Inventive Example-1
[072] Aluminum alloy comprising: Cu at 4.1 wt%, Si at 0.71, 0.2 wt% of Fe, Mn at 0.97 wt%, Mg at 0.38 wt%, 0.08wt% of Cr , 0.01-0.2 wt% of Ti+Zr and other elements with content less than 0.15 wt%, was subjected to direct chill (DC) cast and homogenized at heating rate of around 100 °C/h at 460 °C to 475°C for soaking duration of 10 hours to 12 hours. Homogenized billets were cooled at a rate of > 200 °C/h and extruded in direct extrusion industrial press at billet temperature between 420-430°C with container temperature ranging between 395 °C to 410 °C at maximum ram speed possible (typically between 1.5-4mm/s). The samples were solutionised at 505°C for 120 minutes. The input billet was 230 mm diameter and extruded to final rods of diameter 52 mm. Following this process, the AGG obtained was symmetrical and the typical level of AGG was between 0 to 1.7mm along the length of the extrudate.
[073] Inventive Example-2
[074] Another aluminum alloy comprising Cu at 4.2 wt%, Si at 0.77, 0.27 wt% of Fe, Mn at 0.92 wt%, Mg at 0.465 wt%, 0.05wt% of Cr , 0.01-0.15 wt% of Ti+Zr and other elements with content less than 0.15 wt%, was subjected to direct chill (DC) cast and homogenized at heating rate of around 100 °C/h at 460 °C to 475°C for soaking duration of 10 hours to 12 hours. Homogenized billets were cooled at a rate of > 200 °C/h and extruded in direct extrusion industrial press at various billet temperature ranging between 380-420°C and container temperature of 380°C to 390°C at maximum ram speed possible (typically between 1.5-4mm/s). The samples were solutionised at 505 °C for 120 minutes. The input billet was 230 mm diameter and extruded to final rods of diameter 36 mm with two cavity die. Following this process, the AGG obtained was asymmetrical and the typical level of AGG was from 0 to 1.9 mm along the length of the extrudate.
[075] Inventive Example-3
[076] This example illustrates an inventive alloy which was prepared, homogenized and extruded with parameters as mentioned in Inventive Example-1 above with an intermediate step of homogenization at 380°C for a soaking duration of 4-8h. The samples were solutionised at 500°C for 120 minutes. The input billet was 230 mm diameter, and extruded to final rods of diameter 52 mm. Following this process, the AGG obtained was symmetrical and the typical level of AGG is from 0 to 1.7mm along the length of the extrudate.
[077] Inventive Example-4
[078] Aluminum alloy comprising: Cu 4.2 in wt%, Si at 0.28 wt% max, 0.18 wt% of Fe, Mn at 0.82 wt%, Mg at 1.4 wt%, 0.006wt% of Cr, 0.04 wt% of Ti and other elements with content less than 0.15 wt% was direct chill (DC) cast and homogenized at heating rate of approximately 100 °C/h at 460 °C to 475 °C for soaking duration of 9 hours to 12 hours. Homogenized billets were cooled at a rate of >200 °C/h and extruded in direct extrusion industrial press at billet temperature of 410 °C to 430 °C and container temperature of 390 °C to 395°C at maximum ram speed possible (typically between 1.5-4mm/s). The samples were solutionised at 495 °C for 120 minutes. The input billet was 230 mm diameter and extruded to final rods of diameter 36 mm in two cavity die. Following this process, no AGG was obtained along the length when the process was within the mentioned range.
[079] As observed in the examples and the corresponding Figures, the microstructural features obtained in the present invention result in reduced or no AGG as compared to commercially available alloys.
[080] Table 1: Features of aluminium alloy obtained by commercial and inventive methods:
Microstructure Abnormally grain growth on surface Intermetallic number density/µm2 Intermetallic area fraction Subgrain size, µm (within 10% of surface) Subgrain size, µm (rest of the profile)
COMMERCIAL / EXISTING
Inhomogenous, presence of coarse grains on edges >5% of radius on surface Not specified Not specified Unstable sub-grains replaced with coarse grains NA
Inhomogenous, presence of coarse grains on edges >4% of radius on surface Not specified Not specified Unstable sub-grains replaced with coarse grains NA
INVENTIVE
Homogenous with coarse grains extent less than described <5% of radius on surface or <2mm on radius >1/µm2 >1.0% Stable sub-grains <10 µm
Homogenous with coarse grains extent less than described <4% of radius on surface or <1.5mm on radius >1/µm2 >1.0% Stable sub-grains <10 µm

[081] As observed in Table 1, existing methods are generally performed at non-regulated or uncontrolled process parameters, which result in abnormal sub-grain growth. These alloys reflect inhomogeneous presence of coarse grains on edges. The sub grain growth is more than 5% of radius on surface of commercial alloys. Figures 1 and 2 clearly show an unstable growth of sub-grains in aluminium alloy prepared using existing methods.
[082] The alloys prepared with present process (in accordance with inventive example 1) result in homogeneous structure and if present coarse grain growth with less than 5% of radius on surface. Intermetallic number density and intermetallic area fraction of the alloy was greater than 1/µm2 and 1.0%, respectively. Figure 4 clearly shows aluminium alloy prepared with present process resulting in stable surface without AGG. Figures 5 and 6 also showcase controlled or no AGG in respect of inventive examples 3 and 4.
[083] Overall, it can be observed that the inventive examples of the present invention showcase very low AGG. Moreover, the intermetallic number density of inventive alloys is higher than 1/µm2. On the contrary, the existing alloys and methods result in high AGG. Further, the existing or commercial methods clearly lack a combination of process parameters viz. homogenization, extrusion, solutionising and ageing, as outlined in the present invention. Thus, it may be observed that the combination of the process parameters with the aluminum alloy composition led to homogeneous and coarse grain free microstructure. Furthermore, it has also been observed that the commercially available alloys do not have the specified intermetallic volume fraction or size range, as defined in the present invention. Moreover, unstable subgrains are observed up to 10% from periphery, which transform into large grains after solutionising.
[084] The foregoing description of the present invention has been set merely to illustrate the present invention and is not intended to be limiting. Since the modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to the person skilled in the art, the present invention should be construed to include everything within the scope of the disclosure.
[085] Further, it will be apparent to the person skilled in the art that various changes and modifications may be made without departing from the scope of the present invention as defined in the following claims.
,CLAIMS:We Claim:
1. A method for preparing an aluminium alloy comprising extruding billets of the aluminum alloy comprising:
0.6 wt.% to 7.0 wt.% of copper (Cu);
0.05 wt.% to 1.5 wt.% of silicon (Si);
0.1 wt.% to 0.35 wt.% of iron (Fe);
0.7 wt.% to 1.5 wt.% of manganese (Mn);
0.1 wt.% to 2.0 wt.% of magnesium (Mg);
0.01 wt.% to 0.4 wt.% of chromium (Cr);
0.001 wt.% to 0.25 wt.% of zinc (Zn);
0.001 wt.% to 0.25 wt.% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.%< Zr+Ti < 0.3 wt.%; and
remaining being aluminum;
wherein the wt.% is based on the total weight of the alloy,
at an extrusion ratio ranging between 5 to 60.

2. The method as claimed in claim 1, wherein the aluminum alloy comprises:
3.8 wt.% to 5.0 wt.% of copper (Cu);
0.05 wt.% to 1.2 wt.% of silicon (Si);
0.1 wt.% to 0.3 wt.% of iron (Fe);
0.7 wt.% to 1.2 wt.% of manganese (Mn);
0.2 wt.% to 1.8 wt.% of magnesium (Mg);
0.01 wt.% to 0.1 wt.% of chromium (Cr);
0.001 wt.% to 0.25 wt.% of zinc (Zn);
0.001 wt.% to 0.25 wt.% of each of zirconium (Zr) and titanium (Ti), such that 0.01 wt.%< Zr+Ti < 0.25 wt.%; and
remaining being aluminum.

3. The method as claimed in claim 1 or 2, wherein prior to extruding, the billets are homogenized at a temperature ranging between 430°C to 480°C with a soaking time ranging between 5 h to 20 h followed by cooling to room temperature with heating rate of 20°C/ h to 120°C/h.

4. The method as claimed in claim 1, wherein the billets are extruded at an initial billet temperature ranging between 390°C to 450°C and a container temperature ranging between 380°C to 420°C.

5. The method as claimed in claim 4, wherein an extrusion exit temperature ranging between 450°C to 490°C is maintained to obtain extruded profiles.

6. The method as claimed in claim 5, wherein the extruded profiles are solutionized at a temperature ranging between 450°C to 508°C with a soaking time ranging between 0.25 h to 2 h at a heating rate ranging between 100°C/h to 300°C/h.

7. The method as claimed in claim 6, wherein the extruded profiles are subjected to ageing at a temperature ranging between 120°C to 200°C for a soaking time ranging between 0.5 h to 15 h.

8. The method as claimed in claim 1, wherein the number density of intermetallics in the aluminum alloy is greater than 1 /µm2 and area fraction is greater than 1%.

9. The method as claimed in claims 4 to 7, wherein the aluminium alloy has a homogeneous microstructure with an intermetallic size in the range of 10 nm to 500 nm with region of abnormal grain growth (AGG) restricted to 2mm on a periphery of the extruded profile.

10. The method as claimed in claims 4 to 7, wherein the extruded profile is a rod or tube having a diameter ranging between 30 mm to 100 mm or profiles with an equivalent surface area.

Dated this 13th day of April 2023
Hindalco Industries Limited
By their Agent & Attorney

(Nisha Austine)
of Khaitan & Co
Reg No IN/PA-1390