DESC:FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
THE PATENTS RULES, 2003
As amended by the Patents (Amendment) Rules, 2006

COMPLETE SPECIFICATION
(See section 10 and rule 13)

TITLE OF THE INVENTION
ALUMINUM ALLOYS AND THE PROCESS FOR PREPARING EXTRUDED ALLOY

APPLICANTS
HINDALCO INDUSTRIES LIMITED
of address
Ahura Centre, 1st Floor, B-Wing, Mahakali Caves Road,
Andheri (East), Mumbai- 400 093

PREAMBLE TO THE DESCRIPTION
The following specification particularly describes this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
The present invention relates to aluminium alloys and a process for preparing the same.

BACKGROUND OF THE INVENTION
Al-Mg-Si or AA6xxx series of Al alloys are commonly and widely used for extrusions. The extrudates need superior surface finish in architectural and decorative applications.

Alloy properties such as surface quality and superior surface finish are the key requirements when used in architectural and decorative applications.
Poor surface finish and pick-ups are highly undesirable even for regular extrusion products as it makes the surface rough and undesirable. It is known that, ? type intermetallics formed during the casting are one of the reasons for pick-ups leading to deterioration of surface quality during extrusion. Pick-ups leads to slowing down of extrusion speeds, frequent die changes and overall loss in productivity.

Various reports suggest addition of 0.005 to 0.5 wt% Sr in AA7XXX alloys and more specifically in AA7050 and AA7075 alloys. The benefit envisaged here is in terms of refinement of as cast intermetallics leading to shorter homogenization times. However, the reports are silent on type of intermetallics and any benefit derived therefrom in terms of improved surfaces.

US4406717 describes a method for memory discs’ substrates with the alloys covering AA5xxx and AA7xxx range. US4406717 describes benefit of adding 0.004 to 0.25 wt % of Sr leading to refinement of intermetallics and better anodizing thereafter.

Paray et.al describe effect of 300 ppm Sr addition to AA6061 alloy leading to higher as cast a particles fraction i.e. around 70% as compared to less than 10% in AA6061 Sr free alloy. Paray et.al also mentions increased speeds up to 50% higher than regular one without surface deterioration. The quantified data on surface quality however is lacking.

US3926690 talks about addition of Sr/Ca to levels of 0.01 to 0.5% in AA6063 alloys. The patent describes formation of a intermetallics during casting which leads to lower instance of pick-ups in standard extrusion process. However, US3926690 does not disclose surface roughness obtained in extrudates.

Therefore, there is a need to develop aluminum alloys, specifically AA6xxx series alloys that have lower pick-ups in the extruded profiles, provides higher speed during extrusion and provides lowered surface roughness in the extruded profiles.

SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an aluminium alloy composition having superior surface finish as compared to conventional extrusion alloys.

Another object of the invention is to provide an aluminium alloy composition which offers higher speed during extrusion of aluminium alloy without pick-ups.

Another object of the invention is to provide an aluminium alloy composition that enables alloys to be extruded at higher temperature.

Another object of the invention is to provide an aluminum alloy that can be extruded in long runs without the need for frequent die changes.

Yet another object of the invention is to provide an aluminium alloy composition that enables reduction in peripheral coarse graining of extruded alloy.

Yet another object of the invention is to provide a method for preparing the aluminium alloys having more than 50% of intermetallics of a particles in the as cast microstructure.

For achieving the above object, the invention provides aluminium alloy composition comprising of:
a) 0.1-2 wt% of magnesium (Mg);
b) 0.1-2 wt% of silicon (Si);
c) upto 0.8wt% of Copper (Cu);
d) 0.005-0.1wt% of Strontium (Sr);
e) upto 0.5 wt% of Iron (Fe);
f) upto 0.4 wt% of Chromium (Cr);
g) upto 0.4 wt% of Manganese (Mn);
h) upto 0.15 wt% of Titanium (Ti);
i) upto 0.25 wt% of Zinc (Zn); and
j) the rest being aluminum
wherein Mg to Si ratio is in a range of 1.1-1.6.

Additional objects and advantages of the invention will be set forth in the detailed description that follows, and in part will be obvious from the description, or maybe learned by practice of the invention. The objects and advantages of the invention will be attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing summary, as well as the following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, the details are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein. In the drawings:

Figure 1 is electron microscopic images of Al-Fe-Si intermetallics in AA6061 microstructure(a)As cast; (b) Homogenized.

Figure 2 is electron microscopic images of Al-Fe-Si intermetallics size with: (a) High Fe (0.2wt%) (b) High Mn (0.39wt%) (c) Low Fe (0.11%), Mn (0.05%) and Cr (0.04%).

Figure 3 is a graph comparing (Fe+Mn) vs Si distribution in strontium free and strontium containing alloy, (a)as cast condition; (b) homogenized condition.

Figure 4 is a graph comparing extrusion speeds and surface roughness in strontium containing and strontium free billets.

Figure 5 is a graph comparing extrusion speeds in strontium containing and strontium free billets.

Figure 6 is electron microscopic images of macrostructure of extruded profiles (a) regular aluminium alloy (b) aluminium alloy of present invention.

Figure 7 shows ram speed of regular vs alloy of present application (b) shows the maximum number of billets extruded before die withdrawal.

DESCRIPTION OF THE INVENTION
In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.

As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.

The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention

As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

The term “aluminium alloy” as used herein refers to a chemical composition where other elements are added to pure aluminum in order to enhance its properties, primarily to increase its strength.

The term “surface finish” as used herein refers to the measure of the finely spaced micro-irregularities on the texture of a surface.

The term “die run” as used herein refers to the quantity of alloy that can be extruded in a single run before the surface of the extrudate deteriorates, necessitating a die change.

The term “peripheral coarse grain” as used herein refers to the phenomenon of formation of large, recrystallized grains on and near the surface of the extrudate.

The term “critical quench rate” as used herein refers to the minimum cooling rate during extrusion to ensure maximum mechanical properties after heat treatment.

The invention is based on the object of preparing an aluminium alloy which is suitable for easy extrudability, enabling high elongation in the as-extruded state. The extruded alloys exhibit enhanced surface finish and reduced peripheral coarse graining. In addition, the alloy should be easily weldable and flangeable, able to be riveted and have good mechanical properties.

In an aspect, the invention provides aluminium alloy composition comprising of:
a) 0.1-2 wt% of magnesium (Mg);
b) 0.1-2 wt% of silicon (Si);
c) upto 0.8wt% of Copper (Cu);
d) 0.005-0.1wt% of Strontium (Sr);
e) upto 0.5 wt% of Iron (Fe);
f) upto 0.4 wt% of Chromium (Cr);
g) upto 0.4 wt% of Manganese (Mn);
h) upto 0.15 wt% of Titanium (Ti);
i) upto 0.25 wt% of Zinc (Zn); and
j) the rest being aluminum
wherein Mg to Si ratio is in a range of 1.1-1.6.

The alloy composition of present application is related, but not limited to, AA6xxx series. The total impurity levels in the composition is less than 0.15% and each impurity element is less than 0.05%. The allow composition of the present invention comprises of Mg and Si as major alloying elements. In an embodiment, the ratio of Mg to Si in the alloy composition is maintained in the range of 1.1-1.6, which results in excess Si as compared to balanced alloys. The alloy composition of the present invention has total impurity levels less than 0.15, each impurity element less than 0.05%.

Magnesium and silicon in the alloy facilitates strengthening of the alloy after age hardening due to precipitation strengthening by Magnesium silicide (Mg2Si) formation under the presence of both elements. In an embodiment, preferable lower limit of magnesium is 0.1 wt.% to get the desired strength to the extruded profile. In another embodiment, the upper limit of amount of magnesium in the alloy is 2.0 wt.%.

In an embodiment, preferable lower limit of Silicon is 0.1 wt.%. In another embodiment, the upper limit of amount of Silicon in the alloy is 2.0 wt.%. Presence of Silicon also enhance the extrudability of the alloy.

In an embodiment, the amount of Fe in the alloy is preferably maintained in the range of up to 0.5 wt.%. The increase in Fe content may adversely affect the extrudability and mechanical properties which may not be compensated by other elements. On the other hand, low amount of Fe may lead to unwanted grain growth in the extruded profile adversely affecting the surface properties in the final product.

Manganese and chromium is essential in controlling the grain size in the final product made from the alloy composition. Manganese content is maintained in the range of up to 0.4 wt.%. The higher amount of manganese may interfere with the functions of other alloy components such as strontium. In an embodiment, the amount of Cr in the alloy is preferably in the range of up to 0.4 wt.%.

Lowering of Cr and Mn content in the alloy and restricting it to less than 0.4% helps in reduction of flow stress and makes extrusion easier contributing in achieving higher speeds. Reduction in Fe, Mn and Cr content leads to reduction in volume fraction of dispersoids which gives additional benefit of lowered quench sensitivity. Thus, desirable properties of an extruded product can be achieved without the need of a very fast quench rate after extrusion.

In addition, reducing the amount of chromium in alloy composition leads to reduction in peripheral coarse graining (PCG) occurrence in casted/extruded alloys. Lowering of Cr levels leads to fully recrystallized microstructure in the extrudates and thus prevent peripheral coarse graining.

In an embodiment, titanium is added for refining grain size during extrusion billet casting. In an embodiment, the amount of Ti in the alloy is in the range of upto 0.15 wt.%.

In an embodiment, zinc is added for lowering the galvanic potential of the alloy and increase strength. In an embodiment, the amount of Zn in the alloy is in the range of upto 0.25 wt.%.

Fe and other impurity elements generally are either dissolved in the solid solution or manifests themselves as intermetallics combining with Al, Mg, Si, Cr and Mn. These intermetallics are formed during casting which have the typical (Fe+Mn)/Si weight ratio as 2 with average composition of Al9(Fe, Mn)2Si2, also termed as ß-AlFeSi intermetallics. These ß type intermetallics are one of the major reasons for an extrusion surface defect termed as pick-ups. Pick-ups occur when a string of Al metal with size ranging from few microns to few mm separates out from the extruded surface during interaction with die. This leads to rough surface on extrudate and manifests as score lines along the extrusion direction. If ß type intermetallics are replaced by fine a-AlFeSi (Al8Fe2Si) intermetallics the propensity of pick-ups is minimized. This is achieved in industrial extrusion process through homogenization process.

During homogenization, ß type intermetallics appearing as continuous network at grain boundaries are broken up and transformed into spherodized a particles. The other objective of homogenization is to dissolve coarse magnesium silicide (Mg2Si) particles, which also helps in achieving better surface finish and anodizing response. The presence of a or ß particles is estimated through use of SEM (Figure 1) where a sizeable number of intermetallics are characterized and the ratio of (Fe,Mn)/Si ratio is calculated for the characterized particles.

In an embodiment, strontium is added in the range of 0.005-0.1 wt.%, which leads to major fraction of intermetallics being a particles during the casting. Sr in extrusion alloy promotes conversion of ß-(AlxFeSi) phase to preferred fine a-(AlxFe2Si) phase. This helps in transformation of ß particles to a particles quickly and more effectively in homogenization cycles. a-(AlxFe2Si) phase is more desirable phase for better surface properties. Higher fraction of a particles in as cast structure and almost complete transformation of remaining ß particles to a particles leads to improved extrudability and thus attainment of higher speeds.

In addition, the total volume fraction of intermetallics is reduced by lowering the Iron and Manganese content to less than 0.2%wt. Even without the presence of Sr, transformation of ß particles to a particles greater than 85% can be achieved by increasing amount of Mn, Fe or Cr, however, this will result in the increase of intermetallic size in the range of 5-15 µm which is undesirable.

It is also possible to have alloy having intermetallic size less than 1µm by reducing the amount of Mn, Fe and Cr content in the alloy composition. However, the transformation of ß particles to a particles is less than 85%. This invention achieves intermetallics less than 1µm and transformed ß higher than 85% with modifications in composition ranges.

Another embodiment of the invention relates to a process of preparing an alloy composition as disclosed above. The process comprises of preparing a charge by mixing or blending aluminium with alloying elements as follows.
a) 0.1-2 wt% of magnesium (Mg);
b) 0.1-2 wt% of silicon (Si);
c) upto 0.8wt% of Copper (Cu);
d) 0.005-0.1wt% of Strontium (Sr);
e) upto 0.5 wt% of Iron (Fe);
f) upto 0.4 wt% of Chromium (Cr);
g) upto 0.4 wt% of Manganese (Mn);
h) upto 0.15 wt% of Titanium (Ti);
i) upto 0.25 wt% of Zinc (Zn); and
j) the rest being aluminum
wherein Mg to Si ratio is in a range of 1.1-1.6.

Aluminium is obtained from molten metal, process scrap and other sources. The composition of the charge is checked against the target composition. The charge is then fluxed and skimmed to remove the dross. Degassing and grain refiner additions are done prior to casting of billets.

In an embodiment, the invention also provides a method of forming an extruded profile for near elimination of ß type intermetallics and to be replaced with fine spherical a type intermetallics leading to lower pick-ups and higher speeds during extrusion.

Accordingly, the process of forming an extruded profile from an alloy composition comprising the steps of:
a) casting billet from an alloy composition comprising 0.1-2 wt% of magnesium (Mg), 1-2 wt% of silicon (Si), upto 0.8wt% of Copper (Cu), 0.005-0.1wt% of Strontium (Sr), upto 0.5 wt% of Iron (Fe), upto 0.4 wt% of Chromium (Cr), upto 0.4 wt% of Manganese (Mn), upto 0.15 wt% of Titanium (Ti), upto 0.25 wt% of Zinc (Zn) and the rest being aluminium, wherein Mg to Si ratio is in a range of 1.1-1.6.
b) heating the billet from step (a) at a temperature in the range of 500-600oC preferably in the range 550-590oC for 2-15 hrs;
c) cooling the billet from step (c) to room temperature;
d) heating the billet of step (c) at a temperature in a range of 400-500°C; and
e) extruding the heated billet of step (d) at an exit temperature of 510-590°C.

In an embodiment, heating of step (b) is carried out at a rate of 30-100°C/hr. The heating of step (b) allows for homogenization and uniform distribution of alloy elements by eliminating micro segregation and dissolving any undesirable eutectic. Homogenization of step (b) transform the as-cast ß particles into a particles. The homogenized billet should have a grain size of less than 150 µm, greater than 85% of transformed a intermetallics of size less than 1µm and total volume fraction of intermetallic less than 1%.

The aforesaid process utilizing the alloy composition having components in the specified amounts leads to an improved extrusion speed of greater than 30%, in the range of 40-100%. The surface roughness of the extruded product is less than 2µm even at high extrusion temperature.

High exit temperatures are desirable as it leads to better mechanical properties of the extruded products. Lower critical quench rate is observed. The die withdrawal rate is significantly reduced and die runs 3-4 times higher than usual can be obtained. Peripheral coarse graining in the extruded product is very low or absent in the extruded product. The characteristics of the extruded product is achieved due to the use of specific combination of the components of the alloy composition.

It has been found that the composition having components in specific amounts leads to superior advantages such as reduction or absence of the peripheral coarse graining in the extrudates.

It has also been found that the microstructural features are improved providing maximum benefits. Intermetallics of a AlFeMnSi type are 50% or higher than 50% in as cast state and greater than 85% in homogenized state. Intermetallics size is less than 1µm in the microstructure and total volume fraction is less than 1% in the homogenized state. The grain size is less than 160µm.

Reduced size and lower volume fraction of the intermetallics is integral to the beneficial effect achieved by the alloy composition in terms of excellent surface quality of the extrusion products, high extrusion speed, reduced or negligible pick up.

High volume fraction of intermetallics lead to build up of debris on the dies. This is the primary cause for pick up leading to poor surface quality of the extrusion products. To eliminate this problem, extrusion speeds and/or exit temperatures have to be reduced. This leads to drop in the properties and productivity of the extrusion. Lower volume fractions help to address the issue of die pick and allows higher speeds and higher extrusion temperatures to be achieved. Similarly, small particles lead to reduced abrasion of the die, reduced incidence of die lines and lower surface roughness. This ensures that extrusion can be carried out at higher speeds and die can be used for longer run before having to be changed.

During homogenization, ß type intermetallics appearing as continuous network at grain boundaries are broken up and transformed into spherodized a particles. The presence of a or ß particles is estimated through use of SEM where a sizeable number of intermetallics are characterized and the ratio of (Fe+Mn)/Si ratio is calculated for the characterized particles.

Higher fraction of a particles in as cast structure and almost complete transformation of remaining ß particles to a particles leads to improved extrudability and higher extrusion speeds.

The billets made with the composition as disclosed have exhibits better mechanical properties and faster kinetics of ß to a transformation. The composition having specific amounts of the components lead to 40-100% higher increase in extrusion speeds in comparison to regular alloys without the occurrence of pick-ups. The pick-ups are minimized and surface roughness of the extrudate is reduced to less than 2µm. Reduced pick-ups in turn lead to lower rate of die withdrawals leading to 2-4 times higher extrusion output from a single die.

The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.

WORKING EXAMPLES:
EXAMPLE 1: TRANSFORMATION OF INTERMETALLICS

ID Si Fe Cu Mg Mn Cr Ti Sr
Modified 0.66 0.17 0.25 0.95 0.07 0.09 0.025 350ppm
Regular 0.68 0.16 0.27 0.94 0.05 0.09 0.02 0

The modified alloy has 61% a particles as against 0-20% in alloy with regular alloy composition range and no Sr addition. Cast billets were homogenized in industrial conditions with soaking temperature in the range of 500-590oC for a duration of 3-15 hrs. 90% of ß intermetallics have transformed to a particles in the modified alloy against regular alloys where the transformation is around 60-70% for same duration. Figure 3 provides a comparison of (Fe+Mn) vs Si distribution in Sr free and Sr containing alloy (a) as cast condition; (b) homogenized condition.

EXAMPLE 2: SURFACE ROUGHNESS

ID %Si %Fe %Mn %Mg %Cu %Zn %Cr %Ti Sr
Modified 0.63 0.18 0.13 0.92 0.23 0.005 0.16 0.025 0.028
Regular 0.63 0.19 0.13 0.93 0.25 0.004 0.14 0.02

Cast billets were homogenized in industrial conditions with soaking temperature in the range of 500-5900C for a duration of 3-15 hrs. As cast billet of modified alloy had grain size of 140 µm and homogenized microstructure with 88% transformed a intermetallics of size less than 1µm and total volume fraction less than 1%. Regular billets whereas had intermetallics size less than 2µm and transformed a less than 80%.

Billets were extruded with preheating to 400-5000C with exit temperature of 510-590oC in industrial extrusion press. Speed improvements of >30% were observed with reduction in surface roughness of 50%. Figure 4 shows the comparison of extrusion speeds and surface roughness in Sr containing and Sr free billets.

EXAMPLE 3: EXTRUSION SPEED

ID %Si %Fe %Mn %Mg %Cu %Zn %Cr %Ti Sr
Regular 0.45 0.19 0.02 0.54 0.01 0.002 0.003 0.01 0
Modified 0.47 0.2 0.02 0.56 0.01 0.003 0.003 0.01 0.04

Cast billets were homogenized in industrial conditions with soaking temperature in the range of 500-5900C for a duration of 3-15 hrs. As cast billet of modified alloy had grain size of <150 µm and homogenized microstructure with >90% transformed a intermetallics of size less than 1µm and total volume fraction less than 1%.

Billets were extruded with reheating to 400-5000C with exit temperature of 510-590oC in industrial extrusion press. Speed improvements of 80% were observed in homogenized Sr added billets and speed improvements of around 50% were observed in as cast Sr added billets as against Sr free billets. Figure 5 shows a comparison of extrusion speeds in Sr containing and Sr free billets.

EXAMPLE 4: PERPHIRAL COARSE GRAIN

ID %Si %Fe %Mn %Mg %Cu %Zn %Cr %Ti
Modified 0.64 0.11 0.05 0.86 0.25 0.004 0.04 0.02
Regular 0.774 0.245 0.114 0.99 0.181 0.016 0.182 0.053

Cast billets were homogenized in industrial conditions with soaking temperature in the range of 500-5900C for a duration of 3-15hrs. As cast billet of modified alloy had grain size of <150 µm and homogenized microstructure with >90% transformed a intermetallics of size less than 5µm and total volume fraction less than 1%. In comparison regular billets with high Fe and Cr had intermetallic size >5µm although having similar homogenization practice and extrusion parameters as of modified alloy.

Billets were extruded with reheating to 400-5000C with exit temperature of 510-5900C in industrial extrusion press. Peripheral coarse graining was eliminated in modified alloy as against regular alloy. Figure 6 shows the macrostructure of extruded profiles (a) regular alloy (b) alloy of present application.

EXAMPLE 5: DIE RUN

ID Si Fe Mn Mg Cu Zn Cr Ti Sr
Regular 0.65 0.19 0.1 0.98 0.25 0.002 0.15 0.02
Modified 0.65 0.15 0.07 0.91 0.25 0.002 0.07 0.02 0.025

Cast billets were homogenized in industrial conditions with soaking temperature in the range of 500-5900C for a duration of 3-15hrs. As cast billet had grain size <150 µm and homogenized microstructure with >90% transformed a intermetallics of size less than 1µm and total volume fraction less than 1%. Regular sample had intermetallics less than 2µm and transformed a less than 80%.

The billets were extruded for a rectangular profile and a maximum of 160 billets were extruded whereas in a regular alloy and process a maximum of 39 billets could be extruded before die withdrawal along with improvement in speeds of about 90% than the regular one. Figure 7(a) provides ram speed of regular vs alloy of present application and figure 7(b) shows the maximum number of billets extruded before die withdrawal.

EXAMPLE 6: MICROSTRUCTURE ANALYSIS
Aluminium alloy compositions of present application having varying amounts of individual components were studied to analyse their effect on microstructure.

ID % Si % Fe % Mn % Mg % Cu % Zn % Cr % Ti % Sr %a Mean size(µm)
1 0.63 0.18 0.13 0.92 0.23 0.005 0.16 0.025 0.028 88 ˜1
2 0.63 0.19 0.13 0.93 0.25 0.004 0.14 0.02 - 79 ˜1
3 0.42 0.11 0.014 0.49 0.008 0.003 0.003 0.01 0.018 94 ˜0.5
4 0.64 0.11 0.05 0.86 0.25 0.004 0.04 0.02 - 76 ˜1
5 0.774 0.24 0.11 0.99 0.181 0.016 0.182 0.053 - 96 ˜15
6 0.65 0.16 0.39 0.86 0.29 0.007 0.019 0.022 - 99 ˜9

In the cases where Fe, Mn or Cr content is higher, transformed ? greater than 80% could be achieved but the mean size of intermetallics increase. This then results into surface defects and lowering of speeds. This effect mentioned above is manifested strongly in Fe, Mn and Si in the order mentioned here.

However, if the amount of these elements are reduced i.e. Fe<0.2, Mn<0.1, Cr<0.1 then the mean size of particles is decreased but the amount of transformed a reduces. The best combination is achieved when Fe, Mn and Cr are reduced and Sr content is added which gives fine intermetallic size(1µm) and transformed a greater than 85,90%. This helps in achieving the claims mentioned in the main body of the document.

The microstructural results could be summarised as below:

Without Sr With Sr
Content Cr,Mn Fe Cr, Mn Fe
Low Less a, less size Less a, less size High a, less size, No PCG High a, less size, NO PCG
High High a, high size High a, high size High a, less size, PCG High a, less size, PCG
Table 1

ADVANTAGES:
a) Greater than 50% a particles in as cast microstructure in AA6xxx alloys.
b) Transformation of intermetallic in AA6xxx alloys in homogenized condition leading to >85 % of a particles with size less than 1µm.
c) Attainment of 40-100% higher extrusion speeds without pick-ups.
d) Surface roughness less than 2µm in extrusion products.
e) Higher exit temperatures achievement leading to better end properties.
f) Lower critical quench rate of 70oC/min.
g) Die runs 3-4 times higher than usual.
h) Peripheral coarse grain reduction.

Those skilled in the art can understand now from aforementioned discussion, and information in the present disclosure can be implemented in a variety of forms. Should be understood that this instruction aforementioned be described in be only in nature illustrate, therefore, the variant not departing from this instruction purport is intended in the scope of this instruction. These variants should not be regarded as departure from the spirit and scope of this instruction.

,CLAIMS:Claims
We Claim:
1) An aluminium alloy composition comprises of:
a) 0.1-2 wt% of magnesium (Mg);
b) 0.1-2 wt% of silicon (Si);
c) upto 0.8wt% of Copper (Cu);
d) 0.005-0.1wt% of Strontium (Sr);
e) upto 0.5 wt% of Iron (Fe);
f) upto 0.4 wt% of Chromium (Cr);
g) upto 0.4 wt% of Manganese (Mn);
h) upto 0.15 wt% of Titanium (Ti);
i) upto 0.25 wt% of Zinc (Zn); and
j) the rest being aluminum
wherein Mg to Si ratio is in a range of 1.1-1.6.

2) The composition as claimed in claim 1, comprising more than 50% of a intermetallic particles.

3) The composition as claimed in claim 2, wherein said intermetallic particles are less than 2µ in size.
3) An extruded aluminium alloy comprising 0.1-2 wt% of magnesium (Mg);
a) 0.1-2 wt% of silicon (Si);
b) upto 0.8wt% of Copper (Cu);
c) 0.005-0.1wt% of Strontium (Sr);
d) upto 0.5 wt% of Iron (Fe);
e) upto 0.4 wt% of Chromium (Cr);
f) upto 0.4 wt% of Manganese (Mn);
g) upto 0.15 wt% of Titanium (Ti);
h) upto 0.25 wt% of Zinc (Zn); and
i) the rest being aluminum
wherein Mg to Si ratio is in a range of 1.1-1.6.

4) The extruded alloy as claimed in claim 3, having surface roughness less than 2µm Ra.

5) The extruded alloy as claimed in claim 3, having critical quench rate of 70oC/min.

6) The extruded alloy as claimed in claim 3, having greater than 85% of homogenized transformed a intermetallics.

7) The extruded alloy as claimed in claim 3, having grain size of less than 160µm.
8) A process of forming an extruded profile from an alloy composition comprising the steps of:
a) casting billet from an alloy composition comprising 0.1-2 wt% of magnesium (Mg), 1-2 wt% of silicon (Si), upto 0.8wt% of Copper (Cu), 0.005-0.1wt% of Strontium (Sr), upto 0.5 wt% of Iron (Fe), upto 0.4 wt% of Chromium (Cr), upto 0.4 wt% of Manganese (Mn), upto 0.15 wt% of Titanium (Ti), upto 0.25 wt% of Zinc (Zn) and the rest being aluminium, wherein Mg to Si ratio is in a range of 1.1-1.6.
b) heating the billet from step (a) at a temperature in the range of 500-600oC preferably in the range 550-590oC for 2-15 hrs;
c) cooling the billet from step (c) to room temperature;
d) heating the billet of step (c) at a temperature in a range of 400-500°C; and
e) extruding the heated billet of step (d) at an exit temperature of 510-590°C.

9) The process as claimed in claim 8, wherein heating of step (b) is carried out at a rate of 30-100oC/hr.

Dated 11 November 2021
M. Kisoth
IN/PA-2259
Agent for the Applicant