Title of Invention

"A PROCESS FOR THE PREPARATION OF ALUMINIUM-ZINC-MAGNESIUM-MANGANESE-ZIRCONIUM-SCANDIUM ALLOY "

Abstract

A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium alloy comprising the steps of; preparing a charge mixture of primary aluminium and Al-8.5wt % Mn master alloy; melting at 720 to 730°C the said charge mixture, adding to this molten charge elemental pure Zn followed by raising the temperature of the molten charge as herein described; adding of Al-50wt% Mg master alloy, Mg-28wt% Zr master alloy and Al-2wt% Sc master alloy in the above sequential order to the molten charge followed by super heating for a period of 10 minutes; adding nuclent pellets as herein described at reduced temperature of 735-745°C for grain refinement; degassing the molten melt to remove dissolved gases and pouring the molten melt under indert gas atmosphere, preferably argon atmosphere, into a pre-heated mould; homogenizing, scalping and rolling the sound billets into plates; solution treating, quenching, and stretching the alloy sheets followed by two-step artificial aging.

Full Text

FIELD OF INVENTION This invention relates to a process for preparation of Aluminium-Zinc-Magnesium-Manganese-Zirconium-Scandium alloy. PRIOR ART Aluminium alloys have been used for a variety of armour and structural applications for a considerable number of light armoured vehicles. For these applications, these alloys should have a combination of high strength, SCC resistance and weldability. The first of such alloys is the non-heat treatable Al-Mg-Mn-Cr base AA 5083. However, the ballistic shortcomings of such alloys, due primarily to its low strength (typical 0.2% Y.S. in the commercial H 115 temper being 290 MPa), led to the development of 7xxx series of heat treatable Al-Zn-Mg alloy. These alloys, however, suffer from poor stress corrosion cracking resistance. Al-Mg alloys containing up to 3wt% Mg are well known to be weldable and having excellent corrosion resistance. The Al-Mg alloys containing more than 3wt% Mg experience reduced SCC resistance due to the formation of continuous chain of ß (Al8Mg5) precipitates at the grain boundary. The presence of Ce, a La series element, in such alloys breaks up the ß-plase chain structure, thus improving the SCC resistance of the alloys [Z.Cui and R.Wu, Acta. Metall, Sin (china) 20, 6, pp B323-331, 1984]. Also known in the prior art is the improvement of the SCC resistance of Al-Mg alloys (containing 3-5wt%Mg) by additions of elements such as Zr, Mn, Ti, Sc, Tb and at least one element from the La series in the periodic table [US Patent 6258318, 2001]. Yet another art known in the literature is the improvement of the SCC resistance of Al-Mg alloys (containing 3-5wt% Mg) by addition of elements such as Ni, Sc and Cr [US Patent 6352671, 2002]. In contrast to Al-Mg alloys, Al-Zn-Mg alloys are inherently prone to SCC and report on the influence of the said elements i.e. Ti, Tb, Sc, Ce on the SCC resistance of the Cu-free Al-Zn-Mg alloys are not available. The copper addition to Al-Zn-Mg alloys, on the other hand, considerably improves the SCC resistance. This is because, the resultant alloys could be then subjected to commercial over aging treatments designed for improvement in the SCC resistance. However, the amount in which Cu is present in these alloys gives rise to hot cracking during the solidification of welds. As a result, among the 7XXX series alloys, the Cu free, weldable Al-Zn-Mg alloys remains as the choice for the armour applications. This is despite the fact that these alloys, as described above, has much inferior SCC resistance compared to the Al-Mg base AA 5083 alloy. OBJECTS OF PRESENT INVENTION The primary object of the present invention is to provide a process for preparation of Aluminium-Zinc-Magnesium-Manganese-Zirconiurn-Scandium alloy and an alloy thereof having significantly high SCC resistance. Another object of the present invention is to provide a process for preparation of Aluminium-Zinc-Magnesium-Manganese-Zirconium-Scandium alloy and an alloy thereof having high SCC resistance as well as weldability properties equivalent to that of the alloys without Scandium. Yet another object of the present invention is to provide a process for preparation of Aluminium-Zinc-Magnesium-Manganese-Zirconium-Scandium alloy having improved stress corrosion cracking resistance and an alloy thereof so as to enable the alloys to attain the peak aged strength within 1/4th the aging time required by the alloy without SC to attain the peak aged strength at the commercial aging temperature. STATEMENT OF THE INVENTION According to the present invention, there is provided a process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium alloy comprising the steps of: 1. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium alloy comprising the steps of: (a) preparing a charge mixture of primary aluminium and Al-8.5wt % Mn master alloy; (b) melting at 720 to 730°C the said charge mixture, adding to this molten charge elemental pure Zn followed by raising the temperature of the molten charge as herein described; (c) adding of Al-50wt% Mg master alloy, Mg-28wt% Zr master alloy and Al-2wt% Sc master alloy in the above sequential order to the molten charge followed by super heating for a period of 10 minutes; (d) adding nuclent pellets as herein described at reduced temperature of 735-745°C for grain refinement; (e) degassing the molten melt to remove dissolved gases and pouring the molten melt under indert gas atmosphere, preferably argon atmosphere, into a pre-heated mould; (f) homogenizing, scalping and rolling the sound billets into plates; (g) solution treating, quenching, and stretching the alloy sheets followed by two-step artificial aging. Further according to this invention there is provided an Al-Zn-Mg-Mn-Zr-Sc alloy comprising of Al-(4.8-5.3) Zn-(1.8-2.3) Mg-(0.2-0.4) Mn-(0.11-0.19) Zr-(0.10-0.30) Sc in wt%. The inventive features steps of the present invention have been depicted in the independent claims and the additional features have been mentioned in the dependent claims DESCRIPTION OF FIGURES Further objects and advantages of this invention will be apparent from the ensuing description when read in conjunction with the accompanying tables and figures wherein: Table 1: Stress corrosion crack growth, Aa (mm), status at different initial applied stress intensity factors, K (Mpam1/2) for peak aged (T651) Al-Zn-Mg-Mn-Zr-Sc alloys of present invention. These th those of an Al-Zn-Mg-Mn-Zr alloy known in the prior art i.e. 7017 Al alloy. (CG-crack growth). Table 2: Peak Aged (PA) Tensile Properties (in long transverse direction) and Hardness Data for 15 mm thick plates of the alloy of present invention and those of 7017 Al alloy. Figure 1: Transmission electron micrographs showing development of microstructure in peak aged (a) 85 (b) 7017 Al alloy and (c) & (d) Al-Zn-Mg-Mn-Zr-Sc alloy of present invention. DESCRIPTION OF THE PROCESS According to the present invention, there is provided a process for preparation of Al-Zn-Mg-Mn-Zr-Sc alloy that has significantly higher SCC resistance compared to that of the Cu-free Al-Zn-Mg base alloys known in the prior art. In the present invention Al-Zn-Mg-Mn-Zr-Sc alloys having composition (in wt%) as Al-(4.8-5.3) Zn-(1.8-2.3) Mg-(0.2-0.4) Mn-(0.11-0.19) Zr-(0.10-0.30) Sc are disclosed that retains the superior weldability of the Al-Zn-Mg series of alloys, attains the peak aged strength within I/4th the time required by the base alloy without Sc and possesses significantly high SCC resistance. According to the present invention, the process for the preparation of Al-Zn-Mg-Mn-Zr-Sc alloy having significantly high SCC resistance comprises of following steps: (a) Preparing a charge mixture of 74% by weight of primary aluminium (with 99.85% purity, the balance being 0.09 wt% Fe and 0.06 wt% Si), and 3.80% by weight of the master alloy Al-8.5 wt% Mn. (b) Melting the above charge mixture in an induction furnace by heating at around 720-730°C, and adding to this molten charge 5.4% by weight of elemental pure Zn in the ingot form. (c) Raising the temperature of the molten charge to 740°C, adding to this molten charge 3.7% by weight of master alloy Al-50wt% Mg, 0.60% by weight of master alloy Mg-28wt% Zr, 12.5% by weight of master alloy Al-2wt% Sc, in the sequential order.and superheating the molten alloy to 755- 765°C for about 10 minutes. (d) Reducing the temperature to about 735-745°C and adding O.lkg of suitable nucleant pellets for grain refinement. (e) Degassing the molten melt by adding 0.25kg of suitable degasser pellets. This is to remove the dissolved gasses like hydrogen. The melt, at reduced temperature of 710-720°C, is then poured under argon atmosphere in to a metallic mould, preheated to the temperature of 145-155°C, of appropriate size. (f) Homogenizing the alloy in a temperature range of 462 ± 5°C for 30 to 40 hours followed by fan cooling in air. The homogenization treatment eliminates dendritic segregation in the cast microstructure. (g) Scalping the surface of billet to remove the oxide layers from the surfaces and then subjecting to the nondestructive testing to detect casting defects. (h) Subjecting the sound billets to rolling at initial billet temperature of 425-435°C and at a linear speed of 20 m per minute to finally produce plates having thickness of around 15 mm. The mechanical processing of the billets may be carried out by other deformation routes such as extrusion. (i) The plates obtained by step (h) above are subjected to solution treatment at the temperature range of 458-468°C for 2 hours followed by water quenching at room temperature. (j) Subjecting the plates as obtained by (i) to stretching to obtain 1.5% permanent set for stress relieving purpose. (k) Subjecting the stretched material as obtained by (j) to a two-step artificial aging involving against at 95-105°C for 8 hours followed by aging at 120-125°C for 12 hours. This treatment produces peak strength in the alloy. The invention will now be illustrated with a working example, which is intended to illustrate the working of invention and not intended to take restrictively to imply any limitation on the scope of present invention ORKING EXAMPLE-1 For a 50 kg melt of the alloy of present invention, a mixture of 37 Kg of primary aluminium (purity 99.85% Al and the balance being 0.09% Fe and 0.06% Si impurities) and 1.90 Kg of Al-8.5 wt % Mn master alloy is charged into the induction furnace. The above charge mixture is melted at around 720-730°C. To this molten charge, 2.7 Kg of pure Zn in the ingot form is added. When the charge has melted, the temperature of the molten charge is raised to 740°C, and 1.85 Kg of Al-50% Mg master alloy and 0.30 Kg of Mg-28% Zr master alloy and 6.25kg of Al-2wt% Sc are added in the above sequence. The charge is superheated to 760°C and the whole material is held at this temperature for 10 minutes. The temperature is then reduced to 740°C, and 0.100 Kg of nucleant pellets is added for grain refinement purpose. After 5 minutes, 0.25 kg of degasser pallets is added for degassing purpose. The molten alloy, in the temperature range of 710-720°C, is then poured under argon atmosphere into a preheated (to the temperature of 150°C) metallic mould of suitable size. When the melt was solidified, the ingot was cleared of the portions having casting defects. A rectangular as-cast billet of 340 mm (length) X 300 mm (width) X 100 mm (thickness) was thus obtained. The billet was subjected to the homogenizing annealing at 465°C for 35 hours followed by cooling in air. The billet was scalped and was subjected to the rolling. Rolling was carried out at an initial billet temperature of 430°C and at a linear speed of 20 m per minute. The billet was rolled to 15 mm thick plates. The plates were then subjected to solution treatment at 465°C for 2 hours followed by water quenching at room temperature. The quenched plates were stretched to obtain 1.5% permanent set for stress relieving purposes. The plates were then subjected to artificial aging at 100°C for 8 hours followed by artificial aging at 120°C for 12 hours. This heat treatment produced peak strength in the alloy. These peak-aged materials were then utilized for the quantitative stress corrosion cracking (SCC) test involving alternate immersion tests using 3.5% NaCl solution in accordance with ASTM standard E-1681. EVALUATION OF THE ALLOY The stress corrosion cracking resistance of the alloy of present invention was examined using quantitative test involving alternate immersion tests using 3.5% NaCl solution in accordance with ASTM standard E-1681. Table 1 presents the stress corrosion crack growth at different initial applied stress intensity factors K (MPam1/2), for peak aged (T651) alloy of present invention, as well as that of an Al-Zn-Mg base aluminium armour alloy 7017 known in the prior art [having the composition (in wt%) range of Al-(4-5.2) Zn- (2-3) Mg- (0.10-0.25) Zr-(0.05-0.50) Mn]. It may be noted that the 7017 Al alloy in the prior art shows stress corrosion crack growth at K values > 15 Mpam1/2. Whilst, no such crack growth is observed for the alloy of present invention even after 7000 h of exposure at 15 MPam1/2 and as well as at increased K value of 20 Stress corrosion crack growth, Aa (mm), status at different initial applied stress intensity factors, K (MPam Va) for peak aged (T651) Al-Zn-Mg-Mn-Zr-Sc alloy of present invention. These results are compared with those of an Al-Zn-Mg-Mn-Zr base alloy known in the prior art i.e. 7017A1 alloy (CG- crack growth). (Table Removed) Table 2 presents the hardness data and the tensile properties of the alloy of present invention and those of the 7017 Al alloy. It is noteworthy that the aging time required to attain the peak aged strength is remarkably reduced in the case of the alloy of present invention. Peak Aged (PA) Tensile Properties (in long transverse direction) and Hardness Data for 15 mm thick plates of 7017 Al alloy and those of the alloy of present invention (Table Removed) Figure 1 provides transmission electron micrographs showing development of microstructure in peak aged (a) & (b) 7017 Al alloy and (c) & (d) Al-Zn-Mg-Mn-Zr-Sc alloy of present invention. The grains diffracting strongly in (a), (b) and (c) are in close to Ai orientation. The two-beam orientation in (d) was obtained near Aizone. Comparison of Figure 1 (a) with Figure 1 (c) shows reduced width of PFZs (precipitate free zones) adjacent to the grain boundary and less coarser grain boundary particles in the case of the alloy of present invention (aged for 8 h at 100°C followed by aging for 12 h at 120°C to obtain the peak aged strength) compared to the 7017 Al alloy (aged for 8 h at 100°C followed by aging for 48 h at 120°C to obtain the peak aged strength) comparison of Figure l(b) with Figure l(c) shows the presence of finer strengthening precipitates of n' in the matrix in the alloy of present invention. Figure l(d) shows the presence of a uniform and fine distribution of AbScxZn-x precipitates within the subgrains of the alloy of present invention. Our studies showed that the width of the PFZs (precipitate free zones) adjacent to the grain boundary is much narrower in the case of the alloy of present invention [Compare Figure l(a) with Figure l(b)]. The narrow PFZs adjacent to the grain boundary would reduce areas that could be anodic to the adjacent matrix or could be softer where stain could be concentrated. Also, the interior of the grain / subgrains is dominated by the presence of a uniform and fine distribution of AbScxZn-x precipitates that are unsherable by dislocations during its wake, thus homogenizing the deformation. These microstructural features brought about by suitable alloy design are responsible for the superior SCC resistance of the alloy of present invention. The above results interpretations demonstrate that the alloy of present invention has considerably improved SCC resistance. It is to be understood that the process of the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope present invention, which is further set forth under the following claims:- WE CLAIM: 1. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium alloy comprising the steps of: (a) preparing a charge mixture of primary aluminium and Al-8.5wt % Mn master alloy; (b) melting at 720 to 730°C the said charge mixture, adding to this molten charge elemental pure Zn followed by raising the temperature of the molten charge as herein described; (c) adding of Al-50wt% Mg master alloy, Mg-28wt% Zr master alloy and Al-2wt% Sc master alloy in the above sequential order to the molten charge followed by super heating for a period of 10 minutes; (d) adding nuclent pellets as herein described at reduced temperature of 735-745°C for grain refinement; (e) degassing the molten melt to remove dissolved gases and pouring the molten melt under indert gas atmosphere, preferably argon atmosphere, into a pre-heated mould; (f) homogenizing, scalping and rolling the sound billets into plates; (g) solution treating, quenching, and stretching the alloy sheets followed by two-step artificial aging. 2. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy as claimed in claim 1 wherein the said high purity aluminium has a purity of 99.85% with balance being 0.09% Fe, and 0.06% Fe, and 0.06% Si; the said charge mixture comprises 74% by weight of said high purity primary aluminium and 3.80% by weight of Al-8.5wt% Mn master alloy. 3. A process for the preparation of Aluminium-zinc-Magnesium-Mangenese-Zirconium-Scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy as claimed in any of the preceding claims wherein the said addition of elemental pure zinc in ingot form to the said charge mixture is 5.4% by weight. 4. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy as claimed in any of the preceding claims wherein the said addition of Al-50% Mg master alloy to the said charge mixture is about 3.7% by weight, the said addition of Mg-28% Zr master alloy and Al-2wt% Sc master alloy to the said charge mixture is 0.60% by weight and 12.50% by weight, respectively and the said super heating of the said charge mixture together with the said additions is carried at temperature around 755-765°C. 5. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy as claimed in any of the preceding claims wherein the said nucleant pellets for grain refinement are taken in quantity of 0.1 kg by weight. 6. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy as claimed in any of the preceding claims wherein the said degasser pellets for degassing are taken in quantity of 0.25 kg by weight, and added to the said charge mixture at the temperature of 730-740°C and, the said molten alloy is poured, in the temperature range of 710-720°C, under argon atmosphere into a metallic mould preheated to the temperature of 145-155°C. 7. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy as claimed in any of the preceding claims wherein the said homogenization is carried at a constant temperature range of 462 ± 5°C for 30 to 40 hours and the said heating to homogenization temperature is carried at a constant rate of 25 to 35°C per hour. 8. A process for the preparation of Aluminium-Zinc.Magnesium-Mangenese-Zirconium-Scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy as claimed in any of the preceeding claim wherein the said rolling is carried out at temperature of 425 to 435°C and at a linear speed of 20 m/minute, the said solution treatment is carried out at 458 to 468°C for 2 hours followed by quenching in water at ambient temperature and the said two-step artificial aging is carried at 95-105°C for 8 hours followed by aging at 120- 125°C for about 12 hours. 9. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-Scandium alloy as claimed in claim 1, wherein the said alloy comprising of Al-(4.8-5.3) Zn-(1.8-2.3) Mg-(0.2-0.4) Mn-(0.11-0.19) Zr-(0.10-0.30) Sc in wt%. 10. A process for the preparation of Aluminium-Zinc-Magnesium-Mangenese-Zirconium-scandium (Al-Zn-Mg-Mn-Zr-Sc) alloy having improved stress corrosion cracking resistance and an alloy thereof substantially as herein described with reference to example.

Documents:

1368-DEL-2004-Abstract-(27-02-2009).pdf

1368-del-2004-abstract.pdf

1368-DEL-2004-Claims-(27-02-2009).pdf

1368-del-2004-claims.pdf

1368-DEL-2004-Correspondence-Others (25-01-2010).pdf

1368-DEL-2004-Correspondence-Others-(27-02-2009).pdf

1368-del-2004-correspondence-others.pdf

1368-del-2004-correspondence-po.pdf

1368-DEL-2004-Description (Complete)-(27-02-2009).pdf

1368-del-2004-description (complete).pdf

1368-del-2004-drawings.pdf

1368-DEL-2004-Form-1 (25-01-2010).pdf

1368-del-2004-form-1.pdf

1368-del-2004-form-18.pdf

1368-DEL-2004-Form-2-(27-02-2009).pdf

1368-del-2004-form-2.pdf

1368-DEL-2004-Form-3-(27-02-2009).pdf

1368-del-2004-form-5.pdf

1368-DEL-2004-GPA (25-01-2010).pdf

1368-DEL-2004-Petition-138 (25-01-2010).pdf