Title of Invention

A PROCESS FOR PREPARATION OF ALUMINIUM-ZINC-MAGNESIUM-COPPER-ZIRCONIUM-SILVER ALLOY

Abstract

"A process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium- Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy". This invention relates to a process for preparation of Aluminium-Zinc- Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy comprising steps of melting a charge mixture of high purity primary aluminium, Al-30.7% Cu master alloy and Al-9.8% Ag master alloy; adding to this molten charge elemental pure Zn in the form of ingot, followed by raising the temperature of the molten charge, addition of Al-51% Mg master alloy and Mg-28% Zr master alloy in the above sequential order to the molten charge followed by superheating at higher temperature of 755 to 765°C; adding Al-5% Ti master alloy at reduced temperature of 735-745°C; degassing the molten melt at further reduced temperature to remove dissolved gases and pouring the molten melt under inert gas atmosphere, preferably argon atmosphere, into a preheated mould; homogenizing, scalping and rolling the sound billets into thin sheets; solution treatment, 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-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy having significantly higher weldability compared to the alloy without silver. PRIOR ART Aluminium-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) represents the class of highest strength aluminium alloy that can be produced via ingot metallurgical route. These alloys find applications in aerospace, and in defence where a combination of high strength, fracture toughness and stress corrosion cracking resistance is required. However, these high strength alloys have poor weldability and, therefore, these alloys cannot be used for applications where joining of components by welding is an essential step. One of the high strength and high weldability alloys known in the art is Al-Zn-Mg-Cu-Zr-Sc. A drawback of the above alloy is that scandium (Sc) metal is expensive. Another limitation of the above mentioned alloy is that scandium ores are not available in many countries including India. OBJECTS OF PRESENT INVENTION The primary object of the present invention is to propose a process of preparation of an Al-Zn-Mg-Cu-Zr-Ag alloy having significantly higher weldability Another object of the present invention is to propose a process of preparation of an Al-Zn-Mg-Cu-Zr-Ag alloy that has a combination of high strength and ductility besides high weldability. DESCRIPTION OF THE INVENTION In accordance with the present invention, Al-Zn-Mg-Cu-Zr-Ag alloy is prepared by the process comprising the steps of: B. preparing a charge mixture of 77% by weight of primary aluminium (with 99.85% purity, the balance being 0.09 wt% Fe and 0.06 wt% Si), 6% by weight of the master alloy Al-30, 7% Cu and 5% by weight of the master alloy A1-9.8 wt% Ag 1 ; b. melting the above charge mixture in an induction furnace by o heating at around 720-730 C, and adding to this molten charge 6.6% by weight of elemental pure Zn in the ingot form; c. raising the temperature of the molten charge to 735 to 745 C, adding to this molten charge 4.4% by weight of master alloy A1-- 51% Mg, 0.6% by weight of master alloy, Mg-28% Zr in the sequen— o tial order and superheating the molten alloy to 755 to 756 C for about 10 minutes; DESCRIPTION OF FIGURES In the accompanying figures: Figure 1: Total crack length (mm) vs. augmented strain (%) profiles for the baseline alloy and baseline alloy containing Ag. Figure 2: Photo macrographs revealing the extent of solidification cracks developed in (a) 7010 baseline alloy and (b) 7010 + Ag alloy due to augmented strain of 3.5%. Figure 3: Light micrographs showing development of (a) coarse grain structure in the baseline alloy weld, and (b) finer and equiaxed grain structure in the baseline + Ag alloy weld. Figure 4: Photo macrographs of Houldcroft test samples demonstrating that the use of (a) 7010 + Ag alloy as the filler reduces the length of the solidification crack by 1/9 compared to the case when (b) the 7010 baseline alloy is used as the filler to weld the baseline 7010 alloy. According to this invention there is provided a process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy comprising steps of: a) melting a charge mixture of high purity primary aluminium, Al-30.7% Cu master alloy and Al-9.8% Ag master alloy; b) adding to this molten charge elemental pure Zn in the form of ingot, followed by raising the temperature of the molten charge, c) addition of Al-51% Mg master alloy and Mg-28% Zr master alloy in the above sequential order to the molten charge followed by superheating at higher temperature of 755 to 765°C; d) adding Al-5% Ti master alloy at reduced temperature of 735- 745°C; e) degassing the molten melt at further reduced temperature to remove dissolved gases and pouring the molten melt under inert gas atmosphere, preferably argon atmosphere, into a pre-heated mould; f) homogenizing, scalping and rolling the sound billets into thin sheets; g) solution treatment, quenching and stretching the alloy sheets followed by two-step artificial aging. (d) Reducing the temperature to about 735 to 745°C and adding 0.15 to 0.25% by wt of Al-5% Ti master alloy for grain refinement. (e) Degassing the molten melt by adding 0.2 to 0.3% by wt of suitable degasser pellets. This is to remove the dissolved gases 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 465 ± 5°C for 30 to 40 hours followed by cooling in air. The homogenization treatment eliminates dendritic segregation in the cast microstructure. Heating to homogenisation temperature is carried out at the constant rate of 25 to 35C per hour. (g) Scalping the surface of billet to remove the oxide layers from the surfaces and then subjecting to the non-destructive testing to detect casting defects. (h) Subjecting the sound billets to rolling at initial billet temperature of 435°C and at a linear speed of 20 m per minute to finally produce sheets having thickness of around 5 mm. The mechanical processing of the billets may be carried out by other deformation routes such as extrusion. (i) The sheets obtained by step (h) above are subjected to solution treatment at the temperature range of 465-475°C for 2 hours followed by water quenching at room temperature. (j) Subjecting the sheets as obtained by step (i) to stretching to obtain 1.5% permanent set for stress relieving purpose. (k) Subjecting the stretched material as obtained by step (j) above to a two-step artificial aging at 95-100°C for about 8 hours in the first stage followed by a second stage aging in the temperature range of 120-125°C for about 8 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 is not intended to be taken restrictively to imply any limitation on the scope of present invention. EXAMPLE For a 55 Kg melt of the alloy of present invention, a mixture of 42.445 Kg of primary aluminium (purity 99.85% Al and the balance being 0.09% Fe and 0.06% Si impurities), 3.4 Kg of Al-30.7% Cu master alloy and 2.8 Kg of Al-9.8% Ag master alloy is charged into the induction furnace. The above charge mixture is melted at around 720- 730°C. To this molten charge, 3.63 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 2.41 Kg of Al-51% Mg master alloy and 0.315 Kg of Mg-28% Zr master alloy 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 Al-5% Ti master alloy is added for grain refinement purpose. After 5 minutes, 0.25 kg of degasser pallets was 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 80 mm (thickness) was thus obtained. The billet was subjected to the homogenizing annealing at 465°C for 30 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 435°C and at a linear speed of 20 m per minute. The billet was rolled to 5 mm thick sheets. The sheets were then subjected to solution treatment at 470°C for 2 hours followed by water quenching at room temperature. The quenched sheets were stretched to obtain 1.5% permanent set for stress relieving purposes. The sheets were then subjected to artificial aging at 98°C for 8 hours followed by artificial aging at 122°C for 8 hours. This heat treatment produced peak strength in the alloy. These peak aged materials were then utilized for the quantitative weldability test i.e. Varestraint test, and qualitative Houldcroft test. EVALUATION OF THE ALLOY In order to use the baseline alloy as well as the Ag bearing 7010 alloy as the filler for the qualitative Houldcraft test, 2 mm thick sheets of the above alloys were produced by rolling. Strips of 2 mm width were then cut out of these sheets to use them as the filler during welding of Ag free, peak aged 7010 alloy sheets of 5 mm thickness. The influence of Ag on the weldability of baseline 7010 alloy was examined using a combination of quantitative Varestraint test and qualitative Houldcroft test. In both cases, conventional Gas Tungsten Arc (GTA) welding process was used to weld the alloy sheets. Figure 1 shows the Varestraint test results showing total crack length (mm) as functions of augmented strain (%) for the baseline 7010 alloy as compared to the Ag bearing 7010 alloy of present invention. The most noticeable feature in Figure 1 is the remarkable reduction in the total crack length for the silver-bearing alloy of present invention as compared to the baseline alloy. In fact due to augmented strain values of≥ 3.5 %, the 7010 baseline alloy samples broke into two pieces (as marked by broken line in Figure 1). However, the Ag bearing alloy of present invention remained in one single piece even after subjecting to the augmented strain higher than 3.5%. Figures 2(a) & (b) depict the nature of crack and the status of the test samples after augmented strain value of 3.5% for the baseline alloy and the Ag bearing alloy samples, respectively. Figures 3(a) & (b) represent optical micrographs showing development of coarse grain structure in the baseline alloy weld, and relatively finer and equiaxed grain structure in the Ag bearing weld of present invention, respectively. The beneficial effect of addition of silver on the weldability of 7010 alloys is associated with the refinement of the weld grain structure. Figures 4(a) & 4(b) represent the photo macrographs of the welds corresponding to the use of (a) 7010 + Ag alloy filler and (b) 7010 baseline alloy filler, respectively. It is apparent from Figure 4 that the presence of Ag in the filler remarkably reduced the progress of the crack along the weld centerline. The distance over which the crack propagated in the case of the Ag bearing alloy of present invention is about one ninth of the length of the crack that progressed in the baseline alloy. The tensile properties of the weldments in the as-weld condition for different filler alloys were evaluated. For the 7010 filler alloy, the tensile properties are: 0.2 % Yield strength (YS) = 265 MPa, Ultimate tensile strength (UTS) = 275 MPa, and % elongation (25 mm gauge length) = 1. For the 7010 + Ag alloy of present invention as the filler, the tensile properties are: 0.2 % Y.S.= 280 MPa, UTS = 325 MPa and % elongation (25 mm gauge length) = 2.5. The above results conclusively demonstrate that Ag additions greatly reduce the hot cracking susceptibility of the high strength Al-Zn-Mg-Cu-Zr alloy, and that Ag additions as well improve the tensile properties of the weldments. It is to be understood that the process of the present invention is susceptible to modifications, changes and adaptations by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention which is further set forth under the following claims. Claim; 1. A process for preparation of Aluminium-Zinc-Magnesium-Copper- Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy comprising steps of: a) melting a charge mixture of high purity primary aluminium, Al-30.7% Cu master alloy and Al-9.8% Ag master alloy; b) adding to this molten charge elemental pure Zn in the form of ingot, followed by raising the temperature of the molten charge, c) addition of Al-51% Mg master alloy and Mg-28% Zr master alloy in the above sequential order to the molten charge followed by superheating at higher temperature of 755 to 765°C; d) adding Al-5% Ti master alloy at reduced temperature of 735- 7450C; e) degassing the molten melt at further reduced temperature to remove dissolved gases and pouring the molten melt under inert gas atmosphere, preferably argon atmosphere, into a pre-heated mould; f) homogenizing, scalping and rolling the sound billets into thin sheets; g) solution treatment, quenching and stretching the alloy sheets followed by two-step artificial aging. 2. A process as claimed in claim 1 wherein said high purity aluminium has a purity of 99.85% with balance being 0.09% Fe and 0.06% Si. 3. A process as claimed in claim 1 wherein said charge mixture comprises 77% by weight of said high purity primary aluminium, 6% by weight of Al-30.7% Cu master alloy and 5% by weight of Al- 9.8% Ag master alloy. 4. A process as claimed in claim 1 wherein said melting of charge mixture is carried out at temperature around 720 to 730°C. 5. A process as claimed in claim 1 wherein elemental pure zinc in ingot form added to the said charge mixture is 6.6% by weight. 6. A process as claimed in claim 1 wherein the temperature of the said charge mixture is raised to 735 to 745°C. 7. A process as claimed in claim 1 wherein Al-51% Mg master alloy added to the said charge mixture is 4.4% by weight. 8. A process as claimed in claim 1 wherein Mg-28% Zr master alloy added to the said charge mixture is 0.6% by weight. 9. A process as claimed in claim 1 wherein said Al-5% Ti alloy for grain refinement is taken in quantity of around 0.15 to 0.25% by weight. 10. A process as claimed in claim 1 wherein said degasser pellets for degassing are taken in quantity of around 0.25 to 0.3% by weight. -11- 11. A process as claimed in claim 1 wherein said degasser pellets for degassing are added to the said charge mixture at the said further reduced temperature of 710-720°C. 12.A process as claimed in claim 1 wherein said molten alloy is poured under inert gas atmosphere preferably Argon, into a metallic mould preheated to the temperature of 145-155°C. 13.A process as claimed in claim 1 wherein said homogenization is carried in a temperature range of 465 +5°C for 30 to 40 hours. 14.A process as claimed in claim 1 wherein the heating to homogenization temperature is carried at a constant rate of 25 to 35°C per hour. 15. A process as claimed in claim 1 wherein said rolling is carried out at temperature of 425-435°C. 16. A process as claimed in claim 1 wherein said rolling is carried out at a linear speed of 20m/minute. 17. A process as claimed in claim 1 wherein said solution treatment is carried out at 465-475°C followed by quenching in water at ambient temperature. -12- 18. A process as claimed in claim 1 wherein said two-step artificial aging is carried at 95-100°C followed by aging at 120-125°C. 19. Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg- Cu-Zr-Ag) alloy as prepared by the process as claimed in claim 1 . Dated this 25th day of February, 2002. (MONA SAINI) OF L.S.DAVAR & CO., APPLICANTS ATTORNEY.

Documents:

142-del-2002-abstract.pdf

142-del-2002-claims.pdf

142-del-2002-correspondence-others.pdf

142-del-2002-correspondence-po.pdf

142-del-2002-description (complete).pdf

142-del-2002-drawings.pdf

142-del-2002-form-1.pdf

142-del-2002-form-18.pdf

142-del-2002-form-2.pdf

142-del-2002-form-3.pdf

142-del-2002-gpa.pdf