Title of Invention

"A PROCESS FOR PREPARATION OF ALUMINIUM-ZINC-MAGNESIUM-COPPER-ZIRCONIUM-SILVER ALLOY HAVING IMPROVED WELDABILITY "

Abstract This invention relates to a process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy with improved weldability comprising the steps of preparing a charge mixture of high purity primary aluminium comprising 77.7% by weight of said high purity primary aluminium, 6% by weight of Al-30 wt% of Cu master alloy and 4.1% by weight of Al-9.8 wt% Ag master alloy; melting at 720 to 730°C the said charge mixture of primary aluminium, Al-30wt%, Cu master alloy and Al-9.8wt% Ag master alloy, adding to this molten charge 8.4% by weight elemental pure Zn, followed by raising the temperature of the molten charge as herein described; adding 3.1% by wt of Al-51wt% Mg master alloy, 0.7% by wt of Mg-28wt% Zr master alloy in the above sequential order to the said molten charge followed by super heating at 755 to 765 °C for a period of about 10 minutes; adding 0.1 kg by wt of nucleant pellets at reduced temperature of 735-745 °C for grain refinement; degassing the molten melt to remove dissolved gases and pouring the molten melt under inert gas atmosphere, preferably argon atmosphere, into a pre-heated mould; homogenizing, scalping and rolling as herein described the sound billets into sheets; solution treating, quenching and stretching the alloy sheets followed by two-step artificial aging.
Full Text Filed of Invention
The invention relates to a process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium- Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy having improved weldability and an alloy thereof.
PRIOR ART
Aluminium-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloys represent the class of highest strength Aluminium alloy that can be produced via ingot metallurgical route. These alloys may be suitably processed to obtain a combination of improved strength, fracture toughness and stress corrosion cracking resistance, and the resultant alloys find various applications in areas of aerospace and defence.
However, theses 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.
A weld filler alloy, known in the art (US patent 6, 579,386), has the composition of Al-CU-Ti-Zr-Ag and it was developed for fusion welding Al-Li-Cu-Mg-Ag-Zr base 2195 Al alloy. The presence of Ag in the filler alloy was claimed to reduce the weld crack susceptibility of the alloy 2195, that itself contains Ag as an alloying element. The filler alloy has a Cu content

of as high as 6wt% and has Ti and Zr contents of more than 0.25 wt% each. Both Ti and Zr additions are well known to cause as-cast grain refinement and improve weldability of Al alloys. 2xxx series Al alloys containing high level of Cu is also well known to show reduced tendency toward weld crack susceptibility. It is, therefore, not clear in the art whether the said role of Ag in the filler ally works (1) only in the presence of high levels of both Ti and Zr as well as Cu in the filler alloy or (2) when Ag is also present in the base alloy, and/or (3) only when the base alloy contains high amounts of both Li and Cu.
One of the high strength and high weldability alloys known in the art is Al-Zu-Mg-Cu-Zr-Sc. A major drawback of the above alloy is that scandium (Sc) metal is expensive. Yet another limitation of the above mentioned alloy is that scandium ores are not available in many countries including India.
OBJECTS OF THE INVENTION
The primary object of the present invention is to propose a process of preparation of an Aluminium-Zinc-Magnesium-Copper-Zirconium- Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy having improved weldability and an alloy thereof.Another object of the present invention is to propose a process of preparation of an Al-Zn-Mg-Cu-Zr-Ag alloy and an alloy thereof having high strength.
Yet another object of the present invention is to propose a process of preparation of an Al-Zn-Mg-Cu-Zr-Ag alloy and an alloy thereof using artificial aging treatment so as to enable the alloy to obtain high strength without any loss in ductility.
STATEMENT OF THE INVENTION
According to the present invention, there is provided a process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy with improved weldability comprising the steps of:-
(i) preparing a charge mixture of high purity primary aluminium comprising 77.7% by weight of said high purity primary aluminium, 6% by weight of Al-30 wt% of Cu master alloy and 4.1% by weight of Al-9.8 wt% Ag master alloy; (ii) melting at 720 to 730°C the said charge mixture of primary aluminium, Al-30wt%, Cu master alloy and Al-9.8 wt% Ag master alloy, adding to this molten charge 8.4% by weight elemental pure Zn, followed by raising the temperature of the molten charge as herein described;
(iii) adding 3.1% by wt of Al-51wt% Mg master alloy, 0.7% by wt of Mg-28wt% Zr master alloy in the above sequential order to the said molten charge followed by super heating at 755 to 765 °C for a period of about 10 minutes;
(iv) adding 0.1 kg by wt of nucleant pellets at reduced temperature of 735-745 °C for grain refinement;
(v) degassing the molten melt to remove dissolved gases and pouring the molten melt under inert gas atmosphere, preferably argon atmosphere, into a pre-heated mould;
(vi) homogenizing, scalping and rolling as herein described the sound billets into sheets;
(vii) solution treating, quenching and stretching the alloy sheets followed by two-step artificial aging.
Further according to this invention there is provided an Aluminium-Zinc-
Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy
comprising Al-(7.4-8.2) Zn-(1.6-2.1) Mg- (1.5-2.1) Cu- (0.11-0.19) Zr-
(0.3-.08) Ag.
The invention 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 FIGUES AND TABLE
Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying figure and Table and wherein:
Fig 1: Total crack length (mm) vs. augmented strain (%) profiles fro
Al-Zn-Mg-Cu-Zr-Ag alloy present invention. These results are compared with tse of a high strength Al alloy known in the prior art (7449 Al alloy)
Fig 2: Photo macrographs revealing the extent of solidification
cracks developed in 9a) 7449 Al alloy and (b) present invention due to augmented strain of 3.5%.
Fig 3: Light micrographs showing development of (a) coarse grain
structure in the 7449 Al alloy weld, and (b) finer and equiaxed grain structure in the weld of the alloy of present invention.
Table 1 Tensile properties (in the longitudinal direction) of T651 treated and the alloy of present invention.
DESCRIPTION OF THE PROCESS

According to the present invention, there is provided a. process for the preparation of Al-Zn-Mg-Cu-Zr-Ag alloy that has high strength and significantly high weldability. In the present invention, Al-Zn-Mg-Cu-Zr-Ag alloys having composition (in wt%) as Al-(7,4-8.2) Zn- 91.6-2.1) Ag-(1.5-2.1) Cu-(0,ll-0.19) Zr- (0.3-0.8) Ag. That gives a combinatin of high strength and ductility, besides high weldability, is disclosed.
The process for the preparation of Al-Zn-Mg-Cu-Zr-Ag alloy of the present invention comprises of following steps.
(a) Preparing a charge mixture of 77.7% by weight of primary
Aluminium (with 99.85% purity, the balance being 0.09 wt% Fe
and 0.06 % by weight of the master alloy Al-30wt% Cu and
4.1% weight of the master alloy Al-9.8 wt% Ag.
(b) Melting the above charge mixture in an induction furnace by
heating at around 720-730*, and adding to this molten charge
8.4 % 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 3.1% by weight of master alloy Al-51wt %
M, 0.7% by weight of master alloy Mg-28wt% Zr 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
approximately 0.1 kg of nucleant pellets for grain refinement.
(e) Degassing the molten melt by adding approx 0.25 kg 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 450 ± 5°C for
30 to 40 hours followed by cooling in air. The homogenization
treatment eliminates dendritic segregation in the cast
microstructure.
(g) Scalping the surface of biller 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 biller1 temperature of 420-430°C and at a linear speed of 20 m per minute to finally produce sheets having thickness of around 5mm. The mechanical processing of the billets may be carried our by other deformation routes such as extrusion.
(i) Solution treatmening the sheets obtained by step (b) above to at the temperature range of 450-460 °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 90-100'C for about 8 hours in the first stage followed by a second stage aging in the temperature range of 120-125'Cfor about 24 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 to take restrictively to imply any limitations on the present invention.
WORKING EXAMPLE -1
For a 50 kg melt of the alloy of present invention m a'mixture of 38.85 kg of primary Aluminium (purity 99.85% Al and the, balance being 0.09%Fe and 0.06% Si impurities), 3 Kg of Al-30wt%rCu iriaster alloy and 2.05 Kg of Al-9.8wr% Ag master alloy is charged into the induction furnace, the above charge mixture is melted at arourtd 72(X-730!C. To this molten charge, 4. 2 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 1.55Kg of Al-51wt% Mg master alloy and 0.35 Kg of Mg-28wt% 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 nucleant pellets is added for grain refinement purpose. After 5 minutes, 0.25 g of degasser pellet 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 450° 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 425°C and at a linear seed of 20 m per minute, the billet was rolled to 5 mm thick sheets. The sheets were then subjected to solution treatment at 455°C for 2 hours followed by water quenching at room temperature. The quenched sheets were stretched to obtain 1.5% permanent set for stress reliving purposes. The sheets were then subjected to artificial aging at 100'C for ' 8 hours followed by artificial aging at 120'C for 24 hours. This heat treatment produced peak strength in the alloy. These peak aged material were then utilized for the quantitative weldability test i.e. Varestraint test.

EVALUATION OF THE ALLOY
The weldability of the alloy of present invention was examined using quantitative Varestraint test. The 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 alloy of present invention as compared to the 7449 Al alloy known in the prior art [having the composition (7.5-8.7) Zn- (1.8-2.7) Mg- (1.4-2.1) Cu- (0.10-0.25) Zr]. The most noticeable feature in figure 1 is the remarkable reduction in the total crack length for the alloy of present invention as compared to the 7449 Al alloy.
Figure 2 (a) and (b): represent photo macrographs depicting the nature of crack and the status of the test samples after augmented strain value of 3.5% for the 7449 Al alloy and the alloy of present invention, respectively.
Figures 3(a) and (b): represent optical micrographs showing development of coarse, columnar grain structure in the 7449 Al weld, and relatively finer and equiaxed grain structure in the weld of the alloy of present invention, respectively. The refinement of the weld grain structure is associated with the improved weldability of the alloy of the present invention.
The above results demonstrate the greatly reduced hot cracking susceptibility of the alloy of present invention
Table 1: [resents the tensile properties of the peak aged 7449 ally and htise of the alloy of present invention. It is noteworthy that the tensile properties are comparable.
Table 1: Tensile properties (in the longitudinal directions) of T651 treated 7449 and the alloy of present invention.
(Table Removed)

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.







WE CLAIM;
1. A process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy with improved weldability comprising the steps of:-
(i) preparing a charge mixture of high purity primary aluminium comprising 77.7% by weight of said high purity primary aluminium, 6% by weight of Al-30 wt% of Cu master alloy and 4.1% by weight of Al-9.8 wt% Ag master alloy;
(ii) melting at 720 to 730°C the said charge mixture of primary aluminium, Al-30wt% Cu master alloy and Al-9.8wt% Ag master alloy, adding to this molten charge 8.4% by weight elemental pure Zn, followed by raising the temperature of the molten charge as herein described;
(iii) adding 3.1% by wt of Al-51wt% Mg master alloy, 0.7% by wt of Mg-28wt% Zr master alloy in the above sequential order to the said molten charge followed by super heating at 755 to 765 °C for a period of about 10 minutes;
(iv) adding 0.1 kg by wt of nucleant pellets at reduced temperature of 735-745 °C for grain refinement;
(v) degassing the molten melt to remove dissolved gases and pouring the molten melt under inert gas atmosphere, preferably argon atmosphere, into a pre-heated mould;
(vi) homogenizing, scalping and rolling as herein described the sound billets into sheets;
(vii) solution treating, quenching and stretching the alloy sheets followed by two-step artificial aging.
2. A process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy as claimed in any of the preceding claims wherein the said degasser pellets for degassing are taken in quantity of around 0.25 kg by weight, and added to the said charge mixture at the temperature of 730-740°C, 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.
3. A process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy as claimed in any of the preceding claims wherein the said homogenization is carried at a constant temperature range of 450 + 5 °C for 30 to 40 hours, the said heating to homogenization temperature is carried at a constant rate of 25 to 35 °C per hour.
4. A process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy as claimed in any of the preceding claims wherein the said rolling is carried out at temperature of 420 to 430 °C and at a linear speed of 20 m/minute, the said solution treatment is carried out at 450 to 460 °C for 2 hours followed by quenching in water at ambient temperature, the said two-step artificial aging is carried at 95-100 °C for 8 hours followed by aging at 120-125 °C for about 24 hours.
5. An Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy comprising Al-(7.4-8.2) Zn-(1.6-2.1) Mg-(1.5-2.1) Cu-(0.11-0.19) Zr- (0.3-.08) Ag in wt%.
6. A process for preparation of Aluminium-Zinc-Magnesium-Copper-Zirconium-Silver (Al-Zn-Mg-Cu-Zr-Ag) alloy with improved weldability and an alloy thereof substantially as described and exemplified herein.

Documents:

1369-DEL-2004-Abstract-(05-06-2009).pdf

1369-DEL-2004-Abstract-(17-03-2010).pdf

1369-del-2004-abstract.pdf

1369-DEL-2004-Claims-(05-06-2009).pdf

1369-DEL-2004-Claims-(17-03-2010).pdf

1369-del-2004-claims.pdf

1369-DEL-2004-Correspondence-Others-(05-06-2009).pdf

1369-DEL-2004-Correspondence-Others-(16-04-2010).pdf

1369-DEL-2004-Correspondence-Others-(17-03-2010).pdf

1369-del-2004-correspondence-others.pdf

1369-del-2004-correspondence-po.pdf

1369-DEL-2004-Description (Complete)-(05-06-2009).pdf

1369-DEL-2004-Description (Complete)-(17-03-2010).pdf

1369-del-2004-description (complete).pdf

1369-del-2004-description (provisional).pdf

1369-del-2004-drawings.pdf

1369-DEL-2004-Form-1-(05-06-2009).pdf

1369-del-2004-form-1.pdf

1369-del-2004-form-18.pdf

1369-DEL-2004-Form-2-(05-06-2009).pdf

1369-del-2004-form-2.pdf

1369-del-2004-form-5.pdf

1369-DEL-2004-GPA-(05-06-2009).pdf

1369-DEL-2004-Petition 137-(17-03-2010).pdf


Patent Number 240013
Indian Patent Application Number 1369/DEL/2004
PG Journal Number 30/04/2010
Publication Date 30-Apr-2010
Grant Date 22-Apr-2010
Date of Filing 26-Jul-2004
Name of Patentee THE DIRECTOR GENERAL, DEFENCE RESEARCH & DEVELOPMENT ORGN.
Applicant Address MINISTRY OF DEFENCE, GOVT. OF INDIA, WEST BLOCK VIII, WING-1, SECTOR-1, R.K. PURAM, NEW DELHI-110 066, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 ASHIM KUMAR MUKHOPADHYAY, DEFENCE METALLURGICAL RESARCH LABORATORY, P.O.- KANCHAN BAGH, HYDERABAD-500 058 INDIA
2 GANKIDI MADHUSUDHAN REDDY DEFENCE METALLURGICAL RESARCH LABORATORY, P.O.- KANCHAN BAGH, HYDERABAD-500 058 INDIA
PCT International Classification Number C22C 21/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA