Title of Invention

A PROCESS FOR THE MANUFACTURE OF COMPOSITES

Abstract

This invention describes a process for the manufacture of novel sandwich composites of austenitic stainless steel (SS) and either commercially pure aluminium (A) or any aluminium base alloy of 6xxx series (AA). The process of the invention provides an excellent solid state diffusion bonding between Aluminium or Aluminium alloy (6061) and austenitic stainless steel with the help of metallic inter layers. The composite so prepared has strength superior to aluminium or aluminium alloys and passes standard helium leak test. Unique properties of such composites are generally useful to fabricate base plates and also end-windows in the development of ion chambers. These ionisation chambers are used for a variety of applications including thickness gauge monitoring.

Full Text

FORM 2 THE PATENTS ACT 1970 COMPLETE SPECIFICATION (See Section 10) TITLE A Process For The Manufacture Of Novel Composites Of Austenitic Stainless Steel Aud Either Cuiniiieidaily Pme Aluminium Oi Any Aluminium Base Alluy Of 6xxx Scries APPLICANT Department of Atomic Energy, Government of India, Anushakti Bhavan, Chhatrapati Shivaji Maharaj Marg Mumbai 400 039 The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- Field of Invention The present invention relates to a process for the manufacture of composites of stainless steel and aluminium, as well as composites of stainless steel and any aluminium alloy. This invention particularly relates to a process for the manufacture of composites of austenitic stainless steel and either commercially pure aluminium or any alummium base alloy of 6xxx series. Background and Prior Art Joining of aluminium and aluminium alloys (6061) to stainless steel is frequently required in many cryogenic and detector applications. A variety of alummium and aluminium alloy (6061) are used as storage tanks for cryogenic liquids and transfer links are mostly stainless tubes. This requires a transition between these two alloys. The most common methods employed for joining aluminium and aluminium alloy (6061) to stainless steel are by mechanical means, welding, brazing, friction welding, explosion welding and by solid state diffusion bonding. Mechanical joints require flanges and bolts to produce a compressive seal against the metal pieces. These mechanical joints are generally heavy and are prone to the leakage of the gases during service time. Ishigaki - T, Shibaya-K, Sakaue-H, Be-SH - Journal of Vacuum - Science of Technology- A, Vol.12, No.4 pt.I, July-Aug 1994, p 1705-8 Welding and brazing operations are extremely difficult due to the large difference in the melting points, linear thermal expansion coefficients between aluminium and stainless steel. In addition, the metallurgical incompatibility between these two alloys leads to the development of brittle intermetallic compounds at the reaction zone leading to premature failing of the joints. Friction and explosion welded joints generally results in the development of large residual stresses and structural discontinuities at the interfaces. These aspects lead to cracks in the joints during the service time. This subject is well reviewed in following publications: Hokamata - K, Izyma-T, Fujita-M - Metallurgical Transaction - A, Vol. 24A, no. 10, Oct. 1993; p 2289-97 Tsujino-J, Ueoka-T, Kashino-T; Sugahara-F - Japanese Journal of Applied Physics, Part 1 Vol.28, no.7A, July 1999, p.4254-5 Fukumato-S; Tsubakino - H, Okita-K, Aritoshi-R, Tomita-T Scripta Materialia Vol.42, no.8,14 April 2000, p 807-12 The direct solid state bonding of these two alloys has several inherent difficulties. The basic problem of joining aluminium alloy is the presence of aluminium oxide, which is difficult to remove during the joining process. Several patents have been published on the methodology adopted for removing the aluminium oxide [US patent No3,973,387] and also on the diffusion bonding of aluminium base alloys [US patent Nos 4,890,784, 4,948,457 and 5,222,645]. The joining of aluminium alloy to steel is still more difficult due to the fact that the thermal expansion of aluminium is almost twice that of steel [Brandes, E.A., Smithells Metals Reference Book, sixth edition, 1983 see also S.Fukumoto, T. Inuki, H.Tsubakino, K. Okita, M. Aritoshi and T. Tomita, Materials Science and Technology, , Vol.13, p 679-686 The initial work carried out on the diffusion bonding of aluminium and aluminium alloy (6061) with stainless steel was not successful. This could be due the formation of brittle intermetallic compounds in the diffusion zone. This is essentially because of the interaction of aluminium with iron the basic constituent of the stainless steel resulting in the formation of Al5Fe2, Al2Fe2 , intermetallic compounds as shown in the following research papers: Janssen, M.M.P. and Bieck, G.D. Reactive diffusion and formation of compounds in aluminium and stainless steel, Trans. TMS-AIME, 239, 1967, 1372-1385. Dewing, E.W. and Iyer, S.P., The rate of formation of intermetallic layers between aluminium and steel below 660°C, Metallurgical Transactions, 2 (10), 1971,2931-2934. The formation of brittle intermetallic compounds and the restricted solubility of aluminium in Fe, Cr and Ni would seriously affect the direct bonding between aluminium and stainless steel. Several of the conventional methods like mechanical joining, welding, brazing and direct diffusion bonding have serious limitations for joining aluminium, aluminium - base alloys to stainless steel. These drawbacks can be overcome by joining these materials by solid state bonding by judiciously choosing the interlayer. The layers chosen should prevent the formation of brittle intermetallic compounds and also minimize the linear thermal expansion mismatch. Extensive work is reported in the development of the diffusion bonding between aluminium and stainless steel employing intermediate layers to facilitate bonding and to minimize the formation of brittle intermetallic compounds. Use of metallic copper to form such intermediate layer has been described by Crane, C. H., et al., Welding Journal, 46 (1), 1967, 23-s to 31-s. Use of metallic nickel to form such intermediate layer has been described by Masumato H., Asoda A., Serino M, Bulletin of Kurama Institute of Technology, No. 11,1987, p 9-16. Use of silver films evaporated from hot hollow cathode has been described by Calderon, P. D. et al., Welding Journal, 64 (4), 1985, 104-s to 112s and by Naimon, E.'R. et al., Welding Journal, 60 (11), 1981,17-20. Use of copper foil and silver-copper alloy with 72 wt% Ag - 28%Cu foil as intermediate layer has been described by Hauser, D., et al., Welding Journal, Jan. ,1967, lls-22s. Use of titanium foil as an insert metal has been described by, Momano, T. et al., Journal Japan Inst. Light Metals., 37 (6), 1987, 413-418. Diffusion bonding of aluminium to stainless steel using titanium metal foil inserts has been described by Serino, M., , Trans. Japan Weld, Soc, 22 (1) 1991, 57-65. However, the formation of brittle intermetallic compounds could not be totally prevented at a bonding temperature of 260-315°C for 2 to 4 hrs. of duration. In all these methods the intermediate layers have been in the form of mettalic foils, one formed by electro deposition of metals or one formed by evaporated metal from a hot hollow cathode. In addition, a pressure generally in the range 100 - 200 MPa at a bonding temperature of 260-315 ° C for 2 to 4 hours of duration are employed. Object The main object of the present invention is to provide a process to make a high strength metallurgical bond between aluminium or any aluminium base alloy (6061) and stainless steel by using diffusion bonding technique. Another object is to develop a process technique so that the diffusion bond aluminium or aluminium alloy (6061) and stainless steel is formed at relatively low temperature to minimize the deformation of the base materials. Yet another object is to develop a process technique so that the drawbacks of the prior art diffusion bonding so as to minimize the coefficient of linear thermal expansion mismatch at the interface by providing progressive compatibility between aluminium or aluminium alloy and stainless steel by preventing the formation of brittle intermetallic compounds.. SUMMARY OF THE PRESENT INVENTION Accordingly the present invention provides a process for the manufacture of novel composites comprising of (I) austenitic stainless steel (SS) metal piece and (II) either commercially pure aluminium (A) or any aluminium base alloy of 6xxx series (AA) metal piece, each said metal pieces having atleast one bonding surface that can be placed in juxtaposition contact with one another to form said composite comprising, i. cleaning said bonding surfaces of said metal pieces free of oxide layer by conventional chemical cleaning methods; ii. depositing thin layers of nickel, copper, silver one over other in that order, on the bonding surface of thus cleaned (SS) metal piece forming a three metal layered SS metal piece; iii. depositing thin layers of zinc, copper, silver one over another in that order on the bonding surface of thus cleaned metal piece of pure aluminium (A) or aluminium base alloy (AA) forming three metal layered A or AA metal piece; iv. diffusion bonding of said three metal layered SS metal piece with said three metal layered A metal piece or said three metal layered AA metal piece by placing silver surface of said three metal layered SS metal piece in contact with silver surface of said three metal layered A metal piece or said three metal layered AA metal piece to form the said composite. Description Of The Present Invention One embodiment of the process for manufacture of novel composites of the present invention comprises diffusion bonding of a sandwich of stainless steel piece of regular geometry such as a sheet, a plate, a cylinder or a bar, on one side and aluminium or aluminium alloy piece of the similar geometry on the other side with thin layers of metals in between them deposited partly on SS and partly on A or AA as described above. At first the surfaces of both the metal pieces where the bond is to be established are made geometrically matching . These plates are thoroughly cleaned free of oils/fats or grease and metal oxide layers by chemical treatment.. Then the deposition of layers of nickel, copper and silver is done by conventional electro-deposition techniques while the deposition of a layer of zinc is done by conventional immersion deposition technique. In between all the cleaning and plating cycles rinsing with water is essential to remove adhering chemicals. The materials used in this invention are: Austenitic Stainless Steel comprises mainly Cr. (18-20 wt %), Ni (8-10.5 wt.%), C (0.04 - 0.08 wt%) and balance Fe Preferably Stainless Steel is of the type numbered AISI304. Commercially pure Aluminium of more than 99% purity and preferably, aluminium alloy is of the type Aluminium alloy comprising up to 2.5 wt% Mg. Preferably Aluminium alloy is of the type No.6061 Al, Cu (0.28 wt %); Nickel: Nickel may be preferably deposited electrochemically from Wood's nickel strike bath. This is based on use of chloride salt of nickel, and hydrochloric acid The thin layer of nickel deposited on the bonding surface of said SS sheet/plate is of thickness 2-10 μm. In all the actions to be taken on the bonding surface other surfaces are masked preferably by masking agent (proprietary masking agent - Shailtex Lubricants Pvt. Ltd., Ahmedabad -380 008) Copper: Copper may be preferably deposited electrochemically from dull cyanide copper bath. This is based on use of cyanide salt of copper. The thin layer of copper deposited upon the nickel layer on said SS sheet/plate is of thickness 2-20 μm. Silver: Silver may be preferably deposited electrochemically from conventional silver cyanide bath. Initially a very thin layer of silver is deposited from silver cyanide strike bath and then more silver is deposited on it from silver cyanide plating bath. This is based on use of cyanide salt of silver, The thin layers of silver deposited upon the copper layer on nickel layer over the bonding surface of said SS sheet/plate is of thickness 20 - 50 μm. Zinc: The deposition of a layer of zinc is done by conventional immersion deposition technique. Zinc may be preferably deposited on aluminium or aluminium alloys by dipping them in a sodium zincate bath.. The zinc salts used for this are selected from zinc oxide. The thin layer of zinc deposited on one side of said A or AA sheet/plate is of thickness up to 5 μm. The thin layers of copper may be deposited upon the zinc layer on the bonding surface of said A or AA sheet/plate by conventional electro-deposition technique. The thin layers of copper deposited upon the zinc layer on the bonding surface of said A or AA sheet/plate is of thickness 2-20 μm. Silver may be deposited on the copper layer by conventional electro deposition techniques. The thin layers of silver deposited upon the copper layer which is on zinc layer over the bonding surface of said A or AA sheet/plate is of thickness 20 - 50 μm. The diffusion bonding is performed at 250 - 350 °C for 1 - 5 hrs under a uniaxial pressure of 20-60 MPa under vacuum, preferably, at 290 - 310 °C more preferably at 300 - 320 °C preferably for 2 - 4 hrs., more preferably for 4 hrs., at 45 - 55 MPa more preferably at 50 MPa in conventional diffusion bonding chambers of size 0.5 m § x 0.5 m depth, ram size 0.05 m § , 25,000 kg can operate upto 1100° C with a vacuum better than 10-5 torr. Composites of following combinations are found to be useful: SS is of the type numbered AISI 304 and said A is greater than 99% purity. SS is of the type numbered AISI 304 and said AA is of the type 6061. The present study describes the bonding of aluminium, aluminium- base alloy (6061) to stainless steel (AISI 304) employing chemically/electrochemically deposited Zn, Cu, Ag, Cu and Ni as intermediate layers. According to the present invention, interface layers that can provide progressive compatibility between All Al. (alloy) and SS must be used. The configuration consisting of A/Zn/Cu/Ag - Ag/Cu/Ni/SS or AA/Zn/Cu/Ag - Ag/Cu/Ni/SS would prevent the undesirable intermetallic compounds and also facilitate progressive compatibility between alumimuni/alumimum base alloy and stainless steel, iron based and nickel based alloys. All the layers can be chemically or electrochemically plated in standard baths, (zinc from Alzincate bath, nickel from Wood's nickel strike bath, copper from cynodull copper bath, silver from silver cyanide strike bath / hard additive silver bath for thickness build up). Stainless steel and aluminium alloys (6061) are difficult to coat upon because of the presence of thin film of oxides on the surfaces. A detailed pretreatment procedure was followed to get adherent coatings. EXAMPLES The invention will now be described by the way of Examples. Examples are by the way of illustration only and in no way to restrict the scope of the invention. In the Example I is described a process for manufacture of a novel composite comprising Stainless Steel 304 and pure aluminium. At first before the chemical treatment the two metal surfaces are metallographically polished. In the Example II is described the process for manufacture of another novel composite comprising Stainless Steel 304 and aluminium base alloy 6061. In the Example III is described the process for manufacture of the novel composite comprising Stainless Steel 304 and pure aluminium prepared as in Example I. Example I: Process for manufacture of composite comprising stainless steel 304 and pure aluminium Materials used in this Example Stainless Steel 304 from M/S Mukund Steels, Mumbai, 150mm dia discs 2mm thickness Pure Aluminium ( > 99 % ) purity from M/S Hindusthan Aluminium, Mumbai) 150mm dia discs 5mm thickness Nickel salt - Nickel Sulphate purity 99.92 % and nickel chloride AR grade, purity of nickel salts > 98 %. Copper salt - proprietary cyno dull copper* bath formulation based on copper cyanide. (* M/s Grauer & Weil (India) Ltd., Mumbai) Silver salt - proprietary** silver plating salts based on silver cyanide Initial layer (2-3 μm) of silver from silver strike bath and thickness build-up from hard silver additive bath.(**M/S Apurva Industries, Thane.) Zinc salt - proprietary alzincate* immersion zinc bath based on zinc oxide (* M/s Grauer & Weil (India) Ltd., Mumbai) Nitric acid AR (69 - 72 %), sulphuric acid conc AR (97 - 98 %); chromic acid (purity 99% GR); hydrochloric acid (36 %). Steelex K-20 (Proprietary Graver & Weil) that acts as alkaline cleaner to remove oil or grease from SS surfaces. Ginbond - NS 35 (proprietary Grauer & Weil) that acts as mild alkaline cleaner / etchant for aluminium / aluminium alloys. Procedure a.Coatings on SS disc: A disc of SS was cleaned with acetone to remove any dirt, organic and dust particles. The acetone cleaned disc was cleaned in Steelex K-20 a mild alkaline cleaner to remove oil or grease present on the disc and then dipped it in 30% (v/v) HC1 solution in order to make it free from any oxide present on the surface. Nickel coating was obtained electrolytically by dipping the pretreated disc in the Woods Nickel bath for 5 minutes at 1.5 V, current density 5 A/dm to coat a thickness of 2 μm. Copper was coated electrolytically by dipping the sample in the dull cyanide Cu bath for 10 minutes at 2 V, current density 1 A/dm2 to coat a thickness of approximately 2 μm. An overcoat of silver (2-3 μm) was obtained electrolytically from silver cyanide strike bath at a constant voltage of 3 V followed by electroplating of Ag at 1.5V, current density lA/dm2 to a thickness of 20 μm from silver cyanide plating bath over the nickel, copper coated SS disc. In between, at the end of each cleaning and plating step, the SS disc was thoroughly rinsed with water. b. Coating of aluminium disc: The A-disc was first cleaned with acetone, then in a mild alkaline cleaner Ginbond - NS 35, followed by mild etching in the same solution at a higher temperature and followed by a dip in nitric acid (50%v/v). After cleaning steps, actual coating procedure was followed. In the first step the cleaned A-disc was dipped in zincate bath (20 to 25%) for 1 minute and subsequently the zinc deposit was dissolved in 41% v/v HNO3. The disc was again coated with Zinc. The optimum time period (V2 -1 minute) was maintained in Zinc coating bath to coat a thickness of approximately up to 1-2 μm. Copper was coated electrolytically by dipping the zinc coated disc in the dull cyanide copper bath at 2 V, current density 1 A/dm2 for 10 minutes to coat a thickness of approximately 2 μm. An overcoat of silver (2-3 μm) was obtained electrolytically from silver cyanide strike bath at a constant voltage of 3 V followed by electroplating at 1.5 V, current density 1 A/dm of silver to a thickness of 20 μm from silver cyanide plating bath, over the copper coated disc. In between, at the end of each cleaning and plating step, the A-disc was thoroughly rinsed with water. c. Diffusion Bonding The diffusion bonding between SS and A or AA, was accomplished in a hot vacuum press under a vacuum better than 10"3 Pa. and at a heating rate of 300 °C / h up to the bonding temperature. The diffusion bonding was performed at 250-350 °C for about 4 hrs at a pressure of about 50 MPa. Results of the Example I The product of this process is a novel composite of stainless steel 304 and pure aluminium having following properties as given in Table 1. Example II: Process for manufacture of stainless steel 304 and aluminium base alloy 6061. The process in this Example II is same as that in Example I, except that the cleaning step for Aluminium alloy (6061) requires an additional deoxidising etch in a mixture of concentrated H2 SO4 (10% v/v), CrO3 (3.5 wt%) and H20 after the mild etching treatment and before a dip in nitric acid (50%v/v) as in the pretreatment followed for pure aluminium. Results of the Example II The product of this process is a novel composite of stainless steel 304 and aluminium alloy 6061, having following properties are given in TABLE 1. Example III: Fabrication of stainless steel/Aluminium composite plates The fabrication of the stainless steel/aluminium composite plates involves the following steps. i. Stainless steel circular plate of thickness 2 mm and diameter 150 mm and pure aluminium circular plate of thickness 5 mm and diameter 150 mm were used for fabricating the stainless steel/aluminium composite plates. ii. One of the faces of stainless steel plate as well as one face of aluminium plate was metallographically polished to 1 μm diamond finish. iii. Both these metal discs were chemically cleaned as in Example I iv. The polished face of stainless steel plate was coated with nickel of thickness 2 μm, then electrodeposited with a coating of copper of thickness 2 μm and then overlaid by electroplating with a coating of silver of thickness of 22-23 μm. ( 2-3μm of silver plated from silver strike bath followed by thickness build-up from hard silver additive bath) as in Example I. v. The polished face of aluminium was coated with zinc of thickness up to 1 μam, then electrodeposited with a coating of copper of thickness 2 urn and then electrodeposited with a coating of silver of thickness of 22 - 23 μam, as in Example I. vi. The coated surfaces of stainless steel and aluminium were placed together and diffusion bonded in hot vacuum press under a uniaxial pressure of 50 MPa at the temperature ranging between 300 -315 °C for 4 hrs. in vacuum The characteristics of this novel steel/ aluminium composite are given below in Table 1. Comparative values for steel , aluminium and aluminium alloy are also given in Table 1. TABLE 1 Property Composite SS Al Al alloy Ex l SS/Al Ex 2 SS/Al alloy Ex 3 SS/Al The tensile strength (MPa) 90 240 90 460 85 230 Ductility(%) 18 15 18 40 20 20 standard helium leak test (ASME Section 5 Article2 Subsection A Article 10 Subsection B Article 27) 10-9 std cc/sec 10-9 std cc/sec 10-9 std cc/sec Pressure test (MPa) 0.35 0.35 0.35 - - - Results of the Examples I, II and III show the superiority of the composite over the weaker component. The comparative values also show that this novel composite has strength superior to Al and Aluminium alloys. Advantages Of The Invention The process of the present invention discloses a development in solid state diffusion bonding method. Bonding of SS and A or AA is brought about by interfacing suitable metallic multilayers to prevent the formation of brittle intermetallic compounds at the interface, provide progressive compatibility and also minimise the linear thermal expansion mismatch. The Intermediate layers chosen in this bonding and placed on SS is: Stainless steel/Ni/Cu/Ag and on A or AA is Al/Aluminium (6061 alloy)/Zn/Cu/Ag. The coating process employed involves the standard chemical /electrochemical bath an accepted industrial process. In view of this there is practically no restriction on the size of the specimens to be joined. However, the size restriction is due to the size of the diffusion bonding chamber. The strength of the joints made by the present procedure is more than the strength of the weaker material, aluminium and aluminium alloy (6061). In addition the specimens pass through the standard helium leak test, pressure test (0.350 MPa). The specimens thermal cycled between room temperature and 100 °C for 50 times also pass through the standard helium leak test and pressure test (0.35 MPa). Diffusion bonded stainless steel/aluminium composite plates is generally used to fabricate base plates and also end-windows in the development of ion chambers. These ionization chambers are used for a variety of applications including thickness gauge monitoring. WE CLAIM : 1. A process for the manufacture of novel composites comprising of (I) a first metal piece of austenitic stainless steel (SS) and (II) a second metal piece selected from commercially pure aluminium of better than 99% purity (A) or any aluminium base alloy of 6xxx series (AA), at least one bonding surface of the metal piece is a bonding surface such that such bonding surface may be brought into contact to form the composite, the process comprising, i. cleaning said bonding surfaces of said metal pieces free of oxide layer by conventional chemical cleaning method; ii. depositing thin layers of nickel, copper, silver one over other in that order, on the bonding surface of thus cleaned (SS) metal piece forming a three metal layered SS metal piece; iii. depositing thin layers of zinc, copper, silver one over another in that order on the bonding surface of thus cleaned metal piece of pure aluminium (A) or aluminium base alloy (AA) forming three metal layered A or AA metal piece; iv. diffusion bonding of said three metal layered SS metal piece with said three metal layered A metal piece or said three metal layered AA metal piece by placing silver surface of said three metal layered SS metal piece in contact with silver surface of said three metal layered A metal piece or said three metal layered AA metal piece to form the said composite. A process for the manufacture of novel composites as claimed in claim 1 wherein, said SS comprises, austenitic Stainless Steel mainly Cr. (18-20 wt%) Ni (8-10.5 wt%), C(0.04-0.08 wt%) and balance Fe. A process for the manufacture of novel composites as claimed in claim 1 or 2 wherein said SS is of the type numbered AISI 304. A process for the manufacture of novel composites as claimed in any claim 1-3 wherein, said A comprises commercially pure Aluminium of better than 99% purity. A process for the manufacture of novel composites as claimed in any claim 1-4 wherein, said AA comprises Aluminium alloy comprising up to 2.5 wt% Mg. A process for the manufacture novel composites as claimed in any claim 1- 5 wherein, said AA comprises Aluminium alloy comprising 0.28 wt% Cu. A process for the manufacture of novel composites as claimed in claim 1-6 wherein, said AA is of the type No.6061. A process for the manufacture novel composites as claimed in claim 1-7 wherein, cleaning metal surface free of oxide layer is preferably done after first polishing the surfaces to be bonded metallographically; A process for the manufacture of novel composites as claimed in any claims 1-8 wherein, thin layers of nickel deposited on the bonding surface of said cleaned SS metal piece is of thickness 2-10 μm. A process for the manufacture of novel composites as claimed in any claims 1-9 wherein, thin layers of copper deposited upon the nickel layer on said cleaned SS metal piece is of thickness 2-10 μm. A process for the manufacture of novel composites as claimed in any claims 1-10 wherein, thin layers of silver deposited upon the copper layer on nickel layer over on the bonding surface of said cleaned SS metal piece is of thickness 20 - 50 μm. A process for the manufacture of novel composites as claimed in any claims 1-10 wherein, thin layers of silver deposited upon the copper layer on nickel layer over on the bonding surface of said cleaned SS metal piece is done first by depositing very thin layer of 2-3 urn. by using silver cyanide strike bath and then raising the layer thickness to 20-50 μm. by using silver cyanide plating bath. A process for the manufacture of novel composites as claimed in any claims 1-12 wherein, thin layer of zinc deposited on the bonding surface of said cleaned A and AA sheet/plate is of thickness in the range of l-5um. 14. A process for the manufacture of novel composites as claimed in any claims 1-13 wherein, thin layers of copper deposited upon the zinc layer on the bonding surface of said cleaned A or AA metal piece is of thickness 2-20 urn. 15. A process for the manufacture of novel composites as claimed in any claims 1-14 wherein, thin layers of silver deposited upon the copper layer which is on zinc layer over on the bonding surface of said cleaned A or AA metal piece is of thickness 20 - 50 urn. 16. A process for the manufacture of novel composites as claimed in any claims 1-14 wherein said method employed for chemical cleaning and for metallographically polishing the surfaces to be bonded are as herein described 17. A process for the manufacture of novel composites as claimed in any claims 1-16 wherein said deposition of layers of nickel, copper and silver is done by conventional electro-deposition techniques. 18. A process for the manufacture of novel composites as claimed in any claims 1-17 wherein said deposition of a layer of zinc is done by conventional immersion deposition technique. 19. A process for the manufacture of novel composites as claimed in any claims 1-18 wherein said diffusion bonding is performed at 250-350°C for 1-5 hrs under a uniaxial pressure of 20-60 Mpa under vacuum. 20. A process for the manufacture of novel composites as claimed in any claims 1-19 wherein said solid state diffusion bonding is performed at 290 - 320°C for 2 - 4 hrs under a uniaxial pressure of 45 - 55 Mpa under vacuum. 21. A process for the manufacture of novel composites as claimed in any claims 1-20 wherein said diffusion bonding is performed at 300 - 320°C for 4 hrs under a uniaxial pressure of 50 Mpa under vacuum. 22. A process for the manufacture of novel composites as claimed in any claims 1-21 wherein said SS is of the type numbered AISI 304 and said A is commercially pure aluminium of better than 99% purity. 23. A process for the manufacture of novel composites as claimed in any claims 1-22 wherein said SS is of the type numbered AISI 304 and said AA is of the type 6061. 24. A process for the manufacture of novel composites substantially as herein described in the text and in the Examples. Dated this 12th day of July 2002 SIDDHARTHA NAG Of S. MAJUMDAR & CO Applicant's Agent

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