Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-10-31
2003-07-22
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S518000, C148S528000, C148S535000, C165S177000, C165S905000, C428S648000, C428S654000, C428S658000, C428S668000, C428S674000, C428S675000, C428S680000, C428S926000, C428S935000, C428S936000
Reexamination Certificate
active
06596413
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an aluminium brazing product having a base substrate (
1
) of an aluminium alloy comprising silicon in an amount in the range of 2 to 18% by weight, and on at least one outer surface a layer (
2
) comprising nickel. Further, the invention relates to a brazing sheet product having a core sheet made of an aluminium alloy having on at least one surface of the core sheet clad an aluminium clad layer (
1
), the aluminium clad layer being made of an aluminium alloy comprising silicon in an amount in the range of 2 to 18% by weight, and a layer (
2
) comprising nickel on the outer surface of the aluminium clad layer. The invention also relates to a method of manufacturing such a brazing product and to a brazed assembly comprising at least one component made of the brazing sheet product.
DESCRIPTION OF THE RELATED ART
Brazing, by definition, employs filler metal having a liquidus above 450° C. and below the solidus of the base metal. Brazing is distinguished from soldering by the melting point of the filler metal: solders melt below 450° C.
Brazing sheet products finds wide applications in heat exchangers and other similar equipment. Conventional brazing sheet products having a core sheet, typically an aluminium alloy of the Aluminium Association (AA)3000-series, having on at least one surface of the cores sheet clad an aluminium clad layer, the aluminium clad layer being made of an AA4000-series alloy comprising silicon in an amount in the range of 2 to 18% by weight, and preferably in the range of 7 to 14% by weight. The aluminium clad layer may be coupled to the core alloy in various ways known in the art, for example by means of roll bonding, cladding spray-forming or semi-continuous or continuous casting processes. These aluminium clad layers have a melting point or liquidus temperature typically in the range of 577 to 600° C.
Ideally, the melting point of the aluminium alloy clad layer should be substantially lower than the melting point of the core alloy. Otherwise, the temperature control during subsequent brazing operations becomes very critical. Aluminium alloys having a lower melting point than the melting point of aluminium clad layer cannot be used for the aluminium core layer. Hence, the high melting point of the conventional aluminium clad alloys limits the choice for possibly cheaper and/or stronger aluminium core alloys.
However, there is a market demand for brazing products such as brazing sheet products capable of being brazed into assemblies at a temperature below 570° C.
Controlled Atmosphere Brazing (“CAB”) and Vacuum Brazing (“VB”) are the two main processes used for industrial scale aluminium brazing. Industrial vacuum brazing has been used since the 1950's, while CAB became popular in the early 1980's after the introduction of the Nocolok (trade mark) brazing flux. Vacuum brazing is an essentially discontinuous process and puts high demands on material cleanliness. The disruption of the oxide layer present is mainly caused by the evaporation of magnesium from the clad alloy. There is always more magnesium present in the furnace then necessary. The excess magnesium condenses on the cold spots in the furnace and has to be removed frequently. The capital investment for suitable equipment is relatively high.
CAB requires an additional process step prior to brazing as compared to VB, since a brazing flux has to be applied prior to brazing. CAB is essentially a continuous process in which, if the proper brazing flux is being used, high volumes of brazed assemblies can be manufactured. The brazing flux dissolves the oxide layer at brazing temperature allowing the clad alloy to flow properly. When the Nocolok flux is used the surface needs to be cleaned thoroughly prior to flux application. To obtain good brazing results the brazing flux has to be applied on the total surface of the brazed assembly. This can cause difficulties with certain types of assemblies because of their design. For example, because evaporator type heat exchangers have a large internal surface, problems can arise because of poor access to the interior. For good brazing results the flux has to adhere to the aluminium surface before brazing. Unfortunately the brazing flux after drying can easily fall off due to small mechanical vibrations. During the brazing cycle, corrosive fumes such as HF are generated. This puts a high demand on the corrosion resistance of the materials applied for the furnace.
Ideally, a material should be available that can be used for CAB but does not have the requirements and defects of the brazing flux application. Such a material can be supplied to a manufacturer of brazed assemblies and is ready to use directly after forming of the assembly parts. No additional brazing fluxing operations have to be carried out. Presently, only one process for flux-less brazing is used on an industrial scale. The material for this process can be for example standard brazing sheet made from an AA3000-series core alloy clad on both sides with a cladding of an AA4000-series alloy. Before the brazing sheet can be used the surface has to be modified in such a way that the naturally occurring oxide layer does not interfere during the brazing cycle. The method of achieving good brazing is to deposit a specific amount of nickel on the surface of the clad alloy. If properly applied, the nickel reacts, presumably exothermically, with the underlying aluminium. The nickel can be applied by using a shim of nickel between the two parts to be joined or can be deposited by electroplating. When electroplating is used the adherence of the nickel should be sufficient to withstand typical shaping operations being used in for example heat exchanger manufacture.
The processes for nickel-plating in an alkaline solution of aluminium brazing sheet are known from each of U.S. Pat. No. 3,970,237, U.S. Pat. No. 4,028,200, U.S. Pat. No. 4,164,454, U.S. Pat. No. 4,602,731, and SAE-paper no. 880446 by B. E. Cheadle and K. F. Dockus. According to these documents, nickel or cobalt, or combinations thereof, are most preferably deposited in combination with lead. The lead addition is used to improve the wettability of the clad alloy during the brazing cycle. An important characteristic of these plating processes is that the nickel is preferentially deposited on the silicon particles of the clad alloy. To obtain sufficient nickel for brazing on the surface, the clad alloy should contain a relatively large number of silicon particles to act as nuclei for the nickel deposition. It is believed that to obtain sufficient nucleation sites before pickling a part of the aluminium in which the silicon particles are embedded should be removed by chemical and/or mechanical pre-treatment. This is believed a necessary condition to obtain a sufficient nickel coverage to serve as nuclei for the plating action of the brazing or clad alloy. On a microscopic scale the surface of the Si-containing cladding of the brazing sheet is covered with nickel globules.
However, the use of lead for the production of a suitable nickel and/or cobalt layer on brazing sheet has several disadvantages. The plating baths for electroplating are rather complex and due to the presence of lead comprising components such as salts thereof, these baths are much more environmentally unfriendly than plating baths comprising nickel- or cobalt-components alone. The use of lead for manufacturing products, such as automotive products, is undesirable and it is envisaged that in the very near future there might possibly even be a ban on lead comprising products or products manufactured via one or more intermediate processing steps comprising lead or lead-based components.
In the present application, unless otherwise indicated, all percent compositions are in weight percent.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a brazing product wherein the filler metal has a liquidus temperature below 570° C.
It is an object of the present invention to provide a brazing sheet product wherein th
Wijenberg Jacques Hubert Olga Joseph
Wittebrood Adrianus Jacobus
Corus Aluminium Walzprodukte GmbH
Koehler Robert R.
Stevens Davis Miller & Mosher LLP
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