Nickel-plated brazing sheet product

Details Nickel-plated brazing sheet product Nickel-plated brazing sheet product

2001-07-26

2003-05-27

Dunn, Tom (Department: 1725)

Metal fusion bonding

Process

With protecting of work or filler or applying flux

C228S247000, C228S183000, C228S262510, C428S650000, C428S654000

Reexamination Certificate

active

06568584

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a brazing sheet product in which a layer comprising nickel is plated onto a surface of a clad layer made of an Al—Si alloy containing Si in the range of 2 to 18 weight %, and wherein the brazing sheet product is devoid of a layer comprising zinc or tin as a bonding layer between said outersurface of said aluminum clad layer or layers and said layer comprising nickel. The invention also relates to a brazed assembly comprising at least one component made of the brazing sheet product and to a method of manufacturing an assembly of brazed components.
DESCRIPTION OF THE RELATED ART
For the purpose of this invention brazing sheet is to be understood as a core sheet, for example of aluminum or aluminum alloy, having on at least one side a brazeable aluminum alloy or filler material. Typical brazeable aluminum alloys useful as such a clad layer are the Aluminum Association (AA)4xxx-series alloys, typically having Si in the range of 2 to 18 weight %. The brazeable aluminum alloys may be coupled to the core alloy in various ways known in the art, for example by means of roll bonding, cladding, or semi-continuous or continuous casting, and thermal spraying.
Controlled Atmosphere Brazing (“CAB”) and Vacuum Brazing (“VB”) are the two main processes used for industrial scale aluminum 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 is mainly being caused by the evaporation of magnesium from the clad alloy. There is always more magnesium present in the clad alloy 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 aluminum 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 shaping 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 AA3xxx-series core alloy being clad on both sides with a cladding of an AA4xxx-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 aluminum. The nickel can be applied by using a shim of nickel between 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 of aluminum 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. 4,388,159, 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 is deposited in combination with lead. Alternatively, cobalt is deposited in combination with lead. It is known in the art that instead of nickel, cobalt or combinations thereof, also iron may be used. 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 or cobalt is preferentially deposited on the silicon particles of the aluminum clad alloy. To obtain sufficient nickel or cobalt for brazing on the surface, the aluminum 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 plating a part of the aluminum 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 and/or cobalt coverage to serve as nuclei for the wetting action of the filler alloy or aluminum clad alloy.
However, the use of lead for the production of a suitable nickel 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 are much more environmentally unfriendly than plating baths having nickel or cobalt comprising 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 processing steps comprising lead or lead components.
Some other disclosures of Ni-plating found in the prior art literature will be mentioned below.
General textbook by Wernick and Pinner, “The Surface Treatment and Finishing of Aluminum and its Alloys”, 5th edition, Volume 2, pp.1023-1071. This textbook describes in general immersion processes for plating on aluminum.
FR-A-2,617,868 describes a method of manufacturing aluminum product with a brazeable surface coating of tin or a tin-bismuth alloy, wherein the product is provided with an intermediate layer. This intermediate layer is composed of a first layer of zinc and a second layer of nickel, which nickel has been deposited by electrolysis from a neutral electrolyte. Here, the underlying aluminum or aluminum alloy is not melted in the subsequent brazing process.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a brazing sheet product having a core sheet and filler material, to be melted during brazing, said filler material comprising an aluminum clad layer of an Al—Si alloy and a nickel layer on the aluminum clad layer, in which there is no further need for the addition of lead to the layer comprising nickel while maintaining good brazeability of the brazing sheet product into an assembly.
It is an object of the invention to provide a nickel-plated brazing sheet product which can be used in a vacuum brazing process as well as in an controlled atmosphere brazing process in the absence of a brazing-flux, but ideally suitable for CAB process in the absence of a brazing-flux.
It is another object of the invention to provide a method of manufacturing an assembly of brazed components using the brazing sheet product of this invention.
In accordance with the inventi

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