Fluxless brazing method and compositions of layered material systems for brazing aluminum or dissimilar metals

#The invention addresses the objective of achieving a cladless brazing material system, while maintaining a fluxless brazing system.

#Brazing commonly involves the use of aluminum-silicon clad aluminum brazing sheet composites. Because sophisticated rolling mill practices are required to produce this traditional composite, a premium cost is involved over conventional flat rolled sheet and strip. Also, available alloy compositions are limited by mill product standardization, by casting limitations, or by scrap recovery considerations that affect the economy of the overall casting or mill operation.

#Such conventional brazing alloys can be brazed using fluxless brazing systems, which typically use an electroplated braze-promoting layer. However, there are environmental hazards and liabilities associated with prior art wet electroplating systems for deposition of fluxless braze modifiers. Furthermore, there are limitations on the range of material strip or component dimensions which can be electroplated in high volume production, for example the constraints of fixed size plating cells limit the maximum plateable strip width.

#FIGS. 1 to 3 are photographs illustrating a brazed assembly according to a preferred embodiment of the invention.

#The present invention provides an in-situ filler metal forming material system that may eliminate the need for separately clad filler metal (or separately provided, for example as performs, etc), while maintaining a fluxless brazing method. The present invention also provides an adjustable material braze system, so that for eg, braze fillet size or fluidity may be adjusted according to the product requirements, or on different parts of the same product, for example opposite sides of the same brazing sheet.

#The inventors have also recognized that such a system can be applied to provide a range of filler metal compositions, so that both low and “high”, ie normal Al—Si braze temperatures, could be achieved in a fluxless format. The ability to tailor the material system (filler metal, and braze promoters . . . along with braze modifiers, bonding layers, and temperature modifiers) provides significantly increased flexibility in application to aluminum alloy systems that are either not now brazeable, or not available in forms suitable for brazing. These include, for instance, high alloy content 7xxx, 2xxx, 6xxx or 8xxx series aluminum, or aluminum castings and die-castings. Specific alloys to which a Si eutectic forming layer has been applied include 3003, 5052 (2.8% Mg) and 1100 alloys. The adjustable braze response characteristics are applicable to demanding product applications, such as internal joints of heat exchangers, or brazing of intricate flow field channels formed in metal plate fuel cell engines.

#The inventors have developed PVD deposition methods and layered sequence compositions, including ancillary methods to enable the practical achievement of “dry” material cleaning methods to allow preferred inline deposition processes. Successfully demonstrated dry cleaning techniques such as plasma or ion-cleaning are important steps in minimizing the environmental impact of the brazing process, and have been demonstrated to be practical as well.