Metal fusion bonding – Process – With pretreating other than heating or cooling of work part...
Patent
1994-11-01
1996-08-27
Heinrich, Samuel M.
Metal fusion bonding
Process
With pretreating other than heating or cooling of work part...
228261, 22826251, 228254, B23K 120, B23K10310, B23K 3102
Patent
active
055492398
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of providing particle retaining metal surfaces and more particularly to fluxing of aluminum-based components in order to make possible intimate wetting of their surfaces through removal of oxide films from the surfaces in a subsequent joining of the components, and to the resulting flux retaining metal components.
When aluminum or aluminum alloy (hereinafter both are denominated as "aluminum-based" so as to indicate the possible use of either) components are assembled, for example into a structure like a heat exchanger, they are conventionally fluxed prior to brazing in ambient air or neutral atmosphere such as nitrogen. Fluxes are employed to disrupt/remove a fine film of aluminum oxide readily formed on the aluminum surface.
This oxide film interferes with processes for applying a metallic coating to the surface as well as processes for joining the components by soldering, brazing or welding. The oxide film has to be removed and its reformation prevented in order to establish the metal-to-metal bonds.
Many alternative fluxes are conventionally used to eliminate the oxide layer. Chloride-based fluxes break up the oxide film, but must be subsequently rinsed away due to their hygroscopic and corrosive nature. Fluoride-based fluxes, e.g. mixture of potassium tetrafluoroaluminate and hexafluorotripotassium aluminate, are advantageous since they leave no corrosive residues.
A flux is normally suspended in water or another solvent, and coated onto surfaces of aluminum-based components by spraying or dipping. The flux application is normally part of an on-line process immediately prior to the joining operation. Conventionally, flux application inherently requires an additional step of drying in order to remove water or solvents from the components prior to heating to a brazeable temperature at which a brazing metal melts and the flux is active in order to join together two components. Such drying, however, disrupts the efficiency of the on-line process.
Furthermore, fluxing of the assembled components does not ensure a uniform distribution and access of the flux to the whole surface of the joined components.
An alternative method obviating use of a drying step is electrostatic deposition of dry flux powder upon the components prior to brazing. The electrostatic deposition is, however, inadequately bonded or adherent to permit storage or significant transportation of the components for any significant period of time prior no brazing. Consequently, the conventional art is limited to on-line fluxing.
However, industrial and transportation developments are such that it may be very desirable to extrude or otherwise fabricate and preflux components at one site or at one particular time, and braze them at another location.
U.S. Pat. No. 4,571,352 discloses a method of dipping an aluminum pipe into a molten brazing filler metal through a layer of floated molten flux. Thus a simultaneous coating of Al-pipes with brazing filler metal and application of a flux layer is claimed, avoiding use of brazing sheets in assembling of the components. The disclosed method based on retraction of coated pipes through a molten flux having a solidified surface layer has, however, several drawbacks. First of all, the flux is not properly bonded and can be mechanically removed so that a uniform distribution of firmly adhered flux material is not achieved. Furthermore, the activity of the adhered flux material after repeated melting is deteriorated so that an additional fluxing operation becomes necessary prior to the brazing operation.
It is therefore an object of the present invention to provide a prefluxed aluminum-based component applying conventional brazing/soldering flux in such a manner that the flux remains in place during subsequent transportation, storage and assembly of the coated components, thereby obviating any need for further application of flux.
Another object of the present invention is to provide components having flux present and uniformly distributed at each joint location.
Sti
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Morley Edward J.
Syslak Morten
Heinrich Samuel M.
Norsk Hydro a.s
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