Metal fusion bonding – Process – Plural joints
Patent
1995-12-06
1998-05-26
Heinrich, Samuel M.
Metal fusion bonding
Process
Plural joints
228223, 22826243, 228254, 148 24, 427310, 165133, B23K 100, B23K 35365
Patent
active
057553742
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for the joining together by brazing of two or more metal articles.
Brazing processes for the joining together of two or more articles made of metal such as aluminum-based material are well known. However, in the case of aluminum materials, all the known methods have disadvantages relating to difficulty of process control, and hence reliability of the brazed joint, limitations on materials which may be employed, disadvantages of cost or any combination of these.
One known process, widely employed for the production of aluminum heat exchangers, such as radiators for vehicles for example, requires that the aluminum material of which the radiator is composed is first clad with a brazing alloy of a second aluminum alloy having a lower melting point than the substrate aluminum material. The requirement for cladding, usually carried out by a cold-pressure welding technique such as roll-bonding, is very expensive, and brings with it further disadvantages.
Because of the number of different sized tubes and components used in a radiator assembly, it is only economically feasible to carry out the cladding process on sheet or flat strip material from which the various radiator components are then fabricated.
In the case of tubular components, this necessitates a seam-weld which causes the substrate aluminum material and the cladding alloy to fuse together thus lowering the melting point of the resulting alloy in the welded region, relative to the substrate material, through the complete wall thickness of the tube component in the heat affected zone (HAZ). This results in the generation of high levels of scrap due to the penetration of the braze into the welded tube in the HAZ causing cracks and/or pinhole porosity.
The clad brazing alloy is usually an aluminum-silicon alloy of around eutectic composition. Silicon from the cladding alloy tends to diffuse into the substrate material over the entire clad area lowering the melting temperature even before the clad brazing alloy has melted. This exacerbates the problem of penetration and porosity.
Because of the method of forming the radiator tube sections, they are relatively weak, and even those tubes which do not have readily apparent leaks may be relatively weak and fail prematurely in service due to fatigue from flexing due to the fluctuating water pressure in the radiator. In practice, the use of higher strength aluminum alloy having better fatigue resistance is precluded in the known brazing processes because the fluxes which are employed are known to react with the alloying constituents negating the fluxing effect and also adversely affecting the melting behaviour of the aluminum alloy.
U.S. Pat. No. 3,951,328 describes a process for brazing aluminum heat exchangers as described above. This document describes the use of a flux of potassium fluoaluminate formed by fusing together potassium fluoride and aluminum fluoride such that no free potassium fluoride remains. The fused mixture is then ground into a powder and used as a flux. The document also mentions that the flux powder may be combined with aluminum alloy braze powder to form a physical mixture of the different types of powder particle and used to braze unclad aluminum material components. However, the quality of joint produced by this latter method is relatively poor and has a high reject rate. Thus, industrial processes currently in use employ the clad brazing alloy route.
EP-A-0140267 describes a method of brazing an aluminum material by forming thereon a flux layer of potassium pentafluoaluminate by electrochemical deposition from an aqueous solution to produce a chemical conversion coating either on the material to be joined together or on braze particles.
It is an object of the present invention to provide a reliable brazing process which does not require the aluminum substrate material to be clad.
It is a further object of the present invention to provide a brazing process which allows the use of extruded tube sections having higher strength and/or made from al
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Heinrich Samuel M.
Lexor Technologies Limited
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