Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming multiple superposed electrolytic coatings
Patent
1995-06-01
1997-09-09
Gorgos, Kathryn L.
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming multiple superposed electrolytic coatings
205138, 205139, 205140, 205149, 205213, 205263, 205293, 205300, C25D 510, C25D 706, C25D 544
Patent
active
056652194
DESCRIPTION:
BRIEF SUMMARY
This application is a national stage application of PCT/FR93/01148 filed Nov. 22, 1993.
The present invention relates to a process for continuous manufacture of an electrical conductor at least partially based on aluminium coated with copper and tin.
The invention also relates to an electrical conductor consisting of an aluminium-based central core comprising a metal coating capable of being brazed and resistant to oxidation, consisting of a layer of copper and of a layer of tin.
Aluminium is a metal which offers a good compromise between conductivity, mechanical strength, mass and cost.
Use of conductors made of coated aluminium for manufacturing electrical cables is increasingly widespread in the aeronautics and space industries.
However, the development of aluminium conductors for small-section cables is more difficult and makes it necessary to solve a number of technical problems. The major difficulty stems from the fact that the aluminium central core must be coated to be capable of being resistant to oxidation and brazable with tin alloys. Now, the deposition of a metal layer on aluminium either by an electrolytic route or by immersion in a hot bath is found to be very difficult because of two phenomena which occur during the surface treatments.
The first relates to the chemical displacement of metals on the aluminium because the latter has a very negative electrochemical potential with regard to the majority of metals.
The second is the spontaneous formation of an oxide film on the aluminium surface, this happening even at ambient temperature.
These two phenomena prevent the metal layer from adhering well to the aluminium substrate.
As a result, during soft brazing operations with tin alloys at temperatures of between 210.degree. C. and 250.degree. C. the metal layer which bonds with the filler metal in the molten state tends to separate off from the substrate, thus resulting in a rupture of the soldered joint.
Enormous work has been done with a view to overcoming the difficulties encountered in electrodeposition on an aluminium wire. Some special treatment processes have been established which make it possible to deposit, for example, a nickel coating. However, nickel-coated aluminium wire exhibits a fairly mediocre brazability with tin solders, and this constitutes a major handicap for its electrical application. More recently, research conducted in this field has made it possible to carry out the electrodeposition of silver on an aluminium wire, and this gives it good brazability.
The paper "Electroplating on Aluminium Wire", pages 67-71 of the Transactions of the Institute of Metal Finishing, vol. 61 (1983) describes a process for electrochemical coating of an aluminium wire of 2.1 mm diameter with an underlayer of copper and a layer of tin.
This process consists in pretreating the surface of the aluminium substrate by immersion in various baths for degreasing and priming respectively.
The substrate is next coated with copper by electrodeposition in a first bath at 60.degree. C., containing copper pyrophosphate and potassium pyrophosphate and then the copper coating itself is coated with tin by electrodeposition in a second bath, at ambient temperature, containing tin sulphate and sulphuric acid.
The analysis of the aluminium wire tin-plated according to this process shows that the adhesion between the copper underlayer and the aluminium substrate, as well as that between the tin layer and the copper underlayer are not satisfactory, and this results in problems of brazability of the conductor.
The brazability of a conductor wire is expressed as its wettability by a molten solder. In other words, the bonding of the molten filler metal to the conductor takes place correctly when the surface of the latter is wetted sufficiently by the said liquefied filler metal. The wettability is related to the so-called wetting angle formed by the surfaces of the conductor and of the solder meniscus respectively at their point of junction. The smaller the wetting angle, the better will be the wettability of the
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Axon'Cable SA
Gorgos Kathryn L.
Wong Edna
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