Electric Connectors and methods of making themThis invention relates to electrical connectors, more especially bimetal connectors for interconnecting aluminium and copper conductors, and methods of making them. Conductors of overhead lines and cables for the bulk distribution of electricity are normally made of aluminium (or a high-conductivity aluminium alloy, hereinafter included in the term "aluminium") on account of economic factors and its high conductivity per unit mass. On the other hand, internal wiring in consumers' premises normally uses copper conductors as they more easily form reliable connections, especially in small conductor sizes.
At some appropriate point, therefore, a transition has to be made from an aluminium conductor to a copper one.
For this purpose bimetal connectors of appropriate geometry are used, comprising an aluminium part and a part of copper or a copper alloy (such as brass). The parts need to be secured together in a way that ensures a stable low-resistance current path between them. Mechanical connections are not considered satisfactory, because of the risk of "creep" under thermal cycling resulting in the loss of contact pressure, and bonding processes involving the use of molten metal are excluded by the risk of forming brittle, high resistance Cu/Al phases at the interface and/or by the natural oxide film on the aluminium inhibiting contact.
Present practice is therefore to use bimetal connectors formed by solid-state friction-welding of aluminium and copper (or copper alloy) rods or tubes.
This is expensive and limits design flexibility as the actual aluminium/copper interface must almost inevitably be a nominally plane surface perpendicular to the axis of the initial rods/tubes.
In accordance with the invention, an electric connector comprises a bulk component made of aluminium and a coating on a part of the bulk component made of copper or a conductive copper alloy and applied by thermal spray coating.
In most cases the bulk component can be a standard connector for aluminium conductors, and the invention includes a method of making a bimetal electric connector comprising first forming a connector of aluminium and then coating part of its surface with copper or a conductive copper alloy by thermal spray coating.
Various thermal spray coating techniques can be used, such as plasma spraying, gas-wire metallising and the proprietory technique offered under the designation "Thermospray" by Metco Limited of Chobham, Surrey GU24 8RD: (and in the USA by the Metco division of Perkin-ElmerCorp, of 1101 Prospect Avenue, Westbury NY 11590).
Coatings of pure copper are preferred, but brass might be used in some cases.
If, as is normally the case, the thermal spray coating is porous, we very much prefer to seal it by the application of a suitably penetrating high-solids liquid sealant, several of which are available and recommended for sealing thermal spray coatings used for other purposes. The sealant may also be applied as a corrosionresisting insulating layer extending across the edge of the coating, avoiding the need for the paint banding operation normally used on conventional bimetal connectors. Excess sealant may be abraded away, if found necessary, from the contact areas of the coating.
The invention can be used to make bimetal connectors of many shapes and sizes to suit particular applications.
One important style of connector comprises a tubular part for compression jointing (crimping) onto an aluminium conductor and a flat part (usually referred to as a "palm") with one or more apertures to facilitate bolting to a relatively rigid conductor or terminal of copper or brass; in this case it is preferred that the whole of the area of the palm (including the surface(s) of the hole(s), in particular) plus the transition area between the palm and the tubular part to be coated with copper or copper alloy as described. This results in the formation of a surface bimetal interface in a position where a corrosionresisting seal (either the sealant already described or a separate band of coating, such as paint) can be easily applied and easily inspected.
The invention will be further described by way of example with reference to the accompanying drawings in which figures 1 and 2 are mutually perpendicular view of one form of bimetal connector in accordance with the invention.
The connector comprises a length of electrical grade aluminium tube 1 flattened at one end to form a palm 2 through which are formed holes 3,3 (of circular or other suitable cross-section) for the reception of bolts. As so far described, the connector is a stock item for crimping to a circular aluminium conductor and bolting to other conductors of aluminium or compatible material. In accordance with the invention, the entire surface area of the connector to the left of the line 4 is coated with copper by plasma spraying to the "P55-10" process operated by Netco Limited. Following this operation, the copper coating is sealed by spraying with the 95% soldis airdrying glyceride resin solution sold by Metco Ltd under the designation Metco AG. This sealant is applied not only to seal pores throughout the copper coating but also to form an insulating and corrosion-resisting layer extending for at least a few millimetres on each side of the edge of the coating as a corrosion inhibitor. After standing in air for 15 minutes, excess sealant is wiped from the flat areas of the palm 2 with a clean dry cloth.
It may be desirable, with this particular sealant, to allow a week's storage at ambient temperature (or an hour at 1450C) after evaporation of solvent is complete) for curing, prior to use of the connector.