GB1590836A

GB1590836A - Electrically heated apparatus

Info

Publication number: GB1590836A
Application number: GB4958376A
Authority: GB
Original assignee: Associated Engineering Ltd
Current assignee: Associated Engineering Ltd
Priority date: 1977-11-21
Filing date: 1977-11-21
Publication date: 1981-06-10

Description

(54) ELECTRICALLY HEATED APPARATUS (71) We, ASSOCIATED ENGINEERING LIMITED, a British Company of Ince House, 60 Kenilworth Road, Leamington Spa, Warwickshire CV32 6JZ, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is performed to be particularly described in and by the following statement: This invention relates to electricallyheated apparatus and to methods for its manufacture, the apparatus including but not being limited to cook-ware such as frying-pans, kettles and coffee-pots.

It is desirable to bond an electric element direct to the body of such apparatus, and while the good thermal conductivity of aluminium relative to ferrous metals is well known, problems have been encountered in joining an aluminium-sheathed electric element to an aluminium body. Fusion welding and cold pressure welding lead to gross melting and deformation, while with soldering and brazing problems arise due to the use of fluxes; fluxless brazing on the other hand requires expensive high-vacuum furnaces. Moreover in brazing the temperature control is very critical since it is necessary to work very close to the melting point of the aluminium or aluminium alloy.

From one aspect, the present invention consists in an electrically-heated apparatus having a body and a heating element including a resistance element contained within, and insulated from, a sheath, the body and sheath having surfaces of aluminium or aluminium alloy which are intimately bonded together by a metallurgical bond formed by having used a metal interlayer between said surfaces, there being no intermediate metal remaining between said surfaces in the bonded apparatus.

From another aspect, the invention consists in a method for manufacturing an electrically-heated apparatus having a body and a heating element including a resistance element contained within, and insulated from, a sheath, including placing surfaces of the sheath and the body in contact with one another in a jig, said surfaces being of aluminium or aluminium alloy with an interlayer therebetween, pressing together said surfaces at a temperature at least equal to the eutectic temperature of the combination of the material of the interlayer and the material of said surfaces or the one of the surfaces which has the lower melting point, but below the melting point of the surfaces, preferably in a protective atmosphere, whereby to cause eutectic diffusion bonding between the surfaces.

Preferably the interlayer is of copper or a copper rich alloy and the region adjacent the bond may contain an addition of copper originating from the interlayer.

Advantageously, part of the jig is shaped to conform to the cross-section of the sheath so as to join the sheath of the heating element to the body without damage to the heating element.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawing, which is a cross-section through part of an electrically-heated frying-pan illustrating its method of manufacture.

Referring to the drawing, the frying-pan has an aluminium body 11 of conventional shape, a handle (not shown) and an electric heating element 12. It may also be provided with feet (not shown) of thermallyinsulating material.

The heating element 12 is annular, and consists of a metal resistance element 13, through which current is passed, spaced within a thin triangular-section sheath 14 of commercially pure aluminium by means of electrically-insulating refractory granules 15.

In order to bond the heating element 12 to the body 11, the cooperating surfaces 16, 17 were manufactured to be flat and were degreased and cleaned. The surface 16 of the sheath 14 was copper plated to a thickness of 0.003 mm (0.0001 inch) and washed, and the surfaces 16 and 17 of the sheath 14 and the body 11 were placed in contact with one another, with the copper forming an interlayer therebetween, in a jig, of which one part 18 was such as to support the element 12 in such a way as to spread the load to be applied to it. Another part 19 of the jig was annular and was brought into contact with the upper face of the body 11 opposite the element 12.

The parts 18, 19 were loaded in a furnace press to give a contact pressure of 1.4 MNm-2 (200 Ib/sq.in) and were then heated to 6000C in a furnace in an inert atmosphere.

The surfaces 16, 17 of the sheath 14 and body 11 were intimately bonded together by a metallurgical bond without any intermediate copper remaining therebetween.

Destructive testing showed that the surface 16 of the element was fully bonded around its periphery to the surface 17 of the body 11, with a bond strength of the same order as the ultimate tensile strength of the metal, without damage to the resistance element 13, the insulation 15, or the sheath 14.

In cases where either the sheath 14 and/ or the body 11 is of an aluminium alloy having different melting points or a different melting point to aluminium, then the temperature to which the body 11 and heating element 12 are heated is at least equal to the eutectic temperature of the combination of the copper and whichever of the two materials has the lower melting point. In all cases the heating temperature is below the melting point of the material of the surfaces 16, 17, and the region adjacent the bond contains a small addition of copper from the interlayer which has diffused into the material of the body and the sheath.

WHAT WE CLAIM IS:- 1. An electrically-heated apparatus having a body and a heating element including a resistance element contained within, and insulated from, a sheath, the body and sheath having surfaces of aluminium or aluminium alloy which are intimately bonded together by a metallurgical bond formed by having used a metal interlayer between said surfaces, there being no intermediate metal remaining between said surfaces in the bonded apparatus.

2. Apparatus as claimed in claim 1, wherein the region adjacent to the bond contains an addition of copper originating from the metal interlayer.

3. A method of manufacturing an electrically-heated apparatus having a body and a heating element including a resistance element contained within, and insulated from, a sheath, including placing surfaces of the sheath and the body in contact with each other in a jig, said surfaces being of aluminium or aluminium alloy with an interlayer therebetween, pressing together said surfaces at a temperature at least equal to the eutectic temperature of the combination of the material of the interlayer and the material of said surfaces or the one of said surfaces which has the lower melting point but below the melting point of the surfaces, whereby to cause eutectic diffusion bonding between the surfaces.

4. A method as claimed in claim 3, wherein the interlayer material is copper or a copper-rich alloy.

5. An electrically-heated apparatus substantially as hereinbefore described with reference to the accompanying drawings.

6. A method of manufacturing an electrically-heated apparatus substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. which one part 18 was such as to support the element 12 in such a way as to spread the load to be applied to it. Another part 19 of the jig was annular and was brought into contact with the upper face of the body 11 opposite the element 12. The parts 18, 19 were loaded in a furnace press to give a contact pressure of 1.4 MNm-2 (200 Ib/sq.in) and were then heated to 6000C in a furnace in an inert atmosphere. The surfaces 16, 17 of the sheath 14 and body 11 were intimately bonded together by a metallurgical bond without any intermediate copper remaining therebetween. Destructive testing showed that the surface 16 of the element was fully bonded around its periphery to the surface 17 of the body 11, with a bond strength of the same order as the ultimate tensile strength of the metal, without damage to the resistance element 13, the insulation 15, or the sheath 14. In cases where either the sheath 14 and/ or the body 11 is of an aluminium alloy having different melting points or a different melting point to aluminium, then the temperature to which the body 11 and heating element 12 are heated is at least equal to the eutectic temperature of the combination of the copper and whichever of the two materials has the lower melting point. In all cases the heating temperature is below the melting point of the material of the surfaces 16, 17, and the region adjacent the bond contains a small addition of copper from the interlayer which has diffused into the material of the body and the sheath. WHAT WE CLAIM IS:-

1. An electrically-heated apparatus having a body and a heating element including a resistance element contained within, and insulated from, a sheath, the body and sheath having surfaces of aluminium or aluminium alloy which are intimately bonded together by a metallurgical bond formed by having used a metal interlayer between said surfaces, there being no intermediate metal remaining between said surfaces in the bonded apparatus.