Brazing sheet is a specialised product used in the construction of heat exchangers-for example, car radiators. It is made up of a core sheet of aluminium alloy, containing for example between about 1.0 and 2.5% of alloying elements, that is clad on either one or both sides with a layer of an aluminium alloy of lower melting point. The core provides the required strength. The outer layer, having a lower melting point, provides the metal for brazing (i. e. the process of joining together two pieces of metal, such as the tube and the fins of a radiator, by fusing a layer of a metal between the adjoining surfaces). Usually the core layer is an Al-Mn alloy, typically an AA3000 series alloy such as AA3003, or an alloy of the type disclosed in EP-A-691898 (such as X900 type) or EP-A- 326337 (such as X800 type) and the cladding is aluminium alloy containing 7-12% Si, for example AA4343 or AA4045 for flux brazing and AA4104 for vacuum brazing.

Each layer of cladding accounts for about 5-15%, typically 10%, of the thickness of the brazing sheet. The final thickness of the sheet can be around 0.5 mm or less. The difference in melting point between the cladding and the core is usually about 60°C.

In the production of brazing sheet, the clad layer is applied by hot rolling an ingot of the core material faced on one or both of the major sides with a plate of the cladding alloy. The rolling deformation bonds the

layers very firmly together. They cannot readily be pulled apart. During the rolling process, some material is inevitably scrapped. This scrap has little value since it can be recycle into few other wrought aluminium alloys.

Normally it is sold off either to go into low value applications, such as foundry alloys, or for killing steel.

Previous attempts have been made to reclaim brazing sheet scrap. According to one type of approach, the scrap is completely melted and the high Si content reduced by selective recrystallisation. Such techniques can be complex and have met with only limited success. Other approaches have been tried. JP-A-10219364 makes use of induction heating to preferentially melt the clad layer. JP-A-8013049 describes a technique whereby the scrap is heated until the clad layer is"fused"and flows down to a lower edge of the scrap where it is collecte as a fillet or bead. Presumably, this bead is then sheared off after cooling. The resulting scrap core is expected to retain a film of cladding held in place by surface tension. The bead of cladding will be contaminated with residual core metal.

US 4,203,762 describes a process for processing aluminium clad ferrous substrates to separately recover the aluminium and ferrous components in re-usable form. The clad steel sheet is hung vertically and heated to such high temperature that the aluminium melts and drips off.

The process may include a step which involves the jarring or vibration of the clad sheet and this is said to facilitate dislodgement and gravitational drainage of the molten aluminium from its surface. The process is not suitable for separating the cladding layer from the core of brazing sheet.

There is a need for an economic and straightforward way of recovering the core alloy and the cladding alloy from process scrap arising during hot rolling. This need has become much greater in recent years for the following reasons. Firstly, the use of higher strength core alloys, such as X900 type and X800 type mentioned above, in the production of brazing sheet results in the cladding being smeared out more during the rolling

process. This leads to the formation of a trailing length of cladding hanging over the back of the hot rolled bands. Immediately ahead of that, the cladding is too thin. There is therefore more scrap being cut off and, in consequence, more scrap that it would be desirable to recycle. Secondly, core alloys such as X900 type and X800 type are made from smelter metal to give the required corrosion resistance. Only if the purity of this metal can be preserved by the recovery process, is it possible to recycle such scrap into new core material. Previously, the scrap could be incorporated into core material made from remelt metal. The present invention seeks to meet that need.

According to the present invention there is provided a process for separating the clad layer from the core of brazing sheet and which comprises heating the sheet until the clad layer becomes liquid or semi- liquid but the core remains solid, removing the clad layer in its liquid or semi-liquid state and then recovering the same, and wherein the said clad layer is removed by mechanical means which apply a shear force directly to the clad layer. The invention also provides core alloy and/or cladding alloy which has been recovered from brazing sheet scrap by the aforesaid process, and products made from the said core alloy and/or cladding alloy.

The whole process can be carried out in a reverbatory furnace. The brazing sheet is supported, preferably in a substantially horizontal position, in the furnace which is then heated to a temperature between the solidus of the cladding and the solidus of the core. Depending upon the alloys used, the solidus of the cladding will be about 577°C (for AA4045 and AA4343) and the solidus of the core is from 643-654°C (for AA3003). Preferably, the temperature is held as low as possible to retain the maximum strength in the core but sufficiently high to render the cladding at least partially fluid. The clad layer can then be mechanically removed while it is in this liquid or semi-liquid state and recovered. When sufficient of the cladding has been removed, the core material can be recovered.

The removal of the liquid or semi-liquid clad layer from the brazing sheet is performed by mechanical means which apply a shear force directly to the clad layer. For example the mechanical means may comprise a hand held or automatically operated scraper. The cladding is scraped off the surface of the core material and then recovered.

Alternatively, the mechanical removal may be achieved by the use of gas jets or high pressure flames. The removal of the clad layer is thus effected by a mechanically applied shear force in the plane of the brazing sheet. It is considered surprising that the clad layer can be successfully removed and recovered in this way. The operation is being carried out at a temperature approaching 600°C on a substrate that is close to its melting point and which is very frail. Specifically, it is most surprising that such a frail substrate can be scraped sufficiently hard to effectively remove the liquid or semi-liquid layer. It is even more surprising that the cladding can thereby be removed to such an extent that the core material can be reused (recycled) for alloys such as X900 type and X800 type requiring low silicon contents.

The process enables both the clad layer and the core material to be recovered or reclaimed from brazing sheet scrap and they can then be reused or recycle. It is considered to be particularly surprising that the layer of cladding can be effectively recovered in this way since it was to be expected that this material would have become heavily oxidised by the heating in the furnace. Most surprisingly, the recovered cladding is clean enough (not only from the comparative lack of oxidation, but also sufficiently uncontaminated by the core material) to allow it to be used either in cladding alloys or as a source of high silicon alloy for other applications. The ability for the first time to reuse or recycle both of these components of brazing sheet scrap, by means of a process that is both simple to operate and efficient, is most significant. The process of this invention thus satisfies a long felt want in this branch of the aluminium industry and is expected to produce substantial cost savings.

The process works only with the thicker scrap when the clad layer is thicker than about 0.3 mm. In laboratory tests on sheet about 3 mm thick, it has been found that the clad layer could not be removed.

Very thin liquid layers seem to be held in place by surface tension forces that are difficult to overcome. Plate about 50-75 mm thick can be treated by the process and this accounts for about 33% of all process scrap.

The process of this invention is illustrated by the following example.

EXAMPLE Individual pieces of scrap 50-75 mm thick by about 1 to 5 m2 surface area per side were supported substantially horizontal on rails in an oil fired furnace. The temperature was slowly raised until it was observed that the clad layer began to melt. The resulting semi-liquid was then mechanically removed with a scraper and recovered. It could be recycle as cladding material. When sufficient of the clad layer had been removed, the core was recovered and re-melted as core material.