Aluminium Can Now be Brazed
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D RAZING as a new method of repair ing sheet aluminium was discussed by Mr. C. H. B. Braddle, of the British Oxygen Co., Ltd., at a recent meeting of the Bradford Centre of the Institute of the Motor Industry.
Until comparatively recently, said Mr. Braddle, the only method of joining aluminium which his company recommended was welding in the form of a butt joint. This was because the welding flux was corrosive.
By the application of brazing to aluminium, however, it was now possible to use halt-i. p. lap, r double-lap joints. As one could braze only on to a metal which had a higher melting point than the filler rod, a rod different from those used for the brazing of steel, copper, or brass had to be employed, because it must have a melting point below 659 degrees C. The rod employed was an alloy of aluminium and silicon.
Brazing could not completely take the place of fusion welding, as applied to aluminium, because it was unsuitable for butt joints.
When discussing the technique of welding aluminium, Mr. Braddle said it was rather different from that for mild steel and required a more competent operator He suggested, however, that the fear with which the job was apt to be regarded was largely psychological. One reason why aluminium was rather difficult to weld was that through the welding goggles one could not see it change colour.
Another difficulty was that, whilst the melting point of aluminium was 659 degrees C., that of the metal's oxide was 2,050 degrees C. Thus, superimposed on a substance with a very low melting point was one with an exceptionally high melting point The flame setting must be strictly neutral, for excess of oxygen would cause a thickening of the layer of oxide; and the flux must be capable of melting at a temperature lower than aluminium's melting point, and attacking and breaking down the oxide, despite its much higher melting point.
The blow pipe should be held still, as in the case of steel but instead of using the rod with a piston-like backward motion, the operator should employ a forward brushing motion. This helped the flux to break the oxide layer and permit the metal to come through.