Welding Aluminium.
—In preparing aluminium to be welded, the edges must first be thoroughly cleaned and the welding rod very pure, so as to avoid the incorporation of impurities, which is apt to bring about rapid disintegration in the line of welding. Bevelling the edges to be joined is not necessary below a thickness of 1⁄8 in. From 1⁄8 in. to 3⁄16 in. a slight open bevel is sufficient, 3⁄16 in. thick and above angle of bevel should be about 90°. For thin sheets up to a maximum of 3⁄32 in., welding is facilitated by flanging the edges at right angles. The depth of the flange should be slightly deeper than the thickness of the metal. By this method no welding rod is required, the edges being simply fused. The weld should afterwards be hammered level.
Aluminium should never be welded without a flux. If welding is attempted without a flux, globules consisting of aluminium within and a coating of alumina (oxide of aluminium) will appear. In order to eliminate these by the blowpipe flame it would be necessary to raise the temperature to the melting point of the oxide of aluminium, which is nearly 3,000° C., whilst the melting point of metallic aluminium is only 657° C. To produce a flux which will dissolve the oxide at the low melting point of the metal and at the same time protect the hot metal from contact with the air has obviously not been a simple problem to the chemist and engineer. However, several good fluxes are now obtainable which enable any experienced welder to effect satisfactory welds in aluminium.
A flux consisting of the following ingredients can be recommended: sodium chloride 30 parts, potassium chloride 45 parts, lithium chloride 15 parts, potassium fluoride 7 parts, and bisulphate of potassium 3 parts.
When making fluxes for the welding of aluminium, great care is necessary in order to completely dry the ingredients, thus avoiding their combination with each other. On aluminium above 3⁄32 in. thick, the flux is best applied by dipping the end of the welding rod into the vessel containing the flux. The end of the rod should be first warmed in order that the flux adheres. The welding rod after 1⁄16 in., its diameter should be just about equal to the thickness of the weld, although in practice feeders above 1⁄4 in. diameter are not advisable.
In executing the weld, care must be taken to avoid contact of the white jet of the blowpipe flame with the metal just about to be melted, because the high temperature of this part tends to produce holes which are difficult to fill in. The distance of the white jet should vary according to the power of the blowpipe, say from 1⁄4 in. to 3⁄4 in. The flame should be so adjusted as to furnish an excess of acetylene. There need be but little fear of carbonising the metal, for the reason that the temperature of the work is comparatively low. For thin welds, up to 1⁄8 in. thick, it is preferable to hold the welding rod in front of the blowpipe in the direction of the edges to be welded. As soon as the latter begins to melt it is heated rapidly, and should be lowered to form one molten bath with the metal of the piece. The welding is thus done very rapidly. For great thicknesses it is preferable to obtain fusion of the welding rod, previously heated in the molten bath of the bevel. Directly after welding, the weld should be thoroughly washed in clean warm water in order to remove all remaining traces of the flux, which would otherwise continue to have a chemical action on the metal, thereby setting up corrosion.