—Copper to be welded should have its edges bevelled to enable the welding to penetrate the entire thickness of the metal. Bevelling is not generally practised below a thickness of 3⁄32 in. From 3⁄32 in. to 3⁄16 in., a slight open bevel is sufficient; 3⁄16 in. thick and over, the angle of the bevel should be about 90°. It is not necessary to go beyond this even with great thickness. The bevelling should be regular, especially at the bottom, so as not to produce holes or excess of thickness at the bottom of the bevel.
The edges to be welded and their immediate neighbourhood should be thoroughly cleaned. This can be done with a file, scraper, or sheets of emery. Chemical agents such as spirits of salt or nitric acid are sometimes employed; but it is preferable to precede their use by a mechanical cleaning.
Before beginning the welding the parts should be carefully arranged so that during the welding operation they remain perfectly in position. Owing to the high conductivity of copper, a relatively larger blowpipe tip must be used than when welding either iron or mild steel of the same thickness. The power of a blowpipe of 225 litres with an approximate consumption of 7·75 cub. ft. of acetylene per hour would be suitable, with economical results, for iron or mild steel 1⁄8 in. thick, whereas for copper of the same thickness the power of the blowpipe should be of 300 litres, having an approximate consumption of 10·5 cub. ft. of acetylene per hour. Also, a blowpipe which is too strong tends to melt the metal too rapidly. This should be as carefully avoided as that of melting too slowly.
A pure copper welding rod may be employed for filling in, but it is not so effective as a welding rod made of phosphor copper. The phosphorus is incorporated in a very small quantity, so that none remains in the weld after its execution. A filler rod which contains too much phosphorus lacks fluidity, and melts at a temperature much lower than that of the copper to be welded, thus facilitating adhesion. Moreover, the welds in which the phosphorus remains lack elongation, and therefore do not possess the same mechanical properties as pure copper. The welding rod after 1⁄16 in. of its diameter should be about equal to the thickness of the weld, although in practice feeders about 1⁄4 in. in diameter are not generally employed. Welds made on copper without a deoxidising welding rod properly prepared have a tendency to oxidise, and therefore do not possess the required qualities. In addition, the surface of the metal must be covered with a carefully prepared mixture of potassium phosphate and potassium carbonate to a depth of about 1⁄16 in. Upon the application of the flame, the mixture will melt and form a glaze over the surface of the copper, thus preventing oxidation and assuring good work.
A flux consisting of chloride of sodium, sodium borate, and boracic acid is also recommended. The flux should be sparingly applied by dipping the end of the welding rod into the vessel containing the flux. The end of the rod should be warmed in order that the flux adheres.
Before beginning the actual operation of welding, it is essential to raise the edges of the weld and the parts in the vicinity to a high temperature. The high conductivity of the metal necessitates this, as any supply of molten welding rod before the edges are in a molten state inevitably produces adhesion. The flame of the blowpipe should be perfectly regulated and maintained without excess of either acetylene or oxygen. 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. The distance of the white jet should vary according to the power of the blowpipe, say from 3⁄16 in. to 3⁄8 in. If this distance is increased, the gases resulting from the second phase of combustion, carbonic acid and water vapour, influence the weld. Care must be taken that the fusion of the metal should not be undertaken until the edges of the weld and the parts near have been raised to a high temperature. At this moment the welding rod and the parts to be joined should be melted simultaneously. It is essential that the welding rod should be regularly incorporated in the line of welding, and must not be allowed to fall in drops. The operation should be continuous, taking care to attack regularly the two edges of the metal. The welding is thus executed rapidly.
It is well known that internal strains are always set up in every process of welding, due to the expansion and contraction when a metal is heated and cooled. Copper lacks tenacity when heated; hence contraction of the metal, whose coefficient of expansion is also fairly high; fractures thereby are often produced, especially in the welded part. However, pre-heating the article to a high temperature, maintaining the heating after the operation of welding and slow cooling, enables one in many cases to avoid fractures due to contraction. It is also necessary to hammer the line of welding and its vicinity. After the hammering operation it is essential to reheat the copper, raising it to redness (500° C. to 600° C.). Then plunge into cold water, or cool as rapidly as possible. The structure of the weld is not quite as homogeneous as other parts of the piece welded. This is, however, controlled largely by the skill and workmanship of the operator, who can, at will, make the weld more or less homogeneous.
It is impossible to enumerate in anything like detail all the work in copper which may be executed by oxy-acetylene autogenous welding. However, copper-smiths are advantageously making great use of the system, thereby replacing their old methods of brazing and riveting.
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.