In soldering large pieces, where the ordinary overlap is not allowable, and where a butt joint would be weak, fit the pieces together as at A ([Fig. 45]).
Fig. 45.—Aluminium Fitted Together for Soldering
Solder always flows towards the hottest point. This tendency enables one to direct its course under the blowpipe or blowlamp flame. A large flame should only be employed in “heating” up the part to be soldered on large and heavy work. With a small pointed flame directly on the solder and the parts on which it rests, the solder will flow quickly, and leave a smooth, even surface at completion.
Some aluminium solders now on the market are so hard that it is necessary to heat them and the work to redness before they melt. Sheet aluminium is easily warped by heat, and also contracts badly. If the solder is too high in melting point, the metal must also be brought to that point to cause proper union. If a hole is being filled in, the body of the metal on heating expands all round and partly closes the hole; also both the solder and the patch whilst hot are slightly expanded. In cooling, the hole enlarges, the patch contracts, and the solder also contracts; cracks result. The body of the work, if not exactly evenly made, will warp, which is fatal to engine and similar work. By using a low-heat solder (melting point, about 700° F.) these troubles should be avoided.
Soldering Aluminium to Copper or Brass.
—Aluminium can be readily soldered to copper or brass with fine solder (2 parts of tin and 1 part of lead): tin the metals, using stearin as flux previous to making the required joint. It is essential that both the “tinning” and soldering should be thoroughly done. Do not expect the solder to pull the joint together, but see that the joint is kept under slight pressure until the solder is hard, otherwise the joint will not be perfect. Many workmen refuse to place any reliance in such joints.
Finally, it seems very likely that, at any rate as regards factory work, the use of solder on aluminium objects will be wholly discarded in the future in favour of fusion welding or autogenous soldering, in which process no alloy is interposed between the surfaces to be joined. Information on the subject is given towards the end of this book.