It is well known to those accustomed to the art of soldering that there is no solder which operates with aluminium in the same way that ordinary solders operate with tinplate, copper, brass, etc. Aluminium soldering presents so many difficulties that it has been thought desirable to devote a separate chapter to the subject.

There is more than one reason for the difficulty encountered. Aluminium does not alloy readily with solders at temperatures as low as other metals require; and, secondly, aluminium alloys with lead solders only with great difficulty, and with but a small proportion of lead at that. Consequently, lead solders are not suitable for aluminium. Another and even more serious reason is in respect to the refractory oxide which forms at soldering temperatures, and which is undoubtedly responsible for most of the trouble.

The soldering of aluminium is one of the most debated subjects in metal working. Almost as soon as aluminium was prepared on a large scale, it was discovered that the ordinary solders and fluxes did not answer with it. Either pure tin or pure zinc will wet aluminium, and can, therefore, be used as solder for it; experience shows that the tin soon falls apart, while zinc by itself is brittle and discolours badly. The failure of tin is due to the fact that it forms with the aluminium an alloy that is decomposed by the action of the oxygen present in the air.

Although aluminium is popularly supposed to be non-oxidisable, really the surface is covered with a very thin film of oxide, which prevents solder from alloying with the metal. Aluminium when heated rapidly oxidises. It is customary to scrape the metal before and during the soldering; and although some workers say that it is useless to scrape before soldering because oxidation immediately starts again, it is obvious that a thin film is more easily penetrated than a thick one. Often it answers to scrape with the copper bit during the soldering, previously rubbing off the oxide with emery cloth. The work should, if possible, be backed with asbestos, to keep up heat in the metal. To discover whether the surface is thoroughly tinned, wipe off lightly, and the untinned parts will then soon become apparent. If the oxide is not scraped off beforehand, it will probably mix with the solder and form a scum, which will make a neat flow difficult. Scum should be lightly removed with an old knife blade. It is essential to “tin” every part to be joined, as the solder will not take on any spot that has not been rubbed in some way, unless previously coated.

Solders for Aluminium.

—Hundreds of aluminium solders have been invented, naturally all claimed to be strong and durable, the alloys containing various metals, such as aluminium, antimony, bismuth, cadmium, chromium, copper, lead, manganese, silver, phosphor tin, tin, and zinc. A table of the most approved aluminium solders is here given.

Many of the best solders for aluminium contain a small proportion of phosphor tin. A molten alloy containing phosphorus placed on aluminium tends to absorb oxygen from the impure film as well as the surrounding air.

Compositions of Aluminium Solders

In making the solders here given, it is advisable to avoid loss of the more easily volatile of the metals by adopting the following precautions: The aluminium is melted first, the zinc is added in small pieces, then tin in small pieces, and lastly the phosphor tin.

Inasmuch as zinc alloys with aluminium more readily than does any of the common metals, solders that will readily “tin” aluminium generally contain zinc in varying proportions. The solders found most satisfactory contain zinc, tin, aluminium, and a very small proportion of phosphor tin; but they do not run very freely or fuse so readily as the ordinary tin and lead solders, and it is necessary to use a higher temperature, so high, in fact, that difficulty is found in using these solders with a soldering bit, and it is generally necessary to use a blowlamp.

While there is no solder that allows aluminium to be soldered with the facility and success experienced with other metals, that of Richard’s is extensively used, and seems to have given as good results as any. It consists of the following ingredients: Tin 29 parts zinc 11 parts, aluminium 1 part, and 5 per cent. phosphor tin 1 part—practically the same as that given in the last line of the table. This solder has withstood the test of time better than many of the patented solders, and can be used in jointing aluminium to aluminium, also aluminium to copper or brass, and without the use of a flux. In making the solder it is advisable to avoid loss of the more easily volatile of the metals. The aluminium should be melted first, then the zinc, tin, and phosphor tin in the order named.

When using phosphorus instead of phosphor tin in the making of aluminium solder, it will first be necessary to incorporate it with the tin, for which purpose take a length of 1-in. gas barrel, attach a screwed cap at one end, and close the opposite end with a tin (not tin-plate) plug. Remove the screwed cap, and, having carefully dried between blotting paper the proper proportion of phosphorus, insert the latter in the tube and replace the cap. Now put the plugged end of the tube into the molten tin; this will melt the plug of tin and so allow the phosphorus to come in contact with the molten metal. The ingot of phosphor tin formed is afterwards alloyed with the other ingredients, as already explained.

Fluxes for Aluminium.

—A large variety of fluxes have been tried with more or less success, namely, borax, copper chloride, lithium chloride, paraffin resin, sal-ammoniac, stearin, silver chloride, tin chloride, venetian turpentine, tallow, vaseline, and zinc chloride. Stearin is undoubtedly the most reliable of them all, but no flux is needed for solders containing phosphorus, which is itself a flux.

The Soldering Bit.

—The average temperature required to make a satisfactory and thoroughly sweated joint in aluminium is from 650° F. to 680° F., according to the size of the article. A blowpipe or blowlamp will be of great value, and is frequently preferable to a bit. Should a bit be used, see that it is of aluminium or nickel instead of copper, the point and the soldered joint being kept much cleaner whilst removing the film of oxide during the soldering operation. Another advantage is that the point or “face” of the bit can be “tinned” with the same flux as that which is used for the joint. More care must be taken in the manipulation of the aluminium soldering bit owing to its lower melting temperature than the copper and nickel bits.

The Process of Soldering Aluminium.

—The soldering of aluminium must be performed quickly to be satisfactory, as the metal, if not coated at the first attempt, may be injuriously affected. “Tinning” the parts required to be soldered first is another important factor; also the distance of the overlap of the joints should not exceed more than ¹/₈ in., so as to allow the solder to flow thoroughly through; it does not flow so readily as when soldering other metals.

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