CHAPTER VI
Soldering Aluminium
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
| Tin | Zinc | Silver | Alu- mi- nium | Cop- per | Bis- muth | Phos- phor Tin | Cad- mium | Lead | Anti- mony | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 72 | ·5 | 25 | -- | 1 | ·5 | -- | -- | 1 | -- | -- | -- | ||||||||
| 80 | 20 | -- | -- | -- | -- | -- | -- | -- | -- | ||||||||||
| 97 | -- | -- | -- | -- | 3 | -- | -- | -- | -- | ||||||||||
| 20 | -- | 10 | 70 | -- | -- | -- | -- | -- | -- | ||||||||||
| 90 | -- | -- | 10 | -- | -- | -- | -- | -- | -- | ||||||||||
| 65 | 27 | 5 | ·75 | 2 | ·25 | -- | -- | -- | -- | -- | -- | ||||||||
| 30 | 20 | -- | -- | -- | -- | -- | 50 | -- | -- | ||||||||||
| 99 | -- | -- | -- | 1 | -- | -- | -- | -- | -- | ||||||||||
| 90 | -- | -- | -- | 9 | 1 | -- | -- | -- | -- | ||||||||||
| 6 | 77 | ·5 | -- | 3 | ·25 | -- | -- | -- | -- | 3 | ·25 | -- | |||||||
| -- | 90 | -- | 6 | 4 | -- | -- | -- | -- | -- | ||||||||||
| -- | 80 | -- | 12 | 8 | -- | -- | -- | -- | -- | ||||||||||
| -- | 80 | -- | 20 | -- | -- | -- | -- | -- | -- | ||||||||||
| -- | 90 | -- | 5 | -- | -- | -- | -- | -- | 5 | ||||||||||
| 80 | 17 | -- | 2 | ·25 | -- | -- | ·75 | -- | -- | -- | |||||||||
| 75 | 22 | -- | 2 | ·5 | -- | -- | ·5 | -- | -- | -- | |||||||||
| 70 | 25 | -- | 3 | -- | -- | 2 | -- | -- | -- | ||||||||||