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 | Sil- | Alu- |Cop-|Bis-|Phos- |Cad-| Lead |Anti-
| | ver | min- |per |muth|phor |mium| |mony
| | | ium | | | Tin | | |
----+------+-------+-------+----+----+------+----+------+-----
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 | -- | -- | --
----+------+-------+-------+----+----+------+----+------+-----
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).
[Illustration: 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.