This is a separate page partly because soldering aluminium is quite different in most ways to any other form of soldering. Special solders are made for this. Several types are available and can have varying melting points.
Perhaps the most surprising thing is that no flux is needed. The solder rods made for aluminium do not contain any flux.
If two pieces of aluminium are held together simply by their own weight then they are heated up to be hot enough for the solder to melt when it touches them. In this case the solder used was sold specifying its melting point as being 300ºC. This is a lot less that the melting point of aluminium which is 610ºC.
If this is all that is done nothing will happen. The trick is that a sharp corner of a broken hacksaw blade is pushed through the melted solder and is used to scratch along the joint between the two pieces. Doing this will enable the solder to wet both pieces.
The blade can be used to pull the solder along the joint which, of course, has to be at the required temperature.
fig join on soldering side
It can be seen that there is a real join on this side. What is truly amazing is the when the piece is turned round it can be seen that the solder appears to have gone all the way through even though nothing was done on this side
fig join on the far side
In spite of the low temperature this joint is very strong. It was put in the vice and hit very hard. It bent but did not come apart.
fig bent workpiece
With normal soldering the shape of the pieces being soldered is, of course, completely unchanged by the soldering process. With aluminium soldering, with thinish workpieces, it is possible for either of the workpieces to look very slightly as if the solder has penetrated the workpiece and produced a mark on the other side of it.
Soldering aluminium to other metals
It is said that aluminium solder will solder other metals. However, when soldering aluminium, it works without any flux but it will only solder copper if flux is used. So a piece of aluminium can be wetted with solder without flux. Then a piece of copper can be wetted with solder with flux. When both have been wetted they can be soldered together using the same solder.