If you are joining aluminum to another metal, the aluminum’s properties are usually the limiting factor, so precise identification of the other alloy’s composition may not be necessary. Note that some combinations such as aluminum-steel are extremely difficult or may require specialized welding methods instead of solder.
Technically, filler metals that melt above 840ºF (450ºC) join by brazing instead of soldering. In practice, these are often sold as solders, and the process is similar. Brazing creates a stronger bond, but soldering is preferred for pieces with electrical circuits or other delicate materials. Avoid solders that contain lead whenever possible.
Some brazing fluxes are not intended for use on thin aluminum sheets or wire. Look for “dip brazing” flux for these applications instead.
If torch use is not feasible in your work space, try a 150 watt soldering iron.
Ignore this step if you are using solder to repair a crack or hole in a single object.
Old aluminum with heavy oxidization or other surface debris may require sanding or grinding, or wiping with isopropyl alcohol and acetone.
If the pieces do not fit together smoothly, you will need to make the joined areas smooth through sanding or bending. Because the aluminum should be given as little chance to oxidize as possible, you may wish to clamp the pieces together loosely, clean them while they are clamped, then tighten the clamp.
If soldering wires, dip them in the liquid flux instead. If your flux came in powder form, refer to the label for mixing instructions.
Soldering irons may take up to ten minutes to heat up before they can be used. If the flux turns black, let the area cool, clean it & start over.
If the solder will not bond to the aluminum, it could be that more aluminum oxide formed on the surface, in which case it needs to be cleaned and immediately soldered again. It could also be that you have the wrong type of solder, or your aluminum is actually a difficult-to-join alloy.
