Years ago aluminium was considered a difficult material to weld. When using oxy-acetylene there was no colour change to indicate the metal's temperature so suddenly it could melt and collapse! With the introduction of TIG and MIG welding processes, these welder fears have been put to one side, providing that the properties of aluminium are understood. This article is intended to give a general overview.
Aluminium and its alloys have special properties of lightness, strength, conductivity and malleability making it a particularly useful material in a variety of industries. The metal can be either in cast form or extruded (wrought), which then divides into non-heat treatable and heat treatable. Generally, aluminium is weldable, but it is important to understand some of its characteristics:
Oxidation: In air aluminium immediately forms an oxide layer on its surface, which will increase in thickness with time. This oxide layer must be controlled during the welding process, by chemically and mechanically cleaning the metal, using an aggressive flux or ensure the arc has reverse polarity (electrode positive). Correct gas shielding (argon) will prevent oxides reforming in the weld.
Thermal Conductivity: As aluminium is a very good thermal conductor, it will rapidly disperse heat. Care must be taken to avoid distortion and possibly cracking.
Colour: Unlike steel, there is no change in colour in aluminium as it is being heated. Look out for a 'wet' appearance. For gas brazing, melting of flux powder is a temperature indicator.
Preparation: Smooth all edges of workpiece to minimise trapped dirt. Use a commercial degreaser and stainless steel brush to remove dirt, oil, paint. Dry surface thoroughly. If TIG welding, wipe filler rod clean of any surface oil.
Application: Support the joint to be welded, preferably with a jig, but spot tacking can be used. Keep the arc travelling at the right speed to build up a bead of the right proportions. Do not stop/start on one weld, as this can lead to oxidation/porosity. Carry out the weld quickly to minimise distortion.
Brazing - Sifalumin No 36 and Aluminium Flux
Soldering - SIF 555 Aluminium Solder
Gas welding - Sifalumin No 14 or No 15 and Aluminium Flux.
MIG & TIG welding
SIFMIG 1050 Sifalumin No 14 for pure aluminium
SIFMIG 4043 Sifalumin No 15 contains 5% silicon, for castings and heat treatable alloys 6063, 6061 & 6083.
Weld will discolour if anodised.
SIFMIG 4047 Sifalumin No 16 contains 12% silicon, for castings and automotive applications.
SIFMIG 5356 Sifalumin No 27 contains 5% magnesium, for similar 5xxx alloys and heat treatable alloys 6063, 6061 & 6083.
SIFMIG 5183 Sifalumin No 28 contains 5% magnesium with 0.75% manganese for improved weld strength
SIFMIG 5556 Sifalumin No 37 contains 5.3% magnesium and other closely controlled elements for 5083 military and aerospace applications.