ALUMINIUM AND ITS PROPERTIES
Years ago aluminium was considered a difficult material to weld, since when using oxy-acetylene there was no change in colour to indicate the metal’s temperature and suddenly it would 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.
Aluminium and its alloys have special properties of lightness, strength, conductivity, malleability etc… which make it a very useful material in many industries. The metal can be either in ‘cast’ form or extruded (wrought), which then divides into non-heat treatable and heat treatable. Generally it is readily weldable but it’s important to understand some of its characteristics:
ALUMINIUM AND 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 that the arc has reverse polarity (electrode positive). Correct gas shielding (argon) will prevent oxides reforming in the weld.
Thermal: as aluminium is a very good thermal conductor it will rapidly disperse heat. Care must be taken to avoid distortion or possibly cracking.
Colour: unlike steel, there is no change in colour as it is being treated. 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 and paint. Dry the surface thoroughly. If TIG welding, wipe the filler rod clean of any surface oil.
Application: support the joint to be welded, preferably with a jig, but spot tacking can be used. Keeps 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 avoid distortion.
Consumables for MIG and TIG welding:
4043A (no.15): contains 5% Silicon, for castings and heat treatable alloys 6063, 6061 and 6083. Weld will discolour if anodised. Good all purpose rod.
5356 (no.27): contains 5% Magnesium, for similar 5xxx alloys and heat treatable alloys 6063, 6061 and 6083. Has good corrosion resistance.
For pure aluminium, military, aerospace or significant load-bearing applications please contact us and we will advise on the correct rod for any given application.