TECH. INFO.
ALUMINUM & ALUMINUM ALLOYS
Aluminum and aluminum alloys, like all metals obtained by a fusion process, can in principal be joined by a fusion welding process. As compared to steels, in welding aluminum and its alloys, some specific features of the material have to be taken into account. Since aluminum has a substantially better thermal conductivity as compared to structural steel, penetration depth in welding is reduced and degassing of the weld pool is impeded. As a result, there may occur incomplete fusion or pores in the weld seam. By preheating the workpiece, as well as by accompanying heating during welding of thicker sections, such disturbances can be prevented.
Before initiating welding, the aluminum oxide skin should be completely removed from the weld area by milling or brushing with a stainless steel brush. The walls of the weld joint and neighbouring area (at least 50 mm on each side of the joint) must be clean, free of grease and dry. Good storage practice will generally reduce this work to simplify washing off a solvent the grease left on the prepared edges by proceding machining operation. Besides this, all tools used in processing aluminum should be employed for this materials.
Since aluminum has a highly reflecting surface, protective clothing in welding is a must in order to protect yourself against burns by ultraviolet radiation during welding.
GAS WELDING
Gas welding (oxyacetylene welding) is still sometimes used for welding pure aluminum and non-ageing aluminum alloys because of the relatively low cost and and simplicity of the required equipment.
Welding gases (fuelgas - oxygen): Acetylene and oxygen in gas cylinders are normally used.
The realtively low heat concentration of the oxyacetylene flame, added to the high heat conductivity of aluminum, reduces welding speed and leads to severe contractions with resulting stress and deformation of the work piece. The heat- affected zone is very large and work-hardened or age-hardened parent material in this area becomes soft and loses mechanical strength. The flux required for gas welding is applied by brushing on the sides of the joint and on the filler rod. Removal of flux residues is time-consuming.
MANUAL ARC WELDING
Higher welding speeds are obtained by manual arc welding with flux-coated electrodes.
When welding material thicknesses over 8 mm, it is advisable, in order to obtain a compact, porosity-free weld metal, to preheat the workpiece to at least 200°C. It is necessary to remove all slag residues, to secure corrosion resistance of the weld seam.
GAS-SHIELDED ARC WELDING
Shielding Gases
Apart from its task of excluding air from the weld pool, the shielding gas has a strong influence on the stability of the arc and thus, on weld quality. Up to now, only the inert gases Argon and Helium - each seperately or a mixture of both gases - have proved satisfactory results for welding of aluminum. However, it is important to note that argon will produce a more quiet and stable arc than Ar/He mixture. On the other hand, in using Ar/He mixtures there will be a higher arc performance with the same current setting.
TIG WELDING
Characteristics
The welding arc is established between the workpiece and a non consumable tungsten electrode. The filler metal is added manually. Non-age-hardening alloys can be welded without addition of filler metal. TIG welding of aluminum is normally carried out with alternating current and argon gas shield. Mechanised TIG welding, with and without addition of filler metal is possible.
Applications
Butt welds in one run on material thickness of 1 to 4 mm or one-run fillet welds of corresponding size; double-operator simultaneous vertical welding of wall thicknesses up to 12 mm. TIG welding of heavier sections is not economical because the low heat concentration, compared to MIG welding, imposes a much slower welding speed and multi-pass welding creates very large heat affected zones. Because of the better ability to bridge gaps and the greater security against porosity, TIG welding is neverthless used to thicker materials to weld root runs, especially when a reverse side sealing run is not possible (e.g. on pipelines), the rest of the joint being filled up in one or more passes by MIG welding.
MIG WELDING
Characteristics
The welding arc is established between the workpiece and a consumable wire electrode which is also the filler metal. MIG welding of aluminum is normally carried out with direct current and positive electrode polarity. The metal transfer in fine droplets is known as spray-transfer and is shielded by an inert-gas atmosphere (argon or helium or Ar/He mixture). MIG welding process can be stabilized by using large wire diameters. For smooth welding, wire should be guided through nylon based liner and guides, wire feeding rolls' grooves must be U shaped and contact tips must be longer and should have more tolerant inner diameters. Mechanised MIG welding is also possible and is being increasingly used.
Applications
Butt welds in one run on material thicknesses > 4 mm or fillet welds of corresponding size. Because of the high welding currents and strong penetration, backing supports for the weld pool are mostly used for welding material thicknesses up to 6mm.