Aluminium coil is joined at two distinct points in a production line: coil-to-coil splicing so that a continuous strip can feed a press or roll former, and fabrication welding to build tanks, ducts, frames and enclosures. Both operations face the same two obstacles, namely an oxide film that melts at more than three times the melting point of the base metal, and a thermal conductivity that pulls heat away from the weld pool faster than steel does. Handling both correctly is what separates a sound weld from one that cracks on cooling.
Oxide Removal and Joint Preparation
Aluminium oxide melts at approximately 2070 C while the metal beneath melts at about 660 C, so the film must be removed mechanically or broken up electrically before the arc can establish a pool. Stainless steel wire brushing with a dedicated brush, followed by degreasing, is the normal shop practice, and the cleaned surface should be welded within a few hours. Solvent wiping alone is not sufficient because it removes oil but not the oxide. Edges are usually prepared square for material up to about 4 mm and bevelled at 60 degrees total included angle for heavier sections. Fixtures must be non-magnetic stainless steel or aluminium, since copper backing bars draw heat away too quickly.
Process Selection: TIG, MIG and Laser
Alternating current gas tungsten arc welding with a lanthanated or zirconiated tungsten electrode is the standard choice for material from 1 mm to about 6 mm, because the positive half cycle provides the cathodic cleaning that breaks down the oxide layer. Tungsten electrodes are classified under AWS A5.12, and an electrode with a balled or truncated tip is used on alternating current. Direct current electrode negative is used for thicker sections where penetration matters more than surface cleaning. Gas metal arc welding with pulsed transfer is used from 3 mm upward for higher deposition rates and is common in tank and duct fabrication. Laser welding produces the narrowest heat-affected zone and the least distortion, which makes it attractive for coil splicing and thin-gauge enclosure work, but it demands tight joint fit-up, typically a gap below 0.1 times the material thickness.
Filler Metal Selection
Filler metals for aluminium are classified in AWS A5.10, and two families cover most work. The 4043-type filler, an aluminium-silicon alloy, gives the best crack resistance and the smoothest bead appearance, and is the first choice for 6061 and other heat-treatable alloys that are crack sensitive. Its drawback is a slightly lower deposited strength and a darker anodised colour. The 5356-type filler, an aluminium-magnesium alloy, gives higher deposited strength and matches the parent colour after anodising, which makes it standard for 5052 and 5083 work, but it is more prone to hot cracking in highly restrained joints. Where a 5xxx base metal is being welded to a 6xxx base metal, a 4043-type filler is usually preferred to reduce cracking risk. Filler diameter should match the joint volume; a common mistake on thin coil is using a filler that is too large and dragging the arc.
Distortion and Heat Control
Aluminium expands by roughly twice as much as steel for the same temperature rise, and its thermal conductivity is several times higher, so heat input must be concentrated and travel speed kept up. Practical measures include back-step welding, balanced sequences around the neutral axis, tack welds at close spacing, and the use of chill bars or water-cooled backing on thin material. Preheating is rarely needed on 1xxx, 3xxx and 5xxx material and is normally avoided, because it increases the width of the softened zone without improving penetration. Thick 6061 sections are the exception, where a preheat in the region of 150 C can help avoid porosity. Interpass temperature should be limited, since excessive heat input coarsens the structure and reduces as-welded strength.
Inspection and Acceptance
Weld quality checks normally combine visual examination with at least one volumetric method. Liquid penetrant testing follows ASTM E165/E165M and is useful for surface-breaking defects such as crater cracks and lack of fusion at the toe. Radiographic examination of the completed joint follows ASTM E1032 and is the usual acceptance method for pressure-retaining welds. Mechanical qualification is made by tensile testing to ASTM B557 or ASTM E8/E8M, often supplemented by bend tests on procedure qualification coupons. Porosity is the most common defect and is usually traced to inadequate shielding gas coverage, contamination at the joint, or oxide entrapment rather than to the welding parameters themselves.
Frequently Asked Questions
Q: Why does aluminium welding need different preparation from steel?
A: The oxide film on aluminium melts at about 2070 C while the metal melts at 660 C, so the film must be mechanically removed or broken up by cathodic cleaning before the arc can form a pool.
Q: Which filler should be used for 5052 coil?
A: A 5356-type filler is the normal match because it gives adequate strength and a colour match after anodising. A 4043-type filler is used where hot-cracking risk in a restrained joint outweighs the strength difference.
Q: Is preheating required before welding aluminium coil?
A: Not for 1xxx, 3xxx or 5xxx material in normal thicknesses. Preheating is generally limited to thick 6061 sections, where it helps control porosity and penetration.
Q: How is porosity in an aluminium weld detected?
A: Radiographic examination to ASTM E1032 is the standard volumetric method. Liquid penetrant testing to ASTM E165/E165M detects surface-breaking indications but cannot reveal internal porosity.
Q: What causes crater cracks at the end of a weld run?
A: The crater shrinks rapidly as the arc stops, and the solidifying metal tears if the arc is extinguished abruptly. Filling the crater with a current taper or run-off tabs avoids the defect.
Q: Can aluminium coil be welded to stainless steel?
A: Not by conventional fusion welding without an intermediate transition joint. Direct fusion creates brittle intermetallic compounds, so a bimetallic transition piece or a mechanical joint is used instead.