Welding aluminium sheet assemblies

A 6061-T6 panel 900 mm long, 3 mm thick, seam-welded along one edge, comes back 7 mm out of flat and 4 mm short. The weld is sound and passes a visual check. The drawing called ±2 mm on both. The part is not badly made; it was designed as though aluminium behaved like steel, and it does not.

Welding aluminium sheet assemblies fails in three predictable ways: the heat-affected zone loses a third of its strength, the oxide film prevents fusion until it is mechanically removed, and the thermal expansion pulls the part roughly twice as far as steel would. Each has a design answer, and all three should be settled before the weld symbol goes on the drawing.

The bending and welded assemblies hub covers the surrounding weld, fixture and tolerance material.

Why aluminium moves and softens more than steel

Three material properties set the behaviour. Thermal expansion for 6061 is about 23.6 µm/m·K against roughly 12 µm/m·K for mild steel, so a 900 mm panel grows 0.021 mm per degree of temperature rise instead of 0.011 mm. Weld a seam and the panel either side of it is at a different temperature from the rest, which produces the bow.

Thermal conductivity is about 167 W/m·K for 6061 against roughly 50 W/m·K for steel. Heat does not stay at the joint; it spreads into the panel and the fixture, which is why aluminium welds need a higher current and a faster travel speed for the same thickness, and why the whole part, not just the seam, ends up hot and then contracts.

The third is the oxide. Aluminium oxide melts at about 2,072 °C while the alloy beneath melts between 582 °C and 652 °C, depending on grade. The oxide has to be removed mechanically or displaced by the arc, and it re-forms within hours in ordinary humidity. That is the reason for the cleaning window rather than a cleaning step.

On top of those, aluminium solidifies with about 6% volumetric shrinkage and absorbs hydrogen in the melt that it cannot hold in the solid. Hydrogen solubility falls from roughly 0.69 ml per 100 g in the liquid to 0.036 ml per 100 g at solidification, and the difference comes out as porosity. Cleaning and gas coverage control porosity; current and travel speed control penetration; fixture and sequence control distortion. No single setting fixes all three.

The numbers that decide an aluminium weld

The current range is wide because it depends on joint type. A square-butt at 3 mm needs the upper end and a backing bar; a fillet on the same thickness runs at the lower end. A synergic programme sets wire feed speed and voltage together, so the operator adjusts one dial and the arc stays stable, which is the practical reason to buy a machine with the programme rather than to tune it manually.

Alloy and filler pairing

5052-H32 is the default for formed and welded sheet. It bends well, welds without heat treatment, and its heat-affected zone keeps most of its strength because it is strain-hardened rather than precipitation-hardened. It is not the grade to specify if the part needs 300 MPa.

6061-T6 is the default for machined and welded parts, and it is the grade that causes the disappointment. The T6 temper comes from a precipitation treatment that the weld cycle partially dissolves. The heat-affected zone typically measures 165–200 MPa tensile against 310 MPa parent, so the joint is limited by the softened band, not by the weld metal. Design to 165 MPa and the part is honest.

Filler choice follows the alloy. 4043 (aluminium with 5% silicon) suits 6061, has low crack sensitivity, flows well and anodises to a dark grey. 5356 (aluminium with 5% magnesium) is stronger, matches the colour of 5xxx sheet better after anodising, and is the right choice for 5052 and 5754. On 6061 it raises crack sensitivity in some joint designs, so it is chosen for strength or colour, not by default.

The mismatch shows up in finishing. An anodised assembly welded with 4043 shows a dark seam against a light panel, and no amount of polishing removes it because the difference is metallurgical. If the part is anodised and visible, specify 5356 or design the joint so the weld is hidden.

Cleaning, and the four-hour window

The cleaning sequence is degrease, then brush, then weld, within four hours. Degreasing first matters because brushing a greasy surface pushes contamination into the metal. Acetone or an alkaline cleaner, wiped dry, then a stainless wire brush at low speed with light pressure and a dedicated brush.

The brush has to be stainless and used only on aluminium. A carbon steel brush leaves iron particles embedded in the surface, and those particles rust under paint and start pitting corrosion in service. The same applies to grinding discs, which should be aluminium-rated and free of the iron contamination left by steel work.

Wire brushing removes the oxide but also work-hardens the surface, and the effect is small at light pressure. Heavy pressure with a coarse cup brush gouges the sheet and creates a stress riser at the joint edge. A hand brush or a low-speed pneumatic brush is enough. The weld should follow within four hours; a part brushed on Friday and welded on Monday has an oxide film as thick as the one that was removed.

Filler wire and the workpiece both carry hydrogen, and moisture in the gas line is a common source of porosity. Purge the line at the start of a shift, store wire in a dry cabinet, and check the regulator for a slow leak that lets humid air back in overnight.

Distortion control that works on sheet

Tacking is the first control. Space tacks at 50–75 mm, run each one 10–15 mm long, and grind the start and stop of every tack before the main run, because a tack that is not blended leaves a cold lap. Tack in a sequence that alternates ends and sides rather than working along one edge.

The second control is the backstep. Weld in 150 mm increments, starting each increment 150 mm behind the previous one, so the shrinkage at any moment is local rather than cumulative. On a 900 mm seam that is six increments, and the bow typically halves against a straight run.

The third is the fixture. Aluminium needs support close to the seam, and a copper or aluminium backing bar acts as a heat sink as well as a support. The fixture should hold the panel flat but allow it to contract along the seam; clamping both ends rigidly converts contraction into buckling, which is worse than the bow it replaced.

A pre-set helps on long panels. If the first article bows 6 mm, set the fixture with a 3 mm opposite bow and weld into it, then check the relaxed part. This is empirical and it is recorded per part number, because the right pre-set depends on the seam length and the heat input, not on the alloy alone.

Strength, inspection and what the weld has to prove

The weld quality level decides the inspection. ISO 10042 sets quality levels for arc-welded joints in aluminium and its alloys, covering imperfections such as porosity, lack of fusion, cracks and undercut, with levels B, C and D from most to least demanding. A structural bracket and a cosmetic canopy panel should not be called out at the same level.

Inspection on sheet work is mostly visual plus one destructive check per setup. Look for a smooth toe, a consistent bead width, no undercut beyond the level, and no visible porosity. Then cut a section or bend a coupon from the first-off assembly and confirm fusion on both sides. Radiography is rarely justified on 2 mm sheet, and dye penetrant does not work on the porous oxide surface without etching.

The laser cutting guide for aluminium covers edge quality, because a laser-cut edge with dross or an oxide ridge is a poor faying surface and the cause of a lack-of-fusion defect that gets blamed on the welder.

Welding aluminium sheet assemblies at Ganesh

Aluminium assemblies are welded at the manual MIG/MAG stations within a department of more than 30 MIG/MAG, spot and special-purpose stations, with synergic programmes set per joint and fixtures designed and built in-house. A fixture for aluminium sheet is built with a backing bar and with clearance for contraction, which is the difference between holding a panel flat and buckling it.

Blanks come from the 12 kW fibre laser with a 40 ft × 16 ft bed, which cuts aluminium sheet and plate without a mechanical edge, and forming runs on the 200 T, 5-axis brake with a 4,450 mm bed to 10 mm. The aluminium fabrication capability sets out the material and thickness range, and the bending guidelines for aluminium cover the radius and springback side of the same part.

After welding, assemblies go through seven-tank pretreatment and the 18 ft × 14 ft powder booth; the line settings are confirmed against the alloy before the first batch rather than run to the steel programme. The distortion control guide covers the sequence patterns in more detail.

Ganesh has manufactured sheet metal since 1992 across five units in Mumbai (Vasai) and Nagpur, with ISO 9001:2015 and ISO 14001:2015 certification. PPAP and first article inspection are available, and a first article for a welded aluminium assembly includes a macro section at the joint and a flatness record across the seam.

What to put on the drawing

• Alloy and temper, with the heat-affected-zone strength or the design load, not only the parent grade.
• Thickness and the tolerance on thickness, because a 0.1 mm change moves the current setting.
• Filler alloy, or a note that the supplier proposes one for approval.
• Joint type and preparation, with the root gap and any backing requirement.
• Weld symbols to ISO 2553 and the quality level to ISO 10042.
• Flatness and straightness callouts across the seam, with the measuring method.
• Whether the part is anodised, and if so whether the weld will be visible.
• Any face that must remain free of weld spatter or heat mark.

The RFQ checklist lists the fields that let a welder price an aluminium assembly without guessing at the filler or the fixture.

When welding aluminium is the wrong route

Welding is the wrong route when the part is thin, large and flat. A 1.5 mm aluminium panel 1,500 mm long will bow under any arc process, and a rivet-and-adhesive joint holds it flatter at a comparable cost. Adhesive bonding also avoids the heat-affected zone entirely, which matters on 6061-T6.

It is also wrong when the assembly has to be anodised to a matched colour and the joint is visible. The filler never matches the parent after anodising, and the seam becomes a design feature whether you want one or not. Folded and riveted construction avoids it.

And it is wrong when the load path needs the parent strength. If the joint must carry 310 MPa, the answer is a different alloy, a thicker section, or a mechanical fastening that does not soften the metal around it.

Send the drawing with the alloy, temper, thickness and joint detail. The engineering team will return the filler, the weld programme, the fixture concept and the flatness that the seam will actually hold.