Troubleshooting guide › Material guides › Aluminium welding
Aluminium welds badly for three reasons that have nothing to do with the welder: an oxide skin that melts at three times the temperature of the metal under it, a huge appetite for hydrogen that comes out as porosity, and heat conductivity that pulls the heat away from the weld. Every aluminium problem on this page traces back to one of the three. Get cleaning, gas and heat right and the process behaves.
Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.
| Property | What it means at the torch |
|---|---|
| Oxide skin melts at about 2050 degrees C; the metal at about 660 degrees C | The oxide must be removed before welding and broken up during it (AC cleaning on TIG, DC+ on MIG), or the pool stays trapped under a skin and will not fuse |
| Absorbs hydrogen when molten, rejects it when freezing | Any moisture, oil, grease or hydrated oxide becomes porosity; cleanliness matters more than on any other metal |
| Conducts heat 4-5 times faster than steel | Needs high current and a fast start; the start of the weld is cold and the end is too hot, so preheat thick parts and use down-slope |
| Expands twice as much as steel and has no colour change before melting | Distortion is severe and the pool appears suddenly; tack closely and watch the surface shine instead of colour |
| Some alloys are crack-sensitive when welded | 6xxx (6061, 6063) and 2xxx/7xxx need the right filler; 5xxx and 1xxx are easy |
Oil, grease, marker ink, cutting fluid, moisture in the hydrated oxide, dirty filler.
Argon flow too low, draught, leaking torch, wrong cup; on MIG a long stick-out.
The pool freezes before the gas can escape.
Aluminium shavings in the liner, an old steel liner, drive rolls scraping the wire.
A cylinder that has been used for other gases, or hoses left open.
6xxx alloys crack when welded with their own composition or with the wrong filler; the weld metal has a wide freezing range.
The last pool shrinks and tears across; aluminium does this more than any other metal.
Rigid fixtures and tight sequences stop the weld from shrinking freely.
A deep narrow bead freezes from the sides and tears down the middle.
A 5356 weld diluted heavily into a 6xxx base lands in the crack-sensitive range.
| Control | Set | Effect |
|---|---|---|
| AC balance (cleaning width) | 30-40 % EP for clean plate; 40-50 % for oxidised or cast material; machines that show the scale as 20-80 % start near the middle | More EP: more cleaning, wider zone, tungsten runs hotter and balls. Less EP: more penetration, tungsten stays cooler |
| AC frequency | 60-80 Hz for general work; 100-150 Hz for thin sheet and fillets | Higher: narrower, stiffer arc that is easier to steer; lower: wider, softer arc |
| Current | About 40-50 A per mm of thickness; 3 mm about 120-150 A; 6 mm about 220-280 A | Too low: oxide floats, pool will not wet; too high: burn-through and tungsten balling |
| Tungsten | 2 % lanthanated, blunt point, a small ball forms; 2.4 mm up to about 180 A, 3.2 mm to 250 A, 4 mm above | Pure or zirconiated on transformer machines |
| Gas | Argon 8-12 L/min, gas lens, cup size 7-8; argon-helium mix for plate over 8 mm | Helium adds heat and width |
| Pre-flow / post-flow | 0.5-1 s / 8-15 s | Tungsten should stay silver |
| Start / slope | HF start; up-slope 1-2 s; down-slope 3-8 s to kill the crater | Pedal control is ideal for aluminium |
| Pulse | Off to learn; 1-2 Hz with 40-50 % base for thin sheet and out-of-position | Reduces heat and gives a regular ripple to time the filler |
| Thickness (butt, flat) | Tungsten | Filler | Current | Argon |
|---|---|---|---|---|
| 1.0 mm | 1.0-1.6 mm | 1.6 mm | 40-60 A | 4-7 L/min |
| 1.5 mm | 1.6 mm | 1.6 mm | 60-90 A | 6-9 L/min |
| 3.0 mm | 1.6-2.4 mm | 1.6-2.4 mm | 100-160 A | 7-10 L/min |
| 4.0 mm | 2.4 mm | 2.4 mm | 130-200 A | 10-15 L/min |
| 6.0 mm | 3.2-4.0 mm | 2.4-3.2 mm | 200-280 A | 10-15 L/min |
| 8.0 mm | 3.2-4.0 mm | 3.2-4.0 mm | 200-300 A | 12-18 L/min |
Guide values for AC TIG in the flat position. Preheat 80-110 degrees C for 6 mm and above helps the start.
Soft wire buckles in a long cable, a steel liner, V-groove rolls that deform it, or too much roll pressure.
Wire feed stalls for a moment, or voltage is high for the wire speed.
Cold plate at the start, heat building up along the seam.
Wrong polarity (MIG on aluminium is DC+), CO2 or mixed gas by mistake, long stick-out.
Too much heat for the thickness; spray transfer on thin sheet.
| Thickness (fillet, flat) | Wire | Current | Voltage | Argon |
|---|---|---|---|---|
| 2 mm | 0.9-1.0 mm | 70-100 A (pulse) | 17-19 V | 15 L/min |
| 3 mm | 1.0-1.2 mm | 110-140 A | 19-21 V | 15-18 L/min |
| 5 mm | 1.2 mm | 160-200 A | 22-24 V | 18 L/min |
| 8 mm | 1.2-1.6 mm | 220-280 A | 24-27 V | 18-20 L/min |
| 12 mm | 1.6 mm | 300-360 A | 27-30 V | 20 L/min, preheat |
Spray transfer begins at about 130 A with 1.2 mm wire on argon. Starting points only.
Only with helium shielding on DC-, which is a specialist technique. For normal workshop aluminium you need AC. An AC/DC machine covers aluminium and everything else.
Usually a crater crack or a toe crack that was too fine to see, grown under vibration. Dye-penetrant test the ends of aluminium welds on anything that carries load.
4043 for 6xxx alloys and castings where crack resistance matters and the weld will not be anodised. 5356 for 5xxx alloys, for higher strength, and where the colour must match after anodising. Never 5356 on parts that will run above about 65 degrees C in service for long periods.
The spool has picked up moisture and the oxide has thickened, so it drags in the liner. Keep spools in sealed bags with desiccant and cover the feeder overnight.
Lamps, fault codes and the checks an operator can do outside the cabinet, for the machines this page applies to.
TIG on aluminium needs AC with adjustable balance (cleaning) and frequency, and enough current: about 40-50 A per mm of thickness, so 6 mm plate needs a 250-300 A AC/DC machine with a water-cooled torch. MIG needs a feeder that handles soft wire: U-groove rolls, a PTFE liner, and a push-pull torch or spool gun for anything beyond a 3 m cable. Pulse MIG makes thin aluminium much easier.
Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.
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