Short answer: Aluminum TIG welding uses AC current to break up the surface oxide, with argon shielding and a filler matched to the alloy. Use 4043 for most 6061 work and castings because it resists cracking, and 5356 for 5xxx alloys, higher strength or anodized parts. Aluminum usually cracks because of no filler or the wrong filler, an unfilled crater, contamination, or fatigue at the weld toe.
Aluminum is not hard to weld once you respect how different it is from steel. It conducts heat away fast, it hides under an oxide skin that melts at three times the temperature of the metal, it gives no color warning before it collapses, and some of the most common alloys crack if you weld them with the wrong rod. Almost every bad aluminum weld traces back to one of those four things.
This guide covers the alloy families, how to pick between 4043 and 5356 and when to use something else, the main ways aluminum cracks, and the machine settings and prep that keep porosity out.
Why aluminum welds differently from steel
Pure aluminum melts at about 1,220°F. The aluminum oxide that forms on its surface within seconds of cleaning melts at roughly 3,700°F. If you do not break that oxide up, you end up with molten aluminum sloshing around under a skin that will not fuse. That is the reason aluminum TIG runs on AC. The electrode-positive half of each cycle blasts the oxide off the surface, and the electrode-negative half does most of the heating.
Aluminum also conducts heat roughly three to five times faster than carbon steel, depending on the alloy. The start of a weld feels cold and slow to wet in. Thirty seconds later, the whole part is heat soaked and the puddle wants to run away. A good aluminum welder is backing off the pedal through most of the bead.
Two more differences matter. Aluminum expands about twice as much as steel when heated, so it distorts more and tacks need to be closer together. And it does not glow or change color before it melts, so you cannot read the heat by eye the way you can on steel.
Aluminum alloy families and how weldable they are
Wrought aluminum alloys are grouped by their main alloying element, using a four-digit system from the Aluminum Association. The first digit tells you most of what you need to know about welding it.
- 1xxx (pure aluminum). Very weldable, soft, low strength. Bus bar, chemical tanks.
- 2xxx (copper). 2024 and similar aircraft alloys are generally considered poor candidates for arc welding because they hot crack. Mechanical fastening is the norm.
- 3xxx (manganese). 3003 is weldable and common in sheet work, ducting and fuel tanks.
- 4xxx (silicon). Mostly used as filler metal, including 4043 and 4047.
- 5xxx (magnesium). 5052, 5083, 5086 and 5454. Weldable and strong as welded. The backbone of marine and tank work.
- 6xxx (magnesium and silicon). 6061 and 6063. Heat treatable, weldable with filler, and very common in extrusions, trailers and frames.
- 7xxx (zinc). 7075 is not considered fusion weldable for structural use. A few alloys such as 7005 are made to be welded.
Castings use a three-digit system. A356 and 356 are the common weldable casting alloys you see in marine parts, pump housings and transmission cases. Die castings are often full of trapped gas and can be close to unweldable.
4043 vs 5356 and the other filler metals
For most shop work the choice comes down to two rods. ER4043 is aluminum with about 5% silicon. It flows well, wets in nicely, and resists hot cracking, especially on 6xxx alloys and castings. ER5356 is aluminum with about 5% magnesium. It gives higher strength, particularly in shear, it is stiffer to feed on a MIG gun, and it matches color much better if the part gets anodized. 4043 turns dark gray when anodized.
The table covers the most common combinations. Where your drawing or code names a filler, use that.
| Base metal | Common filler | Notes |
|---|---|---|
| 6061 to 6061 | 4043 or 5356 | 4043 for crack resistance and flow; 5356 for higher shear strength or anodizing |
| 6063 to 6063 | 4043 or 5356 | Architectural extrusions, railings, frames |
| 6061 to be heat treated after welding | 4643 | Designed to respond to post-weld solution treatment and aging |
| 5052 to 5052 | 5356 | Sheet, tanks, boat parts |
| 5083 or 5086 to itself | 5183 or 5356 | Marine hull plate; avoid 4043 |
| 5052 to 6061 | 5356 | Common in boat and trailer work |
| 5454 in hot service | 5554 | For service temperatures above about 150°F |
| 3003 to 3003 | 4043 or 1100 | Ducting, fuel tanks, general sheet |
| A356 casting to itself or 6061 | 4043 | Clean and bake out castings first |
| 2024 or 7075 | Not recommended for arc welding | Hot-crack sensitive; repair by other means |
Two filler rules that prevent failures
First, do not put 4xxx filler on 5xxx alloys that carry more than about 3% magnesium, such as 5083 and 5086. Silicon and magnesium together form brittle magnesium silicide in the weld. Second, avoid 5356 and other high-magnesium fillers, and 5xxx base metals over about 3% magnesium, in parts that run continuously above roughly 150°F. Long exposure to that heat can sensitize them to stress corrosion cracking. Exhaust parts, hot water tanks and heated process equipment fall into this category.
Why aluminum cracks
Aluminum cracking falls into a few well-understood types. Knowing which one you are looking at tells you how to fix it.
Hot cracking in the weld
Solidification cracking, or hot cracking, happens as the weld freezes. Aluminum alloys pass through a range where the metal is part solid, part liquid, and shrinkage pulls the grains apart. 6061 is the classic example. Its own chemistry sits close to the peak of the crack-sensitivity curve, so an autogenous 6061 weld with no filler will often crack straight down the middle. Adding 4043 or 5356 shifts the weld chemistry out of the sensitive range. That is why aluminum joints need enough filler going in, and why a tight square butt on thick 6061 is asking for trouble. Open the joint up so the filler can do its job.
Crater cracks
Aluminum shrinks around 6% by volume as it solidifies. If you snap off the arc at the end of a bead, the crater freezes from the outside in and tears a little star-shaped crack in the middle. Those cracks grow under vibration. Fix it with technique: taper the pedal down slowly, add an extra dab of filler as you finish, or run the bead off onto a tab you cut away later. Most inverter machines have a crater-fill setting for this.
Cracks beside the weld
On 6xxx alloys, the metal right next to the fusion line can partially melt along the grain boundaries and crack, called liquation cracking. Too much heat input and too little filler make it worse. On castings, cracks next to a weld usually mean gas or oil in the metal, or a repair that did not account for the casting’s lower ductility.
Fatigue cracks in service
The cracks that turn up most often in repair work are not welding defects at all. They are fatigue cracks at the toe of a weld on a trailer crossmember, a boat T-top leg, or a bracket on a truck. Aluminum has no fatigue limit the way steel does, so any part that vibrates will eventually crack at its stress risers. Rewelding the same joint the same way usually buys the same life again. Adding a gusset, moving the weld out of the high-stress spot, or smoothing the weld toe makes a lasting repair.
AC TIG settings for aluminum
Exact settings depend on the machine, the alloy and the joint, but these ranges are where most aluminum TIG work lands on a modern inverter:
- AC balance: about 65 to 75% electrode negative on clean material. Drop toward 60% if the part is oxidized or dirty and you need more cleaning action. More EN gives deeper penetration and keeps the tungsten sharper.
- AC frequency: about 80 to 150 Hz for general work. Higher frequency tightens the arc cone, which helps on fillets and thin edges. Older transformer machines run fixed at 60 Hz.
- Tungsten: 2% lanthanated or 2% ceriated, ground to a point and lightly blunted. Pure tungsten balled at the tip is the traditional choice on transformer machines.
- Amperage: a common starting point is about 1 amp per 0.001 inch of thickness, so roughly 125 amps for 1/8 inch, adjusted with the pedal as the part heats.
- Gas: pure argon at 15 to 25 CFH with a gas lens. On thick sections, a helium-argon mix such as 75% helium adds heat and penetration.
Thicker parts, say over 3/8 inch, often benefit from preheat around 200 to 250°F so the weld does not start cold. Keep preheat and interpass at or below about 250°F on 6xxx alloys and on 5xxx alloys with more than 3% magnesium, and do not let them soak there. Use temperature crayons. Aluminum will not show you the heat.
Prep and porosity
Porosity in aluminum is almost always hydrogen, and hydrogen comes from moisture and hydrocarbons: oil, cutting fluid, marker, fingerprints, humidity trapped in the oxide, or damp filler rod. Aluminum absorbs hydrogen readily when molten and rejects it as it freezes, leaving bubbles behind. Our prep order is fixed:
- Degrease first with acetone or a residue-free solvent on a clean rag.
- Then brush with a stainless wire brush that is only ever used on aluminum. Brushing first just drives the oil into the surface.
- Weld the same day. The oxide starts rebuilding immediately and keeps thickening.
- Keep filler rod in a sealed tube, and wipe it before use if it has been sitting out.
On Gulf Coast humidity, condensation on cold parts brought in from a truck bed is a real porosity source. Let them come up to shop temperature first.
What welding does to aluminum strength
Heat-treated and work-hardened aluminum loses strength next to the weld, and buyers should plan for it. 6061-T6 has a typical tensile strength around 45 ksi. AWS D1.2, the structural aluminum welding code, uses a minimum as-welded tensile strength of 24 ksi for 6061-T6, about half. Strain-hardened 5xxx alloys lose less, because their as-welded strength is close to their annealed strength anyway.
Full strength in a 6xxx weldment can be restored with solution heat treatment and artificial aging, but that means quenching, which distorts parts. For most structures the answer is to design for the as-welded strength, keep welds out of the highest-stress areas, and use 5xxx alloys where the joint carries the load.
Repairing cracked aluminum and castings
Repair work has its own sequence. Find the full length of the crack with dye penetrant, because aluminum cracks run farther than they look. Drill a small hole at each end so it stops growing. Grind out a V groove to sound metal and clean it as described above. Castings that have been soaking in oil, like gearboxes and outboard lower units, need to be baked to drive the oil out of the porosity, or the weld will bubble no matter how clean the surface looks.
Our weld repair page covers cracked aluminum and cast parts, and our aluminum TIG welding page covers new fabrication in 5xxx and 6xxx alloys, from tanks to boat parts. Marine owners along the coast can see our Corpus Christi service area. If you are weighing AC TIG against a spool gun for thicker work, the TIG vs MIG comparison lays out the tradeoffs. Send photos, the alloy if you know it, and what the part does, and we will tell you what filler and approach make sense.
Frequently asked questions
Both work on 6061. 4043 flows better and resists hot cracking, so it is the easier choice for most fabrication and repairs. 5356 gives higher shear strength and a much better color match if the part will be anodized. Avoid 5356 if the part runs continuously above about 150°F. Use 4643 if the weldment will be heat treated afterward.
A centerline crack is usually hot cracking. It happens when the weld chemistry is too close to the base metal, often from welding 6061 with too little or no filler, or from a tight joint the filler cannot get into. Add more filler, open the joint up, reduce restraint, and fill the crater at the end of each bead so it does not tear as it shrinks.
On a modern inverter, most aluminum work runs around 65 to 75% electrode negative. That gives good penetration and keeps the tungsten in shape. If the metal is oxidized or dirty and you see black specks in the puddle, shift toward about 60% to add cleaning action. Pair it with a frequency around 80 to 150 Hz for most joints.
7075 is generally not considered fusion weldable for structural use. It is very prone to hot cracking, and the heat-affected zone loses much of its strength. Some cosmetic or non-structural repairs are attempted, but a cracked 7075 part that carries load is usually replaced, or repaired with a bolted doubler or another method designed by an engineer.
Aluminum porosity is almost always hydrogen from contamination: oil, cutting fluid, marker, moisture, a dirty or damp filler rod, or a heavy oxide layer. Degrease with acetone first, then brush with a stainless brush used only on aluminum, and weld the same day. Check for drafts across the torch and a leaking gas line too.