Avoiding Porosity and Cracking in Aluminium Welds

Introduction

In aluminium welding, porosity (gas pores in the weld metal) and cracking (solidification, liquation or crater cracks) are the two defects that most often destroy productivity and confidence. They lead to costly rework, scrap, leaks and reduced fatigue life – and they’re often symptoms of deeper issues with cleanliness, procedure or system design.

This article provides a practical troubleshooting framework: what porosity and cracking actually are in aluminium, the main root causes to check first, and concrete prevention strategies you can build into preparation, parameters and equipment choices – all within the context of a well-matched aluminium welding system.

Porosity in Aluminium Welds

Porosity is gas trapped in the weld metal as it solidifies. In aluminium, it often comes from hydrogen (moisture, oils, contamination), air and other gases entering the arc due to poor shielding, and oxides or contaminants that react or break down in the weld pool.

It can appear as pinholes on the surface, worm-tracks, or subsurface pores detected by radiography or UT. Porosity reduces strength, ductility and leak tightness, and in some aluminium alloys can also become corrosion initiation points.

Common Sources of Porosity

Preventing Porosity: Best Practices

Think of porosity prevention as three layers: cleanliness, shielding and control of hydrogen sources.

Cleaning & Surface Preparation

• Degrease first with suitable solvent or alkaline cleaners to remove oil, coolant and markers.
• Mechanically remove oxide with a dedicated stainless brush or non-contaminating abrasive just before welding.
• Keep brushes and abrasives dedicated to aluminium – no cross-use on steel.
• Ensure fit-up is tight and consistent to minimise gaps that trap contamination.

Shielding Gas & Delivery

• Use high-purity argon for most MIG/TIG work; consider Ar/He mixes for thicker sections.
• Set gas flow to avoid both inadequate shielding and turbulence; stay within typical recommended ranges for your nozzle size.
• Check hoses, connections and O-rings for leaks; fix any damaged or hissing joints.
• Use screens in draughty environments and keep nozzles and diffusers clean.

Learn more about Argon vs Helium Shielding Gases. 

Filler & Wire Handling

• Store aluminium wire and rods in dry, clean conditions to avoid condensation and contamination.
• Avoid handling wire with oily or dirty gloves.
• Maintain clean liners and drive rolls; excessive shavings are a warning sign.

Learn more about Choosing the Right Filler Metal for Aluminium Welding. 

Technique & Parameters

• Use a short, stable arc; long arcs increase air entrainment.
• Limit large, slow weaves; favour controlled travel and slight manipulation.
• Maintain a push angle (around 10–15°) for better cleaning and shielding.
• Avoid excessive heat input; pulse MIG modes can help control heat while maintaining fusion.

Cracking in Aluminium Welds

Cracks are more serious than porosity because they can propagate under load. In aluminium, common forms include:

• Solidification (hot) cracking – forms as the weld metal solidifies and shrinks.
• Liquation cracking – occurs in the HAZ where low-melting constituents tear during cooling.
• Crater cracking – forms at weld terminations where craters are left concave or underfilled.

Cracking risk depends on alloy composition and crack sensitivity, weld metal composition (base + filler), restraint and joint design, and heat input / solidification behaviour.

Alloys & Cracking Risk (Conceptual)

Preventing Cracking: Best Practices

Cracking usually combines composition, stress and cooling. Prevention means managing all three.

Filler Selection

• For many 6xxx alloys, 4043 (Al–Si) filler often improves crack resistance versus 5xxx fillers due to higher silicon and a narrower solidification range.
• 5xxx fillers (e.g. 5356, 5183) can provide higher weld strength but may increase crack risk in highly restrained joints.
• For castings, matching Al–Si fillers (4043, 4047) usually minimise cracking.
• For high-strength 2xxx/7xxx alloys, always consult detailed filler charts and consider whether fusion welding is appropriate.

Joint Design & Restraint

• Avoid overly rigid fixtures and extreme restraint; allow controlled movement where possible.
• Use run-on and run-off tabs to move start/stop imperfections off the part.
• Design joints with realistic root face and throat size; avoid extreme geometries that concentrate stress.

Heat Input & Preheat

• Keep heat input moderate; too low leads to lack of fusion, too high to wide, slow-cooling pools.
• Light preheat on thick sections or cold workpieces can reduce thermal gradients if within alloy limits.
• Observe interpass temperature limits and clean between passes.

Crater Fill & Terminations

• Use crater fill functions or manual back-step techniques to avoid concave, underfilled craters.
• Where possible, run off onto tabs and remove the end sections later.

Process & Parameter Considerations

The same joint can be sound or defect-prone depending on process and procedure.

MIG vs Pulse MIG

Conventional spray MIG offers high deposition but higher heat input and more turbulent pools, which can aggravate porosity and cracking if other variables aren’t under control. Pulse MIG (for example, pulse modes on advanced power sources) gives one-droplet-per-pulse transfer at lower average current, improving control of heat input and pool size, and typically improving arc stability.

Travel Speed & Bead Size

• Too slow → wide, hot pools that cool slowly; more time for gas entrapment and crack formation.
• Too fast → lack of fusion and irregular beads, creating stress raisers.
• Aim for a balanced bead profile – not excessively convex or underfilled at the toes.

Arc Length & Voltage

• Excess arc length (high voltage for the chosen WFS) increases turbulence and gas mixing, promoting porosity.
• Too little voltage can cause an unstable, stubbing arc and lack of fusion.
• On synergic and pre-defined programs, stay within the recommended trim ranges unless re-qualifying a procedure.

System & Equipment Factors

A consistent, aluminium-focused system makes it much easier to stay inside the “no-porosity, no-cracking” window.

Feed & Torch System

Stable wire feeding — U-groove rolls, PTFE or nylon liners, correct tension — avoids the micro-stops and surges that destabilise the pool. Push-pull torches such as the ESAB PP 350w are especially helpful on large structures where long cable runs challenge feeding and arc stability. High-duty manual torches with good cooling and ergonomics help operators maintain consistent distance and angle over long welds. For manual aluminium MIG, the Warrior Edge DX paired with RobustFeed Edge DX and PP 350w push-pull torch delivers a stable, aluminium-ready feed system out of the box.

Learn more about Troubleshooting Aluminium MIG Welding at the Arc.

Power Source & Modes

Modern power sources with aluminium-optimised programs provide stable starts, controlled crater fill, and pulse modes tuned specifically for aluminium. These features widen the safe window where porosity and cracking are unlikely for a given joint and alloy. ESAB's Warrior Edge DX and Aristo Edge are built with these aluminium-specific WeldModes, reducing setup time and giving operators more consistent results across varying material thickness and position.

Filler Wires & Tech Guides

Using well-characterised aluminium wires with clear application guidance removes the guesswork from filler selection. ESAB's OK Autrod wire range covers the most common aluminium alloy families — 4043, 5356, 5183 and beyond — with application data and filler selection charts to ensure every welder starts from proven settings. 

Optimising Your Aluminium System with ESAB

Stable, defect-free aluminium welding is much easier when the whole system is designed for aluminium – not just swapped liners and a different gas.

Reviewing your aluminium setup? ESAB can help align power source, feeder, torch, wire and gas for both manual and robotic aluminium MIG, so you get consistent feedability, controlled heat input and fewer porosity and cracking issues.

Explore ESAB's Aluminium Welding Solutions

Inspection, Rework & When to Stop

Even with a robust system, defects can occur. The key is to catch them early and avoid making things worse.

• Use visual inspection for surface porosity, crater cracks and underfill.
• Apply appropriate NDT (radiography, UT, penetrant) for critical joints according to code.
• When porosity or cracking is found, grind or gouge back to sound metal, clean thoroughly, and review likely causes before re-welding.

If cracking recurs after careful rework and procedure checks, question whether the alloy, joint, restraint or process choice is fundamentally unsuitable; sometimes the right answer is a different filler, joint design or joining process.

Key Takeaways

Porosity is mostly about cleanliness and shielding; cracking is about composition, stress and cooling. Both are strongly influenced by preparation, procedure and how stable your aluminium system is.

Aluminium magnifies small mistakes: wire handling, fit-up, torch angle, gas leaks and cleaning all matter more than in many steels. Pulse MIG and aluminium-specific modes can significantly narrow the gap between “perfectly fine” and “defect-prone” by stabilising the arc and controlling heat input.

A system approach – power source, feeder, torch, filler and gas designed to work together – makes porosity and cracking the exception rather than the rule.

FAQs

Why do I see porosity only on some days with the same settings?

Environmental and cleanliness variables – humidity, draughts, gas changes, less-than-perfect cleaning – often explain “random” porosity. The base settings may be fine, but the margin for error is small.

Is porosity always caused by bad gas?

No. Gas purity and leaks matter, but so do surface contamination, dirty wire, bad liners, long arcs and turbulent gas flow. Gas is one link in the chain, not the whole story.

Why does my 6xxx weld crack with 5356 but not with 4043?

4043’s higher silicon content narrows the solidification range and generally improves crack resistance. 5356 can give higher strength but may increase crack sensitivity depending on alloy, thickness and restraint.

Can I “burn out” porosity by turning up the heat?

Usually not. Higher heat can sometimes make porosity worse by stirring gas and increasing reaction rates. Focus instead on cleaning, shielding and arc stability.

What should I check first when cracking appears?

Start with filler selection and restraint: confirm you’re using a suitable filler for the alloy, and that fixtures and joint design aren’t excessively locking the part. Then look at heat input, crater fill practice and whether run-on/run-off tabs could help.