I’ve lost count of how many times a foundry manager has called me, frustrated, saying: “My aluminum castings look fine coming out of the mold, but after heat treatment – blisters everywhere. My degassing is fine. What’s going on?”
I used to think blisters were always about hydrogen. Turns out, that’s only half the story. Sometimes it’s not the gas. It’s the invisible oxide films that the gas loves to cling to.
Let me explain what we’ve learned from watching dozens of foundries chase their tails on this.
First, What Do These Blisters Look Like?
Two common types:
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Small, round, smooth‑walled bubbles just under the surface. These are classic gas porosity. Hydrogen came out of solution and made little balloons.
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Irregular, rough pits – sometimes with a dark spot in the middle. Those are usually oxide‑related. An oxide film acted like a trap, and gas collected there.
Sometimes you only see them after machining. Sometimes they pop up in heat treat. The latter is a dead giveaway that gas expanded, but it needed a place to nucleate – often an oxide film.
The Usual Suspect: Hydrogen (But Not Always)
Everyone knows aluminum loves hydrogen. Moisture in the air, wet scrap, damp ladles – it all adds hydrogen. When the metal solidifies, hydrogen comes out of solution and forms pores.
If you have high dissolved hydrogen, you’ll get blisters. Period.
Solution: Degas. Use argon or nitrogen. Keep everything dry. Monitor with a reduced pressure test.
But here’s the frustrating part: you can have low hydrogen and still get blisters. I’ve seen foundries with perfect RPT results – under 0.10 ml/100g – and they’re still pulling their hair out over surface blisters.
Why? Because of the other culprit.
The Hidden Culprit: Oxide Films (Bifilms)
When your melt is turbulent – a poor gating design, a splashing pour, a sharp corner in the runner – the surface of the metal folds back into itself. That folded surface is an oxide film, also called a bifilm. It’s like a thin, invisible piece of plastic wrap floating inside your melt.
Here’s what happens next:
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Hydrogen atoms love to collect on that oxide film. It’s a low‑energy place to start a bubble.
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The bubble grows, trapped by the oxide.
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During solidification or heat treatment, that bubble expands and pushes up the surface. Blister.
So even if your total hydrogen is low, if you have oxides, you can still get blisters. The oxides give the gas a home.
How Filtration Helps (Even When Degassing Is Already Good)
A ceramic foam filter does not remove dissolved hydrogen. Let’s be clear about that. You still need degassing.
But a filter does remove solid inclusions – including those oxide films. And when you take out the oxides, you take away the nucleation sites for gas.
I’ve seen this play out many times:
Foundry calls: “We degas like crazy, but blisters won’t go away.”
We ask: “Are you filtering?”
They say: “No, we thought degassing was enough.”
We send them a few 30 PPI alumina filters to try.
They put them in the runner, close to the cavity.
A week later: “Blisters dropped by 70%.”
Degassing lowered the total hydrogen. Filtering removed the places where that remaining hydrogen could collect. Together, they worked.
How to Tell Which Problem You Have
Here’s a quick way to diagnose – no special lab needed.
| If you see… | Likely primary cause |
|---|---|
| High porosity on reduced pressure test | Too much hydrogen – degas better |
| Low porosity on RPT, but still blisters | Oxides – add or improve filtration |
| Blisters only after heat treat | Gas expanding – but oxides likely present as nucleation sites |
| Dark spots or irregular pits | Almost certainly oxide‑related |
Also, cut open a blister and look at it under a decent magnifier. Round, smooth holes = gas. Ragged, dark‑stained cavities = oxides.
Practical Steps to Kill Blisters (Both Kinds)
Step 1: Get your hydrogen under control.
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Rotary degas with argon.
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Keep scrap dry.
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Monitor with RPT or an Alscan.
Target: below 0.12 ml/100g for most castings. For critical stuff, below 0.10.
Step 2: Remove oxides with a ceramic foam filter.
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Use alumina (for aluminum).
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Choose 20 or 30 PPI. Start with 20, go to 30 if blisters persist.
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Place the filter in the runner, as close to the casting as possible.
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Make sure it’s sealed – no metal bypass.
Step 3: Fix turbulence at the source.
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Fill the sprue quickly and keep it full.
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Round sharp corners.
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Avoid long drops.
A filter can catch oxides, but it’s better if you don’t create so many in the first place.
Step 4: Preheat your filter.
Cold filter + hot metal = thermal shock = cracks = unfiltered metal. Preheat to around 350°C for 30‑45 minutes. Worth the extra step.
A Real Example (Anonymous, But True)
A few years ago, a foundry making aluminum oil pans kept rejecting parts for surface blisters after heat treatment. Their RPT numbers were fine – around 0.11 ml/100g. They were degassing.
We visited and watched their pour. The metal splashed into the sprue. The runner had a sharp 90° turn right before the ingate. Oxides were being made right in front of our eyes.
We recommended:
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Install a 30 PPI alumina filter in the runner, just before the turn.
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Round that sharp corner.
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Preheat filters.
They made the changes. Blister scrap went from 8% to under 2% within two weeks. No change to degassing.
The filter didn’t remove hydrogen. It removed the oxide films that were trapping gas.
The Bottom Line
If you’re fighting blisters:
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Don’t assume only hydrogen is the problem.
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Check your gating for turbulence – it may be making oxides.
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Add a ceramic foam filter if you’re not using one.
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If you already filter, try a finer PPI (30 instead of 20) and increase filter area to keep flow.
Degassing + filtration + smooth gating = the closest thing to blister‑proof aluminum casting.
Need a second pair of eyes on your gating or filter setup? Send me a photo or a sketch. I’ll tell you what I’d change – no sales pitch, just practical advice.
Email: info@sf-foundry.com
Tech support: 8618636913699