Gas Porosity in Die Casting: Causes, Prevention, and Why It Blocks T6 Treatment

This is a classic "silent killer" in the foundry. There is nothing more frustrating than pulling a batch of expensive castings out of the oven after a T6 cycle only to find they look like they’ve got a case of the measles.

Thoseblisters aren't a heat treatment problem; they are a casting quality problem that was hiding until you turned up the heat. If you want to run T6 on HPDC parts, you have to master gas control. Here is the reality of why gas porosity happens and why it makesstandard heat treatment a nightmare.

Gas Porosity: The Invisible Saboteur

First, let's distinguish the "bubbles" from the "holes." Unlike shrinkage porosity—which is caused by the metal pulling apart as it freezes—gas porosity consists of smooth, round voids. It’s essentially trapped gas that had nowhere to go.

In my experience, you’re usually fighting a war on three fronts at once:

1. The Shot Sleeve "Air Trap"

This is the big one. When your plunger hits that metal at 50 or 100 m/s, it’s not a smooth flow; it’s a chaotic spray. If your slow shot phase isn't perfectly timed, the metal "folds" over the air in the shot sleeve before it even hits the gate. You’re essentially injecting an air-aluminum aero-chocolate into your die.

2. Drowning the Die in Lube

We’ve all seen operators over-spray the die to "be safe." But if that water-based lubricant isn't fully evaporated, it turns into high-pressure steam the second the 700°C melt touches it. That steam has to go somewhere, and "somewhere" is usually right intothe middle of your casting wall.

3. Hydrogen: The "Melt Quality" Issue

Aluminum is like a sponge for hydrogen, especially on humid days or if you're using dirty, oily scrap. If your hydrogen levels are above0.2 ml/100g, you’re starting the race with a flat tire. No amount of gating design will fix a melt that’s already saturated with gas. Rotary degassing with Argon or Nitrogen isn't optional—it's the bare minimum.

Why T6 and Gas Porosity Don't Mix

This is the part that catcheswrought-aluminum engineers off guard. They expect to T6 a die casting to get that extra 100 MPa of strength, only to end up with scrap.

Here is the physics of the "Blister":During a T6 cycle, you’re heating the part to roughly490–540°Cfor several hours(Solution Annealing). At these temperatures, two things happen simultaneously:

Gas Expansion:According to the gas laws (Boyle/Charles), that tiny bubble of trapped air wants to expand to about2.5 to 3 timesits original volume.

Softening:The aluminum matrix becomes incredibly soft—almost like butter—as it approaches its solidus temperature.

The internal pressure of the expanding gas simply pushes the softened aluminum outward. You get a surface blister, or worse, internal "balloons" that destroy the part's dimensional integrity.

The "T5" Compromise

If your process isn't clean enough for T6, you’re stuck withT5 (Artificial Aging only). You skip the high-temperature solutionizing and go straight to the aging oven at ~160-180°C. You’ll get a decent bump inhardness and yield strength, but you’ll never reach the "Superstar" mechanical properties that a full T6 cycle offers.

How to Actually Fix It

If your customer is demanding T6 properties, "business as usual" won't cut it. You need a two-step strategy:

Vacuum-Assisted Die Casting (VADC):This is the gold standard. You pull a vacuum on the die cavitybeforethe shot. If the air isn't there, it can't get trapped. A solid VADC setup can cut your porosity by 80%, making T6 perfectly viable.

The "Slow Shot" Sweet Spot:Use your machine’s sensors to ensure the metal front in the shot sleeve moves progressively. You want a "wave" that pushes air out, not a splash that traps it.

The Bottom Line

Gas porosity is aninherent "feature" of high-pressure die casting—until you decide it isn't. If you’re seeing blisters after heat treat, don't blame the oven. Go back to your melt prep and your injection parameters. You can't "cook out" gas; you have to keep it out from thestart.