Porosity in Die Casting: Gas vs. Shrinkage, Causes and Fixes

Porosity is the most common reason die castings are rejected for leaks, weak sections or holes opened by machining. There are two different kinds, with different causes and opposite fixes. Adding pressure will not remove trapped air, and better venting will not feed a thick boss. The first job is always to identify which kind you have.

Step 1: Identify the type

Cut the casting through the defect area, or use X-ray, and look at the shape of the cavities and where they sit.

In practice both kinds often appear together: a shrinkage cavity that started on a gas pore is partly round and partly jagged. Treat the gas source first, because gas pores also weaken the feeding of shrinkage.

Gas porosity: where the gas comes from

There are three sources. Rank them before changing anything.

1. Air trapped during the shot. This is the largest source in most dies. Air in the shot sleeve, runner and cavity must leave through the vents before the metal closes them. It is trapped when:

• the sleeve is less than about 40 % full and the slow shot creates a wave that rolls over and folds air into the metal;
• the slow-shot speed is too high or too low for the fill ratio;
• the change to fast shot comes before the metal has reached the gate, so the runner is still full of air;
• vents are too small, blocked with flash or lubricant residue, or closed by the first metal to arrive;
• the runner has enlargements or sharp bends where the stream separates.

2. Gas from die and plunger lubricant. Water that has not evaporated before the die closes turns into steam. Excess plunger lubricant burns in the sleeve. Signs: porosity gets worse after a long spray, on the first shots after a stop, or on the side where the spray nozzles point.

3. Hydrogen from the melt. Less important in high-pressure die casting than in gravity casting, because solidification is fast, but it adds to the total. Wet returns, wet tools and long holding at high temperature raise it.

Fixes for gas porosity

Shrinkage porosity: why it forms

Aluminum alloys shrink by roughly 4–7 % in volume when they solidify. Thin walls freeze first. A thick section that is still liquid then has to be fed through the walls around it, and through the gate, by the intensification pressure. Once the gate or a thin wall on the feed path has frozen, the remaining liquid in the thick section shrinks with nothing to replace it, and a cavity opens in its centre.

So shrinkage porosity depends on four things:

1. how thick the section is compared with the walls that feed it;
2. how long the feed path stays open (gate thickness, biscuit thickness, die temperature along the path);
3. how much pressure is applied and how quickly it builds up;
4. how hot the die is at the thick section.

Fixes for shrinkage porosity

Be careful with higher intensification pressure: it increases the opening force on the die. Check the locking force before raising it, or the result is flash and a casting that is thicker but no denser.

A diagnostic sequence that saves time

1. Section or X-ray three castings from the same cavity and mark the pore positions. Same place every time means shrinkage. Wandering means gas.
2. Read the shot curve. Check the switch point to fast shot, the fast-shot speed actually reached, the pressure build-up time and the final pressure. Compare them with the values on the process sheet.
3. Measure the biscuit. Variation in biscuit thickness means variation in metal dose, which changes the switch point and the pressure transmission.
4. Inspect the die face. Look at vents, overflow gates and flash marks before touching machine settings.
5. Change one thing, run enough shots for the die to reach thermal balance again, and section again.

How to measure and specify porosity

On the drawing, define zones. Ask for low porosity only where it matters: sealing faces, machined bores, threaded bosses and highly loaded sections. A general requirement for "no porosity" cannot be met by this process and makes every casting a discussion. Reference radiograph standards for aluminum die castings give graded levels that both sides can agree on.