How a leak path forms
A leak is rarely one large defect. It is a chain: a pore connected to a cold shut, connected to a micro-shrinkage void, all within a few tenths of a millimetre of the sealing face. None of them would fail inspection on its own. Connected, they form a path that holds until the part sees pressure.
That explains the usual frustration: the part passes X-ray sampling, passes visual inspection, and still leaks on the bench, because the sampling never crossed the connected path. A pressure test proves function, not microstructure.
First steps when leakers arrive
- Contain. Hold suspect lots at both sites and agree a 100% leak screen until the cause is known.
- Record the evidence. Leak location on the part, test method, medium, pressure, dwell, stage (as-cast, machined, coated, assembled) and lot numbers.
- Rule out the test. Fixture seals, plugs, temperature and background helium cause false leaks; repeat on a known-good part.
- Cut or scan the leak site. Section, X-ray or CT the exact location to see whether it is gas porosity, shrinkage, a cold shut or a crack.
- Find the cause in check order (below), then decide: fix the process, repair under approval, or change the route.
Six root causes, in check order
The order runs from the cause cheapest to verify to the one that needs the most evidence. Confirm each before moving to the next.
| Cause | What happens | Evidence to ask the supplier for |
|---|---|---|
| 1. Poor metal flow | Unstable fill or poor gating entrains gas, tears oxide films or leaves two fronts that never fuse | Gating layout, fill parameters, location of cold shuts relative to the gate |
| 2. Venting and overflow | Trapped air becomes hidden porosity on the sealing channel; slow, intermittent leaks | Vent and overflow positions against the leak location |
| 3. Die thermal balance | Steep gradients cause shrinkage and cracks at hot spots and wall changes | Die temperature records, cooling circuit check after long runs |
| 4. Contaminated metal | Hydrogen, moisture or oxides release gas and inclusions during solidification | Degassing practice and hydrogen limit, ingot drying, filtration, recycled-material control |
| 5. Process parameters | Low intensification pressure, unstable speed or wrong hold time leave shrinkage uncontrolled | Parameter window and logged values for the leaking lots |
| 6. Die wear or design | Worn parting faces and poor gating build defects into the sealing region over time | Shot count, die maintenance records, dimensional trend |
Which test finds what
| Method | Use it to |
|---|---|
| Air or water pressure test, pressure decay | Batch screening against a maximum pressure drop over a stated dwell |
| Helium leak detection | Find leaks below the bubble threshold where the part's function depends on it |
| Water immersion | Locate the leak point quickly |
| X-ray or industrial CT | See the porosity, shrinkage or crack behind a failure; CT shows connectivity in 3D |
| Dye penetrant | Show open surface defects: cracks, cold shuts, surface porosity |
More on methods: leak testing aluminum castings and helium leak testing.
Repair or scrap
A leaking part is not automatically scrap, but repair is a controlled decision, approved in advance on any pressure boundary.
| Remedy | Suits | Limit |
|---|---|---|
| Vacuum resin impregnation | Micro-leakage from shrinkage and porosity | Must be permitted on the drawing; reinspect after |
| Weld repair | Accessible cracks, cold shuts, near-surface pores | Defined repair area; re-machine sealing faces; customer approval |
| Re-machining | Shallow surface defects on non-pressure faces | Not for defects that cross the pressure boundary |
A through-wall crack is normally a reject. A repair on a pressure boundary that was not agreed in advance is an unapproved deviation.
When to change the process or the design
If the part must hold pressure with very little defect budget, repeated leak screening sorts bad parts out instead of preventing them. Revisit the design (wall thickness, machining stock at sealing faces) or the process. Squeeze casting applies pressure during solidification, typically 60 MPa and above in our process, which suppresses shrinkage and micro-porosity; it is worth evaluating for sealed parts that keep failing as die castings. It does not remove the need for a leak test.