A drawing that says no porosity is not a specification. It is an instruction that cannot be met, cannot be measured and cannot be argued about usefully. Every casting contains some porosity at some scale, and the question that matters is which porosity, where, at what size and under what acceptance method.
Gas porosity, shrinkage porosity and entrained inclusions behave differently, appear in different places and respond to different process corrections. A specification that lumps them together gives the foundry no way to target its process and gives the inspector no way to decide consistently. Two inspectors looking at the same radiograph will reach different conclusions, and both will be defensible.
The specification has to be written in terms of the function of the region. A boss that carries a bolt has a different requirement from a wall that holds pressure, which has a different requirement again from a rib that carries only bending. One global acceptance criterion across all three is either too strict for the rib or too loose for the wall.
Zoning means dividing the casting into regions and stating a separate acceptance requirement for each. The regions are defined by function and by consequence of failure. A pressure boundary zone, a machined sealing face, a structural zone and a non critical cosmetic zone are four regions with four different requirements, and stating them separately is what makes the specification enforceable.
The zones need to be shown on the drawing, not described in prose. A note that says porosity shall be minimal in critical areas is not a zone. A marked up view with each region identified and a requirement attached to each is a zone, and it can be inspected against.
Zoning also has a commercial effect. A casting with a single demanding requirement everywhere will be priced as though every region were critical, because the foundry has to assume it must achieve the hardest case throughout. Zoning allows the process to be targeted and the price to reflect the actual requirement.
Porosity is measured by a method, and different methods see different things. Visual inspection sees surface indications. Radiography sees internal density variations. Pressure testing sees connected leakage paths. Dye penetrant sees surface breaking defects. A specification that states a limit without naming the method is incomplete, because the limit is meaningless without knowing what is being looked at.
The method also determines the sampling. A radiographic standard applied to every casting is a different proposition from one applied to a first article and a periodic sample. Both may be appropriate, but they must be chosen deliberately and stated, because the cost difference is substantial and the assurance difference is not always what it appears.
Where a pressure test is the acceptance method, the test conditions belong in the specification. A leak test at one pressure is not equivalent to a leak test at another, and a test that is applied after machining tests a different component from one applied before.
A usable porosity note has four parts. It identifies the zones, states the requirement for each, names the acceptance method and states the sampling. Anything less leaves a gap that will be filled by assumption, and the assumption will be whatever the foundry or the inspector finds most convenient.
The note also needs to be consistent with the machining stock and with the casting process. A requirement for sound metal at a finished surface implies a requirement at some depth below the cast surface, because the machining operation will remove the outer layer. Specifying only the finished condition leaves the foundry to infer the as cast condition, and the inference may not match.
Intake and exhaust components sit in a difficult position. They are often thin walled, they often carry pressure or vacuum, and they often have machined interfaces that must seal. The combination means that the regions where porosity is least acceptable are frequently the regions where the casting process is least forgiving, because they are at junctions and at the ends of long flow paths.
A specification that acknowledges this is more useful than one that does not. It allows the foundry to concentrate its process control where the risk is, and it allows the designer to see where the geometry is creating a requirement that the process will struggle to meet. That conversation is far more productive at drawing stage than at first article inspection.
Components that will be welded instead, such as the assemblies in the titanium fabricated components range, carry a different acceptance question, because a weld has its own inspection regime and its own defect categories. The two routes are not interchangeable, and the acceptance specification should make clear which one it is written for.
Porosity acceptance, properly written, is a description of what the part has to do. It says which regions must hold, which must seal, which must merely carry load and which are cosmetic. A drawing that states this is a drawing a foundry can quote against and an inspector can work to. A drawing that says no porosity is a drawing that will be argued about for the life of the programme.
A result without a location is not a result. Each inspection record should say where the indication was found, at what depth and by which method, so that the pattern can be read across several castings. TIG aluminium welding inspection records carry the same information about a joint.
Two inspectors can look at the same film and reach different conclusions, which is why the acceptance note names the method and the zone rather than stating one general limit. aluminium fuel tank range and aluminium water tank range components are specified the same way.
Some indications can be repaired and some cannot, and the drawing should say which. A welded repair to a casting is a joint in its own right, and the custom parts and TIG stainless steel welding standards are the reference for how it should be made.
