Key points
- X-ray grades internal indications in the condition inspected; it does not define a machined face or a leak path.
- Machined-face requirements name the zone, condition after final machining, measured quantity, limit, sampling, evidence, and failure response.
- Machining removes the dense skin and can open gas porosity or shrinkage in thick sections and tapped bosses.
- Sealing faces are decided by the leak test under defined conditions, not by appearance alone.
- Voids at a tapped hole need identification before disposition: breakout, thread defect, gas pore, or shrinkage.
- Impregnation is a buyer drawing decision; where permitted it usually follows final machining and is followed by leak test.
- Shrinkage that keeps returning in one zone is a route or design question once the inspection condition and method are confirmed.
Why parts pass X-ray and still show pores after machining
X-ray is a projection of internal indications; views and beam directions must be agreed, and a clean radiograph is not a leak test. A radiographic grade is assigned by discontinuity type and zone, not as a pore diameter or area percentage on a machined face.
Acceptance must state the stage: as-cast, after heat treatment, after machining, or as supplied. An as-cast X-ray acceptance can be met while final machining opens porosity below the skin.
Machining removes the dense skin and exposes subsurface metal. A pore there may be gas porosity, shrinkage, or a connected path that only a leak test reveals. A part can pass X-ray sampling and visual checks and still leak through connected pores, a cold shut, or micro-shrinkage.
Gas porosity, shrinkage or a machining defect? Identify before you judge
Name the defect from a shared table before applying a limit; the same visible pore can have different causes and next steps.
| What you see on the machined face | Likely type | How to confirm | Typical next step |
|---|---|---|---|
| Rounded, smooth-walled voids, isolated or clustered | Gas porosity | X-ray, sectioning, machined-face inspection | Reject in sealing zones; accept elsewhere only within the drawing limit; leak test decides a sealing face |
| Ragged, clustered voids in a thick section or tapped boss; may appear only after machining or as a leak | Shrinkage | Sectioning, CT, and location against thick sections; keep separate from gas porosity | Review the casting route; reject if outside the drawing limit or leak criterion |
| Voids around a drilled or tapped hole wall; compare clean machined edges with voids around the hole | Machining breakout, thread defect, or porosity the tool broke into | Inspect the hole wall at magnification; record the leak location | Reject or review as a deviation; do not re-machine a pressure boundary without approval |
| A seam or line on the machined face | Cold shut or cold flake | Visual and dye penetrant; leak test on pressure parts | Reject on sealing faces and load paths; review gates against sealing faces |
| Foreign material with irregular shape, color, or hardness difference | Inclusion | Magnification on the machined face; foreign-material grade on the radiograph where specified | Reject where exposed by machining on a functional face |
Write the machined-face requirement by zone
Machined-face porosity acceptance is a zone-based agreement; the buyer's drawing and specification set the values. State the condition after final machining and before coating. Define what is measured: area fraction, projected indication, and volume fraction are not the same result.
| Zone | What to specify | Usual disposition if it fails |
|---|---|---|
| Sealing face / O-ring groove | Condition after final machining; [max size], [count] per [area], [minimum spacing]; flatness, roughness, and seal mating; leak test medium, pressure, method, and pass/fail limit | Reject if outside the sealing-zone limit; appearance alone does not accept or reject; permitted impregnation is followed by leak test |
| Tapped holes and threads | Depth and location of allowable porosity; [max size], [count] per [area]; no void interrupting thread flanks or reducing engagement; magnification and thread gauge | Reject if thread form or pressure boundary is affected; nonfunctional holes may be reviewed as a deviation |
| Bearing or locating bores | Pore limits on the functional surface; [max size], [count] per [area]; condition after final machining; effect on fit, wear, or load path | Reject if outside the drawing limit |
| Non-functional machined faces | Cosmetic or visual limit; [max size], [count] per [area]; defined evaluation area and lighting | Accept within limit; reject outside it |
| As-cast cosmetic faces | Visual limit per zone; reference grade where used; state as-cast condition | Accept within the drawing limit; reject outside it |
A reference standard alone, or a note such as "low porosity", does not complete the agreement. Add sampling, the evidence to keep and the failure response. For porosity in tapped holes in castings, state whether acceptance is based on the hole wall, the thread form, the leak test, or a combination.
How to inspect a machined face
Inspect after final machining and before coating. Use magnification and a pore gauge or measuring loupe, define the evaluation area, and record location, part ID, images, and who interprets.
For sealing parts, run the leak test after the last operation that can open a path. Record where the leak is, not just that the part failed.
For root cause, section or CT-scan the exact site to tell gas porosity, shrinkage, a cold shut or a crack apart; CT also shows connectivity in 3D on first articles. The inspection and leak testing approach page describes the equipment and methods.
Accept, impregnate, re-machine or reject
Accept when the part meets the drawing limit for its zone, the stated condition, and the leak criterion where one applies. A pore on a sealing face is judged by the leak test, not by appearance alone; the sealing-face reject criteria should say so.
Impregnation is allowed only where the drawing permits it. Vacuum impregnation seals micro-porosity that connects through the wall; it does not repair cracks, cold shuts, large shrinkage, or a machining breakout, and it does not raise strength. Where permitted, it usually follows final machining, because machining opens porosity sealed inside the as-cast skin. Specify sealant compatibility, process standard where used, leak test after impregnation, cycles allowed, and marking or records.
Re-machining is only for shallow surface defects on non-pressure faces, not defects that cross the pressure boundary. An unagreed repair on a pressure boundary is an unapproved deviation. Record any concession as an approval, not a silent change. Reject when the zone limit is exceeded or the leak criterion fails after permitted processing.
Prevent it at design and quotation stage
Machining stock must clean up the cast surface without breaking into porosity. Keep sealing faces away from the last-to-freeze region where possible, and place tapped holes out of the thickest section. Review parting line, ejector pins, gates, and overflows against sealing faces. Where machining could affect sealing, agree the casting route and acceptance first, and review any stock change made after tooling.
Alloy and melt quality are part of the porosity cause. The A356 and A356-T6 specification page explains casting versus ingot designations and melt verification. A proposed alloy change to cure porosity is a specification change and needs a documented substitution review; see the alloy substitution approval checklist.
Repeated shrinkage in one zone is a route question. A high impregnation rate sends the same signal. For a porosity-prone high-pressure die-cast part, the guide on moving HPDC porosity problems to squeeze casting covers that review. No process guarantees zero porosity, and a squeeze-cast part uses the same zone-based criteria.
How CEX handles cast and machined parts
CEX machines its own castings on in-house CNC machines and performs T6 in-house before finish machining of critical bores and faces, so final dimensions are checked in the delivered condition (company-reported). X-ray inspection is in-house, CT is agreed separately where a project needs it, and CEX runs water immersion and helium sniffer leak tests and builds the leak-test fixture per part in-house (company-reported). CEX measures critical dimensions on a CMM (company-reported). When impregnation is not stated on the drawing, CEX asks before quoting and does not assume permission (company-reported).
Qingdao Dechengxin Metal Products Co., Ltd. is the manufacturing entity and holds ISO 9001:2015 and IATF 16949:2016 in its own name; the IATF 16949:2016 scope is manufacturing of aluminium alloy die-cast parts. CEX is a combined factory and trading company: the factory plus the trading company Qingdao Chengenxin, under the same owner. The acceptance limits for porosity on machined surfaces still come from the buyer's drawing and specification.
When you send an enquiry, mark the sealing faces, tapped holes, and machined-face limits on the drawing, and state whether impregnation is permitted; the casting RFQ checklist lists the inputs to include.