Best Method and Equipment for Measuring Porosity of Customer supplied castings

[Toefuzz]

Greetings! I'm wondering if any of the metallurgists out there can tell me of a quick and easy method for measuring porosity in castings (typically zinc and aluminam)? Of course I realize there probably isn't a truly 'quick and easy' method but am interested to know how I would go about measuring porosity, equipment involved, time required, etc. I'd like to set up some sort of incoming inspection on customer supplied castings but really don't even know where to start.

[Miner]

I am only familiar with destructive methods, or limiting the inspection to machined areas. The porosity acceptance criteria is typically a maximum size and quantity per area.

Is porosity a concern from a structural strength perspective, a potential leak path or other?

If it is a strength concern, I believe that x-ray inspection may be used, but I am not experienced in that area.

[Caster]

Originally Posted by Toefuzz

Greetings! I'm wondering if any of the metallurgists out there can tell me of a quick and easy method for measuring porosity in castings (typically zinc and aluminam)? Of course I realize there probably isn't a truly 'quick and easy' method but am interested to know how I would go about measuring porosity, equipment involved, time required, etc. I'd like to set up some sort of incoming inspection on customer supplied castings but really don't even know where to start.

X-ray may not be able to resolve microporosity. It will see marco or gross voids for sure.

The best method is destructive. Slow and expensive. Section the part, polish and perfrom image analysis for % voids. There are vendors that can set you up iwith all equipment, ncluding training. Just ask if you need some links.

If you machine the part you could count voids per unit area on the machined face I suppose. It would be fast and easy.

Resonance inspection is coming along but is still R&D. Ask if you want a link, they are looking for R&D partners.

If the part is small and porous enough you may be able to do something with density (weigh in water, weigh in air, compare to theoretical density)

How about the vendor, we are a foundry and customers always push these things back to us. What can they do for you?

Do they understand the porosity spec? How do they check before they ship? Why do you have to check? Please don't tell me these castings come from China!

[ksanders]

Resonance Inspection works well on castings and forgings to detect inclusions, significant shorts, damage, and large porosity, but does not work well for small porosity. The biggest issue with Resonance is that it only tells you the casting is different from the test set. After it is flagged, you have to do the destructive testing to try and determine why. There is at least some risk that you can include an undesirable condition into your 'good' test set by accident.

For X-ray, we use ASTM 155-E, which provides for visual standards for fluoroscope. Soem X-ray equipment suppliers offer Automatic Defect Recognision, which can be effective provided it is properly tuned. Again, the key is managing the level of 'false rejects'.

[rstocum]

This area has been a major headache for my employer for some time now. To detect the presence of porosity inexpensively you can use a pressurized dunk tank test. This means making a fixture that masks off any open areas, using gasket material or o-rings wherever possible. You put a quick disconnect connection in the fixture so you can pop an air hose onto it, and you pressurize the parts. Check any applicable specifications and the drawing to determine if there is a max pressure you should not exceed, or a min the part should withstand. Dunk the pressurized part under water and look for bubbles that come from cast surfaces. If the casting is machined pay special attention to the machined faces. Machining opens up porosity that is just under the cast surface. Having two fixtures so you can load one while the other is in the tank speeds things up a bit.

The disadvantages - SLOW, time consuming, and can be difficult to tell if the problem is porosity if the bubbles are coming from seal joints. Determining this involves using your sealing devices on known good parts that you have to keep for validation purposes. You also have to replace the sealing devices annoyingly often. However it is reusable to a great degree, and as long as you are not 100% inspecting large lots it is relatively inexpensive.

We also use liquid penetrant inspection through a sub-contractor, and it is very good at detection of porosity including microporosity. LPI can be used in conjunction with destructive testing mentioned in a previous post to show that porosity is linking up with shrinkage cracks or other inclusions to form a leak path from the inside of a part to the outside.

X-ray isn't bad, but there are things it can't see due to x-ray "shadows". This is particulary bothersome if your part has multiple cavities. X-ray ain't cheap either.

Resonance is good for 100% testing of large lots, but the equipment is sensitive to a lot of common things in manufacturing environments, and can often need re-calibration. As mentioned by someone else, having a good master part, and mastering to it often is a good practice. We use a resonance testing machine in conjunction with a conveyor line. It's a pain, but if we had to flag parts with porosity by any other method on that process we would'nt be in business much longer.

[ubilalmansoor]

your answer for the porosity inspection by leak testing is used to check the porosity in which the porosity hole connect the external environment. IF the porosity presence in body connects the other hole inside the body, can we able to detect the porosity by leak testing?

[rstocum]

ubilalmansoor,

The dunk tank test I spoke of is not adaptable to check casting porosity from one internal cavity to another. Thankfully, that is not a problem I have had to deal with. That sounds like a major headache from the detection standpoint. One of our customers uses an air gaging system that senses pressure changes from one cavity to another inside a valve casting. These castings are machined, and partially assembled with internal components that seal the cavities off from one another. Something like that might work for you. Otherwise, I think doing non-destructive liquid pentrant testing on castings that you suspect would be a good place to start for you. Once you are sure the problem exists, you could try to improve casting methods, and set up a sampling plan to LPI the parts for inspection purposes. Good luck to you.

[JaxQC]

I’ve used the E155 standard previously and been able to live with the results. The thing I wanted to point out is that all castings have some level of porosity. If you can’t see it, go to the next sized microscope. Seriously it all depends what your part can function with.

The E155 standard references Levels of porosity which you should then define on your print. It can encompass the entire part or have different Levels depending where it is located on the part. ie… thin “arm” area would need to be more pristine (Level 1 porosity) while a thicker “truck” area could afford to be a little worse due to its additional mass providing strength (Level 3). It went from Level 1 (almost pure) to Level 4 (gross and I would reject for most applications (Level 3 being the worst I’ve used for auto parts).

The standard is very specific on defining them so it not as ambiguous as it sounds. You should have each cavity evaluated rather than just a couple since I have seen only specific cavities have problems with flow and heat patterns. RETAIN a copy of the passing x-rays yourself in addition to the supplier rather than just relying on your supplier to have the only copy. The x-rays are able to be put on CD / electronically so keeping them is not the hassle it used to be.

Part B of the equation is to control the process parameters. The x-rays should be used to validate the process settings (temp, pour speed, packing pressure, packing time, re-melts (drives iron content up) etc. If they are not controlled though, it is all for nothing.

Hope it helps explain it a little.