Small Zinc Casting - Help Rating Porosity 1

[twistedneck]

Developing a small zinc casting using Zamak5, its not a precision or a thin walled casting, however extreme voids and porosity can result in a structural failure (yielding of the part).


What would be a good visual inspection method and standard to use? Of couse we have recommendations from our supplier, but i need help form the board here too.

Xray is not possible
Water weighing may work but the enviornment is very dirty
Cutting parts in half with a band saw is quite easy

Thank you

 

[TVP]

I'm very familiar with porosity that can develop in small zinc parts manufactured using high pressure die casting, but I am a little confused by your question. By visual inspection, do you mean optical examination of a metallographically prepared (sectioned, polished) sample? Why is it important that the end use environment is very dirty when discussing the use of water weighing? Wouldn't you have the manufacturer perform precision measurements of the mass using an appropriate technique (digital mass balance) in order to reject parts with excessive porosity? Why can't the manufacturer use x-ray imaging to reject parts if the customer (you) will possibly reject entire lots of parts due to excessive porosity?

 

[twistedneck]

TVP, thanks for responding.

"By visual inspection, do you mean optical examination of a metallographically prepared (sectioned, polished) sample?"

Yes, but at our plant we do not want to be polishing or mounting, only sectioning and looking for large voids - internal porosity does not reduce the function of our part.

"Why is it important that the end use environment is very dirty when discussing the use of water weighing?"

"Wouldn't you have the manufacturer perform precision measurements of the mass using an appropriate technique (digital mass balance) in order to reject parts with excessive porosity?"

In our plant setting its quite dusty and dirty, the zinc casting is enclosed, however there may not be room to setup the mass balance - would slow us down taking it to a clean room. also, since this checks density - general porosity, and that's not important to us - its not value added.. we're looking for large voids.

"hy can't the manufacturer use x-ray imaging to reject parts if the customer (you) will possibly reject entire lots of parts due to excessive porosity?"

X-ray and spectrometer are just too expensive. This is an 'overmold' type casting, with a very low functional and dimensional requirement only. Stress is a non factor, and porosity - fatigue - etc..do not come into play. so its not necessary IMO. but that's why i wrote this thread

i want operators to do functional testing, then if that test falls below process bounds we can section the parts and look for gross porosity and large voids by visually inspecting. i'm looking for a way to rate that. in addition, need your thoughts on the sectioning concept in general.

 

[CoryPad]

With the restrictions you place, it seems like the following might work:

Use a balance to determine part mass - not a mass density determination using Archimedes' principle, but rather, a simple balance that can be used in a dirty environment (plenty of those available). Large porosity means large mass difference.

If you get parts with low mass, use a precision saw (thin diamond or CBN blade) to section the parts. The resulting cut leaves a surface that you could inspect for gross porosity.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Compositepro]

Your concern seems to be "will the part break in use due to an undetected void". How about simply proof testing the part? A simple test jig might work, and it will detect any kind of flaw that will affect strength and only strength.

 

[twistedneck]

Coreypad, thanks for the idea about the precision saw and simple water balance. if it shows its below a certain mass then its known to be suspect - and that can be done quickly w/o cutting or mass balances. thank you!

Compositepro, we toyed with proof testing however it seems to cost too much at the moment. both in terms of facilities and time. right now its an ultimate functional test at the start and end of each shift and well controlled casting parameters, alarms, and understanding of limit parts.

What would be a good visual rating system to use?

I"ve been told that NADCA has a good quick visual standard - thoughts?

 

[CoryPad]

Yes, get it

 

[TVP]

twistedneck,

You can register with NADCA and freely download some information that includes some visual guides on porosity. Use the following and look for Section 7: Quality Assurance, specifically subsection 6 Porosity:

http://www.diecastingdesign.org/resources/scriptcontent/402.cfm?section=unknown&activesection=home

 

[tomwalz]

We measure porosity in tungsten carbide in a size rating

A Porosity:
Pores in the microstructure less than 10 microns in diameter.
Rated from A01 to A08.

B Porosity:
Pores in the microstructure 10-25 microns in diameter.
Rated from B00 to B08.

C Porosity:
Not true porosity. Rather, carbon porosity consists of discrete areas of graphite in the microstructure resulting from an overabundance of carbon.
Rated from C00 to C08.

Free Carbon: A term used to describe C Porosity in excess of C00

You could set a standard such as
1. No porosity allowed over XXX in the largest dimension
2. One porosity allowed per square inch no larger than xxx in the largest dimension
3. No more than 5 incidences of porosity where the largest dimension is between xxx and yyy per square inch.

This makes more sense with photos. The photos should be at

http://www.carbideprocessors.com/level2carbide/level4carbidetecharticles/level4-10wcdefects.htm

We are going through a big web site revamp so I am not entirely sure what you will find.

Feel free to contact me directly.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[arunmrao]

twistedneck,
How difficult is the configuration of your casting,that you are unable to control the occurrence of porosity defects. Is it due to air entrapment misrun or shrinkage? Perhaps this might help you in improving the reliability of the casting. suggestion of using specific gravity method is very simple and reliable. Have you considered magnaflux technique to determine porosity?

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[twistedneck]

Fisrt off, thanks very much again for the new suggestions from TVP, tomwalz, and arunmrao.

I downloaded the quaility manual from NADCA, looks like a great start, i also agree with making a simple visual comparson standard based on pics.

To answer your question arunmrao, the casting is not difficult, the hard part is we are overmolding a steel part, and that adds variability. also in prototype we saw a lot more variablity than we see in production.