Powdered Metal - Die Design and Other Parameters

[goldenfab]

I'm looking at making some prototype powdered metal parts in house which is something I do not have experience with. I'm looking at doing some research on different solid lubricant add mixtures to powdered metal parts which is the reason I am not just having this farmed out. The part is a simple ring as shown in the drawing below. I added a 3 degree draft angle to hopefully simplify the process. I have some 90/10 pre-mix 100 mesh powder order. I plan on placing the pressed part in a vessel purged with argon in an oven with a type K thermocouple to sinter. My question is regarding the die design.

1. Will a two piece top an bottom design with 3 degree tapered walls suffice? My hope is the the part fall out after pressing without needing any sort of ejectors. Is this realistic?

2. I plan on cold pressing via manual operation with a 20 ton shop press, nothing close to a production environment. The cross sectional area for the part is only 0.38in^2 so theoretically I could achieve up to 105,000psi?

3. I want the part about as dense as practical so what pressing pressure should I consider?

4. Is there a rule of thumb on the fill volume vs pressed volume to use when designing the die? I suppose this is easy to figure out for a before and after pressing density but I don't know what is realistic for the pressed density.

5. What shrinkage rate to expect after sintering?

6. From what I have gathered the sintering schedule for something like this would be around 1,500F for 30 minutes. Does this sound right?

 

[replica]

What is the powder? steel?

 

[goldenfab]

Sorry forgot to say it's bronze.

 

[EdStainless]

https://www.copper.org/resources/properties/129_6/characteristics_properties.html#a2

http://www.scmmetals.com/premixed-bronze-powders

http://www.dtic.mil/dtic/tr/fulltext/u2/a344923.pdf

Do you want the finished part at full density or do you want it porous to retain oil?
There are compositions that actually grow when sintered.
Are you sure that you want to use pure Ar? No hydrogen?
We used to use ammonium sterate as a lube, it is more volatile than Ca Sterate, and it is a common additive in cosmetics so you can buy it.

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P.E. Metallurgy, Plymouth Tube

 

[goldenfab]

Thank you very much for the resources. I want as high a density I can get within reason. It will not need to hold oil because it will have solid lubricant particles mixed in - the whole point of this exercise.

The argon was a guess on my part because it is used a lot as a cover gas for things like welding and because I have a lot of it on hand (gas welding bottles). If I need another gas I will get it. Hydrogen sounds dangerous though. I'm not sure I have the process control to ensure there will be no mixing with oxygen.

Was the ammonium sterate used to lube the die when pressing or for actual lubrication properties of the finished part? I forgot to mention I am only interested in the latter as I am trying to develop a self lubricating sealing ring.

 

[EdStainless]

no. we used it as a pressing lube. There usually needs to be one, but since yours is a shallow so you may not need it.
Your draft will be an issue. Since the loose density of the powder is less than 1/2 the pressed density, in order to get a part 0.200" tall you will need a die roughly 0.500" deep. And if it is drafted what size will you make the punch? You really don't want powder between the die and punch.
For lubed bearings the lube is impregnated after the sintering. Often using a solvent and vacuum.

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P.E. Metallurgy, Plymouth Tube

 

[goldenfab]

Thanks for the feedback.

I don't want the draft in the final part. My thinking was it would allow the part to come out of the die. If I get rid of the draft is the part going to come out for a two piece die design? What would you recommend instead?

 

[EdStainless]

You will need to sinter in a reducing atmosphere in order to get consolidation.
I don't know how reducing you need, maybe you can get buy with 10% hydrogen.
You would purge with Ar until the vent gas oxygen is low (like below 0.2%) and then add the hydrogen to the stream.
This will require a higher sinter temp, maybe 1650F or so.

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P.E. Metallurgy, Plymouth Tube

 

[btrueblood]

You don't need any draft for PM parts, typically, they just slide out of the die by using a simple lift (thus the stearate lubricants). By having tight cylindrical fits (as opposed to a draft taper), the press self-cleans the cavity each time the part is lifted/ejected. Also makes it simpler to press, as Ed says.

 

[goldenfab]

Thank you all for the very helpful feedback and resources. I have material on order and will re-design without draft as advised.

 

[WKTaylor]

Your part resembles a 'plain sintered bearing' we use with oil impregnation .

MIL-B-5687 [CX] BEARINGS, SLEEVE, WASHERS, THRUST, SINTERED, METAL POWDER, OIL-IMPREGNATED, GENERAL SPECIFICATION FOR

ASTM B243 Terminology of Powder Metallurgy

ASTM B255 [CX] Standard Specification for Sintered Bronze Structural Parts

ASTM B282 [CX] Standard Specification for Sintered Brass Structural Parts

ASTM B328 Standard Test Method for Density, Oil Content, and Interconnected Porosity of Sintered Metal Structural Parts and Oil-Impregnated Bearings

ASTM B438 Standard Specification for Bronze-Base Powder Metallurgy (PM) Bearings (Oil-Impregnated)

ASTM B439 Standard Specification for Iron-Base Powder Metallurgy (PM) Bearings (Oil-Impregnated)

NOTE.
Word-search ASTM specs for 'sintered', 'powder metal', etc for many other powdered/sintered metal parts, various alloys.

NAS429 Bearing - Sintered, Plain

NAS430 Bearing - Super-Sintered, Plain

NAS470 Bearing - Sintered, Flanged

AMS4805 Bearings, Sintered Metal Powder 89Cu - 10Sn Oil Impregnated

MS17795 [CX] BEARING, SLEEVE, PLAIN TYPE, SINTERED BRONZE, OIL IMPREGNATED

MS17796 [CX] BEARING, SLEEVE, FLANGED, SINTERED BRONZE, OIL IMPREGNATED

MS21783 [CX] BEARING, WASHER, THRUST, SINTERED BRONZE, OIL IMPREGNATED

NOTE. We use a fair amount of high density sintered Tungsten [AMS7725 AND ASTM B777] in aircraft for 'ballast' parts [bars, plates, shapes, etc].

Regards, Wil Taylor

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