Designing investment cast molds help? 1

[geneiusxie]

Hi,

I'm doing some research on whether investment casting would be a good way to reduce production costs for my company, but I want to make sure that the quality of the finished parts is top-notch.
NOTE: some of these questions are kinda noobish haha

How do you make sure the metal seeps into all the little crevices in a complex mold? Couldn't air bubbles form when you pour in the liquid metal - effectively displacing it? Does the ceramic material let air out or something?

Can you cast extruded metals such as 7075 aluminum and then heat treat them to be the same strength as an equivalent CNC machined part? Heat treating might be expensive, but it would still probably cost less than CNC machining.

What if sand particles get into the metal? Would that ruin its fatigue properties?

Would I need to put 7075 aluminum or stainless steel in an inert argon atmosphere for maximum strength?

What are the best tolerances I can expect? Wikipedia quotes 0.1mm or so. If I want to make a complex gear or other piece of machinery, I might need to do some finishing. 0.1mm might not be good enough.

 

[cloa]

Investment casting uses a ceramic so you shouldn't have sand present to become an inclusion.

http://www.eng-tips.com/viewthread.cfm?qid=175504

Looking in Wikipedia is a good start but a full google search including

http://www.eng-tips-com

could answer many of your questions.

 

[geneiusxie]

Hmm so I'm guessing casting 7075-T6 wouldn't be possible? Or is it possible, just much more complex than a simple heat treating to get the T-6 properties?

Also, what about casting high strength maraging steel? Like Vascomax 350? That stuff has a yield strength of >2400 MPa when heat treated, but is it possible to get just by heat treating a casting?

Or is CNC machining the only way to get complex, high-performance parts?

 

[cloa]

http://www.nickelinstitute.org/~/Me...ngSteel_EngineeringProperties_4419_.pdf#page=

Try searching nickelinstitute for maraging steel- it seems that modified grades are suitable for casting.

As I read the thread linked- it certainly difficult to impossible to cast 7075 as casting to net shape screws it up so much that heat treatment is very difficult and destroy the net shape that you wanted.

 

[geneiusxie]

What exactly makes regular maraging steel unsuitable for casting?

Also, why can't 6061 aluminum be cast? It can be welded and then heat treated to restore full T6 properties. And both welding and casting involve completely melting the metal, right?

http://www.lincolnelectric.com/en-u...to/Pages/aluminum-design-mistakes-detail.aspx

The same goes for maraging steel. Welds in maraging steel can be heat treated to full strength. Why not castings? Maraging steel doesn't distort much when heat treating either.

Btw, I think 201 and 206 aluminum would be the best high-performance lightweight alloy, with the exception of maraging steel.

 

[cloa]

Did you read the referenced document for Nickel maraging steels?
Its says the modified grade reduces retained autenite and microsegration.

For Casting 6061 see-

http://www.eng-tips.com/viewthread.cfm?qid=136456

Casting alloys are generally different from wrought alloys. You want to minimise heat treatment with cast alloys. - a net shape casting is going to have significantly different properties to a wrought alloy even without thermomechanical of effects of metalworking as while both are originally cast- wrought can go through more heat treatment to get it to a set of baseline properties with desireable microstructure (distorting a block of metal is no big deal and they can machine it the desired shape). Sure you could cast a wrought alloy but the fluidity may be poor so fine features may not be achieveable and then you have to do considerable heat treatment such as homogenisation with irregular dimensional effects and then do the heat treatment to get your final properties.

 

[cloa]

By the way you have other choices- like powder metallurgy to get the almost exactly same alloy. To get better general answers try contacting the Nickel Institute or other technical organisation. To get answers for your particular part, that takes payment to a consultant or foundry.

 

[NickE]

There are casting alloys, and wrought alloys. Each one has chemistry suitable for the processing to be used, A356 aluminium can be cast and heat treated to relatively high strength levels.

here's a link to an industry body that can get you way more accurate information than wiki.

http://www.investmentcasting.org/

yes investment casting can achieve net shape parts, particularly if you use a finishing method after.

There's plenty of info in any of the metals handbooks, particularly the desktop edition.

https://www.google.com/#hl=en&gs_rn...aWM&fp=494c9f1b679e21a3&biw=1589&bih=810&bs=1

Nick

 

[swall]

Geneieusxie--you are swerving all over the road here. The proper approach in selecting a material and its processing is to first define the component(s). What strength level do they need to be? What is the operating environment? What are your cost targets for the components? What are the production quantities? You have to address those questions first, then you can work on the material and processing.

 

[KENAT]

For aluminum if you're dealing with lower volumes you may want to look into

http://en.wikipedia.org/wiki/Plaster_mold_casting

to see what it can do for you. I've got a couple of parts from these guys

http://www.aljcast.com/,

nothing highly loaded though.

http://en.wikipedia.org/wiki/Aluminium_alloy

gives a list of aluminum casting alloys toward the bottom.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[geneiusxie]

I'm trying to get the highest strength to weight ratio metal parts possible, but I don't want to use cnc machining because it's pretty expensive. I was wondering if cast alloys could offer similar strengths of wrought alloys, but I guess it's not possible to get the same performance of cnc machined 7068 aluminum and 350 and 2800 maraging steel in a cast part.

I'm not sure if powder metallurgy can accept 7068 aluminum or 350 maraging steel, but I'm guessing it can't.

 

[cloa]

Searching for 350 maraging steel and powder metallurgy gets a bundle of hits
e.g.

http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA033231

http://www.ingentaconnect.com/content/maney/pm/1977/00000020/00000003/art00001?crawler=true

Perfectly possible to make it seems. Powder metallurgy is basically mix metal powders and form to final shape and consolidate under high temperature/pressure- the main problem is some residual porosity.

 

[geneiusxie]

I'm not sure if the residual porosity would significantly decrease fatigue life though. But the near net shape and (hopefully) much cheaper processing than cnc machining does seem very promising. And it seems to be a very popular option.

The thing is, I don't really understand the concept of powder metallurgy. How is it that sintering metal powders produces a more unified grain structure than actually melting the powder? The grains probably aren't oriented in any direction, so I guess it's just because the grains are smaller than they would be if the metal was cast?

 

[cloa]

Powder Metallurgy just means created from powders to form the shape- it can be melted. see

http://en.wikipedia.org/wiki/Powder_Metallurgy

and many other references.

 

[tbuelna]

geneiusxie-

Obviously, if you chose to do so, you could melt down any metal alloy and pour it into an investment mold, including 7075 aluminum or maraging steel alloys. But this would not mean your cast parts would have the same properties as the wrought material form. The same would be true of parts made using PM/HIP.

While it is indeed possible to cast parts with very good properties, such as single crystal turbine blades castings. In reality, even parts investment cast using vacuum will still have inferior properties to parts made from wrought material or a forging. Even investment castings can have flaws due to porosity or pieces of loose shell. Porosity problems can be addressed by HIP of the castings.

As for fatigue properties, in critical applications it is normally required to apply an appropriate knock-down factor based on the worst case flaw size and shape that may be present in the finished part structure. Obviously, in this regard any porosity would greatly affect the calculated fatigue life.