Casting aluminum vs equivalent forged aluminum? 1

[GoPDemon]

I'm not sure what to ask honestly, but I feel like I am missing something.

I've read this to help me understand more.

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

If I was making the same part and one was cast and one was forged, and both materials had the same properties like yield, Young's, UTS, etc what's the difference? Forging would have maybe some work hardening and more aligned grain structure, but if the yield strengths etc are rated the same what does it matter?

If you've heard of St Jeans cobapress I guess maybe this is where my questions arise from. Since they cast and then forge the part(so it is possible to forge complicate shapes), it should be stronger than a cast? But actually they use a casting material that is usually rated weaker than something like 6061, it can't gain that much strength through forging, so what is the point of the forging step? Why not just cast from a stronger aluminum instead of casting then forging? Am I missing some cost thing because forging dies seem extremely expensive to me then upgrading the material cost.

 

[WKTaylor]

http://www.st-ji.com/en/Cobapress.html

I noticed that this process appears limited to non-aerospace parts... which is as I suspected.

They KEY is they CAST, then forge the part.

Castings are by definition limited to specific alloys. What they are then [apparently] doing is die-forging the cast part/alloy to improve mechanical properties... which by definition will still have inferior properties to true wrought/forged alloys.

This sounds similar to the concept of Hot Isostatic Pressing [HIP]: processing of castings under high mechanical-crushing-pressures at elevated temperatures [typically uses small ceramic beads as the pressure medium in a very high strength hydraulic reservoir], to 'consolidate' [compress] the casting to near 100% density... and also [supposedly] improving the grain structure, before the final heat treatment. This processing has had limited success, in improving the mechanical and fracture toughness properties of castings.


Regards, Wil Taylor

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[EdStainless]

Even if you got the same UTS and Yield (and of course modulus because that depends on chemistry and not process) you would still be lacking elongation, impact toughness and fatigue strength.
Castings have a wide range of grain size which includes very large, and also significant amounts of compositional segregation. You can end up in similar places, but not identical.

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P.E. Metallurgy, Plymouth Tube

 

[ornerynorsk]

Your assumption that forging dies are expensive is not necessarily correct. I buy a lot of forged and cast components in many different materials, and when it comes to aluminum, I find that a set of tools for forging, if the design allows it, is a fraction of the cost of diecast tools, and, broadly speaking, much more desirable material properties.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Compositepro]

One inherent problem with metal castings are the high probability of large voids, inclusions and other defects. One major attribute of forging is that it squeezes-out or breaks-up these defects into smaller less significant ones.

 

[moltenmetal]

ornerynorsk: that compares die casting to die forging. Of course if you need a smaller run of parts and the tolerances (and properties) aren't as tight, you can sand cast a part for a small fraction of the cost of die casting it.

 

[tbuelna]

While the Cobapress process, where a cast aluminum preform is hot die forged to a finished shape, may provide advantages in cost or cycle times for commercial applications, it probably has limited use in aerospace.

First, if the aluminum component must be produced in one piece and has internal cavities that cannot be machined, then the most practical process for producing the component in quantity is sand casting. The high strength aluminum alloys that can be reliably sand cast are somewhat limited. As wktaylor noted, critical aircraft structural castings are HIP'd to consolidate any internal voids. The presence of limited internal defects in local areas of a casting is mostly a concern with fracture characteristics. HIP'ing can consolidate most internal defects like porosity, shrinkage or minor tears. Any minor defects that are located at an accessible surface usually can be addressed by welding. Most importantly, you cannot die forge most parts that have any hollow internal areas.

Second, if the application demands an aluminum die forging, then you would probably want to use a high-strength wrought alloy rather than a lower-strength casting alloy. As noted, the cost of a set of progressive forging dies for aluminum is probably not much more than permanent/investment cast tooling plus a die forging tool.

Lastly, most critical aircraft components made from forgings require 100% of the as-forged surface to be removed to a certain depth. This is not usually a requirement for commercial or automotive applications, but it would negate some of the claimed cost benefit for the Cobapress process in aerospace.

 

[swall]

I looked at the link to the Cobapress process and could not draw any conclusions about the process. With a casting, you have many degrees of freedom when it comes to configuration, both shape and internal features. With a forging, you are limited to the configuration that can be produced in the plane of the forging dies. Perhaps Cobapress is only forging part of a cast shape to provide enhanced properties in one area of the part?