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Cast versus Wrought aluminum alloy

Cast versus Wrought aluminum alloy

Cast versus Wrought aluminum alloy

(OP)
I have a question about the phrase "cast 7075-T6 aluminum." Every reference I have refers to cast aluminums with designations "AXX.Y", while the wrought aluminum alloys use the 2XXX, 3XXX, etc. designations, with the 'dash' "T-some number" indicating a heat treatment. Does "cast 7075-T6 aluminum" mean an aluminum casting with similar material composition as 7075 with a T6 heat treat, or does it mean the casting has similar mechanical properties of 7075-T6, such as yield, UTS, elongation, etc. Or something else?

RE: Cast versus Wrought aluminum alloy

All wrought products are originally cast in some manner.  Aluminum alloys are either direct chill cast (rounds), ingot cast (rectangles), or continuously cast (usually only sheet).  Perhaps it is one of these processes that is being referred to.  Otherwise it is non-sensical.

RE: Cast versus Wrought aluminum alloy

Prost,

7075 is a pretty weird alloy, and requires IIRC, some combination solution anneal, cold forming, heat treat and rapid quenching to achieve its properties.  There is very little liklihood that a cast version of the alloy could even come close to obtaining the same UTS as properly forged and aged 7075-T6 wrought forms.

RE: Cast versus Wrought aluminum alloy

(OP)
OK, so what does the phrase "cast 7075-T6 mean"? Maybe I am misinterpreting what I am reading. Take a look at this document:
http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA048657

Title: High Strength Aluminum Casting Technology: Squeeze Casting of 7075 Alloy.

Abstract: The objective of this investigation was to develop wrought 7075-T6 mechanical properties in a cast aluminum alloy component of net or near-net shape. ((there's more))

Sounds to me as if they are trying to create a cast aluminum with the same mechanical properties as 7075-T6. Is that a bad interpretation of what I see?

RE: Cast versus Wrought aluminum alloy

Yes, that is what they are doing.
Since they are working with cylinders I am guessing that they are looking to save a ton of machining time.
This isn't a simple casting process that they are using.  This may a high pressure slush casting.
It might be interesting to see what chemistry they are using.  

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
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RE: Cast versus Wrought aluminum alloy

prost,

As EdStainless pointed out, you have the correct understanding of the process that was investigated.  Since this is/was a research project, the investigators used the wrought alloy nomenclature for their casting alloy/process, especially since it is the wrought version of the alloy that they are intending to replace with a casting.  If this were to become standardized, such as in an ASTM casting standard, then the alloy would likely be given a casting designation, e.g. 7xx-T6.

RE: Cast versus Wrought aluminum alloy

As the report was dated Oct 1977 and the world has yet to reap the benefits of cast 7075, I would say they hit a dead end. If I recall,this time frame was when some of the development work was going on with high strength variants of 357 alloy, as well as the silver free version of 201, which became 206.

RE: Cast versus Wrought aluminum alloy

(OP)
Maybe someone knows what is really going on. I am looking at a piece of cast aluminum that a military service uses in a rescue helicopter hoist. It's definitely a casting, not a 'hog out' of a bigger piece of Al alloy. No time to do a detailed material analysis, to figure out what it really is. We've been told by the client that the material is 'cast 7075-T6 aluminum.' The suggestion has been made that since the reference I gave was old, that this was research that didn't pan out. Given I see the hoist in front of me, and there are more out there just like it, I'd say the research did pan out. "Not enough information" of course is a perpetual bugaboo in any analysis, nevertheless one has to try something that makes sense. We are only left to guess the material properties, I suppose, given no other information.

RE: Cast versus Wrought aluminum alloy

The casting must have undergone a Hipping operation. This can certainly have been  die cast to 7075 grade Hipped and then heat treated.

RE: Cast versus Wrought aluminum alloy

One of the leading researchers/experts in squeeze casting is David Weiss at Eck Industries.  They have a squeeze cast machine and have claimed the process is capable of producing as-wrought properties.  You might try contacting him.

RE: Cast versus Wrought aluminum alloy

prost,

Are you sure you aren't looking at a custom-forged shape?

RE: Cast versus Wrought aluminum alloy

MIT lab with Flemings as a principal investigator was a pioneer . Commercially today Bodycote is doing Hipping at their facilities in Europe and US.

RE: Cast versus Wrought aluminum alloy

(OP)
The answer is no, I don't know if it is custom forged. How would I know the shape is custom-forged?

RE: Cast versus Wrought aluminum alloy

If it were me I would do a hardness test and a conductivity test to verify what the client told you.  This will at least verity the alloy and heat treat and is easily accomplished.

RE: Cast versus Wrought aluminum alloy

There are too many instances where conductivity and hardness values overlap with aluminum alloys and you could come to the wrong conclusion. I would opt for a portable spectro analysis.

RE: Cast versus Wrought aluminum alloy

(OP)
Not enough time for all of that testing. If a follow-on project evolves, I'll be sure to include fancy testing as you have suggested.

RE: Cast versus Wrought aluminum alloy

prost

 can you perform a quick macro test to observe the flow lines if any. Absence of flow line will suggest a casting possibly

RE: Cast versus Wrought aluminum alloy

Prost--I don't think you ever described your specific interest in this component. Are you quoting on an order to make new parts? Or are you doing some sort of refurbishment of the existing component?

RE: Cast versus Wrought aluminum alloy

Prost, if this is a piece of hardware for a military chopper, there should be drawings available for it, and those prints should describe what the specific alloy is.

Regarding forged vs cast - aluminum is typically sand-cast, and you would see the typical sand roughness in un-machined areas.  Forgings are much smoother, and you can sometimes see the "flow" of the metal grains if you polish/etch.  More what I was thinking, though, is to look at the part shape, and say to yourself "could I have made this from a forging", since the process is inherently more limited than casting in terms of coring.  Irregardless, you should try and get ahold of copies of the drawings.

RE: Cast versus Wrought aluminum alloy

(OP)
Call what I am doing is "figuring out what kind of damage the part can sustain and still do its mission." Eyeballing the surface, it looks like a casting.

RE: Cast versus Wrought aluminum alloy

Another possibility is that it could be an injection molded part.
Though I've haven't seen any injection molded 7075 parts, I do know that some attempts to make same was attempted several years ago.

We used some pretty complex injection molded parts, mostly 356.

RE: Cast versus Wrought aluminum alloy

It isn't 'cast 7075.'  This alloy as-cast has a terrible microstructure & I guesstimate the mechanical properties as about YS 25 ksi, TS 30 ksi, notched Charpy 1.5 ft-lb, and fatigue strength maybe 5 ksi at 5 x 108 cycles in R. R. Moore type test of smooth specimens.  Criminal if such material flies (except as cargo)!  Note:  Try searching MatWeb by composition for a similar Al casting alloy – you won't find one.  Maybe looks like a casting due to shot peening.  Follow the advice above & track down the actual specifications.

7075 solidifies over the range 635 to 477 oC, with lots of segregation between higher melting intermetallics and a low temperature eutectic. Ingot intermetallics (DC cast, ~16oC/sec) are (Fe,Cr)Al3, (Fe,Cr)3SiAl12, Mg2Si, Mg(Zn2,AlCu) and CrAl7.  It cannot be completely homogenized by heat treatment – significant hot working is required to break up and distribute the intermetallics.  Also, Fe does not diffuse to any significant extent in the Al matrix below 400 oC, and Mn and Cr only above 500-550 oC.  
--- Refs:  various Metals Handbooks (phase diagrams, metallography & non-ferrous alloys), Aluminum Vol. I (1967), and Aluminum: Technology, Application and Environment, 6th Edn. (1998).  

To avoid the casting microstructural problems, alternative methods include powder metallurgy using rapidly solidified powder, and more recently, thixotropic casting (stir casting) and spray casting.  Note that in the article referenced by prost above, squeeze casting didn't solve the segregation problem (nor would HIP).

A few interesting articles on 7075 production and microstructure:

“For 70 years, aluminium alloys have been the materials of choice for both military and commercial aircraft structures. The ingot metallurgy (IM) alloys of the 2000 (Al-Cu-Mg) and 7000 (Al-Zn-Mg-Cu) series used thus far show several disadvantages caused by the production process. Such problems are primarily coarse intermetallic constituent phases, coarse grains, and macrosegregation, resulting in low fracture toughness.”
Mechanical Properties of Spray Cast 7XXX Series Aluminium Alloys (2002)
http://journals.tubitak.gov.tr/engineering/issues/muh-02-26-4/muh-26-4-5-0106-6.pdf

Properties Of 7075 Aluminium Alloy Extruded From Rapidly Solidified Flakes And Conventional Atomised Powder [abstract] (1984)
http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADD458269

'Effects of Dispersoid Particles on Toughness of High Strength 7000 Aluminum Alloys'
http://www.key-to-nonferrous.com/CN/default.aspx?ID=CheckArticle&NM=161

“Q: Why is aluminum alloy 7075 not listed in AWS D1.2, Structural Welding Code - Aluminum?
A: Most aluminum alloys are weldable, but a fair number of them are not, including 7075 aluminum....However, 7075 should not be used for structural work.”  
http://www.arcraftplasma.com/welding/weldingdata/weldingfaq.htm
[Of course, 7075 is weldable, but the results are often bad.  Layers of liquid may form parallel to surface, along the rolling or extrusion direction.]

Thermal Stability of Rapidly Solidified Alloys of Aluminium with Transition Metals [1st page only] (2006) http://www.scientific.net/0-87849-408-1/389/

Finally, you can identify 7075 by its speckled olive coloration when hard anodized.  And, the intermetallics, if not well broken up/dispersed/homogenized, will appear as blisters in the anodize.

RE: Cast versus Wrought aluminum alloy

I agree completely with Ken.  It is almost impossible to have a functional casting that uses the chemistry of 7075.

One important note regarding coarse intermetallic phases in 7XXX alloys.  Because of their detrimental effects, alloy 7075 has been replaced in many critical applications with 7475.  7475 has a modified chemical composition to reduce intermetallic particles and increase fracture toughness.  Here is a comparison:

                Alloy
           7075     7475
           ----     ----
Fe         0.50     0.12
Mn         0.30     0.06
Si         0.40     0.10
Ti         0.20     0.06

For T651 temper:
KIc(MPa√m)   22       37
 

Regards,

Cory

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

RE: Cast versus Wrought aluminum alloy

(OP)
I am surprised there appears to be so much ambiguity in such a simple phrase "cast 7075." Is this normal? I thought that's what the standards (that is, the recipes) that identify things like material composition were for.

RE: Cast versus Wrought aluminum alloy

Of course, there are standards.  Volume 2.02 of the ASTM standards is devoted entirely to Aluminum & Magnesium:; listing of contents: http://www.astm.org/cgi-bin/SoftCart.exe/BOOKSTORE/BOS/TOCS_2006/02.02.html?L+mystore+fdpe8668+1169246775
  
But 7075 (known as 75S in the 1940's & 1950's) isn't a casting alloy & hence isn't in any casting standards. It is only usable as a wrought alloy.  But, few people are really familiar with aluminum metallurgy.  So, the vast maority were easily duped that the only use of 7075-T6 tubes is for uranium enrichment centrifuges (although proven unsuitable by testing in 2002 at Oak Ridge National Labs done at the request of the CIA).  Actual connection of 7075 to WMDs: Material of construction for the B-29s that delivered the atomic bombs ending World War II.

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