Cast versus Wrought aluminum alloy 1

[prost]

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?

 

[TVP]

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.

 

[btrueblood]

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.

 

[prost]

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?

 

[EdStainless]

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.

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[TVP]

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.

 

[swall]

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.

 

[prost]

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.

 

[arunmrao]

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

 

[Flesh]

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.

 

[btrueblood]

prost,

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

 

[arunmrao]

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

 

[prost]

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

 

[Kwan]

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.

 

[swall]

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.

 

[prost]

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.

 

[arunmrao]

prost

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

 

[swall]

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?

 

[btrueblood]

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.

 

[prost]

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.