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?

 

[unclesyd]

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.

 

[kenvlach]

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, ~16^oC/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.

 

[CoryPad]

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:

[tt] 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:
K_Ic(MPa[√]m) 22 37
[/tt]

Regards,

Cory

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

 

[prost]

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.

 

[kenvlach]

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/SoftCar...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.