Grain direction of 7075-T6

[rrewis]

I have searched the archives but couldn't find what I need, so I would like to ask the group. I know nothing about metals. I have a piece of .080 inch 7075-T6 aluminum plate that has been polished. I need to know how to tell which way the grain runs. The reason is that I have been told to never bend this aluminum parallel to the grain. The piece is a fitting on an aircraft.

Any help will be greatly appreciated,
Russell Rewis

 

[Goahead]

Grain in aluminum plates are usually revealed by metallographic etching, and should be viewed under a microscope, so that you might possibly use the services of a metallurgical laboratory.
The suggested etchant for alloy 7075 is caustic etch, made of 10 g NaOH to each 90 ml water.
Depending on the radius of bend and on the degree, it may be difficult to bend 7075-T6 without cracking or without inducing stresses that might promote stress corrosion cracking.
If it is not an exaggeration that you know nothing about metals, and if you are dealing with aircraft fittings, it may be a good idea to have a metallurgist review your procedures.

http://www.welding-advisers.com/

 

[metengr]

rrewis;
I am not in expert in aluminum forming, however I did review some reference material on forming of aluminum alloys, and offer the following information.

Your statement above regarding forming and grain direction is not entirely correct. The grain orientation will obviously effect forming characteristics. However, this could be factored into design use of the part. In other words, if the aluminmum sheet was formed while in the O or annealed condition, forming is probably least affected by rolling direction. You should not bend 7075 in the T6 cindition. From the literature I have seen, the effect of directionality after forming has more to do with fracture toughness properties.

I checked ASM Handbook, Volume 2, and a note appears for 7057 T6 temper applications

Precautions in use. Caution should be exercised in T6 temper applications where sustained tensile stresses are encountered, either residual or applied, particularly in the transverse grain direction. In such instances, the T73 temper should be considered, at some sacrifice in tensile strength.[/quote ASM Handbook]

 

[kenvlach]

The grain in rolled, Al alloy plate looks exactly as expected: Elongated in the longitudinal, broadened in the transverse, and squashed in the short transverse. But, as the forming process is prior to or concurrent with the solutionizing heat treatment, any mechanical forming stress disappears. But, some small difference in mechanical properties. I think the largest is in impact properties:

From MatWeb:
Fracture Toughness 20 MPa-m½ 18.2 ksi-in½ K(IC) in S-L Direction
Fracture Toughness 25 MPa-m½ 22.8 ksi-in½ K(IC) in T-L Direction
Fracture Toughness 29 MPa-m½ 26.4 ksi-in½ K(IC) in L-T Direction

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6

For more complete 7075 info, see Metallic Materials Properties Development and Standardization (MMPDS-1) January 2003 or MIL-HDBK-5J METALLIC MATERIALS AND ELEMENTS FOR AEROSPACE (31-JAN-2003).

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=53876

 

[rrewis]

No, Goahed, no exaggeration about my knowledge of metals, and from all the information you and the others have provided, assures me that I am out of my league and should not attempt to perform any fabricating. Which, by the way, is exactly why I asked the pros.

Thanks Goahead, Metengr and Ken for taking the time to provide your knowledge.

Russell Rewis

 

[kenvlach]

Better to ask a naive question than to do something stupid.

For your application, there is a minimum bend radius (radius of curvature) for cold-forming the material. Tables are available in forming & machining handbooks.
For tighter bends, necessary to preheat the material. This will over-age a T6 temper, so you end up with something similar to a T7 -- not necessarily bad. It's also possible to re-do the entire heat treatment to re-acquire a T6 temper.