Corrosion Control--Material Selection

Lee H. Erb, EAA Chap 1000 Det 5, Arlington TX

Originally published December 1997

MIL-HNBK-5 has 25 different recognized aluminum alloys that may be used in aircraft. Each of these has four or more heat treatments or conditions. The selection of aluminum alloys is usually already made for the homebuilder. In general three alloys are specified: 2024-T3 (or -T4), 6061-T6, and 7075-T73. For homebuilts, I believe 6061-T6 alloy is the best all-around alloy for many reasons.

2024

2024-T3 (for sheet up to .249 thick) and 2024-T4 (for plate .250 and above) was well defined before WWII (known as 24ST) and is still a good choice in most cases. It has a good strength/weight ratio. The -T3 and -T4 have good toughness (tough: strong but pliant; that will bend, twist, etc. without tearing or breaking). Care should be taken because in certain stress orientations they can be susceptible to stress-corrosion cracking if the installation of bushings and fasteners are not well designed for the application.

The fact that 2024-T3 or -T4 can be susceptible to stress corrosion is not as much of a concern as it is with other alloys. This is because once a crack is formed, the toughness of the alloy makes for slow crack growth.

2024-T6 and -T8 tempers are high strength and are less susceptible to stress-corrosion cracking. Some aircraft manufacturers are requiring 2024-T851 instead of other tempers. I have been lead to believe that it is more expensive too.

2024 should not be welded because of the potential for future intergranular corrosion (believed to be because of the redistribution of the copper in the alloy).

6061

I like 6061-T6 for homebuilts. It has a reasonable strength/weight ratio. Homebuilts generally have low loads that can be handled well for sheet aluminum and extrusions with thickness dictated more by handling during assembly (and to lean on at the flightline).

(The first commercial Bell Jet Ranger delivered had to have the forward cowl redesigned and replaced because, although is was plenty strong for the loads, it dimpled when the new owner’s wife leaned on it during the delivery picture taking session.)

6061 has good stress corrosion characteristics and has high toughness even at cryogenic temperatures.

6061 should never remain in the -O condition (true for any alloy) because of its low yield strength (soft and too easy to bend). It is good for welding. If fabricated in the -O condition or welded, it should be heat treated to the -T42 or -T62 condition. This heat treatment gets its stiffness and strength back. A homebuilder will have to go to a commercial heat treat company (which generally is not the way a homebuilder wants to "do business").

-T6 condition for 6061sheet is great but you must use correct bend radius (approximately 3 times or more the thickness).

7075

The airplane designer has many choices depending upon the requirements. "High Performance" aircraft usually use 7075 alloy because of the high strength/weight ratio. It also has other favorable characteristics by selecting the proper heat treat.

7075 does not have good weldability characteristics.

Do not select 7075-T6 for any critically stressed component. (I would not use it for any component.) It is susceptible to stress-corrosion cracking and low toughness characteristics (especially at cryogenic temperatures). The alloy was not well defined after WW II but it looked so good that a lot of people started using it. Allegedly the stress-corrosion cracking caused a few Martin 202 crashes (wing spars failing) before the alloy was well defined.

I specify 7075-T73 where ever high strength and low stress-corrosion cracking are paramount. It has the desirable stress-corrosion cracking and exfoliation characteristics in all stress directions. (The T-38 has been going through replacement of the upper forward longerons with 7075-T73 extrusions. Previous longerons suffered cracking due to notch sensitivity and stress corrosion that resulted in a few inflight breakups.)

5052

I was taught early in my career to not use any 5000 series aluminum except for ground equipment. At cryogenic temperatures 5052 does get stronger and is tough but above 150 degrees F it starts to lose strength rapidly.

Be extremely careful when using 5052 for structure on a airplane. (I will NOT use it for load carrying structure.) It work hardens and then cracks. I know it is used it for gas tanks because of its weldability. Also it does not need heat treatment (it being a non-heat-treatable alloy). This is okay if no airframe loads are introduced.

I’d rather see 6061 used when welding is required and then have it heat treated. (Work hardening in 5052 alloy has the same effect as metal fatigue but has an unpredictable time history.)

Tidbits

Strengthwise and corrosionwise 7075-T73 can be substituted for 2024-T3 (and -T4) and 6061-T6. Likewise 2024-T3 can be used for 6061-T6 on a strength basis.

Like 6061, 2024 and 7075 should not be used for load carrying in the -O heat treat condition. Some aircraft repair procedures (such as for the B-24) do call out the -O condition for use as fillers in repair splices.

If you cannot protect your material from surface corrosion before using, it might be a good idea to purchase alclad material. Remember that the alclad provides no strength and depending upon the thickness an alclad sheet is not as strong as a bare sheet (although the percentage loss is low).

I have been told by purchasing folk that 7075 is becoming less expensive than 2024 because it is becoming more generally used. 6061 is supposed to be even less expensive because of its wide use including machined parts.

I have seen specifications for new Navy aircraft where the minimum thickness of parts is .026. Apparently this is due to thinner parts losing a high percentage of strength before corrosion is detected and repaired.

Selection of material is many times made on the availability of an alloy and the design altered to use what is available.

As far as I am concerned, the bible for material properties is MIL-HDBK-5. (It used to be known as ANC-5.) In general what has been published in Aircraft Spruce and Specialty Company catalogs on aluminum alloys is valid, but be careful if you use the catalog info to select an alloy for a new design.

Various aircraft manufacturers also have proprietary data they have developed over the years for a given alloy and application. With all the recent corporate mergers it is going to be real interesting to see what proprietary data will soon stir debates (contradictory data within a design group composed of individuals from formerly competitive companies). Hopefully much of the competitive data will become available in public domain.

Thanx

Thanks to Bob Urban and Stan Klein for trying to help me understand.

References

1. "Metallic Materials and Elements for Aerospace Vehicle Structures," Military Handbook, MIL-HDBK-5
EAA Chapter 1000 Home Page
E-Mail: Web Site Director Russ Erb at erbman@pobox.com

URL: http://www.eaa1000.av.org/technicl/corrosion/material.htm
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 28 July 1998