Best Aluminum Alloy to use 2

[GTMule]

In an automotive race application what aluminum alloy would you all recommend for the fabrication (CNC machining, mostly) of many of the various suspension and engine brackets on the car, such as those attaching the engine to the car (where the engine is a stressed member of the chassis), "rocker arms" whcih transfer load form the wheel, to the spring/shock combination, or those brackets attaching the suspension A-Arms to the chassis (they'll be relatevely small brackets, which will hold bolts in double shear throuh monoball bearings) and, especially, for uprights (the part at the wheel that the suspension attaches to) considering the type of loads, and relatevely low temperatures these parts see.

We had considered, and, have avaliable (most readily)

6061 T6
7075 T6
and 2024 T6

but were slightly worried about 7075 and especially 2024 being too brittle for some of the applications that will see "shock" loads, and lots of vibration.

Just wanted to get everyone's thoughts on this, cost is interestingly, not as issure, as we've just come into quite alot of various aluminum alloys quite cheaply (offcuts form an aircraft manufacturer)

Thanks.
Chuck Boulware

Chuck Boulware

 

[plasgears]

I vote for 7075T6 based on strength x elongation figures. This is the energy absorption potential of the material.
Results:
Matl UTS x Elong
2024T6 690 ksi x % elong
6061T6 765
7075T6 913

 

[CoryPad]

GTMule,

You use the words brittle and shock, yet neither are useful for engineering design. You want components to withstand the mechanical environment to which they are exposed, and hopefully have minimum necessary mass. This will require certain mechanical properties from the materials, such as: X minimum yield strength, Y minimum fracture toughness and Z maximum fatigue crack growth rate. All three alloys listed possess some plastic strain capacity (fracture strain > 5% for all 3), so brittle doesn't apply to them. "Shock" loading influences material strength and toughness, but should not be a significant factor for the applications you mention since the strain rate should be ~ 10³ s^-1 and aluminium alloys usually show little strain rate sensitivity.

Plasgears shows energy absorption potential for quasistatic loading and material with no flaws that will initiate fracture. This is a good measure for your applications, and I agree that 7075-T6 would be a good choice. If one were to include fracture-based design, the choices may change based on whether the design was in plane strain or plane stress. Also, fatigue should be considered, although 7075-T6 would still be a good choice for this criterion. Good luck.

 

[TVP]

Stress corrosion cracking is usually the biggest concern when using 2xxx or 7xxx series aluminum alloys. You can obtain a great deal of design data on many structural alloys including 7075 from MIL-HDBK-5H, available from either of the following websites:

http://assist2.daps.dla.mil/quicksearch/

(**HUGE DOWNLOAD, ~ 42 MB!**)

or

http://www.grantadesign.com/cgi-bin/stat-tree.cgi?src=MIL5-H.xml&br=0

(individual chapters as .pdfs)

 

[KSshopper]

7075-T6 and 2024-T4 are not good candidates for hogouts (machined parts). Not only do they suffer from stress corrosion cracking problems, but they pick up residual stress during the machining process and they'll warp. 6061-T6 is a good candidate, but is relatively weak in strength. My favorite material to cut parts from is 7075-T73 which is an overaged temper of 7075-T6. Good corrosion, properties and good machineability. You can heat treat you T6 parts to the T73 condition with the appropriate ASTM spec.