7075 T6 Wheel Spindles

[setjo]

I am investigating manufacturing wheel spindles for a 320kg FSAE car out of 7xxx aluminium due to its good fatigue and strength properties. This part will be subject to cyclical bending and torsion although it will only experience limited track time since it will only be used for two years with 6 track days per year before it is decommisioned (approx 100,000 cycles).

Our FSAE team is based in Sydney, Australia and only 7075-T651 is readily available in 7xxx round bar. Will stress corrosion cracking be a significant issue for this kind of application? Is it possible to and should I get the spindles treated to a T73 temper after machining of the T651 round?

Any help would be much appreciated.

 

[kenvlach]

If you stay away from the salt spray at the beaches and avoid storing where condensation may occur, and use wheel bearing grease on the shaft, corrosion shouldn't be a problem.

For insurance against corrosion, consider doing one of the following:
1) Chromate conversion coating,
2) chromic acid anodizing, or
3) sulfuric acid anodizing, clear (non-dyed).
Possibly, SAE may prohibit using a hexavalent chromate. Also, sulfuric acid anodize lowers fatigue; it is 5x stiffer than the alloy, so can initiate cracks.

Yes, it is possible to heat treat from T651 to T7351, but the ASTM standard practice isn't explicit, although mentions greater control of heat treat variables and consideration of the initial level of processing of the T6 material are required. I wouldn't recommend it, as your corrosion resistance requirements aren't severe.

 

[setjo]

Thanks kenvlach,

Having read plenty of threads referring to SCC in 7075 in aircraft applications I am just trying to grasp whether this phenomenon is an issue in the smaller application of FSAE wheel spindles.

FEA indicates a max von mises stress of 150MPa (21.76ksi) at the surface of the spindle shaft in alternating tension/compression. And this is at maximum cornering which does not occur regularly.

Cheers.

 

[TVP]

I agree with Ken, the environment is not too severe as long as you have some type of protection. I would recommend against anodizing and try either a conversion coating or routine cleaning/greasing.

 

[RPstress]

SCC requires sustained surface tension (though you have to keep an eye on shear as well), e.g., in a bolt which has been pre-tensioned, as well as an environment that the material is susceptible to. (Usually for 7xxx this means water is around somewhere. However, for some of the very susceptible materials in T6 just moisture in the atmosphere can be enough.)

Tension can occur in some machinings which are from plate or forging that have built-in internal stresses, where the internal material is exposed to atmosphere by having the outside machined off. (You hear tales of parts which have developed a crack while sitting on the rack in stores.) However, it should be rare for a relatively straightforward part such as a shaft to suffer from this.

As long as the high tension stresses are applied only intermittently and not for sustained periods of time, SCC should not be a worry, even for 7075-T6. This is especially true if it is anodised, primed and painted. Also, 7075-T6 bar is only really vulnerable to stresses in the radial direction; for stress parallel to the bar's axis it gets an 'A' rating in the MMPDS.

One minor area of worry might be a reduction in diameter close to a shrunk-on sleeve or similar. This would expose material in a complicated stress state, possibly with some tensile component in the radial direction.