Hardening of Aluminum

[wantobe]

I have two parts that are matched to each other. One has a key and the other the matched keyway. The key and the keyway will have some abrasions against each other due to vibrations. I can use steel for these two parts, but it cost me too much weight. I would like to use aluminum in combination with some hardening to improve its abrasion resistance. A conventional way to improve hardness is anodizing. A more aggressive way is coating with Nikasil, a method used in engine cylinder wall hardening (I remember someone once told me Rotax 912 also used this).

http://en.wikipedia.org/wiki/Nikasil

Any comment/ critique/ personal experience/ watch-outs/ alternative method?

[nrpetersen]

If you are having to deal with high cycle vibration (i. e. torsional impuolses onto splines, etc) the best way is to preload the joint in a manner that eliminates any possible lost motion, carrying the load/torque thru friction. Say like two poker chips pressed together (called a curvic coupling).

A keyway isn't able to carry cyclic torque without some other friction scheme in parallel to carry the dynamic torque, such as a tapered shaft and a flywheel hub.

The problem with aluminum hard coatings is that they break down into aluminum oxide, which will eat the parent material.

Is there any lubrication present? Can flex plates be used? Gotta know more about the application and needs.

[Bob H]

A "hard anodized" surface tends to be brittle and will crack under cyclic elongation. The debris from cracking would quickly wear sub-surfaces and loosen joint. Does joint have to come apart or can it be semi-permantly joined? Would it be possible to heat shrink the alum members together( interference fit) and avoid relative motion? To open joint, you would heat outer member/chill inner member. When sized properly, a heat shrink joint can carry high torque. Need specifics on what you want to accomplish.

[nrpetersen]

And by the way, anodizing reduces the tensile fatigue strength of aluminum. Not sure about other types of loading. Anodizing isn't necessarily better than bare at any rate.

[Eric Witherspoon]

Another option would be to make one of the parts softer / replaceable. There is an aluminum / phenolic interface on my airplane in a latch subject to vibration. The phenolic part may eventually wear out, but it's small, cheap, and replaceable. The aluminum will maintain its dimension.

[Mike Switzer]

If it is a part that is either subject to wear, repeated loading & unloading, vibration, or needs high strength you probably won't save much (if any) weight making it out of aluminum instead of steel if it is designed properly. If you intentionally design it as a life limited part you might be able to use aluminum & save some weight.

Hardening aluminum parts requires some pretty high tech computer controlled ovens.

[wantobe]

Thanks for the replies. For my case the two parts need to separate regularly, so can't be joined permanently or semi-permanently. The load is not high frequency vibration like the load on engine support holding an engine in a puller, it is more like the load on the bolts that join high wings to an airplane.