Aluminum Grade and Hard Coat Anodize

[vanci]

Good Day,

I have a project to design a multi piece, circular, aluminum track in which a piece of equipment (approx 50lbs) will travel on. The current material is a 6xxx series aluminum that is tempered but uncoated. Manufacturing shop has told me they would like more wear resistance built in to the new tracks. Knowing very little of aluminum alloys would I be better off with 6xxx series hard coated or a 7xxxx series un-coated or hard coated? We will not be performing any welding on the material. The machine uses a nylon "wear pad" on the wear surface of the track which I am considering using a differnt grade to further reduce friction.

Thank you in advance for any suggestions/ reccomendations!

 

[dgallup]

Hard anodize will certainly improve your wear resistance. There are so many different 6xxx and 7xxx alloys I could not say which series would be better. Teflon or Delrin might be a better wear pad. There are also various grades of plastics filled with graphite or MoS2 that might work for you.

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[MintJulep]

Do NOT use a graphite filled material against aluminum.

Consider a Teflon impregnated hard-coat anodize finish on the aluminum.

 

[swall]

I think the 6061 vs 7075 question mainly depends on what product form you are after and the relative availability. I have seen both used in aircraft brake piston bushings and both applications were anodized. Both worked fine. Unlike MintJulep, I am not smitten with the Teflon impregnated hardcoat. I spec'd this out one time for a sticktion problem I was having and after some extensive (and expensive) SEM, Auger and ESCA analysis found that there was no impregnation of the hardcoat by Teflon, but rather there were scattered filaments of Teflon lying on the hardcoat surface. At the suggestion of one anodize vendor, we went to a polishing operation after hardcoat, which worked fairly well.

 

[patprimmer]

The PTFE (Teflon) impregnation of anodised aluminium surface is a critical process.

You need a PTFE that is a known particle size. I would need to find some very old literature to refresh my memory, but sub micron comes to mind.

The pores in the surface of the aluminium need to be slightly bigger and barbell shaped. This is controlled by the make up of the anodising solution, the temperature of the solution, the current density and particular grade of aluminium.

The surface then needs to be sealed to lock the PTFE in.

If done correctly on a suitable grade of aluminium, it can have very good abrasion resistance.

Nylon responds very well to graphite filling to improve bearing, but if it reacts badly with the aluminium I guess it's not a good idea.

MoS2 filling does not actually lubricate nylon. It acts as an extremely good nucleating agent during the moulding process and the high level of superfine crystals at the surface is what improves bearing properties. You can get the same effect from TiO2 rutile type and any other really good nucleating agent.

Regards
Pat
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[TVP]

I agree that either 6061 or 7075 with hardcoat anodize will have improved wear resistance. As the others noted, there are a number of options for incorporating friction reducing agents such as PTFE, graphite, MoS[sub]2[/sub], or silicone additives to the plastic (nylon, acetal) or PTFE to the hardcoat layer. Another option is to hardcoat anodize then apply a dry film lubricant coating (typically epoxy or phenolic with PTFE particles suspended in it). Electroless nickel coatings are yet another option, and these can be co-deposited with PTFE, which have outstanding wear and friction characteristics.

 

[KENAT]

Another option for the wear pad might be

.

Hard anodize can tend to be a bit rough (this may be dependent on the pre anodize surface finish - I can't recall) so a post anodize polish or similar may be required.

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[jagad5]

If your structure is subjected to dynamic loads, watch out for the effects of anodize on fatigue strength. Google "Rateick Aluminum Anodize." His paper includes 6061-T6 results.

Doug