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Epoxy Bonding Aluminum Strength and Application

Epoxy Bonding Aluminum Strength and Application

(OP)
I am currently working on a design for a strut tower brace that involves an aluminum extruded center bar and billet aluminum strut tower caps.  The caps have extruded ends that push into the hollow parts of the somewhat "H" beam profiled bar.  The end product needs to be one solid piece.  Bolting is expensive to machine and supply and Welding is not as aesthetically pleasing especially after anodizing.  

I am currently looking to use some sort of epoxy like some car manufacturers like Lotus have been doing with their frames.  I have a few questions on this subject.  What kind of strength can I hope to achieve compared to welding?  Being that it is being used in the engine compartment it would need to withstand heat and possible lubricants and oils.  Is it possible to anodize the entire assembly after bonding? What kind of equipment is needed to perform this operation?  I know their are epoxies that cure at RT and elevated temps and that surface treatment prior to applying is critical.  I am also looking to see if it would possibly be cheaper to do in house. Any other suggestions or information in this area would be much appreciated.  Thanks.

Shane

RE: Epoxy Bonding Aluminum Strength and Application

Hi Shane,
I do not know much about automotive epoxies but will comment on anodizing (conventional, Type II using sulfuric acid solution).

It's a good idea to anodize all exposed aluminum used in automotive (unless conversion coated + painted).  This gives corrosion protection and the option of coloration.  Anodizing & dyeing is a good quality check for aluminum welds, as any acidic electrolyte seeping out of cracks causes telltale white spots (especially noticeable with black dye).

Not sure what is meant by "aesthetically pleasing," but aluminum filler metal can be chosen for anodizing color match. For welding 6061, filler 5356 is used.
http://www.alcotec.com/atasap.pdf

Remember that welding most aluminum reduces the strength ~50%, as effects of any heat treatment & cold working are lost.  For more info on welding aluminum, see http://www.alcotec.com/techpage.htm
or http://www.welding-advisers.com (lots of links, free registration to their Practical Welding Letter may be required).  

Anodizing is not a D-I-Y process when large objects are involved. Solution agitation & refrigeration, power supply to give correct current density at the proper voltage for the alloy, pretreatment, dye & seal solutions & tanks, DI water, EPA & OSHA...

Anodized aluminum, especially when sealed as normally done (except for hard anodize & chromic or phosphoric acid anodized structural aircraft panels left unsealed for bonding), may be difficult to bond to. Testing required.

Anodizing after bonding is possible but maybe rather infrequent. Some precautions:
The epoxy cannot be porous & must be chemically resistant to any pretreatment solutions (e.g., hot caustic) and dilute nitric acid deox/desmut. The anodize solution (~15-20 wt% sulfuric acid) itself & post-anodizing solutions (e.g., ~180 oF seal) are less aggressive.
All joints should be fully liquid tight* (applies to welded, too).
As epoxy cannot conduct the anodizing current, extra racking may be involved cf. a welded assembly.
Unless someone has info on a specific epoxy, do some test anodizing & carefully inspect the anodize-epoxy interface afterwards.

*For spot welded assemblies, provide weep holes at high & low corners to allow flushing & draining  after anodizing.  Also sometimes done with large hollow objects to avoid floating tendency.

RE: Epoxy Bonding Aluminum Strength and Application

Epoxying aluminum isn't entirely straightforward, since you need to bond to the base metal, not its oxide layer.

However the good news is that the pain is worth the candle. Because you haven't heated the metal up its heat treat remains unchanged, so your 6061 T6 is still nice and strong.

You'll need to contact an adhesive supplier to get details of a suitable preparation regime, and then an epoxy with an appropriate strength. I've always found GE plastics, and CIBA GEIGY (maybe Huntsman now) to be very helpful. Raychem also sell some great stuff, but tend to be a bit spendy. You might even investigate Loctite (not too sure which grade), I've used it to build panhard rods and driveshafts before now.

I strongly recommend a high temperature cure if you use an epoxy, otherwise your car will literally melt in hot sunlight. 80C is a minimum, 120 would be my recommendation.

Your plug and socket design sounds good, /if/ you can control the film thickness. The Loctite TDS shows the effect of film thickness, it is important.

Cheers

Greg Locock

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