How do I solder Aluminum?

There is a list of links following the text.

Below is an article I wrote for our club poop sheet eons ago.   There is a list of materials and sources at the end.  If you practice brazing a couple of beer cans together, you should get a feel for how to do it without causing the base
metal to melt down. 

SOLDERING ALUMINUM
by Ian Maclaughlin

      Joining of aluminum parts by soldering is adequate where high structural strength is not needed, as for assembly of model engine mufflers.  The advantage of soldering over welding to the average modeler is economic.  Aluminum welding requires costly equipment, usually electric arc with filler wire fed from a spool and inert gas to shield the work from oxidation (e.g. MIG).  Soldering can be done with an inexpensive propane torch.  I bought mine at a swap meet for $12.00, a BernzOmatic JTH-7 "Solid Brass Brazing Hose Torch Kit."  The torch head has a handle and is connected to the propane bottle by about 3 ft of flexible hose.  I find it more convenient to use than the type that attaches directly to the propane cylinder, as it is not restricted to use in an upright position.  Also, I have found that propane produces more than adequate heat, and the more expensive oxy/propane or MAPP gas are not needed, at least for thin aluminum sheet or tubing stock.
      There are two keys to successful soldering of aluminum: using the right material, and obtaining fusion with the base metal.  The attached page recommends which material produces good results, and some to be avoided.  Obtaining fusion with the base metal requires preparation and technique to overcome the propensity of aluminum to instantaneously form an oxide layer on its surface, presenting a stubborn barrier to molecular bonding with the solder.  The essence of proper technique is to break through the oxide barrier and deposit the solder, with the solder itself shielding the joint from oxidation.  This can be accomplished by the following steps:

      *Clean the surfaces to be joined down to bare metal.  Smooth  bright surfaces will promote flow better than scratched surfaces.  Polish the surfaces with a stainless steel brush (hand or Dremel tool) as a final preparatory step.  Don't contaminate surface with fingerprints, etc.

      *Jig pieces to be joined firmly in place.  Don't rely on a friction fit, as it will come apart when the pieces expand upon heating.

      *Using a neutral flame (see torch directions), heat the parts to be joined.  If one piece is heavier, direct most of the heat to it, so both pieces come up to the working temperature at about the same time.

      *Apply the rod to the joint, rubbing it in like a crayon. This breaks through the oxide layer, and as noted above, is the key to successful soldering of aluminum.  The temperature should be just above the melting point of the solder, but not as high as the liquidation point.  The proper technique is to heat, test by applying rod, heat, test, etc., until that temperature is reached where material rubs off the end of the rod into the joint.  The rod itself is used as an abrading tool to penetrate the oxide layer, allowing the solder to flow underneath it and fuse with the base metal.

      *Even with great care in the previous step, some spots may appear where abrasion was insufficient to penetrate the oxide and gaps occur in the fillet.  Attempts to fill these with the rod will usually result only in a buildup of excess solder in adjacent areas.  Better results can be obtained by scratching through the stubborn spots with an auxiliary abrading tool, and pulling in solder from adjacent areas.  A stainless "soldering aid" tool is useful for this purpose.  I use one made by Hunter Tools, #51.

      *If necessary, additional material can be applied after the joint is "tinned" by the above steps.  Use a higher temperature to liquefy the solder for good flow.  There is little or no capillary action in this process, so the strength of the joint is in the filleting.  Let the finished assembly cool naturally.  Quenching this material causes brittleness and cracks to develop.

      The above procedure should enable you to make acceptable solder joints in aluminum with a little practice.  For safety's sake, use protective eyewear and gloves, and remember that aluminum is an excellent conductor of heat.  Also note that aluminum at the melting point is not "red hot;" it looks just like it does at room temperature.  Finally, be aware that many metal coatings produce toxic fumes at soldering temperatures. Remove these coatings and provide for adequate ventilation.>

SOLDERING ALUMINUM - MATERIALS

RECOMMENDED
The following products are essentially the same, and have been found to produce satisfactory results.  None of the manufacturers discloses the composition, but it is probably one of the so-called high temperature soft solders, usually cadmium/silver or cadmium/zinc alloys.  These alloys melt at about 640 F and fully liquefy at about 740F.

1. BernzOmatic Aluminum Soldering Rod, Model AL-3
    claimed working temp: 700F
    pkg. of 5 rods $2.99 at Builders Emporium

2. Turner Brazing/Welding Rods, Rod #3, Cat A5040
    claimed working temp: 600F
    pkg. of 5 rods $2.79 at Tool Shack, Escondido

3. Master Marketing Products, ALUMIWELD
    claimed working temp: 730F
    pkg. of 6 rods and stainless brush $5.99 at Harbor Freight

NOT RECOMMENDED
I was unable to obtain satisfactory results with the following:

1. Alpha Metals, Aluminum Braze Solid Wire And Flux
    claimed working temp: 1070F
    0.4 oz wire, 0.25 oz flux, $2.49 at Builders Emporium
    Comments: base metal melted before this filler did

2. Kester Solder, Special Flux Core 100% Tin Aluminum Repair Solder
    claimed working temp: 450F
    0.46 oz coil $1.29 at San Diego Hardware
    Comments: would not wet out and bond with base metal.  Flux 
    caused rapid oxidation of surface, did not break oxide layer.

3. Solder Doctor Aluminum Solder Paste
    claimed working temp: 395F
    est 0.5 oz in plastic syringe $15.99 (!!!) at West Coast Hobby
    Comment: upon heating, solder beaded up on top of the flux
    like water on a fresh Simonize job.  Very poor wetting, no
    adhesion without scrubbing it into the base metal. A rip-off
    price for what appears to be common lead/tin soft solder.

Additional Links

Ian's links to go with above text

http://www.aladdin3in1.com/pages/liqflx.htm
http://www.divescoinc.com/
http://www.bellmanmelcor.com/Alum.htm
http://www.aircondition.com/brazeperfect/
http://durafix.com/durafix.html
http://ishor.com/Solders.htm
http://www.johnsonmfg.com/
http://www.kappalloy.com/alumite.html
http://www.kappalloy.com/fluxes.html#lunar
http://www.ati-intl.com/lacohome.htm
http://www.muggyweld.com/index1.html
http://www.protectic.com/home.html#Aluminum
http://www.efunda.com/materials/solders/types.cfm
http://www.solder-it.com/solderpaste.asp
http://www.superiorflux.com/join.htm
http://www.tinmantech.com/html/aluminum_gas_welding_system.html
http://www.wlv.com/joining/coatedrod.htm

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