I am not doing aerospace parts, but soap and water has always worked well for me.

I have been using "Aircraft grade Simple Green".  Works great and have had no problems over the last couple of years.
The only place I've found it is through  "Grainger".

HTH
Griff

PS
Grainger p/n 22NX99  $18.99US  32oz spray bottle

There at least a couple of threads on this very issue, however I will save you the search.

One of our main products for the military is made from 6061 and over the last year I have experienced more than a fair amount of pain getting this correct.  In the process I brought in 5 CWI's, two of them SCW's and two Weld Engineers.  It all came down to contamination and technique.

If you have dirty 6061, you have two choices: a) Get cleaner aluminum or b) Clean the aluminum in Bio Circle Alustar 100.  We use a tank with an agitator and heater.  We hang the parts in the bath for a couple of hours.  This also works well for cleaning reworked weldments before repair.

Bag the parts to limit the reformation of aluminum oxide as much as possible unless they are going directly to weld after drying.

When ready to weld, wipe the joint areas with Arcal Weld-O cleaner, then wire brush in ONE DIRECTION ONLY.  Brushing in both directions encourages oxide to form.  Pratt and Whitney uses wire brushing in both directions to cause oxide to form on jet engine parts.  Do not use power wire brushes...that did not go well for us.

Some additional points:

Do not run the parts through an abrasive deburing machine as it will create grooves that hold dirt and makes it MUCH harder to clean.  Hand debur edges.

Do not grind carbon steel in the same end of the building that you are welding aluminum.  It WILL contaminate the aluminum and create nightmares with the finished welds.

Ensure that your MIG and TIG fill are in sealed containers.  Use only good quality fill, not fill purchased from companies that buy up excess and respool it for resale.  Cover the spools on the machines when not in use.

Watch out for CRATER CRACKS.  Check a good size sample of the welds under 10X for crater cracks.  Ensure that your welders terminate their welds properly.  Turning the TIG cup away too soon at the end of a weld or poor MIG technique will result in nightmares, from crater cracks to throat cracks.  If they are not using a crater fill setting, ensure they are filling the crater.

Feel free to ask more questions if needed and I can point you to some very good websites such as: http://www.thefabricator.com/author/frank-armao

Going to add a couple of things:

The first step is the cutting process.  6061 gives our laser issues as it is reflective and we end up setting fire to the bellow.  We use a water jet, and if you use a water jet, keep in mind that the water is under pressure with garnet sand suspended in it.  While the garnet does not cause all that much trouble (a surprise to us) the street water with all the minerals does and when you add the gunk that forms in the tank, you have the first step in the chain of woe and doom.   That is what the agitated bath is for.  To remove the poo from the parts.   Bagging the parts in plastic bags if they will not be used directly as it keeps contaminants off them.

By deburing machine I mean the belt sander type commonly known by the generic term "time saver".  People frequently forget to change belts between carbon steel and aluminum.  This is the second step in the chain of woe and doom.

We use the Bio cleaner in the wash tank as it can be put directly down the sanitary sewer and it is specially formulated for aluminum.  Simple Green did not do quite as good a job.   DO NOT let the parts sit in the tank for an extended amount of time else they will end up looking like the sacrificial anode on your outboard motor...after a long sit in salt water.

Hope this all helps!

I forgot to note that the consensus from all the CWI's and Weld Engineers is that alcohol does little to nothing.  Acetone works much better.  If you can't use the Acetone, use the Weld-O.  You will be surprised at how much oxide and dirt it pulls off.  Also with regard to Acetone and Alcohol...it proved not to be a great idea to have very flammable liquids around a weld booth.  Alcohol burns with out much of a visible flame and it takes a moment to realize you have a fire.

It's all hard learned...

I am trying like mad to attend the upcoming AWS Aluminum weld conference in FL, if for no other reason than to camp out next to Frank Armao and soak up every comment he makes like a huge sponge.  For some odd reason I have an obsession with learning everything there is to know about aluminum welding issues and their causes.

How often do you change your tanks?

       M.G.