"Weldcome" to the forum mig welder!
Are you talking about this stuff?
http://www.alcoa.com/adip/catalog/pdf/6561_Alcoa_PipeTube78_98.pdf

Hi Dave!
Is there a possibility that you're thinking carbon steel when you mentioned what you did because, according to these .pdf's - their chemical compositions for both 6061 and 6082 Al show no carbon at all... Why? There is'nt any carbon in these grades of Aluminum. You were probably thinking along the lines of the Aluminum oxide layer that any grade of aluminum will have on it as soon as it's exposed to the atmosphere...
http://www.alcotec.com/us/en/education/knowledge/qa/Storage-and-Preparation-of-Aluminum-Base-Alloys-and-Filler-Alloys.cfm

Here are the chemical compositions for 6061 & 6082:
http://www.alcoa.com/adip/catalog/pdf/Extruded_Alloy_6061.pdf
http://www.alcoa.com/adip/catalog/pdf/Extruded_Alloy_6082.pdf
http://www.kaiseral.com/proalloy/pdf/tp_6061.pdf

Now as far as the differences between each alloy, one has to look at the slight differences in chemistry because the physical and mechanical properties are almost identical.

Finally, Read this from AlcoTec to find out what is really being burned off:
http://www.alcotec.com/us/en/education/knowledge/qa/Atmosperic-Conditions-Affect-Weld-Quality.cfm

Which leads me into answering your question regarding preheating Aluminum -Specifically the alloy 6082...
According to "The Procedure Handbook of Arc Welding" 13th Edition by Lincoln Electric in Section 9, "Welding Aluminum and Aluminum Alloys", on page 9.4-8 it recommends for GTAW Aluminum "Preheating: preheating the work for AC GTA welding of Al is necessary for heavier sections, where the heat is conducted away from the joint so fast that the welding arc cannot produce fusion. Preheating can be done with a gas torch or any othe suitable means. Unless the Ambient temperature is below 40 degrees F, no preheating is used on plate up to 3/8" thick and is optional between 3/8" & 1/2" thick. Plates 3/4" and thicker may require preheating but, the maximum preheat temperature should not exceed 350 degrees F... Maximum temperature of heat treatable alloys are particularly critical because of the narrow temperature range that mast be maintained to avoid adverse affects of the heat of welding on the mechanical properties of the joint."

So in a nutshell, Al was correct in suggesting for you not to use preheating for your specific situation. The only exception to this would be if the ambient temp went below 40 Degrees F and or the relative humidity is higher than acceptable levels... If these potentially detrimental conditions do exist, then one could preheat to 120-150 degrees F maximum especially for the grade of Al your working with. I would'nt preheat any higher than 150 F for the thicknesses you're welding together. Remember the primary purpose for preheating this grade of Aluminum is for moisture removal as opposed to other factors that can necessitate the need for preheating low alloy steels or some Chrome-Moly steels.

Here are some of the reasons for preheating the joint in preparation to welding:

"a)  To drive away moisture from the weld area:  Typically, this is performed by heating the surface of the material to a relatively low temperature, just above the boiling point of water.  This will dry the plate surface and remove the undesirable contaminants that may otherwise cause porosity, hydrogen embrittlement, or cracking through the introduction of hydrogen during the welding process.

b)  To lower the thermal gradient:  All arc welding processes use a high temperature heat source.  A steep temperature differential occurs between the localized heat source and the cool base material being welded.   This temperature difference causes differential thermal expansion and contraction and high stresses around the welded area.  Reducing the temperature differential by preheating the base material will minimize problems associated with distortion and excessive residual stress.  If preheating is not carried out, a large differential in temperature can occur between the weld area and the parent material.  This can cause rapid cooling, leading to the formation of martensite and probable cracking when welding some materials with high hardenability." Reference: http://www.alcotec.com/us/en/education/knowledge/weldinginspection/What-The-Welding-Inspector-Should-Know-About-Preheating-And-Postweld-Heat-Treatment.cfm
I hopes this helps in answering your query ;)

P.S. BTW, here's the main link to the Knowledge center ofr AlcoTec:
http://www.alcotec.com/us/en/education/knowledge/index.cfm

Respectfully,
Henry

Having Tig welded tons of aluminum lemme pitch in on this one with just a few notions and practical tips.

Aluminum is #2 of all metals in thermal conductivity, just below copper.   This means that a second or two after you have struck an arc (probably before you add your first dab if filler) you have a torch generated preheat of well over 250 degrees radiating several inches from the arc zone, this includes metals that are up to an inch thick or more.  If proper surface prep has occured, any mositure will be driven off long before the arc reaches it.

If it is not feasable to pre-heat as Mr. Armao suggested (I think warm is a better term since as Al has alerted us, preheat is not advisable for 60XX alum). All one needs to do is focus the tungsten slightly toward the thicker member of the fillet or joint. If the torch is placed exactly centered between the legs of the fillet, the thinner member will take more filler and the fillet legs will be uneven every time. In the case of aluminum, technique is very important because the heat will go to the heat sink naturally and this means you sometimes must exaggerate a little bit where the tungsten is pointed.

This same technique can be observed and demonstrated with fillets with members of equal thickess.... For example; Take a fillet of 0.063 aluminum and clamp it to a table. The horizontal member (the one clamped to the table) will drink up and conduct away more heat, so in order to get a weld with equal legs the operator must focus the arc a little toward the member clamped to the table.  The vertical leg has only the atmosphere to help it conduct away the heat so it gets hotter quicker and is far more prone to suckback.

So if distortion is not an issure.... Just focusing the arc on the thicker member and striking an arc and letting it just pause (I call it a pilot arc) for a moment to allow some heat to soak into the aluminum is a technique that is much more productive than a time consuming pre-heat technique. The proof that this technique works is that the operator will be able to back off on the foot pedal amperage after just a few dips of wire and continue to back off as the weld progresses when the (pilot arc method is used.)

While it is true that aluminum wants to be GTA welded hot and fast... A little patients in the beginning will make for a much better outcome.

Hello Henry;

No disrespect for your response was intended. I understood what you were trying to communicate, but my concern was that not everyone would take the time and effort to utilize the valuable website link you provided. The information is great; I was simply attempting to reinforce the point that the microstructure "martensite" is associated with ferrous metals.

Common sense is not all that common. What you and I take for granted isn't common knowledge to everyone. There are so many "wife's tales" in the welding industry it hard to imagine we why don't have more catastrophic failures than we do.

How many times have you heard welders say the purpose of preheating is to "drive moisture out of the base metal," or "to prevent shocking the molecules"? I often wonder how those "tales" got their beginning.

Another bit of "misinformation" I heard recently was that "low hydrogen electrodes work best when they are wetted before use, they almost weld as good as regular pipe wire." It is a scary world out there!

It's so scary I sometime hesitate to repeat some of the things I've heard for fear people will only hear part of the quote and misapply the information. That happened once when I stated that SAW was an underutilized welding process. The question was then asked, "If it is never used, why even talk about it!" Some parents simply don't realize how much money they wasted sending their offspring to college.

You are on the mark regarding Basic Metallurgy 101, that's what we're all about.

Best regards - Al

Nobody said that one cannot preheat 60XX aluminum Kix!

We're just saying that it is'nt always a requirement... There are certain situations where preheat is necessary but when it comes to the thicknesses described in the initial post by "mig welder", there's really no need for it... in fact unless the environmental conditions dictate otherwise, more harm than good would be the result of preheating the joint.

Respectfully,
Henry

Hello mig welder, this milky white finish that you are referring to is likely the results of the cleaning action of the DC positive side of the welding current as well as a portion of the high-frequency bombardment that helps to break-up surface oxides, it can also be affected by arc focus through AC hertz frequency adjustment. Adjusting the AC balance, high-frequency intensity, and AC hertz frequency, if your machine is so equipped, can help to minimize this effect. It is cosmetic only and not detrimental to the welds integrity. Some fabrication and manufacturing companies do not like this to show up on their products so they take the steps necessary to tune this out by adjusting their equipment. Just my $.02 Regards, aevald

I suspect the "milky white finish" is the area adjacent to the weld that was cathodically cleaned, i.e., the surface oxides are "blasted" off the surface by the cathodic action of the welding arc.

The 6XXX series can also experience something the military welding standards refer to as "arc errosion". The outer envelop of the arc melts the very surface of the base metal and very fine shallow craze cracks can form. You will see them if you perform a penetrant test. They can usually be removed easily with a stiff stanless steel brush.

Good luck - Al

Thanks again all help and advice is greatfully recieved.
Laura