"Weldcome" to the forum mig welder!
Are you talking about this stuff?
http://www.alcoa.com/adip/catalog/pdf/6561_Alcoa_PipeTube78_98.pdfHi 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.cfmHere are the chemical compositions for 6061 & 6082:
http://www.alcoa.com/adip/catalog/pdf/Extruded_Alloy_6061.pdfhttp://www.alcoa.com/adip/catalog/pdf/Extruded_Alloy_6082.pdfhttp://www.kaiseral.com/proalloy/pdf/tp_6061.pdfNow 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.cfmWhich 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.cfmI 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.cfmRespectfully,
Henry