Do a search on "6061 Aluminum," and you'll find quite a few threads on the topic.
Here's the link in case you're not familiar with the search function in this forum... Click any one of the threads to see if you can find the potential solution(s) to your problem(s):
http://www.aws.org/cgi-bin/mwf/forum_search.pl?words=6061+Aluminum&user=&board=0&min=&max=&field=body&sort=time&order=descBe prepared to do some reading, then repost your query/question with as much SPECIFIC information as possible such as: diameter wire, gas flow rate, voltage, current/wire feed speed, travel speed, type of application, whether or not post weld heat treatment is being used afterwards, thickness of material, etc. Hopefully either you might find the answer to your dilemma or someone here may be able to help out but, remember to include as much of the SPECIFICS involved in your welding operation, and application of final product as possible from now on or, use the search feature ;)
Respectfully,
Henry