6xxx series alloys are composed of Al-Mg-Si.
6061 is used extensively for extrusions.
6061 is the most commomnly fusion welded of the Aluminum alloys that are heat treatable.
Strength of 6061 is on the low end of the scale compared to other heat treatable Aluminum alloys.
Some links:
matweb.com
metalsonline.com
aluminumassociation.org
asminternationl.org
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