i was just looking through my Maxall “guide for aluminum welding” book and i see in the book they say, ” the 6xxx series base metals have 30% higher thermal conductivity than the 5xxx series base metals, making it more difficult to produce consistent quality welds in the 6xxx base metals".

can anyone explain this further ?

i weld a lot of thick 6061 aluminum at work, 5/8" to 3/8" and 5/8" to 3/16" all day every day, using 4043 filler, (required by the weld spec the customer provided), and from time to time i can weld pretty good appearing beads all day, then the next day its a struggle to get a good looking bead.
its starting to drive me crazy, seriously.... its very frustrating, but after just finding that statement, im asumming maybe it isnt just me ?

thanks guys for any help you can provide