[Gary Dillard]

ASTM B308 gives UTS/YS for 6061-T6 of 38/35 ksi, AMS4156 for 6063-T6 gives UTS/YS of 30/25 ksi, so it does have 30% lower strength. If you're welding it you will lose the temper in the heat affected zone either way and can only count on about 9 ksi there. It pretty much depends on how much margin you have in the design; if this is your entire section and it's 1/16 thick it doesn't sound like there's much but if this is part of a box frame then you may be OK. Chemical compositions are pretty similar so they'll form and machine about the same.