I didnt get any bites in Metallurgy so Ill throw this here

My question is very general and many.  We are doing tensile testing on some 6061-T6.  We have 3 PQR's one done with AC, one with DC+, and one with AC/DC+.  These plates have been heat treated back to T-6 condition.  Now before we did these plate we were sent samples to pull and we could not reach the yield the customer was aiming for in the 34K+ range.  We did a set of tensiles exactly like D1.2 states to cut them and got into the mid 24K range for yield.  We are using a a united testing tensile machine computer aided with extensometer.   After we got those results we changed the dimensions to narrow up the sample.  From a reduced section thickness of 1.5" to 1" and another to 3/4".  The 1 inch and 3/4 both yielded above 34K.  Why is that?  Also will a shorter reduced section cause higher yield?  Lastly if the whole plate is theoretically welded the same, should they all be proportional to their area in the reduced section?

A bit towards metallurgy question:  If aluminum is welded hot and gets too hot it can become brittle in the weld zone.  Now if its taken back up T6 in heat treat, will that phase change "fix" the over heated weld? or will the weld still hold some qualities of the less than favorable welding?  Basically can heat treat fix anything structurally that would normally fail in the as welded condition.

Thanks
-Jay