The degradation is heavily influenced by the time at temperature. The higher the interpass temperature and the longer the time, the more severe the degradation of mechanical properties is. The problem is called overaging. Normally the alloying constituents clump together in the solidified grain causing the 6061 to become harder and stronger. Overaging causes the alloying elements to precipitate to the grain boundaries thereby weakening the metal.

Consider this: even when everything is done properly, most welding standards accept the fact that the tensile strength of 6061-T6 will drop from 42ksi to 24ksi and it is considered to be acceptable.

AWS D1.2 can be reviewed, but it doesn't give your the "whys" or "how" things do what they do. There are books by Kaiser and others that explain what happens to different aluminum alloys when you weld them.

Best regards - Al

Copied from "The Welding of Aluminium and its Alloys"
Gene Mathers
Cambridge

Woodhead Publishing ISBN 1 85573 567 9
CRC Press ISBN 0-8493-1551-4
Published by Woodhead Publishing Limited, Abington Hall, Abington
Cambridge CB1 6AH, England
www.woodhead-publishing.com
Published in North America by CRC Press LLC, 2000 Corporate Blvd, NW
Boca Raton FL 33431, USA
First published 2002,Woodhead Publishing Ltd and CRC Press LLC
© 2002,Woodhead Publishing Ltd

In the overheated zone in the HAZ closest to the fusion line, partial melting of the grain boundaries will have taken place. Temperatures have been high enough and cooling rates sufficiently fast that solution treatment has taken place, enabling some ageing to occur after welding. Adjacent to this is the partially solution-treated zone where some of the precipitates have been taken into solution, enabling some post-weld hardening to occur, but those not dissolved will have been coarsened. Outside this will be the overaged zone where precipitate coarsening has taken place and there has been a large drop in strength.
The strength losses in the 6000 alloys are less in the naturally aged metal than in the artificially aged alloys.The strength of the weld and HAZ in the artificially aged condition generally drop to match that of the naturally aged alloy with a narrow solution-treated zone either side of the weld and an overaged zone beyond this, which is weaker than the T6 condition. With controlled low-heat input welding procedures the strength of the weldment will not drop to that of an annealed structure but will be close to that of the T4 condition.