Hi Les,
Yes, there's a valid concern with the excessive heating of 6061-T1 materials and developing hot cracking. The 300 degree preheat was used to do the PQR on 1/4" material and most importantly, is also the maximum interpass temperature allowed. So the reasoning is if it's safe for the interpass, so should it be for the preheat temperature!
In addition, it's essential when welding 6XXX series that adequate dilution (alloying) of the base material and filler metal occurs. This is described throughout technical publications but was impressed upon me by an old friend that's an experienced metallurgist. NEVER "wash-pass" (without adding filler) a 6XXX series weld!
That’s the best I can do for you Les. I’m certain that either Dr. Crisi or Niekie3 could explain it in far greater detail though.
Good luck.
When welding Al, you essentially always end up with a weld that is substantially weaker than the parent metal.
In the case of the heat treatable alloys such as 6061, the strength is obtained by a precipitation hardening mechanism. Typically small amounts of Cu is added to achieve this precipetation hardening. To achieve the hardening, the material is typically quenched from a solution heat treatment temperature and then "aged" by applying a moderate temperature to it, (Typically 300 - 350°F for this alloy) to help form the precipitates. If this temperature is too high, or held too long, the material "over-ages" which leads to a weaker material. It is for this reason that welding will always result in a weaker HAZ, because you will always have an over-aged material. It is also for this reason that you want to keep the pre-heat low.
In the case of the non-heat treatable alloys, strengthening is usually obtained by strain hardening. (Cold deformation.) When the material is heated up, the effect of the strain hardening is lost due to recrystallization and simmilar effects. That is why welding will laways leave a weakened zone in the HAZ of these alloys. That is why a pre-heat in these materials will result in lower strengths and wider HAZ's, so you want to keep the pre-heats low.
What is the moral of the story? - Make sure your welds are not in highly stressed areas of your components. The design philosophy in Al is somewhat different to that of steel and other alloys where you can get a joint efficiency of 1.
The story with the addition of filler being important is so because:
1) Some compositional ranges of alloying additions are very crack sensitive. The filler is needed to move the weld away from these ranges.
2) Si has a very important effect on the solidification range of Al. The wider the solidification range, the bigger the chances of hot cracking. At 12% Si, the solidification range is basically zero. It is for this reason that many casting grades of Al have around 12% Si.
Hope this helps
Regards
Niekie Jooste