Now this PQR is on a 1/2" plate so wouldnt the heat still be the same across that plate? I could see that being a point on a single or maybe even a 2 pass plate but these have many passes. Controlling heat over many passes is a different challenge than on a single pass... I'm not sure how to explain to you the thermal conductivity with aluminum... Heat builds and grows in the direction of travel.. The end of the weld will be hotter than the start. When you get to the end of a joint, no matter if it is thick or thin, there is less mass for the heat to run to. Increased travel speed can mitigate this to some degree.
Would then an over heated weld hold up longer in tension then if its the opposite of brittle? Therefore adding yield? NO ! Brittle material (typically) has greater tensile strength (holds up longer in tension) than softer (more ductile) materials.
Lastly curious where I can read more about the Hydrogen in the weld creating the bumpy surface. Ive seen it before and didnt know that was a sign of that. Would be great to know more
Aluminum being soluble to hydrogen when it is molten is no real news.
Way back machine... 1974 Welding Journal
https://app.aws.org/wj/supplement/WJ_1974_03_s97.pdfASM Handbook Vol 6 has more data.