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Larger included angle usually but not always equates to less residual stress in the joint afterwards, of course the geometry does differ when one uses a process with less heat input also. ;) What it comes down to is the behavior of each process, and how it relates to the amount of the heat input, and how large of a HAZ is made which one wants to minimize as much as possible...
Then of course, travel speed does change this also. So in effect, The primary reason is to minimize residual stresses as much as possible for each joint geometry as it relates to each of the various processes as well as insuring uniform fusion and penetration within the entire joint and this also is dependent on the process being used. ;)
Respectfully,
Henry