I can only chime in here from an engineering standpoint as I am not a rigger. I am sure Brion will have a better reply. The holes you drill are standard practice to stop a crack from spreading. They spread the stress out over a larger area so it no longer has enough force to crack the aluminum. The diamond piece you are welding on has a similar function. You don't want to make one part of the mast too strong compared to the section right next to it.
That would put too much stress right at that junction. The diamond will make your weak point very strong and then gradually reduce the strength as you go up and down the diamond. The radius does the same thing although personally I would not see a problem with a sharp point going up or down as it would not be a stress concentrator but a stress reliever. I like the radius on the sides of the diamonds. But in general, no sharp corners, no abrupt transitions.
I personally have had good luck with TIG wilding of some aluminum although they were not boat related. I had a very good welder and he did an excellent job. There must be a lot of variation in welders as I had people tell me both jobs were impossible.
Allen
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