selecting the right rod for aluminum and steel fence fabrication also right amperage

My dad just purchase a lincoln 225 arc welder and I would like to know what rod we need to weld those square pipes sold at homedepot they're about 1" and hollow inside and we plan to stick em to a 2" side bar with the same thickness.

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for steel, I would choose a 6011 rod. probably 3/32 diameter because the wall thickness on what you describe is probably only about .063"

for aluminum, unless you have done quite a bit of stick welding, its gonna be a lot more difficult than the steel.
I would recommend going to a local welding supply and buying a small qty of aluminum stick rod and give them a try before you jump into anything.