welding aluminum frame?

we're welding together a frame, that COULD be made out of aluminum, as it is made out of old bicycles, we're welding the useable pieces of metal from the bicycles into a motorcycle frame. We planned on just cutting the useable parts out with a grinder, and a cutoff wheel. Here are my questions, I know that welding together an aluminum frame isnt recommended if the frame has a crack or internal/external failure, but since these bikes are in good shape, and just being cut up and rewelded into something else, will it hold up. There wont be any pre and post heating stuff, I want to know if I can just take the bicycles, take a grinder, cut the useable metal out, weld it into a motorcycle frame, and ride it when we build it, while being able to rely on it, not fearing that my back end is going to fly off and kill someone.