CoryPad may be right if you are looking at high production type work. I don't think that tubes to plate can be friction stir welded yet though, but friction welding may be a good idea. Depending on the diameter of the tube, you may also look at stud welding equipment if they are short tubes. Laser welding could also be a solution. If the above ideas are not applicable to your situation, a GTA (TIG) weld may be your best and most conventional bet. Due to the thickness differences you will probably have to preheat the thicker plate. This would have to be determined by experimentation. Unless your joint is a square groove, filler metal will generally have to be added also. 6061-T6 aluminum is readily weldable, but I do not know about the 3015-H12. The tube manufacurer may be able to give you weldability and filler metal recommendations.