Welding different aluminium alloys??

I have treated myself to an AC / DC TIG welder, and am busy experimenting and practicing. My first attempts at aluminium welding were on soft pipe, the type a race car might use for rad or intercooler plumbing. Down to user ineptitude it wasn't very successful at all, the material melting too fast for me to keep up, with holes, runs and all the usual nasties i kind of expected at a first attempt! I then tried welding some aluminium alloy extrusions, angle and channel. This stuff is HARD, will break rather than bend, but I have had a lot more success getting a weld bead on this material. Is it simply because it melts at a higher temperature? Thanks