Hello Sazid, I believe the short answer to your question is because of the alloying elements that are used in certain grades of aluminum, namely zinc. When welding is attempted on these specific grades there is a volatilization of the zinc causing possible internal porosity and also reduction of the zinc possibly causing considerable mechanical strength reduction as well. I'm sure that there are others who could address this much more specifically, yet I believe I have given you a bit of the reason. If you google "aluminum alloys and their weld ability", I believe you will find many more of the specifics to support this train of thought. Try looking for Alcoa Aluminum, or possibly Reynolds Aluminum, I'm sure there are others but I can't think of them right now. A little bit to consider. Best regards, Allan