as lowracer said, heat treatment. Welding 6061 will cause it to lose half it's strength and then after welding you will need to get it heat treated again to regain it's strength and regain its integrity. This is why i stay away from using aluminium in projects and rather use steel. I believe it is also quite expensive to get the heat treatment done propoerly. Not sure how much though.
But it's up to you in the end.
