6061 T6 is a heat treatable alloy. With the heat treatable aluminum alloys, the last heat treatment step heats the metal to approximately 400° F. But when welding, the material around the weld becomes much hotter than 400° F so the material tends to lose some of its mechanical properties. Therefore, if the operator doesn't perform postweld heat treatments after welding, the area around the weld will become significantly weaker than the rest of the aluminum, by as much as 30 to 40 percent. If the operator does perform postweld heat treatments, the properties of a heat treatable aluminum alloy can be improved. The strength of heat treatable alloys are decreased by the heat of welding. However, these alloys require a substantial time at temperature to fully soften them. As a result, the mechanical properties of the heat affected zone in an as welded joint are higher than those of fully annealed base metal. The properties may vary considerably with heat input and cooling rate, but they are generally lower than those of weld heat affected zones in many of the 5XXX and 7XXX series of alloys. The speed of welding has a significant effect upon the mechanical properties of arc welds in heat treatable alloys. High welding speed contributes to low heat input and a narrow heat affected zone. This, in turn, reduces the possibility of eutectic melting, grain boundary precipitation, overaging, grain growth, or a combination of these. Mechanical properties of joints welded with GMAW are generally higher than those of equivalent joints welded with GTAW because of the higher speeds employed with the former process. Material thickness also has an effect on the properties of arc welds. Thinner materials will have better strength properties and can be welded at higher speeds and lower heat inputs. Other factors that affect weld strength are the size of the weld, the joint design, and the amount of reinforcement. Heat treatable alloys develop their strength by solution heat treating followed by either natural or artificial aging. Cold working before or after aging may provide additional strength. Heat treated alloys may be annealed to provide maxium ductility with a sacrifice in strength properties. Annealing is achieved by heating the component at an elevated temperature for a specified time, and then cooling it at a controlled rate. Welding of heat treatable alloys in the solution heat treated condition rather than in the aged condition will often minimize cracking. Welds in solution heat treated materials will have a more uniform microstructure, and the base material will impose less restraint during weld metal solidification. Repair welding is usually more difficult with heat treatable alloys because of microstructural changes that take place during the original welding. Restraint is usually greater when repair welding, and this may cause cracking in the heat affected zone or in the previously deposited weld metal. Joint strength may also be lower after repairs are made. Here's a link that may also be helpful:
http://www.lincolnelectric.com/knowledge/articles/content/comistakes.asp