The paper deals with accumulation of strain and acoustic emission in a welded sample of aluminum-magnesium alloy obtained by friction stir welding under loading at temperatures in the range from room temperature to 500 °С. The accumulation of strain in a welded sample is demonstrated as a two-step process: a low-temperature (Area I) step controlled by dislocation climb, and a hightemperature (Area II) step controlled by grain boundary sliding. Transition from low-temperature area to hightemperature area is characterized by the boundary temperature T_l. at which structural transition occurs. In the hightemperature area the energy parameter J_{Region II} grows with increase of mechanical loading in the cycle. This fact indicates an increase of vibrational energy concentration in a deformed part of the sample that contributes to the activation of deformation processes. It is shown that the temperature of the structural transition point is shifted to lower temperatures with increase of loading in cycles, complying to inversely proportional dependence of the temperature from the voltage. Plastic resource of the welded sample under cyclic thermomechanical loading is almost seven times lower than the plastic resource of a sample cut from metal alloy AMg6 flat-rolled product.