Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-T7451

Aluminum alloy 7050 was friction-stir welded (FSW) in a T7451 temper to inv estigate the effects on the microstructure and mechanical properties. Resul ts are discussed for the as-welded condition (as-FSW) and for a postweld he at-treated condition consisting of 121 degrees C for 24 hours (as-FSW + T6) , Optical microscopy and transmission electron microscopy (TEM) examination of the weld-nugget region show that the FS welding process transforms the initial millimeter-sized pancake-shaped grains in the parent material to fi ne 1 to 5 mu m dynamically recrystallized grains; also, the FS welding proc ess redissolves the strengthening precipitates in the weld-nugget region. I n the heat-affected zone (HAZ), the initial grain size is retained, while t he size of the strengthening precipitates and of the precipitatefree zone ( PFZ) is coarsened by a factor of 5, Tensile specimens tested transverse to the weld show that there is a 25 to 30 pet reduction in the strength level, a 60 pet reduction in the elongation in the as-FSW condition, and that the fracture path is in the HAZ. The postweld heat treatment of 121 degrees C for 24 hours did not result in an improvement either in the strength or the ductility of the welded material. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW + T6 condition, a t a stress ratio of R = 0.33, shows that the FCG resistance of the weld-nug get region is decreased, while the FCG resistance of the HAZ is increased. Differences in FCGRs, however, are substantially reduced at a stress ratio of R = 0.70. Analysis of residual stresses, fatigue-crack closure, and fati gue fracture surfaces suggests that decrease in fatigue crack growth resist ance in the weld-nugget region is due to an intergranular failure mechanism ; in the HAZ region, residual stresses are more dominant than the microstru cture improving the fatigue crack growth resistance.