Application of positron lifetime annihilation spectroscopy for characterization of friction stir welded dissimilar aluminum alloys

Abstract

Positron annihilation lifetime spectroscopy (PALS) was utilized to characterize dissimilar aluminum alloy workpieces of 2017A-T451 and 7075-T651 joined by friction stir welding. Since the interphase boundaries between the aluminum matrix and second phase particles provided open volumes to trap the positrons, positron lifetimes in the welds and in the heat-affected and thermomechanically-affected zones corresponded with the distribution of secondary phases. The 7075 alloy displayed higher positron lifetimes than the 2017A alloy. The positron lifetime profiles across the weld showed numerous local maxima and minima on the advancing and retreating sides, which also paralleled the hardness behavior. These variations in positron lifetime and hardness away from the weld center stemmed from the temperature distribution in these areas relative to the critical temperatures for secondary phase nucleation and/or dissolution in the two alloys. Weld temperatures on the advancing side were relatively higher than those on the retreating side and consequently promoted more precipitation on the advancing side away from the weld center. This behavior was reflected by the higher positron lifetime on the advancing side compared to the retreating side, at the same distances from the weld center.