Local deformation response and failure behavior of AA6061-AA6061 and AA6061-AA7075 friction stir welds

Abstract

Friction stir welding (FSW) is generally associated with a gradient in plastic deformation and temperature, which results in the evolution of heterogeneous microstructure thereby, the variation of mechanical properties along the weld. It becomes critical to study this variation in mechanical properties to understand the mechanism involved in the failure of the FSW joints. In the present study, the local strain response is investigated using digital image correlation (DIC) for FSW joints of similar AA6061-AA6061 and dissimilar AA6061-AA7075 alloys (placing AA6061-T6 and AA7075-T6 in advancing and retreating side respectively). Local strain evaluated through DIC revealed the localization of strain at the heat-affected zone (HAZ) in similar FSW joint and joint failed from the HAZ. Opposing to this, in dissimilar FSW joint the localization of strain occurs at two sections, in HAZ and at the stir zone (SZ) edge in the advancing side (AS) of the joint. However, the failure occurred at SZ edge containing defects such as microvoids. On the other hand, dissimilar FSW joint without defects failed from HAZ in AS of the joint. The microhardness measurement revealed low hardness in HAZ of the weld. Notch tensile tests are carried out to allow the failure to occur in different zones found in the similar and dissimilar FSW joint. The HAZ in a similar FSW has shown the least resistance to failure induced by the notch. Fractography of the similar FSW joint revealed a ductile failure. However, the fractography of dissimilar FSW suggested a failure affected by defects and weak bonding at SZ edge in AS.