Characterisation and modelling of toughness in 6013-T6 aerospace aluminium alloy friction stir welds

The effect of the friction stir welding process on the toughness properties of AA6013-T6 sheet has been investigated. The alloy was received and welded in the peak aged T6 condition and the toughness measured at intervals across the weld by means of a notched tear test, with subsequent fractographic examination via field emission gun scanning electron microscope (FEGSEM) and microstructural characterisation via optical microscopy and energy dispersive X-ray (EDX). It is shown that the controlling factors for toughness in AA6013-T6 following FSW are the population and distribution of the coarse α-(Al,Fe,Si,Mn) intermetallic particles, with strength variations caused by precipitate dissolution, coarsening and transformation representing a secondary consideration. Minimum toughness occurs at the boundary between the weld nugget and the heat-affected zone due to the alignment and concentration of coarse particles at this point by the FSW process. A simple model is implemented and provides a reasonable prediction of the weld toughness from simple microstructural observations.