Microstructure and tensile response of friction stir welded Al–Cu–Li (AA2198-T8) alloy

Abstract

Friction stir welds (FSWs) can be considered as an ensemble of elements of material with composite microstructures connected in series. In the present study, bead-on-plate FSW runs were made on an Al–Cu–Li alloy with varying rotation speeds ranging from 400 to 800 rpm. Microstructure of the FSW region was investigated by using optical microscope, electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). Thermal stability of various precipitates was evaluated with differential scanning calorimetry (DSC) measurements. Strength variation across FSW cross sections was mapped by microhardness measurements. Average as well as local mechanical properties were evaluated using a digital image correlation (DIC) technique. Irrespective of the process parameters, FSW samples showed similar tensile and strain hardening behaviour along with serrations in stress-strain curves while local strength values showed increasing trend with rotation speed. The FSW alloy produced at intermediate rotation speed exhibited different mechanical behavior and is correlated with the resultant substantial changes in the microstructure. Strain localization occurred at the boundary of nugget zone and thermo-mechanically affected zone which led to failure of the FSW tensile specimens within weld regions. Fractography investigation revealed that the failure is initiation controlled, that is void nucleation at coarse precipitate-matrix interfaces.