Effect of weld metal microstructure on the monotonic and cyclic mechanical behavior of tig welded 2091 Al–Li alloy joints

The effect of TIG welding on the mechanical behavior of Al–Li alloy 2091-T851 has been investigated by means of tensile and fatigue testing. Compared to the base alloy, as-welded joints were found to display significant reductions in both tensile and fatigue strengths and to suffer a drastic degradation in ductility. Such behavior is primarily related to the presence of large elongated R phase precipitates at the weld metal grain boundaries, which leads to intergranular crack propagation through that part of the joint. Substantial microstructural changes, though, can be induced in the weld metal by subjecting the welded joints to a post-weld T6 type heat treatment. This promotes dissolution of the R phase, thus enhancing the tensile properties as well as the fatigue strength, consistent with the predominance of transgranular fracture of the weld metal in the heat treated joints. Finally, the microstructural aspects presented above indicate that the weld metal is plastically more heterogeneous than the base alloy and therefore is expected to work-harden faster, which is borne out by the tensile test results.