  Microstructure-property relationships of resistance spot welded 6061-T6
aluminum alloy lap joints were investigated via mechanical testing and
microscopy techniques. Quasi-static tensile and novel shear punch tests were
employed to measure the mechanical properties of the distinct weld regions.
Quasi-static tensile and shear punch tests revealed constantly decreasing
strength and ductility as the weld center was approached. For instance, the
ultimate tensile strength of the fusion zone decreased by ~52% from the parent
material (341 MPa to 162 MPa) while the yield strength decreased by ~62% (312
MPa to 120 MPa). The process-induced microstructures were analyzed with
scanning electron microscopy and optical microscopy to elucidate the underlying
cause of the reduced mechanical properties. Fractography reveals void growth
from particles being the dominant damage mechanism in the parent material as
compared to void nucleation in the fusion zone. Overall, significant changes in
the mechanical behavior across the weld are the result of a change in
microstructure congruent with a loss of T6 condition (precipitate coarsening).
