Quantifying structure-property relationships during resistance spot   welding of an aluminum 6061-T6 joint
  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). 
S. A. Turnage, K. A. Darling, M. Rajagopalan, W. R. Whittington, M. A.   Tschopp, P. Peralta, K. N. Solanki
cond-mat.mtrl-sci