  The single lap shear test is widely used to measure the strength of
dissimilar welds even though such a test brings limited understanding of the
intrinsic weld toughness. The present study proposes a numerical finite element
(FE) analysis and experimental characterization of dissimilar joints presenting
various microstructures (thickness of the intermetallic layer (IML) and
hardness profile). For this purpose, Friction Melt Bonding (FMB) and Friction
Stir Welding (FSW) were used to join aluminum AA6061 and Dual Phase steel
(DP980). The FE simulations allowed calculating the evolution of the J-integral
near this notch tip. It shows that crack initiation depends significantly on
the plastic properties of the welded metallic alloys around the notch tip and
the width of the welded zone, which both are significantly different for FSW
and FMB processes. Nevertheless, a similar weld fracture toughness J_C of
approximately 1 kJ.m-2 is estimated from the analysis for both FMB and FSW.
This is three orders of magnitude higher than the fracture toughness of the
intermetallic layer, revealing that the plastic dissipation in the Al and steel
plates around the crack tip has a major effect on the weld toughness.
