Failure Prediction of Aluminum Laser-Welded Blanks

The formability of aluminum-3% magnesium laser-welded blanks manufactured by 3kW YAG laser welding process is investigated in this paper. Previous experimental results showed that shear failure occurs in the welds of these blanks during tensile testing with the weld line perpendicular to the tensile axis. In order to further understand the failure in these blanks under uniaxial and biaxial straining conditions, finite element computations under generalized plane strain conditions are conducted to understand the effects of weld geometry and strength on the shear failure. The results indicate that the average plastic stress-strain behavior of the weld should be softer than that in the base metal in the direction perpendicular to the weld line. The results also show that the weld geometry has significant effects on strain localization in the weld under the tensile loading perpendicular to the weld line. The stress-strain histories of the material elements of the weld metal in tensile specimens obtained from the finite element computation are used for a failure/localization analysis. The imperfection approach of Marciniak and Kuczynski is employed to examine the failure/localization in the weld of the tensile specimens. The failure/localization analysis is based on an elastic-viscoplastic constitutive law that accounts for the potential surface curvature, material plastic anisotropy, material rate sensitivity and softening due to nucleation, growth and coalescence of microvoids. The material imperfection parameters for void nucleation and growth are obtained by fitting the results of the failure/localization analysis to those of the uniaxial tensile tests. The material imperfection parameters are then used to predict the failure strains of the blanks under biaxial straining conditions based on the stress-strain histories of the material elements obtained from finite element computations under generalized plane strain conditions. The results indicate that the forming limits of the blanks under plane strain tension and equal biaxial tension are reduced significantly when compared with that under uniaxial tension. Finally, a finite element simulation of the welded sheet under uniaxial tension is carried out with consideration of void nucleation and growth for all the material elements in the weld. Our computational results indicate that without consideration of the failure/localization analysis presented in this paper, an unrealistically high volume fraction for void nucleation must be assumed in our finite element simulation of the tensile tests in order to match the failure strains observed in the uniaxial tensile tests.