Numerical laser welding simulation of dissimilar Steel-Aluminum overlap joints

The current dissertation aims to develop an efficient finite element (FE) model for the structural simulation of dissimilar St-Al overlap joints. In particular, a prediction of residual stresses and distortions after welding is of interest. In the thesis, the known computational methods were analyzed and expanded regarding their application for the deep laser welding of dissimilar St-Al joints in steel-on-aluminum overlap configuration. The influence of the mixing in the weld pool on the mechanical properties of the St-Al weld metal and the subsequent influence of the weld on the structural behavior of the joints are the central questions of the study.
The developed temperature FE model enables accurate computation of the temperature distribution in the overlap joint as well as a calculation of fractions of melted alloys in the St-Al weld.
The presented experimental investigation provides the first insight into the elastic-plastic behavior of St-Al weld metal. The mechanical properties were determined using the indentation technique and then correlated to aluminum concentration. Hardness and yield strength showed a nonlinear increase with an increasing aluminum concentration. Young’s modulus and strain hardening exponent were found to be nearly insensitive to changes in weld chemical composition. Based on the obtained relations, the weld material model was designed and coupled with the FE thermomechanical model.
The structural simulations of the St-Al overlap joints were validated on welding experiments through the comparison of measured and computed distortions. The performed sensitivity analyses indicated that the application of the developed weld material model is crucial for achieving reliable accuracy of structural simulations for St-Al joints.