Computational analysis of the intermetallic formation during the dissimilar metal aluminum-to-steel friction stir welding process

Abstract

The extent of inter-material mixing and the formation of intermetallic compounds play a critical role in the structural integrity and mechanical properties of the joints in the case of dissimilar metal friction stir welding. In general, there is a critical volume fraction of the intermetallic compounds in the mix zone of the friction stir welding-joint at which the mechanical properties of the joint are maximized. That is, insufficient inter-material mixing and the accompanying sub-critical volume fraction of the intermetallic compounds results in insufficient inter-material bonding and inferior joint strength. Conversely, super-critical volume fraction of the intermetallic compounds typically gives rise to the joint embrittlement. To address the problem of the effect of the friction stir welding process parameters on the extent of intermetallic compound formation, a multi-physics computational framework has been developed and applied to the case of dissimilar metal friction stir welding involving commercially pure (CP) aluminum and AISI 1005 low-carbon steel. The multi-physics framework comprises the following main modules: (a) finite-element-based friction stir welding-process modeling; (b) quantum-mechanics, atomistic and CALPHAD-type continuum material thermodynamics analyses of the intermetallic compound-nucleation process; (c) a continuum-type analysis of multi-component diffusion-controlled growth of the intermetallic compounds; and (d) Kolmogorov–Johnson–Mehl–Avrami type analysis of the evolution of the intermetallic compound volume fraction within the friction stir welding joint as a function of the friction stir welding process parameters. The results obtained revealed that: (i) the extent and the spatial distribution of the intermetallic compounds is a sensitive function of the friction stir welding-process parameters; and (ii) among the six potential Al-Fe intermetallic compounds, FeAl and Fe3Al are associated with the largest volume fractions and, hence, play a key role in both attaining the required joint strength and in the potential loss of the joint fracture toughness.