Multiscale study of the properties of hybrid laser-welded Al-Mg-Si alloy joints

Abstract

Hybrid laser welding has been increasingly used in joining aluminium alloys. Due to the nature of hybrid laser welding, the welded joint made from Al alloys typically fractures in the fusion zone, indicating that the fusion zone is the softest part of the joint. Thus, modifying the microstructure in the fusion zone and analysing the according mechanical properties is both academically and industrially of interest. In the first part of this research, it was focused on altering the microstructure of the fusion zone using two different filling materials, Al-Mg alloys and Al-Si alloys. The results showed that the mechanical properties of joints with Al Si as filling material were stronger since the fusion zone features with smaller grain size and multiple-alloys solid solution. However, the fatigue properties and corrosion resistance of joints with Al-Si as filling material were weaker. The reason for such a distinct difference was investigated via theoretical calculation and fracture mechanism. The microstructure properties relationship of hybrid laser welded AA6061 joints at macroscale was well understood through this research. Motivated by understanding the mechanical properties at multiscale, the nano/microscale deformation of a single crystal of the fusion zone was investigated via pillar compression tests. These experiments showed that the strength of the fusion zone with Al-Mg as filling material was size-dependent, showing a “smaller is stronger” trend. Such size-dependent strength disappeared when the pillar’s diameter was greater than 3.3 µm. A theoretical model was built and used to analyse the observed size-dependent strength. Interestingly, the strength of the single crystal of the fusion zone with Al-Si as filling material was not size-dependent. Strong solid-solution effect was proposed for this unusual size effect according to theoretical calculation. With increasing dislocation densities, the size- and orientation-dependent strength for pillars in both FZs disappeared. A theoretical model was proposed to quantitively VII analyse the effect of solute elements, dislocation density, and size on the strength of pillars in FZs. To link the microplasticity with macroscale plasticity, crystal plasticity finite element (CPFE) simulation was conducted. Firstly, microscale-mechanical-properties prediction of single crystals was applied to prove the validity and obtain the material parameters of CPFE simulation. Then, CPFE was successfully utilized to simulate the mechanical properties of the welded joint at macroscale based on the microplasticity obtained by the pillar compression at microscale. Show more