A Study Of Failure Mechanism Of Important Structural Parts Of Aluminum Alloy

Al-Zn-Mg alloys are typical age-hardening weldable aluminium alloys. They are wildly used in aerospace, trains and buildings as important structural materials. In previous studies about the effects of microstructure on crack initiation and propagation, the microstructural heterogeneity and the complexity of failure condition were always ignored due to employing the laboratorial homogeneous materials and single factor experiments. So the conclusion was contradictory and useless for actual production.In this work, scanning electron microscopy(SEM), electron back scattering diffraction(EBSD), scanning transmission electron microscopy(STEM) and energy dispersive spectrometry(EDS) were employed to investigate the relationship between microstructures and crack initiation and propagation in two practical welding structural parts. On the one hand, the influences of microstructural heterogeneity on crack propagation behavior are studied systematically; on the other hand, the effects of weld defects and parent metal on crack propagation are analyzed. The main innovation points are summarized as follows:(1) Materials forming process and recrystallization lead to microstructural heterogeneity. The different microstructure impeded the cracks in different degrees. When the fracture plane parallel to the interface between dissimilar grain structures, cracks trend toward propagate at the interface.(2) Intergranular cracks tend to propagate along the high-angle grain boundaries and be blocked, even closured at low-angle grain boundaries. The transgranular cracks are easily impeded or deflected through the large angle grain boundaries due to a large angle between two favorable slip planes in the neighboring grains. Even the fracture mode will change.(3) Compared to coarse grains and fine recrystallized grains, elongated deforming grains have a greater hindrance to the crack. In partially recrystallized regions, the cracks prefer to propagate along the high-angle grain boundaries between equiaxial recrystallized grains. In few cases, cracks penetrate the elongated deforming grains via cleavage fracture under the concentration of stress.(4) In general, fusion line is the most vulnerable zone through the whole welded structure. Cracks prefer to initiate and propagate in this area via microvoid accumulation fracture with the help of gas holes and other welding defects.(5) Due to the influence of heat input introduced during welding, microstructures and mechanical properties in the based materials next to the weld, i.e., heat affected zone(HAZ), will be changed. In this study, grain refinement in HAZ is observed via reversion and recrystallization of deforming grains. Finer grain structures make the plasticity/toughness of the joint decreased, which is responsible for the fracture of the welded parts.