Fine Equiaxed Zone Induced Softening And Failure Mechanism Of High Strength Aluminum Alloy Fusion Welds

There has been great development in high-speed railway vehicles.The lightweight and high durability of critical components have received a great deal of attention.In comparison with the most widely used 5000-series and 6000-series aluminum alloy,medium-and high-strength 7000-series aluminum alloy with higher specific strength have shown advantages in lightweight design of high-speed railway carbody shells.Zirconium(Zr)is regarded as the beneficial addition element in aluminum alloy.However,a fine equiaxed zone(FQZ)can be formed along the fusion boundary in Zrcontaining aluminum alloy fusion welds,posing a great threat to the structural integrity of welded structures.An in-depth investigation on the formation mechanism,microstructural feature,softening behavior and failure mechanism of the FQZ is significantly important.The fiber laser and pulse arc hybrid heat sources were adopted to produce 7050-T7451 aluminum alloy welds,in which 7050-T7451 contains 0.08-0.15 wt % Zr.This thesis attempts to identify the critical microstructural features associated with the FQZ,both in terms of its softening and the grain boundary weakening,and to reveal the softening and intergranular failure mechanisms.This thesis is expected to serve a basis reference for the improvement and evaluation of the in-service performance of the welded components containing FQZ.The main research contents and conclusions are as follows:The softening and intergranular failure behavior of the FQZ were investigated using nanoindentation testing and in situ synchrotron radiation X-ray computed microtomography,respectively.Both the region softening and grain boundary softening were verified.FQZ had the lowest hardness across the weld zone,with a softening degree of ~54% relative to the base metal.The true stress-strain curves were inferred from the load-displacement curves using dimensionless functions-based reverse analysis.The results showed that FQZ also had the lowest yield strength and strainhardening exponent.This indicated that FQZ had the poor resistance to both plastic deformation and continued plastic deformation as it is plastically deformed.It was observed that some micro-voids were nucleated preferentially in the FQZ when the welded joints were subjected to uniaxial monotonic tensile loading.Once formed these micro-voids linked up rapidly to give failure through the most softened FQZ.Postmortem fractography was characterized by intergranular cracking,displaying low levels of plasticity over the fracture surface.Through a variety of microstructural characterization techniques,a comprehensive investigation on the microstructural features across the weld cross-section was conducted.The remarkable microstructural features in the FQZ were very fine grains(~7 μm),significantly high fraction of high-angle grain boundaries(~87%),random grain orientation,higher Schmid factor(~0.45),lower local misorientation(~0.86°),lower geometrically necessary dislocation density(~8.07×1012 m-2)and very low volume fraction of precipitates in the interior of the grains(~0.2%).Besides,the grain boundaries were decorated with a network of ω(Al7Cu2Fe)and S(Al2Cu Mg)phases.The contribution of precipitates,grain size,dislocation density and solute element to the therotical yield strength was calculated using classical strengthening models,including precipitation strengthening,grain refinement strengthening,dislocation strengthening and solid solution strengthening.The drastic softening of the FQZ arises,despite the fine grain size,from the significant reduction in precipitates due to the serious segregation of Zn,Mg,and Cu to grain boundaries and the fast cooling limiting the reprecipitation of remaining solute elements.The multiscale correlative tomography investigation was carried out.We have threaded together in-situ scanning electron microscopy testing,high-resolution X-ray nanotomography and transmission electron microscope(TEM)-energy dispersive spectrometers(EDS)characterization all for the same region.It was found that the damage nucleation and crack propagation preferentially occurred in the FQZ when the welded joints were subjected to uniaxial monotonic tensile loading.The damage nucleation modes included micro-voids nucleated at grain boundary triple points,micro-voids nucleated at particle-grain boundary interfaces and micro-cracks caused by fracture of the large-sized particle fragments.The morphology of the intergranular phases was anisotropic.They tended to fill between the grains along the weld thickness.The micro-voids were primarily nucleated at grain boundary triple points and at particle-grain boundary interfaces in a ductile mechanism.The size of the micro-voids was anisotropic.They were prone to nucleating and growing along the weld direction perpendicular to the loading direction.Almost the nucleation of all micro-voids were induced by intergranular phases.It was indicated by TEM-EDS characterization that the micro-voids nucleated in the FQZ were closely related to the coarse Al Cu Mg phases at the high-angle grain boundaries.A minor variation in the grain orientation and crystal rotation was found during in situ electron backscattered diffraction testing and the Schmid factor hardly changed.Both the small-angle grain boundaries and local misorietation increased significantly.These increased small-angle grain boudnaries and the region with higher local misorientation were located along the high-angle grain boundaries.The combined effects of the grain boundaries and intergranular phases lead to the serious stress concentration at the grain boundaries,becoming the potential damage or micro-void nucleation sites.Two failure modes were found in the high-cycle fatigue testing.The cracks originated from the weld toe tended to propagate along the FQZ,and the cracks originated from the porosity defects tended to propagate along the weld center.The Kitagawa-Takahashi(KT)diagram was constructed to explore the variation in fatigue strength with the defect size.The fatigue crack growth rate curve of the FQZ was predicted by the combination of the improved long and physically short(i LAPS)crack growth model and the elastic-plastic properties inferred from the nanoindentation loaddisplacement curves using dimensionless functions-based reverse analysis.Within the framework of defect tolerance and damage tolerance,the remaining life due to the cracks propagating along the FQZ and weld metal was evaluated using the i LAPS and NASGRO models,respectively.The extended KT diagram(defect-stress-life threeparameter model)was proposed for the fatigue strength design and remaining life evaluation,which not only contained the safe-life region in the traditional KT diagram but the defect-determined finite life region.