Research On Cyclic Mechanical Behavior And Fatigue Performance Of Welding HAZ Of 7XXX Aluminum Alloy

7XXX aluminum alloy is widely used in the manufacturing of bullet train and civil airplane due to its high specific strength,good malleability,high corrosion resistance and excellent weldability.And welding is the one of the most common methods to joint aluminum components.As a typical brand of age-hardenable aluminum alloy,welding thermal cycle will undoubtedly impact the microstructure and mechanical performance of welding HAZ.According to the statistics,more than 70% of failure accidents are induced by fatigue crack.Early studies on fatigue failure of aluminum alloy welding mostly focused on overall properties of welding joints,lacking a deep-going analysis of the evolution of microstructure and performance of welding HAZ.In essential,fatigue failure is a process of plastic damage accumulating constantly under cyclic loads until fracture,and the rate of crack propagation mainly depends on the passivation effect of plastic zone of crack tip.Therefore,researching cyclic mechanical behaviors of welding HAZ and its influence on crack propagation may provide meaningful theoretical guidance to the process optimization and life-time increase of welding component of high-strength aluminum alloyIn present study,non-isothermal heat treatments,representative of welding thermal histories in a Heat-Affected Zone(HAZ),were imposed on different 7XXX specimens to obtain specific welding microstructures.Then an uniaxial cyclic tension-compression test and a fatigue crack propagation rate test were performed on these specimens,combining semi-quantitative microstructure characterization like OM,TEM&SAD,SEM&EDS and DSC.The objective of this study is to:(i)characterize a qualitative relationship among welding thermal cycle,microstructure and mechanical performance;(ii)obtain the quantitative expression between precipitate distribution and crack propagation rate.The results show that entire evolution of HAZ microstructure can be divided into the growth-decomposition of preprecipitated phases and the nucleation-growth-decomposition of reprecipitated phases.To be more precise,the relationship between microstructure and fatigue performance is: The more the strengthening phases nucleate,the less the aluminum matrix gets hardening while fatigue loads going on,thus the plastic zone of crack tip tends to be stable and impeding the propagation of fatigue crack;The more the strengthening precipitates get dissolved,the more intense the plastic zone hardens and shrinks,leading to a faster fracture of welding joints.