| Friction stir welding of 2 series aluminum alloy can improve the problems of weld porosity and hot crack initiation existing in the traditional welding process,while cryogenic treatment process can further improve or improve the mechanical properties of strength of materials by changing the microstructure of metal materials through ultra-low temperature environment.In this thesis,2024 aluminum alloy is taken as the research object,and the friction stir welding and cryogenic treatment process are combined to study the influence of different welding process parameters and cryogenic treatment on its metallographic structure,mechanical properties,residual stress and fatigue crack growth.The main research content is as follows:(1)The 2024 aluminum alloy friction stir welded joints were studied by changing the sitrring speed and welding speed,and the metallographic structure,tensile strength,microhardness,and fatigue crack propagation of 2024 aluminum alloy friction stir weled welding joints under different welding parameters were compared and analyzed.It was found that the most severe softening area of welded joint was the thermal-mechanical affected zone.The tensile strength showed an initial increas and then a decreas with the increase of stirring speed and welding speed.At the early stage,the fatigue crack growth rate of a welded joint is lower compared to that of the base metal,and this decrease becomes more pronounced as the stirring speed increases.The optimal parameter combination was obtained in this experiment.(2)The coupled eulerian-lagrangian simulation analysis method is used to establish the2024 aluminum alloy friction stir welding thermo mechanical coupling finite element model through simulation analysis software,using the material constitutive equation parameters that consider the temperature,strain rate effect and dynamic recrystallization process.The simulation is carried out under different welding parameters,and the influence of welding parameters on heat input and temperature field distribution in different stages are obtained.(3)The 2024 aluminium alloy friction stir weled welded joints with the optimal parameters obtained from the experiment were subjected to cryogenic treatment for different durations.The changes in metallographic structure,microhardness,tensile strength and residual stress of the welded joints before and after cryogenic treatment were compared and analyzed.The results show that the microstructure of the weld nugget zone is significantly refined after deep cryogenic treatment;the overall microhardness and tensile strength have been improved.The transformation of surface residual stress into residual compressive stress hinders the propagation of fatigue cracks. |
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