| Friction stir welding(FSW)is a novel solid-state joining technology,which has been widely applied to the welding of aluminum,magnesium and nonferrous alloys.The experiments and engineering applications of FSW show that there are two main factors which can significantly affect the final mechanical properties and performance of FSW.One is the fatigue life of FSW tool and the other one is the grain microstructure of the welded joints in the nugget zone.To improve and optimize FSW,the simulation and design of welding tool and microstructural evolutions in FSW are necessary.Based on Computational Fluid Dynamics(CFD)model,the friction stir welding of aluminum alloy is simulated in this dissertation.The analytical model for tool force calculation and the fatigue model are proposed.In combination with Monte Carlo method(MC),the recrystallization and the grain growth are further calculated.With the validation from experimental tests,the influences of welding and geometrical parameters on the tool force and fatigue and the microstructural evolutions in FSW,are fully investigated.The main contents in all five chapters are summarized below:In the first chapter,the distinguishing features and wide applications in industry of FSW are discussed.This chapter reviews the research works focusing on FSW welding tools and microstructure evolutions of welded plates.The deficiency of current research works has been pointed out.This chapter also summarizes the research works of this dissertation.The second chapter mainly talks about the CFD model of FSW of AA6061-T6 plates.FSW experiments of AA6061-T6 plates are used to validate the model.Temperature fields and pattern of material flows have been investigated based on computed results.The width of stirring zone with different welding parameters are predicted based on streamlines.The third chapter proposes the methods for predicting fatigue stress and fatigue life of welding tool in FSW.Firstly in this chapter,the formation mechanism of forces,the distribution of welding forces and their relationship with welding parameters and geometries are studied in detail.Then analytical and numerical methods are used to calculate the fatigue stress on welding pin in high temperature.At last,fatigue life of the welding tools is predicted with different welding parameters and tool geometries.In chapter four,a model combined with MC method and CFD model has been proposed to simulate the microstructure evolution in stirring zone.Firstly the correlation between MC model and CFD model is built.Then this chapter discusses the influence of DRX recrystallization and precipitation in the stirring zone of FSW.Simulation algorithms of DRX and precipitation have been coupled in the MC model.At last part of this chapter,the influence of welding parameters and locations in stirring zone have been analyzed during grain evolutions.A software with pre-process and post-export functions for the simulation of grain evolution during FSW has been established.The last chapter summarized the main conclusions of this dissertation.The main innovation points and future works based on this dissertation have been discussed in detail. |
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