Microstructure Evolution and Strengthening Mechanism of the Probeless Friction Stir Spot Welding of a Al-Li Alloy
The microstructure evolution and strengthening mechanism of probeless friction stir spot welded(P-FSSW)joints of 2198 Al-Li alloy are studied.The results show that the stir zone(SZ)is characterized by equiaxed grains,which is dominated by the continuous dynamic recrystallization,but the geometrical effect of imposed strain and limited discontinuous recrystallization are also involved.It is accompanied by shear textures,like A/(A)({111 }<110>),B/(B)({112}<110>),A1*/A2*({ 111 }<112>),which is attributed to the plastic flow induced by shear action.In addition,the primary strengthening phase of T1(Al2CuLi),which is semi-coherent/incoherent with the crystal structure of the base metal,is completely dissolved in the stir zone(SZ),but only a few of dispersed β'phases are observed.The combined effect of extensive dissolution of the strengthening phases and reduction of dislocation density due to thermal cycle significantly decrease the hardness in the joint,while grain refinement and solution strengthening are considered as main factors affecting the SZ.
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