Microstructure Transformation Mechanism And Mechanical Properties Analysis Of Friction Stir Processed Titanium Alloy

Titanium alloys have been widely used in aerospace,chemical industry,biomedicine and other fields due to their excellent specific strength,corrosion resistance and good biocompatibility.At present,the research on friction stir processing of titanium alloys is mainly concentrated in the superplasticity,the mechanical properties of friction stir processed titanium alloys at room temperature have rarely been reported.In this paper,the microstructure and mechanical properties of pure titanium and TC4 dual-phase titanium alloys processed by friction stirring were systematically studied,and the mechanism of microstructure transformation in the processing zone was deeply discussed.The research shows that continuous recrystallization is the main mechanism of grain refinement in friction stir processing of pure titanium.Due to the influence of temperature gradient,the recrystallization degree of the upper layer is greater than that of the lower layer,which results in the grain size of the upper layer being smaller than that of the lower layer.Through grain orientation analysis of the processing zone layers,it is found that the upper layer of the processing zone is mainly P₁ texture.In addition to P₁ texture,B texture exists in the lower layer.The results show that the stress mode of upper and lower grains in the process of friction stirring is different.The upper layer of the processing area is acted by the axle shoulder,and the cylindrical slip mainly occurs.The lower layer of the processing area is acted by the stirring needle,and the cylindrical and base slip mainly occurs.Through grain orientation analysis at three locations of BM→TMAZ→PZ region,it is found that the texture of P₁ tends to become stronger from scratch,and B texture appears in PZ region.When the deformation is in the initial stage,alpha grains coordinate deformation mainly through cylindrical slip.With the increase of deformation degree,cylindrical slip and base slip coordinate deformation together.The microstructure hardness and tensile strength of Friction Stir Processing Zone are significantly improved compared with the base metal,but the tensile plasticity is worse.Heat input has an important influence on the structure of friction stir processing zone of TC4 titanium alloy.α→βphase transition occurs when the temperature reaches theβtransition temperature.When the temperature reaches a higher temperature in the two-phase region,the diffusion of beta-stable elements is fast,spheroidization completed.When the temperature is high but not enough to spheroidize,recrystallization of alpha grains occurs.The hardness and tensile strength of TC4 titanium alloy processing zone are obviously improved compared with the base metal under the two parameters,but the tensile plasticity becomes worse.Initial microstructures also have significant effects on the microstructural transformation of TC4 titanium alloy processing zone.,the strain required for recrystallization of equiaxed initial microstructure is larger than that of lath initial microstructures.After friction stir processing,the grain refinement in the lower layer of the equiaxed initial microstructure processing zone is insufficient.The residual equiaxedαgrains are mainly P₁ texture.This texture is also the most easily occurring texture in pure titanium after deformation ofαgrains.The hardness and tensile strength of the processing zones with the two initial microstructures are improved compared with the base metal,but the tensile plasticity is worse.