Numerical Simulation And Experimental Study On Grain Refinement Of Pure Titanium By Friction Stir Process

In this paper,the variations of temperature,stress-strain and grain refining phenomenon of pure titanium has been investigated during friction stir processing,which were studied by temperature field,stress strain field and grain refinement simulations for the grain refinement behavior of pure titanium surface fabricated by friction stir processing.In this paper,commercially pure titanium(CP Ti)TA2 was used.Firstly,some basic material parameters have been added to the material library of Deform-3D software.Secondly,the heat source model for CP Ti was established during friction stir processing,and the variations of temperature field,equivalent stress-strain field have been measured during the pressure stage,pre-heating stage and advancement stage of stirring tool,as well as rotational speed,processing speed,and pressure reduction.Thirdly,a CA model was established to study the grain refinement processing of CP Ti during friction stir processing,which was related to the different state variables,such as deformation temperature,equivalent strain and strain rate for the tracking points and characteristic points of stirring zone.Finally,experimental verification has been conducted according to the simulation results.The main results are as follows:(1)In the advancement stage,the temperature of the characteristic points on the upper surface of CP Ti plate showed the following changes along the fabricating direction.At first it increased,and then kept stable,finally decreased.And the heating rate was higher than the cooling rate.At the cross section of stirring zone,the closer to the heat source,the sooner and higher the peak temperature of the characteristic point.It was found that the equivalent stress exhibited a double-peaked “M” shape on both sides of the agitation center parallel to the processing surface.And the peak value of stress appeared on the advancing side,which existed at the stirring zone about 1/4 from the center of the stirring needle.The rising gradient of equivalent stress on the receding side was smaller than that of the forward side.(2)Through the control variable method,the simulation results show that the processing parameters had different influence on the heat production,first rotation speed,then processing speed,finally down pressure in this study.The maximum value of equivalent strain was improved with increasing the rotation speed,but decreased with theincrease of processing speed and the pressed depth.(3)Based on the tracking point method,it was found that the fibrous stripes were formed during hot deformation,and the hypoeutectic structure was formed during the dynamic recovery process in CP Ti and the dislocation density decreased.During the friction stir processing,entanglement of a large number of dislocations led to grain refinement,which decreased from 30 to 3.4?m,and the dislocation density decreased significantly.According to the simulation results of the characteristic points at stirring zone,the homogeneity of CP Ti grains decreased with the decrease of processing speed,which decreased from 3.68?m to 2.99?m.When the temperature kept stable,the increase of equivalent strain and strain rate led to grain refinement on the upper surface of CP Ti plate.When the temperature and equivalent strain rate kept stable,the smaller the grain size,the larger the equivalent strain.(4)By use of the friction stir processing parameters obtained from the simulation,a fine-grained sample of CP Ti was prepared by friction stirs processing.It was found that the cross-section morphology of the friction stirring processed zone showed “V” shape.A good gradient distribution of grain size was obtained,and the grain size gradually increased along the surface of processing zone to the interior,which corresponded to the simulation results.