Effect Of Electropulsing On The Microstructure And Mechanical Properties Of TA1

Pure titanium is one of the most widely used biomedical materials in practical medical field due to its superior biocompatibility.However,the application range of pure titanium is greatly limited due to its relatively low strength.Although the strength of pure titanium can be significantly improved after conventional cold working processes such as rolling,the plasticity is significantly reduced.Electropulsing treatment(EPT)is one of new heat treatment technologies,which not only has similar effects as conventional heat treatment but also can relieve defects and promote recrystallization.Therefore,traditional cold rolling process and EPT were combined in this present study,and the effects of different cold rolling and EPT parameters on the microstructure and mechanical properties of pure titanium were studied,in order to provide theoretical basis for obtaining medical pure titanium with good matching between strength and plasticity,and expand the application of pure titanium.The annealed TA1 sheet was subjected to multi-pass cumulative rolling under different deformation,followed by EPT with different parameters.OM,SEM and TEM were used to observe the micro structure of TA1 before or after EPT under different process parameters,and the mechanical properties of TA1 under different process parameters were tested by a tensile testing machine.The research results show that the tensile strength of 20%cold-rolled TA1 after 3 kA-20Hz EPT is reduced by 8.4%compared with that without EPT.But with the deformation increases,the tensile strength of cold-rolled TA1 after EPT is gradually enhanced compared with that without EPT,and the tensile strength is significantly increased by 9.5%if the deformation is 80%,while the elongation is still maintained at 13%.When the pulse frequency is constant(20 Hz),the tensile strength of 80%cold rolled TA1 after EPT firstly increases and then decreases with the increase of current intensity.If the current intensity is 3 kA,the tensile strength reaches 691 MPa,while recrystallization occurs at 4 kA.When the pulse current intensity is constant(2 kA),the strength of the 80%cold-rolled TA1 decreases with the increasing of the pulse frequency.However,the tensile strength of cold rolled TA1 at different pulse frequencies is still higher than that without EPT.Microstructure observations results show that cold-rolled structure of 80%deformed TA1 after EPT below the recrystallization temperature parameters(1?3 kA)is still exist,but nanocrystal grains appear at the boundary of shear bands.That is the reason why the tensile strength of 80%deformed TA1 after EPT increases.And with the pulse current intensity increases,the number of nanocrystal grainsincreases,so the tensile strength of 80%deformed TA1 after EPT at 3 kA is highest.However,the dislocation density within the shear bands decreases rapidly due to the recovery.If the pulse current intensity is 2 kA,the amount of nanocrystal grains at the boundary of shear bands is relatively less.That is why the improvement of tensile strength of 80%deformed TA1 after EPT at 2 kAis relatively low,and the strength decreses as the pulse frequency increase.Therefore,the effect of pulse current on the strength of 80%cold-rolled TA1 is the combination between nanocrystal strengthening effect and the softening effect caused by the decrease of dislocation density.To further explore the effect of EPT on cold rolled TA1,80%cold-rolled TA1 were annealed at the same temperature range,and the results show that the tensile strength of cold-rolled TA1 after annealing gradually decreases with the increasing of the annealing temperature.Compared with the annealing treatment,a non-uniform high-pressure temperature field forms in the high-density dislocation region at the boundary of shear bands in 80%cold-rolled TA1 during EPT.So,under that combined action of thermal compressive stress and electronic wind force,the dislocation movement is promoted,and finally nanocrystals form.While if the dislocation density is greatly reduced during EPT,the effect of the non-uniform high-pressure temperature field is weakened and the grain growth is limited.If the deformation is 20%,the tensile strength of cold rolled TA1 gradually decreases with the increasing of pulse current intensity.That is because there are not a lot of high-density dislocations if the deformation degree is low.EPT has the similar effects as heat treatment,which could promote the disappearance of point defects and dislocation motion,causing the dislocations density decrease,which leads to the softening of the cold rolled pure titanium.