Effect Of High-energy Shot Peening On Thermal Oxidation And Carburizing Of TA2

Commercial pure titanium has the characteristics of high specific strength,good corrosion resistance,and good biocompatibility.It is widely used in aerospace,energy chemical industry,biomedical and other fields,but its hardness is low,friction coefficient is high and unstable,poor wear resistance.In order to improve the mechanical properties of pure titanium,high energy shot peening method was firstly used to treat TA2 industrial pure titanium,and then thermal oxidation and carburizing were carried out respectively.Through the characterization of microstructure and mechanical properties,the microstructure evolution and strengthening mechanism of TA2 were studied,and a new way to optimize the properties of TA2 was explored.The main research contents are as follows:In this study,high-energy shot peening was used to treat industrial pure titanium,and the evolution of microstructure was analyzed by optical microscope,electron backscattering diffraction(EBSD)and transmission electron microscope(TEM).The mechanical properties were tested by nano indentation experiment,and the high-energy shot peening process was simulated by finite element.It is found that the microstructure of high-energy shot peening is gradient distributed on the surface of pure titanium,in which the grain fragmentation occurs in the range of 40 ?m from the surface due to the severe plastic deformation,and the nanostructure is formed.With the increase of depth,the grains remain coarse-grained,but a large number of {11-22<111-2-3 compression twins and {10-12<110-1-1 tensile twins are formed.High-energy shot peening can transform the hexagonal close-packed hexagonal structure(HCP)of TA2 into a face centered cubic structure(FCC),(0001)//(001).The dimensional analysis of nano indentation curve shows that the high energy shot peening significantly improves the surface hardness and yield strength.The wear test shows that the friction coefficient decreases to 1/3 of the original sample after high-energy shot peening.In the study,the high-energy shot peening samples and the original samples were subjected to thermal oxidation treatments at different temperatures and times.The study found that high-energy shot peening promoted the thermal oxidation of TA2.The surface oxygen content of the high-energy shot peened sample is significantly increased after 60 min of thermal oxidation at 650 ?,while the non-shot peened sample needs to be thermally oxidized at 750 ? to observe the increase in surface oxygen content.With the increase of thermal oxidation temperature,both thermal oxidation samples and direct thermal oxidation samples formed obvious oxide film after high-energy shot peening,but the thickness of oxide film of high-energy shot peening is significantly higher than that of direct thermal oxidation samples.The surface hardness of the high-energy shot peening sample can reach 1219 HV after thermal oxidation treatment at 950 ? for 60 min,while that of the direct thermal oxidation sample is only 803 HV.For the thermal oxidation at850 ?,the high energy shot peening samples also obtained higher hardness.With the further decrease of temperature,the hardness increase is not obvious.For thermal oxidation at 750 ? and 850 ?,the friction coefficient of thermal oxidation samples after high-energy shot peening is lower than that of direct thermal oxidation samples,while at950 ?,the friction coefficient of thermal oxidation samples after high-energy shot peening is slightly higher than that of direct thermal oxidation samples.The solid carburization study of the high-energy shot peening sample and the original sample shows that,on the one hand,the high-energy shot peening promotes the formation of the carburized layer on the TA2 surface.Observation of surface morphology and phase analysis showed that after 12 h of solid carburizing at 750 ? and 850 ?,more TiC flakes were formed on the surface of the high-energy shot peened samples.After carburizing at950 ? for 12 h,a thicker TiC coating was formed on the surface of the high-energy shot peened sample.On the other hand,high-energy shot peening promotes the diffusion of carbon atoms in the surface region of TA2.Vickers hardness and nanoindentation tests show that after high-energy shot peening samples are carburized at 850 ? and 950 ?,the hardness and the depth of the hardened area at the same depth from the surface are higher than those of the direct carburized samples.In the friction experiment with GCr15 steel as the friction pair,the friction coefficients of the high-energy shot peened and direct carburized samples at 750 ? are close,which are significantly lower than the high-energy shot peened samples;at 850 ? and 950 ? The friction coefficient of carburized samples after high-energy shot peening is lower.