Investigaton Of Laser Shock Peening On Tensile Properties And Micro-strengthening Mechanism Of TA2 CP-Ti Under Different Temperatures

Commercially pure titanium(CP-Ti)and titanium alloys have been widely applied in aerospace,ocean development and automotive industry fields because of their high specific strength,good mechanical properties,corrosion resistance and other characteristics.Traditional surface modification methods cannot meet the demand of the strengthening of CP-Ti and Ti alloys structural member.Laser shock peening(LSP)is a rising surface strengthening technology,widely used in surface modification of metallic materials,titanium structural member service temperature below 500 °C.The strengthening mechanism of LSP on titanium alloy at different temperatures is not found,and its microstructure evolution and plastic deformation behavior is not reported.Therefore,the elevated temperature tensile properties and micro-structure evolution of TA2 CP-Ti induced by LSP was studied in this paper.Three different respects were involved,i.e.elevated temperature tensile properties,fracture characteristic,hardness distribution,micro-structure evolution and micro-strengthening mechanism of TA2 CP-Ti subjected to LSP treatment.Summarized the following points:First of all,the elevated-temperature tensile properties and fracture characteristic of CP-Ti subjected to LSP treatment were investigated,meanwhile the correlation the plastic deformation behavior and fracture characteristic at high temperature were analysed.The gauge part of tensile specimens were treated by massive LSP impacts,and the tensile tests of LSPed and as-machined specimens were carried out at 20 °C,150 °C,250 °C and 350 °C,respectively.Results showed that the ultimate tensile strength(UTS)decreased with increasing temperature.Necking phenomenon of the LSPed specimen was more obvious than the as-machined specimen.Fracture morphology showed that the LSPed specimen had a better plasticity at the same temperature.Brittle fracture was replaced of mixed fracture,and then turned to fracture toughness.Secondly,the correlation of micro-structure evolution and micro-hardness distribution of CP-Ti subjected to LSP treatment were investigated.It is found that the grain refinement deformation is obvious,and there are a large number of deformation twins,dislocation and lentoid martensite phase.The martensite morphology of titanium alloy at laser shock deformation region,with the characteristics of obvious plate(needle)shape,is finer and more uneven than steel's,and its transformation is also incompletely.Therefore,the martensite(? ?)is fine.When the temperature rises to 350 °C,the reverse phase change(? ?? ?)of the martensitic transformation occurs,the intracrystalline martensite gradually disappears with the increase of the temperature.It is found that the grain size is larger,but the range is not very uniform,and the growth of sub-grain is more obvious.Compared to the original material,laser shock makes the hardness of commercial pure titanium(CP-Ti)has great improvement.With tensile temperature increasing,the hardness decreases,but the magnitude is not very large.Due to severe plastic deformation resulted from the tensile test,the dislocation density in metallic materials has been greatly increased.The accumulation,tangles,growth and decline of the dislocation between each other allow more mechanical twinning and sub-grain,which result in grain refinement.In addition,the greater degree of grain refinement contributes to the greater hardness of the material.To sum up,the offset grow and decline between the softening of the temperature rise and the hardening of the tensile mechanical deformation make the hardness changes not be great.Finally,micro-mechanism of plastic deformation of CP-Ti subjected to LSP treatment at different temperatures and deformation behavior mechanism in different regions were investigated.The results show that the deformation behavior of tensile specimens at different temperatures can be explained by the model of dislocation's activation and twins' disintegration.The twins of commercial pure titanium(CP-Ti)were generated induced by Laser shock peening.Because the stacking fault energy of pure titanium is low,when the line defect is activated by the high temperature and external force,the line defect in the twin passes through the twin boundary.When the temperature and external force reach a critical value,the twin boundary disappears,and the line defect of the product is even.At the same time,the twin boundary will absorb the reactant-incomplete dislocation,thus improving the plastic properties of the material.When the temperature is above 350 °C,the model of dislocation's activation and twins' disintegration of plastic deformation behavior is almost over.The nucleation and motion of dislocations are the main mechanism in the plastic deformation process.The microstructure of the fracture zone is different,which is mainly the result of twin boundary.The twin crystal of TA2 commercially pure titanium is formed in the severe plastic deformation region,and the existence of twin boundaries hinders the movement of dislocations.