Effect Of Laser Peening On Surface Integrity And Fatigue Property Of Additive Manufactured Titanium Alloy

Additive manufacturing can directly produce metal parts with complex geometries while significantly reducing costs.But compared with the wrought TC4,the less satisfactory performance of additive manufactured TC4 in some properties limits its use in structurally critical parts.Laser peening is an innovative surface post treatment technology,well-known to induce the high amplitude residual compressive stress of deeper thickness into the subsurface of parts and significantly improve the fatigue performance of the material.Aiming at the existing problems of metal additive manufacturing parts,the effects of laser peening process on the microstructure,mechanical properties,fatigue properties and pore defects of additive manufactured titanium alloy parts are discussed.The main research contents include the following aspects:(1)The effect of laser peening process on the surface integrity of additive manufactured TC4 titanium alloy was analyzed.The microstructure difference of additive manufactured TC4 titanium alloy before and after laser peening was analyzed by means of metallographic microscope and scanning electron microscope.The microhardness and residual stress were measured by micro-hardness tester and X-ray diffractometer.The results were compared with the ones of the same grade forged titanium alloy samples.The results show that laser peening has a significant strengthening effect on the surface integrity of additive manufactured titanium alloy.It can introduce high amplitude and large depth residual stress field on the surface of the material under the premise of ensuring surface quality,and generate a large number of dislocation structures,which significantly increase the micro-hardness of the surface layer.(2)The effect of laser peening on the pore defects of TC4 titanium alloy was investigated.A femtosecond laser was used to fabricate a series of holes with different sizes and different depth positions on the additive manufactured TC4 sample.Changes of the hole size before and after the laser peening on the additive manufactured sample were investigated,compared with the simulation results.It turns out that laser peening can effectively reduce the size of the subsurface pores by causing plastic deformation on the surface.In addition,the relative density of the samples before and after laser peening was measured by the Archimedes method to verify the effectiveness of reducing the porosity by laser peening.(3)The effect of laser peening on the fatigue properties of additive manufactured TC4 titanium alloy was also analyzed.High cycle fatigue tests at the same stress level were carried out using the samples with and without laser peening treatment.The results were compared with the ones of the same grade forged titanium alloy.The fracture morphology of each samples was also analyzed using the scanning electron microscope.It turns out that the laser peening process can significantly improve the fatigue life of additive manufactured titanium alloy.Based on the research work above,the feasibility of laser peening as a post-processing enhancement process for metal additive manufacturing technology was verified.The experimental results show that the laser peening process can effectively improve the micro-hardness of the additive manufactured titanium alloy,introduce the compressive residual stress field of high amplitude and large depth into the subsurface,reduce the size of the pores on the subsurface,reduce the porosity of the surface layer,and thus significantly increase the fatigue life.