Anti-hydrogen Embrittlement Performance And Micro-strengthening Mechanism Of Laser Peened TC4 Titanium Alloy

Laser peening is an emerging surface strengthening process that improves the surface properties of materials by inducing the formation of high-value compressive residual stresses and beneficial microstructures on the surface of metallic materials.TC4 titanium alloy has a hydrogen embrittlement tendency in a hydrogen-rich environment,and hydrogen induced cracking in a high pressure environment may cause premature failure of in its service life.The research on improving the hydrogen embrittlement resistance of TC4 titanium alloy by laser peening is still in its infancy.In this paper,through theoretical analysis and experimental research,the anti-hydrogen embrittlement performance of laser peened TC4 titanium alloy was studied.The macroscopic and microscopic mechanical response of the laser peened specimen under different hydrogen charging conditions was also investigated,and the microscopic mechanism of laser peening on the hydrogen embrittlement resistance of TC4 titanium alloy was analyzed.The main research work of this paper is as follows:(1)Theoretically,the hydrogen embrittlement mechanism of metallic materials was preliminarily discussed.In particular,the intrusion and diffusion process of hydrogen in metals and the influence of hydrogen content on the microstructure and mechanical properties of the material were analyzed.The effects of laser peening-induced tissue strengthening and stress strengthening on the hydrogen embrittlement resistance of the alloy were also discussed,which laid a firm theoretical foundation for the experimental research.(2)The laser peening test on TC4 titanium alloy under different laser pulse energies was carried out,after which the laser peened and non-laser peened samples were subjected to electrochemical hydrogen charging treatment for 24 hours and 48 hours.Subsequently,the residual stress,surface morphology and chemical phase composition of the various samples were tested.The surface microstructures of the laser peened samples before and after hydrogen charging were observed by OM and TEM.The results indicated that laser peening could induce compressive residual stress of about-517 MPa on the surface layer of the alloy,which effectively inhibits the initiation of hydrogen induced cracks,as well as promoting crack closure.At the same time,laser peening could improve the grain refinement rate of the alloy surface layer to reach 54.18%,as well as inducing high-density dislocations,dislocation entanglements and other microstructures in the plastic deformation zone.These microstructures are interwoven with the plastic slip zone to hinder the invasion of hydrogen atoms into the grain boundary to form hydrides,which reduces the mobility of hydrogen atoms and ultimately reduces the hydrogen embrittlement sensitivity of the TC4 titanium alloy.(3)The tensile test of different treated samples was carried out,and the macroscopic mechanical properties of the tensile samples,including ultimate strength,elongation and elongation loss,were systematically studied.The results show that under the same hydrogen charging conditions,the elongation loss rate and hydrogen hardening rate of the laser peened samples were significantly lower than their corresponding nonlaser peened samples;and although hydrogen induced degree of elongation loss increased with increasing hydrogenation,its adverse effects were reduced with the increase in laser pulse energy.Hydrogen induced elongation loss rate was used to characterize the hydrogen embrittlement index.It was found that hydrogen charging time is one of the key factors that affect the hydrogen embrittlement index,and laser peening can significantly reduce the hydrogen embrittlement index of the alloy under hydrogenation conditions.(4)Microscopic tensile fracture morphology test was carried out to compare the fracture characteristics of the laser peened and non-laser peened specimen under different hydrogen charging conditions,and the anti-hydrogen embrittlement mechanism of laser peened TC4 alloy was expounded.The results show that the fractures of the hydrogenfilled samples exhibited hydrogen-induced plasticity loss characteristics.However,compared with the non-laser peened specimen,the laser peened ones displayed good fracture toughness,and the fracture toughness got better with the increase in laser pulse energy;which indicated that laser peening could reduce the hydrogen induced plasticity loss of the hydrogenated sample.Laser peening induced compressive residual stress and grain refinement effect as well as high density dislocation structure were considered to be the main reasons for improving the hydrogen embrittlement resistance of the TC4 titanium alloy.