Experimental Study And Numerical Simulation Of Hydrogen Penetration And Hydrogen Diffusion Resistance Of TC4 Titanium Alloy Subjected To Laser Peening

TC4(Ti-6Al-4V)titanium alloy has excellent properties such as high specific strength,low density and strong corrosion resistance.The alloy is widely used in the field of marine engineering equipment applications as a result of these excellent qualities.However,titanium alloy also has strong environmental sensitivity when operating in an ocean.The hydrogen-rich environment will result in hydrogen-induced cracking on the surface of titanium alloy components,causing hydrogen-induced cracking and major safety accidents.Laser peening(LP)surface modification technology uses high-intensity shock wave pressure to induce stress strengthening and microstructure strengthening effect on the surface of the materials,which can effectively reduce the probability of hydrogen embrittlement(HE)and stress corrosion cracking of key structural parts of marine engineering equipments.In this paper,TC4 titanium alloy was used as the research object.Using theoretical research,experiment and FEM simulation,the differences of hydrogen adsorption and hydrogen diffusion behavior in the samples before and after LP strengthening were analyzed.The macro-microscopic strengthening mechanism of the TC4 titanium alloy against hydrogen permeation finally achieved the purpose of prolonging the service life of the marine structural equipment.The main content of the research is as follows:(1)Based on the mechanism of HE,the causes of HE of titanium alloy were analyzed.The adsorption and diffusion process of hydrogen in materials were as well discussed.The theory of hydrogen permeation,the governing equations and constitutive equations of hydrogen diffusion were also discussed.From the perspective of elastoplastic mechanics,the dynamic response mechanism of the LPed material was analyzed,and the residual stress field induced by LP was also evaluated.The mechanism of enhancing the hydrogen permeability resistance of TC4 titanium alloy by LP,such as compressive residual stress,grain refinement and dislocation proliferation was as well analyzed.(2)LP treatment,electrochemical hydrogenation and hydrogen permeation tests of typical TC4 titanium alloy samples under different laser power densities were carried out to explore the residual stress,microhardness and grain size of the surface of the hydrogen-filled sample before and after LP.The macroscopic and microscopic performance changes such as dislocation configuration,and their effects on hydrogenadsorption and diffusion behavior were subsequent analyzed.The results show that with the increase of laser power density,the compressive residual stress amplitude,the depth of the LPed layer increased,grain refinement rate and dislocation density of the surface of the hydrogen-charged sample increased.The hardening rate was gradually reduced,and the increase in dislocation density provided more hydrogen capture sites,and the uniform distribution of dislocations inside the material helped in reducing the probability of hydrogen enrichment at the grain boundaries.According to the hydrogen permeation curve generated from the hydrogen permeation test,the hydrogen diffusion coefficient,hydrogen concentration in the crystal lattice and the hydrogen trap density were obtained.The results show that LP is beneficial in hindering the adsorption of hydrogen atoms on the surface layer of TC4 titanium alloy material,leading to a reduction of hydrogen in the hydrogen permeation curve.The diffusion coefficient in the material lattice was decreased,and the decrease of the diffusion rate made it difficult for hydrogen to continue to penetrate into the material;thereby,effectively reducing the HE sensitivity of the TC4 titanium alloy.(3)Based on ABAQUS software,a digital analysis platform integrating the LP process parameters,residual stress and hydrogen diffusion characteristics was developed.The stress and strain results obtained by LP finite element analysis were used as a predefined field for hydrogen diffusion to conduct stress-assisted hydrogen diffusion.The sequential coupling calculation was used to study the influence of compressive residual stress distribution on the hydrogen permeation resistance of TC4.On this basis,the mechanism of hydrogen permeation resistance of LPed TC4,realizing the reasonable prediction of HE sensitivity of titanium alloy.Comparing the numerical simulation and experimental results of the surface residual stress induced by LP,it shows that the simulated value is consistent with the change trend of the experimental value.Finally,based on comprehensive analysis experiments and simulation results,the mechanism of LPed TC4 titanium alloy against hydrogen permeation is proposed.