Effect Of Ultrasonic Impact Treatment On Fine Structure And Hydrogen Damage Resistance Of Ti6Al4V Surface Layer

TC4 titanium alloy has excellent mechanical properties,thermal properties and good corrosion resistance,and is widely used in marine environments.However,titanium is prone to hydrogen-induced failure or hydrogen-induced cracking when it is in service.Ultrasonic rolling technology can induce the formation of an oxide film of titanium alloy to achieve a hydrogen retardation effect,and introduce beneficial residual compressive stress,which is expected to alleviate the hydrogen embrittlement sensitivity of TC4.Therefore,in this paper,two methods of ultrasonic rolling 5 times and 1 rolling combined with low temperature thermal oxidation are used to treat the surface of TC4 titanium alloy,to discuss the influence of different treatment processes on the surface structure of titanium alloy,and to comprehensively analyze the ultrasonic treatment on its resistance to hydrogen damage.Influence and mechanism of action.This paper uses SEM,OM,XRD and other technologies to analyze the surface structure of TC4 before and after different rolling processes.The results are as follows:5 times single pass of ultrasonic rolling has formed a plastic deformation layer with a thickness of about 35.0μm on the surface of TC4,and the stress-inducedβphase→αphase transition occurred;a dense oxide film with a thickness of about 6.30μm was formed in multiple passes,and the oxide phase of the film was composed of rutile Ti O₂and Ti₆O.The plastic deformation on the surface of the TC4 produced by ultrasonic rolling for one pass is very small,and there is no obvious phase change,but it will affect the thickness and quality of the thermal oxide film.After ultrasonic rolling pretreatment,the thermal oxide film is thickened from 1.4μm to 3.0μm,the quality of the film is significantly improved,but the phase of the oxide film remains unchanged,still Ti₆O phase.Through the slow strain rate tensile test,the influence of different ultrasonic schemes on the hydrogen embrittlement susceptibility of the material was studied.The results show that:5 times of ultrasonic rolling treatment can reduce the hydrogen embrittlement index of TC4 from 21.2%to 1.2%;1 pass of ultrasonic pretreatment can reduce the hydrogen embrittlement index of low temperature thermal oxidation of TC4 from 16.6%to 8.7%.Ultrasonic rolling can reduce the hydrogen embrittlement index of TC4 from 16.6%to 8.7%.Effectively reduce the material’s hydrogen embrittlement sensitivity and improve its resistance to hydrogen and hydrogen damage.Fracture analysis found the presence of a bright white new phase Ti H_x,and further statistics on its content found that the original sample after ultrasonic treatment showed that the hydrogen embrittlement of TC4 was caused by solid solution hydrogen and hydride.The stress test shows that the ultrasonic treatment can introduce beneficial residual compressive stress,which makes the diffusion and penetration of hydrogen atoms difficult.The hydrogen charging test on the original and the samples processed by the two rolling schemes found that the hydride will be in theαphase and the junction of the two phases.It is preferentially generated,and the presence ofβphase will promote the formation of hydrides;the oxide film of the samples treated by ultrasound can realize hydrogen blocking through O capturing H mechanism,which protects the samples from forming hydride.Comprehensive structural characteristics and performance analysis show that the ultrasonic treatment improves the hydrogen damage resistance mainly in terms of the increase in dislocation density and uniform distribution caused by plastic deformation,the decrease inβphase content,the residual compressive stress layer on the surface,and the oxide film layer on the surface.Under the combined action,the penetration and diffusion of hydrogen are hindered,and the solubility of hydrogen in the surface layer of the sample increases.This will increase the threshold for hydride formation,which is not conducive to the rapid formation of titanium hydride,which makes the ultrasonically treated sample resistant to Improved hydrogen damage performance.