A Shortened Process Of α+β Titanium Alloy To Generate Trimodal Microstructure Based On Electropulsing Treatment Technology

Due to high strength,good plasticity,excellent fatigue strength,and superior biocompatibility,titanium alloys are widely used in aerospace and biomedicine.Among the typical microstructure of titanium alloys,the trimodal microstructure has excellent overall mechanical properties inαandα+βtitanium alloys,which combine the advantages of both equiaxed and basket-weave microstructures.In particular,the trimodal microstructure combines excellent creep resistance and fatigue properties,resulting in good service life despite high temperatures and alternating stresses.However,the rapid preparation of trimodal microstructures is still a complex problem.Currently available processes require the sample to undergo a special manufacturing process(additive manufacturing)or deformation and multi-step heat treatment to form a trimodal microstructure.This paper used a combined process of electropulsing treatment+heat treatment to achieve a short process preparation of the trimodal microstructure by inducing rapid spheroidization of the lamellar grains.The influence of process parameters on the formation process of the trimodal microstructure and its effect on the mechanical properties of the material was investigated by microstructure characterization and mechanical property testing.The mechanism of rapid spheroidization of lamellar grains was also revealed.The main findings of this paper are as follows:(1)Ti-6Al-4V alloy is treated with the pulsed current to form a fully martensitic microstructure with partially low aspect ratio martensite grains.This microstructure requires only one step of two-phase temperature heat treatment(950℃)to generate a trimodal microstructure with spherical grains,lamellar grains,and martensite.(2)The rapid heating process of the electropulsing treatment limits the growth of high-temperatureβgrains,which affects the morphology of the martensite,and the inadequate diffusion of the elements leads to the appearance of partially Al-rich and V-poor low aspect ratio martensite in the material.This type of martensite does not undergo phase transformation when heated in the two-phase temperature but rather spheroidization and grain growth.(3)The electropulsing treatment parameters show an important influence on the spheroidization process of the grains.As the electropulsing parameter increases,the high-temperatureβgrain size gradually increases,and the elements diffuse more homogeneously,which leads to a rise in the aspect ratio of the quenched martensite and,thus to a decrease in the proportion of spherical grains in the samples after heat treatment in the two-phase temperature.The decrease in the proportion of spherical grains leads to a reduction in material plasticity and work-hardening rate.The process with optimized parameters(7.23×107 A/m~2–400 ms+950°C-2 h)resulted in an optimal strength-plasticity combination of the material(σ_b=1110 MPa,δ～22%).(4)Grain spheroidization occurs in the heat treatment process after the electropulsing treatment,and the proportion of spherical grains and elemental concentration gradients within the material shows an increasing trend with the increase of holding time.The increase in the proportion of spherical grains within 1 h of insulation at 950°C is due to the continuous spheroidization of the low aspect ratio grains,and the spheroidization process follows the cylindrical mechanism.In comparison,the increase in the proportion of spheroidized areas of the sample after the insulation time of more than 1 h is attributed to the growth of spherical grains,and the growth process of the grains follows the Ostwald mechanism.The degree of elemental partitioning increases with increasing heating duration,which leads to an increase in the hardness difference between theαphase and martensite,which in turn increases the work-hardening rate of the alloy.