| TC4(Ti-6A1-4V)titanium alloy is the earliest developed and successfully applied dual-phase titanium alloy,which is also the most widely used currently.However,due to the strength and hardness of TC4 titanium alloy strengthened by conventional heat treatment are still can't meet the performance requirements of the components under certain special circumstances.Therefore,it is necessary to develop a valid heat treatment process to further improve the mechanical property of TC4 titanium alloy,which is of great significance for improving its safety and broadening its application fields.In this paper,widely used in engineering,TC4 titanium alloy was taken as the research object,and the heat treatment process of solution+cold deformation+aging was considered.The effects of different heat treatment processes on microstructure and mechanical properties of TC4 titanium alloy were investigated and analyzed by means of optical microscope,X-ray diffractometer,scanning electron microscope,microhardness meter and electronic universal testing machine.The following main conclusions are obtained.(1)The original TC4 titanium alloy with equiaxed structure has moderate strength and highest elongation compared with the alloy after heat treatment.The results show that the original microstructure of TC4 titanium alloy consists of about 86.5%a phase and 13.5%? phase.At room temperature,the original TC4 alloy possess the moderate strength(?O.2=821 MPa,?b=872 MPa),the excellent plasticity(the elongation rate is 22.5%),the inferior hardness and the moderate comprehensive performance.(2)TC4 titanium alloy after 940 ? and 955 ? solution treatment is suitable for predeformation and aging.Compared with direct aging after solution,the effect of solution temperature on the microstructure and mechanical properties of the TC4 titanium alloy after solution treatment and aging with predeformation are present difference.When the TC4 titanium alloys were aged directly after solution treatment at different temperatures,with the increase of solution temperature,the content and size value of equiaxed primary a phase decreased,and the number of structure of ?-transformation gradually increased.The strength and hardness of the alloy increased firstly and then decreased with the increase of solution temperature.When the solution temperature is 955 °C,the maximum values of the strength and hardness are obtained.However,both elongation and reduction of crosssection area decreased monotonously with the increase of solution temperature.The strength,hardness and ductility of TC4 titanium alloy after 940 ? solution+7%deformation value+aging treatment are all higher than those of samples with 955 ? solution+7%deformation value+aging treatment,and the change trend of strength with temperature is different from that of direct aging without predeformation after solution.(3)The addition of predeformation to TC4 titanium alloy after solution can promote the precipitation of secondary a phase during aging process,and when the solution temperature is 940 ?,the promoting efficiency of predeformation on the precipitation of secondary a phase is higher than that at 955 ?.Under the same conditions,with the increase of predeformation amount,the content of secondary a phase increased,but its size decreased,and the strength and hardness of the alloy increased after aging.The tensile strength and hardness of TC4 titanium alloy after 940 ?solution and aging with predeformation are evidently higher than that of without predeformation,at the same time,excellent plasticity is maintained.Under the added predeformation,compared with the solution temperature of 955 ?,the temperature of 940 ?improves the strength and hardness of TC4 titanium alloy effectively.(4)When the aging temperature is too high,the secondary a phase coarsened,which is unfavorable to the mechanical properties of TC4 titanium alloy.In the case of the same solution temperature,predeformation value and aging time,with the increase of aging temperature,the width of the secondary a phase increased and the volume fraction decreased.The strength and hardness of the alloy increased firstly and then decreased,while the transformation of elongation rate is irregular with the increase of aging temperature.When the aging temperature is 500 ?,the highest strength and excellent plasticity was obtained.However,the alloy achieved the worst comprehensive performance after aging at 600 ?.(5)The content of secondary a phase in TC4 titanium alloy increased firstly and then coarsenen after reaching equilibrium with the prolongation of the aging time,the strength,hardness and plasticity of TC4 titanium alloy vary with the microstructure.When the other parameters are constant,at the initial stage of aging,the amount of secondary a phase precipitated in TC4 titanium alloy gradually increased with the prolongation of the aging time.In the later of aging period,with the extension of the aging time,the microstructure of TC4 titanium alloy began to coarsen.The strength,hardness,and plasticity of the alloy increased firstly and then decreased during the whole aging process. |
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