The Effect Of Asymmetric Rolling And Heat Treatment On The Microstructure And Properties Of Titanium And Titanium Alloy

Titanium and titanium alloy are widely used in aerospace field, marine, military equipment, medical devices and other top science fields by its excellent properties, which are called "the third-largest metal" and "strategic metal". The fine-grain strengthening is drawn public attention to enhance the properties of titanium alloy, which is an effective method to improve the strength and toughness. Severe plastic deformation and thermal mechanical control processing are two major methods to improve the microstructure and refine grain in the material forming field. The asymmetric hot rolling is a processing method with two characteristic of SPD and TMCP in the industrial production. On the one hand, the severe plastic deformation could refine the grain by cross shear zone in the asymmetric rolling; On the other hand, hot rolling could improve the problem of the hard deformation metal with the HCP structure and control the size of grain by recrystallization and cooling. Asymmetric hot rolling was successfully applied in aluminium alloy, but it was rarely reported about titanium alloy. So the paper is the tentative exploration of the asymmetric hot rolling being used in titanium alloy.The pure titanium TA2 and titanium alloy TC4 were studied in this paper. The new 400mm two-high asymmetric hot rolling mill and the 450mm asymmetric hot rolling mill with different-diameter rollers were the major experimental equipments. It was the main research contents that the effect of the paramenters of asymmetric hot rolling and heat treatment on the microstructure and properties of titanium alloy. The main research contents were carried out in this thesis as follows:(1) The α/β phase transformation point of TA2 was 899℃, and the α+β/β phase transformation point of TC4 was 991℃ by the DAT experiments. The results of grain coarsening experiment showed that the grain size increased when the heating temperature increased, but there was no evident coarseness. So we could choose higher temperature,which should be lower than the β phase transformation point, to heat the titanium in order to facilitate the hot deformation.(2) The thermal simulation experiments were used to research the effect of deformation parameters on the dynamic recrystallization, microstructure and hardness. The result showed that TA2 occured dynamic recovery and TC4 occured dynamic recrystallization in the hot compression experiments, which was beneficial to refine grain size. It was found that deformation resistance of TA2 and TC4 enhanced, grain size reduced and microhardness increased with the decrease of deformation temperature and the increase of deformation rate. Therefore, to make for the excellent comprehensive performance titanium alloy, the heat temperature under the phase transformation point, the low deformation temperature, deep deformation degree and the high strain rate should be used.(3) The experiments of asymmetric hot rolling were carried out on the asymmetric hot rolling experiment mills. The result showed that the rolling forces of TA2 and TC4 reduced by 35.6% and 17.32%, which had rolled with the smooth rolled 。 and the asynchronous speed ratio of 1.2, comparing with symmetric rolling under the similarity conditions. The strength of annealed TC4 which is rolled by knurled rollers was higher than smooth rollers. The tensile strength increased by 98MPa, and the yield strength increased by 39MPa.(4) The double direction rolling process was more beneficial to refine grain than single direction rolling process. The potential of distortion could be stored and the recrystallization nucleation points could be provided by quenching between rolling schedules in the process of TC4 asynchronous rolling process, which was beneficial to refine microstructure and improve integrated property.(5) It was obtained the annealed sheet of TA2 whose tensile strength,yield strength and elongation respectively were 559MPa,423MPa and 31.0% and the annealed sheet of TC4 whose tensile strength.yield strength and elongation were 1102MPa,960MPa and 12.1% by asymmetric rolling with high asynchronous speed ratio, crave rollers, three rolling schedules and quenching between two rolling schedules. The mechanical properties of two sheets were better than the national standard.(6) It was investigated that the heat treatment of asymmetric hot rolled TA2 and TC4 to investigate the best the optimum heat treatment process paraments. The results showed that low temperature annealing was beneficial to maintain the fine equiaxed microstructure, eliminate deformation zone, and uniform the microstructure. So the optimum process of TA2 was annealing in 580℃ for 30min. It was investigated that the influencing factors of solution and aging of TC4 at two-phase region. High solution temperature and short solution time were beneficial to form more metastable phases and disperse phases by aging process. Under the same solution process, high aging temperature and short holding time,or low aging temperature and long holding time was beneficial to precipitate secondary phases. So the optimum solution and aging process was heating in 900℃ for 20min and aging in 430℃ for 6h or 530℃ for 3h.