Papers by Author: Ze Kun Yao

Abstract: The primary purpose of the present work is to investigate the effects of annealing after equal channel angular pressing (ECAP) on microstructure and microhardness of TA15 titanium alloy. A study was performed by annealing treatment on the microstructure evolution and microhardness variation of ECAPed TA15 alloy. The results state that, static recrystallization occurred distinctly during annealing after ECAP. Since a sample was annealed at a proper temperature and for proper time after ECAP, a larger amount of well globularized and more homogeneous equiaxed α phase has been attained, grains have not grown observably, and the relief of residual stress and work hardening for subsequent processing and using has also been achieved. Accordingly, the optimum annealing parameters of ECAPed TA15 alloy were optimized to be 973 K and 1 hour. The microhardness level of the sample annealed after ECAP was lower than that unannealed, and the microhardness level decreased with the increasing annealing temperature and time.

Abstract: Dissimilar joining of Ti-22Al-25Nb alloy and an α+β titanium alloy TC11 were carried out using electron beam process and linear friction process, respectively. The microstructure and tensile properties of the joints were investigated. The results show that both EBW and LFW could be adopted to the dissimilar joining of Ti-22Al-25Nb/TC11. The EB welded joint exhibits higher strength than that of the LF welded joint, due to the precipitation of O phase in the fusion zone of the former one. These results can be expected to be of great technical interest as basic data for the use of EBW or LFW in aerospace industry.

Abstract: A study was conducted by optical microscope (OM) and transmission electron microscope (TEM) on the microstructure evolution of TA15 alloy by severe plastic deformation (SPD) and annealing after SPD. In this study, equal channel angular pressing (ECAP) was taken as the method of SPD. The chief aim of the present work is to investigate the temperature effects of ECAP and annealing after ECAP on microstructure of TA15 alloy. The results indicate that equiaxed microstructure has been obtained by ECAP at the temperatures of α+β phase region, and that with the increase in pressing temperature, equiaxed grains have become coarser and the content of α phase has reduced. β grains have been coarsened severely since the pressing temperature was above the α-β transformation temperature (Tβ). Annealed at proper temperature after ECAP, the α phase of TA15 alloy has been more homogeneous, prior α phase has been well globularized, and grains have not grown obviously. According to the testing of TA15 alloy, the optimized temperature parameters of ECAP and annealing after ECAP are 900°C and 700°C. Observation and Analysis of the TEM morphological images illustrate that a quantity of twinning deformations have been produced by ECAP at the temperatures below Tβ, which leads to the continued plastic deformation through the restarting of many slip bands.

Abstract: In this paper the effect of isothermal forging process parameters on the microstructure and the mechanical properties of Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy was researched. The results of the tests indicate that, in the temperature range of 755~905 °C and the deformation degree range of 20~60 %, with the increase of deforming temperature, the volume of primary α-phases decrease, but the globularization extent of the α-phases increases and partial secondary α-phases transform into equiaxed shape. At the temperature of 860 °C, the alloy exhibits excellent strength and plasticity, as the uniform and fine duplex microstructure formed after isothermal forging. When the deformation degree increased from 20% to 60%, primary and secondary α-phases were gradually broken and the recrystallization energy was continually accumulated, which ceaselessly strengthened the properties of the alloy. With the increase of forging times, the globularization extent of decreases, leaving the chain of α-phases, which damages the strength and plasticity of the alloy.

Abstract: Thermomechanical treatment (TMT) of PM FGH4096 superalloy were carried out to futher improve the mechanical strength and refine the γ’ particle, and this processing route as follows: near-isothrmal forged at deformation temperature of 1130°C and strain rate of 0.1 s-1, followed by subsequence oil quenching, and then held at 760°C for 16 h. OM, SEM and TEM were used to investigate the microstructure of TMTed alloy. It was found that the advanced mechanical strength originated form the dispersion strengthening of fine γ’ particle and stain hardening reserved from deformation after TMT. But TMT had no obvious effect on improving the chemical segregation of original materials. Fracture analysis of TMTed alloy shown that cracks origined from Ti and Nb chemical segregation and presented rose-pattern and ladder-pattern at room temperature and 750°C temperature.

Abstract: The microstructures and room temperature and 600°C tensile properties of Ti-5.8Al-4.0Sn-4.0Zr-0.7Nb -0.4Si-1.5Ta alloy after isothermal forging have been studied. The forging temperature range was from 850°C to 1075°C, and the constant strain rate of 8×10-3/S-1 was adopted. With the increase of forging temperature, the volume fraction of primary α phase decreased and the lamellar α phase became thicker when the temperatures were in range of 850°C -1040°C; The grain size became uneven and the α phase had different forms when the forging temperature was 1040°C and 1075°C respectively; The tensile strength was not sensitive to the temperature and the most difference was within 20MPa. Tensile strength and yield strength attained to the maximum when temperature was 1020°C; the ductility decreased with the increase of forging temperature, and this trend became more obvious if forging temperature was above the β-transus temperature.

Abstract: The influences of different hot working conditions on the microstructures and mechanical properties at weld zone of the Ti3Al/TC4 dual alloy have been investigated. The experimental results show that forging and heat treatment has significant influence on room temperature tensile strength (RTTS for short) of the weld zone. The influence of forging strain and temperature on its elongation at temperature 500°C is also conspicuous. The results of intuitional and variance analysis show that affecting factors of its RTTS are heat treatment, forging strain, forge temperature and condition of billet in sequence. The affecting factors of its room temperature (RT for short) ductility and the tensile strength (TS for short) at temperature 500 °C are forging strain, forge temperature, heat treatment and condition of billet. Good comprehensive properties are obtained under optimal conditions: forging at 1020°C, annealing at 700°C for 12 hours, air cooled, about 40% strain and as-rolled billet. The microstructures of the weld zone worked according as those conditions are uniform and the grain size is also fine.

Abstract: Ultrafine-grained treatment (multiple forging and heat treatment) were carried out on FGH4096 alloy to get ultrafine grain with grain being of 1-5μm. OM, SEM, TEM and tensile test were used to study the action of ultrafine-grained treatment on improving structure and properties. The tensile properties of the alloy processed under ultrafine-grained treated have shown quite satisfactory levels, which ultimate strength (UTS) reach 1730Mpa and yield strength (YS) values reach 1470Mpa at room temperature. The apperance of advanced strength has originated chiefly form not only the persence of fine grains, but also structure of grain boundaries and diaperce pricipitations of secondary phase. A better combination of mechanical properties achieved by use of ultrafine-grained treatment makes it possible to explore the dual property turbine disc to its maximum potential for FGH 4096 alloy.

Abstract: In this paper the effect of isothermal forging process parameters on the microstructure and the mechanical properties of TA15 titanium alloy was researched. The results of the tests indicate that, in the range of temperature of 850 °C~980 °C and deformation degree of 20%~60%, with the increase of temperature or deformation, as the reinforcement of deformation recrystallization, the primary α-phase tends to the spherical shape and secondary α-phase transforms from the acicular shape to fine and spherical shape with disperse distribution, which enhance the tensile properties at room and high temperature. With the increment of forging times, the spheroidization of primary α-phase aggrandizes and secondary α-phase transforms from spherical and acicular shape to wide strip shape, which decrease the tensile properties at room and high temperature. The preferable isothermal forging process parameters are temperature of 980 °C, deformation degree of 60%, and few forging times.

Abstract: The superplastic properties of Ti3Al base alloy have been investigated in the range of 900~1020°C and strain rate range of 3.3×10-4~3.3×10-2s-1 .The largest elongation of 1570% was obtained under the optimal superplastic deformation condition of 980°C and 3.3×10-4 s-1. The m-value varied from 0.46 to 0.59, its maximum value was obtained at 980°C.The microstructure of the alloy after heat treatment was composed of two phases of small spherical α2 phase particles distributing in βtransus matrix. Microstructure change has an obvious effect on the superplastic properties of Ti3Al-based alloy. At 980°C,when theα2 phase grain size changed from large to small then large and the cavitations in the fracture surface were larger and deeper with decreasing strain rate, the alloy showed the larger elongation and the lower flow stress.