Microstructure characterization and mechanical properties of TC4-DT titanium alloy after thermomechanical treatment

Influence of thermomechanical treatments （mill annealing, duplex annealing, solution treatment plus aging and triple annealing） on microstructures and mechanical properties of TC4-DT titanium alloy was investigated. Results showed that thermomechanical treatments had a significant influence on the microstructure parameters and higher annealing and aging temperature and lower cooling rate led to the decrease of the volume fraction of primaryαand the size of prior-βand the increase of the width of grain boundary αand secondary α. The highest strength was obtained by solution treatment and aging due to a large amount of transformedβand finer grain boundaryαand secondaryαat the expense of slight decrease of elongation and the ultimate strength, yield strength, elongation, reduction of area were 1100 MPa, 1030 MPa, 13%and 53%separately. A good combination of strength and ductility has been obtained by duplex annealing with the above values 940 MPa, 887.5 MPa, 15%and 51%respectively. Analysis between microstructure parameters and tensile properties showed that with the volume fraction of transformedβphase and the prior-βgrain size increasing, the ultimate strength, yield strength and reduction of area increased, but the elongation decreased. While the width of grain boundary α and secondary α showed a contrary effect on the tensile properties. Elimination of grain boundaryαas well as small prior-βgrain size can also improve ductility.

Hot deformation behavior and microstructure evolution of TC4 titanium alloy

The hot deformation behavior of Ti-6Al-4V （TC4） titanium alloy was investigated in the temperature range from 650℃ to 950℃ with the strain rate ranging from 7.7×10^-4 s^-1 to 7.7×10^-2 s^-1. The hot tension test results indicate that the flow stress decreases with increasing the deformation temperature and increases with increasing the strain rate. XRD analysis result reveals that only deformation temperature affects the phase constitution. The microstructure evolution under different deformation conditions was characterized by TEM observation. For the deformation of TC4 alloy, the work-hardening is dominant at low temperature, while the dynamic recovery and dynamic re-crystallization assisted softening is dominant at high temperature.

Effect of grain size on high-temperature stress relaxation behavior of fine-grained TC4 titanium alloy

In order to analyze the effect of grain size on stress relaxation(SR) mechanism,the SR tests of TC4 alloy with three kinds of grain size were performed in a temperature range of 650-750℃.A modified cubic delay function was used to establish SR model for each grain size.A simplified algorithm was proposed for calculating the deformation activation energy based on classical Arrhenius equation.The grain size distribution and variation were observed by microstructural methods.The experimental results indicate that smaller grains are earlier to reach the relaxation limit at the same temperature due to lower initial stress and faster relaxation rate.The SR limit at 650℃ reduces with decreasing grain size.While the effect of grain size on SR limit is not evident at 700 and 750℃ since the relaxation is fully completed.With the increase of grain size,the deformation activation energy is improved and SR mechanism at 700℃ changes from grain rotation and grain boundary sliding to dislocation movement and dynamic recovery.

Effect of undercut defect on deformation behavior TC4 titanium alloy laser welded butt joint under static tensile loading

By measuring and analyzing infrared thermal image of the specimen in static load tensile test process, it was studied that the influence of the undercut defects and double-sided dressing method on the deformation behavior of the laser welded joint specimens of TC4 titanium alloy. The results showed that for the unmodified specimens, the yield phenomenon occurs first in the region of the joint, but the undercut value has an effect on the stress and strain of starting to yield phenomenon, and a great effect on the plastic deformation behavior.When the undercut is less than a certain value, the large plastic deformation occurs in the base metal region and the plasticity of the specimen is comparable to that of the base metal, but the larger undercut defect results in a concentrated plastic deformation in the joint region and rapidly failed in this region. But the double-sided dressing specimen is significantly different. The physical yield is no longer concentrated in the joint region, but at the same time occurs in the several regions including joint and the base metal. And the plastic deformation mainly occurs in the base material area, similar to that of the base material.

Effects of Cryogenic Treatment on the Thermal Physical Properties of TC4 Titanium Alloy

A cryogenic treatment test was carried on TC4 titanium alloy,and the effects of cryogenic treatment on the thermal physical properties of TC4 titanium under the temperature range of 25~600°C were investigated through comparing the variation of the thermal conductivity and thermal expansion coefficient of the alloy before and after cryogenic treatment.The results show that the thermal conductivity of the cryogenic treated alloy is first less and then greater than that of the untreated alloy,however,its thermal expansion coefficient is first greater and then less than that of the untreated alloy with the increase of temperature.When the temperature is up to 600°C,the thermal conductivity and thermal expansion coefficient of the cryogenic treated alloy are 7.24 W·m-1·K-1 and 11.71×10-6°C-1,and increased and decreased by 3.18%and 1.65%than that of the untreated alloy at the same temperature,respectively.

Influence of high pulsed magnetic field on tensile properties of TC4 alloy

The tensile tests of TC4 alloy are carried on electronic universal testing machine in the synchronous presence of high pulsed magnetic field（HPMF） parallel to the axial direction.The effects of magnetic induction intensity（5 = 0,1 T,3 T,and 5 T） on elongation（5） of TC4 alloy are investigated.At 3 T,the elongation arrives at a maximum value of12.41%,which is enhanced by 23.98%in comparison with that of initial sample.The elongation curve shows that 3 T is a critical point.With B increasing,the volume fraction of α phase is enhanced from 49.7%to 55.9%,which demonstrates that the HPMF can induce the phase transformation from β phase to α phase.Furthermore,the magnetic field not only promotes the orientation preference of crystal plane along the slipping direction,but also has the effect on increasing the dislocation density.The dislocation density increases with the enhancement of magnetic induction intensity and the 3-T parameter is ascertained as a turning point from increase to decrease tendency.When B is larger than 3 T,the dislocation density decreases with the enhancement of B.The influence of magnetic field is analyzed on the basis of magneto-plasticity effect.The high magnetic field will enhance the dislocation strain energy and promote the state conversion of radical pair generated between the dislocation and obstacles from singlet into triplet state,in which is analyzed the phenomenon that the dislocation density is at an utmost with B = 3 T.Finally,the inevitability of optimized 3-T parameter is further discussed on a quantum scale.

TC4钛合金环热应力校形的实验研究

TC4钛合金的屈服强度和弹性模量比值高,故其成形后回弹量大,成形后不精确,难以成形出合格零件,需要进行校形,以得到合格零件。因此对钛合金进行了校形实验,得到了感应加热工艺参数对材料校形的影响规律。并运用大型商用模拟软件ANSYS建立了钛合金热应力校形的模型,利用软件中LS-DYNA求解器,对钛合金热应力校形的过程进行了模拟。从而找出钛合金环热应力校形中的一些规律和校形最佳参数,期望能够将其用于指导生产。

Weld shape and microstructure of TC4 laser welding with activating flux of Na_(2)SiF_(6)

Na_(2)SiF_(6) was used as surface activating flux for laser welding of TC4 titanium alloy. The effect of Na_(2)SiF_(6) on TC4 titanium alloy laser welding was determined by observing the weld surface. The morphological characteristics of the high temperature plasma above the workpiece was observed and analyzed by using high-speed digital camera system. The variation of weld depth,width and microstructure were analyzed by optical microscope. The experimental results show that laser weld of TC4 titanium alloy has good appearance with activating flux of Na_(2)SiF_(6), weld penetration increases by about 0.8%–12%, while weld surface width decrease by about 10%–29%, the depth to width ratio is effectively improved. The inhomogeneity of weld microstructure was improved, and the crystallization direction of β columnar crystals on the upper part of the weld was changed, the grain size and microstructure of the weld were refined by Na_(2)SiF_(6).

Effects of boronizing treatment on microstructural development and mechanical properties of additively manufactured TC4 titanium alloys

Microstructure and mechanical properties of additively manufactured TC4 alloys were investigated,following boronizing treatment.The treatment process was carried out at temperatures ranging from 950 to 1050°C and held for 8-15 h.The microstructural features of fabricated boride layers were examined by optical microscopy.The phase compositions of the boride layers were analyzed by X-ray diffraction.The hardness profile through the boride layers was also determined.The results showed that the boride layer of additively manufactured TC4 had a thickness of 51 pm and was composed of an outer TiB2 layer on the top of TiB layer;TiB whiskers wedged into the matrix,forming a strong bond between the boride layer and substrate.The diffusion activation energy was determined to be 80.9 kJ/mol.The matrix was found to transfer from needlelikeα'martensite phase toα+βbiphasic compounds.The newly formed boride layer can reach 1680 HV in hardness,thus imparting a strong protection to 3D-printed part.

Improving performance of macro electrolyte jet machining of TC4 titanium alloy:Experimental and numerical studies

Electrolyte jet machining(EJM)is a promising method for shaping titanium alloys due to its lack of tool wear,thermal and residual stress,and cracks and burrs.Recently,macro-EJM has attracted increasing attention for its high efficiency in machining wide grooves or planes.However,macro-EJM generates large amounts of electrolytic products,thereby increasing the difficulty of rapid product removal with a standard tool and reducing the surface quality.Therefore,for enhanced product transport,a novel tool with a back inclined end face was proposed for macroEJM of TC4 titanium alloy.For comparison,also proposed were ones with a standard flat end face,a front inclined end face,and both front and back inclined end faces.The flow field distributions of all proposed tools were simulated numerically,and experiments were also conducted to validate the simulation results.The results show that one with a 5°back inclined end face can decrease the lowvelocity flow zone in the machining area and increase the high-velocity flow zone at the back end of tool,thereby promoting rapid product removal.A relatively smooth bright-white groove surface was obtained.The same tool also resulted in the highest machining depth and material removal rate among the tested ones.In addition,rapid product removal was beneficial to the subsequent processing.Because of its rapid product removal,the machining depth and material removal rate during deep groove machining using the tool with a 5°back inclined end face were respectively 7%and14%higher than those produced using a standard one.Moreover,the lowest bottom height difference of 0.027 mm can be obtained when the step-over value was 8.2 mm,and a plane with a depth of0.285 mm and a bottom height difference of 0.03 mm was fabricated using the tool with a 5°back inclined end face.

TC4钛合金的固体渗硼

采用固体粉末渗硼法对TC4钛合金基体表面进行渗硼试验。通过扫描电镜(SEM)、能谱(EDS)与X射线衍射(XRD)研究TC4钛合金渗硼后的物相组成和组织形貌,讨论渗硼过程中元素的扩散行为。结果表明:在1000,1050和1100℃分别保温5,20h后,渗层由外表层的TiB2和内表层的TiB晶须组成,渗层厚度范围为0.8～15μm。XRD分析表明:TC4钛合金渗硼后形成TiB2与TiB双相硼钛化合物层,随着温度的升高,TiB2与TiB的峰位增多;EDS分析得出表层B原子被TC4钛合金吸附后与基体的Ti化合导致过渡区域内的Ti含量减少,同时Al和V元素开始向基体扩散并在近界面处富集。渗层的显微硬度呈梯度分布,TiB2到TiB晶须维氏硬度值的变化范围为22000～11000MPa,过渡区的硬度值要高于基体的硬度值。

RHEOLOGICAL FORMING APPLICATION RESEARCH ON THE LIGHT METAL ALLOY MATERIALS

On the bases of high temperature creep experiments, the research on engineering application of rheological forming is carried out on two kinds of light metal alloy parts named cylindrical shell of Lc4 aluminum alloy and vane disk with complex curved surface of TC11 titanium alloy. Moreover, the mechanical property tests under room and high temperatures for the workpieces produced by this new technique are also done, the results showed that they are much improved evidently compared with those produced by traditional method.