Papers by Author: Zhi Qiang Li

Abstract: The tensile deformation process and dislocation behavior of primary α-Ti of Ti-6Al-4V were studied by the in-situ tensile test combined with EBSD (electron backscatter diffraction). The initiation, evolution and distribution of dislocation slips at different strains were discussed. The results showed that the microtexture of the material had a significant influence on slip behavior. Typically, basal and prismatic <a> slips initiated first, but the dominant slip type was related to the local texture characteristics. Sometimes, the basal and prismatic <a> slips could still initiate when their Schmid factors were relatively low, while the pyramidal slips usually need a higher Schmid factor to initiate. With the increase of strain, the second slip system inside one grain was activated to accommodate the plastic deformation. When the deformation was localized in a specific microtextured region, basal <a> slips were dominant, but eventually the crack initiated from the <c+a> slip bands inside the grain.

Abstract: CAD models were built for a three-layer SPF/DB cylinder component. FEM method was employed to simulate the SPF/DB process with different girder structure at appropriate temperature/strain rate combinations. The results show that topology parameters of the structure influence the thickness of outer sheet and surface quality. When the thickness ratio of outer sheet to inner sheet exceeded 3:1 and the angle between adjacent ribs was great than 110 degree, ribs were formed and the cylinder surface was perfect. The simulation result was verified by experiment. The experimental results showed that the optimized cylinder structure can be formed at 920°C, 1×10^-4 strain rate perfectly, and the size of blank cylinder diameter can be controlled accurately by SPF/DB mold and the size deviation is less than 0.5mm.

Abstract: Ti₃Al intermetallic alloy is a light and high-temperature resistant material which substitutes for superalloy. It is promising for wide use in aerospace yield. In this paper, the Ti₃Al alloy was joined by diffusion bonding with and without Ti-6-4 alloy interlayer respectively. Diffusion bonding quality and shear strength of the joints were measured and compared. The results show that there was almost no defect found in joints with interlayer and maximum shear strength is 523 MPa in conditions of 940°C and 2.5 MPa pressure for 1.5h. In contrast, defect rate (percentages of sum of defects length in optical micrograph) of joints without interlayer exceeds 80% and maximum shear strength is only 150 MPa in same conditions. According to fracture analysis, the ductile fracture occurred at Ti-6-4 interlayer of joint with interlayer. Fracture of joint without interlayer belongs to mixed type of ductile fracture and brittle fracture.

Abstract: The effect of cryogenic treatment on the microstructure and properties of Ti-6Al-4V has been studied in this paper. The program controlled SLX cryogenic box was used to conduct the cryogenic treatment and the subsequent low temperature temper. The scanning electron microscope was used to study the morphology of microstructure and fracture surface. As the results show that the cryogenic treatment increases the elongation of Ti-6Al-4V from 16.5 percent to 24.5 percent, at the same time, the strength increases slightly, this indicates that cryogenic treatment can improve the comprehensive mechanical properties. The microstructure measurement revealed that there is a tendency of reduction in the precipitated particles after cryogenic treatment. The cross section is flat and the size of dimples is more uniform. It is concluded that the change in the precipitation particle had a great influence in the mechanical properties.

Abstract: The superplasticity of fine-grained 1420 Al-Li alloy which was prepared by two-stage aging and turning rolling process was studied by constant-strain-rate tensile test. The results showed that: good superplasticity was attained at temperature range of 460°C~520°C and deformation strain rate 1×10-4s-1~5×10-3s-1. An elongation of 650% was obtained at the temperature of 650°C and strain rate of 1×10-4s-1. Furthermore, the microstructures of material before and after deformation were examined using OM and TEM. Equiaxed grains were still kept, and the second-phase particles and dislocations were observed in the deformed samples.

Abstract: The high pressure waterjet peening (WJP) was adopted to strengthen the surfaces of 7075-T651 aluminum alloy. The results demonstrate that the processing parameters such as shooting distance, holding time and water pressure affected the abrasive capacity of gas cavities significantly. Compared with non-peening and shot peening (SP) specimens, the fatigue life of as-WJPed specimens was improved by 22% and 6.6%, respectively. The maximum residual compressive stress generated by WJP and SP was approximately equivalent, which is −350 ≤ σ ≤ −400MPa. While the residual compressive stress near the strengthened surface introduced by WJP was higher than that of SP, hence the fatigue life of WJPed specimen increased much more. The surface roughness strengthened by WJP was also better than that of SP. Because the surface profile was well protected in WJP, the good surface qualities also insured a long fatigue life.

Abstract: This paper deals with the cavity formation and growth behavior of fine-grained 1420 Al-Li alloy during superplastic forming. The results indicated that there were many sub-micron cavities pre-existing at the particle-matrix interface and these sub-micron cavities grew initially under deformation. Different from uniaxial tension, the cavities nucleation under biaxial tension was combinable effect of stress concentration and matrix/particle de-cohesion. With the strain and temperature increasing, the total number and the average size of cavities increased. By the calculations, it was seen that diffusional growth process dominates the initial stage of void growth, and for void radii>~1.7μm, void growth was mainly controlled by plasticity.

Abstract: Effects of hydrogen on superplastic deformation behavior were investigated through high temperature tensile experiment in this paper. It is found that reasonable hydrogen contents can improve the superplastic behavior such as lowering flow stress and temperature and increasing m value. While addition of 0.1wt% hydrogen in Ti-6Al-4V alloy, peak flow stress decrease to 53%, deformation temperature decrease 60°C. The influence of hydrogen on microstructure transition by means of optic microscope, SEM, TEM and XRD was also researched. The results show that β phase amounts in the hydrogenated alloy increase with hydrogen contents, while hydrogen contents reach to 0.2wt%, martensite becoming coarser with the increase of hydrogen contents. Moreover, dislocations density of hydrogenated alloy after deformation is lower than that of unhydrogenated alloy because of hydrogen action.

Abstract: Ti2AlNb orthorhombic alloys exhibit great potential as advanced aerospace and structural materials serviced at elevated temperature. In this paper, pre-heat treatment of as-received hot rolling Ti-22Al-25Nb alloy was conducted. Fine, stable and equiaxed grain structure was obtained by the pre-heat treatment. The volume fraction of B2 increased when annealing at 980°C. The Ti-22Al-25Nb alloy showed characteristics of superplastic deformation when tested at 960°C. Maximum elongation of 280% has been obtained at strain rate of 1.0×10-4s-1.

Abstract: Tensile testing was performed on fine-grained GH4169 superalloy sheet at elevated temperatures (920°C~980°C) and at different initial strain rates (10-4~10-2s-1). The maximum elongation obtained was 280%. By TEM, active dislocation movement was observed and compared at different strain level and strain rate. Dynamic recovery and recrystallization was also found during superplastic deformation that played an important role of softening. At last the superplastic deformation mechanism of GH4169 alloy was discussed.