Papers by Author: Hua Ding

Abstract: Ti-6Al-4V alloy sheet with two kinds of initial microstructure (base material (BM) and heat-treated material) was subjected to friction stir processing (FSP). These two kinds of friction stir processed (FSPed) materials show similar characteristics that the microstructure exhibited equiaxed ultrafine-grained (UFG) structure with high fraction of high angle grain boundaries (HAGBs) and random crystallographic orientations. Furthermore, slightly finer grains were observed in FSPed heat-treated material than that in FSPed BM. Superplastic tensile tests were conducted utilizing the FSPed heat-treated material in the strain rates range from 1×10^-2 to 1×10^-4 s^-1 at 650°C, and the optimal superplasciticy with an elongation of 1220% was achieved at 1×10^-3 s^-1. The superplastic deformation mechanism appears to be grain boundary sliding.

Abstract: Elongation control played a vital role for the production of cold-rolled strip. In the production process, especially during tension disturbances or parameter variations, the conventional PID control method can not meet the actual demand well. Therefore, the intelligent control algorithm was introduced in this paper. A fuzzy self-adaptive PID closed-loop control strategy which combines the fuzzy control algorithm with the conventional PID control algorithm was applied to elongation control system. It is proved in the simulation study that the fuzzy self-adaptive PID control system has both high dynamic performance and static performance as well as strong robustness, which can greatly improve control accuracy and anti-jamming capability of elongation control system of the tension leveller.

Abstract: The hot deformation characteristics of Al-12.7Si-0.7Mg alloy were investigated on an Instron5500 electronic universal testing machine at strain rates ranging from 1.67×10^-4 s^-1 to 1.67×10^-3 s^-1 and testing temperatures ranging from 460 °C to 520 °C. The results show that strain rate had a great effect on the flow stress. The flow stress increased with increasing the strain rate. The true stress-strain curves of the Al-12.7Si-0.7Mg alloy were obtained, and the strain rate sensitivity and hot deformation parameters of deformation activation energy were calculated. The hot deformation constitutive equation with hyperbolic sine form was also obtained. The test calculations demonstrate that the constitutive equation describes the deformation features of the Al-12.7Si-0.7Mg alloy well.

Abstract: By using the finite element method (FEM), the stress-strain curves of multiphase steels was simulated based on the stress-strain curves of single-phase ferrite, bainite, and martensite steels; then the measured result was compared with the simulated one. Effective factors such as the different distribution of microstructure, the volume fraction of hard phase and the yield stress ratio between single-phase hard phase steel with single-phase ferrite steel in multiphase steel are discussed in this work. The results show that the simulated result closely fits the measured one, which proves that this FEM built in this work is correct. The coarser the microstructure, the higher the drag effects of bulk structure, and the larger deformation degree of ferrite phase. With the increase of bainite and martensite volume fraction, the maximum stress rise gradually and the maximum strain decreases gradually. Meanwhile, the effects of volume fraction of hard phase on the stress-strain curve of multiphase steels are larger than that of yield stress ratio between single-phase hard phase steels and single phase ferrite steel.

Abstract: Multi-pass of equal channel angular pressing (ECAP) at a single temperature as low as room temperature from 200 °C were measured using electronic back scatter diffraction (EBSD). The effect of texture and grain size on mechanical properties was investigated to realize the strengthening and large plastic deformation mechanism. A room temperature ECAP with multi-pass procedure is effective to product high strength and large plastic Mg, as a result of submicron grain structure and texture strengthening.

Abstract: Multi-pass ECAP process of pure Al for square samples (Φ＝90º, Ψ＝37º, pressing speed of 1mm/s), was simulated by using 3D finite element method (FEM).The distribution of equivalent strain for two and four passes was compared. The results showed that route A leads to non-uniform distribution of equivalent strain during multi-pass ECAP. The distribution of equivalent strain is uniform in BC route, but the value of equivalent strain is larger in C route. The simulation results were compared with the experimental ones with previous work in the literature. The simulation also shows that equivalent strain and load increase with pass increasing.

Abstract: The effect of the partial substitution of Si by Al on the microstructures and the mechanical properties of cold rolled C-Mn-Si TRIP steel was investigated. The results show that the partial substitution of Si by Al could refine the microstructures, increase the volume fraction of ferrite and retained austenite. In addition, the excellent mechanical properties of the Al partial substituted TRIP steel could be obtained, the tensile strength, total elongation and strength-ductility of C-Mn-Si-Al TRIP steel are 739MPa, 38% and 28082MPa%, respectively.

Abstract: The microstructures of Ti6Al4V alloy after hydrogenation have been investigated and analysed by optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the influence of hydrogenation on the hardness of α and β phases has been analysed by microhardness testing. The microstructure observation revealed that δ hydride (fcc structure) precipitated in the specimens with 0.278 wt.% and 0.514 wt.% hydrogen, and a lot of dislocations and twins have been found simultaneously. The diffraction peaks moved to the lower angles because of the lattice expansion of β phase with the solution of hydrogen atoms. The result of microhardness testing shows that the hardness of α and β phases increases synchronously with increasing of hydrogen and the increment of β is larger than that of α. It is considered that the formation of δ hydrides, lattice defects and alloying element diffusion are the major factors leading to the microhardness change.

Abstract: Superplastic behavior and microstructure evolution of an isothermally forged Ti-47Al-1Cr-1V-1.5Mo-1.5Nb alloy were investigated. The results showed that the strain rate sensitivity index, m, increased with strain during the superplastic deformation, and it kept as a constant when the strain reached a certain value. The maximum value of m was 0.53 at 900°C and strain rate of 5x10-4 s-1. During the superplastic deformation, the as received material with lamellae and subgrains were refined due to dynamic recrystallization, and small and equiaxed grains with high angle boundaries were formed, creating a better condition for superplastic deformation. Grain boundary sliding and boundary migration were the main superplastic deformation mechanisms and slip and twining were also very important during the superplastic deformation of the alloy.

Abstract: In this study, superplastic tensile tests were carried out for Ti-6Al-4V alloy using different initial grain sizes (2.6 μm, 6.5μm and 16.2 μm) at a temperature of 920°C with an initial strain rate of 1×10-3 s-1. To get an insight into the effect of grain size on the superplastic deformation mechanisms, the microstructures of deformed alloy were investigated by using an optical microscope and transmission electron microscope (TEM). The results indicate that there is dramatic difference in the superplastic deformation mode of fine and coarse grained Ti-6Al-4V alloy. Meanwhile, grain growth induced by superplastic deformation has also been clearly observed during deformation process, and the grain growth model including the static and strain induced part during superplastic deformation was utilized to analyze the data of Ti-6Al-4V alloy.