Papers by Author: Zheng Yi Jiang

Abstract: A novel microforming process - Micro Cross Wedge Rolling (MCWR) has been developed, which is promising for fabricating micro stepped components used in micro electro-mechanical systems. Numerical simulations have been established and the effect of geometrical and process parameters such as forming angle α, stretching angle β and reduction ΔA have been studied. Micro stepped components have been fabricated successfully on a MCWR testing rig by adopting flat wedge tools. The effects of initial surface roughness of tool, grain size and cross section area reduction on surface morphology have been assessed quantitatively.

Abstract: The various chemical compositions of stainless steels can result in complication of the formation of oxide scales in hot rolling process. The time for formation of tertiary oxide scale during finishing rolling is short. In the present study, the oxidation tests with short time period were carried out on eight stainless steel grades by Gleeble 3500 thermal mechanical simulator in a simulated water mist environment. Multi-layers of oxide scale have been developed on all the steel grades during short time oxidation. Internal and intergranular oxides were formed in the steel matrix underneath the inner spinel oxide layer. The cross section of the oxide scales were examined and measured by SEM to understand the oxide scale cross section morphology and thickness.

Abstract: A micro hydromechanical deep drawing is carried out using the pure titanium and the effect of fluid pressure on formability of pure titanium is investigated. The experiments are performed using the two kinds of pure titanium foils (TR270C-H and TR270C-O) and stainless steel foil (SUS304-H) with 50 thickness and the cylindrical and conical punches. As a result, it is found that the peeling off the oxide film of pure titanium can be reduced by applying the fluid pressure because the friction force and contact pressure between the blank and die decreases. However, the formability is lower for pure titanium than that for stainless steel because the tensile strength is low and the friction force is easy to increase as the friction force increases. In contrast, due to the low young modulus of pure titanium, the restriction of wrinkling, decrease of friction force and friction holding effect can be obtained at low fluid pressure.

Abstract: A 3D elastic-plastic finite element method (FEM) model of cold strip rolling for 6-high continuous variable crown (CVC) rolling mill was developed. The rolling force distributions were obtained by the internal iteration processes. The calculated error has been significantly reduced by the developed model. the absolute error between the simulated results and the actual values is obtained to be less than 10μm, and relative error is less than 1%. The developed model is significant in investigating the profile control capability of the CVC cold rolling mill in terms of work roll bending, intermediate roll bending and intermediate roll shifting.

Abstract: In the last few decades, there is a global interest in micro products, and micro forming of metals is a promising micro manufacturing method. However, a comprehensive understanding of this process is absent. Therefore, this study aims to investigate micro deep drawing process via experimental and analysis work. Simulation results are in good agreement with the experimental data. The comparison between the finite element method (FEM) simulation and experimental results shows the feasibility of FEM simulation for micro deep drawing process. This research also lays a fundament of investigating micro forming process, especially micro deep drawing.

Abstract: In the present work, detailed studies were made on the transformation characteristics, microstructure and mechanical properties of heavy shell ring (HSR) in the spray cooling process. The spray cooling device of HSR was designed. The 2.25Cr1Mo0.25V steel used in production of HSR for hydrogenation reactor was selected as the testing material. The simulation of spray cooling of HSR was carried out on ABAQUS. The constitutive model and continuous cooling transformation (CCT) diagram of 2.25Cr1Mo0.25V were determined. CCT diagram, metallograph and SEM results show that the bainite forms throughout the cooling rate range from 0.5 to 10 ℃/s, and martensite begins to be produced by increasing the cooling rate higher than 60℃/s; when the cooling rate is 10 ℃/s, with the increase of the deformation degrees, the ferrite grain size becomes small, the yield strength and tensile strength increase, the elongation decrease, So it is good for refining the grain to increase the deformation. The yield strength, tensile strength and elongation were obtained under different cooling technology.

Abstract: An experimental method was developed to examine oxidations of austenitic and martensitic stainless steels. The results show that the surface roughness along both rolling and transverse directions decreases with an increase of reduction. When the reheating time is increased, the average thickness of oxide scales of stainless steels increases, which results in relatively rough surface after hot rolling. The effects of oxide scale on the friction condition and surface roughness transfer in hot rolling depend on the oxide scale generated during reheating. The calculated surface roughness is close to the experimental results, which verifies the developed FEM model.

Abstract: The compressive strain hardening behaviour of a novel bimetal with pearlitic steel and low carbon steel was investigated by computational analysis based on the isothermal compression tests in a wide range of deformation temperature and strain rate. The Hollomon’s equation was employed to calculate the strain hardening exponent (SHE) with the assistance of mathematical manipulation. The result shows that the logarithmic relationship between the flow stress and plastic strain of the bimetal is highly non-linear, which results in the variation of the SHE of the bimetal. This variation reflects the dynamic competition between the strain hardening and softening mechanism by the varying value of the SHE in the range of 0.4 to-0.4. Furthermore, the influences of deformation temperature and strain rate on the SHE are significant. With decreasing temperature and increasing strain rate, the strain hardening of the bimetal was enhanced, while the dynamic recrystallisation was activated under the opposite conditions with the evidence of negative SHE value.

Abstract: Microforming technology has attracted more and more attention because of its high utilization in almost every field. However, due to size effect, the conventional scale mechanical processing theories could not be applicable. Further, the characteristic of each single grain involved in the deformed area activates to play a significant role in the manufacturing process. In order to reflect and investigate the relationship among these grains better, this paper represents a pre-process modeling method with Voronoi tessellation to reveal the grained heterogeneity of workpiece numerically in order to obtain high accuracy and prediction result in finite element (FE) modelling of microforming process. Corresponding micro V-bending experiments have been carried out, and the experimental results are in good agreement with simulation results in terms of final angles after micro bending with consideration of springback.

Abstract: A high carbon steel (HCS) and low carbon steel (LCS) bimetal was fabricated by centrifugal composite casting. Two different thermomechanical treatments (TMT1 and TMT2) were employed to improve the mechanical properties of the bimetal. TMT1 process includes 60% of overall reduction by hot compression with temperatures of 1100 and 800 ^oC, respectively. While TMT2 process involves 60% of overall reduction using the two-step deformation method, which is a combination of non-isothermal compression cooling from 1100 to 800 ^oC and isothermal compression at 800 ^oC. The flow stress behavior, microstructural evolution and microhardness variation were analysed. Experimental results show that both TMT processes contributed to the improvement in mechanical properties resulting from a refinement of the grain size and an increase of density of pearlitic lamella in HCS layer. However, TMT2 process gave a better efficiency and a more significance in improvement of properties with the evidence of the same overall reduction leading to a higher microhardness.