Papers by Author: Zhao Yao Zhou

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Authors: Tungwai Leo Ngai, Yuan Yuan Li, Zhao Yao Zhou
Abstract: Increasing density is the best way to increase the performance of powder metallurgy materials. Conventional powder metallurgy processing can produce copper green compacts with density less than 8.3g/cm3 (a relative density of 93%). Performances of these conventionally compacted materials are substantially lower than their full density counterparts. Warm compaction, which is a simple and economical forming process to prepare high density powder metallurgy parts or materials, was employed to develop a Ti3SiC2 particulate reinforced copper matrix composite with high strength, high electrical conductivity and good tribological behaviors. Ti3SiC2 particulate reinforced copper matrix composites, with 1.25, 2.5 and 5 mass% Ti3SiC2 were prepared by compacting powder with a pressure of 700 MPa at 145°C, then sintered at 1000°C under cracked ammonia atmosphere for 60 minutes. Their density, electrical conductivity and ultimate tensile strength decrease with the increase in particulate concentration, while hardness increases with the increase in particulate concentration. A small addition of Ti3SiC2 particulate can increase the hardness of the composite without losing much of electrical conductivity. The composite containing 1.25 mass% Ti3SiC2 has an ultimate tensile strength of 158 MPa, a hardness of HB 58, and an electrical resistivity of 3.91 x 10-8 Ω.m.
Authors: Jing Zhao, Wei Xia, Feng Lei Li, Zhao Yao Zhou, Zheng Qiang Tang
Abstract: . An analytical model is developed for the prediction of residual stresses in burnishing. The model is simplified as a concentrated force pressing on elastic-plastic half-space using the solution to the Boussinesq-Flament problem. The treated material admits the elastic-plastic properties with hardening using a power law constitutive relation. Trial computation using Johnson-Cook model on AISI 1042 steel is presented and the results are verified with the experimental results given by Bouzid’s previous work. The residual stresses in the feed direction show the same trend with the experimental results while some differences still exist near the surface because of the concentrated normal force assumption and such stresses increase with the increase of burnishing force, decrease with the increase of depth and turn to zero beyond the plastic deformation boundary.
Authors: Jian Yi Pan, Yao Wang, Zhao Yao Zhou, Shou Bin Dong
Abstract: In order to improve the service life of extrusion dies with long cantilever structure, a design scheme of porthole die for half-hollow profiles with long cantilever was introduced. Using numerical simulation method, compared with conventional method for the half-hollow porthole design method of a typical profile die model, the equilibrium of the material flow at the outlet, the stress and deformation of the die were analyzed in detail. With selection of the cantilever thickness shrinkage as the objective function, experiment was done to verify the result of simulating analysis. The results indicated that there was only little difference for the equilibrium of material flow between the two design schemes, but the stress load and deformation of the design scheme were greatly improved.
Authors: Feng Lei Li, Wei Xia, Zhao Yao Zhou, Jing Zhao, Zheng Qiang Tang
Abstract: Vibration Assisted Burnishing (VAB) is an advanced burnishing form incorporating dynamic force resulting from vibration into burnishing not only to change the loading type and contact method to greatly reduce friction and wear, but also to produce an excellent nanocrystalline surface by severe plastic deformation induced by high speed impact. The expression describing the relation between decrease of surface roughness and ball burnishing force is given. The dynamic model of ball VAB is established. The relations between VAB depth, VAB force and VAB time and their maximum values are derived, and the required maximum power of the vibration generator is then obtained. The theoretical equivalent burnishing force of ball VAB is only about 47.55% that of conventional burnishing, which prove validity of ball VAB.
Authors: Jian Yi Pan, Zhao Yao Zhou, Shou Bin Dong
Abstract: Detail analysis is presented for the influence of elastic deformation on profile wall thickness, the metal flowing and stress load situation of an asymmetry multi-mandrel die. Through the combination of numerical simulation and traditional experimental design, the method on complex section extrusion dies by quantitative prediction of the elastic deformation situation, can improve the accuracy and success rate effectively. In addition, profiles can be extruded stably when the exit velocity fluctuation is under a corresponding range. Under this range, deformation stress caused by uneven metal flowing is under the critical stress, which relates to the thickness to width ratio t/b.
Authors: Fei Wu, Zhao Yao Zhou, Meng Long Dong, Bi Bo Yao
Abstract: A new method of mechanical vibration assisted induction heating hot-pressing was proposed. A set of apparatus was developed to perform the forming of mechanical vibration assisted induction heating hot-pressing. A series of experiments of induction heating temperature characteristics of different kinds of powder was carried out, such as Fe powder, Al powder and SiC powder. The results show that different kinds of powder reach different equilibrium temperatures and have different temperature characteristics under the same condition of setting temperature and cyclic heating times. SiCp/Al composites were prepared by induction heating and vibratory forming and observed by metallographic microscope. It turns out that completely dense and homogeneous powder metallurgy compaction is compacted and sintered at the same time by the method of mechanical vibration assisted induction heating hot-pressing with low pressure and short time.
Authors: Wen Jun Deng, Wei Xia, Zhao Yao Zhou, Wei Ping Chen, Yuan Yuan Li
Abstract: A three-dimensional coupled thermal-mechanical model of ball burnishing process is developed using commercial explicit finite element code MSC. Marc. The workpiece is modeled as being elastic-plastic, while its flowstress is taken as a function of strain, strain-rate, and temperature. Temperature-dependent material properties are also considered in this analysis. The burnishing ball is considered as rigid and only heat transfer analysis is carried out for it. In the zone of the workpiece and tool contact, the Coulomb friction is taken into account. Effects of ball burnishing parameters (burnishing force, feed rate, speed, ball diameter and number of tool passes) on residual stresses are analyzed. The results show that burnishing force, ball diameter, number of passes and burnishing feed have the most significant effect on the residual stresses. However, burnishing speed seems to just produce little effect on those. Larger burnishing force, larger number of passes, smaller ball diameter and small feed rate seem to be more effective in increasing in the maximum compressive residual stress and the depth of the layer at the compressive stress state.
Authors: Feng Lei Li, Wei Xia, Zhao Yao Zhou, Tian Zhang
Abstract: Burnishing, an ultra-precision superficial plastic deformation process, is used increasingly as a surface enhancement finishing treatment after machining operations not only to give a mirror-like and work-hardened surface but also to impose favorable compressive residual stress in it. To analyze the feasibility of turning-burnishing hybrid process, the Taguchi’s L27(313) orthogonal array method with the analysis of variance (ANOVA) were used to analyze the influence of the initial turning process on surface integrity of roller burnished AISI 1045 steel such as surface roughness, surface microhardness. three turning parameters, namely the cutting feed, cutting depth and cutting speed, three burnishing parameters, namely the burnishing feed, burnishing depth and burnishing speed were selected as the experimental factors in Taguchi’s design of experiments to determine which one has the dominant influence and how it works on burnishing effects, namely the surface roughness and surface microhardness, the interactions between cutting feed, burnishing feed and burnishing depth were considered. The experimental results agreed well with the theoretical analysis and the conclusion is cutting feed has dominant influence on burnished surface integrity.
Authors: Ming Jun Liu, Wei Xia, Zhao Yao Zhou, Pu Qing Chen, Jun Jun Wang, Yuan Yuan Li
Abstract: The rolling compaction can produce porous or dense strips with special functions. The mechanical behaviors in rolling compaction are hard to predict accurately and efficiently by traditional means. The numerical simulation based on the Finite Element Method (FEM) provides a flexible and efficient way for such problems. This paper introduces three-dimensional (3-D) FEM simulations for the rolling compaction of the iron matrix powders. The elliptical yield criterion, elasto-plastic constitutive relationship and the friction model were analyzed. Simulations were based on the second-developed user subroutine in MSC.Marc. Effects of friction and rolling velocity on the rolling force, distribution of the density and some other parameters were analyzed.
Authors: De Zhi Yang, Meng Long Dong, Xin Wei Lu, Zhao Yao Zhou
Abstract: Fully developed turbulence single phase convection heat transfer of water in pipes filled with sintered porous metal inner rings or solid inner rings was investigated numerically respectively. Numerical calculations were conducted with the Fluent 6.3 code, using the SST k-ω turbulence model. Comparing to solid-ring turbulator pipes, porous-ring turbulator pipes have better comprehensive heat transfer effect. The maximum PEC for porous-ring turbulator pipes is 4.4 and the PEC of solid-ring turbulator pipes is less than 1. It was also analyzed effect of geometric structures on porous-ring turbulator pipe performance. f/f0 for porous-ring turbulator pipes increases with the increasing of Re while Nu/Nu0 decreases with the increasing of Re ,and PEC decreases with the increasing of Re. With the same Re, if the width of the porous ring is equal to the width of groove, f/f0, Nu/Nu0 increases and PEC decreases with the increasing height of porous ring. When the height of porous ring is constant, the f/f0, Nu/Nu0 and PEC decreases with the increasing height of porous ring under the same Re.
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