Zhang L.,Key Laboratory for Advanced Materials Processing Technology |
Feng X.,Key Laboratory for Advanced Materials Processing Technology |
Li Z.,Beijing Institute of Control Engineering |
Liu C.,Key Laboratory for Advanced Materials Processing Technology
Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture | Year: 2013
Finite element models were established to simulate the quenching processes of aluminum alloy 2024 blocks under different quenching conditions. The surface cooling curves of aluminum blocks in different quench media were obtained by experiment. Inverse heat-transfer calculations were applied to determine the heat-transfer coefficient curves. Finite element model was used to predict residual stresses developed after quenching process by quenching aluminum blocks with different thicknesses (20mm ≤ d ≤70 mm) into two kinds of quenchants (water and polyalkylene glycol solution). The influence of block thickness and quenchant on residual stresses was investigated by both finite element method simulation and experiments. To validate the simulation results, the X-ray diffraction method was used to measure the residual stresses developed in aluminum alloy 2024 blocks after quenching process. The agreement between model and experiment was good. It was concluded that both block thickness and quench media had critical influences on residual stresses. When the thickness of the aluminum block increased from 20 to 50 mm, the residual stress value at the center of the block increased by 100% and the increase of the residual stress became insignificant after 50 mm. In d = 20 and 30mm cases, polyalkylene glycol quench conditions were proved to cause lower levels of residual stress than water quench conditions. © IMechE 2013.
Yan D.-Y.,Tsinghua University |
Yan D.-Y.,Key Laboratory for Advanced Materials Processing Technology |
Wu A.-P.,Tsinghua University |
Wu A.-P.,Key Laboratory for Advanced Materials Processing Technology |
And 3 more authors.
Materials and Design | Year: 2011
Friction stir welding (FSW) has been widely applied in aluminum alloy manufacturing with its advantages of low residual stress and small deformation. But some FSW cases indicated that the residual distortion was still significant if a large size sheet be welded. In order to reduce the residual distortion of large aluminum alloy sheet after FSW, a general method adopted in this paper was that some stiffeners were designed on the sheet before it be welded. The process of FS-welded structure (sheet with stiffeners) was numerically simulated by finite element analysis (FEA) method. Based on the numerical simulation, the residual distortion of the structure was predicted and the effect of stiffeners was investigated. First, a three-dimensional FEA model was developed to analyze the welding process on a sheet, and the simulation results were verified by FSW experiments. Then, the verified model was applied on structure to compute residual distortion. The prediction displayed that the distortion pattern of structure was convex in longitudinal direction and concave in transversal direction after FSW. In simulation results, the distortion shape of structure was similar to that of sheet, but the distortion value of structure was much smaller. The comparison of simulation results showed that the residual plastic strain generated by FSW on the sheet and the structure was nearly the same, so the part of distortion which was reduced by stiffeners was the distortion induced by buckling. © 2010 Elsevier Ltd.
Du M.,Tsinghua University |
Zhang H.W.,Tsinghua University |
Zhang H.W.,Key Laboratory for Advanced Materials Processing Technology |
Li Y.X.,Tsinghua University |
Li Y.X.,Key Laboratory for Advanced Materials Processing Technology
Surface and Coatings Technology | Year: 2015
A new method was explored for inner surface alloying on pores of lotus-type porous copper. Zinc was deposited on the inner surface of pores by electroless plating with thorough supersonic vibration. Then, the Cu substrate together with Zn coatings was transformed into a brass layer by annealing treatment. From images filmed by a digital single lens reflex camera and a field emission scanning electron microscopy, it can be observed that the appearance and microstructure of lotus-type porous copper surface and pore walls were changed after the sequential treatments. The statistical thickness of the alloy layer at the distance of 0.5. mm from pore openings increased with the electroless plating time increase, attaining a maximum of about 1.7. μm when the time exceeded 1.5. h. The alloy layer thickness at the sample height of 2.5. mm along the pore axial direction was determined as about 1.5. μm by the nano-indentation technique. Uniform coating and alloy layer can be achieved on the inner surface of 5. mm-long pores. The influence of annealing temperatures on phase compositions was studied by X-ray diffraction. © 2014 Elsevier B.V.
Ouyang A.,Tsinghua University |
Ouyang A.,Key Laboratory for Advanced Materials Processing Technology |
Liang J.,Tsinghua University |
Liang J.,Key Laboratory for Advanced Materials Processing Technology
RSC Advances | Year: 2014
Porous chitosan beads are widely used as adsorption media in environmental and biomedical areas. We present two simple methods to tailor the adsorption behavior of chitosan beads by dynamic adsorption and structural modification. Compressing the chitosan beads repeatedly in solutions containing dye molecules or nanoparticles leads to significant improvement in adsorption rate by enhancing internal molecular diffusion, compared with statically placed beads. We also create a hierarchical core-shell structure consisting of a single-walled carbon nanotube network uniformly wrapped around each chitosan bead, in which the outside nanotube network can block or slow down the diffusion of larger size nanoparticles without influencing adsorption of small sized molecules and nanoparticles. Our strategy involving dynamic adsorption and fabrication of hierarchical porous structures might be applied to many other porous materials to tailor their adsorption properties. © the Partner Organisations 2014.
Jiang G.,Tsinghua University |
Li Y.,Key Laboratory for Advanced Materials Processing Technology |
Li Y.,Tsinghua University
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2011
The documented experimental results of hydrogen solubility in different liquid metals were summarized according to the periodic table. It is found that the hydrogen solubility in liquid transition metals is much higher than in others and it changes regularly with the atomic number to form a V-shaped curve. It is supposed that the electron interaction between the hydrogen and metal atoms of the liquid is the major factor in determining hydrogen solubility in the liquid metal. Based on this consideration, a model was proposed for characterizing electron interaction and calculating the hydrogen solubility in various liquid metals according to the nearly free-electron theory. The calculated hydrogen solubility in liquid transition metals agrees well with the documented experimental results, and some undocumented results could be predicted. © 2010 The Minerals, Metals & Materials Society and ASM International.
Liu X.,Tsinghua University |
Liu X.,Key Laboratory for Advanced Materials Processing Technology |
Li Y.,Tsinghua University |
Li Y.,Key Laboratory for Advanced Materials Processing Technology
Journal of Materials Processing Technology | Year: 2012
In the batch type gas injection foaming process, there must exist some melt residual. A differential equation relating the particle content and gas injection depth in the batch type process was presented, by which the melt residual ratio was predicted with the stability criterion as the boundary condition. It was found that the particle diameter increased during the foaming process but the thickness of the foam cell wall can be regarded as a constant. The adsorption coefficient and the injection depth were found to have an inverse relationship, with an exponential of -0.89. Based on the adsorption coefficient at different injection depth, the residual ratio of the melt containing given size particles was able to be predicted. To decrease melt residual needs to increase the particle content and the initial injection depth, and the particle diameter and the critical coverage ratio should be decreased. © 2011 Elsevier B.V. All rights reserved.
Yang M.,Tsinghua University |
Xiong S.-M.,Tsinghua University |
Xiong S.-M.,Key Laboratory for Advanced Materials Processing Technology |
Guo Z.,Tsinghua University
Acta Materialia | Year: 2016
In solidification, dendritic morphology was observed to change accordingly if either the type or quantity of the additional element was modified. To gain insight into this phenomenon, the 3D dendrite morphology of different binary magnesium alloys, including MgAl, MgBa, MgCa and MgZn alloys was characterized using synchrotron X-ray tomography and electron backscattered diffraction. Results showed that for most Mg-based alloys, the dendrite exhibited a typical 18-branch morphology with preferred growth orientations along (1120) and (1123), whereas for MgZn alloys, the dendrite morphology would change from the 18-branch pattern to 12-branch if the Zn content increased, i.e. the so-called dendrite orientation transition (DOT) took place. This DOT behaviour of the Mg alloy dendrite was then successfully modelled using the 3D phase field method by changing the magnitude of related parameters in the specially formulated anisotropy function based on spherical harmonics. © 2016 Acta Materialia Inc. All rights reserved.
Zhang H.,Key Laboratory for Advanced Materials Processing Technology |
Zhang H.,Tsinghua University |
Chen L.,Tsinghua University |
Liu Y.,Key Laboratory for Advanced Materials Processing Technology |
And 3 more authors.
International Journal of Heat and Mass Transfer | Year: 2013
A special kind of micro-channel heat sink was made by using lotus-type porous (also named Gasar) metals with long cylindrical pores formed during unidirectional solidification of metal-gas eutectic system. Copper was selected as the matrix metal because of its high heat conductivity. The heat transfer performance of lotus-type porous copper heat sink with a length of 20 mm along the axial direction of pores was studied on a testing platform designed and set up in this paper. The experimental results show that the lotus-type porous copper heat sink cooled by water has excellent heat transfer performance and a heat transfer coefficient of 5 W/(cm2 K) is attainable when the porosity is 29% and mean pore diameter is 400 μm. An even larger heat transfer coefficient of 9 W/(cm2 K) can be reached after simply cutting the porous copper along the vertical direction of pore axis into four or eight equal sections alined in the direction of pore axis, because that reducing the length of porous copper heat sink along the direction of pore axis will increase the penetrative porosity, result in increase of flow rate, and then enhance the heat transfer performance of the heat sink. Thus some methods have to be taken to increase the pore length and penetrative porosity when fabricating lotus-type porous copper heat sink. © 2012 Elsevier Ltd. All rights reserved.
Guo Z.,Tsinghua University |
Guo Z.,Key Laboratory for Advanced Materials Processing Technology |
Xiong S.M.,Tsinghua University |
Xiong S.M.,Key Laboratory for Advanced Materials Processing Technology
Computer Physics Communications | Year: 2015
An algorithm comprising adaptive mesh refinement (AMR) and parallel (Para-) computing capabilities was developed to efficiently solve the coupled phase field equations in 3-D. The AMR was achieved based on a gradient criterion and the point clustering algorithm introduced by Berger (1991). To reduce the time for mesh generation, a dynamic regridding approach was developed based on the magnitude of the maximum phase advancing velocity. Local data at each computing process was then constructed and parallel computation was realized based on the hierarchical grid structure created during the AMR. Numerical tests and simulations on single and multi-dendrite growth were performed and results show that the proposed algorithm could shorten the computing time for 3-D phase field simulation for about two orders of magnitude and enable one to gain much more insight in understanding the underlying physics during dendrite growth in solidification. © 2015 Elsevier B.V. All rights reserved.
Wang X.,Key Laboratory for Advanced Materials Processing Technology |
Shi Q.,Key Laboratory for Advanced Materials Processing Technology |
Zhang Z.,Key Laboratory for Advanced Materials Processing Technology
Materials Science and Engineering A | Year: 2010
Ti-6Al-4V lamella microstructure obtained by β annealing, which had slow fatigue crack propagation rate and high propagation resistance, was used as base metal and welded by tungsten-inert-gas welding (TIG). Three kinds of orientations were designed to study the influences of precrack orientations and locations on fatigue crack growth rate in as-weld welded joints. In comparison, the classical total-life fatigue performances of the joints were also studied. The results showed that, no matter the precrack was initiated in the center of the weld, near the fusion-line or in HAZ, the fatigue crack propagation rates in the initial stage were all slower than that of the base metal. The fatigue crack in the central region of the weld seam propagated by striation mechanism in the initial propagation stage, and the weld metal exhibited lower fatigue crack propagation rate and higher threshold stress intensity than the base metal and the other joint specimens. © 2009 Elsevier B.V. All rights reserved.