Materials Science Forum Vols. 704-705

Abstract: In order to solve the problem of crack appeared in Francis turbine runner, the welding residual stress of Francis turbine runner is regulated and controlled by means of numerical simulation from the view of subsection welding, local heating and local peening. The results show that the length of welding section of blade outlet and the welding direction in the subsection welding process influence the residual stress of the turbine blade. For the local heating technology, the decreasing effect on residual stress increases with the increase of heating time, heating temperature and heating area. The welding residual stress of dangerous region decreases and then increases with the increase of heating distance. Moreover, the peening effect is very clear when the temperature of material in the peening region reaches its own plastic temperature.

Abstract: Based on a novel arc welding device which can provide hybrid ultrasonic pulse current, the effect of hybrid pulsed current parameters on the arc pressure and weld formation during the aluminum alloy welding process was investigated. The experiment results show that, compared with normal VPTIG process, hybrid ultrasonic pulse VPTIG process gets greater arc pressure. Under the condition of keeping the RMS of positive current unchanged, lowering duty ratio has an evident effect on increasing arc pressure. At proper frequency range, increasing the pulse frequency and decreasing the pulse duty ratio can increase the depth and width of the weld, improve weld penetration rate correspondingly, and be beneficial to improve weld quality.

Abstract: In this paper, Modified Embedded Atom Method (MEAM) method, one of the molecular dynamic (MD) methods, was utilized to investigate the wettability of Sn droplet on the copper plate to simulate the wetting and spreading phenomena of the solder process. And the experiment of spreading and wetting process of Sn droplet on the copper plate was performed by sessile-drop as well to get the contact angles. The results show that the MEAM method is fit to simulate the soldering process. Keywords: Wettability; Soldering; MEAM; Sessile-drop; Molecular dynamic

Abstract: Based on the FEM analysis of the distributions and varieties of the stresses and the strains of the large-size dissimilar steel welded tube-plate structure under cyclic loading, the fatigue crack initiation sites of this structure were predicted. The low-cycle fatigue test was performed to verify the prediction and to study the cracks propagating paths as well. The results indicated that the weld toes linked to the two surfaces of the plate were the positions with the peak values of the maximum stress and the stress amplitude. The plastic deformations in the first loading cycle introduced the tensile-compressive repeated stress cycle at the weld toes during the subsequent loading cycles. It induced the fatigue cracks initiating at the weld toes linked to the two surfaces of the plate respectively. After initiating from the surfaces the cracks propagated along the fusion lines with a short distance then turn into the base metal in the sections vertical to the surfaces. The depth of the crack initiating from the compressed surface was shorter than the one from the tensioned surface.

Abstract: Based on the gas tungsten arc welding process this paper investigated the welding temperature field measurement and simulation of 945 steel plate and AZ31B magnesium alloy, using a non-contact and fast response measured temperature method-Infrared Radiation (IR)-which utilized the baffle to remove the arc and hot tungsten electrode interference, the top-face temperature of AZ31B magnesium alloy plate and 945 steel plate. The results indicated that the lengthwise isothermal map of 945 steel plate is much longer than that of AZ31B magnesium alloy plate, which leads to the most temperature field information loss, this is because the thermal conductivity of steel is small and decreases with temperature increase, the thermal conductivity of AZ31B magnesium alloy is much bigger that that of the 945 steel, about 1.5 times of 945 steel, and increased with temperature increase. The results also showed using IR to measure the welding temperature of magnesium alloy is more difficult than 945 steel. Keywords: magnesium alloy, high strength steel, welding temperature field, IR

Abstract: Carbon nanotubes (CNTs) over hydroxyapatite (HA) as catalyst carrier were synthesized successfully using transition metal by chemical vapor deposition (CVD). The influences of catalyst types on the synthesis of CNTs were investigated when using Fe, Co and Ni as transition metal catalyst respectively. The results showed that CNTs synthesized by Fe catalyst normally possess more ideal morphology and higher crystallinity than those by the other two. But the yield rates of CNTs synthesized by CVD were in the order of Ni>Fe>Co. On the basis of this, HA matrix composites reinforced by CNTs in-situ were prepared and their mechanical properties were studied preliminarily. This study supplies valuable information for controlling the property of CNTs/HA composite by the selection of catalyst.

Abstract: Aluminum foams, as a representative of metallic foams, are a kind of very useful and promising functional materials. This paper reports progress in three-dimensional numerical simulations of gas bubble-metallic melt turbulent flows during the foaming process of aluminum foams, in which air is injected into molten aluminum composites and the melt is mechanical stirred by a pitched-blade impeller with an inclined shaft. The bubble-melt two phase flow in the tank is described with an Eulerian-Eulerian two fluid model, the impeller flow region is simulated based on the Multiple Reference Frames (MRF) method. Influences of gas flow rate, impeller rotation speed and initial bubble diameter on the characteristics of the liquid flow field and gas fraction distribution are examined. Computational results show that bubbles tend to accumulate behind the impeller blades and have an approximately uniform distribution near the top surface of the liquid. Gas holdup values are increased with increasing the impeller speed and gas flow rate and decreased with the bubble diameter.

Abstract: In order to gain a deep understanding of energy absorption property and failure mechanism of closed-cell foam metals, the dynamic energy absorption property of closed-cell foam metals with matrix Al, matrix Al-Si6, matrix Al-Si12 CCAF, Mg matrix, aluminum-fiber and RE-Al alloys are discussed based on the compression and energy absorption test. The influence of different basis materials on absorption property of closed-cell foam metals is discussed, too. It also shows the micro topography of CCAF during the compression process. In addition, micro-mechanics failure mechanism of CCAF is discussed. Results obtained in this research include: (1) the compression deformation curves of different foam metals indicate that matrix Al CCAF is better for energy absorption materials, because of the best energy absorption property and the high strength. (2) the strength of metal foams with matrix Al-Si6, matrix Al-Si12 and matrix Mg are higher than that of matrix Al CCAF, and also have better energy absorption property. However, the failure characteristics of brittle fracture indicate obvious in matrix Al-Si6, matrix Al-Si12 and matrix Mg. (3) the micro-topography of CCAF with matrix Al is simple and uniform. Also the fracture has obvious tear trace. Aluminum-fiber and RE-Al alloys have better compression property, but lower strength and no strength increasing stage lead to destroy suddenly. (4) the curves of energy absorption for matrix Al and matrix Al-Si6CCAF are on the rise. And the maximum of energy absorption is about 2.1MJ/m³. (5) micropore and crack of closed-cell foam metals provide a good space for the compression and energy absorption. Meanwhile, it is the main failure parts of damage. Keywords: closed-cell foam metal; dynamic energy absorption; micro-topography; micro-mechanics

Abstract: This paper start with a discussion on various types of strain energy functions of rubber like materials. Theoretical analysis based on the strain energy function given in by Y.C.Gao in 1997 is proposed. The material parameters of strain energy function were curve-fitted from the uniaxial tensile test. The selected constitutive relation of rubber like materials was implemented into a finite element code MSC.Marc as a user material subroutine to analyze the thermal and mechanical behavior of rubber seal under the plane strain conditions. Contact force and distribution of the contact stress between lip seal and shaft are analyzed and coupled thermal mechanical analysis of rubber seal was proposed. The contact pressure distribution is readily obtainable from the nonlinear finite element analysis and the coupled thermal mechanical analyses results indicate that the thermal stress only have minor influence on the deformed shape of rubber seal, which will be a useful technique for predicting the properties of rubber seal and providing reference for engineering design. Keywords:rubber like materials, nonlinear finite element, contact analysis, thermal mechanical analysis

Abstract: The goal of the present work is to investigate the influence of concrete on failure mode and stress distribution of the reinforced concrete specimens under axial tension by using a numerical test code named Realistic Failure Process Analysis. It can be found that, the periodically distributed fracture spacing phenomenon and tension stiffening phenomenon exist in the failure process of the reinforced concrete structure. Besides, the effect of concrete characteristics on the mechanical behavior and crack spacing of reinforced concrete was also studied in three samples with different concrete strength. The concrete strength value is considered to be an important factor not only to significantly influence the average crack spacing but also to influence the initial peak load of the specimen. In addition, the average fracture spacing is increased and the initial peak load is also increase with the increasing of the concrete strength value, but the mechanical capacities of the concrete has little influence on the ultimate load capacities of the specimen. Keywords: Numerical test; reinforcement concrete, crack distribution, 3D