Materials Science Forum Vol. 941

Abstract: In Low Pressure Casting (LPC), the counter gravity filling at low velocity and the protective gas atmosphere above the metal can potentially reduce gas and oxides entrapment in the metal. However, the relationship between the imposed gas pressure evolution and the melt filling dynamics cannot be analytically determined as it is geometry-dependent. This issue is the missing link to master and automate the filling step in LPC process. In this work, the filling dynamics is numerically investigated for different mold geometries and pressure ramps. The simulation, carried out using ANSYS Fluent® simulation software, is combined with an analytical model. As the results are quantitatively predictive of the filling flow, it permits to develop a numerical study, considering different sudden or progressive section changes and pressure ramps. The impact of the different process parameters on the flow dynamics is analyzed, particularly the transition smoothing impact.

Abstract: Most welding methods in use today involve heating and subsequent cooling of the substrates for joining. Not surprisingly, understanding of associated thermal cycles implicit with the various processes has been a key facet of welding research. While the tools are available for sophisticated numerical solutions, much insight can be gained from simplified analytical approaches. A wide range of joining technologies in use today can be addressed by nominal one-dimensional heat transfer analyses. These include, for example, resistance spot, flash-butt, and linear friction welding. In addressing heat transfer problems, the mathematical constructs for heat transfer are analogous to those for mass (diffusion) transfer. Not surprisingly, one dimensional heat transfer problems can be greatly simplified by adapting the Zener approximation from mass transfer. The work described here employs the Zener approximation to address the direct spot welding of aluminum to steel. The Zener approximation is used to understand heat flow progressively from the steel into the aluminum and finally the copper electrodes. The results are used to understand weld morphology and implicit cooling behavior

Abstract: Ductility is the property of a given material to deform without fracture. In other words, is the capacity to maintain a structural stability under stresses. It is an important property that is difficult to predict since many microstructural and experimental factors play a role. A review of the most important approaches on ductility is given in this work with special emphasis in the high temperature deformation and the deformation mechanisms. The stability of materials is also analyzed and new concepts on the conditions for hot working are included. Stability maps are analyzed and conclusions on the various stability criteria are given on the base of magnesium alloys.

Abstract: The modeling of the forming of materials at high homologous temperatures allows obtaining optimum forming parameters, reduced costs and improving final properties of the finished product. In this work, the behavior of the ZK30 Magnesium alloy was characterized by means of compression tests at temperatures 300 to 450oC and strain rates between 0.1 and 8.7 s^-1. Using data from these tests, the parameters of the Garofalo equation are calculated. In addition, by means of the second Lyapunov stability criterion, the optimum temperature at a given temperature is determined which should minimize the appearance of deformation bands and cracks during hot working. This temperature was found to be 641 K (368oC) at 8.7 s^-1.

Abstract: The high-temperature viscosity of metallic glass-forming liquids is investigated by using the Bond Strength-Coordination Number Fluctuation (BSCNF) model developed by the authors. For many glass-forming liquids, a salient change in the structural relaxation is observed above the melting point. The temperature dependence of the structural relaxation exhibits a deviation from an Arrhenius-like behavior, and upon cooling it transforms to a non-Arrhenius-like one. In the present study, we show that the BSCNF model describes well the high-temperature viscosity behaviors of metallic liquids. The analysis based on the BSCNF model also enables to extract a characteristic temperature at high temperature. The results of the present study show that such characteristic temperature can be a good indicator for the evaluation of the range of the transition from the Arrhenius-like to the non-Arrhenius-like relaxation behavior.

Abstract: Unless corrected by so called anti-trapping currents, phase field models of solidification display a dependence upon the diffuse interface width, δ, used in the simulation. This is most commonly manifest as a reduction in solute partitioning, which is both growth velocity and interface width dependent, resulting in a serious impediment to quantitative simulation. However, such anti-trapping currents are often restricted to very simple materials thermodynamics, appropriate only to dilute ideal solutions. Here we propose a form of the anti-trapping current which can be implemented for arbitrary thermodynamics, including both Redlich-Kister solution phases and sub-lattice models for intermetallic growth. The effect of the new anti-trapping current is illustrated with respect to Pb dendrites growing from a Pb-Sn melt containing either 25% or 30% Sn. The new anti-trapping current is shown to render the solutions independent of the diffuse interface width both with regard to solute partitioning and other growth metrics such as solidification velocity and dendrite tip radius.

Abstract: Superplastic forming has already been proven as a practical solution for manufacturing lightweight components in niche applications such as the aerospace and luxury cars industries. The demand to produce such components will continue with the limited nature of the energy resources available today. Therefore, superplastic materials are expected to stay as potential candidates in such applications. In addition, superplastic forming offers many unique advantages over conventional forming techniques including greater design flexibility, relatively low tooling cost, and no spring back. However, the full potential of the process has not yet been fulfilled due to concerns about the nonuniformity of the produced parts thickness profiles and the need for heating to achieve the superplastic properties of the material. In this paper the authors address the main challenges that hinder the wide spread of the process. It is of great practical importance, for example, to develop accurate simulations of the superplastic forming process. Such simulations are required for identifying the optimum process parameters for high quality components. The results of any such simulations or experimental investigations should be translated into simple and clear industrial guidelines. In addition, they discuss the current trends and the prospects of this process.

Abstract: Using a first-principles methodology we have investigated the interfacial and bonding characteristics of the Al(001)/Fe (0-11) interface. The Al/Fe interface model was developed using a face-to-face matching method. Among many possible interface structures, the Al (001)/ Fe(0-11) orientation relation gave the minimum lattice misfit along the a and b directions (a=b= -0.47%). Hence, this interface structure provided the minimum energy value and was used for this study. To predict the interface strength and stability, the work of separation and interfacial energy were calculated. Here, all systems were calculated under exactly the same conditions (k-point mesh, cutoff energy, lateral lattice strain etc). In order to predict the bonding nature at the interface, charge density difference plot was evaluated, which showed charge gain at the interface. The aim of this study is to describe the adhesive behavior between Al and Fe, provide some insights about strength and stability of this interface structure for galling, and provide reference interface system for Al/Fe welding.

Abstract: We have developed a combined approach of metaheuristic optimization algorithms (MOA), such as the genetic algorithm, with an ab-initio materials simulation engine. Concurrent run of the ab-initio calculations with each different parameter set selected by the MOA searches the optimum condition within a given input-parameter space. Using this methodology, the optimum dopant and its position/structure at a graphene edge are found to be a multiple N-atoms doping at graphitic sites, which predicts to lead to better charging/discharging performance when it is used as an anode material of Li-ion battery.

Abstract: Secondary dendrite arm spacing (SDAS) is a macrosegregation parameter directly linked to content of macrosegregation through cooling rates. The aim of this paper is to highlight the effect of cooling rate on the SDAS and macrosegregation patterns in a high strength steel. For this purpose, directionnal solidification in a cylinder was modeled with a plane-front solidification. Two cylinders were modeled with different boundary conditions (T_surface = 1000°C and 1200°C). Using the FEM software Thercast, 3D macrosegregation maps were generated with thermomechanic algorithm taking into account metal shrinkage. Using Won’s equation, the influence of cooling rates in the mushy zone on SDAS was determined. The results indicated that a 72% lower difference in the area of negative macrosegregation zone (macrosegregation ratio (r_seg) < -0.016%) for lower cooling rate (T_s = 1200°C). The difference of the area for positive segregation was 85% lower for higher cooling rates.