Key Engineering Materials Vols. 611-612

Abstract: In injection molding of highly filled polymers, such as plastic-bonded hard ferrite, mold life decreases significantly due to wear and surface damage. This deterioration process leads to lower accuracy concerning the surface finish and the dimensional and geometric tolerance of the molded bonded magnets. In this work, the wear of a steel mold caused by the molding of a plastic-bonded ferrite magnet has been studied with the aim of determining the main wear factors. Furthermore, two different surface treatments have been tested, namely gas nitriding and DLC superlattice coating. Friction and wear induced by part de-molding was investigated by means of tribological pin-on-disc tests. The results of such experiments were analyzed in terms of loads, surface roughness evolution and surface investigations by means of Scanning Electron Microscope observations.

Abstract: Tailor-welded tubes are widespread in aircraft and automotive industries due to their advantages of low cost, reduction in part weight and flexibility in mass production. It is necessary to obtain the stress-strain relationship of tailor-welded tubes to study deformation behaviors of tubes and simulate deformation tests of tubes. Then a method via digital image correlation (DIC) method based on uniaxial tensile test (UTT) is proposed in this paper to establish stress-strain relationship of tailor-welded tubes. Material parameters of tailor-welded tubes obtained from three methods, i.e. the method based on UTT, the iso-strain method based on a rule of mixtures and the proposed method, were compared in this paper. To assess the accuracy of material parameters obtained from these three methods, UTTs were simulated, and load-displacement curves and maximal loads obtained from simulations were compared with those obtained from UTTs. In simulations of UTTs, finite element models of specimens of sole parent metal and mixed specimens were established, respectively. The results show that: When HAZ included in the specimen has large proportion of the specimen, the proposed method is more reliable than the iso-strain method based on a rule of mixtures on determining the material parameters of the weld; load-displacement curve and maximal load obtained from the proposed method are more close to those obtained from UTT than those obtained from the method based on UTT.

Abstract: In this study, dynamic recrystallization during nonisothermal hot deformation was numerically simulated by finite element analysis and new physically based dynamic recrystallization model. The dynamic recrystallization model was developed based on mean field approach by assuming grain aggregate as representative volume element. For each grain aggregate, changes of state variables such as dislocation density and grain size were calculated using three sub-models for work hardening, nucleation, and nucleus growth. The developed dynamic recrystallization model was validated by comparing with isothermal hot compression of pure copper. Finally, developed dynamic recrystallization model was combined with finite element method to predict the local changes of microstructure and average grain size during nonisothermal hot compression of pure copper and hot tube extrusion of austenitic stainless steel. The simulation results were in reasonably good agreement with experimentally determined microstructures.

Abstract: Development of reliable algorithm for generation of statistical representations of microstructure morphologies is the subject of the present work. The implemented cellular automata sphere growth model is presented first. Obtained grain size distribution of digital microstructure is compared with experimental measurements to prove efficiency of the proposed algorithm. Then, the grain growth model was additionally combined with the genetic algorithm optimization to extend its microstructure generation capabilities. Finally, possibilities of practical applications of generated digital material representation for modelling texture evolution during channel die test is presented.

Abstract: Development of an efficient and user friendly application (framework) for modelling microstructure evolution during thermo-mechanical processing using Cellular Automata (CA) method and WorkFlow approach is the subject of the present work. Description of the major assumptions and functionality of the developed framework is presented first. Then, major assumptions of the implemented cellular automata models dealing with simulation of phase transformation and static recrystallization are presented. Finally, the idea of the WorkFlow methodology is described and used to join the two CA microstructure evolution models into one complex solution. Examples of obtained results of microstructure behaviour during thermo-mechanical processing are also presented within the paper.

Abstract: Material models that couple the evolution of flow stress to the evolution of the microstructure are important for the simulation of hot working processes in which the microstructure undergoes large changes. Among the microstructural evolution mechanisms in hot working, dynamic recrystallization (DRX) plays a central role as it occurs during deformation. When the workpiece deforms, the element shape may deteriorate, which makes re-meshing necessary. At the same time, certain regions of the finite element mesh undergo DRX and a sharp interface between recrystallized and non-recrystallized portions of the workpiece develops. Elements of the old mesh that are cut by the interface contain nodes with a non-zero recrystallized volume fraction and nodes where the recrystallized volume fraction is zero. During re-meshing, when the microstructural data is transferred to the new mesh, nodes or integration points that are actually in region of the workpiece that is not yet recrystallized may be assigned a non-zero recrystallized volume fraction. As a consequence, the interface moves, which is unwanted and may produce large errors when re-meshing is frequently done. In this paper, the problem of the propagation of the DRX interface during re-meshing is treated. It is shown that the propagation occurs with standard data mapping algorithms and produces a large error at the interface. A re-meshing scheme is proposed that uses a smooth mesh-free interpolating function based on radial basis functions to interpolate the recrystallized volume fraction. The interface is the zero level set of this interpolant. Performing the mapping as a least squares fit of the interpolant allows for a substantial reduction of the mapping error and suppresses the propagation of the DRX front.

Abstract: Friction Stir Welding (FSW) is a welding technique the more and more demanded in industry by its multiple advantages. Despite its wide use, its physical foundations and the effect of the process parameters have not been fully elucidated. Numerical simulations are a powerful tool to achieve a greater understanding in the physics of the problem. Although several approaches can be found in the literature for simulating FSW, all of them present different limitations that restrict their applicability in industrial applications. This paper presents a new solution strategy that combines a robust approximation method, based on natural neighborhood interpolation, with a solution separated representation making use of the Proper Generalized Decomposition (PGD), for creating a new 3D updated-Lagrangian strategy for addressing the 3D model while keeping a 2D computational complexity

Abstract: This article deals with numerical simulation of necking. It draws the attention onto the importance of the description of strain-hardening and the effects on the evolution of necking. In order to compare necking evolution in relation with different plasticity models, a tracking procedure which consists in determining the evolution over time of discharged volumes of the sample is adopted. Models that take into account physical phenomena at the microscopic level and especially the heterogeneities of materials from a mechanical point of view seem well suited to fit experimental evidence connected to necking.

Abstract: In the present work the disc compression test used to determine the balanced biaxial strain-ratio $r_b$ is analyzed in terms of the influence of contact friction using non-linear finite element analysis (FEA). The FEA results reveal an unexpectedly strong sensitivity of the $r_b$-value on contact friction, which is discussed in detail. The most important outcome of the present work is that the FEA can reproduce the $r_b$-value imposed by the utilized yield function very well, but only when the prescribed Coulomb friction coefficient has a very small value; for increasing friction coefficients a gradual deviation from the imposed $r_b$-value can be observed. This finding implies that in experiments contact friction must be eliminated to a larger extent than commonly expected, otherwise the determined $r_b$-value using disc compression testing will considerably deviate from the actual one, particularly when $r_b$ is far from one.

Abstract: In this paper anisotropic mechanical behavior of AA2024 aluminum and Ti6Al4V titanium alloys were studied using three different approaches: unified, multi-mechanism and polycrystalline. The theoretical formulations of studied elastoplastic models are first described. Thereafter, some numerical results concerning the simulation of a uniaxial tension test applied to thin metallic sheets are presented. Comparison between experimental results (taken from the literature) and numerical simulations shows that the multi-mechanism and polycrystalline models describe slightly better the anisotropy when considering all the directions. Finally, numerical simulations of a deep drawing test of AA2024 aluminum thin sheets will be analyzed.