Near-surface axial tensile residual stresses (from manufacturing) are reportedly detrimental to the yield strength of cold-drawn wires. Therefore, a reliable evaluation of their magnitude is ...necessary. The size and geometry of electrical wires can pose challenges for experimental measurement of those residual stresses. For that reason, the finite element analysis can prove useful. However, great care must be taken with the right choice of strain hardening law for a sound assessment of residual stresses. Given the complex loading condition during cold drawing, cyclic loading arises through the wire cross section even in single-pass drawing. As a result, it is of crucial importance to account for associated backstresses. The current study makes a comparison between two different hardening laws’ prediction of axial residual stress profiles in numerically cold-drawn Cu–Al composite wires of various Al volume fractions. The impact of die geometry on this prediction was also examined for a 25%Al-wire. To that end, a combined isotropic-kinematic law and a pure isotropic constitutive equation were considered. The results imply a possible overestimation of residual stresses by the pure isotropic model at relatively low Al volume fractions. The difference between the maximum magnitudes of tensile or compressive residual stresses (predicted by the two models) could be as large as about 100 MPa (larger than the yield strength of the starting materials). Furthermore, the tooling geometry minimally affects the prediction of the hardening models. In conclusion, backstresses are not to be overlooked for accurate estimations of drawing residual stresses at low Al volume fractions.
This work focuses on the thermal modeling of the Directed Energy Deposition of a composite coating (316L stainless steel reinforced by Tungsten carbides) on a 316L substrate. The developed finite ...element model predicts the thermal history and the melt pool dimension evolution in the middle section of the clad during deposition. Numerical results were correlated with experimental analysis (light optical and scanning electron microscopies and thermocouple records) to validate the model and discuss the possible solidification mechanisms. It was proven that implementation of forced convection in the boundary conditions was of great importance to ensure equilibrium between input energy and heat losses. The maximum peak temperature shows a slight increase trend for the first few layers, followed by an apparent stabilization with increasing clad height. That demonstrates the high heat loss through boundaries. While in literature, most of the modeling studies are focused on single or few layer geometries, this work describes a multi-layered model able to predict the thermal field history during deposition and give consistent data about the new materiel. The model can be applied on other shapes under recalibration. The methodology of calibration is detailed as well as the sensitivity analysis to input parameters.
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•A thermal model for the Directed Energy Deposition of a novel composite coatingis presented.•The conducted analyzes explain the dissolution of carbides and the formation of a carbon network around the grains.•Thermo-physical properties of a new Metallic Composite Material are here presented.•This work presents an exhaustive explanation of the methodology of calibration of 2D models for additive manufacturing.
Typical computer-based parameter optimization and uncertainty quantification of the additive manufacturing process usually requires significant computational cost for performing high-fidelity heat ...transfer finite element (FE) models with different process settings. This work develops a simple surrogate model using a feedforward neural network (FFNN) for a fast and accurate prediction of the temperature evolutions and the melting pool sizes in a metal bulk sample (3D horizontal layers) manufactured by the DED process. Our surrogate model is trained using high-fidelity data obtained from the FE model, which was validated by experiments. The temperature evolutions and the melting pool sizes predicted by the FFNN model exhibit accuracy of
99
%
and
98
%
, respectively, compared with the FE model for unseen process settings in the studied range. Moreover, to evaluate the importance of the input features and explain the achieved accuracy of the FFNN model, a sensitivity analysis (SA) is carried out using the SHapley Additive exPlanation (SHAP) method. The SA shows that the most critical enriched features impacting the predictive capability of the FFNN model are the vertical distance from the laser head position to the material point and the laser head position.
The microstructure directly influences the subsequent mechanical properties of materials. In the manufactured parts, the elaboration processes set the microstructure features such as phase types or ...the characteristics of defects and grains. In this light, this article aims to understand the evolution of the microstructure during the directed energy deposition (DED) manufacturing process of Ti6Al4V alloy. It sets out a new concept of time-phase transformation-block (TTB). This innovative segmentation of the temperature history in different blocks allows us to correlate the thermal histories computed by a 3D finite element (FE) thermal model and the final microstructure of a multilayered Ti6Al4V alloy obtained from the DED process. As a first step, a review of the state of the art on mechanisms that trigger solid-phase transformations of Ti6Al4V alloy is carried out. This shows the inadequacy of the current kinetic models to predict microstructure evolution during DED as multiple values are reported for transformation start temperatures. Secondly, a 3D finite element (FE) thermal simulation is developed and its results are validated against a Ti6Al4V part representative of repair technique using a DED process. The building strategy promotes the heat accumulation and the part exhibits heterogeneity of hardness and of the nature and the number of phases. Within the generated thermal field history, three points of interest (POI) representative of different microstructures are selected. An in-depth analysis of the thermal curves enables distinguishing solid-phase transformations according to their diffusive or displacive mechanisms. Coupled with the state of the art, this analysis highlights both the variable character of the critical points of transformations, and the different phase transformation mechanisms activated depending on the temperature value and on the heating or cooling rate. The validation of this approach is achieved by means of a thorough qualitative description of the evolution of the microstructure at each of the POI during DED process. The new TTB concept is thus shown to provide a flowchart basis to predict the final microstructure based on FE temperature fields.
The critical dimension (e.g. thickness for sheet, diameter for cylindrical part) of small components used in a miniaturized system is often a crucial factor affecting the mechanical behavior and the ...forming process, since in such length scale ranges the grain size is comparative to the mechanical part dimension. To have a better understanding of the mechanical behavior of microsized metallic parts, the size effects in 500 μm thick samples of nickel sheet were studied in tensile and shear loading states. The modifications of the mechanical behavior due to different numbers of grains across the thickness were thoroughly investigated at room temperature and 573 K. New experimental results obtained by shear tests at high temperature revealed that the transition from polycrystalline to multicrystalline behavior is more pronounced in tensile loading than in shearing conditions and that different surface sample state is observed. It was demonstrated that the reduced stress level effect depends not only on the temperature, but also on the stress state. In addition, with a moderate increase in temperature, the surface effects leading to the multicrystalline behaviors became more predominant in tensile condition than in shearing condition.
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•Grain size effects, coupled with stress state and temperature, were investigated in nickel specimens.•Specific shear samples were employed to allow medium temperature tests.•Temperature promotes the apparition of quasi single crystalline behavior.•Surface effects are not only sensitive to temperature and stress state but also to deformation control mode.•Surface effects present a higher level in tensile conditions at 573 K than in shear loading.
•A strain gradient crystal plasticity law models Ni single crystal.•External surface state is featured by its dislocation mobility.•Internal behavior is affected by the surface state.•Capabilities of ...3 numerical methods to model the phenomena of surface permeability.
The behavior of dislocations in the neighborhood of a metallurgical interface or a free surface can be totally different depending on the boundary conditions. Dislocations cease to move and accumulate around impermeable interfaces, such as grain boundaries or hard (i.e. coated or oxidized) external surfaces. On the contrary, dislocations annihilate on free surfaces, as supported by the image force concept. However the behavior of dislocations depends on the true surface permeability, which falls between these two idealized cases. In this paper, two different numerical methods are applied to model the intermediate surface behaviors: the virtual image geometrically necessary dislocation approach and the generalized elastic foundation approach while a strain gradient crystal plasticity constitutive law simulates the response of a Ni single crystal. It is demonstrated that a uniform dislocation density field on the surface can only be obtained by a variant of the generalized elastic foundation approach.
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Within the large Additive Manufacturing (AM) process family, Directed Energy Deposition (DED) can be used to create low-cost prototypes and coatings, or to repair cracks. In the case of M4 HSS (High ...Speed Steel), a reliable computed temperature field during DED process allows the optimization of the substrate preheating temperature value and other process parameters. Such optimization is required to avoid failure during the process, as well as high residual stresses. If 3D DED simulations provide accurate thermal fields, they also induce huge computation time, which motivates simplifications. This article uses a 2D Finite Element (FE) model that decreases the computation cost through dividing the CPU time by around 100 in our studied case, but it needs some calibrations. As described, the identification of a correct data set solely based on local temperature measurements can lead to various sets of parameters with variations of up to 100%. In this study, the melt pool depth was used as an additional experimental measurement to identify the input data set, and a sensitivity analysis was conducted to estimate the impact of each identified parameter on the cooling rate and the melt pool dimension.
Full range constant strain rate tests are required for accurately characterizing initial yield point, strength differential effect and direct identification of constitutive laws describing the ...plastic behavior of materials. These tests require the use of a closed-loop control in order to achieve the constant strain rate, however this feature is not available in many laboratories. A method for full range constant strain rate with testing machines that can be configured for user-defined displacements of the cross head prior to testing is presented here. Tests performed at a constant die speed include a variable strain rate response for the specimen involved. Significant deformation rate variation occurs between the elastic and plastic range with consequences for initial yield point identification. To overcome this drawback, appropriate user-defined displacements can be computed and applied, allowing for both tensile and compression tests to be performed at a constant strain rate. The method is validated using a compression test of Ti6Al4V alloy at room temperature, as well as a 3D digital image correlation (DIC) system exhibiting a constant strain rate value equal to 10
−3
s
−1
, for both elastic and plastic ranges. A non-negligible inhomogeneous strain field was measured on the surface of the compression specimen using DIC and was corroborated by numerical modeling. Results identified the source of the non-homogeneous strain field, thereby proposing a quantitative indicator of plastic anisotropy. The initial yield stress and strain hardening rates of the alloy at several temperatures were obtained with both testing method, conventional constant cross-head speed, and the constant strain rate; these were then used to determine the influence of the small strain rate variations on the mechanical response of Ti6Al4V alloy.
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