A computational phase transformation model for selective laser melting processes

Noll, Isabelle; Bartel, Thorsten; Menzel, Andreas

doi:10.1007/s00466-020-01903-4

Cited by 16 publications

(11 citation statements)

References 46 publications

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“…In this section, the phase transformation approach used for the SLM process shall be introduced based on the framework presented in [28],which enhances the model introduced in [4], and the references cited therein. By means of this method, a model based on energy densities, rather than on enforcing a phase transition purely by the melting temperature, see, for example, [11,21,43], is used for the heat source model.…”

Section: Phase Transformation Approachmentioning

confidence: 99%

On the incorporation of a micromechanical material model into the inherent strain method—Application to the modeling of selective laser melting

2021

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When developing reliable and useful models for selective laser melting processes of large parts, various simplifications are necessary to achieve computationally efficient simulations. Due to the complex processes taking place during the manufacturing of such parts, especially the material and heat source models influence the simulation results. If accurate predictions of residual stresses and deformation are desired, both complete temperature history and mechanical behavior have to be included in a thermomechanical model. In this article, we combine a multiscale approach using the inherent strain method with a newly developed phase transformation model. With the help of this model, which is based on energy densities and energy minimization, the three states of the material, namely, powder, molten, and resolidified material, are explicitly incorporated into the thermomechanically fully coupled finite-element-based process model of the micromechanically motivated laser heat source model and the simplified layer hatch model. K E Y W O R D Sadditive manufacturing, finite element method, inherent strain, multiscale framework, phase transformation INTRODUCTIONAdditive manufacturing (AM) of metallic parts-such as the selective laser melting (SLM) process-has gained high interest in the industry, as it allows the manufacturing of components layer by layer which offers a new design freedom and a production of custom-made assemblies. A detailed representation of the SLM process is illustrated in Figure 1, where a moving laser beam heat source melts powder particles of the corresponding layer according to the defined model. Afterwards, the building platform is lowered and a new layer of powder is applied. This process is conducted until the part is finished. Due to the high temperature impact, the particles melt at first and then solidify after a cooling period, so that a dense part is formed. The layer height and the hatching distance have to be chosen according to the melting pool, so that a complete fusion of neighboring layers and melt lines can be assured. Due to this procedure, complex and in particular coupled thermal, mechanical, and metallurgical processes arise during the production. Especially high temperature gradients caused by the laser beam heat input and phase changes influence the characteristics of the part,This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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Section: Phase Transformation Approachmentioning

confidence: 99%

On the incorporation of a micromechanical material model into the inherent strain method—Application to the modeling of selective laser melting

2021

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“…Compared to more involved material models, e.g. [3,37], this simple approach can easily be integrated in standard material libraries and FEM codes while still consistently capturing the most essential aspects of residual stress generation in PBFAM. This capability has been verified in a series of elementary test cases and via reference solutions stated for the sharp-interface limit.…”

Section: Discussionmentioning

confidence: 99%

“…This assumption is ubiquitous in the macroscale PBFAM simulation literature [3,6,15,16] and seems justified as strain-rate dependent heating effects are not relevant for a quasi-static solid mechanics problems. A model that considers the two-way coupled thermomechanical problem can be found in [37].…”

Section: Mathematical Problem Statementmentioning

confidence: 99%

“…Unfortunately, most existing works do not go into detail how a material model that was initially designed for a single solid phase is applied to a three-phase mixture of powder, melt and solid, and how the aforementioned condition of a stress-free solidification start can be incorporated in such models. A notable exception are the recent contributions [2,37], where a constitutive model for the three-phase mixture powder-melt-solid has been consistently derived on basis of iso-stress homogenization and energy minimization. In this interesting contribution, the phase fractions for powder, melt and solid are treated as (independent) internal variables with associated evolution equations and compatibility (inequality) constraints.…”

Section: Introductionmentioning

confidence: 99%

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A simple yet consistent constitutive law and mortar-based layer coupling schemes for thermomechanical macroscale simulations of metal additive manufacturing processes

Proell¹,

Wall²,

Meier³

2021

Preprint

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This article proposes a coupled thermomechanical finite element model tailored to the partscale simulation of metal additive manufacturing processes such as selective laser melting. A first focus lies on the derivation of a consistent constitutive law on basis of a Voigt-type spatial homogenization procedure across the relevant phases, powder, melt and solid. The proposed constitutive law accounts for the irreversibility of phase change and consistently represents thermally induced residual stresses. In particular, the incorporation of a reference strain term, formulated in rate form, allows to consistently enforce a stress-free configuration for newly solidifying material at melt temperature. Application to elementary test cases demonstrates the validity of the proposed constitutive law and allows for a comparison with analytical and reference solutions. Moreover, these elementary solidification scenarios give detailed insights and foster understanding of basic mechanisms of residual stress generation in melting and solidification problems with localized, moving heat sources. As a second methodological aspect, mortar meshtying strategies are proposed for the coupling of successively applied powder layers. This approach allows for very flexible mesh generation for complex geometries. As compared to collocation-type coupling schemes, e.g., based on hanging nodes, these mortar methods enforce the coupling conditions between non-matching meshes in an L 2 -optimal manner. The combination of the proposed constitutive law and mortar meshtying approach is validated on three-dimensional examples with rather complex geometry, representing a first step towards part-scale predictions.

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“…Part-scale demonstration: Microstructure evolution for an exemplary SLM processIn the context of SLM process simulation, macroscale thermo-mechanical models are typically applied to predict the temperature evolution, residual stresses and dimensional warping[55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70]. In the following, an advanced macroscale…”

mentioning

confidence: 99%

A Novel Physics-Based and Data-Supported Microstructure Model for Part-Scale Simulation of Laser Powder Bed Fusion of Ti-6Al-4V

Nitzler¹,

Meier²,

Müller³

et al. 2021

Preprint

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The elasto-plastic material behavior, material strength and failure modes of metals fabricated by additive manufacturing technologies are significantly determined by the underlying process-specific microstructure evolution. In this work a novel physics-based and data-supported phenomenological microstructure model for Ti-6Al-4V is proposed that is suitable for the part-scale simulation of selective laser melting processes. The model predicts spatially homogenized phase fractions of the most relevant microstructural species, namely the stable β-phase, the stable α s -phase as well as the metastable Martensite α m -phase, in a physically consistent manner. In particular, the modeled microstructure evolution, in form of diffusion-based and non-diffusional transformations, is a pure consequence of energy and mobility competitions among the different specifies, without the need for heuristic transformation criteria as often applied in existing models. The mathematically consistent formulation of the evolution equations in rate form renders the model suitable for the practically relevant scenario of temperature-or time-dependent diffusion coefficients, arbitrary temperature profiles, and multiple coexisting phases. Due to its physically motivated foundation, the proposed model requires only a minimal number of free parameters, which are determined in an inverse identification process considering a broad experimental data basis in form of time-temperature transformation diagrams. Subsequently, the predictive ability of the model is demonstrated by means of continuous cooling transformation diagrams, showing that experimentally observed characteristics such as critical cooling rates emerge naturally from the proposed microstructure model, instead of being enforced as heuristic transformation criteria. Eventually, the proposed model is exploited to predict the microstructure evolution for a realistic selective laser melting application scenario

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A computational phase transformation model for selective laser melting processes

Cited by 16 publications

References 46 publications

On the incorporation of a micromechanical material model into the inherent strain method—Application to the modeling of selective laser melting

On the incorporation of a micromechanical material model into the inherent strain method—Application to the modeling of selective laser melting

A simple yet consistent constitutive law and mortar-based layer coupling schemes for thermomechanical macroscale simulations of metal additive manufacturing processes

A Novel Physics-Based and Data-Supported Microstructure Model for Part-Scale Simulation of Laser Powder Bed Fusion of Ti-6Al-4V

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