Development of Holistic Homogeneous Model of Selective Laser Melting based on Lattice Boltzmann Method: Qualitative Simulation

Additive manufacturing (AM) technologies are developing fast in recent years. Many of them, such as Selective Laser Sintering/Melting (SLS/SLM) and also Laser Cladding, deal with materials in different states of matter that are changed during processing. The holistic numerical model based on Lattice Boltzmann and cellular automata methods (LBM-CA) is currently being developed for simulation of the laser assisted AM processes where changes of the physical state of matter are essential. The presented qualitative results are mainly related to melting and solidification of the powder bed under the influence of a moving laser beam considering free surface flow, wettability, surface tension and other relevant physical phenomena showing effectiveness of the proposed holistic and homogeneous modelling approach. In this work, we also discuss the potential 3D extension and applications of the model

Energy transfer including solid-liquid phase transformation aspects in modelling of additive layer manufacturing using Lattice Boltzmann-Cellular automata methods

A new holistic numerical model based on Lattice Boltzmann and cellular automata methods (LBM-CA) is currently under development within a frame of an integrated modelling approach applied for studying the complex relationship between different physical mechanisms taking place during laser assisted additive layer manufacturing (ALM). The entire ALM process has been analysed and divided on several stages considering a powder bed deposition, laser energy absorption and heating of the powder bed by the moving highly concentrated energy source leading to powder melting, fluid flow in the melted pool and through partly or not melted material and solidification. The presented earlier results included the entire structure of the model consisting of different modules connected together demonstrating the homogeneity of the proposed holistic model. The modules, considering the mentioned above physical phenomena, were developed to different extent leaving many aspects of this integrated numerical approach for further consideration and analysis. The aim of this work is more detailed analysis of energy transfer including solid-liquid phase transformation during the ALM process. The presented results are mainly related to consideration of melting and solidification of the powder bed including of the free surface flow, wettability, surface tension and other relevant phenomena. Initially, the absorbed thermal energy spreads by heat diffusion. The solid-liquid phase transformation starts when the temperature in the affected zone exceeds the solidus temperature. After consuming latent heat, when the volume of liquid phase exceeds a threshold, the solid particulate material exhibit signs of liquid behaviour, where heat transport is described by diffusion or convection including radiation and convection heat transfer from the liquid surface. The excess heat in the area of the liquid phase is dissipated by heat conduction into the deeper layers of the powder bed leading to re-solidification of the melt pool. The different stages and fragments of LBM-CA model development are presented and discussed. The validation of the general aspects of the obtained modelling data showed that the developed numerical algorithm is in good agreement with available experimental results and theoretical predictions. For example, Fig. 1 illustrates predictive abilities of the algorithm in terms of melting, free surface flow, wettability and solidification of the droplets on the solid basement

A Multiphysics Simulation Approach to Selective Laser Melting Modelling based on Cellular Automata and Lattice Boltzmann Methods

This paper presents a novel approach to numerical modelling of selective laser melting (SLM) processes characterised by melting and solidification of the deposited particulate material. The approach is based entirely on two homogeneous methods, such as cellular automata and Lattice Boltzmann. The model components operate in the common domain allowing for linking them into a more complex holistic numerical model with the possibility to complete full-scale calculations eliminating complicated interfaces. Several physical events, occurring in sequence or simultaneously, are currently considered including powder bed deposition, laser energy absorption and heating of the powder bed by the moving laser beam leading to powder melting, fluid flow in the melted pool, flow through partly or not melted materials and solidification. The possibilities and benefits of the proposed solution are demonstrated through a series of benchmark cases, as well as model verifications. The presented case studies deal mainly with melting and solidification of the powder bed including the free surface flow, wettability, and surface tension. An example of process simulation shows that the approach is generic and can be applied to different multi-material SLM processes, where energy transfer including solid-liquid phase transformation is essential, by integrating the developed models within the proposed framework

Numerical modelling of grain refinement around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials by duplex technique

Microstructure evolution around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials have been investigated and discussed in the present work. Conditions leading to grain refinement during co-rolling stage of the duplex processing technique are analysed using the multi-level finite element based numerical model combined with three-dimensional frontal cellular automata. The model was capable to simulate development of grain boundaries and changes of the boundary disorientation angle within the metal structure taking into account crystal plasticity formulation. Appearance of a large number of structural elements, identified as dislocation cells, sub-grains and new grains, has been identified within the metal structure as a result of metal flow disturbance and consequently inhomogeneous deformation around oxide islets at the interfaces during the co-rolling stage. These areas corresponded to the locations of shear bands observed experimentally using SEM-EBSD analysis. The obtained results illustrate a significant potential of the proposed modelling approach for quantitative analysis and optimisation of the highly refined non-homogeneous microstructures formed around the oxidised interfaces during processing of such laminated materials

Additive manufacturing of multi layered bioactive materials with improved mechanical properties: modelling aspects

Multilayered laminate structures obtained by coating of ultrafine-grained metallic materials with bioactive and multifunctional composite coatings are considered for biomedical applications. Laser-assisted densification of multiple materials using laser cladding and selective laser melting is an alternative route to reduce the risk of early implant failure allowing for faster and cheaper fabrication. To understand the cooperative relationships between different factors that cam influence the manufacture of such bioactive laminates reflecting in their bioactivity and mechanical properties, the multi scale numerical modelling is applied. This work presents resent advances on development of integrated numerical models including generation, melting and solidification of the powder bed, considering surface flow, wettability, surface tension and other physical phenomena, specific mechanical and thermo-mechanical aspects and microstructure evolution

An extended laser beam heating model for a numerical platform to simulate multi‑material selective laser melting

A laser beam heating model (LBHM) is an important part of a platform for numerical modelling of a multi-material selective laser melting process. The LBHM is utilised as a ray-tracing algorithm that is widely applied for rendering in different applications, mainly for visualisation and very recently for laser heating models in selective laser melting. The model presented in this paper was further extended to transparent and translucent materials, including materials where transparency is dependent on the material temperature. In addition to refection and surface absorption, commonly considered in such models, phenomena such as refraction, scattering and volume absorption were also implemented. Considering associated energy transfer, the model represents a laser beam as a stream of moving articles, i.e. photons of the same energy. When the photons meet a boundary between materials, they are reflected, absorbed or transmitted according to geometric and thermal interfacial characteristics. This paper describes the LBHM in detail, its verification and validation, and also presents several simulation examples of the entire selective laser melting process with implemented LBHM

Modeling with FCA-based model of microstructure evolution in ultra-thin wires of MgCa0.8 alloy during hot drawing

Magnesium alloys are widely applied in medicine due to their high biocompatibility and solubility in human body. For example, they could be applied for surgical threads used for integration of tissue [1,2]. This application requires wires diameter of 0.1 mm and smaller. A new manufacturing process of thin wires, including drawing in heated dies, was developed by Authors [3,4] for biocompatible Mg alloys. An occurrence of recrystallization is the main condition of such a process, which does not use intermediate annealing between the deformations. Because the trial and error method is very expensive and ineffective, a numerical modeling was applied for process design and its optimization. A model of recrystallization of MgCa0.8 alloy in macro scale was developed previously [5]. This model allows for prediction and optimization of drawing process parameters. However, basing on the results of the study [5], we conclude that some microstructural phenomena should be additionally considered in the case of ultra-thin wire drawing in the heated tools. An analysis of the effect of the wire diameter on recrystallization kinetics was an object of interest, especially when wire diameter is comparable with grain size. Study of influence of such a geometrical parameter was fulfilled with use of FCA-based model. The modelling shows that an approaching of the wire diameter to the grain size elongates the recrystallization process with other conditions the same. For example, a decrease of the diameter from 200 to 20 µm extends the recrystallization time by 30%. From the practical point of view, the results detached such a geometrical parameter can be implemented into simpler models of recrystallization, e.g. JMAK-based models

Application of Cellular Automata and Lattice Boltzmann Methods for modelling of Additive Layer Manufacturing

Purpose - The holistic numerical model based on cellular automata (CA) and Lattice Boltmann Methods (LBM) is being developed as part of an integrated modelling approach applied to study the interaction of different physical mechanisms in laser assisted additive layer manufacturing (ALM) of orthopaedic implants. Several physical events occuring in sequence or simultaneously are considered in the holistic model. They include a powder bed deposition, laser energy absorption and heating of the powder bed by the moving laser beam leading to powder melting or sintering, fluid flow in the melted pool, flow through partly or not melted material and solidification. Design/methodology/approach - The mentioned physical events are accompanied by heat transfer in solid and liquid phases including interface heat transfer at the boundaries. The sintering/melting model is being developed using LBM as an independent numerical method for hydrodynamic simulations originated from lattice-gas cellular automata (LGCA). It is going to be coupled with the CA based model of powder bed generation. Findings - The entire laser assisted ALM process has been analised and divided on several stages considering the relevant physical phenomema. The entire holistic model consisting of four interrelated submodels has currently been developed to a different extent. The submodels include the CA based model of powder bed generation, the LBM-CA based model of heat exchange and transfer, the thermal solid-liquid interface model and the mechanical solid-liquid interface model for continuous liquid flow. Practical implications – The results obtained can be used to explain the interaction of the different physical mechanisms in ALM, which is intensively developing field of advanced manufacturing of metal, non-metal and composite structural parts for instance in bio-engineering among others. The proposed holistic model is considered to be a part of the integrated modelling approach being developed as a numerical tool for investigation of the co-operative relashionships between multiphysical phenomena occurred in sequence or simultaneousely during heating of the power bed by the moving moving high energy heat source leading to selective powder sintering or melting, fluid flow in the melted pool and through partly (or not) melted material and also to solidification. The model is compatible with the earlier developed CA based model for generation of the powder bed allowing for decrease of the numerical noise. Originality/Value - The present results are original and new for the study of the complex relathionships between multifysical phenomena occurring during ALM process based on selective laser sintering or melting (SLS/SLM) including fluid flow and heat transfer among others identified as crucial for obtaining the desirable properties