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

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

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

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

Interfacial oxidation in processing of nanocrystallised metallic materials using duplex technique - experimental and modelling aspects

Duplex techniques are attempted to be developed combining nanocrystallisation processes with a subsequent thermomechanical processing in order to produce multilayered bulk structures with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure. However, bonding imperfections at the interfaces due to interfacial oxidation among other reasons during the duplex process can significantly influence the final properties. Moreover, the interface oxidation occurring during duplex processes influences microstructure development around the interfaces depending on whether the oxide scale is a continuous layer or a layer of discontinuous oxide clusters with heterogeneous thicknesses. Effectively the oxide scale becomes a part of microstructure development in such nano-crystallised multilayered structures. This paper deals with understanding of the underlying events around the highly reactive interfaces explaining the microstructure evolution applying advanced experimental and numerical modelling techniques. The research encompassed surface mechanical attrition treatment followed by constrained compression testing and hot rolling of the assembly of steel strips supported by multilevel numerical analysis using combined finite element (FE) and cellular automata (CA) methods. Shear banding has been observed near metal-metal contact between the oxide clusters at the interfaces. The shear banding can be considered as bonding enhancement creating channels for the base metal of the different laminates to come into contact through the oxidised interface. Temperature, texture and grain refining are among the factors influencing the shear banding. In the simulations, the meso-level of the developed multi-level FE-based model is combined with the advanced 3D frontal CA numerical model allowing for both appearance of the new boundaries and rotation of dislocation cells (sub-grains and grains) simultaneously

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

Modelling of Transient Temperature Field and Phase Transformation Change: A way for Residual Stress Management in Large Scale Forgings

The paper is devoted to development of the modelling approach based on 3D finite-element (FE) analysis of the transient temperature fields and the thermally induced phase transformations as a way towards residual stress management in large size forgings. Heating, holding and cooling stages are under consideration and modelling of both the austenite formation and decomposition are taken into account. The thermal-mechanical FE model capable of taking into account changes in the specific volume during ferrite/austenite transformation is coupled with the relevant phase transformation model in order to allow simulation of the transient stresses due to both thermal contraction and the dilatometric effect. The model is capable of taking into account different boundary conditions for the heat transfer problem based on the available data. To improve the predictive abilities, the following two commercial FE codes, such as MSC Marc 2013.1.0 and Abaqus/Standard 6.12, are used for solving the non-steady state 3D problem of the metal expansion/contraction during consecutive heating, holding and cooling stages. Although all the mentioned process steps are considered, the model is dedicated to be used for modelling the cooling stages of large forgings and castings

Analysis of the porosity degree during laser-assisted cladding of bioactive glass on titanium substrates with highly refined grain structure

Titanium alloys, due to their exceptional mechanical properties and biocompatibility, are commonly used to produce medical implants nowadays. However, the presence of such elements as aluminium and vanadium can be harmful to human health. One of the possible solutions could be replacing the titanium alloys with commercially pure titanium (cpTi) with highly refined grain structure. One of the most promising methods in manufacturing medical implants with improved biological fixation is laser cladding in which bioactive glass coatings are imposed on metallic substrates. The aim of this work is to present a 3D numerical modelling of the above mentioned additive manufacturing process. The obtained model is able to predict the stress-strain and temperature distributions as well as porosity degree during the processing. Porosity affects the bioactivity of medical implants as it significantly improves their ability to bonding with host tissues