Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

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

ELECTRON FIELD-EMISSION FROM CARBON NANOTUBES FOR NANOMACHINING APPLICATIONS

The ability to pattern in the nanoscale to drill holes, to draw lines, to make circles, or more complicated shapes that span a few atoms in width is the main driver behind current efforts in the rapidly growing area of nanomanufacturing. In applications ranging from the microprocessor industry to biomedical science, there is a constant need to develop new tools and processes that enable the shrinking of devices. For this and more applications, nanomanufacturing using electron beams offers a window of opportunity as a top-down approach since electrons, unlike light, have a wavelength that is in the order of the atomic distance. Though the technology based on electron beams has been available for more than twenty years, new concepts are constantly being explored and developed based on fundamental approaches. As such, a tool that utilizes electron field-emission from carbon nanotubes was proposed to accomplish such feats. A full numerical analysis of electron field-emission from carbon nanotubes for nanomachining applications is presented. The different aspects that govern the process of electron field-emission from carbon nanotubes using the finite element method are analyzed. Extensive modeling is carried here to determine what the effect of different carbon nanotube geometries have on their emission profiles, what energy transport processes they are subject to, and establish what the potential experimental parameters are for nanomachining. This dissertation builds on previous efforts based on Monte Carlo simulations to determine electron deposition profiles inside metals, but takes them to next level by considering realistic emission scenarios. A hybrid numerical approach is used that combines the two-temperature model with Molecular Dynamics to study phase change and material removal in depth. The use of this method, allows the determination of the relationship between the amount of energy required to remove a given number of atoms from a metallic workpiece and the number of carbon nanotubes and their required settings in order to achieve nanomachining. Finally, the grounds for future work in this area are provided, including the need for novel electron focusing systems, as well as the extension of the hybrid numerical approach to study different materials

NUMERICAL INVESTIGATION AND PARALLEL COMPUTING FOR THERMAL TRANSPORT MECHANISM DURING NANOMACHINING

Nano-scale machining, or Nanomachining is a hybrid process in which the total thermal energy necessary to remove atoms from a work-piece surface is applied from external sources. In the current study, the total thermal energy necessary to remove atoms from a work-piece surface is applied from two sources: (1) localized energy from a laser beam focused to a micron-scale spot to preheat the work-piece, and (2) a high-precision electron-beam emitted from the tips of carbon nano-tubes to remove material via evaporation/sublimation. Macro-to-nano scale heat transfer models are discussed for understanding their capability to capture and its application to predict the transient heat transfer mechanism required for nano-machining. In this case, thermal transport mechanism during nano-scale machining involves both phonons (lattice vibrations) and electrons; it is modeled using a parabolic two-step (PTS) model, which accounts for the time lag between these energy carriers. A numerical algorithm is developed for the solution of the PTS model based on explicit and implicit finite-difference methods. Since numerical solution for simulation of nanomachining involves high computational cost in terms of wall clock time consumed, performance comparison over a wide range of numerical techniques has been done to devise an efficient numerical solution procedure. Gauss-Seidel (GS), successive over relaxation (SOR), conjugate gradient (CG), d -form Douglas-Gunn time splitting, and other methods have been used to compare the computational cost involved in these methods. Use of the Douglas-Gunn time splitting in the solution of 3D time-dependent heat transport equations appears to be optimal especially as problem size (number of spatial grid points and/or required number of time steps) becomes large. Parallel computing is implemented to further reduce the wall clock time required for the complete simulation of nanomachining process. Domain decomposition with inter-processor communication using Message Passing Interface (MPI) libraries is adapted for parallel computing. Performance tuning has been implemented for efficient parallelization by overlapping communication with computation. Numerical solution for laser source and electron-beam source with different Gaussian distribution are presented. Performance of the parallel code is tested on four distinct computer cluster architecture. Results obtained for laser source agree well with available experimental data in the literature. The results for electron-beam source are self-consistent; nevertheless, they need to be validated experimentally

Cutting Edge Nanotechnology

The main purpose of this book is to describe important issues in various types of devices ranging from conventional transistors (opening chapters of the book) to molecular electronic devices whose fabrication and operation is discussed in the last few chapters of the book. As such, this book can serve as a guide for identifications of important areas of research in micro, nano and molecular electronics. We deeply acknowledge valuable contributions that each of the authors made in writing these excellent chapters

Optimization of a die insert produced through metal powder bed fusion

The study described in this paper is a reference application of HPDC and AM simulation coupling the benefits of the two manufacturing processes. The thermo-mechanical performance of traditional diecasting insert is improved by conformal cooling channels. The SLM simulation validate the 3D printing of steel material and conformal channels. The cost-benefits analysis supports the decision to maximize the benefits and reducing costs