Abstract
Additive Manufacturing technologies consist a real revolution in the perception of functional components’ design and manufacturing. As a matter of fact, 3D printing is a class of different methods and technologies that share the characteristic of building a component by a CAD design through selective addition and adhesion of elemental volumes of material. AM differ from each other according to the utilized material, the physical principles that employ and the final usage of the produced component. Selective Laser Melting (SLM) is probably the most common method for manufacturing metallic components. A focused beam selectively melts the material, which has the form of a thin layer of powder. The study methods of SLM are categorized as experimental and numerical. The later are subdivided in micro-, meso-, macro- and multiscale, based on their temporal and space scale. The current Ph.D. Thesis develops and presents a modeling methodology in microscale of the SLM process by using the Finit ...
Additive Manufacturing technologies consist a real revolution in the perception of functional components’ design and manufacturing. As a matter of fact, 3D printing is a class of different methods and technologies that share the characteristic of building a component by a CAD design through selective addition and adhesion of elemental volumes of material. AM differ from each other according to the utilized material, the physical principles that employ and the final usage of the produced component. Selective Laser Melting (SLM) is probably the most common method for manufacturing metallic components. A focused beam selectively melts the material, which has the form of a thin layer of powder. The study methods of SLM are categorized as experimental and numerical. The later are subdivided in micro-, meso-, macro- and multiscale, based on their temporal and space scale. The current Ph.D. Thesis develops and presents a modeling methodology in microscale of the SLM process by using the Finite Element Method (FEM). It is divided in to 6 chapters that include the necessary review of the literature, the aim and purpose of the essay and solutions to the scientific issues that were originally set. Finally, the main conclusions are summarized and directions for further investigation are suggested. The 1st chapter introduces the basic concepts of manufacturing, including AM. A brief presentation of the manufacturing processes is followed by a history review regarding AM and a comparison between CNC machining and AM. The basic pros, cons and challenges of AM are listed. The first chapter is concluded with an overview of the most common and widely adopted AM methods. The aim is to clarify what distinguishes SLM from the rest AM technologies. The 2nd chapter focuses on the Laser Powder Bed Fusion (L-PBF), which includes SLM. The basic operation principles of a L-PBF set up are presented along with their main systems and subsystems. The Selective Laser Sintering (SLS) methods are distinguished from the SLM, by pointing out both their differences and their similarities and the various materials and alloys usually used, such as ferrous, titanium and nickel alloys, are described. Moreover, the main machining parameters and the most common defects that occur are covered. The second chapter is concluded with the classification of the SLM study approaches in experimental and numerical, with the latter being further divided in micro-, meso-, macro- and multiscale. With chapter 3 that concerns the modeling of SLM process in microscale by using FEM the literature review is completed. At first, a distinction between the modeling methodologies for heat transfer and fluid dynamics is made. Special attention is paid to which specific phenomena and physical mechanisms have to be taken into consideration since they have a significant impact on the machining results, while, at the same time, the developed model will have realistic demands in computational time and power. Moreover, the different main modes of the melting pool in SLM (conduction mode-CM, transition mode-TM and keyhole mode-KM) are listed, and which modeling methodology can be considered as optimal for the simulation of each mode is suggested. The main deduced conclusion is the wide utilization of semiempirical relations and coefficients to describe the underlying physical mechanisms and the lack of a common modeling methodology frame. Thus, the main purpose of the current Ph.D. Thesis is specified in presenting a robust modeling methodology to simulate the SLM process in microscale, with the minimum use and need of semiempirical coefficients. In the 4th chapter a heat transfer model to simulate the CM is developed and presented. The laser beam is modeled as a volumetric heat source with Gaussian distribution as to the powder bed level and the powder bed thickness. The use of semiempirical coefficients is avoided, if possible, while extra attention is paid to the exact determination of the thermophysical properties and the absorptivity of the powder bed. The validation of the model was made based on experimental results from the relevant literature. The main conclusion was that the current thermal model can accurately simulate the melting pool in CM, and predict its main dimensions and characteristics for different Volumetric Energy Densities (VED). Moreover, although the model does not solve the fluid dynamics differentials equations, it can predict the balling formation for certain machining parameters. Finally, it was deduced that the heat loss due to material ablation is the most significant, followed by the heat losses due to radiation and convection. In the 5th chapter, again, a thermal model was developed and presented to simulate the CM, TM and KM of the melting pool. The laser bean now was approached by a 2D Gaussian surface heat source, while, the change in power density due to different incident angles during the process was estimated by utilizing position vectors. The material ablation was simulated by using deformable geometry, with the material recombination factor being the only coefficient that needs to be determined. It was concluded and proposed a linear correlation between the material recombination factor and the VED, while an exponential relation of the material ablation rate as function of VED was deduced. Finally, a 2D model was developed and proposed for the quicker determination of the material ablation rate coefficient. The physical system came down to a 2D where only a plane was modeled and simulated. The moving laser beam was approached as a Gaussian 2D surface heat source with time dependent power and radius. Again, the aforementioned models were validated through experimental results from the literature. In the 6th chapter, coupling between the heat transfer and the fluid mechanics equations is made, to develop the thermohydraulic model. For keeping the computational power and time requirements in feasible level the model is again solved as 2D based on the methodology that was previously presented in chapter 5. The recoil pressure, along with the stresses due to the curvature of the free surface and the temperature gradients were considered, while the different phases of the material (solid, semifluid and liquid) was modeled by utilizing pointwise constraint and a proper smoothing function for the semifluid phase. Finally, the free surface velocity is calculated and captures not only the material ablation rate but also the fluid movement. From the simulation results it is deduced that the temperatures that are calculated by the pure thermal model are higher than the respective ones from the hydrothermal model, since in the hydrothermal model both the heat transfer due to conduction and convention are considered. The velocities are typically lower than 10 m/s, while there are some points that in certain times reach higher velocity near 20 m/s, velocity values that are in line with the respective literature. Finally, it was pointed out that FEM is not suggested as a numerical method for solving the fluid dynamics differential equations since convergence problems may occur, especially when large deformations of the free surface is taking place. In the final two sections the main conclusions that were emerged by the current Ph.D. Thesis were listed and presented in brief, while, based on these conclusions, guidelines and topics for future research were suggested.
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