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Computational Models for Non-linear Mechanical Loading Analyses of Lattice Structures Made by Laser Powder Bed Fusion
Červinek, Ondřej ; Dr. Michael R. Tucker (oponent) ; Haas/, Franz (oponent) ; Koutný, Daniel (vedoucí práce)
The development of additive technologies in recent years has enabled the manufacturing of metamaterials with porous internal architecture, called lattice structures, from several types of metal alloys. With these structures, it is possible to develop lightweight parts with potential in the field of mechanical energy absorption. Their implementation in vehicle deformation zones can increase the safety of passengers. The properties of structures allow to design absorbers with specific type of behavior which reduce the overload applied on the vehicle crew during an accident. To use these parts for specific applications, it is necessary to estimate their deformational behavior. Recent research has shown that the parent material of these structures has properties different from those of conventional bulk components produced by the same technologies. It means that, for efficient use of lattice structures, their specific properties and deformation characteristics must be accurately mathematically described. However, a mathematical model that would consider a description of all significant deformation characteristics of lattice structures is not available. Therefore, this thesis focuses on development of non-linear numerical model of lattice structures loading with inclusion of the most significant geometrical imperfections, specific properties of multi-strut samples and dynamic effects. The structures are made of aluminum alloy AlSi10Mg and stainless steel 316L using the selective laser melting technology. Two different finite element analysis approaches are used to create the geometry model that allows inspection of the deformation features in detail. The results of both models confirm that geometrical imperfections related to a change in shape and cross-sectional area of the strut have a significant impact on the resulting mechanical properties. Their inclusion in the geometry model improves the accuracy of the simulation results. Furthermore, the mechanical properties of lattice structures determined by multi-strut samples significantly better represent properties of structures for quasi-static and dynamic loading. The final parameter verification simulation of lattice structures loading at several velocities shows good agreement between the experiment and the computational solution. A similar parametrical study can lead to the finding of efficient structure configurations determined for a specific amount of absorbed energy without prior manufacturing and testing.
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Processing of metallic materials by Selective Laser Melting at elevated temperatures
Malý, Martin ; Filho, Sergio de Traglia Amancio (oponent) ; Dr. Michael R. Tucker (oponent) ; Koutný, Daniel (vedoucí práce)
This dissertation thesis deals with the influence of preheating on the components produced using Selective Laser Melting (SLM), also known as Laser Powder Bed Fusion (LPBF) technology. The thesis contains an overview of the current state of knowledge in the field of preheating and the physical nature of preheating. Furthermore, the work contains an overview of the effect of preheating on specific types of materials. These types of materials included in the state of the art are titanium, intermetallic, nickel and aluminium alloys, and copper. From the state of knowledge, promising research areas were identified, where preheating could lead to more efficient production using LPBF technology and to expansion of the area of processable materials. These areas include the investigation of the effect of preheating in combination with other process parameters on the residual stresses of Ti6Al4V alloy, the effect of preheating on nickel alloy Inconel 939 and copper. The premise of the Ti6Al4V and Inconel 939 topics was that preheating would reduce residual stresses, and thus will be possible to reduce the necessary amount of support structures. The results can lead to more cost-effective production using LPBF technology. This hypothesis was rejected. Despite the reduction in residual stresses in Ti6Al4V, they were not fully eliminated and, in addition, a rapid degradation of unused powder was detected, which increases production costs. The preheating of the Inconel 939, against the assumption based on behaviour of other materials, led to higher deformations and thus residual stresses, due to the evolution of precipitates. Another selected area where preheating could lead to an increase in the portfolio of processable materials is the processing of copper. Copper is a difficult to process material using LPBF technology due to its high thermal conductivity and laser reflectivity. The experiments confirmed a very positive effect of preheating on the relative density of the samples. The samples reached relative density values of over 99% when fabricated with preheating at 400 °C. Thus, preheating can significantly improve the process ability of reflective and high conductive materials. All of the results lead to a better understanding of the behaviour of the materials during processing by LPBF technology and may lead to its further expansion to more industries. The results are summarized in three publications that have been published in scientific journals.
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