Příspěvek k analýze zbytkového napětí povrchových a podpovrchových vrstev po 3D tisku kovů

Abstract

The presented dissertation thesis addresses the issue of residual stress in materials produced by Selective Laser Melting (SLM), specifically austenitic 316L stainless steel. With the increasing importance of additive manufacturing in industries, it has become necessary to thoroughly understand how the processes associated with additive printing affect the mechanical properties of materials, particularly in the area of residual stress generation and control. Residual stress can have a major impact on the structural stability and long-term reliability of components, making its effective management one of the main objectives in metal additive manufacturing. In the theoretical part of the thesis, the influence of different factors on residual stress generation, such as printing strategies, heat treatment and characteristics of the material itself, is discussed in detail. Extensive research has been carried out to map the existing knowledge on SLM and the mechanisms of residual stress formation during printing and subsequent finishing. This part of the thesis focuses on the physical principles of the selective laser melting process, including the melting and cooling rates of the material, which are the main factors influencing the stress states in the final products. In the context of different approaches to the theoretical description of residual stress generation, attention has been also given to analytical and numerical stress prediction methods used for manufacturing process simulations. The next part of the paper focuses on the optimization of printing parameters and strategies in terms of residual stress minimization. Additive technologies allow a wide variability in the setting of printing parameters. The selection of an appropriate printing strategy is crucial to influence the microstructure and mechanical properties of the final product, which has a direct impact on the formation and distribution of residual stresses. In this thesis, the Meander, Chessboard and Stripe printing strategies are described and analysed in terms of their influence on the residual stress s in 316L steel samples. An important part of the present thesis is the evaluation of the effect of heat treatment on residual stress reduction. Heat treatments such as annealing or quenching are often used to relax residual stresses in a material, improving its mechanical properties and overall structural integrity. In this work, different heat treatment temperatures and time regimes were investigated to determine the optimum conditions for residual stress reduction. The role of heat treatment in the context of stress relaxation and its effect on the microstructure of the material is discussed in this part of the dissertation. The experimental part of the thesis focuses on the evaluation of mechanical properties of samples produced using selected printing strategies subjected to different heat treatments. X-ray diffraction analysis was used to measure the residual stresses, which allows a non-destructive evaluation of the surface tension in the samples. Microhardness analysis was also performed to evaluate the mechanical properties of the surface layers, and the porosity of the samples was measured. These experimental methods provided deeper insight into the relationship between printing strategies, heat treatment and the resulting mechanical properties of 316L steel. The work provides new insights into the interrelationship between printing parameters, residual stress generation and mechanical properties of the material. The results of this dissertation have the potential to contribute to the optimization of metal additive manufacturing processes and can be used in industrial practice. The thesis concludes by proposing recommendations for further research in the field of additive manufacturing, which should include a deeper investigation of the influence of printing strategies and thermal treatments on the internal structure and mechanical properties of materials. Other areas of research include the optimization of printing parameters with respect to specific applications where it is important to achieve the lowest residual stress and the best possible material homogeneity.