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TY - THES AU - Munk, Juri TI - Einfluss der Geometrie auf die Eigenschaften der mittels pulverbettbasierten Laserschmelzen gefertigten Legierung Ti-6Al-4V PB - Rheinisch-Westfälische Technische Hochschule Aachen VL - Dissertation CY - Aachen M1 - RWTH-2024-09537 SP - 1 Online-Ressource : Illustrationen PY - 2024 N1 - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University N1 - Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2024 AB - Laser Powder Bed Fusion of Ti-6Al-4V allows manufacturing of components with complex geometric shapes from various metallic alloys, especially for lightweight applications. In LPBF, the geometry of the component defines the thermal history during the build process and therefore the microstructure and the mechanical properties. Geometrically complex shapes lead to locally different thermal histories over the component, resulting in inhomogeneous microstructures. This is in contrast to the fact that microstructural and mechanical characterization of LPBF-made material is carried out with standardized sample geometries that do not represent the geometric complexity and inhomogeneous thermal history. In the present work, the influence of the geometry on the material properties of Ti-6Al-4V was investigated. For measuring the impact of the geometry effect, the β phase fraction was selected as it depends on the thermal history on the one hand and defines the mechanical properties on the other hand. Furthermore, α lamellae width, tensile strength and fatigue life was characterized. In this work, two mechanisms were described that lead to heterogenous microstructures: Firstly, intrinsic heat treatment (IHT) that represents the thermal influence of all following layers above the point of interest. Secondly, reduced cooling rate during solidification of the point of interest and the thermal influence of the few directly following layers. The mechanisms were investigated separately by samples that have been built under conditions solely with one mechanism. The resulting morphology of the β phase matched well with literature and confirmed the described mechanisms. In order to evaluate the thermal histories of the different sample geometries finite element simulations have been carried out. In detail, super layer approach was applied which means that multiple real LPBF layers were summarized in a lumped package of one super layer. Two characteristic values were extracted from the simulated thermal histories: Holding time of the intrinsic heat treatment Δt_IHT and cooling constant k for describing the reduced cooling rate. It was shown that for the majority of the investigated sample geometries Δt_IHT is capable of predicting the intrinsic heat treatment. For evaluation of the second mechanism reduced cooling rate the characteristic value k worked for all investigated sample geometries. The time between two subsequent layers in the LPBF process is called inter layer time. It has been shown that the influence of the geometry increases by reducing the inter layer time. The mechanism of reduced cooling rate was only observed for inter layer time below 28 s but intrinsic heat treatment remained present even at the highest investigated inter layer time of 45 s for some sample geometries. For investigation of the thermal influence of directly following layers above the point of interest, sample geometries with varied number of following layers were analyzed. The first local maximum of the β phase fraction and the α lamellae width were both observed on samples that have only one single following layer above the point of interest. As a possible explanation it was assumed that the cooling rate in the relevant range from the β transus temperature is the most reduced in this specific condition. The observed morphology of the α lamellae confirmed the more dominant role of the mechanism of reduced cooling rate. For a built-up thickness of 0,6 mm above the point of interest, representing 10 following layers, the next increase of α lamellae width and β phase fraction was observed and could be explained by the mechanism of intrinsic heat treatment. The influence of geometry on the mechanical properties was also identified. Samples with fast increase of cross-sectional area in build direction were characterized by a reduced strength, both under static (ultimate tensile strength) and dynamic (fatigue life) condition. The influence of the geometry on ultimate tensile strength was observed to be present even at highest inter layer times of more than 75 s. To make the geometry influence predictable, a multiple linear regression model was established to provide a correlation of simulated thermal history with β phase fraction and tensile strength, respectively. Discretization of the thermal histories was done by use of multiple partitions of the integrated time-temperature-curve. For prediction of the β phase fraction and the ultimate tensile strength the adjusted coefficient of determination was 99.8 LB - PUB:(DE-HGF)11 DO - DOI:10.18154/RWTH-2024-09537 UR - https://publications.rwth-aachen.de/record/994774 ER -