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- 9.6 Additive Fertigung metallischer Komponenten (28) (entfernen)
Any fracture mechanics based determination of the fatigue strength of weldments requires different input information such as the local weld geometry and material data of the areas the crack is passing through during its propagation. The latter is so far not a trivial task as the fatigue crack is usually initiated at the weld toe at the transition from the weld metal to the heat affected zone and it subsequently propagates through the different microstructures of the latter to eventually grow into the base material and to cause final fracture. This paper describes how the material input information has gained particularly for heat affected zone material by thermo-mechanically simulated material specimens for two steels of quite different static strength. The data comprise the cyclic stress-strain curve, the crack closure effect-corrected crack growth characteristics, long crack fatigue crack propagation thresholds, the dependency of the parameter on the crack length and monotonic fracture resistance. The substantial experimental effort was necessary for the validation exercises of the IBESS approach, however, within the scope of practical application more easily applicable estimating methods are required. For that purpose the paper provides a number of appropriate proposals in line with its check against the reference data from the elaborate analyses.
Ferritic spheroidal graphite cast iron (SGI) materials have a remarkable technical poten-tial and economic impact in modern industry. These features are closely related to the question of how the cast materials can be produced without structural defects and graphite degenerations such as for example chunky graphite. Although the chunky graphite degeneration superficially seems to be well known, its metallurgical background is still controversially discussed, appropriate field-tested non-destructive tools for its quantification in castings are lacking and the knowledge on its impact on material properties is fairly limited. Addressing this status, the article is providing a current overview on the subject. Existing theories on formation and growth mechanisms of chunky graphite are briefly reviewed. Furthermore, from a metallurgical point of view, causes for the appearance of chunky graphite as well as preventive measures are concisely summarized. Particular attention is paid to the morphology of chunky graphite and how it can be characterized by destructive and non-destructive techniques. Special emphasis was laid on providing a comprehensive overview on the impact of chunky graphite on strength, ductility, fatigue limit, fatigue crack growth rate as well as fracture tough-ness of ferritic SGI materials based on experimental data. Moreover, conclusions for the assessment of castings affected by chunky graphite are drawn.
Additive manufacturing (AM) of metals and in particular laser powder bed fusion (LPBF) enables a degree of freedom in design unparalleled by conventional subtractive methods. To ensure that the designed precision is matched by the produced LPBF parts, a full understanding of the interaction between the laser and the feedstock powder is needed. It has been shown that the laser also melts subjacent layers of material underneath. This effect plays a key role when designing small cavities or overhanging structures, because, in these cases, the material underneath is feed-stock powder. In this study, we quantify the extension of the melt pool during laser illumination of powder layers and the defect spatial distribution in a cylindrical specimen. During the LPBF process, several layers were intentionally not exposed to the laser beam at various locations, while the build process was monitored by thermography and optical tomography. The cylinder was finally scanned by X-ray computed tomography (XCT). To correlate the positions of the unmolten layers in the part, a staircase was manufactured around the cylinder for easier registration. The results show that healing among layers occurs if a scan strategy is applied, where the orientation of the hatches is changed for each subsequent layer. They also show that small pores and surface roughness of solidified material below a thick layer of unmolten material (>200 µm) serve as seeding points for larger voids. The orientation of the first two layers fully exposed after a thick layer of unmolten powder shapes the orientation of these voids, created by a lack of fusion.
In the automotive industry, hot stamping has been established as a key technology for manufacturing safety-relevant car body components with high strength-to-weight ratio. However, hot stamping tools are stressed by cyclic thermo-mechanical loads, which leads to severe wear and high friction during the forming operation. Consequently, the quality of the parts, the durability of the tools and the efficiency of the process are negatively affected. Within the scope of this work, a promising approach named laser implantation process has been investigated for improving the tribological behavior of hot stamping tools. This technique enables the fabrication of highly wear resistant, separated and elevated micro-features by embedding hard ceramic particles into the tool via pulsed laser radiation. Hence, highly stressed tool areas can be modified, which influences the thermal and tribological interactions at the blank-die interface. To clarify these cause-effect relations, numerical simulations, quenching tests as well as tribological investigations have been conducted. In this context, laser-implanted tools reveal a significantly improved tribological performance while offering the possibility to adjust the thermal properties within hot stamping. Based on these results, a tailored tool modification can be pursued in future research work, in order to enhance the effectiveness of hot stamping tooling systems.
Laser powder bed fusion is used to create near net shape metal parts with a high degree of freedom in geometry design. When it comes to the production of safety critical components, a strict quality assurance is mandatory. An alternative to cost-intensive non-destructive testing of the produced parts is the utilization of in-situ process monitoring techniques. The formation of defects is linked to deviations of the local thermal history of the part from standard conditions. Therefore, one of the most promising monitoring techniques in additive manufacturing is thermography. In this study, features extracted from thermographic data are utilized to investigate the thermal history of cylindrical metal parts. The influence of process parameters, part geometry and scan strategy on the local heat distribution and on the resulting part porosity are presented. The suitability of the extracted features for in-situ process monitoring is discussed.
The safe fatigue design of metallic components fabricated by additive manufacturing (AM) is still a largely unsolved problem. This is primarily due to (a) a significant inhomogeneity of the material properties across the component; (b) defects such as porosity and lack of fusion as well as pronounced surface roughness of the asuilt components; and (c) residual stresses, which are very often present in the as‐built parts and need to be removed by post‐fabrication treatments. Such morphological and microstructural features are very different than in conventionally manufactured parts and play a much bigger role in determining the fatigue life. The above problems require specific solutions with respect to the identification of the critical (failure) sites in AM fabricated components. Moreover, the generation of representative test specimens characterized by similar temperature cycles needs to be guaranteed if one wants to reproducibly identify the critical sites and establish fatigue assessment methods taking into account the effect of defects on crack initiation and early propagation. The latter requires fracture mechanics‐based approaches which, unlike common methodologies, cover the specific characteristics of so‐called short fatigue cracks. This paper provides a discussion of all these aspects with special focus on components manufactured by laser powder bed fusion (L‐PBF). It shows how to adapt existing solutions, identifies fields where there are still gaps, and discusses proposals for potential improvement of the damage tolerance design of L‐PBF components
This article is an outcome of a workshop on Fatigue of Additive Manufactured Metallic Components jointly organized by the Federal Institute for Materials Research and Testing (BAM) Berlin, Germany and the National Institute of Standards and Technology (NIST) Boulder, CO, U.S.A. The aim of the workshop was a comprehensive discussion of the specific aspects of additively manufactured (AM) components in regard to failure under cyclic loading. Undoubtedly, a better understanding and the further development of approaches for damage tolerant component design of AM parts are among the most significant challenges currently facing the use of these new technologies.
This article presents a thorough overview of the workshop discussions. It aims to provide a review of the parameters affecting the damage tolerance of AM parts with special emphasis on the process parameters intrinsic to the AM technologies, the resulting defects and residual stresses. Based on these aspects, concepts for damage tolerant component design for AM are reviewed and critically discussed.
Undetected and undesired microstructural variations in components produced by laser powder bed fusion are a major challenge, especially for safety-critical components. In this study, an in-depth analysis of the microstructural features of 316L specimens produced by laser powder bed fusion at different levels of volumetric energy density and different levels of inter layer time is reported. The study has been conducted on specimens with an application relevant build height
(>100 mm). Furthermore, the evolution of the intrinsic preheating temperature during the build-up of specimens was monitored using a thermographic in-situ monitoring set-up. By applying recently determined emissivity values of 316L powder layers, real temperatures could be quantified. Heat accumulation led to preheating temperatures of up to about 600 °C. Significant differences in the preheating temperatures were discussed with respect to the individual process parameter combinations, including the build height. A strong effect of the inter layer time on the heat accumulation was observed. A shorter inter layer time resulted in an increase of the preheating temperature by more than a factor of 2 in the upper part of the specimens compared to longer inter layer times. This, in turn, resulted in heterogeneity of the microstructure and differences in material properties within individual specimens. The resulting differences in the microstructure were analyzed using electron back scatter diffraction and scanning electron microscopy. Results from chemical analysis as well as electron back scatter diffraction measurements indicated stable conditions in terms of chemical alloy composition and austenite phase content for the used set of parameter combinations. However, an increase of the average grain size by more than a factor of 2.5 could be revealed within individual specimens. Additionally, differences in feature size of the solidification cellular substructure were examined and a trend of increasing cell sizes was observed. This trend was attributed to differences in solidification rate and thermal gradients induced by differences in scanning velocity and preheating temperature. A change of the thermal history due to intrinsic preheating could be identified as the main cause of this heterogeneity. It was induced by critical combinations of the energy input and differences in heat transfer conditions by variations of the inter layer time. The microstructural variations were directly correlated to differences in hardness.
The detection of internal irregularities is crucial for quality assessment in metal-based additive manufacturing (AM) technologies such as laser powder bed fusion (L-PBF). The utilization of in-process thermography as an in situ monitoring tool in combination with post-process X-ray micro computed tomography (XCT) as a reference technique has shown great potential for this aim. Due to the small irregularity dimensions, a precise registration of the datasets is necessary as a
requirement for correlation. In this study, the registration of thermography and XCT reference datasets of a cylindric specimen containing keyhole pores is carried out for the development of a porosity prediction model. The considered datasets show variations in shape, data type and dimensionality, especially due to shrinkage and material elevation effects present in the manufactured part. Since the resulting deformations are challenging for registration, a novel preprocessing methodology is introduced that involves an adaptive volume adjustment algorithm which is based on the porosity distribution in the specimen. Thus, the implementation of a simple three-dimensional image-to-image registration is enabled. The results demonstrate the influence of the part deformation on the resulting porosity location and the importance of registration in terms of irregularity prediction.
This paper is focused on the approximation of absorbed Charpy impact energy-temperature curves (KV-T) and the management of sub-size test piece results in Charpy pendulum impact tests. Both subjects are frequently underestimated but may have a strong impact on the assessment of product quality and structural integrity. The status of related test standards, product standards, structural integrity assessment procedures and corresponding research is reviewed.
It is concluded that the state of regulation is partly insufficient and wrong. There is a wide gap between the state of knowledge in the field of materials testing and the requirements of product standards and structural integrity assessment procedures. The present paper aims at boosting and supporting the necessary discussion among the parties involved to raise the level of technical safety. Correspondingly, a large experimental database was created by Charpy impact testing of popular modern structural Steels with nominal yield strength from 235 MPa to 960 MPa. KV-T data were generated using test pieces with thicknesses between 2.5 mm and 10 mm. Different strategies for KV-T curve fitting, definition of transition temperatures, as well as upper and lower shelf energy levels were followed, and different scaling procedures were applied to the sub-size test piece results. The results show that proportional scaling of sub-size to full-size test piece results in terms of KV should be disallowed because it is a wrong approach.
Regulations still incorporating such procedures should be revised as soon as possible. Existing nonproportional scaling and conversion procedures were applied. The results obtained illustrate that neither an overall nor consistent conservatism nor an adequate conservatism level can be achieved. In summary, the standardizing bodies should enforce the discussion to revise relevant test standards considering more precise specifications on Charpy data fitting. Furthermore, proposals are made for an improved management of sub-size test piece results.