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- 7 Bauwerkssicherheit (1)
- 7.1 Baustoffe (1)
- 8.3 Thermografische Verfahren (1)
- 8.4 Akustische und elektromagnetische Verfahren (1)
Despite of the significant advances in additive manufacturing (AM) process optimization there is still a lack of experimental results and understanding regarding the mechanical behavior and its relationship with the microstructural features of AM-parts, especially in loading conditions typical for safety-relevant applications. Within the scope of the presented ongoing investigations, a basic microstructural characterization, tensile tests at room and elevated temperature (400°C) as well as a characterization of the fatigue behavior of additively manufactured Ti-6Al-4V in the low cycle fatigue regime are carried out in the as-built state. After failure, different techniques are used to describe the failure mechanisms of the specimens. The AM-Specimens are provided by the Fraunhofer institute for production systems and design technology and investigated at the BAM following the philosophy of the TF-Project AGIL.
For a deep process understanding of the laser powder bed fusion process (PBF-LB/M), recording of the occurring surface temperatures is of utmost interest and would help to pave the way for reliable process monitoring and quality assurance. A notable number of approaches for in-process monitoring of the PBF-LB/M process focus on the monitoring of thermal process signatures. However, due to the elaborate calibration effort and the lack of knowledge about the occurring spectral directional emissivity, only a few approaches attempt to measure real temperatures. In this study, to gain initial insights into occurring in the PBF-LB/M process, measurements on PBF-LB/M specimens and metal powder specimens were performed for higher temperatures up to T = 1290 °C by means of the emissivity measurement apparatus (EMMA) of the Center for Applied Energy Research (CAE, Wuerzburg, Germany). Also, measurements at ambient temperatures were performed with a suitable measurement setup. Two different materials—stainless steel 316L and aluminum AlSi10Mg—were examined. The investigated wavelength λ ranges from the visible range (λ-VIS = 0.40–0.75 µm) up to the infrared, λ = 20 µm. The influence of the following factors were investigated: azimuth angle φ, specimen temperature TS, surface texture as for PBF-LB/M surfaces with different scan angles α, and powder surfaces with different layer thicknesses t.
The porosity and the surface roughness are recently discussed problems for SLM parts. The influence of SLM process parameters on porosity is well studied for different materials. Nevertheless, the build angle (i.e. the angle between part orientation and build plate) needs to be understood as an additional SLM process parameter, as it has been shown, that the microstructure and hence the mechanical performance of various materials depend on the build angle. The inherent build angle of each strut as a part of a lattice structure is the motivation to investigate the influence of the build angle on the porosity and roughness on round-shaped (1 mm diameter) struts by means of CT. Conventional Coordinate Measuring Machine (CMM) has the limitation towards small and round shaped samples. The need for Computed Tomography (CT) regarding investigations of SLM parts will increase because no other non-destructive technique allows the assessment of complex geometries with inner laying surfaces. We used CT to assess the pores and the strut surface. Seven struts out of the nickel alloy Inconel 625 with build angles from 30° to 90° were studied. It was found that the number of pores is smaller, and the size of pores is larger for the 90° strut. In case of 30° strut, the number of pores is increased towards down-skin side, additionally, this strut orientation showed to have the largest number of attached powder particles. The elongated pores exist exclusively near the strut surface. While the roughness at the down-skin surface is highly depending of the biud angle, the roughness at the up-skin surface is the same for all struts. The mechanisms of pore and surface roughness formation is not mainly driven by gravity.
Using non-optimum combination of manufacturing parameters in selective laser melting (SLM) may lead to reduction of quality of component: defects generation, distortion of geometry and even cracking. Usually, the optimization of parameters is performed by changing volumetric energy density (Ev) and selecting parameters giving low porosity values. However, not only low porosity but also stable microstructure and low residual stresses will help to achieve advanced mechanical behavior of the component.
In present work, we investigated cuboid-shaped Ti-6Al-4V samples produced with different manufacturing parameters. The parameters leading to the same Ev were considered as well as parameters which are not included in Ev. Residual stresses in subsurface region were investigated by synchrotron X-ray diffraction, which allows to penetrate around 100 µm from the surface therefore overcome the problem of high roughness of SLM components without additional sample preparation. Only tensile stresses were found along the building direction, that can play critical role especially during cyclic loading. In parallel, using X-ray computed tomography we also observe that porosity is mainly concentrated in the contour region, except in case where the laser speed is small. However, by using some process parameters it was possible to decrease residual stresses and obtain uniform α+β Ti microstructure and relatively low porosity. Additionally, it was found that not included in Ev (e.g., base plate position, focus distance) should be considered as additional manufacturing parameters during SLM process.
Compact nuclear magnetic resonance (NMR) instruments make NMR spectroscopy and relaxometry accessible in industrial and harsh environments for reaction characterization and process control. Robust field integration of NMR systems have to face explosion protection or integration into process control systems with short set-up times. This paves the way for industrial automation in real process environments.
The design of failsafe, temperature and pressure resistant flow through cells along with their NMR-specific requirements is an essential cornerstone to enter industrial production plants and fulfill explosion safety requirements. Additionally, if fast reactions are monitored, suitable mixing devices need to be placed in close vicinity to the measuring volume to mix the reactants properly.
NMR-specific requirements aim at full quantitative pre-magnetization and acquisition with maximum sensitivity while reducing sample transfer times and dwell-times. All parameters are individually dependent on the applied NMR instrument.
Luckily, an increasing number of applications are reported together with an increasing variety of commercial equipment. However, these contributions have to be reviewed thoroughly.
The performance of sample flow cells commonly used in online analytics and especially for low-field NMR spectroscopy was experimentally and theoretically investigated by 1H-NMR experiments and numerical simulations. Especially, the applicability of 3D printed zirconium dioxide for innovative flow cell designs was of interest. Here, we demonstrate and discuss an automated test method to determine the critical parameters of flow through cells for quantitative online NMR spectroscopy. The setup is based on randomized setpoints of flow rates in order to reduce temperature related effects. Five flow cells and tubing were assessed and compared for high-field as well as low-field NMR spectrometers.
Currently, mandatory requirements and recommendations for the detection of irregularities in laser beam welded joints are based on classic micrographs as set out in the standard ISO 13919-1:2019. Compared to classic micrographs, computed tomography enables a non-destructive, three-dimensional and material-independent mode of operation, which delivers much more profound results. Even in building material testing, methods with limited informative value can be checked and supplemented by CT examinations.
The interest to additively manufacture Nickel-based superalloys has substantially grown within the past decade both academically and industrially. More specifically, additive manufacturing processes such as laser powder bed fusion (LPBF) offer the ability to produce dense parts within a single manufacturing step. In fact, the exceptional freedom in design associated with the layer-based nature of the processes is of particular interest for the complex shapes typically required in turbine applications. In certain cases, the overall part performance can be achieved by tailoring the microstructure and the crystallographic texture to the specific application. However, these advantages must be paid at a price: the large local temperature gradients associated with the rapid melting and solidification produce parts that inherently contain large residual stress in the as-manufactured state. In addition, the presence of pores in the final part may further affect the in-service part failure. As among Nickel-based alloys Inconel 718 exhibits excellent weldability, this alloy has been widely studied in open research in the domain of LPBF. However, significant microsegregation of the heavier alloying elements such as Niobium and Molybdenum accompanied by dislocation entanglements may preclude the application of conventional heat treatment schedules. Therefore, different post processing heat treatments are required for laser powder bed fused Inconel 718 as compared to conventional variants of the same alloy.
In this study, we investigated two different heat treatment routes for LPBF Inconel 718. In a first routine, the samples were stress relieved and subsequently subjected to hot isostatic pressing (HIP) followed by a solution heat treatment and a two-step age (referred to as FHT). In a second routine, the samples were subjected to a single-step direct age post stress relieving heat treatment (referred to DA). We investigated the consequences of such heat treatment schedules on the microstructure, texture, and mechanical behavior. We show that by applying a DA heat treatment the typical columnar microstructure possessing a crystallographic texture is retained, while an equiaxed untextured microstructure prevails in case of an FHT heat treatment. We further evaluate how these heat treatments affect the mechanical behaviour on the macroscopic and microscopic scale.