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- Zeitschriftenartikel (30) (entfernen)
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- Additive manufacturing (9)
- Laser powder bed fusion (5)
- Additive Manufacturing (3)
- Electron backscatter diffraction (EBSD) (3)
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- AlSi10Mg alloy (2)
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- Creep (2)
- Diffraction elastic constants (2)
- Digital image correlation (DIC) (2)
- EBSD analysis (2)
- Electron backscatter diffraction (2)
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- Heat treatment (2)
- Inconel 718 (2)
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- Residual stress analysis (2)
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- A357-T6 cast aluminum alloy (1)
- A357-T6 casting (1)
- AGIL (1)
- AISI 316L (1)
- Al-3.85%Mg alloy (1)
- Aluminium alloys (1)
- As-built LPBF IN718 alloy (1)
- Bragg-edge neutron 2D imaging (BENI) (1)
- Braking load history (1)
- Cellular structures (1)
- Components influence (1)
- Compression testing (1)
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- Crystal plasticity finite element modeling (CPFEM) (1)
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- Distortion upon baseplate removal (1)
- Effect of scanning strategies (1)
- Electron microscopy (1)
- Ernel average misorientation (KAM) (1)
- Evolution of bulk properties (1)
- Fatigue crack propagation (1)
- Fatigue performance (1)
- Fractals (1)
- Fractographic observations (1)
- Friction braking (1)
- Geometric descriptor (1)
- Graphite induced hysteresis (1)
- Hystersis (1)
- IN718 PBF-LB/M (1)
- Imaging (1)
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- Kernel average misorientation (1)
- L-PBF (1)
- L-PBF IN718 material (1)
- Laboratory XµCT (1)
- Laminated metallic composites (1)
- Laser Powder Bed Fusion (1)
- Laser beam melting (1)
- Laser powder bed fusion (LPBF) (1)
- Laser-based additive manufacturing (1)
- Macroscopic stress (1)
- Mechanical behavior (1)
- Metal additive manufacturing (MAM) (1)
- Metallic matrix pad (1)
- Microstructure (1)
- Microstructure and texture (1)
- Near-surface X-ray diffraction (1)
- Neutron Diffraction (1)
- Neutron and X-ray diffraction (1)
- Nickel-based superalloys (1)
- Nonlinear stress-strain curve (1)
- Online Process Monitoring (1)
- Pore size (1)
- Powder analysis (1)
- Powder metallurgy (1)
- Power law and power-law breakdown (1)
- Power-law breakdown (1)
- Pure aluminium (1)
- Qquantification and segmentation (1)
- Re-entrant surface feature (1)
- Refractory (1)
- Residual stress in AM (1)
- Residual stress state (1)
- Residual stresses (1)
- SIF evaluation (1)
- Scan strategies (1)
- Scan strategy influence (1)
- Scanning electron microscopy (1)
- Scanning electron microscopy (SEM) (1)
- Semi-metallic brake-pad material (1)
- Semi-metallic sintered material (1)
- Si network disintegration (1)
- Sintering (1)
- Stainless Steel (1)
- Statistical image analysis (1)
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- Steady-state creep (1)
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- Strain-free lattice references (1)
- Strain-free lattice spacing (1)
- Stress balance (1)
- Stress balance condition (1)
- Surface roughness analysis (1)
- Synchrotron X-ray diffraction (1)
- Synchrotron X-ray refraction radiography (1)
- Synchrotron computed tomography (1)
- Tensile testing (1)
- Thermally induced porosity (TIP) (1)
- Thermography (1)
- Ti-6Al-4V (1)
- Tiatanium (1)
- Torsional in situ fatigue testing (1)
- Toughening mechanisms (1)
- Uniaxial compression tests (1)
- X-ray Diffraction (1)
- X-ray analysis (1)
- X-ray and Neutron Diffraction (1)
- X-ray and neutron diffraction (1)
- X-ray refraction (1)
- X-ray synchrotron tomography (1)
- pure aluminum (1)
- subgrain (1)
Organisationseinheit der BAM
- 8 Zerstörungsfreie Prüfung (30) (entfernen)
Due to the complexity of friction materials, the characterization of the tribological properties is prioritised over the bulk material properties even though the tribology is expected to be influenced by the material behaviour. The extent of this relationship is still unknown and further knowledge is required to account for the load history and evolution of the bulk properties. With this view, the compression behaviour and microstructure of a semi-metallic friction material with reduced formulation were investigated before and after a braking program. The thermal loading was monitored with inserted thermocouples. Uniaxial compression tests coupled with Digital Image Correlation (DIC) show significant changes in the worn material, which develops a compression behaviour similar to that of a tri-layered material. The microstructural analysis indicates microcracking of the metallic matrix and carbon diffusion in the Fe-phase. The thermal loading was found to be the key parameter controlling both the friction behaviour and evolution of the material properties. The expected effects of material evolution on the contact uniformity, durability and tribology are discussed.
The quality of components made by laser beam melting (LBM) additive manufacturing is naturally influenced by the quality of the powder bed. A packing density <1 and porosity inside the powder particles lead to intrinsic voids in the powder bed. Since the packing density is determined by the particle size and shape distribution, the determination of these properties is of significant interest to assess the printing process. In this work, the size and shape distribution, the amount of the particle’s intrinsic porosity, as well as the packing density of micrometric powder used for LBM, have been investigated by means of synchrotron X-ray computed tomography (CT). Two different powder batches were investigated: Ti–6Al–4V produced by plasma atomization and stainless steel 316L produced by gas atomization. Plasma atomization particles were observed to be more spherical in terms of the mean anisotropy compared to particles produced by gas atomization. The two kinds of particles were comparable in size according to the equivalent diameter. The packing density was lower (i.e., the powder bed contained more voids in between particles) for the Ti–6Al–4V particles. The comparison of the tomographic results with laser diffraction, as another particle size measurement technique, proved to be in agreement.
Connecting Diffraction-Based Strain with Macroscopic Stresses in Laser Powder Bed Fused Ti-6Al-4V
(2020)
The laser powder bed fusion (LPBF) production process often results in large residual stress (RS) in the parts. Nondestructive techniques to determine RS are badly needed. However, a reliable quantification of macro-RS (i.e., stress at the component level) by means of diffraction-based techniques is still a great challenge, because the link between diffraction-based strain and macro-RS is not trivial. In this study, we experimentally determine (by means of in-situ synchrotron radiation diffraction) this link for LPBF Ti-6Al-4V. We compare our results with commonly used models to determine the so-called diffraction elastic constants (DECs). We show that LPBF materials possess different DECs than wrought alloys, simply because their microstructural and mechanical properties are different. We also show that the existing models can be used to calculate DECs only if high accuracy of the RS values is not required. If the peculiarities of the microstructure have to be taken into account (as is the case of additively manufactured materials), a radically new approach is desirable.
The high-cycle fatigue, small crack propagation behavior of an A357-T6 cast aluminum alloy is investigated. Laboratory X-ray micro-computed tomography (μCT) is used to assist in the manufacturing of two flat fatigue specimens containing subsurface shrinkage pores of different sizes (Pore 1 √A=522μm against Pore 2 √A=280μm). Surface crack monitoring is performed by means of optical microscopy and the cracked specimens are analyzed via scanning electron microscopy and electron backscatter diffraction techniques. The subsurface pores tend to induce intergranular crack nucleation, principally when the grain boundaries are oriented perpendicular to the loading direction. Pore 1 induces a fatigue life reduction of 500.000 cycles when compared to Pore 2. The crystallography is able to influence small crack propagation by slightly decelerating the crack growth rates as well as by altering the crack path topography. Tailoring of the crystallography for improved fatigue resistance requires an investigation of the optimal largest defect to grain size ratio.
The mechanisms controlling the fatigue response of an A357-T6 cast aluminium alloy under cyclic torsional loading are investigated. Surface crack monitoring coupled with Electron BackScattering Diffraction (EBSD) analysis is used to study crack initiation. Determination of S-N curve combined with interrupted in situ fatigue testing using synchrotron tomography allows the study of the propagation behaviour. It is observed that fractographic morphologies depend on the stress level. At intermediate-low stress levels ( τmax < 100 MPa), the grain structure controls the crack initiation and propagation periods. Cracks are usually nucleated in mode II from slip planes close to the specimen axis or perpendicular to it. Mode II crack growth dominates the early stages of crack propagation as mode III inward crack growth is rapidly decelerated. This behaviour leads to the formation of characteristic shallow surface cracks. Once the crack is long enough for the mode II driving force to be overtaken by mode III, propagation branches into ~45 ◦mode I.
The relationship between residual stresses and microstructure associated with a laser powder bed fusion (LPBF) IN718 alloy has been investigated on specimens produced with three different scanning strategies (unidirectional Y-scan, 90° XY-scan, and 67° Rot-scan). Synchrotron X-ray energy-dispersive diffraction (EDXRD) combined with optical profilometry was used to study residual stress (RS) distribution and distortion upon removal of the specimens from the baseplate. The microstructural characterization of both the bulk and the nearsurface regions was conducted using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). On the top surfaces of the specimens, the highest RS values are observed in the Y-scan specimen and the lowest in the Rot-scan specimen, while the tendency is inversed on the side lateral surfaces. A considerable amount of RS remains in the specimens after their removal from the baseplate, especially in the Y- and Z-direction (short specimen Dimension and building direction (BD), respectively). The distortion measured on the top surface following baseplate thinning and subsequent removal is mainly attributed to the amount of RS released in the build direction. Importantly, it is observed that the additive manufacturing microstructures challenge the use of classic theoretical models for the calculation of diffraction elastic constants (DEC) required for diffraction-based RS analysis. It is found that when the Reuß model is used for the calculation of RS for different crystal planes, as opposed to the conventionally used Kröner model, the results exhibit lower scatter. This is discussed in context of experimental measurements of DEC available in the literature for conventional and additively manufactured Ni-base alloys.
The effect of two types of scanning strategies on the grain structure and build-up of Residual Stress (RS) has been investigated in an as-built IN718 alloy produced by Laser Powder Bed Fusion (LPBF). The RS state has been investigated by X-ray diffraction techniques. The microstructural characterization was performed principally by Electron Backscatter Diffraction (EBSD), where the application of a post-measurement refinement technique enables small misorientations (< 2°) to be resolved. Kernel average misorientation (KAM) distributions indicate that preferably oriented columnar grains contain higher levels of misorientation, when compared to elongated grains with lower texture. The KAM distributions combined with X-ray diffraction stress maps infer that the increased misorientation is induced via plastic deformation driven by the thermal stresses, acting to self-relieve stress. The possibility of obtaining lower RS states in the build direction as a consequence of the influence of the microstructure should be considered when envisaging scanning strategies aimed at the mitigation of RS.
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses.
Laser-based additive manufacturing methods allow the production of complex metal structures within a single manufacturing step. However, the localized heat input and the layer-wise manufacturing manner give rise to large thermal gradients. Therefore, large internal stress (IS) during the process (and consequently residual stress (RS) at the end of production) is generated within the parts. This IS or RS can either lead to distortion or cracking during fabrication or in-service part failure, respectively. With this in view, the knowledge on the magnitude and spatial distribution of RS is important to develop strategies for its mitigation. Specifically, diffraction-based methods allow the spatial resolved determination of RS in a non-destructive fashion. In this review, common diffraction-based methods to determine RS in laser-based additive manufactured parts are presented. In fact, the unique microstructures and textures associated to laser-based additive manufacturing processes pose metrological challenges. Based on the literature review, it is recommended to (a) use mechanically relaxed samples measured in several orientations as appropriate strain-free lattice spacing, instead of powder, (b) consider that an appropriate grain-interaction model to calculate diffraction-elastic constants is both material- and texture-dependent and may differ from the conventionally manufactured variant. Further metrological challenges are critically reviewed and future demands in this research field are discussed.
Layer-by-layer additive manufacturing (AM) by means of laser-powder bed Fusion (L-PBF) offers many prospects regarding the design of lattice structures used, for example, in gas turbines. However, defects such as bulk porosity, Surface roughness, and re-entrant features are exacerbated in nonvertical structures, such as tilted struts. The characterization and quantification of these kinds of
defects are essential for the correct estimation of fracture and fatigue properties.
Herein, cylindrical struts fabricated by L-PBF are investigated by means of X-ray computed tomography (XCT), with the aim of casting light on the dependence of the three kinds of defects (bulk porosity, surface roughness, and re-entrant features) on the build angle. Innovative analysis methods are proposed to correlate shape and position of pores, to determine the angular-resolved Surface roughness, and to quantify the amount of re-entrant surface features, q. A meshing of the XCT surface enables the correlation of q with the classical Surface roughness Pa. This analysis leads to the conclusion that there is a linear correlation between q and Pa. However, it is conjectured that there must be a threshold of surface roughness, below which no re-entrant features can be build.