Ingenieurwissenschaften und zugeordnete Tätigkeiten
Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (50)
- Vortrag (49)
- Posterpräsentation (11)
- Beitrag zu einem Tagungsband (10)
- Dissertation (1)
- Sonstiges (1)
Schlagworte
- Microstructure (122) (entfernen)
Organisationseinheit der BAM
- 9 Komponentensicherheit (40)
- 5 Werkstofftechnik (36)
- 8 Zerstörungsfreie Prüfung (23)
- 5.1 Mikrostruktur Design und Degradation (20)
- 8.5 Röntgenbildgebung (20)
- 5.2 Metallische Hochtemperaturwerkstoffe (19)
- 9.3 Schweißtechnische Fertigungsverfahren (19)
- 9.4 Integrität von Schweißverbindungen (13)
- 7 Bauwerkssicherheit (12)
- 7.1 Baustoffe (8)
Paper des Monats
- ja (1)
Influence of deposition hatch length on residual stress in selective laser melted Inconel 718
(2017)
Additive Manufacturing (AM) by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts in comparison to the traditional subtractive manufacturing strategies. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
Copolymer structure elucidation by multidimensional techniques with focus on UPLC x ESI-TOF-MS
(2017)
Structure elucidation of complex synthetic copolymers still represents a challenge. An one-dimensional separation technique cannot give the answer to the question: What are the molar mass distribution (MMD), the functionality distribution (FTD), the chemical composition distribution (CCD), the monomer sequence distribution (MSD), the topology differences within a single broad distributed polymer sample?
Since the first LC/ ESI-MS experiment of the Nobel prize winner John B. Fenn in 1984, the coupling of liquid chromatographic to mass spectrometric techniques gained a continuous rapid development.
Often the deficiencies of stand-alone methods can be bridged. LC, blind to structural information needs mass spectrometry as one of the most powerful detectors able to give detailed information on e.g. the repeat units, functionalization or copolymer composition of the chromatographic separated constituents. A separation prior to MS reduces radical the dispersity which is one of the reasons for failing of MS. Also problems with different ionization probabilities in complex mixtures can partly be overcame.
Different LC separation techniques as size exclusion chromatography (SEC), liquid adsorption chromatography (LAC), liquid chromatography at critical conditions (LCCC) and gradient elution liquid chromatography (GELC) combined with Matrix assisted Laser Desorption Ionization (MALDI) respectively Electrospray Ionization (ESI) Time of Flight (TOF) mass spectrometry are able to give information which otherwise are completely inaccessible. In some cases CID tandem mass spectrometry is applied. Fragmentation of suitable precursor ions resulted in typical fragment ion patterns. This technique enables an additional information on e.g. sequences, structural defects and topology of complex polymer mixtures.
Herein a new approach is demonstrated to provide evidence of different functionalities and short block sequences in statistical EO-PO copolymers. Furthermore silsesquioxane mixtures and Polyglycerols are investigated concerning occurring topology effects.
Magnesium (Mg) alloys have attracted much attention due to their merits of meeting requirements of lightweight, energy-efficient and environmental friendly engineering. In this study, a modified refill friction stir spot welding (refill-FSSW) method is proposed to weid AMSO Mg alloy, in which pin and sleeve rotate at different states. Effects of process parameters on the microstructure, material flow, and mechanical properties of welds were studied. Results showed that, the modified refill-FSSW technology could enhance the intennixing of material by changing the flow state. Lap shear strength of welds could be significantly improved with changed failure modes. The modified refill FSSW technology is a competitive welding method for Mg alloy.
The present work describes the shear creep behavior of the superalloy LEK 94 at temperatures between 980 and 1050 °C and shear stresses between 50 and 140 MPa for loading on the macroscopic crystallographic shear system (MCSS) (011)[01-1]. The strain rate versus strain curves show short primary and extended secondary creep regimes. We find an apparent activation energy for creep of Qapp = 466 kJ/mol and a Norton-law stress exponent of n = 6. With scanning transmission electron microscopy, we characterize three material states that differ in temperature, applied stress, and accumulated strain/time. Rafting develops perpendicular to the maximum principal stress direction, gamma channels fill with dislocations, superdislocations cut gamma' particles, and dislocation networks form at gamma/gamma' interfaces. Our findings are in agreement with previous results for high-temperature and low-stress [001] and [110] tensile creep testing, and for shear creep testing of the superalloys CMSX-4 and CMSX-6 on the MCSSs (111)[01-1] and (001)[100]. The parameters that characterize the evolving gamma/gamma' microstructure and the evolving dislocation substructures depend on creep temperature, stress, strain, and time.
Additive Manufacturing (AM) by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts in comparison to the traditional subtractive manufacturing strategies. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
Trapping in T24 steel weld joints – Effects on activation energy for hydrogen diffusion during TDA
(2017)
Failure cases in the past decade exhibited severe cracking in T24 welds and showed that generally hydrogen-assisted cracking (HAC) occurring up to 200°C cannot be excluded. A basic understanding is necessary on how hydrogen diffusion is influenced by the weld process. In this regard, both weld microstructures HAZ and weld metal have particular influence on hydrogen diffusion compared to the base material. In general, hydrogen diffusion at a certain temperature is described by diffusion coefficients representing an effective value of combined lattice diffusion and effects of reversible hydrogen traps. Those traps are typically precipitates, interstitials, grain boundaries and so on. A common approach to describe the trap character and its effect on diffusion is the determination of so-called activation energy. This can be done by respective thermal desorption analysis (TDA) with linear heating. In the present study, different T24 as-welded microstructures (BM, HAZ, WM) were investigated. For that purpose, electrochemically hydrogen charged specimens were analyzed by TDA with linear heating using a mass spectrometer for detection of ultra-low hydrogen amounts. The results showed that typically the as-welded HAZ had higher energy traps than the tempered base material. Nonetheless two important effects were ascertained: (1) it is strictly necessary to monitor the sample temperature due to its great impact on the hydrogen desorption peak temperature and (2) the real heating rate in the specimen vs. the applied heating rate has to be considered. Both influence the calculated activation energy, i.e. the assigned hydrogen trap character (moderate or strong trap), which changed up to a factor of two in terms of the calculated activation energy. This effect can be much more important compared to the microstructure effect itself. Hence, suitable experimental boundary conditions should be mandatory for the determination of hydrogen trap kinetics.
Evaluation of high spatial resolution imaging of magnetic stray fields for early damage detection
(2017)
The paper discusses the evaluation of elastic and plastic strain states in two low-carbon steels of the same steel group with high spatial resolution GMR (giant magneto resistance) sensors. The residual stress distributions of tungsten inert gas welded plates were determined by means of neutron diffraction as a reference. The normal component of local residual magnetic stray fields arise in the vicinity of the positions of maximum stress. The experiments performed on flat tensile specimen indicate that the boundaries of plastic deformations are a source of stray fields. The spatial variations of magnetic stray fields for both the weld and the tensile samples are in the order of the earths magnetic field.
The aluminum alloy 2618A is an Al-Cu-Mg alloy with additions of Fe and Ni, which was designed for long-term operation at elevated temperature in transportation and aerospace industries. Typical applications include aircraft parts and structures (sheet material) or engine components such as turbo charger centrifugal compressor wheels (forged material). Such components are subjected to prolonged aging during service, (e.g. 50 000 h) at temperatures which are close to their age hardening temperature (ca. 190 °C).
The microstructural evolution during creep exposure is studied.
The aluminum alloy 2618A is an Al-Cu-Mg alloy with additions of Fe and Ni, which was designed for long-term operation at elevated temperature in transportation and aerospace industries. Typical applications include aircraft parts and structures (sheet material) or engine components such as turbo charger centrifugal compressor wheels (forged material). Such components are subjected to prolonged aging during service, (e.g. 50 000 h) at temperatures which are close to their age hardening temperature (ca. 190 °C). The microstructural evolution was investigated.
The current study reports on the effect of the sintering temperature and secondary carbide (VC, Mo2C and TiC) additions on the microstructure and concommitant Vickers hardness (HV30) and fracture toughness of Ni-bonded NbC cermets. All cermets were prepared by pressureless sintering in vacuum. Detailed microstructural investigation was performed by electron probe microanalysis (EPMA) and X-ray diffraction (XRD) analysis. Sintering results indicated that both the sintering temperature and secondary carbide additions had a significant effect on the properties of NbC-Ni cermets. Nickel pools and residual pores were observed in the cermets sintered at temperatures ≤ 1340 °C. Increasing of the sintering temperature up to 1420 or 1480 °C resulted in fully densified NbC-Ni based cermets composed of homogeneous contrast cubic NbC grains for the single carbide (VC or Mo2C) modyfied system, whereas core-rim structured NbC grains were observed with the additon of TiC + VC or TiC+Mo2C. The secondary carbide doped cermets with 5–10 vol.% VC/Mo2C and 10 vol.% TiC showed a significantly improved hardness and fracture toughness, as compared to the plain NbC-Ni cermets.
In general, hydrogen assisted cracking is a result of a critical combination of local microstructure, mechanical load and hydrogen concentration. In that connection, welded microstructures of low-alloyed creep-resistant steels can show different hydrogen trapping kinetics. That influences the adsorbed hydrogen concentration as well as the diffusion itself in terms of moderate or strong trapping. A common approach to describe trapping is by the activation energy that is necessary to release hydrogen from a specific trap site. In the present study, T24 base material and weld metal were investigated. For that purpose, electrochemically hydrogen charged specimens were analyzed by thermal desorption analysis(TDA) with linear heating using a mass spectrometer. The results showed a microstructure effect on hydrogen trapping kinetics at elevated temperatures. Additionally, it is necessary to monitor the specimen temperature. A comparison between idealized temperature profile and real specimen temperature showed that the calculated activation energy varied up to a factor of two. Thus, the assigned trap character(moderate or strong) changed. In case of high temperature peaks, this effect could be more important compared to the microstructure effect itself.
Full densification of Fe, Co and Ni bonded NbC based cermet’s was achieved by pressure less liquid phase sintering in vacuum for one hour at 1420°C. The hardness and toughness of the NbC matrix cermet’s can be mainly tailored by the binder composition and secondary carbide additions. Ni binder based NbC cermet’s allow the combination of high hardness and improved toughness. The addition of lesser amounts of VC/Cr3C2 in a NbC partially substituted WC-Co cemented carbide increased significantly the hardness in combination with a moderate fracture toughness.
Hydrogen-assisted cracking is a critical combination of local microstructure, mechanical load and hydrogen concentration. Welded microstructures of low-alloyed creep-resistant Cr-Mo-V steels show different hydrogen trapping kinetics. This influences the adsorbed hydrogen concentration as well as the diffusion by moderate or strong trapping. A common approach to describe hydrogen traps is by their activation energy that is necessary to release hydrogen from the trap. In the present study, Cr-Mo-V steel T24 (7CrMoVTiB10-10) base material and TIG weld metal were investigated. Electrochemically hydrogen charged specimens
were analyzed by thermal desorption analysis (TDA) with different linear heating rates. The results show two different effects. At first, the microstructure effect on trapping is evident in terms of higher hydrogen concentrations in the weld metal and increased activation energy for hydrogen release. Secondly, it is necessary to monitor the real specimen temperature. A comparison between the adjusted heating rate and the real specimen temperature shows that the calculated activation energy varies by factor two. Thus, the trap character in case of the base material changes to irreversible at decreased temperature. Hence, the effect of the experimental procedure must be considered as well if evaluating TDA results. Finally, realistic temperature assessment is mandatory for calculation of activation energy via TDA.
From simple homopolymers to functionalized, 3-dimensional structured copolymers, the complexity of polymeric materials has become more and more sophisticated. With new applications for instance in the semiconductor or pharmaceutical industry, the requirements for the characterization have risen with the complexity of the used polymers. For each additional distribution, an additional dimension in analysis is needed. Small, often isomeric heterogeneities in topology or microstructure can usually not be simply separated chromatographically or distinguished by any common detector, but affect the properties of materials significantly. For a drug delivery system for example, the degree of branching and branching distribution is crucial for the formation of micelles. Instead of a complicated, time consuming and/or expensive 2d-chromatography or ion mobility spectrometry (IMS) method, that also has its limitations, in this work a simple approach using size exclusion chromatography (SEC) coupled with electrospray ionization mass spectrometry (ESI) is proposed. The online coupling allows the analysis of reconstructed ion chromatograms (RIC) of each degree of polymerization. While a complete separation often cannot be achieved, the derived retention times and peak widths lead to information on the existence and dispersity of heterogeneities. Although some microstructural heterogeneities like short chain branching can for large polymers be characterized with methods such as light scattering, for oligomers where the heterogeneities just start to form and their influence is at the maximum, they are inaccessible with these methods. It is also shown, that with a proper calibration even quantitative information can be obtained. This method is suitable to detect small differences in e. g. branching, 3d-structure, monomer sequence or tacticity and could potentially be used in routine analysis to quickly determine deviations.
A study was made on the effect of creep loading on the precipitate radii evolution of the aluminum alloy 2618A.
The overageing process of the alloy was investigated under load at a temperature of 190 °C with stresses between 79 and 181 MPa and compared to stress free isothermal ageing. The precipitates responsible for strength were characterized using dark-field transmission electron microscopy (DFTEM). This allows the experimental Determination of radii distributions of the rod-shaped Al2CuMg precipitates and the evaluation regarding their mean precipitate radius. It was found that the mean precipitate radius enables the comparison of the different microstructural conditions of crept and uncrept samples. The mean precipitate radii of the samples experiencing creep are significantly higher than those of undeformed samples. It was shown that the acquired radii distributions are viable to determine averaged particle radii for comparison of the aged samples. A ripening process including pipe diffusion along dislocations describes the data on coarsening very well for the creep samples.
Low heat input gas metal arc welding for dissimilar metal weld overlays part II: the transition zone
(2018)
Dissimilar metal weld overlays (DMWOL) of nickel base alloys on low alloy steel components are commonly used in the oil and gas, petrochemical, and power generation industries to provide corrosion and oxidation resistance in a wide range of service environments and temperatures. Traditionally, dissimilar weld overlays are produced using cold or hot wire gas tungsten arc welding. This study aims to identify and evaluate potential advantages of low heat input gas metal arc welding processes over the conventional gas tungsten arc welding in the production of dissimilar weld overlays. In order to evaluate the quality of these overlays regarding resistance against hydrogen-assisted cracking, their transition zone region is investigated in this part of the publication series. Metallurgical characterization, including energy-dispersive x-ray spectroscopy, is performed on Alloy 625/grade 22 steel overlays. The transition zone is characterized by a narrow planar growth zone and steep compositional gradients from the fusion boundary towards the weld metal. Evidence of low carbon contents in the planar growth zone, as well as for carbide precipitation in the cellular growth zone was found. The microstructure in the transition zone region of the fusion zone shows characteristics known to be suitable for good resistance against hydrogen embrittlement.
Three high-strength Nb-,.Ti- and Ti+ V-bearing S690QL steels were welded to investigate and compare the effects of microalloy addition on heat-affected zone (HAZ) toughness. Charpy V notch impact tests from three microalloyed welds under different cooling rates have been performed. Fractographic examination shows that several factors, including large-sized grain, upper bainite or hard second phase, interact to determine brittle fracture and impaired toughness in Nb-bearing weld with high heat input. In contrast to this reduced toughness, Ti-bearing welds exhibits satisfied toughness regardless of at fast or slow cooling. This is attributed to its limited austenite grain and refines favorable intragranular acicular ferrite structure. Moreover, in the case of such refined structure as matrix, TiN particles are found to be irrelevant to the facture process. The crystallographic results also confirm that high-angle boundaries between fine ferrites plates provide effective barriers for crack propagation and contribute to improved toughness.