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In situ ED XRD und Radiographie während eines Zugversuchs an wasserstoffbeladenem Supermartensit
(2019)
Die Ergebnisse von in-situ Experimenten am Berliner Elektronenspeicherring (BESSY II) werden vorgestellt. Es wurden Zugversuche an wasserstoffbeladenen und wasserstofffreien supermartensitischen Proben durchgeführt und Diffraktionsspektren und radiographische Bilder aufgenommen. Die energiedispersive Röntgendiffraktion ermöglichte dabei die dehnungsinduzierte Phasenumwandlung von Restaustenit zu Martensit in-situ zu beobachten. Die Radiographiebilder der zerreisenden Probe gaben Einblicke in das Bruchverhalten in Abhängigkeit vom Wasserstoffgehalt des Supermartensits.
Chemical composition of the welding seam and HAZ has significant effect on weld quality.
Current inspection methods for weld seams measure and investigate post factum and cannot prevent weld defects. Element burn-off may significantly affect solidification leading to embrittlement or cracking. Control of weld chemical compositions insures a controlled solidification and resistance to related weld defects. “Industry 4.0” is the current trend of automation and data exchange in manufacturing technologies. With this trend “intelligent” welding machines will be developed. LIBS can provide significantly important online data for such systems.
Laser powder bed fusion (L-PBF) is one of the most promising additive manufacturing (AM) technologies for the production of complex metallic real part components. Due to the multitude of factors influencing process conditions and part quality and due to the layer-wise characteristic of the process, monitoring of process signatures seems to be mandatory in case of the production of safety critical components. Here, the iterative process nature enables unique access for in-situ monitoring during part manufacture. In this talk, the successful test of the synchronous use of a high-frequency infrared camera and a camera for long time exposure, working in the visible spectrum (VIS) and equipped with a near infrared filter (NIR), will be introduced as a machine manufacturer independent thermal detection monitoring set-up. Thereby, the synchronous use of an infrared camera and a VIS NIR camera combines the advantages of high framerate and high spatial resolution. The manufacture of a 316L stainless steel specimen, containing purposely seeded defects and volumes with forced changes of energy inputs, was monitored during the build. The measured thermal responses are analysed and compared with a defect mapping obtained by micro X-ray computed tomography (CT).
The first results regarding methods for data analysis, derived correlations between measured signals and detected defects as well as sources of possible data misinterpretation are presented in this talk.
Residual stresses are crucial when assessing the performance of welded components. The present work deals with the possibilities of transferring the real-life boundary conditions of welding, which influence the residual stress, into the laboratory. The possibilities of a test system with a load capacity of 2 MN specifically developed for online monitoring of stress formation and cracking are shown. Due to the structural design, global process, geometry and material-dependent stresses are induced, which can be quantified in-situ during welding and post weld heat treatment. Examples are presented how the conditions to be found during production are simulated in the laboratory. It is shown how welding residual stresses in high-strength steels are affected by the heat control. Elevated working temperatures significantly increase the tensile residual stresses in the heat affected zone (HAZ). The effect of mechanical stresses resulting from welding on stress relief cracking is demonstrated by the example of a creep resistant steel. Reheat cracks were monitored online during post weld heat treatment.
Im Rahmen des Themenfeld Projektes „Process Monitoring of AM“ (ProMoAM) evaluiert die BAM gegenwärtig die Anwendbarkeit verschiedenster ZfP-Verfahren, darunter die Thermografie, zur Prozessüberwachung in der additiven Fertigung von Metallen in Hinblick auf die Qualitätssicherung.
In diesem Beitrag werden SWIR-Thermografiemessungen während des Bauprozesses mittels Laser Pulver Auftragsschweißen (LPA) vorgestellt. Eine Herausforderung im Rahmen dieser Messungen liegt in der Positionierung der Kamera, welche entweder fixiert am Schweißarm, also mitbewegt, oder fixiert in der Baukammer, also stationär, erfolgen kann, wobei beide Varianten mit individuellen Vor- und Nachteilen verbunden sind. Eine stationäre Befestigung der Kamera ermöglicht zwar eine einfachere Zuordnung der Messdaten zu der jeweiligen Position im Bauteil, führt jedoch bei komplexeren Geometrien zwangsläufig zu Problemen durch Abschattungen und zu defokussierten Bereichen. Zur Auswertung von Thermogrammen, welche durch eine mit dem Schweißarm mitbewegte Kamera aufgenommen wurden, sind hingegen für jedes Bild akkurate Positionsdaten der Kamera nötig um die Messdaten einer Position im Bauteil zuzuordnen. Da die Positionsdaten des Schweißarmes im allgemeinen Fall durch die Anlagensoftware nicht zur Verfügung gestellt werden, muss diese Information durch zusätzliche Messungen aufgezeichnet werden. Hierzu verwenden wir einen an der Kamera befestigten Beschleunigungssensor. Dieser ermöglicht einen zeitlichen Abgleich mit dem vorprogrammierten Verfahrweg des Schweißarmes, welcher im Allgemeinen noch Unsicherheiten bezüglich genauer Geschwindigkeiten und Beschleunigungen offenlässt. Weiterhin untersuchen wir den Einfluss des empfindlichen Spektralbereiches der IR-Kamera durch den Vergleich von Messungen mit verschiedenen schmalbandigen Bandpassfiltern (25 nm FWHM) in einem Bereich von 1150 nm bis 1550 nm.
By allowing economic on demand manufacturing of highly customized and complex workpieces, metal based additive manufacturing (AM) has the prospect to revolutionize many industrial areas. Since AM is prone to the formation of defects during the building process, a fundamental requirement for AM to become applicable in most fields is the ability to guarantee the adherence to strict quality and safety standards. A possible solution for this problem lies in the deployment of various in-situ monitoring techniques. For most of these techniques, the application to AM is still very poorly understood. Therefore, the BAM in its mission to provide safety in technology has initiated the project “Process Monitoring of AM” (ProMoAM). In this project, a wide range of in-situ process monitoring techniques, including active and passive thermography, optical tomography, optical emission and absorption spectroscopy, eddy current testing, laminography, X-ray backscattering and photoacoustic methods, are applied to laser metal deposition (LMD), laser powder bed fusion and wire arc AM. Since it is still unclear which measured quantities are relevant for the detection of defects, these measurements are performed very thoroughly. In successive steps, the data acquired by all these methods is fused and compared to the results of reference methods such as computer tomography and ultrasonic immersion testing. The goal is to find reliable methods to detect the formation of defects during the building process. The detailed acquired data sets may also be used for comparison with simulations.
Here, we show first results of high speed (> 300 Hz) thermographic measurements of the LMD process in the SWIR range using 316L as building material. For these experiments, the camera was mounted fixed to the welding arm of the LMD machine to keep the molten pool in focus, regardless of the shape of the specimen. As the thermograms do not contain any information about the current spatial position during the building process, we use an acceleration sensor to track the movement and synchronize the measured data with the predefined welding path. This allows us to reconstruct the geometry of the workpieces and assign the thermographic data to spatial positions. Furthermore, we investigate the influence of the acquisition wavelength on the thermographic data by comparing measurements acquired with different narrow bandpass filters (50 nm FWHM) in a spectral range from 1150 nm to 1550 nm.
This research was funded by BAM within the Focus Area Materials.
In this study, the influence of the welding speed and the arc power on the solidification crack formation for partial penetration laser hybrid welded Thick-Walled plates were investigated. Experimentally, a linear correlation between the welding velocity and the crack number was observed. That is by reducing the welding velocity the crack number was reduced.
The reduced welding velocity showed a strong impact on stress, as the model demonstrated a very lower stress amount in comparison to the reference case. The reduction of the welding speed could be a helpful technique to reduce the hot cracking. The wire feed speed showed a very slight influence on the crack formation. That can be returned to the large distance between the critical region for cracking and the arc region.
Laser beam welding of aluminum die casting is challenging. A large quantity of gases (in particular hydrogen) is absorbed by aluminum during the die-cast manufacturing process and is contained in the base material in solved or bound form. After re-melting by the laser, the gases are released and are present in the melt as pores. Many of these metallurgic pores remain in the weld seam as a result of the high solidification velocities. The natural (Archimedean) buoyancy is not sufficient to remove the pores from the weld pool leading to process instabilities and poor mechanical properties of the weld. Therefore, an electromagnetic (EM) system is used to apply an additional buoyancy component to the pores. The physical mechanism is based on the generation of Lorentz forces, whereby an electromagnetic pressure is introduced into the weld pool. The EM system exploits the difference in electrical conductivity between poorly conducting pores (inclusions) and the comparatively better conducting aluminum melt to increase the resulting buoyancy velocity of the pores. Within the present study, the electromagnetic supported degassing is investigated in dependence on the laser beam power, welding velocity and electromagnetic flux density. By means of a design of experiments a systematic variation of these parameters is carried out for partial penetration laser beam welding of 6 mm thick sheets of wrought aluminum alloy AlMg3 and die-cast aluminum alloy AlSi12(Fe) where the wrought alloy serves as a reference. The proportion of pores in the weld seams is determined using X-ray images, computed tomography (CT-) images and cross-section images. The results prove a significant reduction of the porosity up to 70 % for both materials as a function of the magnetic flux density
Over the past years economic and environmental considerations have led to a markedly increased demand for efficiency and flexibility in petrochemical plants. The operational temperatures and pressures required today can only be achieved by using new creep-resistant grades of steel. The modified 13CrMoV9-10 vanadium steel shows a good resistance against creep and compressed hydrogen and has been in use for the construction of petrochemical reactors since the mid-1990s. Nevertheless, processing of this type of steel requires extreme care during the welding procedure. This is due to its low toughness and high strength in the welded state when not post weld heat treated combined with increased susceptibility to cracking during stress relaxation. Previous research into crack formation in creep-resistant steels has largely focused on thermal and metallurgical factors; however, little knowledge has been gathered regarding the influence of the welding procedure on crack formation during post weld heat treatment considering real-life manufacturing conditions. The influence of heat control on the mechanical properties has been investigated by simulating the welding and subsequent post weld heat treatment operations during the construction of petrochemical reactors using a special 3-D testing facility on the laboratory scale.
This work is subdivided in two parts. In part I of this study the stresses resulting from preheating, welding, dehydrogenation heat treatment and the final post weld heat treatment were analyzed during experiments under varied heat control. In all experiments stress relief cracks formed during post weld heat treatment could be observed. The total crack lengths correlated with the welding induced stresses.
Part II of this work is dedicated to the characterization of the cracks and the microstructure. The application of a special acoustic emission analysis indicated that the cracks formed in a temperature range between 300 °C and 500 °C during the post weld heat treatment. In comparison to small scale specimens welded without additional shrinkage restraint, the toughness of the restrained welds was significantly decreased. SEM and TEM analyses of all samples revealed accelerated aging due to early precipitation of special carbides during post weld heat treatment under component relevant restraint.
Over the past years economic and environmental considerations have led to a markedly increased demand for efficiency and flexibility in petrochemical plants. The operational temperatures and pressures required today can only be achieved by using new creep-resistant grades of steel. The modified 13CrMoV9-10 vanadium steel shows a good resistance against creep and compressed hydrogen and has been in use for the construction of petrochemical reactors since the mid-1990s. Nevertheless, processing of this type of steel requires extreme care during the welding procedure. This is due to its low toughness and high strength in the welded state when not post weld heat treated combined with increased susceptibility to cracking during stress relaxation. Previous research into crack formation in creep-resistant steels has largely focused on thermal and metallurgical factors; however, little knowledge has been gathered regarding the influence of the welding procedure on crack formation during post weld heat treatment considering real-life manufacturing conditions.
In this work, the influence of heat control on the mechanical properties has been investigated by simulating the real-life manufacturing conditions prevailing during the construction of petrochemical reactors using a special 3-D testing facility. The stresses resulting from preheating, welding, dehydrogenation heat treatment and the final post weld heat treatment were measured during experiments under varied heat control. In all experiments stress relief cracks formed during post weld heat treatment could be observed. The total crack lengths correlated with the stresses due to welding. The application of a special acoustic emission analysis indicated that the cracks formed during post weld heat treatment in a temperature range between 300 °C and 500 °C. In comparison to small scale samples welded without additional shrinkage restraint, the toughness of the restrained welds was significantly decreased. SEM and TEM analyses of all samples revealed accelerated aging due to early precipitation of special carbides during post weld heat treatment under component relevant restraint.
Over the past years economic and environmental considerations have led to a markedly increased demand for efficiency and flexibility in petrochemical plants. The operational temperatures and pressures required today can only be achieved by using new creep-resistant grades of steel. The modified 13CrMoV9-10 vanadium steel shows a good resistance against creep and compressed hydrogen and has been in use for the construction of petrochemical reactors since the mid-1990s. Nevertheless, processing of this type of steel requires extreme care during the welding procedure. This is due to its low toughness and high strength in the welded state when not post weld heat treated combined with increased susceptibility to cracking during stress relaxation. Previous research into crack formation in creep-resistant steels has largely focused on thermal and metallurgical factors; however, little knowledge has been gathered regarding the influence of the welding procedure on crack formation during post weld heat treatment considering real-life manufacturing conditions. The influence of heat control on the mechanical properties has been investigated by simulating the welding and subsequent post weld heat treatment operations during the construction of petrochemical reactors using a special 3-D testing facility on the laboratory scale.
This work is subdivided in two parts. In part I of this study the stresses resulting from preheating, welding, dehydrogenation heat treatment and the final post weld heat treatment were analyzed during experiments under varied heat control. In all experiments stress relief cracks formed during post weld heat treatment could be observed. The total crack lengths correlated with the welding induced stresses.
Part II of this work is dedicated to the characterization of the cracks and the microstructure. The application of a special acoustic emission analysis indicated that the cracks formed in a temperature range between 300 °C and 500 °C during the post weld heat treatment. In comparison to small scale specimens welded without additional shrinkage restraint, the toughness of the restrained welds was significantly decreased. SEM and TEM analyses of all samples revealed accelerated aging due to early precipitation of special carbides during post weld heat treatment under component relevant restraint.
Because of its excellent corrosion resistance, a high tensile strength together with a high ductility, duplex stainless steel 2205 offers many areas of application in the chemical and the offshore industry, to name just two. Though welding, especially laser beam welding accompanied by high cooling rates, duplex steels tend to perform higher ferrite contents in weld metal upon cooling down from melting temperature as the base metal. This trend leads to a reduction of the ductility as well as the corrosion resistance of the weld joint. To overcome this problem a solution, based on buffering the plate edges by laser metal deposition with material containing higher Ni concentrations prior to the laser welding was suggested. This method offers more benefits in comparison to conventional usage of higher Ni-alloyed filler wire due to the better control over Ni-distribution in the weld seam, resulting in balanced austenite- ferrite ratio everywhere in the weld metal.
In this context different mixtures of duplex and nickel powder were investigated as well as different process parameters, that enable a smooth surface structure with slightly reduced ferrite contents. In a second step the possibility of welding those edges defect free with standard parameters while achieving balanced austenite- ferrite ratio was verified with metallographic analysis of the microstructure, Electron Backscatter Diffraction (EBSD) and impact testing according to Charpy. The improved corrosion resistance of the welds in comparison to unbuffered ones was observed with the ASTM G48 standard test method.
High heat input leads to grain coarsening and softening in WM and HAZ; the tensile strength is reduced. Low heat input leads to inadmissible hardening in the WM; the impact strength is reduced.
The proposed t8/5-time of 3 s to 15 s could be achieved through the reduced welding velocity.
The concept of electromagnetic weld pool support system allowed single-pass welds in flat position without gravity drop-outs even for reduced welding speeds; in this way the heat input can be controlled.
The adaptation of the electromagnetic weld pool support system to laser and laser hybrid welding process can dramatically increase the potential field of application of these technologies for real industrial implementation.
Heute werden in Stahlkonstruktionen zunehmend hochfeste Baustähle (Streckgrenze ≥ 960 MPa) eingesetzt. Daher sind auch in Schweißnähten solche hohen Festigkeiten gefordert. Jedoch können hohe Eigenspannungen aufgrund geringerer plastischer Dehnungsreserven in diesen Schweißnähten die Sicherheit der Bauteile vermindern. Diese sind insbesondere im Zusammenhang mit Wasserstoff und Härtegefügen risskritisch. Niedrige Bauteilbeanspruchungen und Kosteneinsparungen lassen sich heutzutage mit modernen fokussierten Lichtbogenvarianten und engeren Nahtspalten erreichen. Die vorliegende Studie zeigt allerdings, dass diese modernen Schweißprozesse und die angepasste Nahtgeometrie höhere Wasserstoffkonzentrationen und signifikant gesteigerte oberflächennahe Zugeigenspannungsniveaus bedingen. Anhand mehrlagiger Schweißversuche an S960QL unter Variation von Wärmeführung, Lichtbogenvariante und Nahtöffnungswinkel wurden die Effekte und Wechselwirkungen unter Zuhilfenahme von Eigenspannungsanalytik mittels Röntgendiffraktometrie und Wasserstoffanalytik mittels Trägergasheißextraktion systematisch untersucht. Das Schweißgefüge führte unter kritischer mechanischer Beanspruchung und erhöhtem Gehalt an diffusiblem Wasserstoff insbesondere bei geringem Nahtöffnungswinkel und fokussiertem Lichtbogen zu einer deutlichen Ausbildung von Mikrorissen. Die Mikrorisse führten teils zu einer makroskopischen Ausprägung. Durch Analysen zur Nachwärmung aus der Schweißwärme heraus konnten suffiziente Wärmeführungsparameter für eine effektive Prävention solcher Risse erarbeitet werden. Die vorliegende Arbeit zeigt anhand des Interaktionssystems bei der wasserstoffunterstützten Kaltrissbildung Gefüge-Beanspruchung-Wasserstoff, welche Wärmeführungs- und Schweißparameter sich für die hochfesten Feinkornbaustähle zur Kaltrissvermeidung eignen.
Der Implant-Test nach ISO 17642-3 gehört zur Gruppe der fremdbeanspruchten Kaltrisstests, bei dem eine äußere Beanspruchung auf einen zylindrischen Stab (Implant-Probe) definiert aufgebracht wird. Mit dieser Kaltrissprüfung wurde vergleichende Analysen zur Kaltrissempfindlichkeit der modernen hochfesten Feinkornbaustähle S960MC und S960QL durchgeführt.
Heute werden in Stahlkonstruktionen zunehmend hochfeste Baustähle (Streckgrenze ≥ 960 MPa) eingesetzt. Daher sind auch in Schweißnähten solche hohen Festigkeiten gefordert. Jedoch können hohe Eigenspannungen aufgrund geringerer plastischer Dehnungsreserven in diesen Schweißnähten die Sicherheit der Bauteile vermindern. Diese sind insbesondere im Zusammenhang mit Wasserstoff und Härtegefügen risskritisch. Niedrige Bauteilbeanspruchungen und Kosteneinsparungen lassen sich heutzutage mit modernen fokussierten Lichtbogenvarianten und engeren Nahtspalten erreichen. Die vorliegende Studie zeigt allerdings, dass diese modernen Schweißprozesse und die angepasste Nahtgeometrie höhere Wasserstoffkonzentrationen und signifikant gesteigerte oberflächennahe Zugeigenspannungsniveaus bedingen. Anhand mehrlagiger Schweißversuche an S960QL unter Variation von Wärmeführung, Lichtbogenvariante und Nahtöffnungswinkel wurden die Effekte und Wechselwirkungen unter Zuhilfenahme von Eigenspannungsanalytik mittels Röntgendiffraktometrie und Wasserstoffanalytik mittels Trägergasheißextraktion systematisch untersucht. Das Schweißgefüge führte unter kritischer mechanischer Beanspruchung und erhöhtem Gehalt an diffusiblem Wasserstoff insbesondere bei geringem Nahtöffnungswinkel und fokussiertem Lichtbogen zu einer deutlichen Ausbildung von Mikrorissen. Die Mikrorisse führten teils zu einer makroskopischen Ausprägung. Durch Analysen zur Nachwärmung aus der Schweißwärme heraus konnten suffiziente Wärmeführungsparameter für eine effektive Prävention solcher Risse erarbeitet werden. Die vorliegende Arbeit zeigt anhand des Interaktionssystems bei der wasserstoffunterstützten Kaltrissbildung Gefüge-Beanspruchung-Wasserstoff, welche Wärmeführungs- und Schweißparameter sich für die hochfesten Feinkornbaustähle zur Kaltrissvermeidung eignen.
A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes’ temperature by the melting temperature.
The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g. the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.