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Peculiarities of the process of orbital laser-arc welding of thick-walled large-diameter pipes
(2010)
New brilliant high-power lasers such as disc laser and fibre laser open up new fields of applications for laser-hybrid welding, e.g. power generation, shipbuilding and pipeline construction. For the use of laser-hybrid welding in pipe laying, insensibility towards tolerances and the possibility of orbital welding are very important factors in addition to the weld seam quality.
Within the scope of a basic research project, first promising results for laser-hybrid welding of thick materials could be demonstrated. As base material, the typical pipeline steel API 5L X65 was selected. With the help of a 20 kW fibre laser in combination with an arc-welding process it was possible to produce high-quality welds in plates of up to 20 mm thickness in a single pass and of up to 32 mm in three to five passes, both welded in position PA. Various joint preparations were examined to weld 20 mm thick plates in one pass. Besides different welding positions, gap bridgeability and misalignment were studied. Results for microhardness and Charpy toughness are presented to proof the applicability of laser-hybrid welding for joining in pipe laying.
Continuous cooling transformation (CCT) behavior affects the transient state of
material properties employed in a numerical welding simulation, having a direct influence
on the developing stress state. Three different CCT diagrams for S355J2 steel
were employed to understand the influence of variations in CCT behavior on the numerical
calculation of welding-induced residual stresses. The CCT diagrams were
constructed from transformation data contained in the Sysweld software database,
measured dilatometric data from Gleeble experiments, and transformation data calculated
from the JMatPro software. The calculated transverse and longitudinal residual
stress distributions provided a qualitative correction only in comparison to experimental
measurements, with the largest deviation occurring near the weld
interface. Overall, the results indicate a weak dependency of the calculated residual
stresses due to anticipated CCT variations. The most significant effect on the calculated
residual stresses was shown to be related to the proportion of formed martensite.
It is suggested that CCT data of approximate accuracy is sufficient for reliable
calculation of welding-induced residual stresses.
Numerical simulation of component and assembly behaviour under different loading conditions is a main tool for safety design in automobile body shell mass production. Knowledge of local material behaviour is fundamental to such simulation tests. As a contribution to the verification of simulation results, the local deformation properties of spot-welded similar and dissimilar material joints in shear tension tests were investigated in this study for a TRIP steel (HCT690T) and a micro-alloyed steel (HX340LAD). For this reason, the local strain distribution was calculated by the digital image correlation technique (DIC). On the basis of the hardness values and microstructure of the spot welds, the differences in local strain between the selected material combinations are discussed. Additionally, the retained austenite content in the TRIP steel was analysed to explain the local strain values. Results obtained in this study regarding similar material welds suggest significant lower local strain values of the TRIP steel HCT690T compared to HX340LAD. One reason could be the decrease of retained austenite in the welded area. Furthermore, it has been ascertained that the local strain in dissimilar material welds decreases for each component compared with the corresponding similar material weld.
A method is reported with which heat affected zone (HAZ) cracks in a specific region of spot welded joints, made of advanced high strength steels, could be realised. The influence of HAZ cracks on the fatigue behaviour, which is the focus of the contribution, was studied by evaluating the stiffness characteristics as a function of the number of cycles N in comparison with crack-free specimens, i.e. by analysing HAZ and fatigue cracks on the specimen surface and the surface of fatigue fractures containing beach marks. It was found that the fatigue cracks are not initiated at the HAZ cracks. The number of cycles to failure Nf is not significantly reduced for specimens with HAZ cracks. Overall, the examinations did not reveal any influence of HAZ cracks on the fatigue strength of resistance spot welded joints.
Welding simulation is a powerful, cost-efficient tool to predict welding induced distortion. Nevertheless, effects on calculation result quality are often unknown, thus, sensitivity analyses should be performed to evaluate the influences of certain parameters on distortion development.
In the present paper, a single-layer gas metal arc (GMA) weld of 5 mm thick structural steel S355J2+N is experimentally and numerically investigated. Subsequent to welding, the numerical modeling begins with a mesh analysis based on modal analyses. Hereby, the influence of different coarsening methods and element edge length (EEL) in welding direction on the deformation behavior or the stiffness of the discrete geometry is the focus of the analysis. Secondly, phase transformations in structural steels such as S355J2+N are decisive for final product properties. The sensitivity of welding-induced distortion is examined regarding different continuous cooling transformation (CCT) diagrams for S355J2+N.
The present investigations deal with different relevant influences on numerical calculation of welding-induced distortion. The quality and quantity of these effects are clarified based on the experimental and numerical set-up employed. Consequently, prediction of welding-induced distortion is possible and potential for pre-production optimization is present.
The paper presents bounded volume heat sources and the corresponding functional-analytical expressions for the temperature field. The power density distributions considered here are normal, exponential and parabolic. The sources model real heat sources like the welding arc, laser beam, electron beam, etc., the convection in the weld pool as well as the latent heat due to fusion and solidification. The parameters of the heat source models are unknown a priori and have to be evaluated by solving an inverse heat conduction problem. The functional-analytical technique for calculating 3D temperature fields in butt welding is developed. The proposed technique makes it possible to reduce considerably the total time for data input and solution. It is demonstrated with an example of laser beam welding of steel plates.
The development of high-strength structural steels with yield strengths up to 1000 MPa results in the requirement of suitable filler materials for welding. Recently designed low transformation temperature (LTT) alloys offer appropriate strength. The martensitic phase transformation during welding induces compressive residual stress in the weld zone. Therefore, the mechanical properties of welded joints can be improved. The present paper illustrates numerical simulation of the residual stresses in LTT-welds taking into account the effect of varying Ms/Mf-temperatures, and therefore different retained austenite contents, on the residual stresses. Residual stress distributions measured by synchrotron diffraction are taken as evaluation basis. A numerical model for the simulation of transformation affected welds is established and can be used for identification of appropriate Ms-temperatures considering the content of retained austenite.
Resistance spot welded joints made of two advanced high strength steels (AHSS) and one high strength steel (HSS) were examined. In the high cycle fatigue regime tests were realised for one loading condition, and the load and the displacement were measured. From the load and displacement range, the stiffness was calculated and in situ normalised to its value after 1000 cycles. It was thus possible to use the normalised stiffness evaluated during the experiment as failure criterion for the fatigue test of resistance spot welded joints. In order to evaluate this failure criterion, fatigue cracks were produced in the three different steel grades mentioned above according to user-specified levels of the normalised stiffness. It was found that the fatigue crack area increases linearly and the through-sheet-thickness crack length exponentially with decreasing stiffness. The comparison of the fatigue crack propagation during the fatigue test of the three different tested base metals showed very similar values. The comparability of fatigue tests performed on spot welded joints might thus be improved by the in situ evaluated stiffness as a failure criterion. This can be also used in further work to verify results of computer simulations of the stress intensity factor at spot welds with the help of experimental results.
Die Ermittlung lokaler Werkstoffkennwerte im Schweißbereich ist für die Auslegung sicherheitsrelevanter Bauteile insbesondere durch Finite-Elemente-Berechnungen von großer Bedeutung. Konventionelle Prüfmethoden können lediglich globale Kennwerte bereitstellen, die jedoch gerade für die Berechnung widerstandspunktgeschweißter Bauteile häufig nicht ausreichen, da diese durch das Vorliegen örtlich eng begrenzter Fügezonen gekennzeichnet sind. Moderne Prüfmethoden wie die optische Dehnungsfeldmessung ermöglichen hingegen eine orts- und zeitkonkrete Bestimmung der Werkstoffkenndaten. Um das Potenzial optischer Dehnungsfeldmessungen für das Charakterisieren des lokalen Werkstoffverhaltens von Punktschweißverbindungen zu ermitteln, wurden Scherzugversuche sowie Zugversuche an widerstandspunktgeschweißten hochfesten Stählen durchgeführt. Die Ergebnisse zeigen das hohe Potenzial dieses Verfahrens sowohl für die Quantifizierung des lokalen Dehnungsverhaltens im Fügebereich als auch für die Ermittlung lokaler Werkstoffkenndaten.
Direct measurement and quantification of phase transformation in a low-alloyed transformation induced plasticity steels depending on the tensile load as well as determination of the real true stress and true strain values were carried out in-situ using high energy synchrotron radiation. Digital image correlation technique was used to quantify more precisely the true strain values. The aim of the work was to obtain a better understanding of the phase transformation of commercial low-alloyed transformation induced plasticity steel depending on the true strain and true stress values.
Peculiarities of the process of orbital laser-arc welding of thick-walled large-diameter pipes
(2010)
Manganese alloyed stainless steels represent a cost-effective alternative to conventional CrNi- stainless steels due to strong fl uctuations of the market prices for nickel seen during the last years. In CrMnNi steels, nickel is partially replaced by lower-cost manganese and small amounts of nitrogen for stabilization of the austenitic phase. This also brings benefi ts regarding the mechanical properties, as it results in an increased material strength. Laser beam welding of such materials was investigated for direct comparison with Standard CrNi steels. Main emphasis was laid on fi nding adequate process parameters to achieve a stable welding process and obtain a good weld quality. Two different laser sources, a 4.4 kW Nd:YAG and a 5 kW CO2 laser, were used to weld 1.5 mm stainless steel sheets in continuous wave mode. A high-Mn austenitic (1.4376) and a lean duplex (1.4162) steel, as well as the standard austenitic (1.4301) and duplex (1.4362) grades were selected as test materials. Both butt and lap joint confi gurations were studied. Experiments were carried out systematically, varying the welding speed, laser power and focal point position in order to determine adequate process windows. The infl uence of the shielding gas type and fl ow rate on the process stability and the weld quality were investigated. The effects of weld edge preparation on the weld appearance and quality levels attained were also examined. The obtained welded joints were subjected to radiographic tests for detection of internal imperfections. Also a metallurgical characterization of the samples regarding the resulting phase composition or balance and hardness depending on the welding process parameters was conducted. Furthermore, tensile and potentiodynamic tests were performed to evaluate the mechanical and corrosion properties, respectively. The results provide an insight into the advantages and limitations of the laser beam welding process for joining high-manganese alloyed stainless steels. Conditions for the production of defect-free and corrosion-resistant welds having good mechanical properties could be determined.
Grain refinement is an important opportunity to improve mechanical properties of fusion welds and the weldability (susceptibility to solidification cracking) of the base metal. In this study, grain refinement was achieved in aluminium welds through additions of grain refiner to the weld metal. Increasing grain refiner additions led to a decrease of the weld metal mean grain size (down to −86 %). The grain refinement efficiency was the highest in commercial pure Al (Alloy 1050A, Al 99.5), followed by Alloy 6082 (Al Si1MgMn) and Alloy 5083 (Al Mg4.5Mn0.7). To investigate this clear influence of alloy content on grain size, the undercooling parameters P and Q were calculated. Temperature measurements revealed that solidification parameters such as solidification growth rate or cooling rate vary significantly along the solidification front, dependent upon torch speed and alloy. On the basis of this comparison, an analytical approach was used to model the columnar to equiaxed transition (CET). Moreover, wavelength dispersive X-ray spectroscopy (WDS) and transmission electron microscopy (TEM) analyses revealed particles rich in Ti and B that are probably TiB2 particles coated by Al3Ti nucleating Al grains during solidification. Also, Ti/B contents needed in commercial filler wires to allow optimum weld metal grain refinement were calculated dependent upon base alloy and welding process.
This work addresses the fatigue strength of friction-welded joints for the drive pinion in heavy-duty trucks. Three different friction welding (FW) variants were tested for the joint between the pinion and the bevel of the shaft: (1) conventional FW using series materials, (2) conventional FW from dissimilar materials for every part, and (3) dissimilar joints by applying joint-site structure of FW. Each joint variant was compared to the series production part using a 4-point bending test. Results showed that the higher strength material slightly improved the fatigue strength of the conventional friction-welded joints. Despite the light weight of the component joined structurally at the site, the joint revealed a lower endurance limit compared to other variants. However, there has been little discussion on the weld zone position. The applied equivalent stresses on the weld zone of the joint-site structure are less similar to the position for the conventional FW. Characterization of the failure explains the extraordinary behaviors in relation to S-N curves.
Laser remote welding is a dynamic and flexible joining technology which enables various new approaches in welding sequence and weld position planning. However, the challenge is to find the optimal set-up within this large parameter space. In this article, numerical welding simulation is used to deal with this challenge. For a car door, different welding sequences and weld positions were numerically analyzed and compared with respect to its welding distortion behavior.
Life Cycle Assessment (LCA) and Social Life Cycle Assessment (SLCA) are applied in evaluating possible social and environmental impacts of the state-of-art welding technologies, such as Manual Metal Arc Welding (MMAW), Manual Gas Metal Arc Welding (GMAW), Automatic GMAW and Automatic Laser-Arc Hybrid Welding (LAHW). The LCA results indicate that for 1 meter weld seam, MMAW consumes the largest amount of resources (like filler material and coating on electrodes) and energy, which contributes to comparatively higher environmental impacts in global warming potential, acidification, photochemical ozone creation potential and eutrophication than other chosen processes. With regard to social aspects, the health issues and fair salary are under survey to compare the relative potential risk on human health caused by fumes in different welding technologies, and to indicate the sufficiency of current salary of welders in Germany. The results reflect that the wage status of welders is still fair and sufficient. The manual processes bring much higher potential risk of welders health than the automatic processes, especially MMAW.
Welding is one of the most critical operations for the construction of reliable metal structures in everything from ships to reactor vessels. When welds fail, the entire structure often fails—so expectations on weld quality have never been higher. Any process that uses a localized heat source, such as welding, is likely to result in some distortion. The welding process of very thick metal components is not inherently stable and is barely controllable without external forces.
This paper deals with the use of an adaptive control system for compensating the variation in the gap height of a fillet welded lap joint. Gap bridging requires the input of additional filler material and is related to an increased energy input. Hence, the aim was a compensation of the effect of an increased heat input, in order to maintain the weld pool and excessive penetration, which can prevent consequently root reinforcement and burn-through. The findings achieved in this work show possibilities for a real-time controlled adjustments of the welding parameters in automated metal active gas (MAG) welding for compensating a higher heat input, in particular by means of the torch offset relative to the top sheet at the fillet weld on a lap joint.
Understanding the influence of tool/pin shapes on the thermal and material flow behaviors in friction stir welding is of great significance for the optimal design of tool/pin based on a scientific principle. In this study, a numerical method based on computational fluid dynamics is employed to quantitatively analyze the thermo-physical phenomena in friction stir welding with two tools of different pin shapes (axisymmetrical conical tool and asymmetrical triflat tool). Through combining a steady state model with a transient state model, both the computation efficiency and accuracy are ensured. The boundary conditions of heat transfer and material flow are determined with considering a partial sticking/sliding contact condition at the toolworkpiece interface. The total heat generation, heat density and temperature distribution during the welding process with triflat tool are elucidated and compared with that of conical tool, and the material flow patterns and deformation regions of various pin orientations are illustrated in detail. It is found that the deformation zone caused by triflat tool is larger than that by conical tool, which is validated by the weld macrographs. The computed thermal cycles and peak temperature values at some locations are in good agreement with the experimentally measured ones.
Widerstandspunktschweißen ist insbesondere im Automobilbau eine der wichtigsten Fügetechniken. Bislang erfolgt die Qualitätssicherung überwiegend durch stichprobenartige zerstörende Prüfung. Eine zerstörungsfreie Prüftechnik würde neben der Reduzierung der Prüfkosten auch eine Optimierung des Punktschweißverfahrens bedeuten, da prinzipiell jeder Schweißpunkt geprüft werden könnte und somit auch eine Reduzierung der Anzahl der Schweißpunkte möglich ist.
Es wird ein Verfahren vorgestellt, bei dem die Punktschweißverbindung zwischen zwei Stahlblechen optisch auf einer Blechseite mittels Laser oder Blitzlicht erwärmt wird. Die aufgeschmolzene Zone, die sogenannte Schweißlinse, stellt dabei neben der mechanischen Verbindung auch eine Wärmebrücke zwischen den beiden verschweißten Blechen dar, die bei diesem Verfahren ausgenutzt wird. Durch den verbesserten thermischen Kontakt zwischen den verschweißten Blechen an der Schweißlinse kontrastiert diese deutlich mit dem umgebenden Blechmaterial, bei dem der Wärmeübertrag zwischen den Blechen vergleichsweise gering ist. Dieser Kontrast im thermischen Verhalten kann mittels zeitabhängiger Thermografie gemessen werden.
Durch das hier vorgestellte Verfahren kann mittels aktiver Thermografie sowohl in Transmissions- als auch in Reflexionsanordnung die Größe des thermischen Kontaktes zwischen den beiden Blechen ermittelt werden, welche ein Maß für die Größe der Schweißlinse und damit für die qualitative Güte der Schweißung darstellt. Ein Vorteil des entwickelten Verfahrens ist seine Anwendbarkeit auf Bleche ohne Oberflächenbehandlung.
The presented apparatus enables laser beam welding of thick materials under local reduced pressure conditions, thus improving the quality of welds and reducing the laser beam power necessary for complete penetration welding. The vacuum cap presented in this article uses a local reduced ambient pressure environment in a tight zone around the welding area and, in contrast to a conventional vacuum chamber, it is movable in the welding direction. The mobile installation is very compact and reaches pressure values of around 200 mbar. The reduced pressure in the vacuum cap is sufficient to generate 50 % higher penetration depth in comparison to welding under ambient pressure conditions. The low pressure around the keyhole reduces the vapour-plasma plume and therefore prevents a defocusing and scattering of the laser radiation. This allows to raise the amount of laser beam power entering the keyhole as well as the effective power density.
Surface modification of mild steel was undertaken using two covered electrodes and two tubular electrodes. The two covered electrodes are DIN 8555: E6-UM-60 and E10-UM-60GR; however, the two tubular electrodes are E10-GF-60GR1 and E10-GF-60GR2. Chemical analysis for the weld metal and XRD of the flux covering for covered electrodes and flux core for tubular electrodes were conducted. The most important factor in determining the wear resistance is the microstructure of the deposit layers. In similar carbon equivalent electrodes E10-UM-60GR and E10-GF-60GR1, electrode E10-GF-60GR1 (tubular) shows larger carbides area fraction and better wear resistance than electrode E10-UM-60GR (covered). This could be attributed to a lower dilution associated with tubular electrode compared with covered electrode. The two tubular electrodes showed higher wear resistance than the covered electrodes which is mainly due to the larger Fe-Cr carbide precipitation area fraction.
This paper presents an approach to reconstruct the three-dimensional transient temperature field for fusion welding processes as input data for computational weld mechanics. The methodology to solve this inverse heat conduction problem fast and automatically focuses on analytical temperature field models for volumetric heat sources and application of global optimisation. The important issue addressed here is the question which experimental data is needed to guarantee a unique reconstruction of the experimental temperature field. Different computational-experimental test cases are executed to determine the influence of various sets of discrete experimental data on the solvability of the optimisation problem. The application of energy distributions utilised for laser beam welding allows reconstructing the temperature field efficiently. Furthermore, the heat input into the workpiece determined by the simulation contributes to the evaluation of the thermal efficiency of the welding process.
Die Anwendung der additiven Fertigung nimmt stark zu. Eine neue Designfreiheit erlaubt es hier den Konstrukteurinnen und Konstrukteuren, die Bauteilfunktion in den Vordergrund der Entwicklungen zu stellen. Leichtbaustrukturen sowie Funktionsintegration machen es möglich, vormals Bauteile aus vielen Einzelkomponenten jetzt in einem einzigen Bauteil zu vereinigen. Experten am Fraunhofer IPK qualifizieren das Laser-Pulver-Auftragschweißen für die additive Fertigung und legen dabei besonderen Wert auf die Automatisierung des Verfahrens.
Ob in der Automobilindustrie, beim Rohrleitungs- oder beim Kraftwerksbau – anspruchsvolle und hochwertige Fügeverbindungen entscheiden oft über die Qualität des Endproduktes. Um z. B. das Verzugsverhalten von Karosseriebauteilen vorab einschätzen zu können, setzen Hersteller auf die numerische Simulation der geschweißten Komponenten. Besondere Aufmerksamkeit gilt hier den Fügestellen. Da diese oft nur wenige Millimeter klein sind, kommen konventionelle Prüfverfahren wie der Zugversuch hier jedoch nicht ohne weiteres in Frage.
Forscher des Fraunhofer IPK untersuchen deshalb eine alternative Methode, um die negativen Begleiterscheinungen des thermischen Schweißens im Vorfeld am PC zu analysieren und nötige Gegenmaßnahmen ableiten zu können.
Laser beam processes are increasingly used in the field of additive manufacturing.
Prominent methods are either powderbed-based like Laser Metal Fusion (LMF), or utilizing a powder nozzle like Laser Metal Deposition (LMD). While LMF allows the manufacturing of complex structures, build rate, part volumes and material flexibility are limited. In contrast, LMD is able to operate with high deposition rates on existing parts, and materials can be changed easily during the process. However LMD shape complexity is limited. Utilizing their respective strengths, a combination of these two additive technologies has the potential to produce complex parts with high deposition rates and increased material flexibility. In this paper, combined manufacturing with additive technologies LMF and LMD is described. Its benefit for industry with emphasis on turbomachinery is shown. As reality test for the innovation, an industrial turbine blade is manufactured.
Die Reduktion des Fahrzeuggewichts ist ein wesentlicher Ansatz zur Ver-ringerung des Energie- und Ressourcenverbrauchs und damit zur Senkung der CO2-Emissionen im Automobilbau. In der Karosserieentwicklung kann der vermehrte Einsatz von Aluminiumwerkstoffen einen bedeutenden Beitrag dazu leisten. Im preissensitiven Umfeld des Karosseriebaus etabliert sich das bei Stahlanwendungen genutzte Widerstandspunktschweißen zunehmend auch für Aluminiumverbindungen. Verfahrensbedingte Herausforderungen, wie verkürzte Elektrodenstandzeiten und mangelnde Kenntnis über den Einfluss von Imperfektionen auf die Festigkeit, begrenzen dennoch die Weiterverbreitung des Verfahrens und stellen die Prozessrobustheit insgesamt in Frage. Im Rahmen des hier vorgestellten Forschungsvorhabens wurden das Auftreten verschiedener Brucharten experimentell untersucht und Prognosefunktionen zur Abschätzung der Tragfähigkeit von Widerstandspunktschweißverbindungen unter verschiedenen Belastungsfällen erstellt. Anschließend wurde der Einfluss von Oberflächenrissen und Rissen in der Schweißlinse auf die Scherzugfestigkeit sowohl experimentell als auch simulativ analysiert.
Using the novel optical measurement technique together with the optical flow algorithm, a twodimensional deformation analysis during welding has been conducted. This technique provides for the first time a measurement of the strain field locally in the immediate vicinity of the assumed solidification front. The described procedure of the opticalmeasurement allows to determine the real martial-dependent values of critical strain and strain rate characterising transition to the hot cracking during laser welding processes. The local critical strain that obtained in the assumed solidification showed that the local critical strain decreases as the strain rate increases. Moreover, this phenomenon has also been shown with results from the CTW-test, since the global strain decreases with an increase in the external strain rate.
The process of laser cladding has become more important during recent years because of its broad application for cladding, repair or additive manufacturing. In the field of mechanical engineering, one use is the repair of turbine blades. For high quality and reliability of the repaired components, it is necessary to adjust the weld bead geometry to the specific repair task. The bead geometry influences the metallurgical bonding and the degree of dilution as well as the formation of defects like pores or cracks. Therefore, it is important to know the effects of the different parameters on the welding bead. A valuable tool to meet this industrial challenge is the design of experiments (DoE). In this context, the user can choose between a huge number of test plans. Greater Profit of information is expected by a larger test range. In order to confirm the acceptance, a five-step full factorial test plan is compared to a central composite design in this paper. Moreover, the limits of the experimental range are indicated and restrictions can be derived. As the results show, the essential effects are detected with a full factorial test plan as well as with a central composite design. Merely the effect strength could not always be specified unambiguously. On this account and in consideration of cost efficiency, the use of central compound design is recommended in industrial applications.
Distortions in Additive Manufacturing (AM) Laser Metal Deposition (LMD) occur in the newly-built component due to rapid heating and solidification and can lead to shape deviations and cracking. This paper presents a novel approach to quantify the distortions experimentally and to use the results in numerical simulation validation. Digital Image Correlation (DIC) is applied together with optical filters to measure in-situ distortions directly on a wall geometry produced with LMD. The wall shows cyclic Expansion and shrinking with the edges bending inward and the top of the sample exhibiting a slight u-shape as residual distortions. Subsequently, a structural Finite Element Analysis (FEA) of the experiment is established, calibrated against experimental temperature profiles and used to predict the in-situ distortions of the sample. A comparison of the experimental and numerical results reveals a good agreement in length direction of the sample and quantitative deviations in height direction, which are attributed to the material model used. The suitability of the novel experimental approach for measurements on an AM sample is shown and the potential for the validated numerical model as a predictive tool to reduce trial-and-error and improve part quality is evaluated.
Licht ist grüner als Strom
(2017)
The paper deals with a numerical finite element simulation of the residual stress evolution in friction stir welding of 6 mm thick aluminum alloy AA2024-T3. The transient thermal field during the welding process was calculated with the commercial code COMSOL Multiphysics 5.0. Therefore, a thermal-pseudo-mechanical (TPM) heat source was implemented. A subsequent mechanical simulation was performed with varying hardening models for different welding speeds of 60 and 300 mm/min. The influence of softening of the material, which was due to hardening precipitation dissolution associated to the heating, was also investigated. Experiments in terms of thermocouple measurements as well as Vickers hardness and X-ray measurements of the residual stresses were conducted and compared to numerically obtained results. A qualitatively as well as quantitatively good agreement was found for different applied welding speeds.
Single-wire gas metal arc welding (SGMAW) and high power tandem GMAW (TGMAW) are evaluated with respect to energy efficiency. The key performance indicator electrical deposition efficiency is applied to reflect the energy efficiency of GMAW in different material transfer modes. Additionally, the wall-plug efficiency of the equipment is determined in order to identify the overall energy consumption. The results show that energy efficiency can be increased by 24% and welding time is reduced over 50% by application of the tandem processes. A comparative life cycle assessment of a 30-mm-thick weld is conducted to investigate the influences of the energy efficiency on the environmental impacts. The environmental impacts on the categories global warming potential, acidification potential, eutrophication potential, and photochemical ozone creation potential can be reduced up to 11% using an energy-efficient TGMAW process.
This paper investigates gas metal arc welding (GMAW) with respect to energy consumption and its associated environmental impacts. Different material transfer modes and power levels for single wire GMAW (SGMAW) and tandem GMAW (TGMAW) are evaluated by means of the indicator electrical deposition efficiency. Furthermore, the wall-plug efficiency of the equipment is measured in order to describe the total energy consumption from the electricity grid. The results show that the energy efficiency is highly affected by the respective process and can be significantly enhanced by a TGMAW process. The wall-plug efficiency of the equipment shows no significant dependency on the power range or the material transfer mode. Moreover, the method of life cycle assessment (LCA) is adopted in order to investigate the influences of energy efficient welding on the environmental impacts. In the comparative LCA study, the demand of electrical energy is reduced up to 24%. In consequence, the indicator values for global warming potential (100), acidification potential, eutrophication potential, and photochemical ozone creation potential are reduced up to 11%.
Build-up strategies for temperature control using laser metal deposition for additive manufacturing
(2018)
The track geometry created with laser metal deposition (LMD) is influenced by various parameters. In this case, the laser power has an influence on the width of the track because of an increasing energy input. A larger melt pool is caused by a rising temperature. In the case of a longer welding process, there is also a rise in temperature, resulting in a change of the track geometry. This paper deals with the temperature profiles of different zigzag strategies and spiral strategies for additive manufacturing. A two-color pyrometer is used for temperature measurement on the component surface near the melt pool. Thermocouples measure the temperatures in deeper regions of a component. The welds are located in the center and in the edge area on a test part to investigate the temperature evolution under different boundary conditions. The experiments are carried out on substrates made from mild steel 1.0038 and with the filler material 316L. The investigations show an influence on the temperature evolution by the travel path strategy as well as the position on the part. This shows the necessity for the development and selection of build-up strategies for different part geometries in additive manufacturing by LMD.
Ultrasonic vibration enhanced friction stir welding (UVeFSW) is a recent modification of conventional friction stir welding (FSW), which transmits ultrasonic vibration directly into the localized area of the workpiece near and ahead of the rotating tool. In this study, a high strength aluminium alloy (2024-T4) was welded by this process and conventional FSW, respectively. Then tensile tests, microhardness tests and fracture surface analysis were performed successively on the welding samples. The tests results reveal that ultrasonic vibration can improve the tensile strength and the elongation of welded joints. The microhardness of the stir zone also increases.
The geometry of the melt pool in laser beam welding plays a major role to understand the dynamics of the melt and its solidification behavior. In this study, a butt configuration of 15 mm thick structural steel and transparent quartz glass was used to observe the weld pool geometry by means of high-speed camera and an infrared camera recording. The observations show that the dimensions of the weld pool vary depending on the depth. The areas close to the weld pool surface take a teardrop-shape. A bulge-region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a 3D transient thermal-fluid numerical simulation was performed to obtain the weld pool shape and to understand the formation mechanism of the observed bulging effect. The model takes into account the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature. The numerical results showed good accordance and were furthermore used to improve the understanding of the experimentally observed bulging effect.
The sound numerical prediction of welding-induced thermal stresses, residual stresses, and distortions strongly depends on the accurate description of a welded material’s thermomechanical deformation behaviour. In this work, we provide experimental data on the viscoplastic deformation behaviour of a grade-s960ql steel up to a temperature of 1000 ◦C. In addition, a multi-phase viscoplastic material model is proposed, which accounts for the experimentally observed isothermal deformation behaviour of grade-s960ql steel base and austenitised material, as well as for athermal contributions that originate from solid-state phase transformations. The multi-phase viscoplastic and a classic rateindependent isotropic hardening material model were applied in the numerical simulations of both-ends-fixed bar Satoh tests and a single-pass gas metal arc weld. The influence of material modelling choices on the agreement between numerical simulation and experimental results is discussed, and recommendations for further work are given.
Evaluating innovative process technologies has become highly important within the last decades. As standard tools different Life Cycle Assessment methods have been established, which are continuously improved. While those are designed for evaluating single processes they run into difficulties when it comes to assessing environmental impacts of process innovations at macroeconomic level. In this paper we develop a multi-step evaluation framework building on multi regional inputeoutput data that allows estimating macroeconomic impacts of new process technologies, considering the network characteristics of the global economy.
Our procedure is as follows: i) we measure differences in material usage of process alternatives, ii) we identify where the standard processes are located within economic networks and virtually replace those by innovative process technologies, iii) we account for changes within economic systems and evaluate impacts on emissions.
Within this paper we exemplarily apply the methodology to two recently developed innovative technologies: longitudinal large diameter steel pipe welding and turning of high-temperature resistant materials. While we find the macroeconomic impacts of very specific process innovations to be small, its conclusions can significantly differ from traditional process based approaches. Furthermore, information gained from the methodology provides relevant additional insights for decision makers extending the picture gained from traditional process life cycle assessment.
Laser beam oscillation, applied one- or two-dimensional to the actual welding process, influences the welding process in terms of compensation of tolerances and reduction of process emissions like spatter and melt ejections that occur in industrial applications, such as in body-in-white manufacturing. If the welding process could be adapted to these tolerances by the momentarily demanded melt pool width to generate sufficient melt volume or to influence melt pool dynamics, e.g. for a better degassing, laser welding would become more robust. However, beam oscillation results are highly dependent on the natural frequency of the melt pool, the used spot diameter and the oscillation speed of the laser beam. The conducted investigations with an oscillated 300 μm laser spot show that oscillation strategies which are adjusted to the joining situation can bridge gaps to approximately 0.6 mm at metal sheet thickness of 0.8 mm. However, the complex behaviour of the melt pool has to be considered to generate proper welding results. This work puts emphasis on showing aspects of beam oscillation in fillet welding in lap joints.
Modern advanced high-strength steel sheets for automotive applications are mostly zinc coated for corrosion resistance. However, the presence of zinc can—besides its positive effects—increase the material’s susceptibility to liquid metal embrittlement (LME) during resistance spot welding (RSW). Zinc and its eutectics are, due to their low melting point, present in liquid state during the welding process. This fact can, in combination with other factors like tensile strains or stresses, lead to the formation of brittle, intergranular cracks in the weld, and heat-affected zone. This phenomenon is commonly called liquid metal embrittlement. In order to understand the process from a practical perspective, one must learn what factors facilitate it. In this study, industry-relevant parameters are investigated regarding their influence on the occurrence of LME, embodied by the formation of surface cracks. It was found that electrode wear has less of an influence on the cracking susceptibility than welding current or tensile stresses. Finite element analysis is believed to provide a powerful tool in order to gain insights on the formation process. Modeling of the process shows promising initial results, revealing the underlying local stress and strain fields, unmeasurable with common techniques.
Comparison between GTA and laser beam welding of 9%Ni steel for critical cryogenic applications
(2018)
IncomparisonwithGTAweldedjoints,highertensilestrengthcomparabletothatofthebasemetalwasobtained for laser beam welded joints due to fine martensitic microstructure. Impact fracture toughness values with much lower mismatching were obtained for laser beam welded joints due to similarity in the microstructures of its weld metal and HAZ. In this case, the lower impact fracture toughness obtained (1.37J/mm2) was much higher than that of the GTA welded joints (0.78J/mm2), which was very close to the specified minimum value (≥0.75J/mm2). In contrast to other research works, the overall tensile and impact properties are influenced not only by the fusion zone microstructure but also by the size of its hardened area as well as the degree of its mechanical mismatching, as a function of the welding process. A better combination of tensile strength and impact toughness of the concerned steel welded joints is assured by autogenous laser beam welding process.
In order to generate a material data base for computational welding mechanics, temperature and strain-rate dependent stress-strain experiments were performed by using a Gleeble®3500 testing system. The object of the investigation was HSLA transformable steel S960QL and related solid phases as bainite, martensite and austenite. For the production of these solid phases, the base material was heat treated according to an average weld temperature cycle which was extracted within the heat affected zone of a thermal numerical weld simulation of a GMA weld. The hot tensile tests were carried out via cost-saving flat specimen geometries. Two experimental series with different strain-rates were conducted, where the longitudinal strain-rate was controlled by specification of the transversal strain-rate applying Poisson’s-ratio. Subsequently, the resulting stress-strain curves were approximated in accordance with the Ramberg-Osgood-materials law. Consequently, it is shown that the temperature and strain-rate dependent stress-strain behavior of metals can be successfully characterized by means of a Gleeble®-system.
However, this requires a control of the longitudinal strain-rate by specification of the transversal strain-rate. The related experimental procedure and the method of evaluation are explained in detail. With regard to all tested solid phases, a significant strain-rate dependency can only be observed upwards from temperatures of 400 °C. Based on experimental results, Ramberg-Osgood-parameters will be presented to describe the stress-strain behavior of steel S960QL and related solid phases for temperatures between 25 °C and 1200 °C. Furthermore, the use of costsaving flat specimen-geometry appears reasonable.
A 3D electro-thermomechanical model is established in order to investigate liquid metal embrittlement.
After calibration to a dual phase steel of the 1000 MPa tensile strength class, it is used to analyse the thermo-mechanical system of an experimental procedure to enforce liquid metal embrittlement during resistance spot welding. In this procedure, a tensile stress level is applied to zinc coated advanced high strength steel samples during welding. Thereby, liquid metal embrittlement formation is enforced, depending on the applied stress level and the selected material.
The model is suitable to determine and visualise the corresponding underlying stresses and strains responsible for the occurrence of liquid metal embrittlement. Simulated local stresses and strains show good conformity with experimentally observed surface crack locations.
Using a novel optical measurement technique together with the optical flow algorithm, a two-dimensional deformation analysis during welding was conducted. The presented technique is the first to provide a measurement of the full strain field locally in the immediate vicinity of the solidification front.
Additionally, the described procedure of the optical measurement allows the real material-dependent values of critical strain and strain rate characterizing the transition to hot cracking during laser welding processes to be determined. Furthermore, the above-mentioned technique is independet on the welding process, which means, it can be also used for arc welding processes. Dependency between the external strain rate and the critical local strain and strain rate has been observed. That is to say, the critical local strain and strain rate is increased with an increase of the strain rate.Moreover, this technique allows automatic identification of the cases that can be critical for the solidification crack formation by monitoring the state of strain on the crack-sensitive region within the mushy zone.
The shape of the weld pool in laser beam welding plays a major role to understand the dynamics of the melt and its solidification behavior. The aim of the present work was its experimental and numerical investigation. To visualize the geometry of the melt pool in the longitudinal section a butt joint configuration of 15 mm thick structural steel and transparent quartz glass was used. The weld pool shape was recorded by means of a high-speed video camera and two thermal imaging MWIR and VIS cameras. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop shaped weld pool. A bulge-region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a transient numerical simulation was performed until reaching a steady state to obtain the weld pool shape and to understand the formation mechanism of the observed bulging phenomena. A fixed keyhole with an experimentally obtained shape was used to represent the full-penetration laser beam welding process. The model considers the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature. It was found that the Marangoni convection and the movement of the laser heat source are the dominant factors for the formation of the bulging-region. Good correlation between the numerically calculated and the experimentally observed weld bead shapes and the time-temperature curves on the upper and bottom surface were found.
The paper describes an experimental investigation of the electromagnetic porosity reduction in partial penetration laser beam welding of 6 mm thick aluminum die casting AlSi9MnMg. The Investigation reveals that the usage of an electromagnetic field leads to a significant reduction of the porosity as well as to a surface smoothing of aluminum die casting. Based on the reference case without an electromagnetic influence, the porosity area was reduced gradually up to 76%.
Metallurgical pores as well as process pores were removed from the weld pool. Also the weld reinforcement was improved up to 78%. Best results were reached with a frequency of 4325 Hz and a magnetic flux density of 348 mT. Although a complete prevention of porosity was not achieved, the best weld seam reached a high quality and can be ranked in valuation group B of DIN EN ISO 13919-2:2001–12.
Micrometric surface topologies are required for a wide range of technical applications. While lowered surface features have been used for many years to improve the tribological behavior of contacting surfaces, there are also other fields of application, where the potential of elevated surface features is known, e. g. for metal forming tools. However, the demand for a high wear resistance of these structures often inhibits an industrial application. A solution is offered by structuring techniques that use additional material. A promising approach is the localized dispersing of hard ceramic particles by pulsed laser radiation, the so-called laser implantation. This paper describes the potential to adjust the geometry as well as the mechanical properties of laser implanted surfaces by means of microstructural and topological investigations. Afterwards, results of a wear test are given and different applications for this structuring technique are discussed. It can be shown that dome-shaped or ring-shaped structures on a micrometric scale can be produced with high hardness and wear resistance.
An approach to develop an arc sensor for gap width estimation during automated NG-GMAW with a weaving electrode motion is introduced by combining arc sensor readings with optical measurements of the groove shape to allow precise analyses of the process. The two test specimen welded for this study were designed to feature a variable groove geometry in order to maximize efficiency of the conducted experimental efforts, resulting in 1696 individual weaving cycle records with associated arc sensor measurements, process parameters and groove shape information. Gap width was varied from 18 mm to 25 mm and wire feed rates in the range of 9 m/min to 13 m/min were used in the course of this study. Artificial neural networks were applied as a modelling tool to derive an arc sensor for estimation of gap width suitable for online process control that can adapt to changes in process parameters as well as changes in the weaving motion of the electrode. Wire feed rate, weaving current, sidewall dwell currents and angles were defined as inputs to calculate the gap width. The evaluation of the proposed arc sensor model shows very good estimation capabilities for parameters sufficiently covered during the experiments.
A transient simulation including the impact of the laser energy, the melting of the metal and the development of the weld pool was conducted to observe the evolution of the vapor capillary and the solidification of the melt in pulsed laser beam welding of AISI 304 steel. The phase field method was implemented to investigate the evolution and behavior of the liquid-gas interface during welding and to describe the condensed and vapor phases. The effects of phase transition, recoil pressure, thermo-capillary and natural convection, vaporization and temperature dependent material properties were taken into account. A Gaussian-like heat source under consideration of the Fresnel absorption model was used to model the energy input of the laser beam. The heat source model was extended by a newly developed empirical approach of describing multiple beam reflections in the keyhole. To validate this new model, the numerical results were compared to experimental data and good agreement regarding the size and shape of the weld pool was observed.
The paper describes an experimental investigation of high power laser beam welding with an electromagnetic weld pool support for up to 20 mm thick plates made of duplex steel (AISI 2205) and mild steel (S235JR). The results of the welding tests show a successful application of this technology at ferromagnetic metals. Irregular sagging was suppressed successfully. An ac-power of less than 2 kW at oscillation frequencies between 800 Hz and 1.7 kHz is necessary for a full compasation of the hydrostatic pressure. Thus, it was demonstrated that the electromagnetic weld pool support is not only limited to non-ferromagnetic metals like austenitic steels. For future studies with duplex steel, the use of filler material has to take into account with regard to the balance of the mixed austenitic and ferritic phases.
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 remelting 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 6mm 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 images, and cross-sectional images. The results prove a significant reduction of the porosity up to 70% for both materials as a function of the magnetic flux density.
Electromagnetic stirring is known to promote material flow, reduce porosity, uniform elements distribution, and refine grain in laser beam welding (LBW), which enhances the applicability of LBW in various industries. In this study, a phase-field model of dendrite growth in AA5754 Al alloy electromagnetic stirring laser beam welding was established. The model considered the thermal electromagnetic Lorentz force resulting from the interaction between the electric field generated by the Seebeck effect and the magnetic field, as well as the temperature gradient and solidification rate of the solidification interface obtained from the computational fluid dynamics electromagnetic stirring LBW model. The variation rules of dendrite growth with different magnetic parameters and effects are analyzed. Comprehensively, the magnetic field promotes the solidification rate, thus promoting interfacial instability and a large magnetic flux density leads to a faster interface instability. The solidification rate as well as the temperature gradient affect the growth rate, and the accelerated growth caused by the so lidification rate with a high frequency and a large magnetic flux density effectively inhibits the slow growth caused by the temperature gradient. The thermal electromagnetic Lorentz force is the main factor for the branch increment at low frequencies, while both thermal electromagnetic Lorentz force and temperature gradient in crease the number of branches at high frequencies. The calculated average branch numbers considering various factors in the stable stage under different magnetic parameters were consistent with the results of the scanning electron microscope tests.
Through experimental observation and auxiliary numerical simulation, this investigation studies the different types of grain refinement of 5754 aluminum alloy laser beam welding by applying a transverse oscillating magnetic field. Scanning electron microscope results have proved that the application of a magnetic field can reduce the average crystal branch width and increase its number. The interaction between the induced eddy current generated by the Seebeck effect and the applied external magnetic field produces a Lorentz force, which is important for the increase in the number of crystal branches. Based on the theory of dendrite fragmentation and the magnetic field-induced branches increment, the grain size reduction caused by the magnetic field is studied. Furthermore, the effects of the magnetic field are analyzed by combining a phase field method model and simulations of nucleation and grain growth. The grain distribution and average grain
size after welding verify the reliability of the model. In addition, the introduction of a magnetic field can increase the number of periodic three-dimensional solidification patterns. In the intersection of two periods of solidification patterns, the metal can be re-melted and then re-solidified, which prevents the grains, that have been solidified and formed previously, from further growth and generates some small cellular grains in the new fusion line. The magnetic field increases the building frequency of these solidification structures and thus promotes this kind of grain refinement.
The amount of absorbed energy in the keyhole as well as its spatial and temporal distribution is essential to model the laser beam welding process. The recoil pressure, which develops because of the evaporation process induced by the absorbed laser energy at the keyhole wall, is a key determining factor for the macroscopic flow of the molten metal in the weld pool during high-power laser beam welding. Consequently, a realistic implementation of the effect of laser radiation on the weld metal is crucial to obtain reliable and accurate simulation results. In this paper, we discuss manyfold different improvements on the laser-material interaction, namely, the ray tracing method, in the numerical simulation of the laser beam welding process. The first improvement relates to locating the exact reflection points in the ray tracing method using a so-called cosine condition in the determination algorithm for the intersection of reflected rays and the keyhole surface. A second correction refers to the numerical treatment of the Gaussian distribution of the laser beam, whose beam width is defined by a decay of the laser intensity by a factor of 1/e2, thus ignoring around 14% of the total laser beam energy. In the third step, the changes in the laser radiation distribution in the vertical direction were adapted by using different approximations for the converging and the diverging regions of the laser beam, thus mimicking the beam caustic. Finally, a virtual mesh refinement was adopted in the ray tracing routine. The obtained numerical results were validated with experimental measurements.
Hybrid laser-arc welding (HLAW) was applied for butt welding of 14.5 mm thick plates of ferritic cryogenic steel X8Ni9 containing 9% Ni, which is used for manufacturing storage and transport facilities of liquefied natural gas (LNG). The weld seam formation and the achievable metallurgical and mechanical properties of the hybrid welds were investigated experimentally for two types of filler wire, an austenitic wire dissimilar to the base metal (BM) and an experimentally produced matching ferritic wire. Safe penetration and uniform distribution of the austenitic filler metal in the narrow hybrid weld could only be achieved in the upper, arcdominated part of the weld. The pronounced heterogeneous distribution of the austenitic filler metal in the middle part and in the root area of the weld could not ensure sufficient notched impact toughness of the weld metal (WM). As a result, a decrease in the impact energy down to 17±3 J was observed, which is below the acceptance level of ≥34 J for cryogenic applications. In contrast, the use of a matching ferritic filler wire resulted in satisfactory impact energy of the hybrid welds of up to 134±52 J at the concerned cryogenic temperature of -196 °C. The obtained results contribute to an important and remarkable conversion in automated manufacturing of LNG facilities. In other words, the results will help to develop a new laser-based welding technology, where both quality and productivity are considered.The efficiency of the developed welding process has been demonstrated by manufacturing a prototype where a segment of the inner wall of large size LNG storage tank was constructed. In this concern, hybrid laser arc welding was conducted in both horizontal (2G) and vertical (3G) positions as a simulation to the actual onsite manufacturing. The prototype was fabricated twice where its quality was confirmed based on non-destructive and destructive examinations.
AbstractLaser beam welding has become widely applied in many industrial fields in recent years. Solidification cracks remain one of the most common welding faults that can prevent a safe welded joint. In civil engineering, convolutional neural networks (CNNs) have been successfully used to detect cracks in roads and buildings by analysing images of the constructed objects. These cracks are found in static objects, whereas the generation of a welding crack is a dynamic process. Detecting the formation of cracks as early as possible is greatly important to ensure high welding quality. In this study, two end-to-end models based on long short-term memory and three-dimensional convolutional networks (3D-CNN) are proposed for automatic crack formation detection. To achieve maximum accuracy with minimal computational complexity, we progressively modify the model to find the optimal structure. The controlled tensile weldability test is conducted to generate long videos used for training and testing. The performance of the proposed models is compared with the classical neural network ResNet-18, which has been proven to be a good transfer learning model for crack detection. The results show that our models can detect the start time of crack formation earlier, while ResNet-18 only detects cracks during the propagation stage.
Liquid metal embrittlement (LME) cracking is a phenomenon observed during resistance spot welding (RSW) of zinccoated advanced highstrength steels (AHSS) in automotive manufacturing. In this study, severe cracks are observed at the edge of the sheet under reduced flange widths. These cracks, traversing the AHSS sheet, culminate at the edge with a width of approximately 1.2 mm.
Through combined numerical and experimental investigations, and material testing, these cracks are identified and validated as a new type of LME crack. The mechanism behind this crack formation is attributed to unique geometric conditions that, when compared to center welding, amplify radial material flow by ninefold to 0.87 mm. The resultant tangential tensile stresses approximate 760 MPa, which exceed the yield strength of the examined advanced highstrength steel (AHSS) under heightened temperature conditions, and when combined with liquid zinc, promote the formation of this new type of LME crack.
The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.
Im Additive Manufacturing Verfahren Directed Energy Deposition (DED) wird bei der Verarbeitung von Werkzeugstahl in der Regel reines Argon als Schutzgas verwendet. Dabei kann die Verwendung von speziellen Schutzgasgemischen, auch bei geringen Anteilen zugemischter Gase, durchaus die Bauteilqualität positiv beeinflussen.
In Vorarbeiten der Messer SE & Co. KGaA zeigte ein gewisser Sauerstoffanteil im Schutzgas die Tendenz, den Flankenwinkel von Schweißspuren beim DED zu verbessern. In der vorliegenden Studie wurde daher detailliert untersucht in wie weit unterschiedliche Schutzgasgemische einen Einfluss auf die Qualität sowie die geometrischen Eigenschaften der additiv gefertigten Strukturen des Werkzeugstahls 1.2709 beim Laser-DED ausüben. Es erfolgten zunächst Testschweißungen in Form von Einzelspuren mit unterschiedlichen Gemischen aus dem Basisschutzgas Argon mit geringen Anteilen verschiedener Gase. Dabei wurde der Einfluss der Zusätze auf die Spurgeometrie und Aufbauqualität untersucht. Auf Basis dieser Vorversuche wurde eine Auswahl vielversprechender Gasgemische getroffen und Detailuntersuchungen in Form von Spuren, Flächen und Quadern unter Zugabe verschiedener Mengen an Zusätzen durchgeführt. Zur Bewertung des Einflusses der Schutzgasbeimengungen wurden der Flankenwinkel, die Porosität und das Gefüge der Proben anhand metallografischer Schliffe untersucht. Es zeigte sich, dass eine Zugabe von geringen Anteilen an Zusätzen zunächst zu einer Vergrößerung des Flankenwinkels im Vergleich zu reinem Argon führt. Mit steigendem Anteil der Gase nimmt dieser Winkel jedoch ab. So kann je nach Menge des zugesetzten Gases eine individuelle Benetzung des aufgetragenen Materials an der Oberfläche erreicht werden. Auch die Porosität ließ sich durch Schutzgasgemische beeinflussen und zeigt ein abweichendes Verhalten im Vergleich zu reinem Argon.
The strain field can reflect the initiation time of solidification cracks during the welding process. The traditional strain measurement is to first obtain the displacement field through digital image correlation (DIC) or optical flow and then calculate the strain field. The main disadvantage is that the calculation takes a long time, limiting its suitability to real-time applications. Recently, convolutional neural networks (CNNs) have made impressive achievements in computer vision.
To build a good prediction model, the network structure and dataset are two key factors. In this paper, we first create the training and test sets containing welding cracks using the controlled tensile weldability (CTW) test and obtain the real strain fields through the Lucas–Kanade algorithm. Then, two new networks using ResNet and DenseNet as encoders are developed for strain prediction, called StrainNetR and StrainNetD. The results show that the average endpoint error (AEE) of the two networks on our test set is about 0.04, close to the real strain value. The computation time could be reduced to the millisecond level, which would greatly improve efficiency.
An accurate and efficient description of the spatial distribution of laser energy is a crucial factor for the modeling of laser material processing, e.g., laser welding, laser cutting, or laser-based additive manufacturing. In this study, a 3D heat transfer and fluid flow model coupled with the volume-of-fluid algorithm for free surface tracking is developed for the simulation of molten pool dynamics in high-power laser beam welding. The underlying laser-material interactions, i.e., the multiple reflections and Fresnel absorption, are considered by a raytracing method. Two strategies of free surface reconstruction used in the ray-tracing method are investigated: a typical piecewise linear interface calculation (PLIC)-based method and a novel localized level-set method. The PLIC-based method is discrete, resulting in noncontinuous free surface reconstruction. In the localized level-set method, a continuous free surface is reconstructed, and, thus, the exact reflection points can be determined. The calculated spatial laser energy distribution and the corresponding molten pool dynamics from the two methods are analyzed and compared. The obtained numerical results are evaluated with experimental measurements to assure the validity of the proposed model. It is found that distinct patterns of the beam multiple reflections are obtained with the different free surface reconstructions, which shows significant influence not only on the molten pool behaviors but also on the localized keyhole dynamics.
The Directed Energy Deposition process is used in a wide range of applications including the repair, coating or modification of existing structures and the additive manufacturing of individual parts. As the process is frequently applied in the aerospace industry, the requirements for quality assurance are extremely high. Therefore, more and more sensor systems are being implemented for process monitoring. To evaluate the generated data, suitable methods must be developed. A solution, in this context, was the application of artificial neural networks (ANNs). This article demonstrates how measurement data can be used as input data for ANNs. The measurement data were generated using a pyrometer, an emission spectrometer, a camera (Charge-Coupled Device) and a laser scanner. First, a concept for the extraction of relevant features from dynamic measurement data series was presented. The developed method was then applied to generate a data set for the quality prediction of various geometries, including weld beads, coatings and cubes. The results were compared to ANNs trained with process parameters such as laser power, scan speed and powder mass flow. It was shown that the use of measurement data provides additional value. Neural networks trained with measurement data achieve significantly higher prediction accuracy, especially for more complex geometries.
The application of hybrid laser-arc welding (HLAW) for joining closed circumferential welds is a challenge due to the high risk of forming a defective overlap area with a shrinkage void or solidification cracks in the material thickness. A series of HLAW experiments were performed to understand the development of a faulty overlap area when closing the circumferential weld. Welding trials on flat specimens and pipe segments were supported by numerical analyses in which the thermomechanical behavior of the welds in the overlap area was investigated. Different process control strategies were tested, including variations in defocusing levels and the overlap length. The newly developed HLAW head, including laser optics with a motor-driven collimation system, made it possible to defocus the laser beam during welding without disturbing the stability of the welding process. High-level defocusing of the laser beam of more than 40 mm relative to the specimen surface with a resulting beam diameter of > 2.9 mm, and in combination with a short overlap length of 15 mm, was promising with respect to the formation of a desired cup-shaped weld profile that is resistant to solidification cracks.
During resistance spot welding of zinc-coated advanced high-strength steels (AHSSs) for automotive production, liquid metal embrittlement (LME) cracking may occur in the event of a combination of various unfavorable influences. In this study, the interactions of different welding current levels and weld times on the tendency for LME cracking in third-generation AHSSs were investigated. LME manifested itself as high penetration cracks around the circumference of the spot welds for welding currents closely below the expulsion limit. At the same time, the observed tendency for LME cracking showed no direct correlation with the overall heat input of the investigated welding processes. To identify a reliable indicator of the tendency for LME cracking, the local strain rate at the origin of the observed cracks was analyzed over the course of the welding process via finite element simulation. While the local strain rate showed a good correlation with the process-specific LME cracking tendency, it was difficult to interpret due to its discontinuous course. Therefore, based on the experimental measurement of electrode displacement during welding, electrode indentation velocity was proposed as a descriptive indicator for quantifying cracking tendency.
This article presents a cost-effective and reliable method for welding 30 mm thick sheets of shipbuilding steel EH36. The method proposes to perform butt welding in a two-run technique using hybrid laser arc welding (HLAW) and submerged arc welding (SAW). The HLAW is performed as a partial penetration weld with a penetration depth of approximately 25 mm. The SAWis carried out as a second run on the opposite side. With a SAWpenetration depth of 8 mm, the weld cross-section is closed with the reliable intersection of both passes. The advantages of the proposed welding method are: no need for forming of the HLAW root; the SAW pass can effectively eliminate pores in the HLAWroot; the high stability of the welding process regarding the preparation quality of the weld edges. Plasma cut edges can be welded without lack of fusion defects. The weld quality achieved is confirmed by destructive tests.
With regard to efficient production, it is desirable to combine the respective advantages of additively and conventionally manufactured components. Particularly in the case of large-volume components that also include filigree or complex structures, it makes sense to divide the overall part into individual elements, which afterwards have to be joined by welding.
The following research represents a first step in fundamentally investigating and characterizing the joint welding of Laser Powder Bed Fusion (L-PBF) components made of Inconel 718. For this purpose, bead-on-plate welds were performed on plates manufactured using the L-PBF process and compared with the conventionally manufactured material. Conventional laser beam welding was used as welding process. The weld geometry was investigated as a function of the L-PBF build-up orientation. It was found that the welding depth and weld geometry differ depending on this orientation and in comparison to the conventional material.
This study presents mechanisms of the evolution of a narrowed region in the weld pool center during deep penetration laser beam welding. In numerous numerical studies presented in this study, it was also found that the local reduction of the weld pool size can cause detrimental effects on the melt flow behavior and the resulting properties of the welds. A particularly large influence of this effect was identified in three aspects. First, the local variation of the solidification sequence of the weld pool causes an increase in the hot-cracking susceptibility due to a locally delayed solidification. Second, it was proven that a change in the local length and width of the weld pool is associated with an adverse impact on the potential flow routes of the molten material that induces stronger local variations of its solidification. Thus, the element mixing, e.g., during the welding with filler materials, is blocked. This leads to a non-homogeneous chemical composition of the final weld and can cause undesired effects on the final material properties. Finally, another observed effect is related to the reduced ability of process pores to reach the top surface. As this type of porosity is usually produced around the keyhole tip, the change of the fluid flow regime above this area plays a significant role in determining the final path of the pores until the premature solidification in the middle of the weld pool captures them. This study summarizes mainly numerical results that were supported by selected experimental validation results.
Recent studies have confirmed the widening of the weld pool interface, known as a bulge effect, during deep penetration high power laser beam welding. The link between such geometric particularities of the weld pool shape and the hot cracking phenomena is significant. The present work seeks to extend the level of understanding by investigating their relationship. A coupled multiphysics, multiscale numerical framework is developed, comprising a series of subsequent analyses. The study examines the influences of the bulge on the three most dominant effects causing hot cracking, namely the thermal cycles, the mechanical loading, and the local microstructure. The bulge in the weld pool shape forms approximately in the middle of the plate, thus correlating with the location of hot cracking. It increases the hot cracking susceptibility by enhancing the three dominant effects. The numerical results are backed up by experimental data.
The present contribution is the first research reporting full penetration HLAW joints in 15 mm thick EH36 steel butt T-welds with square grooves on 2F welding position by single-sided welding. The effects of welding parameters were investigated to increase the quality of the joints.
Conditions leading to defect-free full penetration welds fulfilling naval regulations includes a laser power of 12.5 kW, a welding speed of 1.6 m/min and the vertical laser offset distance from the flange of 1 mm. Advanced characterization of selected welds included a microstructural identification by optical microscopy, SEM, and XRD, revealing the presence of acicular, polygonal and Widmanstätten ferrite, lath martensite, and some retained austenite at FZ. Hardness and microhardness mapping tests showed values of 155 HV at base metal and 200 to 380 HV at the fusion zone connecting the web to the flange.
A self-restraint hot cracking test (free edge test) was used in combination with a novel optical measurement technique to determine the critical straining conditions for solidification cracking for the stainless steel grade 1.4828 (AISI 309). The Lucas-Kanade algorithm for the optical flow (OF) calculation was implemented to obtain the full-field displacement and then the full-field strain.
The use of external laser illumination with appropriate filters allows to obtain good image quality with good contrast. The critical straining conditions required for solidification cracking can be obtained by means the proposed technique in the immediate vicinity of the solidification front. A very good repeatability was demonstrated for the used measurement technique. The critical straining conditions for solidification cracking for the tested steel und under this welding conditions has been detected
Directed energy deposition additive manufactured parts have steep stress gradients and an anisotropic microstructure caused by the rapid thermo-cycles and the layer-upon-layer manufacturing, hence heat treatment can be used to reduce the residual stresses and to restore the microstructure. The numerical simulation is a suitable tool to determine the parameters of the heat treatment process and to reduce the necessary application efforts. The heat treatment simulation calculates the distortion and residual stresses during the process. Validation experiments are necessary to verify the simulation results. This paper presents a 3D coupled thermo-mechanical model of the heat treatment of additive components. A distortion-based validation is conducted to verify the simulation results, using a C-ring shaped specimen geometry. Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to compare the distortion of the samples with different post-processing histories.
Directed energy deposition is a metal additive manufacturing process that builds parts by joining material in a layer-by-layer fashion on a substrate. Those parts are exposed to rapid thermo-cycles which cause steep stress gradients and the layer-upon-layer manufacturing fosters an anisotropic microstructure, therefore stress relief heat treatment is necessary. The numerical simulation can be used to find suitable parameters for the heat treatment and to reduce the necessary efforts to perform an effective stress relieving. Suitable validation Experiments are necessary to verify the results of the numerical simulation.
In this paper, a 3D coupled thermo-mechanical model is used to simulate the heat treatment of an additive manufactured component to investigate the application of a C-ring geometry for the distortion-based validation of the numerical simulation.
Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to quantify the distortion after each process step.
Dieser Beitrag stellt Ergebnisse der Untersuchungen zum Auftragschweißen als Plasma-Laserstrahl-Hybrid-Prozess dar. Es hat sich gezeigt, dass ein Laserstrahl, der in einer gemeinsamen Prozesszone mit einem Plasma-Pulver-Auftragschweißprozess vorlaufend angeordnet ist, diesem Prozess eine erhebliche Geschwindigkeitssteigerung sowie eine Verbesserung der Stabilität ermöglicht. Der Hybrid-Prozess konnte mit Verschleißschutzwerkstoffen
sowie dem Korrosionsschutzwerkstoff Inconel 625 bei Vorschubgeschwindigkeiten von bis zu 10 m/min erfolgreich validiert werden.
Im Hinblick auf die aktuellen Entwicklungen zu Hochgeschwindiglceits-Laserstrahlauftragschweißen kann der Plasma-Laserstrahl-Hybrid-Prozess
zwischen diesen und den konventionellen Verfahren eingeordnet werden.
To increase the competitiveness of jacket substructures compared to monopiles a changeover from an individual towards a serial jacket production based on automated manufactured tubular joints combined with standardized pipes has to be achieved. Therefore, this paper addresses fatigue tests of automatically welded tubular X-joints focusing on the location of the technical fatigue crack. The detected location of the technical crack is compared to numerical investigations predicting the most fatigue prone notch considering the structural stress approach as well as the notch stress approach. Besides, the welding process of the automated manufactured tubular X-joints is presented.
Notch impact toughness of laser beam welded thick sheets of cryogenic nickel alloyed steel X8Ni9
(2020)
The paper deals with the investigations of the impact toughness of laser beam welded 14.5 mm thick sheets made of cryogenic steel X8Ni9 as a function of preheating. This 9% nickel alloyed steel is widely used in the liquefied natural gas (LNG) industry. An application of highly efficient welding processes such as high-power laser beam welding (LBW) in LNG sector requires an understanding of the interactions between the LBW process parameters and weld properties, in particular the impact toughness. The results show that the original fine-grained martensitic microstructure of the base metal (BM) is significantly changed by melting and crystallization during the LBW, what can lead to a decrease in the impact toughness of the weld metal (WM) below the required level. An optimal preheating temperature range leads to the favorable thermal welding cycle and is of remarkable importance for maintaining the notch impact toughness of laser beam welded joints of these thick steel sheets. A parameter window was identified in which V-notch impact toughness comparable to that of the BM at -196 °C was achieved.
The study deals with the application of the high-power hybrid-laser arc welding process on up to 15 mm thick pipe segments with the intention to avoid end crater imperfections during closing of the circumferential welds, where the pipes were turned during welding in 1G- and 2Gpositions.
Different techniques such as laser power ramp-down, abrupt switch-off of the laser power and change of the magnification of the laser spot diameter and defocusing of the laser beam relative to the workpiece were tested to remove the laser energy from the process. It could be shown that a high defocusing of the optic system above 40 mm with a resulting beam diameter > 2.9 mm in a short overlap length of approx.
20 mm leads to the formation of a cup-shaped weld seam, which is preferred for avoidance of cracks and pores in the end crater. A laser optics with motor-driven lens system was used for the welding experiments to defocuse the laser beam without changing the position of the arc.
The transport phenomena in the wire feed laser beam welding are investigated numerically. A three-dimensional transient heat transfer and fluid flow model coupled with free surface tracing and element transport is developed. A ray-tracing method with local grid refinement algorithm is used to calculate the multiple reflections and Fresnel absorption on the keyhole wall. The filler material flows backward along the lateral side of the weld pool, and subsequently flows forward along the longitudinal plane. The occurrence of the bulging phenomenon may further prevent the downward transfer of the additional elements to the root of the weld pool.
The increasing adoption of Open Science principles has been a prevalent topic in the welding science community over the last years. Providing access to welding knowledge in the form of complex and complete datasets in addition to peer-reviewed publications can be identified as an important step to promote knowledge exchange and cooperation. There exist previous efforts on building data models specifically for fusion welding applications; however, a common agreed upon implementation that is used by the community is still lacking. One proven approach in other domains has been the use of an openly accessible and agreed upon file and data format used for archiving and sharing domain knowledge in the form of experimental data. Going into a similar direction, the welding community faces particular practical, technical, and also ideological challenges that are discussed in this paper. Collaboratively building upon previous work with modern tools and platforms, the authors motivate, propose, and outline the use of a common file format specifically tailored to the needs of the welding research community as a complement to other already established Open Science practices. Successfully establishing a culture of openly accessible research data has the potential to significantly stimulate progress in welding research.
the joints throughout the structure. The flat units are constituted by butt welded flat plates, on which longitudinal T-welded reinforcements are placed to rigidize the structure. Among the different welding technologies, Hybrid Laser Arc Welding (HLAW) is becoming a mature process, profitable and highly productive. In addition, more innovative welding equipment are being developed nowadays, offering greater work flexibility, and raising expectations of achieving better quality, and economic viability. Another key point of HLAW to keep in mind is that structural distortions are reduced, resulting in decreasing the cost and time of straightening work.
In the present contribution, the influence of HLAW parameters on the quality of fillet joints of naval steel has been analysed. Experimental HLAW tests were performed with a high power disk laser to join EH36 naval steel plates, with a T configuration. The influence of different processing parameters has been analysed, as the laser power, welding speed, wire feed rate and the configuration of the HLAW processes (including head angle and laser/arc leading process). In addition, FEM simulations were carried out in order to estimate residual stresses and distortion of welded part. The distortion values provided by FEM presented excellent agreement with the measured experimental results. To evaluate the welds, non destructive tests including X-ray tests, metallographic analysis of cross sections, and microhardness mapping tests were performed.
Full penetration 8 mm T welds were obtained for the first time at an industrially applicable 2F position with a reasonable HLAW head angle, in one single step without sealing root, and using zero gap square groove Edge preparation. The present contribution presents welding rates up to 2.2 m/min for 2F T-joints of this steel thickness, a much higher processing velocity than previously reported for industrial applications.
Ein wesentliches Ziel der deutschen Industriepolitik besteht darin, Rahmenbedingungen zu schaffen, welche die Positionierung nachhaltiger Produkte und Prozesse innerhalb globaler Wertschöpfungsketten ermöglichen und ökoeffiziente Fertigungsprozesse langfristig sichern. In diesem Beitrag werden aktuelle Forschungsarbeiten zur Bewertung der Umweltwirkungen der schweißtechnischen Fertigungskette anhand ausgewählter Schmelzschweißverfahren unter Berücksichtigung vor- sowie nachgelagerter Fertigungsschritte dargestellt. Eine weit verbreitete und standardisierte Methode zur Abschätzung der Umweltwirkungen eines Produkts oder Prozesses ist die Ökobilanzierung. Hierbei stellen die Sachbilanzierung, das heißt die Ermittlung sämtlicher relevanter Energie- und Ressourcenverbräuche
während der schweißtechnischen Fertigung, aufgrund des hohen Dokumentationsaufwands, sowie die Ableitung assoziierter Wirkkategorien das größte Hindernis für eine Etablierung und Akzeptanz der Ökobilanzierung in der Praxis dar. Gleichwohl wird seitens Unternehmen, welche als Zulieferer von OEM’s (engl.: original equipment manufacturer) agieren, die Dokumentation des fertigungsspezifischen CO2-Verbrauches gefordert.
Ein weiterer Aspekt der Arbeiten beinhaltet Methoden zur automatisierten Erfassung von schweißtechnischen Produktionsdaten sowie deren Nachverfolgbarkeit. Anhand unterschiedlicher Schweißverfahren werden die aus den Produktionsdaten abgeleiteten Energie- und Ressourcenverbräuche automatisiert in die relevanten Umweltwirkungen überführt. Die analysierten Schweißprozesse umfassen dabei ein breites für kleine und mittlere Unternehmen (KMU) relevantes Spektrum. Durch die softwareseitige Bereitstellung der aufgestellten Umweltprofile ist der Anwender in der Lage, Schweißprozesse unter ökologischen Aspekten zu bewerten und die effizienteste Variante zu identifizieren.
Additive manufacturing, and therefore directed energy deposition, is
gaining more and more interest from industrial users. However, quality assurance for the components produced is still a challenge. Machine learning, especially using artificial neuronal networks, is a potential method for ensuring a high-quality standard. Based on process Parameters and monitoring data, part quality can be predicted. A further advantage is the ability to constantly learn and adopt to slight process changes.
First tests using artificial neural networks focus on the prediction of track geometry. The results show that even a small data set is enough to provide high accuracy in the predictions. In this work, an artificial neural network for the predictive analysis of relative density in laser powder cladding has been developed. A central composite experimental design is used to generate 19 data sets. Input variables are laser power, feed rate and powder mass flow. Cubes are built up where density is considered as a target value. Several neural networks are trained and evaluated with these data sets. Different topologies and initial weights are considered. The best network reaches a confidence level of around 90 % for the prediction of relative density based on the process parameters. Finally, the optimization of the generalization performance is investigated. To this purpose, methods of variation in error limit as well as cross-validation are applied. In this way, density is predictable by an artificial neural network with an accuracy of about 95 %.
The application of the electromagnetic stirring from an oscillating magnetic field can improve the metal mixing in wire feed laser beam welding. However, the extra parameters introduced in this technique make the selection of an optimal combination of process parameters more difficult. In the current study, besides the commonly concerned magnetic flux density and frequency, the influence of the magnetic field orientation (magnetic field angle) on the transport of filler metal is studied numerically and experimentally. Ex-situ X-ray fluorescence spectrometer measurements are used to map the metal mixing in the final weld. A three-dimensional transient multi-physical model is developed to reveal the deeper physical essence, considering the coupling between heat transfer, fluid flow, keyhole dynamics, element transport and magnetohydrodynamics. The spatial distribution of the laser energy on the keyhole wall is calculated by a ray tracing algorithm. The results show that the magnetic field with smaller angle with respect to the transverse direction provides better penetration capacity, and its stirring effect can reach the lower part of the molten pool. Therefore, the smaller magnetic field angle produces better metal mixing. A constant downward flow is formed at the lower part of the molten pool when magnetic field of 10° angle is applied, which brings the filler metal to the root region. As the magnetic field angle increases to 40°, the beneficial downward flow changes into a constant upward flow, resulting in a concentration of the filler metal in the upper region. This study provides further insight of the underlying physics in the electromagnetically enhanced laser beam welding, which may guide the optimization of parameters to achieve property homogeneity or to avoid potential defects.
In diesem Beitrag werden Ergebnisse der Untersuchungen eines Verfahrens zum fehlerfreien Laserstrahl-MSG-Hybridschweißen von geschlossenen Rundnähten vorgestellt. Das Verfahren zielt auf die Vermeidung von Schweißnahtunregelmäßigkeiten im Überlappbereich einer hybridgeschweißten Rundnaht. Eine Strategie der Prozessführung beim Schließen der Rundnaht wurde entwickelt, mit der ein fehlerfreier Überlappbereich durch die Kontrolle der Erstarrungsbedingungen am Schweißnahtende erreicht wird. Die kontrollierte Wärmeführung wird durch eine Anpassung der Parameter von beiden beteiligten Schweißprozessen, dem Laserstrahl sowie dem Metallschutzgas (MSG)-Schweißprozess, realisiert. Experimentelle Untersuchungen wurden an 12 bis 15 mm dicken Rohrabschnitten durchgeführt. Der Einfluss von Prozessparametern wie der Veränderung des Abbildungsmaßstabs und der Defokussierung des Laserstrahls auf die Erstarrungsbedingungen am Ende der Rundnaht wurde untersucht, um eine optimale Strategie zum Herausführen der Prozessenergie zu finden.
Im Rahmen der experimentellen Untersuchungen konnte gezeigt werden, dass eine Defokussierung des Laserstrahls im Bereich zwischen 60 mm und 100 mm über einen kurzen Auslaufbereich der Naht von etwa 15 mm zu einer deutlich besseren Nahtausbildung im Überlappbereich führt. Es konnte eine günstige kelchförmige Schweißnaht ohne eine Tendenz zur Rissbildung erzielt werden. Die Laseroptik mit motorisch angesteuertem Linsensystem ermöglichte dabei eine Vergrößerung des Laserstrahldurchmessers ohne eine Veränderung der Position des MSG-Lichtbogens relativ zur Bauteiloberfläche.
A technique for calculating the main solidification parameters for a two-dimensional columnar crystal growth during complete penetration laser beam welding of thin steel sheets was developed. Given that the weld pool interface is described by Lamé curves (superellipses) within the horizontal plane of growth, general analytical solutions were derived for the geometry of the crystal axis and the corresponding growth rate and cross-sectional area of the crystal. A dimensionless analysis was performed to provide insights on the dependence of the solidification parameters on the shape and dimensions of the rear part of the weld pool boundary. The derived solutions were applied for the case of complete penetration laser beam keyhole welding of 2 mm thick 316L austenitic chromium-nickel steel sheets. It was shown that the reconstruction of the weld pool boundary with Lamé curves provides higher accuracy and flexibility compared to results obtained with elliptical functions. The validity of the proposed technique and the derived analytical solutions was backed up by a comparison of the obtained solutions to known analytical solutions and experimentally determined shapes and sizes of the crystals on the top surface of the sheet. The dimensions of the calculated crystal axis correlated well with the experimentally obtained results.
It is already known that the laser beam welding (LBW) or hybrid laser-arc welding (HLAW) processes are sensitive to manufacturing tolerances such as gaps and misalignment of the edges, especially at welding of thick-walled steels due to its narrow beam diameter. Therefore, the joining parts preferably have to be milled. The study deals with the influence of the edge quality, the gap and the misalignment of edges on the weld seam quality of hybrid laser-arc welded 20-mm-thick structural steel plates which were prepared by laser and plasma cutting. Single-pass welds were conducted in butt joint configuration. An AC magnet was used as a contactless backing. It was positioned under the workpiece during the welding process to prevent sagging. The profile of the edges and the gap between the workpieces were measured before welding by a profile scanner or a digital camera, respectively. With a laser beam power of just 13.7 kW, the single-pass welds could
be performed. A gap bridgeability up to 1 mm at laser-cut and 2 mm at plasma-cut samples could be reached respectively.
Furthermore, a misalignment of the edges up to 2 mm could be welded in a single pass. The new findings may eliminate the need for cost and time-consuming preparation of the edges.
МОДЕЛИРОВАНИЕ ГИДРОДИНАМИЧЕСКИХ И ТЕПЛОВЫХ ПРОЦЕССОВ ПРИ ЛАЗЕРНОЙ СВАРКЕ СО СКВОЗНЫМ ПРОПЛАВЛЕНИЕМ
(2020)
Разработана модель физических процессов при сварке плавлением на основе концепции эквивалентных источников теплоты. Модель включает в себя две части: термогидродинамику сварочной ванны и теплопроводность свариваемого тела вне ванны. В задаче термогидродинамики учитываются температурные зависимости свойств материала, форма парогазового канала, термокапиллярная и естественная конвекция, фазовые превращения и другие физические явления.Приведено решение задачи термогидродинамики методом конечных элементов на примере сварки стальной пластины толщиной 15 мм со сквозным проплавлением лазерным лучом (по технологии "замочная скважина"). Показано, что термокапиллярная конвекция жидкого металла является основной причиной сложной выпукло-вогнутой формы границы ванны с увеличенными размерами в приповерхностных областях. Получено удовлетворительное совпадение расчетных и экспериментальных размеров сварочной ванны.
This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone.
Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials’ plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve; (2) profile of the indented surface; (3) analyzing of the 3D-measurement image; and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good Agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface.
The present work deals with the development of a strategy for the prevention of end crater defects in high-power laser welding of thick-walled circumferential welds. A series of experiments were performed to understand the influence of the welding Parameters on the formation of end crater defects such as pores, cracks, root excess weld metal and shrinkage cavities in the overlap area. An abrupt switch-off of the laser power while closing the circumferential weld leads to a formation of a hole which passes through the whole welded material thickness. A laser power ramp-down causes solidification cracks which are initiated on the transition from full-penetration mode to partial penetration. Defocusing the laser beam led to promising results in terms of avoiding end crater defects. Cracks and pores in the overlap area could be effectively avoided by using defocusing techniques.
A strategy for avoiding of end crater imperfections was tested on flat specimens of steel grade S355J2 with a wall thickness of between 8 mm and 10 mm and then transferred on the 10 mm thick pipe sections made of high-strength pipeline steel API5L-X100Q.
Among the various welding technologies, resistance spot welding (RSW) and laser beam welding (LBW) play a significant role as joining methods for the automobile industry. The application of RSW and LBW for the automotive body alters the microstructure in the welded areas. It is necessary to identify the mechanical properties of the welded material to be able to make a reliable statement about the material behavior and the strength of welded components. This study develops a method by which to determine the mechanical properties for the weldment of RSW and LBW for two dual phase (DP) steels, DP600 and DP1000, which are commonly used for the automotive bodies. The mechanical properties of the resistance spot weldment were obtained by performing tensile tests on the notched tensile specimen to cause an elongation of the notched and welded area in order to investigate its properties. In order to determine the mechanical properties of the laser beam weldment, indentation tests were performed on the welded material to calculate its force-penetration depth-curve. Inverse numerical simulation was used to simulate the indentation tests to determine and verify the parameters of a nonlinear isotropic material model for the weldment of LBW. Furthermore, using this method, the parameters for the material model of RSW were verified. The material parameters and microstructure of the weldment of RSW and LBW are compared and discussed. The results show that the novel method introduced in this work is a valid approach to determine the mechanical properties of welded high-strength steel structures. In addition, it can be seen that LBW and RSW lead to a reduction in ductility and an increase in the amount of yield and tensile strength of both DP600 and DP1000.
Due to rapid, localized heating and cooling, distortions accumulate in additive manufactured laser metal deposition (LMD) components, leading to a loss of dimensional accuracy or even cracking. Numerical welding simulations allow the prediction of these deviations and their optimization before conducting experiments. To assess the viability of the simulation tool for the use in a predictive manner, comprehensive systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to Cluster these products in new assembly oriented product families for the optimization.
The laser-based direct energy deposition (DED) as a technology for additive manufacturing allows the production of near net shape components. Industrial applications require a stable process to ensure reproducible quality. Instabilities in the manufacturing process can lead to faulty components which do not meet the required properties.
The DED process is adjusted by various parameters such as laser power, velocity, powder mass flow and spot diameter, which interact with each other. A frequently used comparative parameter in welding is the energy per unit length and is calculated from the laser power and the velocity in laser welding. The powder per unit length comparative parameter in the DED process has also be taken into account, because this filler material absorbs energy in addition to the base material.
This paper deals with the influence of mass energy as a comparative parameter for determining the properties of additively manufactured parts. The same energy per unit length of 60 J/mm as well as the same powder per unit length of 7.2 mg/mm can be adjusted with different parameter sets. The energy per unit length and the powder per unit length determine the mass energy. The laser power is varied within the experiments between 400 W and 900 W. Energy per unit length and powder per unit length are kept constant by adjusting velocity and powder mass flow. Using the example of Inconel 718, experiments are carried out with the determined parameter sets. In a first step, individual tracks are produced and analyzed by means of micro section. The geometry of the tracks shows differences in height and width. In addition, the increasing laser power leads to a higher dilution of the base material. To determine the suitability of the parameters for additive manufacturing use, the individual tracks are used to build up parts with a square base area of 20x20 mm². An investigation by Archimedean principle shows a higher porosity with lower laser power. By further analysis of the micro sections, it can be seen that at low laser power, connection errors occur between the tracks.
The results show that laser power, velocity and powder mass flow have to be considered in particular, because a constant mass energy can lead to different geometric as well as microscopic properties.
The influence of heat input and welding speed on the microstructure and mechanical properties of single-pass hybrid laser arc welded 20mm thick plates of high-strength pipeline steel X120 were presented. The heat Input was varied in the range of 1.4 kJ mm−1 to 2.9 kJ mm−1, while the welding speed was changed between 0.5m min−1 and 1.5m min−1. A novel technique of bath support based on external oscillating electromagnetic field was used to compensate the hydrostatic pressure at low welding velocities. A major advantage of this technology is, that the welding speed and thus the cooling time t8/5 can be variated in a wide parameter window without issues regarding the weld root quality. The recommended welding thermal cycles for the pipeline steel X120 can be met by that way. All tested Charpy-V specimens meet the requirements of API 5 L regarding the impact energy. For higher heat inputs the average impact energy was 144 ± 37 J at a testing temperature of −40 °C. High heat Input above 1.6 kJ mm−1 leads to softening in the weld metal and heat-affected-zone resulting in loss of strength. The minimum tensile strength of 915 MPa could be achieved at heat inputs between 1.4 kJ mm−1 and 1.6 kJ mm−1.
Advanced high strength steels are usually coated by a zinc layer for an increased resistance against corrosion. During the resistance spot welding of zinc coated steel grades, liquid metal embrittlement (LME)mayoccur. As a result, cracking inside and around the spot weld indentation is observable. The extent of LME cracks is influenced by a variety of different factors. In this study, the impact of the used electrode geometry is investigated over a stepwise varied weld time.
A spot welding finite element simulation is used to analyse and explain the observed effects. Results show significant differences especially for highly increased weld times. Based on identical overall dimensions, electrode geometries with a larger working plane allow for longer weld times, while still preventing LME within the investigated material and maintaining accessibility.
Study on the transition behavior of the bulging effect during deep penetration laser beam welding
(2022)
The present work is devoted to the study of the transition behavior of the recently confirmed widening of the weld pool, known as the bulging effect, during high-power deep penetration laser beam welding of thick unalloyed steel sheets. A three-dimensional transient multi-physics numerical model is developed, allowing for the prediction of the bulge formation and the study of its temporal behavior. The model is generalized to account automatically for the transition from partial to complete penetration. Several experimental measurements and observations, such as drilling period, weld pool length, temperature, efficiency, and metallographic cross-sections are used to verify the model and assure the plausibility of the numerical results. The analysis of the calculated temperature and velocity distributions, as well as the evolution of the keyhole geometry, shows that the formation of a bulging region strongly depends on the penetration depth of the weld. Based on the numerical results, the bulge is found to occur transiently, having its transition from a slight bulge to a fully developed bulging between penetration depths of 6 mm and 9 mm, respectively.
Characterization of Ti-6Al-4V fabricated by multilayer laser powder-based directed energy deposition
(2022)
Laser powder-based directed energy deposition (DED-L) is increasingly being used in additive manufacturing (AM). As AM technology, DED-L must consider specific challenges. It must achieve uniform volume growth over hundreds of layers and avoid heat buildup of the deposited material. Herein, Ti–6Al–4V is fabricated using an approach that addresses these challenges and is relevant in terms of transferability to DED–L applications in AM. The assessment of the obtained properties and the discussion of their relationship to the process conditions and resulting microstructure are presented. The quality of the manufacturing process is proven in terms of the reproducibility of properties between individual blanks and with respect to the building height. The characterization demonstrates that excellent mechanical properties are achieved at room temperature and at 400 °C.
Herein, an improved ray-tracing routine using a virtual mesh refinement approach is adopted in a 3D transientmultiphysics computational fluid Dynamics model for deep penetration wire feed laser beam welding. In a previous study, it was shown that the improved localization of the reflection points of the subrays within the keyhole leads to a more realistic development of the keyhole Depth being validated with experimental results. Another effect investigated in Detail herein is a drastic change in the flow behavior in the weld pool, which promotes the occurrence of a necking area in the solidification line and subsequent bulging under specific circumstances. This has a detrimental effect on the filler material element transport in the weld pool, leading to an inhomogeneous dilution of the added material. The numerical observations are backed up by experimentally obtained data, allowing to provide a clear physics-based explanation of the reduced mixing behavior of the filler wire in the melt pool.