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Some zinc coated advanced high strength steels (AHSS), under certain manufacturing conditions, are known to be prone to liquid metal embrittlement (LME) during resistance spot welding. LME is an undesired phenomenon, which can cause both surface and internal cracks in a spot weld, potentially influencing its strength. An effort is made to understand influencing factors of LME better, and evaluate geometry-material combinations regarding their LME susceptibility. Manufacturers benefit from such knowledge because it improves the processing security of the materials. The experimental procedure of welding under external load is performed with samples of multiple AHSS classes with strengths up to 1200 MPa, including dual phase, complex phase and TRIP steels. This way, externally applied tensile load values are determined, which cause liquid metal embrittlement in the samples to occur. In the future, finite element simulation of this procedure gives access to in-situ stress and strain values present during LME formation. The visualization improves the process understanding, while a quantification of local stresses and strains allows an assessment of specific welded geometries.
The additional element from the filler wire in the laser beam welding is usually distributed inhomogeneously in the final weld due to the high solidification rate of weld pool. It has been found that the electromagnetic stirring produced by an external oscillating magnetic field can enhance the material mixing in the weld pool to achieve a more uniform element distribution. However, the magnetic field has a highly non-linear and multi-coupled interaction with the weld pool behavior, which makes the quantitative explanation of the physical mechanism difficult. In this study, the effect of electromagnetic stirring on the transport phenomena in the wire feed laser beam welding is investigated by a numerical modelling. A 3D transient multi-physical model considering the magnetohydrodynamics, heat transfer, fluid flow, keyhole dynamics and element transport is developed. The multiple reflections and the Fresnel absorption of the laser on the keyhole wall are calculated using the ray tracing method. The numerical results show that a Lorentz force produced by the oscillating magnetic field and its induced eddy current gives significant influence on the transport phenomena in the molten pool. The forward and downward flow is enhanced by the electromagnetic stirring, which homogenizes the distribution of the additional elements from a nickel-based filler wire in a steel weld pool. The numerical results show a good agreement with the high-speed images of the molten pool, the fusion line from the optical micrograph and the element distribution from the energy dispersive X-ray spectroscopy. This work provides a physical base for the electromagnetic-controlled laser beam welding and some guidance for the selection of electromagnetic parameters.
The control of friction and wear is a major concern in many industrial applications. A promising method for a tailored surface modification is the so-called laser implantation technique. This method combines surface texturing and material optimization in one processing step by a localized dispersing of hard ceramic particles using pulsed laser radiation. Wear resistant, protruding micrometric features (implants) with defined geometry can be created in deterministic pattern where needed on highly stressed surfaces, i.e. on forming or cutting tools. However, in order to maintain the implants over the tool’s lifetime, a suitable selection of hard ceramic particles is a prerequisite. They must provide a defect-free Metal Matrix Composite with a high share of homogeneously distributed particles and especially a high implant hardness.
In this study TiN, TiC and TiB2 hard particles were compared as implant materials for the first time. By a systematic variation of the pulse power and pulse duration, their dispersing behavior and influence on the material properties of AISI D2 tool steel was investigated. Although all powder materials had grain sizes smaller than 10 µm, it was possible to disperse them by pulsed laser radiation and to obtain defect-free protruding implants. The highest share of dispersed particles (~64 %) was observed for TiB2. By scanning electron microscopy and energy dispersive X-ray spectroscopy, it was also shown that a significant share of the pre-placed particles was dissolved by the laser beam and precipitated as nanometer sized particles within the matrix during solidification. These in-situ formed particles have a decisive influence on the material properties. While the TiN and TiC implants have shown maximum hardness values of 750 HV1 and 850 HV1, the TiB2 implants have shown the highest hardness values with more than 1600 HV1. By X-ray diffraction, it was possible to ascribe the lower hardness values of TiC and TiN implants to high amounts of retained austenite in the metal matrix. By implanting TiB2, the formation of retained austenite was successfully suppressed due to the in-situ formation of TiC particles, which was proven by electron backscatter diffraction. In conclusion, all the implant materials are basically suitable for laser implantation on AISI D2 tool steel. However, TiB2 has shown the most promising results.
Gut kombiniert: Laser-Plasma-Auftragschweißen
Für den Schutz vor starkem abrasiven Verschleiß (Panzern) oder vor korrosiven Medien (Plattieren) sowie für das Auftragen einer Pufferlage können das Laser-Pulver-Auftragschweißen oder das Plasma-Pulver-Auftragschweißen zum Einsatz kommen. Während das Laser-Pulver-Auftragschweißen eine geringe thermische Belastung verspricht, ermöglicht das Plasma-Pulver-Auftragschweißen hohe Auftragraten. Wissenschaftler des Fraunhofer IPK erarbeiten neuartige Verfahrenskombinationen und Düsenkonzepte, die die jeweiligen Vorteile dieser bestehenden Verfahren vereinen und somit eine hohe Qualität und Effizienz garantieren. So kann beispielsweise die punktgenaue Energie der Laserstrahlung den Plasmalichtbogen stabilisieren und den Wärmeeintrag in das Bauteil besser kontrollieren.
Plasma-Transferred-Arc (PTA) welding is a process that enables high deposition rates, but also causes increased thermal 9 load on the component. Laser based Direct Energy Deposition (DED) welding, on the other hand, achieves a high level of 10 precision and thus comparatively low deposition rates, which can lead to high processing costs. Combining laser and arc 11 energy aims to exploit the respective advantages of both technologies. 12 In this study, different possibilities of this process combination are presented using a PTA system and a 2 kW disk laser. 13 This includes the combination in a common process zone as a highspeed plasma laser cladding technology (HPLC), which 14 achieves process speeds of 10 m/min. Besides that it is being examined whether a pre-running plasma arc can be used to 15 coat difficult-to-weld rail steel with a carbon content of 0.8 % due to a preheating effect. Furthermore, a smoothing of the 16 coating by a plasma arc following the laser is investigated.
A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes’ temperature by the melting temperature.
The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g. the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.
The weld pool dynamics and shape play a fundamental role in keyhole mode welding. The presented work aims the experimental and numerical investigation of the influence of the weld pool characteristics on the formation of hot cracking. The experimental procedure allows recording the molten pool in the longitudinal section of a butt joint configuration of 15 mm thick structural steel and transparent quartz glass by using a high-speed video camera and two thermal imaging 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 are observed approximately in the middle of the depth of the weld pool, where hot cracking appears. A numerical framework including models for the weld pool dynamics, global temperature field, transient stress state, crystal growth, diffusion and macro-segregation and subroutines for their one-way couplings is developed. The numerically obtained and experimentally observed results are in a good agreement. It is shown that the bulge-region leads to a delay in the solidification behavior, increased temporal tensile stresses and accumulation of impurities in the defect region and hence enhance the probability of hot cracking formation.
The geometrical characteristics of the weld end crater are commonly used for the validation of numerical results in welding simulation. A semi-analytical model calculating the cooling stage of the welding process after the moving energy source is turned off has been developed. A solution for various combinations of heat sources and workpieces has been found. The theoretical limits for the heat transfer of the absorbed energy during cooling in a thin plate and a semi-infinite body were studied. It is shown that after turning off the energy source, an additional melting of the base material in longitudinal direction may occur. The developed technique is applied to complete-penetration keyhole laser beam welding of a 2 mm thick austenitic chromium-nickel 316L steel plate at a welding speed of 20 mm/s and a laser power of 2.3 kW. The results show a theoretical increase of the weld end crater length in comparison to the length of the steady-state weld pool of up to 19 %. A shift of the centre of the end crater, in which the solidification of the liquid metal ends, towards the tail of the end crater relative to the axis of the heat source at the time of its termination, was computed. The speed and the direction of crystallization of the molten material in the weld pool and the end crater were found to be different. A good agreement between the computational results and the welding experiments was achieved.
The talk will give an overview of past, ongoing and future activities at BAM welding divisions, specifically those regarding prospects and challenges of the ongoing digital transformation and the move to more accessible research data (open science) with concerns to welding research. We will discuss current ways to publish and share research results inside the welding community and highlight approaches and advancements from other scientific fields to improve accessibility and reproducibility. We would also like to discuss the feasibility of integrating open science principles into the current IIW landscape of meetings, publications and education. In Addition we will introduce our upcoming series of workshops organized by BAM focusing on defining and implementing an open source file format specifically designed to publish and exchange high quality welding research data.
WelDX is a newly started research project at BAM publicly funded by the German Federal Ministry of Education and Research. Over the course of three years the project aims to build the foundation for a publicly accessible file format and to foster research collaboration in arc welding on a national and international level.
The talk will motivate benefits and discuss challenges of using a common file format designed to describe, store and share arc welding research data in the scope of "Open Science". By using common open source software and tools, welding data will be made more accessible and reusable so that new scientific practices may emerge.
The proposed file format aims to be easy to use for common welding applications while also offering the possibility to describe complex experiments for state of the art welding research.
In addition the talk will illustrate how in the future other facilities and researchers will be able to use experimental arc welding data generated at BAM for their own research, for example to conduct their own data analysis or welding process and thermo-mechanical simulations.
Ziel des WelDX Forschungsvorhabens ist es, den Austausch wissenschaftlicher Forschungsdaten aus dem Bereich der Schweißtechnik auf Basis eines einheitlichen Open Source Dateiformates zu ermöglichen und damit die Einhaltung und Dokumentation geeigneter Qualitätsstandards zu fördern. Im Zuge der Projektarbeit wird die Zusammenarbeit und der Wissenstransfer schweißtechnischer Institute zunächst auf nationaler Ebene gefördert und die Bildung eines von der Fachcommunity getragenen Kuratoriums zur Betreuung und Weiterentwicklung des Dateiformates und der Ausarbeitung zugehöriger Qualitätsstandards angestrebt.
Zunächst erfolgt die Erstellung und Veröffentlichung eines geeigneten Dateiformates zum Austausch großvolumiger, detaillierter Forschungsdaten im Bereich mechanisierter und automatisierter Schweißverfahren. Durch die Nutzung des Dateiformates zur Dokumentation der Forschungsarbeiten soll auch für komplexe Schweißaufgaben und Versuchsaufbauten eine deutlich gesteigerte objektive Vergleichbarkeit der Messdaten und Ergebnisse im Sinne der guten wissenschaftlichen Praxis zwischen den Instituten erzielt werden.
Der Vortrag gibt einen Überblick über Arbeiten im Rahmen der Themenstellung "photonische Prozessketten" des SimCoLas Projekts. Es werden Ergebnisse zur Datendurchgängigkeit der Prozesskette Umformen/Laserstrahlschweißen sowie erste Ergebnisse zu numerischen Schweißstruktursimulationen zur Bewertung der Heissrisskritikalität eines tiefgezogenen Napfes beim Laserstrahlschweißen gegeben.
Die Präsentation gibt einen Überblick bezüglich der von der BAM im Rahmen des SimCoLas Verbundprojekts durchgeführten Arbeiten zum Thema "Photonische Prozessketten". Die numerisch abgebildete Prozesskette Tiefziehen/Laserstrahlschweißen wird darsgestellt und diskutiert. Fokus der Diskussion ist die entwickelte Methodik zur konsistenten Abbildung numerischer Prozessketten mit der Applikation Heissrisskritikalität beim Laserstrahlschweißen.
The deep penetration laser beam welding (LBW) has developed to one of the most promising metal joining methods in the modern manufacturing industry. It has well-known advantages of good penetration capacity, low heat input, high reachable welding speed and low welding distortion in comparison to conventional arc welding techniques. However, there are still challenges in LBW making the realization of the advantages difficult, such as porosity or inhomogeneous element distribution when using filler material. The magnetohydrodynamics technique is a promising way to solve these issues by introducing a suitable electromagnetic field, and correspondingly Lorentz force, to control the Transport phenomena in the weld pool.
The underlying physics in wire feed laser beam welding with electromagnetic stirring were investigated numerically and experimentally. A three-dimensional transient heat transfer and fluid flow model coupled with dynamic keyhole, magnetic induction and element transport was developed for the first time. The electromagnetic behaviour as well as the temperature and velocity profiles, solidification parameters, keyhole evolution and element transport are calculated.
The model is well tested against the experimental results. The beneficial effects from electromagnetic stirring (element homogenization and grain refinement) are explained quantitatively using the numerical data and the results from high-speed imaging, OM, EDX and EBSD.
Laser beam welding of aluminum die casting is challenging. A large quantity of gases (in particular hydrogen) is absorbed by aluminum during the die-cast manufacturing process and is contained in the base material in solved or bound form. After re-melting by the laser, the gases are released and are present in the melt as pores. Many of these metallurgic pores remain in the weld seam as a result of the high solidification velocities. The natural (Archimedean) buoyancy is not sufficient to remove the pores from the weld pool leading to process instabilities and poor mechanical properties of the weld. Therefore, an electromagnetic (EM) system is used to apply an additional buoyancy component to the pores. The physical mechanism is based on the generation of Lorentz forces, whereby an electromagnetic pressure is introduced into the weld pool. The EM system exploits the difference in electrical conductivity between poorly conducting pores (inclusions) and the comparatively better conducting aluminum melt to increase the resulting buoyancy velocity of the pores. Within the present study, the electromagnetic supported degassing is investigated in dependence on the laser beam power, welding velocity and electromagnetic flux density. By means of a design of experiments a systematic variation of these parameters is carried out for partial penetration laser beam welding of 6 mm thick sheets of wrought aluminum alloy AlMg3 and die-cast aluminum alloy AlSi12(Fe) where the wrought alloy serves as a reference. The proportion of pores in the weld seams is determined using X-ray images, computed tomography (CT-) images and cross-section images. The results prove a significant reduction of the porosity up to 70 % for both materials as a function of the magnetic flux density
The introduction of an external magnetic field in laser beam welding can bring a lot of beneficial effects e. g. a homogeneous element distribution and grain refinement. However, the physical mechanism has not been fully revealed because it is difficult to describe the weld pool behavior quantitatively. In this work, the influence of an oscillating magnetic field in wire feed laser beam welding is studied using a three-dimensional transient multi-physical model. The magnetohydrodynamics, heat transfer, fluid flow, keyhole dynamics and element transport are all taken into consideration. Under suitable electromagnetic parameters, a significant electromagnetic stirring can be produced in the weld pool by the induced Lorentz force. It shows important influence on the keyhole stability and the fluid flow. The keyhole collapses more frequently at the upper part under the oscillating magnetic field. The forward and downward flow along the central plane of the weld pool is enhanced, which can bring the additional element from the filler wire to the root of the weld pool. There is a good agreement between the numerical results and the experiment results from the high-speed imaging, the optical micrograph and the energy dispersive X-ray spectroscopy. This work provides a fundamental understanding of the relationship between electromagnetic control, weld pool behavior and weld property.
The shape of the parts, created by the technology of direct laser metal deposition (DLMD), is influenced by various parameters, for example, the power and diameter of the laser source spot. The contribution of energy from the laser affects the temperature distribution in the formed layers. The changing temperature in the working area entails a change in the geometry of the layers and affects the stability of the process. In this paper, experiments on the measurement of temperature cycles in the DLMD process with different directions of the filling track are carried out. An infrared camera was used to measure thermal cycles. The calibration of the acquired data (i.e. correspondence table between the intensity of thermal radiation of the material and the absolute temperature) was done with help of two-color pyrometer ex situ and in situ measurements. The experiments are carried out on two materials 316L and Inconel 718. The effect of the maximum temperature on the layer height is shown, and thermal cycles in the formation of layers for different filling strategies are presented.
In this study, the influence of the welding speed and the arc power on the solidification crack formation for partial penetration laser hybrid welded Thick-Walled plates were investigated. Experimentally, a linear correlation between the welding velocity and the crack number was observed. That is by reducing the welding velocity the crack number was reduced.
The reduced welding velocity showed a strong impact on stress, as the model demonstrated a very lower stress amount in comparison to the reference case. The reduction of the welding speed could be a helpful technique to reduce the hot cracking. The wire feed speed showed a very slight influence on the crack formation. That can be returned to the large distance between the critical region for cracking and the arc region.
In this study, the influence of the welding speed, the arc power and the laser focal position on the solidification crack formation for partial penetration laser hybrid welded thick-walled plates were investigated. Experimentally, a direct correlation between the welding speed and the crack number was observed. That is by reducing the welding velocity the crack number was decreased. The focal position shows also a significant influence on the crack number. Since by focusing the laser on the specimen surface, the crack number has been significantly diminished.
The wire feed speed showed a very slight influence on the crack formation. That is due to the large distance between the critical region for cracking and the arc region.
The numerical model shows a high-stress concentration in the weld root for both components (vertical and transversal).
Numerically, the reduced welding speed showed a strong impact on stress, as the model demonstrated a lower stress amount by decreasing the welding speed. The solidification cracking in the weld root is a result of interaction between metallurgical and geometrical and thermomechanical factors.
Single-pass Hybrid Laser Arc Welding of Thick Materials Using Electromagnetic Weld Pool Support
(2019)
Hybrid laser-arc welding process allows single-pass welding of thick materials, provides good quality formation of joints with minimal thermal deformations and a high productivity in comparison with arc-based welding processes. Nevertheless, thick-walled steels with a thickness of 20 mm or more are still multi-pass welded using arc welding processes, due to increased process instability by increasing laser power. One limitation factor is the inadmissible formation of gravity drop-outs at the root. To prevent this, an innovative concept of electromagnetic weld pool support is used in this study. With help of such system a stable welding process can be established for 25 mm thick steel plates and beyond. Sound welds could be obtained which are tolerant to gaps and misalignment of the welded parts. The adaptation of this system to laser and hybrid laser-arc welding process can dramatically increase the potential field of application of these technologies for real industrial implementation.