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Multi-material structures are a promising solution to reduce vehicle weight and save fuel or electric energy in automotive design. However, thermal joining of steel and aluminum alloys is a challenge to overcome due to different material properties and the formation of brittle intermetallic phases. In this study, a new joining approach for producing overlap line-shaped joints is presented. The lower joining partner (EN AW 5754) is melted by a laser beam and this melt is displaced into a line-shaped cavity of the upper joining partner (1.0330) by induced Lorentz forces. The melt solidifies in the cavity to a material and form-fitting joint. This approach needs no auxiliary joining elements or filler materials. Previous investigation to produce spot-shaped joints by using this approach showed that quality and reproducibility were limited by known melt pool dynamics of aluminum alloys (keyhole collapses). For line-shaped joints, the melt displacement can take place behind the keyhole. This allows the displacement process to be spatial uncoupled from the influence of the keyhole collapses. The study shows that this improved the process stability and the quality of the joint. The created line-shaped joints were microstructurally characterized by transversal sections. Intermetallic phases were identified by electron backscatter diffraction (EBSD) and EDX-Analysis. The detected intermetallic phases consist of a 5 µm - 6 µm compact phase seam of Al5.6Fe2 and a needle shaped phase of Al13Fe4. Tensile shear tests were carried out to quantify the load capacity. It was possible to create a joint with a load capacity of about 2 kN.
The weld quality and the possible defect formation are directly determined by the weld pool shape and the thermo-fluid dynamics therein. In this paper, an untypical weld pool profile, i.e., elongated at its top and bottom but narrowed at the middle, is found experimentally and numerically in the wire feed laser beam welding. The detrimental influence of the weld pool narrowing on the element transport is analyzed and discussed. A magnetohydrodynamic technique is utilized to suppress the narrowing, aiming at a more homogenous element distribution. It is found that a low-temperature region is formed in the middle of the weld pool due to the interaction of the two dominant circulations from the top and bottom regions. The weld pool is significantly narrowed due to the untypical growth of the mushy zone in the low-temperature region, which results in a direct blocking effect on the downward flow and the premature solidification in the middle region. The Lorentz force produced by a transverse oscillating magnetic field shows the potential to change the flow pattern into a single-circulation type and the low-temperature-gradient region is mitigated. Therefore, the downward transfer channel is widened, and its premature solidification is prevented. The numerical results are well validated by experimental measurements of metal/glass observation and X-ray fluorescence element mapping.
Numerical study on the formation of a bulging region in partial penetration laser beam welding
(2022)
A transient three-dimensional thermo-fluid dynamics numerical model was developed to study the formation of a bulging region in partial penetration laser beam welding. The model accounts for the coupling between the fluid flow, the heat transfer, and the keyhole dynamics by considering the effects of multiple reflections and Fresnel absorption of the laser beam in the keyhole, the phase transitions during melting and evaporating, the thermo-capillary convection, the natural convection, and the phase-specific and temperature-dependent material properties up to the evaporation temperature. The validity of the model was backed up by experimentally obtained data, including the drilling time, the weld pool length, the local temperature history outside the weld pool, the process efficiency, and a range of metallographic cross-sections. The model was applied for the cases of partial penetration laser beam welding of 8 mm and 12 mm thick unalloyed steel sheets. The obtained experimental and numerical results reveal that the bulging region forms transiently depending on the penetration depth of the weld, showing a tendency to transition from a slight bulging to a fully developed bulging region between penetration depths of 6 mm and 9 mm, respectively.
A three-dimensional numerical model was developed to accurately predict the steady-state weld pool shape in full penetration laser beam welding. The model accounts for the coupling between the heat transfer and the fluid dynamics by considering the effects of solid/liquid phase transition, thermo-capillary convection, natural convection, and phase-specific and temperature-dependent material properties up to the evaporation temperature. A fixed right circular cone was utilized as a keyhole geometry to consider the heat absorbed from the laser beam. The model was used to analyze the influence of the thermodynamic activity of solutes and turbulence on the weld pool shape. A mesh sensitivity analysis was performed on a hybrid mesh combining hexahedral and tetrahedral elements. For the case of full penetration laser beam welding of 8 mm thick unalloyed steel sheets, the dependence of the weld pool shape on the surface-active element sulfur was found to be negligible. The analysis of the results showed that a laminar formulation is sufficient for accurately predicting the weld pool shape since the turbulence has a minor impact on the flow dynamics in the weld pool. The validity of the numerical results was backed up by experimental measurements and observations, including weld pool length, local temperature history, and a range of metallographic cross-sections.
Environmental protection, resource conservation and CO2 reduction require new joining concepts for effective multi-material design in automotive lightweight construction. The thermal joining of dissimilar materials, especially the combination of aluminum alloys and steel, is associated with difficulties. Different material properties, such as melting point and coefficient of thermal expansion, complicate joining processes. Furthermore, the insolubility between these materials results in the formation of intermetallic phases. These brittle phases reduce the load-bearing capacity and quality of the joints. The thickness of the intermetallic phases should not reach the critical value of 10 μm to ensure good mechanical properties of the joint. It is known that a two-phase reaction layer consisting of Al5Fe2 and Al3Fe (also known as Al13Fe4) forms at the interface between solid steel and liquid aluminum. Due to high cooling rates, as it is the case in laser beam welding, metastable intermetallic compounds can form different from the equilibrium state. The formation of the intermetallic reaction layer is a diffusion-controlled process. So, it is only possible to control the growth of these phases. This problem has led to use joining methods that do not require the melting of both joining partners. A promising joining method is laser beam welding-brazing, whereby only one joining partner is melted and wets the solid joining partner. Conventional laser beam welding-brazing only results in material-fit joints and often requires the use of expensive filler materials. An additional form-fit could optimize the mechanical performance of the joint. For this reason, a new joining method for overlap configurations of dissimilar materials was developed. A laser beam melts the lower joining partner through a cavity of the upper joining partner. The created melt pool is moved upwards into the cavity of the upper joining partner due to contactless induced Lorentz forces of an AC-magnetic system. The displaced melt creates a form- and material-fit joint after solidification. The advantage of this joining technology is the absence of filler materials, flux agents and expensive auxiliary joining elements.
The present work deals with the recently confirmed widening of the weld pool interface, known as a bulging effect, and its relevance in high power laser beam welding. A combined experimental and numerical approach is utilized to study the influence of the bulge on the hot cracking formation and the transport of alloying elements in the molten pool. A technique using a quartz glass, a direct-diode laser illumination, a high speed camera, and two thermal imaging cameras is applied to visualize the weld pool geometry in the longitudinal section. The study examines the relevance of the bulging effect on both, partial and complete penetration, as well as for different sheet thicknesses ranging from 8 mm to 25 mm. The numerical analysis shows that the formation of a bulge region is highly dependent on the penetration depth and occurs above 10 mm penetration depth. The location of the bulge correlates strongly with the cracking location. The obtained experimental and numerical results reveal that the bulging effect increases the hot cracking susceptibility and limits the transfer of alloying elements from the top of the weld pool to the weld root
The study deals with the determination of the influence of an externally applied oscillating magnetic field on the melt pool dynamics in high power laser beam and hybrid laser arc welding processes. An AC magnet was positioned under the workpiece which is generating an upward directed electromagnetic force to counteract the formation of the droplets. To visualise the melt flow characteristics, several experiments were carried out using a special technique with mild steel from S355J2 with a wall thickness of up to 20 mm and a quartz glass in butt configuration. The profile of the keyhole and the melt flow were recorded with a high-speed camera from the glass side. Additionally, the influence of the magnetic field orientation to the welding direction on the filler material dilution on laser hybrid welding was studied with variating oscillation frequency. The element distribution over the whole seam thickness was measured with X-ray fluorescence (XRF). The oscillation frequency demonstrated a greater influence on the melt pool dynamics and the mixing of the elements of the filler wire. The high-speed recordings showed, under the influence of the magnetic field, that the melt is affected under strong vortex at the weld root, which also avoids the formation of droplets
Solidification cracking is still a particular problem in laser beam welding, especially in the welding of thick-walled plates. In this study, the influence of weld pool geometry on solidification cracking in partial penetration welding of thick plates is a subject of discussion. For this purpose, a special experimental setup of steel and quartz glass in butt configuration and lateral with high speed camera was used to capture the weld pool shape. Additionally, laser beam welding experiments were carried out to compare the crack positions and the cross section with the high-speed camera observations. The results showed a bulge in the weld pool root separated from the upper region by a nick area. This leads to the fact that three different longitudinal lengths with different solidification areas are taking place. This temporal sequence of solidification strongly promotes the solidification cracks in the weld root.
The use of the oscillating magnetic field as a backing support for the welding of thick components is already known. The influence of the magnetic field and the induced Lorentz forces in the melt on the melt pool geometry and the fluid flows is not yet fully investigated. The study deals with the determination of the influence of an externally applied oscillating magnetic field on the melt pool dynamics in high power laser hybrid welding process. An AC magnet was positioned under the specimen to create an electromagnetic force directed upwards to oppose dropping. To visualise the flow characteristics of the melt, several experiments were carried out using a technique specifically designed for this purpose with mild steel made of S355J2 with a wall thickness of 20 mm and a quartz glass in a butt configuration. A high-speed camera was used to monitor the geometry of the melt pool through the glass. The influence of the magnetic field orientation to the welding direction and the oscillation frequency on the molten pool was investigated for the case of the metal-glass configuration and for laser hybrid welding. The high-speed recordings were analysed with the Optical Flow Algorithm to characterise the flow within the melt pool. The element distribution over the whole seam thickness was evaluated by X-ray fluorescence (XRF). The high-speed analysis showed that in the melt pool two vortices are formed, one in the upper part and the other in the lower part. In the region where the two vortices come together, a narrow region (necking region) forms in the melt pool. The evaluation of the high-speed recordings shows that the depth of the region where the two vortices meet is strongly influenced by the oscillation frequency. Additionally, the oscillation frequency demonstrated a greater influence on the melt pool dynamics and the mixing of the elements of the filler wire.
One of the challenges of the high-power hybrid laser welding of thick steels is the sensitivity of the process of the process to manufacturing tolerances. This usually leads to a time-consuming preparation of the welding edges, such as milling. The study deals with the influence of the edge quality of milled and plasma-cut steel made of S355J2 with a wall thickness of 20 mm on the laser hybrid welded seam quality. Furthermore, the gap bridgeability and the tolerances towards edge misalignment was investigated. An AC magnet was used as backing support to prevent sagging and positioned under the workpiece, to generate an upwards directed electromagnetic pressure. The profiles of the edges and the gap on the top and root side were measured using a digital camera. Single-pass laser hybrid welds of plasma-cut edges could be welded using a laser beam power of just 13.7 kW. A gap bridgeability up to 2 mm and misalignment of edges up to 2 mm could be achieved successful. Additionally, the independence of the cutting side and the welding side was shown, so that samples were welded to the opposite side to their cutting. For evaluation of internal defects or irregularities, X-ray images were carried out. Charpy impact strength tests were performed to determine the toughness of the welds.
Because of its excellent corrosion resistance, high tensile strength and
high ductility, duplex stainless steel 2205 offers many areas of application. Though laser beam welding accompanied by high cooling rates, duplex steels tend to perform higher ferrite contents in weld metal as the base metal, which leads to a reduction of ductility and corrosion resistance of the weld joint. To overcome this problem, a solution, based on buttering the plate edges by laser metal deposition (LMD) with material containing higher Ni concentrations prior to laser welding was suggested.
In this context different process parameters for LMD and different mixtures of duplex and nickel powder, were investigated. In a second step the possibility of welding those edges defect free while achieving balanced austenite-ferrite ratio was verified with metallographic analysis, Electron Backscatter Diffraction (EBSD) and impact testing according to Charpy. The improved corrosion resistance was observed with ASTM G48 standard test method.
In order to reduce weight of vehicles, the interest in multi-material-design has been growing within the last few years. For vehicles the combination of steel and aluminium alloys offers the most promising compromise between weight, strength and formability. Thermal joining of these dissimilar materials is still a challenge to overcome. A possible approach is a new joining technology, whereby a combination of laser beam welding and contactless induced electromagnetic forces are used to displace the generated melt of one joining partner into a notch of the other. This paper presents the working principle and shows numerical analyses to improve the understanding of this joining process. The simulations help to calculate the thermal development of the joining partners, which is important for the formation of intermetallic phases. Furthermore, the calculation of the time required for a complete displacement is possible. The numerical results are validated by experimental results.
Notch impact toughness of laser beam welded thick sheets of cryogenic nickel alloyed steel X8Ni9
(2020)
This research work 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.
Using a novel optical measurement technique together with the OF algorithm, a two-dimensional deformation analysis during welding was conducted.
The OF technique was the first of its kind to provide a measurement of the full strain field locally in the immediate vicinity of the solidification front. The local critical strain and strain rate in vicinity of the solidification front has observed 1.57%, 28 %/s, respectively.
The distribution of the alloying elements from the filler wire in laser beam welding plays an important role on the property of the final weld. In this study, 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 to bring the additional alloying elements to the forepart of the weld pool. The occurrence of the bulging phenomenon may further prevent the downward transfer of the additional elements to the root of the weld pool. The numerical results show a good agreement with the experimental fusion line and element distribution.
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 development of high energy laser sources enables single-pass welds of thick plates up to 30 mm, but often additional materials are needed to influence the properties of the weld seams. However, the homogenous distribution of filler materials in form of e.g. electrodes is only possible up to 7 mm while the elements are only traceable up to a depth of 14 mm. To overcome this problem a two-step process is used where first the edges of the weld partners are coated with the filler material by laser metal deposition (LMD) and afterwards are welded by laser beam. Single-pass welds with electromagnetic weld pool support of 30 mm thick S355 J2+N-plates with austenitic AISI 316L-coatings were investigated as well as the influence of the coatings to the penetration depth of the laser beam without electromagnetic weld pool support in double-sided joints. The weld seams were tested by X-ray inspection and cross sections.
High power laser beam welding has been already introduced in ship building industry since last two decades and is characterized by higher performance as conventional arc welding technologies through its higher welding speed, penetration depth and therefore higher productivity. Moreover, high quality weld joints are achievable thanks to a stronger automatization degree and better repeatability of the process. A further advantage of the laser beam welding is a manufacturing of parts with very low distortion according to much lower heat input, so that rework amount can be reduced significantly. Nevertheless, there are still some unresolved problems limiting the range of application of the technology in part manufacturing within ship building industry, restricted by development and application of new materials or new design of ship vessels.
The present contribution deals with some novel aspects of laser beam welding technology allowing to widen this range significantly. One of the possibilities is to increase the weld seam thickness for single pass laser or laser-hybrid welding is to apply contactless EM-support system based on generation of the Lorentz force counteracting the liquid metal drop out. Also, application of laser-hybrid welding technology for welding novel materials like cold resistant steel X8Ni9 used for construction of LNG tanks brings technological and design advantages comparing to conventional welding technologies by using of a similar alloyed filler wire instead of expensive Ni-based filler material. Finally, it is shown how the application of computational techniques can be used to helps to improve the quality of laser welded joints and avoid the critical weld failure by optimization of the welding process parameters.
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.
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.