Claus Thomy

Magnetic stirring during laser welding of aluminum

The use of magnetic fields to influence melt flow is a well-established method, e.g., in foundry technology. For arc welding processes (especially tungsten inert gas welding), a process called magnetic stirring was first proposed, patented, and investigated in the 1970s. The central aim was to improve (by the help of an alternating magnetic field coaxial with the arc axis) weld quality especially in aluminum by the combined effects of arc current and magnetic field. It was established that such fields are indeed capable of influencing weld bead appearance, of increasing the degree of dilution, of decreasing pore formation and of producing a finer grain structure. However, the process never gained wide industrial acceptance. In the late 1990s, some efforts have been taken to apply constant magnetic fields to laser welding processes, aiming at influencing weld geometry, reducing porosity, and potentially increasing welding speed. However, neither have any detailed statements been made on possibilities to in...

Laser-Mig Hybrid Welding Of Aluminium To Steel — Effect Of Process Parameters On Joint Properties

Laser MIG hybrid welding was recently suggested as a feasible process for joining of aluminium to steel for both structural as well as tailored blank applications. To promote an understanding of the process and the effect of process parameters on joint properties, laser MIG hybrid welding experiments were performed to join aluminium alloy AA6016 to DC05 zinc-coated steel sheets, in the thickness range of 1 mm, in a butt joint configuration. Among the process parameters varied were laser power, MIG arc power, wire feed rate, welding speed and arc position relative to the abutting edges. By metallographic cross-sections and tensile tests, the effect of these process parameters on joint properties such as wetting length, intermetallic phase layer thickness and tensile strength could be elucidated. Based on these results, a process parameter envelope resulting in adequate and reproducible joint properties (sound weld bead, sufficient and regular wetting, thin intermetallic phase layer, tensile strength exceeding 180 MPa) was established. Within this parameter envelope, corrosion behaviour was rated not critical, and forming behaviour showed promising results.

Magnetic stirring during laser welding of aluminium

The use of magnetic fields to influence melt flow is a well-established method e.g. in foundry technology. For arc welding processes (especially TIG welding), a process called magnetic stirring was first proposed, patented and investigated in the 1970s. The central aim was to improve (by the help of an alternating magnetic field coaxial with the arc axis) weld quality especially in aluminium by the combined effects of arc current and magnetic field. It was established that such fields indeed are capable of influencing weld bead appearance, of increasing the degree of dilution, of decreasing pore formation and of producing a finer grain structure. However, the process never gained wide industrial acceptance. In the late 1990s, some efforts have been taken to apply constant magnetic fields to laser welding processes, aiming at influencing weld geometry, reducing porosity and potentially increasing welding speed. However, neither have any detailed statements been made on possibilities to influence melt flow and, consequently, filler metal distribution, nor on the application of alternating magnetic fields on laser welding.To help this situation, basic studies on magnetically influencing melt flow during laser welding of Aluminium have been conducted. To that end, alternating fields have been coaxially applied with magnetic flux densities up to 60 mT and frequencies in the the range of 0 to 20 Hz. It was demonstrated by the help of a specially developed method that, depending on the parameters chosen, such fields indeed are capable of influencing melt flow and weld pool dilution, thus “stirring” the weld metal. The results have been so promising that further investigations will be conducted, also focussing on laser hybrid welding especially of hot-cracking sensitive aluminium alloys.The use of magnetic fields to influence melt flow is a well-established method e.g. in foundry technology. For arc welding processes (especially TIG welding), a process called magnetic stirring was first proposed, patented and investigated in the 1970s. The central aim was to improve (by the help of an alternating magnetic field coaxial with the arc axis) weld quality especially in aluminium by the combined effects of arc current and magnetic field. It was established that such fields indeed are capable of influencing weld bead appearance, of increasing the degree of dilution, of decreasing pore formation and of producing a finer grain structure. However, the process never gained wide industrial acceptance. In the late 1990s, some efforts have been taken to apply constant magnetic fields to laser welding processes, aiming at influencing weld geometry, reducing porosity and potentially increasing welding speed. However, neither have any detailed statements been made on possibilities to influence melt flow ...

Laser-Mig Hybrid Welding of aluminium to steel — A straightforward analytical model for wetting length

Laser-MIG hybrid welding was recently suggested as a feasible process for joining of aluminium to steel for both structural as well as tailored blank applications. In this process, the aluminium is molten to create a brazed bond to the zinc-coated steel sheet, which remains solid. Using thin-sheet materials in a thickness range of approximately 1 mm in butt joint configuration, this process allows a joining speed of more than 100 mm/s at a laser power of 4 kW whilst retaining some gap bridging ability. In such a case, the wetting length of the aluminium melt on the top and bottom sides of the zinc-coated steel sheet is of significance for the properties of the joint. Therefore, an understanding of the parameters governing wetting length is required. To promote this understanding, a model to calculate wetting length in dependence of processing conditions is suggested. Based on the assumption that there is a correlation between the temperature field on the steel sheet and wetting length, an analytical model for the temperature field is developed and correlated with experimental results on the material combination AA6016 T4 (thickness 1.15 mm) and DC05+ZE (1 mm). Both model and experiments show that the wetting length is correlated to the melting isotherm of the zinc coating (provided a sufficient amount of melt is created). These results shall contribute to an improved understanding of the process.

Welding with fiber lasers from 200 to 17000 W

Latest developments in laser physics have made available single-mode fiber lasers of several hundreds of Watts and multi-mode high-power systems up to 17 kW and potentially more at excellent beam qualities for the respective power ranges. Adding to these properties their high energetic efficiency, their considerable estimated lifetime and their compact size, they might well be considered to be a viable alternative to both conventional laser systems for welding in the micro as well as in the macro range. However, due to the novelty of these systems, only few experiences on their usability for materials processing and in special for welding are existing.To help this situation, on the one hand a 200 W single-mode fiber laser has been used for micro-welding experiments at highest beam intensity. For a variety of materials the process boundaries were investigated with respect to penetration, welding speed and gap bridging in overlap position.On the other hand, after preceding work with fiber lasers with 7 kW and 10 kW beam power, respectively, currently a high-power fiber laser of the 17 kW class is used at the BIAS for developing welding applications for a variety of industries such as pipelaying and welding of aluminium extruded profiles for the railway industries.Latest developments in laser physics have made available single-mode fiber lasers of several hundreds of Watts and multi-mode high-power systems up to 17 kW and potentially more at excellent beam qualities for the respective power ranges. Adding to these properties their high energetic efficiency, their considerable estimated lifetime and their compact size, they might well be considered to be a viable alternative to both conventional laser systems for welding in the micro as well as in the macro range. However, due to the novelty of these systems, only few experiences on their usability for materials processing and in special for welding are existing.To help this situation, on the one hand a 200 W single-mode fiber laser has been used for micro-welding experiments at highest beam intensity. For a variety of materials the process boundaries were investigated with respect to penetration, welding speed and gap bridging in overlap position.On the other hand, after preceding work with fiber lasers with 7 kW a...

Influence of Magnetic Fields on Dilution during Laser Welding of Aluminium

In order to minimize the occurrence of hot-cracking phenomena in laser welding of hotcracking sensitive aluminum alloy sheets, it is a common technique to introduce silicon-containing filler wire into the weld metal. However, to achieve an optimum result, a homogenous distribution of not less than 2 % of silicon throughout the weld metal is strongly recommendable. Under certain circumstances, this may be a difficult task. One potential solution to achieve sufficient dilution and, consequently, a very homogenous silicon distribution might be the application of alternating magnetic fields. In foundry technology, the use of magnetic fields to influence melt flow is a wellestablished method. For TIG welding, a process called magnetic stirring was first investigated in the 1970s. It was sufficiently demonstrated by the help of an alternating magnetic field coaxial with the arc axis, that, among other effects, the degree of dilution can be increased and a refined grain structure is achieved. Since the late 1990s, some efforts have been taken to apply constant magnetic fields to laser welding processes. However, neither alternating fields nor potential effects on dilution have been in the focus of these investigations. To help this situation, basic studies on magnetically influencing melt flow during laser welding of aluminum have been conducted. To that end, alternating fields have been coaxially applied with magnetic flux densities up to 60 mT and frequencies in the range of 0 to 20 Hz. It was demonstrated by the help of a specially developed method that, depending on the parameters chosen, such fields are indeed capable of influencing melt flow and weld pool dilution, thus “stirring” the weld metal.

Humping in welding with single-mode fiber lasers

The availability of lasers with highest beam quality at laser powers of 1 kW or more (such as single-mode fibre laser, which nowadays come close to the theoretical limits) provides a unique tool to investigate a wide range of welding process phenomena with penetrations of some 10 µm to penetrations of some mm. Thus covering the field of micro welding as well as of macro welding, scalability of welding processes as well as size effects associated with the underlying physical phenomena may be of significance.In this paper, the humping effect will be given a closer look, as this periodic melt pool instability is an important limitation to welding speed both in the micro and the macro range. Based on experimental investigations with a single-mode fiber laser (YLR-1000, laser power 1 kW, M2 < 1.05), a model including a modification of Rayleigh’s considerations on the stability of an inviscid incompressible fluid which is freely suspended in space and maintained only by surface tension is developed and discussed. It is shown that, within the scope of the investigations, humping to a considerable extent can be explained by a modified version of Rayleigh’s theory, permitting to neglect a direct influence of three-dimensional melt flow.The availability of lasers with highest beam quality at laser powers of 1 kW or more (such as single-mode fibre laser, which nowadays come close to the theoretical limits) provides a unique tool to investigate a wide range of welding process phenomena with penetrations of some 10 µm to penetrations of some mm. Thus covering the field of micro welding as well as of macro welding, scalability of welding processes as well as size effects associated with the underlying physical phenomena may be of significance.In this paper, the humping effect will be given a closer look, as this periodic melt pool instability is an important limitation to welding speed both in the micro and the macro range. Based on experimental investigations with a single-mode fiber laser (YLR-1000, laser power 1 kW, M2 < 1.05), a model including a modification of Rayleigh’s considerations on the stability of an inviscid incompressible fluid which is freely suspended in space and maintained only by surface tension is developed and discusse...

Welding of aluminium and steel with high-power fibre lasers

High-power fibre lasers for materials processing have undergone a rapid development process within the past two years. The modular structure of the systems has, in this short period of time, enabled the scaling of laser power from several hundreds of Watts to 10 Kilowatts and potentially even more. Due to its specific advantages, among which there are a beam quality significantly improved compared to other solid-state lasers, a high energetic efficiency and a compact size, they might well be considered to be a future alternative (in view of technical and economic aspects) to both Nd:YAG- as well as to CO2-Lasers. However, principle studies on its readiness for welding steel and aluminium alloys are still lacking.Thus, at the BIAS Bremer Institut fur angewandte Strahltechnik GmbH, a basic experimental programme on welding of various steel and aluminium materials with a fibre laser possessing a maximum beam power of 6.9 kW was performed. The central aim of the experiments was to explore the potentials of this new type of laser and demonstrate its ability to enhance the process limitations considering welding speed and sheet thickness previously regarded to be inevitable when welding with solid-state lasers.Compared to Nd:YAG-Lasers with a maximum beam power of 4 kW (nowadays a standard laser in industry), the penetration depth could be significantly increased. E.g., a penetration depth of up to 8 mm with a sufficiently narrow seam width could be achieved, and for sheets of 3 mm thickness, an increase of laser power from 4 to 7 kW nearly doubles the possible welding speed. For aluminium materials, very stable processes have been obtained even at a welding depth of more than 6 mm.In general, the results have been so promising for all materials tested that first studies on other industrially relevant joints such as overlap joints of zinc-coated steel or aluminium have been conducted.High-power fibre lasers for materials processing have undergone a rapid development process within the past two years. The modular structure of the systems has, in this short period of time, enabled the scaling of laser power from several hundreds of Watts to 10 Kilowatts and potentially even more. Due to its specific advantages, among which there are a beam quality significantly improved compared to other solid-state lasers, a high energetic efficiency and a compact size, they might well be considered to be a future alternative (in view of technical and economic aspects) to both Nd:YAG- as well as to CO2-Lasers. However, principle studies on its readiness for welding steel and aluminium alloys are still lacking.Thus, at the BIAS Bremer Institut fur angewandte Strahltechnik GmbH, a basic experimental programme on welding of various steel and aluminium materials with a fibre laser possessing a maximum beam power of 6.9 kW was performed. The central aim of the experiments was to explore the potentials of th...

Distortion effects in micro welding with fibre laser

While effects associated with distortion are already understood quite well in keyhole laser welding in the macro range, this is not yet the case for welding in the micro range. In special, the effect of sheet thickness on distortion in micro welding of aluminium sheets with single-mode fibre laser is of interest. To investigate this effect, linear bead-on-plate welds were carried out, whilst total nominal heat input was scaled with sheet thickness by scaling laser power. Distortion (out-of-plane) was measured using a laser triangulation sensor. To improve understanding of the effect of heat input and sheet thickness, a straightforward analytical model from thermal bending was modified and applied.It was found that average (out-of-plane) distortion will increase with decreasing sheet thickness, leading to a displacement out of plane of up to approximately double the sheet thickness. A simplistic model for buckling in thermal forming was applied and found to be in good correlation with the experimental data, giving a dependence of distortion on sheet thickness s0 according to approximately 1/s02/3.While effects associated with distortion are already understood quite well in keyhole laser welding in the macro range, this is not yet the case for welding in the micro range. In special, the effect of sheet thickness on distortion in micro welding of aluminium sheets with single-mode fibre laser is of interest. To investigate this effect, linear bead-on-plate welds were carried out, whilst total nominal heat input was scaled with sheet thickness by scaling laser power. Distortion (out-of-plane) was measured using a laser triangulation sensor. To improve understanding of the effect of heat input and sheet thickness, a straightforward analytical model from thermal bending was modified and applied.It was found that average (out-of-plane) distortion will increase with decreasing sheet thickness, leading to a displacement out of plane of up to approximately double the sheet thickness. A simplistic model for buckling in thermal forming was applied and found to be in good correlation with the experimental data...

Humping Effect in Welding of Steel with Single-Mode Fibre Laser

The availability of lasers with highest beam qualities at laser powers of 1 kW (such as single-mode fibre laser, which nowadays come close to the theoretical limits) provides a unique tool to investigate welding process phenomena in a wide range of potential applications from welding with penetrations of some 50 μm to penetrations of some mm. Thus covering the field of micro welding as well as of macro welding, scalability of welding processes as well as size effects associated with the underlying physical phenomena may be of significance. In this paper, the humping effect will be given a closer look, as this periodic melt pool instability is an important limitation to possible welding speed both in the micro and the macro ranges. Based on experimental investigations with a single-mode fibre laser (YLR-1000, laser power 1 kW, BPP < 0.4 mm*mrad), a model based on a modification of Rayleigh’s considerations on the stability of an inviscid incompressible fluid, which is freely suspended in space and maintained only by surface tension, is developed and discussed. It is shown that, within the scope of the investigations, humping to a large extent can be explained by Rayleigh’s theory, permitting to neglect the influence of three-dimensional melt flow.