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A three-dimensional multi-physics numerical model was developed for the calculation of an appropriate equivalent volumetric heat source and the prediction of the transient thermal cycle during and after fusion welding. Thus the modelling process was separated into two studies. First, the stationary process simulation of full-penetration keyhole laser beam welding of a 15 mm low-alloyed steel thick plate in flat position at a welding speed of 2 m/min and a laser power of 18 kW was performed. A fixed keyhole with a right circular cone shape was used to consider the energy absorbed by the workpiece and to calibrate the model. In the calculation of the weld pool geometry and the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature were taken into account. The obtained local temperature field was then used in a subsequent study as an equivalent heat source for the computation of the transient thermal field during the laser welding process and the cooling stage of the part. The system of partial differential equations, describing the stationary heat transfer and the fluid dynamics, were strongly coupled and solved with the commercial finite element software COMSOL Multiphysics 5.0. The energy input in the transient heat transfer simulation was realised by prescription of the nodes temperature. The prescribed nodes reproduced the calculated local temperature field defining the equivalent volumetric heat source. Their translational motion through the part was modelled by a moving mesh approach. An additional remeshing condition and helper lines were used to avoid highly distorted elements. The positions of the elements of the polygonal mesh were calculated with the Laplace’s smoothing approach. Good correlation between the numerically calculated and the experimentally observed weld bead shapes and transient temperature distributions was found.
One of the main factors affecting the use of lasers in the industry for welding thick structures is the process accompanying solidification cracks. These cracks mostly occurring along the welding direction in the welding center, and strongly affect the safety of the welded components. In the present study, to obtain a better understanding of the relation between the weld pool geometry, the stress distribution and the solidification cracking, a three-dimensional computational fluid dynamic (CFD) model was combined with a thermo-mechanical model. The CFD model was employed to analyze the flow of the molten metal in the weld pool during the laser beam welding process. The weld pool geometry estimated from the CFD model was used as a heat source in the thermal model to calculate the temperature field and the stress development and distributions. The CFD results showed a bulging region in the middle depth of the weld and two narrowing areas separating the bulging region from the top and bottom surface. The thermo-mechanical simulations showed a concentration of tension stresses, transversally and vertically, directly after the solidification during cooling in the region of the solidification cracking.
A numerical model for simulation of the steady-state thermal behaviour in keyhole mode welding has been developed. It is based on the equivalent heat source concept and consists of two parts: computational thermo-hydrodynamics and heat conduction. The solution of the thermo-hydrodynamics problem by the finite element method for a bounded domain results in a weld pool interface geometry being the input data for a subsequent heat conduction problem solved for a workpiece by a proposed boundary element method. The main physical phenomena, such as keyhole shape, thermo-capillary and natural convection and temperature-dependent material properties are taken into consideration.
The developed technique is applied to complete-penetration keyhole laser beam welding of a 15 mm thick low-alloyed steel plate at a welding speed of 33 mm/s and a laser power of 18 kW. The fluid flow of the molten metal has a strong influence on the weld pool geometry. The thermo-capillary convection is responsible for an increase of the weld pool size near the plate surfaces and a bulge formation near the plate middle plane. The evaluated and experimental molten pool, cross-sectional weld dimensions and thermal cycles of the heat affected zone are in close agreement.
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.
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 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.
A simplified model for numerical simulation of laser metal deposition process with beam oscillation
(2018)
A simplified model for the numerical simulation of the laser metal deposition process with beam oscillation is proposed. The model studies circular and lateral oscillations in order to reduce the porosity of the deposited part, to increase the process efficiency and the gap bridging ability as well. The deposition rate is increased by modifying the shape and the width of the molten pool through an optimized laser beam power distribution and oscillation amplitude. The relationship between the process conditions and the shape of the fabricated part are determined. It is found that an increase of the amplitude by a lateral oscillation of the beam reduces the heat flux and hence the shape of the deposited wall. A good correlation between the numerically calculated results and the experimental measurements is obtained.
This work aims to find the thermal cycles during and after fusion welding through simulation by first calculating the resulting local temperature field in the quasi-stationary part of the process. Here complete-penetration keyhole laser beam welding with a laser power of 18 kW on a 15 mm thick slab of a low-alloyed steel at a welding speed of 2 m/min is considered. In order to physically depict the laser material interaction a multi-physics numerical model including the effects of phase transformation, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature is developed.
It uses a fixed keyhole geometry with a right truncated circular cone shape to introduce the laser beam energy to the workpiece. In a subsequent study, the resulting local temperature field is then used as an equivalent heat source in order to predict the unsteady thermal cycle during and after fusion welding. The translational movement of the laser beam through the workpiece is represented by a moving mesh approach. For the simulation, stationary heat transfer and fluid dynamics are described by a system of strongly coupled partial
differential equations. These are solved with the commercial finite element software COMSOL Multiphysics 5.0. The results of the numerical simulation are validated by experiments, where the weld bead shapes and the thermal cycles show good correlation.
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.
A simplified model for numerical simulation of laser metal deposition process with beam oscillation
(2018)
A model of laser metal deposition with beam oscillation has been developed. The proposed model consists of two coupled sub-models calculating the heat transfer in the deposited part and the free surface of the molten pool, respectively. The heat transfer simulation of the deposited part solves a three-dimensional quasi-stationary heat conduction problem. The free surface of the molten pool are determined by solving the Laplace-Young equation. The developed model enables the layer-by-layer prediction of the shape of the deposited part and the resulting temperature field. It is shown that for an oscillation amplitude equal to the beam radius the peak value of the heat flux decreases by about 53% and 73% in the case of lateral oscillation and circular oscillation, respectively. Lateral oscillating laser beam results in a higher penetration depth due to the higher thermal efficiency. The amplitude of the laser beam oscillation effects the shape of the deposited wall and the deposition rate. A good correlation between the numerically calculated and experimentally observed results is obtained.
In this study, a three-dimensional CFD-simulation model was developed to simulate the fluid flow in the weld pool. The CFD-model showed a bulging region in the middle of the depth, which is separated from the top surface and bottom surface by two narrowing regions. It can be concluded that the interaction of the movement of the laser source with the Marangoni vortex leads to a teardrop shape at the upper and bottom surface of the workpiece. Additionally, it shows that the bulging in the weld is a result of the backflows on the upper and lower sides due to the thermo-capillary-driven flows. The weld pool shape was used as a heat source in a two-dimensional thermomechanical model, which allows a highly accurate transformation of the weld pool dimensions obtained from the CFD model. This developed technique allows the consideration of physical aspects, which cannot be considered when using traditional heat sources. The mechanical model has shown that the chronological order of the solidification of the weld has a significant influence on the nature and distribution of the stresses in the weld. High tensile stress has been observed in the bulging region, i.e. in the susceptible region for solidification cracking, when compared to the other narrowing regions, which show compressive stress.
Effect of Inter-Layer Dwell Time on Distortion and Residual Stresses of Laser Metal Deposited Wall
(2019)
The laser metal deposition is an advanced manufacturing technology enabling the production of large-sized parts and partially or completely elimination of machining and welding. The process is characterised by non-uniform local heating of the buildup leading to a stress distribution, which may exceed the yield strength of the material and leads to loss of dimensional accuracy. The interlayer dwell time has a strong influence on the temperature field. The effect of the interlayer dwell time on the distortion and the stress distribution during laser metal deposition of a single-pass wall on the edge of 2 mm thick plate was studied experimentally and numerically. The deposited material was IN625 and the substrate material was AISI 316. A decrease of the residual displacement, due to a uniform shrinkage after the deposition of the last layer and a lower level of the residual compressive longitudinal plastic strain, has been observed in the studies without dwell time. The peak increment of the free edge displacement corresponds to the first layer and hence the subsequent layers will be deposited on the already plastically deformed buildup. The tensile residual longitudinal stress near the top of the buildup and transverse stress near the edges of the buildup is higher than yield strength in the studies with dwell time.
The introduction of electromagnetic stirring to laser beam welding can bring several beneficial effects e.g. element homogenization and grain refinement. However, the underlying physics has not been fully explored due to the absence of quantitative data of heat and mass transfer in the molten pool. In this paper, the influence of electromagnetic stirring on the thermo-fluid flow and element transport in the wire feed laser beam welding is studied numerically and experimentally. A three-dimensional transient heat transfer and fluid flow model coupled with dynamic keyhole, magnetic induction and element Transport is developed for the first time. The results suggest that the Lorentz force produced by an oscillating magnetic field and its induced eddy current shows an important influence on the thermo-fluid flow and the keyhole stability. The melt flow velocity is increased by the electromagnetic stirring at the rear and lower regions of molten pool. The keyhole collapses more frequently at the upper part. The additional Elements from the filler wire are significantly homogenized because of the enhanced forward and downward flow. The model is well verified by fusion line shape, high-speed images of molten pool and measured element distribution. This work provides a deeper understanding of the transport phenomena in the laser beam welding with magnetic field.
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 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.
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.
Приведено аналитическое решение задачи теплопроводности после прекращения действия подвижного источника теплоты для различных комбинаций источника и свариваемого тела. Показано, что после выключения источника возможно дополнительное плавление основного металла за счет перегретого жидкого металла сварочной ванны. Например, при лазерной сварке со сквозным проплавлением стальной пластины толщиной 2 мм длина кратера шва может быт на 19% больше установившейся длины сварочной ванны. Установлено, что центр кратера, в котором заканчивается затвердевание жидкого металла, смещен в сторону хвостовой части кратера относительно оси теплового источника в момент прекращения его действия. Скорость и направление кристаллизации металла сварочной ванны и кратера различны.
This work aims to find the thermal cycles during and after fusion welding through simulation by first calculating the resulting local temperature field in the quasi-stationary part of the process. Here complete-penetration keyhole laser beam welding with a laser power of 18 kW on a 15 mm thick slab of a low-alloyed steel at a welding speed of 2 m/min is considered. In order to physically depict the laser material interaction a multi-physics numerical model including the effects of phase transformation, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature is developed. It uses a fixed keyhole geometry with a right truncated circular cone shape to introduce the laser beam energy to the workpiece. In a subsequent study, the resulting local temperature field is then used as an equivalent heat source in order to predict the unsteady thermal cycle during and after fusion welding. The translational movement of the laser beam through the workpiece is represented by a moving mesh approach. For the simulation, stationary heat transfer and fluid dynamics are described by a system of strongly coupled partial differential equations. These are solved with the commercial finite element software COMSOL Multiphysics 5.0. The results of the numerical simulation are validated by experiments, where the weld bead shapes and the thermal cycles show good correlation.