Research on fluid dynamics during laser deep penetration welding

The influence of fluid flow phenomena on the laser beam welding process

This study presents mechanisms of the evolution of a narrowed region in the weld pool center during deep penetration laser beam welding. In numerous numerical studies presented in this study, it was also found that the local reduction of the weld pool size can cause detrimental effects on the melt flow behavior and the resulting properties of the welds. A particularly large influence of this effect was identified in three aspects. First, the local variation of the solidification sequence of the weld pool causes an increase in the hot-cracking susceptibility due to a locally delayed solidification. Second, it was proven that a change in the local length and width of the weld pool is associated with an adverse impact on the potential flow routes of the molten material that induces stronger local variations of its solidification. Thus, the element mixing, e.g., during the welding with filler materials, is blocked. This leads to a nonhomogeneous chemical composition of the final weld and can cause undesired effects on the final material properties. Finally, another observed effect is related to the reduced ability of process pores to reach the top surface. As this type of porosity is usually produced around the keyhole tip, the change of the fluid flow regime above this area plays a significant role in determining the final path of the pores until the premature solidification in the middle of the weld pool captures them. This study summarizes mainly numerical results that were supported by selected experimental validation results.

The effect of laser beam scattering on capillary stability and melt dynamics is investigated simulatively for deep penetration laser beam welding. The mesh-free Lagrangian Smoothed Particle Hydrodynamics (SPH) method is coupled with a ray-tracing scheme. The SPH model covers fluid and thermodynamics, including temperature-dependent surface tension, recoil pressure, heat conduction, and phase transitions. The ray tracer models the laser-material interaction by tracking the propagation of light rays within the capillary according to the laws of geometrical optics, and taking into account multiple reflections and scattering. Surface scattering due to surface roughness, and volume scattering due to condensed droplets or local changes of the complex refractive index in the vapor plume are evaluated separately. For surface scattering, the reflections at the material surface are divided into a specular part and a diffuse part with randomly reflected rays. For volume scattering, the Henyey-Greenstein model is used to specify the angular distribution of the scattered light ray. The effect of scattering is examined for laser beam welding of aluminum and titanium. The results show that volume scattering has a stabilizing effect on the capillary for the highly reflective material aluminum, while the effect of surface scattering is small for both materials.

Liquid motion in fusion welding plays the important role in the heat and mass transfer as well as stability of bead formation. In an arc weld pool, many works have been intensively conducted experimentally and theoretically. In a keyhole laser welding, however, there is few comprehensive study on the metal flow and its related phenomena in weld pool. In the present work, liquid motion in the laser weld pool was directly observed by the high-speed optical and X-ray transmission methods using tracing particles put in the pool. Slag motion on the pool surface revealed that there was a quick stream flowing around the side of keyhole and it changed the direction to forward at the rear part of weld pool. Namely, there was a big circulation of flow in the rear part of pool surface. Liquid flow inside the pool was observed by tracing the X-ray transmission images of Tungsten particles that were initially sandwiched between two thin sheets. The flow pattern inside the pool was very complex and there observed a strong eddy just behind the keyhole. The flow speed inside the pool was 0.3 to 0.5 m/s and this fast stream hindered the bubbles of metallic vapor and shielding gas to float up by buoyancy force and resulted in the formation of characteristic porosity.Liquid motion in fusion welding plays the important role in the heat and mass transfer as well as stability of bead formation. In an arc weld pool, many works have been intensively conducted experimentally and theoretically. In a keyhole laser welding, however, there is few comprehensive study on the metal flow and its related phenomena in weld pool. In the present work, liquid motion in the laser weld pool was directly observed by the high-speed optical and X-ray transmission methods using tracing particles put in the pool. Slag motion on the pool surface revealed that there was a quick stream flowing around the side of keyhole and it changed the direction to forward at the rear part of weld pool. Namely, there was a big circulation of flow in the rear part of pool surface. Liquid flow inside the pool was observed by tracing the X-ray transmission images of Tungsten particles that were initially sandwiched between two thin sheets. The flow pattern inside the pool was very complex and there observed a str...

Stability of the weld pool during laser keyhole welding depends on a number of interrelated physical phenomena. Understanding these phenomena is essential for good control of the welding process. A considerable body of theoretical work has been published concerning identification of the keyhole and weld pool dynamics in penetration laser welding of thin steel sheets. The present paper describes a method for obtaining information about weld bead formation. This technique provides data about the three dimensional shape of the weld pool and has been used to reconstruct the weld pool in 0.75 and 1.2 mm sheet steel. The method has been used to identify different keyhole penetration modes and reveal differences in weld pool solidification for specified thicknesses. Solidification rates in different regions of the weld pool have also been calculated and presented. This study provides a methodology that can be used to support modelling of weld pool formation during laser keyhole welding.Stability of the weld pool during laser keyhole welding depends on a number of interrelated physical phenomena. Understanding these phenomena is essential for good control of the welding process. A considerable body of theoretical work has been published concerning identification of the keyhole and weld pool dynamics in penetration laser welding of thin steel sheets. The present paper describes a method for obtaining information about weld bead formation. This technique provides data about the three dimensional shape of the weld pool and has been used to reconstruct the weld pool in 0.75 and 1.2 mm sheet steel. The method has been used to identify different keyhole penetration modes and reveal differences in weld pool solidification for specified thicknesses. Solidification rates in different regions of the weld pool have also been calculated and presented. This study provides a methodology that can be used to support modelling of weld pool formation during laser keyhole welding.

In an industrial joining process, exemplified by deep penetration laser beam welding, ensuring a high quality of welds requires a great effort. The quality cannot be fully established by testing, but can only be produced. The fundamental requirements for a high weld seam quality in laser beam welding are therefore already laid in the process, which makes the use of control systems essential in fully automated production. With the aid of process monitoring systems that can supply data inline to a production process, the foundation is laid for the efficient and cycle-time-neutral control of welding processes. In particular, if novel, direct measurement methods, such as Optical Coherence Tomography, are used for the acquisition of direct geometric quantities, e.g., the weld penetration depth, a significant control potential can be exploited. In this work, an inline weld depth control system based on an OCT keyhole depth measurement is presented. The system is capable of automatically executing an inline control of the deep penetration welding process based only on a specified target weld depth. The performance of the control system was demonstrated on various aluminum alloys and for different penetration depths. In addition, the ability of the control to respond to unforeseen external disturbances was tested. Within the scope of this work, it was thus possible to provide an outlook on future developments in the field of laser welding technology, which could develop in the direction of an intuitive manufacturing process. This objective should be accomplished through the use of intelligent algorithms and innovative measurement technology—following the example of laser beam cutting, where the processing systems themselves have been provided with the ability to select suitable process parameters for several years now.

Influence of oscillating magnetic field on the keyhole stability in deep penetration laser beam welding

Modeling keyhole oscillations during laser deep penetration welding at different spatial laser intensity distributions

Profiles of joints resulting from deep penetration laser beam welding of a flat workpiece of carbon steel were computed. A semi-analytical conduction model solved with Green's function method was used in computations. In the model, the moving heat source was attenuated exponentially in accordance with Beer-Lambert law. Computational results were compared with those in the experiment.

Simulation of Fluid Flow in a Channel Induced by Three Types of Fin-Like Motion

Modelling the evolution of weld metal microstructure requires knowledge of cooling rates at various locations in the fusion zone. In the recent past, significant advances have been made in the calculation of transient three-dimensional temperature fields, considering convective heat transfer and fluid flow in the weld pool. However, very little effort has been made to use these accurate cooling rates to understand fusion zone microstructures. The present paper demonstrates the advantages of microstructure calculations using fundamentals of transport phenomena and phase transformation theory. The velocity and temperature fields, the shape and size of the fusion zone, and the cooling rates at different locations were calculated by solution of the equations for conservation of mass, momentum, and energy in three dimensions. The time-temperature transformation (TTT) diagrams were calculated for a series of steels with varying carbon and manganese contents using a phase transformation model. The TTT diagrams and the computed cooling rates were then used to obtain the continuous cooling transformation (CCT) diagrams and the microstructures. The computed volume fractions of the various microstructural constituents were then compared with the experimental results. Good agreement between the computed and the experimental results indicates significant promise for predicting weld microstructure from the fundamental principles of transport phenomena and phase transformation theory.

Modelling of fluid flow phenomenon in laser+GMAW hybrid welding of aluminum alloy considering three phase coupling and arc plasma shear stress

The compressible metallic vapor plume or plasma plume behaviors in the keyhole during deep penetration laser welding have significant effects on the joint quality. However, these behaviors and their responses to process parameter variations have not been well understood. In this paper, we first systematically study the dynamic characteristics and mechanisms of compressible metallic vapor plume behaviors in transient keyhole during fiber laser welding of 304 stainless steels based on a multiple timescale multiphase model. The time-dependent temperature, pressure, velocity and Mach number distributions of vapor plume under different process parameters are theoretically predicted. It is found that the distributions of the main physical characteristics of vapor plume such as pressure, velocity as well as Mach number in keyhole are usually highly uneven and highly time dependent. The peak difference of the velocity, pressure, temperature and Mach number of the vapor plume in a keyhole could be greater than 200 m/s, 20 kPa, 1000 K and 0.6 Mach, respectively. The vapor plume characteristics in a transient keyhole can experience significant changes within several hundreds of nanoseconds. The formation mechanisms of these dynamic characteristics are mainly due to the mesoscale keyhole hump (sized in several tens of microns) dynamics. It is also demonstrated that it is possible to suppress the oscillations of compressible vapor plume in the keyhole by improving the keyhole stability through decreasing the heat input. However, stabilizing the keyhole could only weaken, but not eliminate, the observed highly uneven and transient characteristics. This finding may pose new challenges for accurate experimental measurements of vapor plume induced by laser welding.

Gas metal arc (GMA) fillet welding is one of the most important processes for metal joining because of its high productivity and amiability to automation. This welding process is characterized by the complicated V-shaped joint geometry, a deformable weld pool surface, and the additions of hot metal droplets. In the present work, a three-dimensional numerical heat transfer and fluid flow model was developed to examine the temperature profiles, velocity fields, weld pool shape and size, and the nature of the solidified weld bead geometry during GMA fillet welding. The model solved the equations of conservation of mass, momentum, and energy using a boundary fitted curvilinear coordinate system. Apart from the direct transport of heat from the welding arc, additional heat from the metal droplets was modeled considering a volumetric heat source. The deformation of the weld pool surface was calculated by minimizing the total surface energy. Part I of this article is focused on the details of the numerical model such as coordinate transformation and calculation of volumetric heat source and free surface profile. An application of the model to GMA fillet welding of mild steel is described in an accompanying article (W. Zhang, C.-H. Kim and T. DebRoy, J. Appl Phys. 95, 5220 (2004)).

In the present study, the vorticity of melt motion in the keyhole and weld pool has been evaluated in case of high power CO2 laser beam welding. The circulation of vorticity is obtained as a function of Reynolds number for a given keyhole volume which is linked to Mach number variation. The shear stress and thermal fluxes present in the turbulent pool are linked to diffusivity and Prandtl number variation. It was shown that below a critical value of Rayleigh number, the conduction mode of melt transfer signifying beam absorption becomes dominant. Above this value, convective heat transfer indicates melting and evaporation occurring in the weld pool during laser welding. The evaporative recoil pressure expels the liquid while surface tension and hydrostatic pressure help to retain the melt in the keyhole cavity in this high power laser beam welding. The understanding of several hydrodynamic phenomena occuring in the weld pool is valuable not only for understanding basic mechanistic aspects but also for process optimization involved in laser beam welding.