Molten Pool Dynamics Research Articles

Keyhole-induced porosity formation during laser welding

Through laser welding experiment with glass, the dynamics of keyhole and molten pool could be directly observed to reveal the mechanism of porosity formation using high speed camera. The high speed images showed that large fluctuation of keyhole was responsible for the increased bubble formation, and bubble merging resulted in the formation of large pore. The porosity of the welded samples was characterized qualitatively and quantitatively by combining super depth microscope and image processing. Porosity characteristics such as pore number, pore diameter (maximum and mean), porosity volume and porosity ratio varied with changes in welding speeds and laser powers. Increased energy density led to a decrease in the number but an increase in the other characteristics, indicating that the higher the energy density the more violent the bubble merging and giving rise to an increased possibility to form large pores.

Influence of additive multilayer feature on thermodynamics, stress and microstructure development during laser 3D printing of aluminum-based material

A transient three dimensional model for describing the temperature behavior, thermo-capillary convection, microstructure evolution and the resultant mechanical properties during selective laser melting of AlN/AlSi10Mg composite is proposed. The powder-solid transformation, temperature dependent physical properties and the preservation of the heat are taken into account. The effect of the additive manufacturing multilayer feature on the molten pool dynamics, cooling rate, crystal size, microstructure morphology, micro-hardness and types of the residual stress has been investigated. It shows that the operating temperature and the thermo-capillary convection obtained within the molten pool generally increases as the processing multilayers are successively added, while the thermal effect depth is negatively reduced. The preferential direction of the heat diffusion generally changes from a downward pattern, then to the slightly strengthened horizontal direction and finally to a typically horizontal one for various deposited layers being processed. Therefore, the microstructure of the solidified part along the building direction (Region I to Region V) undergoes an interesting transformation: directional columnar cellular microstructure, crosswise-extended cellular microstructure, refined cellular microstructure, fragmentation microstructure and the coarse cellular microstructure. The tensile stress and the compressive stress are comprehensively obtained within the finally solidified layers, significantly influencing the micro-hardness.

Investigation on Weld Pool Dynamics in Laser Welding of AISI 304 Stainless Steel With an Interface Gap Via a Three-Dimensional Dynamic Model and Experiments

This paper reports on numerical and experimental investigations involving examination of the effects of an interfacial gap in the range of 0–0.3 mm on keyhole and molten pool dynamics. A numerical model was developed to investigate the three-dimensional transient dynamics of the keyhole in lap welding processes with an interface gap. The model was able to reliably predict the weld profile. In addition, the modeling results provided detailed information regarding the interaction between the molten pool and the solid/liquid boundary that led to the extended weld width. Experimentally, AISI 304 stainless steel was joined in a lap welding configuration using an IPG YLR-1000 fiber laser. The tensile shear and T-peel testing of the lap joints showed that adding an adequate amount of interface gap improves weld strength.

Effects of beam configurations on wire melting and transfer behaviors in dual beam laser welding with filler wire

Butt joints of 2mm thick stainless steel with 0.5mm gap were fabricated by dual beam laser welding with filler wire technique. The wire melting and transfer behaviors with different beam configurations were investigated detailedly in a stable liquid bridge mode and an unstable droplet mode. A high speed video system assisted by a high pulse diode laser as an illumination source was utilized to record the process in real time. The difference of welding stability between single and dual beam laser welding with filler wire was also compartively studied. In liquid bridge transfer mode, the results indicated that the transfer process and welding stability were disturbed in the form of “broken-reformed” liquid bridge in tandem configuration, while improved by stabilizing the molten pool dynamics with a proper fluid pattern in side-by-side configuration, compared to sigle beam laser welding with filler wire. The droplet transfer period and critical radius were studied in droplet transfer mode. The transfer stability of side-by-side configuration with the minium transfer period and critical droplet size was better than the other two configurations. This was attributed to that the action direction and good stability of the resultant force which were beneficial to transfer process in this case. The side-by-side configuration showed obvious superiority on improving welding stability in both transfer modes. An acceptable weld bead was successfully generated even in undesirable droplet transfer mode under the present conditions.

Role of melt behavior in modifying oxidation distribution using an interface incorporated model in selective laser melting of aluminum-based material

A transient three dimensional model for describing the molten pool dynamics and the response of oxidation film evolution in the selective laser melting of aluminum-based material is proposed. The physical difference in both sides of the scan track, powder-solid transformation and temperature dependent physical properties are taken into account. It shows that the heat energy tends to accumulate in the powder material rather than in the as-fabricated part, leading to the formation of the asymmetrical patterns of the temperature contour and the attendant larger dimensions of the molten pool in the powder phase. As a higher volumetric energy density is applied (≥1300 J/mm3), a severe evaporation is produced with the upward direction of velocity vector in the irradiated powder region while a restricted operating temperature is obtained in the as-fabricated part. The velocity vector continuously changes from upward direction to the downward one as the scan speed increases from 100 mm/s to 300 mm/s, promoting the generation of the debris of the oxidation films and the resultant homogeneous distribution state in the matrix. For the applied hatch spacing of 50 μm, a restricted remelting phenomenon of the as-fabricated part is produced with the upward direction of the convection flow, significantly reducing the turbulence of the thermal-capillary convection on the breaking of the oxidation films, and therefore, the connected oxidation films through the neighboring layers are typically formed. The morphology and distribution of the oxidation are experimentally acquired, which are in a good agreement with the results predicted by simulation.

Influence of thermodynamics within molten pool on migration and distribution state of reinforcement during selective laser melting of AlN/AlSi10Mg composites

A transient three dimensional model for describing the thermo-capillary convection and migration behavior and the resultant distribution state of reinforcing particles during selective laser melting of AlN/AlSi10Mg is proposed. The powder–solid transformation, temperature dependent physical properties and interaction between the reinforcement and the melt are taken into account. The effect of the laser energy per unit length (LEPUL) on the molten pool dynamics, cooling rate and the resultant sizes and distribution state of AlN reinforcement has been investigated. It shows that the thermo-capillary convection pattern changes from inward flow pattern to outward one, due to the appearance of the oxidation in molten pool. Therefore, the morphology of the top surface undergoes a continuous variation from the balling phenomenon, to the discontinuous tracks and finally to the formation of a flat and dense one. Meanwhile, both the clockwise and counterclockwise convection patterns are produced in the molten pool, caused by the interaction of reinforcing particles and the melt. An increase in LEPUL will significantly intensify the thermo-capillary convection whereas result in a decrease in the cooling rate of the molten pool. As LEPUL decreases from 1800J/m to 450J/m, the distribution state of AlN particles changes from the severe aggregation, then to the formation of partial aggregation and finally to the homogeneous distribution in the solidified matrix. The particle sizes of AlN reinforcement are experimentally acquired, which are in a good agreement with the results predicted by simulation.

Effect of metal vaporization behavior on keyhole-mode surface morphology of selective laser melted composites using different protective atmospheres

A selective laser melting (SLM) physical model of the change from heat conduction to keyhole-mode process is proposed, providing the transformation of the thermal behavior in the SLM process. Both thermo-capillary force and recoil pressure, which are the major driving forces for the molten flow, are incorporated in the formulation. The effect of the protective atmosphere on the thermal behavior, molten pool dynamics, velocity field of the evaporation material and resultant surface morphology has been investigated. It shows that the motion direction of the evaporation material plays a crucial role in the formation of the terminally solidified surface morphology of the SLM-processed part. For the application of N2 protective atmosphere, the evaporation material has a tendency to encounter in the frontier of the laser scan direction, resulting in the stack of molten material and the attendant formation of humps in the top surface. As Ar protective atmosphere is used, the vector direction of the evaporation material is typically upwards, leading to a uniform recoil pressure forced on the free surface and the formation of fine and flat melt pool surface. The surface quality and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.

Multiscale modeling of electron beam and substrate interaction: a new heat source model

An electron beam is a widely applied processing tool in welding and additive manufacturing applications. The heat source model of the electron beam acts as the basis of thermal simulations and predictions of the micro-structures and mechanical properties of the final products. While traditional volumetric and surface heat flux models were developed previously based on the observed shape of the molten pool produced by the beam, a new heat source model with a physically informed foundation has been established in this work. The new model was developed based on Monte Carlo simulations performed to obtain the distribution of absorbed energy through electron-atom collisions for an electron beam with a kinetic energy of 60 keV hitting a Ti---6Al---4V substrate. Thermal simulations of a moving electron beam heating a solid baseboard were conducted to compare the differences between the new heat source model, the traditional surface flux model and the volumetric flux model. Although the molten pool shapes with the three selected models were found to be similar, the predicted peak temperatures were noticeably different, which will influence the evaporation, recoil pressure and molten pool dynamics. The new heat source model was also used to investigate the influence of a static electron beam on a substrate. This investigation indicated that the new heat source model could scientifically explain phenomena that the surface and volumetric models cannot, such as eruption and explosion during electron beam processing.

Effects of shielding gas composition on arc profile and molten pool dynamics in gas metal arc welding of steels

In gas metal arc welding, gases of different compositions are used to produce an arc plasma, which heats and melts the workpiece. They also protect the workpiece from the influence of the air during the welding process. This paper models gas metal arc welding (GMAW) processes using an in-house simulation code. It investigates the effects of the gas composition on the temperature distribution in the arc and on the molten pool dynamics in gas metal arc welding of steels. Pure argon, pure CO2 and different mixtures of argon and CO2 are considered in the study. The model is validated by comparing the calculated weld profiles with physical weld measurements. The numerical calculations reveal that gas composition greatly affects the arc temperature profile, heat transfer to the workpiece, and consequently the weld dimension. As the CO2 content in the shielding gas increases, a more constricted arc plasma with higher energy density is generated as a result of the increased current density in the arc centre and increased Lorentz force. The calculation also shows that the heat transferred from the arc to the workpiece increases with increasing CO2 content, resulting in a wider and deeper weld pool and decreased reinforcement height.

Effect of gap on plasma and molten pool dynamics during laser lap welding for T-joints

The aim of the present research is to discuss the effect of gap on plasma plume, keyhole, and molten pool dynamics during laser lap welding for T-joints. The authors observe plasma plume, keyhole opening, and molten pool images by high-speed camera in different gaps during CO2 laser overlap welding of T-joints. The results show that gap causes beam energy fluctuations in the keyhole and leads to the instability of welding process. In laser spot welding, zero-gap and small gap greatly affect the stability of plasma and keyhole, which causes the formation of cavities in the weld metal, while a proper gap can help prevent porosity formation. In laser continuous welding, the disruption and closure of front keyhole wall at the gap periodically changes with the gap, which causes the formation of plenty of porosities at the gap. The instability of keyhole is closely related to dynamics of plume and molten pool, which gives an insight into the mechanism of porosity formation during laser overlap welding.

Investigation of atypical molten pool dynamics in tungsten carbide-cobalt during laser deposition usingin-situthermal imaging

An atypical “swirling” phenomenon observed during the laser deposition of tungsten carbide-cobalt cermets by laser engineered net shaping (LENS®) was studied using in-situ high-speed thermal imaging. To provide fundamental insight into this phenomenon, the thermal behavior of pure cobalt during LENS was also investigated for comparison. Several factors were considered as the possible source of the observed differences. Of those, phase difference, material emissivity, momentum transfer, and free surface disruption from the powder jets, and, to a lesser extent, Marangoni convection were identified as the relevant mechanisms.

Numerical simulation of molten pool dynamics in high power disk laser welding

A single-phase problem is solved rather than a multiphase problem for numerical simplicity: and the solution is based on the assumption that the region of gas or plasma can be treated as a void because solid or liquid steel has a greater density level than gas or plasma. The volume-of-fluid method, which can calculate the free surface shape of the keyhole, is used in conjunction with a ray-tracing algorithm to estimate the multiple reflections. Fresnel's reflection model is simplified by the Hagen–Rubens relation for handling a laser beam interaction with materials. Factors considered in the simulations include buoyancy force, Marangoni force and recoil pressure; furthermore, pore generation is simulated by means of an adiabatic bubble model, which can also lead to the phenomenon of a keyhole collapse. Models of the shear stress on the keyhole surface and of the heat transfer to the molten pool via a plasma plume are introduced in simulations of the weld pool dynamics. Analysis of the temperature profile characteristics of the weld bead and molten pool flow in the molten pool is based on the results of the numerical simulations. The simulation results are used to estimate the weld fusion zone shape; and the results of the simulated fusion zone formation are compared with the results of the experimental fusion zone formation and found to be in good agreement. The effects of laser beam profile (Gaussian vs. measured), vapor shear stress, vapor heat source and sulfur content on the molten pool behavior and fusion zone shape are analyzed.

Effect of process parameter on turbulent transport in a laser surface alloying process

A three-dimensional, transient numerical model is used for analyzing turbulent momentum, heat, and mass transport in typical laser surface alloying processes. The Reynolds averaged conservation equations are solved using a finite volume approach. The turbulent transport is accounted for by a suitably modified high Reynolds number k-ε model in the presence of a continuously evolving phase-change interface. Phase change aspects of the problem are addressed using a modified enthalpy-porosity technique. Subsequently, the developed turbulent transport model is used to simulate single pass laser surface alloying processes with different sets of processing parameters, such as laser power, scanning speed, and powder feedrate, in order to assess their influences on molten pool geometry and dynamics, cooling rates, as well as on species concentration distribution inside the substrate. In order to investigate the effects of turbulence, the parametric studies for the present turbulent simulation are compared with corresponding laminar simulation results. Significant differences are observed on comparing the laminar and turbulent simulation results, which provide valuable insights for controlling the process parameters based on the manufacturing needs.

Physical and Thermal Processes During Electron Beam Welding

Complicated processes of the seam formation during electron beam welding are described. The physical processes in the weld pool and in the vapor-plasma mixture emitted in the cavity into the liquid metal created by intensive high energy beam are discussed. The dynamics of the liquid metal in the weld pool are interconnected with the energy distribution of the electron beam in the cavity which influences the weld depth and width and controls the defect formation in the weld such as pores (blow holes), non-uniformity of the weld root (spiking) and rippled weld surface. The moving heating source, working in the weld pool of metal samples is steady only as a first approximation due to the molten pool dynamics. A more exact heat model of the electron beam welding must take into account the unstable nature of beam transport within vapors and gas plasma in the cavity as well as the variations of energy dissipation on the crater walls and of heat transfer through molten metal of the weld pool.

Dynamics of keyhole and molten pool in laser welding

In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in very high-power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.