Molten Pool Surface Research Articles

Study of the solidified thin layer on molten pool surface in laser deep penetration welding

Morphology and temperature are two important features of molten pools in laser deep penetration welding. An experimental platform equipped with a high-speed camera and a two-color pyrometer was built to observe the variations in the morphology and temperature, and a floating layer on the surface of a molten pool was discovered. The floating layer’s morphology, temperature, and thickness were described, and there was a correlation between the morphology of the floating layer and the temperature distribution of the molten pool. The results revealed that the variation range of floating layer thickness is primarily related to three factors: the floating layer area, the molten pool’s temperature, and the inclination angle of the floating layer immersed in the molten pool. Surface tension is an important factor in allowing the solidified thin layer to float on the surface of the molten pool. During laser deep penetration welding, the floating layer area goes through three stages during laser welding: growth, reduction, and dynamic stabilization. Finally, the floating layer abbreviated as “STL”, is defined as the solidified thin layer. As the laser power increases, the STL area decreases and the weld reinforcement is lower. Weld reinforcement is linearly related to the STL area. In summary, this study’s results provide a theoretical foundation for predicting the welding quality in laser deep penetration welding in the future.

A novel liquid-shielded welding solution for diffusible hydrogen content restriction and metal transfer controlling in underwater FCAW condition

Hydrogen produced by water decomposition caused high diffusible hydrogen content in deposited metal, but also produced numerous spatter and unstable fragments which were considered as the main reason that deteriorated the weld formation during underwater wet flux cored welding (FCAW). Drawn inspiration from onshore gas shielded welding, in this paper, a liquid shielded welding method was invented. As a by-product of biodiesel, glycerol was chosen as liquid protectant. The results suggested that the diffusible hydrogen content in deposited metal was reduced by 62.4% with glycerol shielded due to its higher decomposition threshold and less hydrogen in decomposition products. Arc bubbles were attached to the molten pool surface and their expansion rate and rising velocity were slowed down resulted in more stable droplet transfer process and less spatter owing to high viscosity glycerol protectant. Almost porosity-free joints were obtained and the mechanical properties of the welded joints were also improved.

Power density effect on the laser beam-induced eruption of spatters in fiber laser keyhole welding

Violent eruption of spatters is one of the major problems restricting fiber laser keyhole welding technology development. In this study, the fiber laser power density variation was attained by altering the laser power and laser spot diameter for analyzing its impact on the eruption of spatters. The severity of spatters’ eruption can be indicated by the number of spatters and the mass loss of the weld seam. The correlation between the number of spatters and laser power density can be positive (if the latter is adjusted by changing the laser power) or negative (if the laser power density is varied by changing the laser spot diameter). When the latter diameter is changed, it has a strong impact on the vapor induced by the laser on the front keyhole wall (FKW), the FKW inclination angle, and the molten pool surface tension. The number of spatters also positively correlates with the laser-induced vapor on the FKW or the molten pool width and negatively correlates with the FKW inclination angle. Reducing the laser spot diameter can increase the laser power density, enhancing the welding process stability and suppressing the eruption of spatters to a certain extent.

Investigation of molten pool dynamic behaviors in laser additive manufacturing by numerical simulation

Laser additive manufacturing (LAM) can provide innovative approaches for designing a lightweight product with load-adapted structures and be widely used in the automotive and aerospace industries. A numerical model taking into account wire feeding and laser absorption is developed in this paper to investigate the dynamic behaviors of a mixing molten pool in the LAM process. Based on this model, flow field distribution and the evolution of molten pool are numerically calculated. The surface of the mixing molten pool during the LAM process is tracked and the evolution of the molten pool is analyzed. The droplet transfer mode of molten wire adding into the molten pool and the influence of Marangoni convection on the surface evolution of the molten pool are discussed by way of simulation results. It is found that the molten pool dynamic behaviors have great influence on the morphology of the formed layer after molten metal solidification in the molten pool. The calculated height and width of the formed layer are validated by the experimental results and good agreement between them is found. Therefore, the proposed method is of importance for controlling the molten metal flowing in the molten pool and the morphology of the formed layer in LAM.

Molten Pool Behaviors in Double-Sided Pulsed GMAW of T-Joint: A Numerical Study

The T-joint is one of the essential types of joints in aluminum welded structures. Double-sided welding is a preferable solution to maintain high efficiency and avoid significant distortion during T-joint welding. However, interactions between double-sided molten pools make flow behaviors complicated during welding. Numerical simulations regarding molten pool behaviors were conducted in this research to understand the complex flow phenomenon. The influences of wire feed rates and torch distances were simulated and discussed. The results show that droplet impinging drives the fluid to flow down to the root and form a frontward vortex. Marangoni stress forces the fluid to form an outward vortex near the molten pool boundary and flatten the concave-shaped molten pool surface. With an increased wire feed speed, the volume of the molten pool increases, and the root fusion is improved. With an increased torch distance, the width of the front molten pool decreases while the length increases, and the rear molten pool size decreases slightly. Both wire feed speeds and the torch distances have limited influences on the basic flow characteristics.

Influence of spatial laser beam profiles on thermal-fluid transport during laser-based directed energy deposition

ABSTRACT The effects of the two types of typical spatial laser beam profiles (SLBP), including the super-Gaussian profile (SGP) and Gaussian profile (GP), on the thermal-fluid transport during laser-based directed energy deposition (L-DED) were numerically investigated. Considering the laser-powder-pool coupling effect, a comprehensive heat transport model including an input source model, laser-powder coupling model and molten pool thermal-fluid transport model is established. It is found that the SLBP has an important influence on the temperature gradient at the solidification interface and the flow field. Compared with the GP, the temperature gradient at the solidification interface under the SGP is high. Because the Marangoni stress and fluid momentum are in opposite directions, the fluid velocity distribution on the molten pool surface presents an ‘M’ shape. The fluid velocity on the molten pool surface under the SGP is obviously different from that under the GP.

Numerical simulation of slag movement from Marangoni flow for GMAW with computational fluid dynamics

This study analyzed the behavior of molten slag, which is an oxide generated during spray mode gas metal arc welding, with computational fluid dynamics. The molten slag, composed mainly of silicate (SiO2), is located on the molten pool surface. It is generally assumed that the molten slag is generated around the arc plasma boundary. Therefore, in the numerical simulation in this study the slag is modeled as a spherical particle, which has a specific density and size. To compare the Marangoni flow effect, this study investigated two different cases where the surface tension gradients were different (positive and negative). In both the numerical simulation and experimental results it was found that negative surface tension gradient formed trapped slag on the bead edge while the positive surface tension gradient formed trapped slag on the center of the top surface.

Modeling and simulation of heat transfer, fluid flow and geometry morphology in GMAW-based wire arc additive manufacturing

In this study, we develop a 3D transient mathematic model to simulate the heat transfer, fluid flow, and geometry morphology in GMAW-based wire arc additive manufacturing (WAAM). The processes of droplet formation, growth, and detachment from the end of wire electrode, who travels dynamically along the scanning direction, are coupled with molten pool for the first time by considering their own mechanical conditions and solving the transport equations in the whole solution domain. By the developed model, the simulations of single-pass multi-layer of WAAM of Al-5%Mg are performed. The calculated results indicate that when the droplet falls into the molten pool, the maximum velocity inside the droplet reaches 0.9m/s, resulting in that liquid metal in the middle flows toward the bottom of the molten pool and a depressed region is formed. On the surface of molten pool, the liquid metal dominated by Marangoni force flows from center to periphery, and on the bottom of molten pool, a clockwise circulation is formed. In addition, the interlayer idle time contributes to the formation of deposit with higher height and narrow width. Finally, to validate the model, the deposit profiles are also compared between simulated and experimental results.

Experimental sensing of molten flow velocity, weld pool and keyhole geometries in ultrasonic-assisted plasma arc welding

• Comparative experiments were performed to investigate the ultrasonic effect in plasma arc welding. • The molten flow velocity on the weld pool surface was measured by tracer particle approach. • With ultrasonic vibration, the front wall and the back wall of the keyhole became steeper. • With ultrasonic vibration, the backward flow velocity of the molten metal decreased obviously. • Ultrasonic vibration causes small sizes of weld pool and keyhole but enhanced keyholing/penetrating capability. Comparative experiments were performed to investigate how the ultrasonic vibration exerted on the tungsten electrode affect the molten metal flow on weld pool surface and the sizes of weld pool and keyhole in plasma arc welding (PAW). The fluid flow velocity on the molten pool surface was measured by tracer particle approach. The images of front molten pool and keyhole entrance were captured synchronously from different directions with two CCD cameras. Special algorithms were developed to calibrate and integrate the weld pool and keyhole images into a single picture in one coordinate system. It was found that when ultrasonic vibration was applied, the keyhole shape was changed, the front wall and the back wall of the keyhole became steeper, and the backward flow velocity of the molten metal decreased obviously. The size of the molten pool and the keyhole entrance in ultrasonic-assisted PAW was smaller than that in PAW, and the keyholing/penetrating capability was enhanced.

The formation mechanism of special globular surface grain during the solidification of laser surface remelted near β titanium alloys

During laser materials processing, the cooling rate (R) and thermal gradient (G) near the molten pool surface region are much higher than those in the traditional casting conditions, which may result in special solidification microstructures. In this study, the surface microstructure of near β titanium alloys was investigated by conducting both laser surface remelting experiments and phase-field simulations. It was found that the grains nucleated near the surface region of the molten pool exhibit near globular shape with 〈110〉-oriented side branches, which are very different from the ordinary dendrite morphology in casting conditions. Phase-field simulations were used to detect the formation mechanism of such special surface grain morphology. It was illustrated that the surface grain morphology is determined by the combined effect of high cooling rate (R) and high vertical thermal gradient (G). The high R results in the morphology transition from needle-like dendrite to near globular shape by increasing the undercooling and reducing the impact of anisotropy of interface energy. The high G promotes the grain contour to be more close to globular and facilitates the fins-like side branches to develop along 〈110〉 direction. Furthermore, a selection map of surface grain morphology at various thermal conditions (R and G) was established, which can be applied to predict the surface grain morphology.

Study on keyhole coupling and melt flow dynamic behaviors simulation of 2219 aluminum alloy T-joint during the dual laser beam bilateral synchronous welding

In this research, the keyhole coupling and melt flow behaviors of 2219 aluminum alloy T-joint during dual laser beam bilateral synchronous welding (DLBSW) process are investigated by simulation, which involves fluid heat transfer and fluid flow. The calculated mesh model of the T-joint is established by simplifying the equal boss of the skin. The combined Gaussian laser heat source is adopted to simulate the laser energy loaded on the base metal during the welding process. The obtained simulation results are reliably matched with the experimental results. Subsequently, not only the coupling process of two keyholes and the evolution characteristics of the coupled keyhole are analyzed, but also the characteristics of molten pool and the flow behaviors of the liquid metal are studied in detail. The results demonstrate that the keyhole profile’s oscillations continuously exist, whether before or after the keyhole coupling. The flow direction of the liquid metal near the coupled keyhole wall includes the flow parallel to the keyhole wall and that around the keyhole wall. Besides, the maximum depth of the molten pool slightly lags behind the rear wall of the keyhole. On the coupled keyhole wall and molten pool surface, the melt flow velocities are significantly greater than that inside the molten pool. The vortexes are produced near the upper and lower regions of the coupled keyhole wall.

Polarimetric imaging method for surface quality evaluation of a liquid metal pool obtained during welding

Conventional methods used to inspect the molten pool surface induced by arc welding processes are based on post-welding evaluation using destructive or semi-destructive testing methods. Many attempts were performed to develop alternative and nondestructive tests. In this context, a polarimetric vision methodology is evaluated. The polarimetric parameters are measured from the thermal radiations emitted by the hot liquid metal at a wavelength within an arc plasma blind spectral window. The Stokes parameters and their segmentation within a Gaussian mixture model allow to discriminate artifacts at the surface of the liquid metal. After weld pool solidification, the use of scanning electron microscopy and energy dispersive spectroscopy allow to identify the artifacts, i.e., floating alumina particles.

Development mechanism and solidification morphology of molten pool generated by laser cladding

Due to the complex transient characteristics, it is difficult to directly measure the formation and development of a molten pool formed by laser cladding. Considering the change of thermophysical properties at high temperatures and the coupling between the heat source and the molten pool surface, a new numerical simulation method was proposed and verified. The thermal history and formation of the molten pool, the convective pattern, and the solidification morphology of the cladding have been analyzed. The results show that the molten pool can reach steady-state in a very short time, and both considerable temperature gradient and fast circular convection exist in the molten pool. The bottom of the molten pool mainly showed columnar growth, while the top was equiaxed. Columnar to equiaxed transition occurred at the center of the molten pool.

Mechanism investigation of the influence of the magnetic field on the molten pool behavior during laser welding of aluminum alloy

Magnetic field-assisted welding provides a promising approach to suppress defects and improve weld bead properties significantly. In this paper, a full factorial experiment was conducted to study the mechanism of the influence of the magnetic field on the weld profile of aluminum alloy laser welding. Furthermore, a three-dimensional numerical simulation model simultaneously considering the free surface of the keyhole, and the effect of the magnetic field is developed to reveal the mechanism of the influence of the magnetic field on the weld bead profile by analyzing molten pool behavior. It was found that the magnetic field can reduce the width of the weld bead while increasing the depth and middle width of the weld bead. The magnetic field distribution in the weld bead was numerically simulated and validated. The simulation results demonstrate that the liquid motion in the magnetic field can induce a Lorentz force in a direction opposite to the flow velocity. The magnetic field can suppress both the lower-to-upper region and center-to-edge molten flows of the molten pool, which reduces the heat transfer from the tip of the keyhole to the molten pool surface and from the center to the boundary of the molten pool. This can provide a concept to reveal the underlying mechanism of the influence of the magnetic field on the weld bead profile. This work could contribute to gain insight into the influence of a magnetic field on aluminum alloy laser welding.

Numerical analyses of molten pool evolution in laser polishing Ti6Al4V

Abstract As one advanced manufacturing process, laser polishing has been widely used in precision machining of metal or nonmetal materials. However, the evolution mechanism of molten pool formed during polishing process has not been thoroughly studied. This paper studies the polishing process of Ti6Al4V with a continuous wave (CW) fiber laser under the top-hat distributed heat source. A two-dimensional (2D) axisymmetric numerical transient model is established to simulate the formation process of the surface of Ti6Al4V after laser heating to form a molten pool and then cooling. It reveals that the evolution mechanism of physical processes involved in heat transfer, heat radiation, heat convection, melting and solidification during the polishing process. Particularly, the capillary and thermocapillary regimes (Marangoni flow) play a key role in smoothing the free-form surface of molten pool. The freedom surface can be smoothed effectively before the solidification of molten pool. In order to verify the correctness of the simulation model and explore the influence of laser irradiation duration on the material, the surface topography of the polished sample was investigated. It is found that the errors of the depth and width of the remelting zone are respectively less than 8 % and 22 % by the comparison of simulation and experimental results. Likewise, the depth and width of molten pool expand with the laser radiation duration increasing. Meanwhile, the polished surface topography becomes smoother.

Effect of Al[sbnd]Si coating on laser spot welding of press hardened steel and process improvement with annular stirring

Laser spot welding method was employed to fabricate AlSi coated press-hardened steel (PHS) joints in overlapped configuration. Al and Si segregate at the notch root of a laser spot weld of AlSi coated PHS resulting in pre-mature cracking and weakening of the joint strength. The thickness-oriented δ-ferrite strips induced by the Al and Si segregation was mainly responsible for the mechanical deterioration though the dispersive δ-ferrite in the fusion zone also contributes. A supplemental annular stirring with beam oscillation is proposed to break up the segregation. With the annular stirring, the lap-shear peak load and energy absorption increased 57% and 80% respectively, which is attributed to the removal of δ-ferrite strips and the reduction in the fraction of dispersive δ-ferrite. Thermo-fluid numerical simulations suggest that the annular stirring promoted the melt flow and extended the duration of the melt pool, which accelerated and prolonged the diffusion of Al and Si atoms into the fusion zone, thus preventing the segregation of Al and Si and formation of δ-ferrite strips. Meanwhile, the prolonged melt flow pulled additional dissolved Al onto the molten pool surface forming the Al oxide film, resulting in further reduction of Al content and fraction of dispersive δ-ferrite.

Observing molten pool surface oscillations during keyhole processing in laser powder bed fusion as a novel method to estimate the penetration depth

Various in-situ monitoring techniques have been developed for the detection of process drifts in the Laser Powder Bed Fusion (LPBF) process. Currently, optical emission monitoring can retrieve information regarding molten pool characteristics, such as temperature, width, length and area which provide substantial process signatures. Nonetheless, a fundamental indicator for the retrieval of a complete set of spatially distributed information is missing: the molten pool depth. Within the present investigation, a system for the estimation of the penetration depth based on the detection of molten pool surface oscillations is reported. Initially, the fundamentals of the monitoring technique are presented. The principle relies upon the observation of molten pool surface ripples through the measurement of probe light reflections in the melt area. Proof of concept testing of the sensing principle was conducted through an experimental investigation on a prototypal platform. A monitoring system (consisting of a high-speed camera and a secondary illumination light) was employed to view the process while realising both bead-on-plate material remelting and single track powder bed fusion depositions of AISI316L at different levels of laser emission power. Oscillation frequencies were extracted from the high-speed imaging acquisitions after image processing and signal analysis. The surface wave oscillations were measured to be in the range of 3.5–5.5 kHz in keyhole conditions. Metallographic cross-sections allowed to observe the effective molten pool penetration depth and cross-sectional area and were correlated to oscillation frequencies. Higher values of oscillation indicated shallower penetration and consequently a smaller mass of molten material.

Numerical analysis of the influence of molten pool instability on the weld formation during the high speed fiber laser welding

The dynamic behaviors of the molten pool play a critical role in determining the weld bead formation process and the welded joints quality. A numerical simulation modeling method is proposed to investigate the instability of the molten pool and understand its influence on the formation process of the weld and the related welding defects during the high speed fiber laser welding. In the modeling, the molten metal flowing dynamic behaviors and free surface evolution of molten pool are coupling taken into account. The instable phenomena including the profiles variation of the molten pool surface in the vertical direction and edges oscillation in the horizontal direction are demonstrated and discussed in the simulation results in details. The Rayleigh instability of the molten pool is numerically approved and the relationship between the molten pool profile and Rayleigh instability is revealed, which is kept consistent with the experiment and literature results. It is found that the welding defects formation process in the weld is greatly affected by the instable phenomena in the molten pool. The oscillation and fluctuation of the molten pool surface in the horizontal direction and vertical direction during the cooling process can directly result in the defects formation in the weld.

Stirring effect of the rotating arc on the molten pool during non-axisymmetric tungsten NG-GTAW

Abstract The stirring effect of the rotating arc during non-axisymmetric tungsten narrow-gap GTAW (NT-NG-GTAW) on the molten pool is investigated. Tracer particles are embedded in the groove and plate metal surface, respectively. During welding, the floating tracer particles are captured with a high-speed camera to reflect the flow characteristics of the liquid metal. Compared with conventional GTAW processes, mechanical force (Fc) and inertial centrifugal force (Fl) in NT-NG-GTAW have more significant effect on the molten pool. They are the main driving forces to change the flow pattern of the molten pool under the rotating arc. As the arc rotate to the boundary of the molten pool, liquid metal layer on molten pool surface forms a vortex at the other edge to impact on the sidewall. When the arc rotates clockwise, the molten pool in bead-on-plate rotates counterclockwise, whereas the molten pool in the narrow gap rotates clockwise with the same rotation frequency as the arc.

Effect of overlapped adjacent tracks on surface morphology in plasma beam polishing of austenitic stainless steel

The adoption of high energy beam in the area of metallic surface polishing is a novel technique to produce glossy surfaces. In particularly, plasma beam finishing is emerging as a workable and economical option to realize high quality irradiated surfaces. In this paper, effect of overlap between the adjacent tracks on surface morphology during plasma beam finishing was researched through using different offset distances, and AISI 304 austenitic stainless steel was selected to be the test material. Based on considering surface roughness, certain statistic features (e.g. material ratio function and power spectral density function) were analyzed to comprehensively reflect the influence of offset distance. To aid a deeper understanding of the molten pool flow and surface evolution during polishing process, a two-dimension transient model, which consists of magnetic field, electric field, laminar flow and heat transfer, was developed by means of the finite element approach. The multi-track polishing tests showed that although the irradiated surface became smoother, significant bulges were found in the overlapped region between adjacent tracks. The simulation model then verified that these bulges varied with different offset distances, reflecting different dynamic processes. Therefore, the selection of offset distance is important for the optimization of plasma beam polishing.