Deep Penetration Laser Welding Research Articles

Numerical Simulation of Preheating Temperature on Molten Pool Dynamics in Laser Deep-Penetration Welding

In this paper, a heat-flow coupling model of laser welding at preheating temperature was established by the FLUENT 19.0 software. The fluctuation of the keyhole wall and melt flow behavior in the molten pool under different preheating temperatures were analyzed, and the correlation between keyhole wall fluctuation and molten pool flow with spatters and bubbles was obtained. The results indicate that when the outer wall in the middle of the rear keyhole wall is convex, the inner wall is concave, which causes spatter or the bottom of the keyhole to collapse. When the metal layer in the middle of the rear keyhole wall turns into obliquely upward flow, welding spatter is generated. In contrast, the metal layer in the middle of the rear keyhole wall changes to flow into the keyhole, and the bottom of the keyhole collapses. When the preheating temperature is 300 K (ambient temperature), 400 K, and 500 K, the inner wall in the middle of the rear keyhole wall is concave. With the increase in the preheating temperature, the area of the concave gradually increases, and the size of the liquid column behind the keyhole opening gradually decreases. When the preheating temperature is 300 K, there are more spatters above the molten pool. In comparison, when the preheating temperature is 400 K or 500 K, there are less spatters, and the bottom of the keyhole collapses.

Simulation of the Effect of Keyhole Instability on Porosity during the Deep Penetration Laser Welding Process

The quality of a laser deep penetration welding joint is closely related to porosity. However, the keyhole stability seriously affects the formation of porosity during the laser welding process. In this paper, a three-dimensional laser welding model with gas/liquid interface evolution characteristics is constructed based on the hydrodynamic interaction between the keyhole and molten pool during the laser welding process. The established model is used to simulate the flow and heat transfer process of molten. The Volume of Fluid (VOF) method is used to study the formation and collapse of the keyhole and the formation of bubbles. It is found that bubbles are easy to form when the keyhole depth abruptly changes. There are three main forms of bubbles formed by keyhole instability. The front wall of the keyhole collapses backward to form a bubble. The back wall of the keyhole inclines forward to form a bubble. The lower part of the keyhole produces a necking-down effect, and the lower part of the keyhole is isolated separately to form a bubble. In addition, when the keyhole does not penetrate the base metal, the stability of the keyhole is high and the percentage of porosity is low.

Measurement of pulsed laser welding penetration based on keyhole dynamics and deep learning approach

The keyhole instability is a key concern in laser deep-penetration welding of high reflectivity materials, potentially impacting the penetration status and weld quality. Monitoring and control the keyhole behavior still remain a great challenge for obtaining a desired welded joint. For the pulsed laser welding of thin-sheet aluminum alloy, an active visual monitoring system was established to systematically probe the dynamic keyhole behavior from multi-view sensing. Combining with the image processing method and process analysis, the keyhole surface area and depth were extracted to quantify the keyhole formation dynamics under different welding conditions. Furthermore, a data-driven deep learning model with hyperparameter optimization was constructed to identify different penetration states and it has a high accuracy and good reliability. The experiment results show that our proposed measurement scheme based on multi-view monitoring and deep learning approach could guide the development of real-time control of the pulsed laser welding process.

Solidification cracking of a nickel alloy during high-power keyhole mode laser welding

Nickel alloy Inconel 740H, a candidate material for use in ultra-supercritical power plants, is susceptible to solidification cracking during high power deep penetration laser welding. Here we examine how cracking is affected by welding variables and determine the locations where the cracks occur experimentally and theoretically. We use a solidification cracking model to calculate the effects of welding variables on cracking and the locations where the cracks form during high power laser keyhole mode welding of IN 740H. The parameters needed for the cracking model are obtained from a well-tested numerical heat transfer and fluid flow model for keyhole-mode welding. Model predictions of cracking and their locations for different welding conditions are verified by experiments. • Solidification cracking of nickel alloys during laser welding is poorly understood. • We propose a novel methodology to understand why, when, and where cracking occurs. • A heat and fluid flow and solidification model, and experimental data are used. • Model predictions of cracking locations and conditions are verified by experiments. • The work can be used to prevent weld failures in thick section laser keyhole welds.

Numerical Simulation of Molten Pool Dynamics in Laser Deep Penetration Welding of Aluminum Alloys

In this paper, the numerical simulation of molten pool dynamics in laser deep penetration welding of aluminum alloys was established based on the FLUENT 19.0 software. The three-dimensional transient behavior of the keyhole and the flow field of molten pool at different welding speeds were analyzed, and the influence of the welding speed on the molten pool of aluminum alloys in laser welding was obtained. The results indicated that the generation of welding spatters was directly related to the fluctuation of the diameter size in the middle of the keyhole. When the diameter in the middle of the keyhole increased by a certain extent, welding spatters occurred. When welding spatters occurred, the diameter in the middle of the keyhole became smaller. In addition, the size of the spatters at the welding speed of 9 m/min was larger than that of the spatters at the welding speeds of 3 m/min and 6 m/min. The welding spatter formed in laser deep penetration welding included: spatter created by an inclined liquid column behind the keyhole; splash created by a vertical liquid column behind the keyhole; small particles splashed in front of the keyhole. With the increase of the welding speed, the tendency of the welding spatter to form in front of the keyhole and to form a vertical liquid column behind the keyhole became weaker. When the welding speed was 9 min, only an obliquely upward liquid column appeared on the molten pool surface behind the keyhole. Compared with the welding speeds of 6 m/min and 9 m/min, the maximum flow velocity fluctuation of the molten pool at the welding speed of 3 m/min was obviously higher.

Effect of magnetic field orientation on suppressing porosity in steady-magnetic-field-assisted aluminum alloy deep-penetration laser welding

The steady magnetic fields were utilized to decrease the porosity of aluminum alloy laser welded joints, and the effect is related to the magnetic field orientation. The influence of the magnetic field orientation on the suppression of porosity was investigated by establishing a three-dimensional heat flow model coupled with magnetohydrodynamics (MHD) and free surface of a keyhole. The Lorentz force decreased the backflow velocity toward the keyhole wall when the magnetic fields were horizontally parallel to the welding direction (magnetic field X-axis, MFX) and perpendicular to the welding direction (magnetic field Y-axis, MFY), thereby clearly improving the keyhole stability. Additionally, the melt flows in the rear region of the molten pool were suppressed downward and forward under the MFX and MFY, respectively, leading to additional heat accumulation in the rear periphery of the molten pool. As a result, a long molten pool in the longitudinal section and a small penetration depth were observed under the MFX and MFY. Therefore, the porosity defects decreased from 9.4% to 3.5% and 3.2% under the MFX and MFY, respectively. However, many pores were observed when the magnetic field orientation was vertical (magnetic field Z-axis, MFZ), owing to the high penetration depth and poor keyhole stability. This study provides a deeper understanding of the effect of magnetic fields on the laser welding of aluminum alloys and potentially lays a foundation for the adjustment of suitable magnetic field parameters toward obtaining the desired effect.

Direct observation of keyhole with an innovative layer-by-layer imaging method during deep penetration laser welding

In order to solve the difficult problem of observing the keyhole in deep penetration laser welding of metal materials, an innovative keyhole direct observation method based on layer-by-layer imaging technology is first proposed in this paper. In this novel method, a specially-designed composite workpiece consisting of an upper metal wedge and a lower glass trapezoidal prism is used. During deep penetration laser welding, a keyhole will form in and penetrate the metal wedge, whose cross section image that appears on the inclined bottom surface of the wedge is directly observed by a high speed video camera through the transparent glass prism. Then, the deep penetration laser welding is carried out with a fiber laser unit, by using the above proposed keyhole direct observation method, the keyhole cross section images at different depths are captured by the high speed camera. Finally, the images of the keyhole cross sections are processed and the outlines of the keyhole cross sections are extracted, from which the variation of the keyhole cross section during deep penetration laser welding of metal material is discussed. The result shows that from the top to the bottom, on one hand, the size of the keyhole cross section decreases gradually in general, but fluctuates to some extent, which indicates that the laser beam energy may not be uniformly absorbed into the keyhole along the depth and the evaporation site may change its position depending on the energy equilibrium there, on the other hand, the position of the keyhole cross section moves backward in the opposite direction of the welding velocity, whose motion tend agrees with the bending keyhole in the real case of laser welding.

Prediction of penetration based on plasma plume and spectrum characteristics in laser welding

The plasma characteristics generated in laser welding are closely related to the weld penetration state and the resultant weld properties. In this paper, a real-time monitoring platform was built to monitor the plasma plume and spectral in laser deep penetration welding of the H340LA high-strength steel tailor rolled blanks. The image processing method and the spectral processing method were used to investigate the correlation of the penetration on the plasma plume and spectral. The weld penetration state was divided into three categories: excessive penetration, moderate penetration and partial penetration. It was found that the penetration was mainly affected by centroid height, plasma area and intensity. The weld gradually transitioned from partial penetration, moderate penetration to excessive penetration with the centroid height, plasma area and spectral intensity decreased. When the centroid height was 2.42–4.19 mm, the area was 6.96–17.58 mm2, and the spectral intensity was 447.98–1453.95 a.u., moderate penetration weld can be obtained. A plasma plume-spectrum based multi-features Back Propagation (BP) neural network model was purposed to predict the weld penetration state with an accuracy of 97%, which is greatly higher than that of the single-feature neural network model. This work provides technical guidance for realizing real-time control of weld penetration.

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.

Effects of reduced ambient pressure on the OCT-based weld depth measurement signal in laser welding of aluminum and steel

For the process monitoring during laser deep penetration welding, optical coherence tomography (OCT) has emerged as a promising method for the in-situ measuring of the keyhole depth or, as a result, of the weld penetration depth. Prior research on this topic with steel substrates has shown that reducing the ambient pressure significantly improves the detectability of keyhole bottoms. In addition to OCT measurements, high-speed recordings of the process zone enable the evaluation of the keyhole opening behavior in this study. Bead-on-plate welds in steel demonstrate that the reduction of the ambient pressure improves the measurements unambiguity and decreases the average keyhole diameter while the keyhole opening stability seems to remain mainly unchanged. Therefore, it can be concluded that the higher measurement unambiguity at lower ambient pressure is not caused by a higher keyhole accessibility of the OCT probe beam but rather by changed conditions inside the keyhole. Experiments with aluminum substrates validated the beneficial effects of the reduced ambient pressure on the detectability of the keyhole bottom.

Root formation and mechanical properties in laser keyhole welding of 15 mm thick HSLA steel

Deep penetration laser welding is promising in joining thick (> 10 mm) steel sections. Focused laser beam by drilling vapour cavity, the keyhole, generates deep and narrow welds. Full penetration single-pass joining has a persistent problem with root quality where humping is one of the most frequent imperfection. This strongly hampers the use of high-power laser for thick plate welding. A 16 kW disk laser was used for single-pass welding of 15 mm thick plates in a butt joint configuration. Root humping occurred within a wide range of welding parameters. This provides narrow processing window. By adding an arc source to the laser beam process, the tendency of root humping increases. To achieve humping-free welds and consistent root quality over length, a delicate balance of process parameters is required. High heat input (> 0.50 kJ/mm) was positive to achieve a combination of low hardness (< 325 HV) and good Charpy toughness at -50 °C (> 50 J).

Beam shaping solutions for stable laser welding

AbstractThe laser beam is a highly flexible tool that is used for many material processing applications. New spatial beam shaping technology enables even further functionality to create up to four focal spots in an axial or lateral direction, which enables the distribution of the energy to be tailored. It was shown that certain beam shapes help to suppress spattering and increase the bridgeable gap size during deep penetration laser welding.

Numerical simulation of the temperature field, weld profile, and weld pool dynamics in laser welding of aluminium alloy

A transient 3-d thermo-fluid model is developed to simulate the heat transfer and weld pool dynamics during the deep penetration laser welding of aluminium alloy AA2024. Numerical modelling and non-dimensional analyses are carried out to investigate the effect of driving forces on melt pool dynamics and geometry. The temperature-driven surface tension and its influence on weld pool geometry is analysed quantitatively by varying the input parameters. The normalised dimensions of the weld pool are plotted against dimensionless numbers viz. Peclet, Reynolds, Marangoni numbers to understand the uncertainty and changes in the process. The melt flow in weld pool is found to be laminar for the considered combination of material and process parameters. The conclusions derived in this work could explain the Marangoni convection in materials with a negative gradient of surface tension and provide an insight into the defects formation and their reduction in laser welding of aluminium alloys. The developed model is validated with experimental data and found in good agreement.

Study on laser welding of copper material by hybrid light source of blue diode laser and fiber laser

Focusing on the problem of high-power laser welding of high reflectivity of copper, in this article, a physical model of energy absorption for the temperature field of hybrid laser deep penetration welding of copper materials is established by using a Gaussian rotating body heat source of 450 nm blue diode laser and 1060 nm fiber laser. The main factors are analyzed such as welding joint design, laser energy density, focal position, and shielding gas. When the 1000 W fiber laser is combined with a 300, 500, and 700 W blue diode laser to weld copper material, the relationship between the welding temperature field and the light source absorption rate is analyzed on the condition of a hybrid heat source distance of 2.2–6.2 mm and a welding speed of 10–30 mm/s. The results show that the absorption rate of a fiber laser welded by double-beam hybrid welding can be increased by 20% compared with that of a single light source; that is, 300, 500, and 700 W blue diode laser together with 1000 W fiber laser combines to weld copper materials, and the absorption rate of the light source reaches the highest when the hybrid heat source distance and the welding speed are 3.0 mm and 20 mm/s, respectively.

Effects of energy density attenuation on the stability of keyhole and molten pool during deep penetration laser welding process: A combined numerical and experimental study

The energy density attenuation of focused laser beam could induce inconsistent thermodynamic behaviors from near field to far field, which leads to the deterioration of the welding stability. However, the influence of energy density attenuation on welding process remains unclear. In this work, a set of innovative characterizing methods are presented to quantitatively evaluate the features of the front keyhole wall and molten pool dynamics. The energy density attenuation and correlative phenomena are numerically studied using a multiphase model. This model incorporates an improved ray-tracing method which can describe the actual beam profile more accurately. Based on experimental and numerical results, a unique melting behavior, i.e., the alternate melting between keyhole wall and keyhole bottom due to the energy density attenuation, is discovered. This melting behavior is primarily caused by movement of protrusions along the front keyhole wall, which can change the energy absorption efficiency significantly. Besides, it is found that the fluctuation amplitude of molten pool is closely related to this melting behavior. By sustaining protrusion movement process, the melting behavior can be regulated and hence improve the molten pool stability. Finally, an optimization method by adjusting the beam defocus and inclination is proposed and testified to improve the welding stability.

Numerical study on molten pool dynamics and solute distribution in laser deep penetration welding of steel and aluminum

To shed new light on the molten pool behavior and transmission mechanism during the laser welding of dissimilar metals, a 3D transient numerical model was developed in this study. The fluid flow, heat transfer, keyhole evolution, and mass transport between two dissimilar metals were studied. The volume of fluid (VOF) method was used to track the free surface. The effects of recoil pressure and surface tension on the keyhole wall were mainly considered. The effects of convection, diffusion, and keyhole formation on the mass transfer were also considered. The relative importance of convection and conduction and the effects of various driving forces on the convection in the molten pool were analyzed in terms of dimensionless numbers. The results showed that the maximum temperature and liquid flow velocity of the molten pool slightly fluctuate at the peak value after the molten pool enters the quasi-steady state, and the stability of the keyhole depends on the dynamic balance between the recoil pressure and the surface tension. Because of the influence of the formation mechanism of the keyhole, the weld zone contained significant amounts of aluminum elements. The results also showed that the welding parameters significantly affect the transition layer thickness and element distribution, which is closely related to the weld quality. The numerical simulation results correspond well with the experimental results. This study provides new insights into the weld pool dynamics in the laser welding of dissimilar metals and can help guide the selection of the welding parameters.

Melt flow regularity and hump formation process during laser deep penetration welding

In this study, the relationship between melt flow and hump formation was directly revealed during laser deep penetration welding of transparentmaterial. In partial-penetration welding, the melt flowed obliquely upwards along the bottom of the keyhole to the rear surface of the molten pool. And its flow rate increased first and then decreased with the increase of the welding speed. There was a one-to-one correspondence between the humps formed on the weld surface and the fluctuation of the inclination angle of the front keyhole wall (FKW) near the Brewster angle. The maximum eruption rate of the laser-induced vapor on the FKW exceeded 30 m/s. In full-penetration welding, the melt flow direction changed and humping was suppressed. The laser-induced vapor on the FKW surface can act on the rear keyhole wall, and this is the main reason for melt flow. When the inclination angle of the FKW is close to the Brewster angle, the amount of laser-induced evaporation on the FKW will increase to form a high speed jet of vapor which will drive melt flow in the molten pool behind the keyhole, and this is the main reason for hump formation on the rear surface of the molten pool.

Spatter Characteristics of High-Power Fibre Laser Deep Penetration Welding and Effect of Defocus on Spatter

Spatter Characteristics of High-Power Fibre Laser Deep Penetration Welding and Effect of Defocus on Spatter

Geometry and stability of the capillary during deep-penetration laser welding of AlMgSi at high feed rates

The influence of the feed rate on the geometry of the capillary and the process stability during laser beam welding of the AlMgSi aluminum alloy AA6082 was investigated by means of online-X-ray videography. Processes at feed rates between 4 m/min and 50 m/min with laser powers of up to 16 kW were considered. The X-ray images were used to generate three dimensional reconstructions of the geometry of the capillary which allowed to calculate the absorptance and the local distribution of the laser irradiance inside of the capillary by means of a ray-tracing approach. The stability of the capillary increased with increasing feed rates, which resulted in the avoidance of both depth fluctuations and processing pores. The stabilization of the capillary is attributed to the change of the capillary’s geometry which leads to a corresponding change of the absorptance of the laser power.

An Experimental Approach for the Direct Measurement of Temperatures in the Vicinity of the Keyhole Front Wall during Deep-Penetration Laser Welding

The formation of defects such as pores during deep-penetration laser welding processes is governed by the melt pool dynamics and the stability of the vapor capillary, also referred to as the keyhole. In order to gain an insight into the dynamics of the keyhole, the temperature in the transition region from the liquid to the gaseous phase, i.e., near the keyhole wall, is a physical value of fundamental importance. In this paper, a novel method is presented for directly measuring temperatures in the close vicinity of the keyhole front wall during deep-penetration laser welding. The weld samples consist of pure aluminum with a boiling point of 2743 K. The measurement is performed using high-speed pyrometry with a refractory tantalum probe capable of detecting temperatures that significantly exceed the boiling point of the sample material. Temperature curves are recorded from the beginning of the welding process until the moment the probe is finally destroyed through direct laser-tantalum interaction. With an effective spatial resolution up to 0.3 µm in the welding direction, a detailed investigation into the temperature ranging from the prerunning melt pool front to the keyhole center is possible, exhibiting temperatures of up to 3300 K in the vicinity of the keyhole front wall.