Keyhole Bottom Research Articles

Study on the keyhole oscillation mechanism of laser welding based on electro-mechano-acoustical analogy theory

The acoustic emission phenomena generated by the laser welding process are closely related to the keyhole's forced oscillation and the dynamic pressure balance at the keyhole bottom. In this study, we constructed the mechanical equation of keyhole bottom pressure balance and used the “electro-mechano-acoustical” analogy theory to convert it into an acoustic equation, the acoustic resonant frequency as well as the magnification amplification factor of the keyhole oscillation in different penetration states were derived. A monitoring platform combining acoustic sensor and high-speed photography was established to obtain acoustic emission signals and lateral keyhole images. The acoustic and image features extracted from the monitoring data validated the derived formulas. Additionally, the acoustic phenomena of different frequency-energy components were explained according to the derived formulas in turn. The results demonstrated that the derived theory is compatible with experimental comparisons, showcasing its great interpretability and applicability. The introduced analogy theory effectively establishes a bridge between keyhole forced pressure and acoustic emission vibration, aiding in the understanding of keyhole oscillation characteristics. A novel model of keyhole oscillation was proposed and verified using process monitoring data in this study, which provides a new approach for the investigation of keyhole dynamic.

Implementation of energy interaction between laser beam and keyhole wall based on algebraically reconstructing free surface: An experimentally validated numerical framework

The interaction of the laser beam with the material is critical for determining laser processing quality, with ray tracing methods mostly used in energy transfer research. These techniques generally rely on only the volume of fluid (VOF) method or level set (LS) function to track the molten pool free surface, resulting in either accuracy loss or difficulty in ensuring mass conservation of the interface. The few coupled algorithms are built on geometric notions that are difficult to understand while being limited to structural grids. In this paper, we have proposed a revised ray tracing framework, so-called ARCLS, to derive the exact intersections of the rays with the keyhole wall which is represented by a continuous LS surface algebraically rebuilt on top of the VOF interface. It is easy to code and convenient for unstructured meshes. This framework is tested through a V-groove case and a set of Ti6Al4V titanium alloy laser welding examples. The findings reveal that the reconstructed LS surface perfectly matches the VOF interface, and the predicted molten pool profile agrees with the actual data well. The keyhole morphology and molten flow patterns are also consistent with the physical truths. Spattering, porosity, necking, and swelling are observed in the developing keyhole, which are the possible reasons for the potential welding defects. Further investigation suggested that the ARCLS-based algorithm collected more laser rays at the keyhole bottom compared to the standard technique, achieving a deeper keyhole profile and a reflection pattern that was more optically logical. The work presents a reliable approach for estimating laser energy distribution, which can be effectively applied to the heat-fluid coupling simulation for laser manufacturing processes in order to forecast the dynamic behaviors of keyhole and molten pool.

Welding process with hybrid arc by compositing two free arcs into constraint arc

Controlling heat output and force in the arc source is important to improve the keyhole stability during welding. In this study, a double-layer hybrid arc torch was developed by adding two outer tungsten electrodes symmetrically to the orifice in the plasma arc torch. Two free arcs forming from the two outer tungsten tips are absorbed into the center constraint arc to form a hybrid arc. The arc pressure of the hybrid arc was measured, and welding experiments were carried out to test the application possibility in the keyhole welding process. It was found that: the arc pressure of the hybrid arc with two free arcs is symmetrical. The additional heat input provided by free arcs can be transferred into the keyhole bottom, and a lower constraint arc current is needed to form a stable fully penetrated keyhole. With the left-right arranged outer tungsten electrodes, arc heat is easier to be deposited into the keyhole bottom, and less sum of arc current is needed to form a fully penetrated keyhole. With the leading-rear arranged outer tungsten electrodes, the reflected arc will bridge the weld pool and the rear tungsten, and the coupling state between the free arc and the constraint arc is disturbed. When the constraint arc current is 115A and the free arc current is 40A, a fully penetrated keyhole can be formed with the left-right arranged outer tungsten electrodes but cannot be formed with the leading-rear arranged outer tungsten electrodes. The proposed hybrid arc system has advantages over the previous model in terms of a wider range of usable arc current and symmetrically distributed arc.

Keyhole-in-keyhole formation by adding a coaxially superimposed single-mode laser beam in disk laser deep penetration welding

Laser deep penetration welding is characterized by the formation of a vapor capillary (keyhole) and a high overall absorption (coupling rate) of the laser radiation in comparison to heat conduction welding. Due to multiple reflexions within the highly dynamic keyhole, the absorption mechanisms are very complex. The questions, how a laser beam interacts with the existing keyhole, and, if shadowing effects occur, are a matter of concern for several researchers. In this study, the hypothesis that the beam of a laser source can be transmitted to a significant extent through an existing keyhole and acts predominantly at the keyhole bottom was investigated. Experiments were carried out on mild steel and technical pure aluminum, using a specially designed optical system, which allows to coaxially combine two laser beams and to adjust their settings like the energy and the focal plane individually. A combination of a disk laser with a focal diameter of 390 µm and a single-mode fiber laser with a focal diameter of 30 µm was used. Weld depths and seam shapes were analyzed in cross and longitudinal sections. Based on the results, it is shown that the much smaller focal diameter of the single-mode laser beam acted at the bottom of the keyhole and caused a kind of “keyhole-in-keyhole formation” in the form of a local maximum in weld depth whose expression depended on the focal plane of the single-mode laser beam.

A study on the flow behavior and bubble evolution of circular oscillating laser welding of SUS301L-HT stainless steel

The flow characteristics, bubble formation and inhibition mechanisms of circular oscillating laser beam welding (OLBW) at distinct stages were elucidated in present work. The formation processes of bubbles mainly included the following manners. First, one or more of vortex P2, the recoil pressure flow P3, the migration flows FMF and BMF impinged the keyhole wall. The necking generated, gradually destabilized, and finally collapsed. Second, the self-instability of needle-like keyhole bottom (NKB) with large curvature induced the bubble formation. Third, complex melt flow broke the abnormal deformation of the keyhole or the bubble wall to form a new bubble. The bubble suppression mechanisms were summarized to three categories. First, the recoil pressure on the inner wall of bubble was at a low level, and the volume of bubble continually decreased until it collapsed. Second, high recoil pressure penetrated the metal liquid bridge between keyhole and bubble. Third, when recoil pressure was greater than the resultant force between surface tension and melt dynamic pressure, the bubble constantly fluctuated and gradually merged with the moving keyhole. As the frequency increased from 10 Hz to 200 Hz, porosity showed an exponential decrease from 7.16% to 0.35% in circular oscillation mode. A unified porosity prediction curve based on the Reynolds number was built with a coefficient of determination of 0.989. When fluid changed from laminar to turbulent flow, the critical Reynolds number was set as 1000, the predicted porosity was 0.912%, and the critical frequencies in circular and infinite oscillation modes were 143 Hz or 115 Hz, respectively, which were consistent with experiment results.

Dynamic keyhole behaviors and element mixing in paraxial hybrid plasma-MIG welding with a gap

A paraxial hybrid plasma-MIG welding process is developed to join middle-thick high strength steel plates with a butting gap. The arc and metal vapor characteristics are investigated by an advanced three-dimensional spectroscope measurement system. The dynamic keyhole behaviors and element mixing are studied by a three-dimensional numerical model. The numerical and experimental results show that the Fe vapor ejected from the wire surface, other than the plasma keyhole, has a great influence on arc characteristics. The butting gap affects the keyhole stability and molten metal flow, as well as the element mixing. With a gap, the keyhole opens and closes periodically. The opposing flows at the front and rear keyhole bottom walls, and the pressure imbalance in the keyhole bottom contribute to the keyhole collapse. The gap promotes the downward flow below the MIG arc center, which helps to transport the Ni element from the top surface of the MIG pool region to the bottom. The “Pull-Push” molten metal flows in the top molten pool suppress the Ni element to flow from the MIG pool region to the plasma pool region. As a result, the Ni content is inhomogeneous in the hybrid pool, and higher in the bottom of the MIG pool region.

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.

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.

Material flow analyses of high-efficiency joint process in VPPA keyhole flat welding by X-ray transmission system

The consideration of environment-friendly and low energy consumption tends to control and guide the progress of metal joint manufacturing processes. Variable polarity plasma arc (VPPA) welding is an environment-friendly, high quality and efficiency joint method for aluminum alloys due to its characteristics of such as little welding fume, and high energy density etc. However, the three-dimensional morphology and physical phenomena of penetration weld pool in VPPA welding are not clearly understood, which slows down its process development and optimization. This work aims to clarify the three-dimensional keyhole detouring flow of VPPA welding for aluminum alloy. The liquid metal distribution of penetration weld pool was generally obtained by metallographic observation. The measurement results show that the liquid metal layer attached to the three-dimensional keyhole boundary is thin. And the liquid metal mainly distributes in the bottom side of the keyhole. In order to understand the weld pool dynamics, the stereo synchronous imaging of tungsten tracer particles using two sets of X-ray transmission system was adopted, for the first time, to measure the 3D velocity field of convection flow inside weld pool of VPPA welding of aluminum alloy in real time. The flow on the weld pool surface at different locations of keyhole boundary was also measured by tracing zirconia and slag particles. The liquid metal flows simultaneously to the top and around the keyhole. The metal at the bottom of keyhole’s front wall, flows around the keyhole towards the backside, then separately flows to top side and bottom side, forming a vertical stationary point. The key to weld and avoid defect proofing are to ensure stable detouring flow on the keyhole bottom and proper stationary point position.

Modeling of Keyhole-Induced Pore Formation in Laser-Arc Hybrid Welding of Aluminum Alloy with a Horizontal Fillet Joint

A three-dimensional transient model is proposed to investigate the weld pool dynamic behavior in laser + metal inert gas (MIG) hybrid fillet welding of aluminum alloy in the horizontal position, which allows for the joint configuration and coupling of the keyhole, droplet and weld pool as well as the heat and mass exchange between gas and liquid phases and is able to simulate the temperature distribution, fluid flow and formation process of a keyhole-induced pore in hybrid welding with a horizontal fillet joint. The weld porosity is also measured using x-ray nondestructive testing technology. Keyhole behavior and the formation mechanism of keyhole-induced porosity were analyzed. The calculated results are in generally good agreement with the experimental ones. A clockwise vortex always exists at the middle part of the weld pool. The formation and growth of the molten metal bulge on the keyhole wall are responsible for the occurrence of a gas bubble, which has a variation in size and shape and can be split during welding. The keyhole collapses easily at its middle or upper part in horizontal fillet welding, and the capture of the bubble by the upper molten pool boundary enhances the possibility of porosity formation to some degree. The keyhole-induced pore is mainly formed at the regions near the keyhole bottom and the upper fusion line of the weld pool.

Synchronized Monitoring of Droplet Transition and Keyhole Bottom in High Power Laser-MAG Hybrid Welding Process

This present paper addresses the process monitoring of laser-MAG hybrid welding by using a double high-speed camera sensing system. Different welding parameter experiments are conducted under the normal penetration condition. The real-time surface physical phenomena of top and bottom in welding process are captured by the sensing system. The image processing method is proposed to quantify the process status, such as droplet transition, spatter, keyhole bottom, and metal vapor. The statistical method combined with detail analysis method is used to analyze the effects of droplet transition on keyhole status under different welding parameters. Also, the time distribution of opened keyhole bottom is analyzed. The synchronization analysis is conducted to demonstrate the status of the droplet transition and laser-induced keyhole, and the status of the opened keyhole bottom is separated as two types: opened keyhole bottom occurs during droplet transition or during droplet growing. The experimental analysis results demonstrate that the droplet transition mainly affects the status of keyhole under normal penetration condition, and the opened time of keyhole bottom is quite different which means the effects are unstable. Furthermore, the experimental results confirmed that the process monitoring method in this paper is an effective way for laser-MAG hybrid welding evaluation.

Dynamic keyhole behavior and keyhole instability in high power fiber laser welding of stainless steel

A three-dimensional numerical model, considering the real-time multiple reflections of a laser beam, adiabatic bubble model and shear stress, was developed to study the dynamic keyhole behavior and keyhole instability in fiber laser welding of stainless steel. The inner dynamic keyhole behavior and weld defect formation were directly observed in a high resolution assisted by transparent glass. The numerical and experimental results showed that the keyhole width reached the quasi-steady state earlier than the keyhole depth did during fiber laser welding of stainless steel. Due to the large recoil pressure at rear keyhole wall caused by the irradiation of laser energy reflected by the bulge at the front keyhole wall, the rear keyhole wall was severely deformed at keyhole bottom and keyhole middle. The rear keyhole wall was collapsed due to the high surface tension pressure and hydrostatic pressure. The whole keyhole collapse was attributable to the capillary instability of the keyhole associated with large depth/width ratio and the strong flow of the bulges at the keyhole wall. When the laser power was increased, the keyhole depth/width ratio was increased, so the keyhole was more capillary instable. The average inclined angle of the front keyhole wall was decreased.

Elucidation of keyhole induced bubble formation mechanism in fiber laser welding of low carbon steel

Laser welding experiment with the aid of glass, and numerical simulation are carried out to study the keyhole behavior and keyhole-induced bubble formation. Two mechanisms are responsible for keyhole-induced bubble formation. The first mechanism is the strong fluid flow inside the weld pool, and the capillary instability of the whole keyhole, causing the collapse between rear keyhole wall and front keyhole wall. This mechanism contributes to most of keyhole-induced bubble formation. The second mechanism is the instability of the rear keyhole wall caused by the increase absorption of laser energy reflected by the bulge at front keyhole wall. The breaking of molten bridge is analyzed based on static pressure balance. The molten bridge with large curvature and low temperature is difficult to be broken. Bubble coalescence can be clearly observed at the bottom of the weld pool. Large bubble and small bubble have high coalescence efficiency. The bubbles at keyhole bottom have more time to escape without broken by laser beam, and the bottom part of rear keyhole wall is more easily depressed, so bubbles are easily formed at the keyhole bottom.

Status analysis of keyhole bottom in laser-MAG hybrid welding process.

The keyhole status is a determining factor of weld quality in laser-metal active gas arc (MAG) hybrid welding process. For a better evaluation of the hybrid welding process, three different penetration welding experiments: partial penetration, normal penetration (or full penetration), and excessive penetration were conducted in this work. The instantaneous visual phenomena including metallic vapor, spatters and keyhole of bottom surface were used to evaluate the keyhole status by a double high-speed camera system. The Fourier transform was applied on the bottom weld pool image for removing the image noise around the keyhole, and then the bottom weld pool image was reconstructed through the inverse Fourier transform. Lastly, the keyhole bottom was extracted from the de-noised bottom weld pool image. By analyzing the visual features of the laser-MAG hybrid welding process, mechanism of the closed and opened keyhole bottom were revealed. The results show that the stable opened or closed status of keyhole bottom is directly affected by the MAG droplet transition in the normal penetration welding process, and the unstable opened or closed status of keyhole bottom would appear in excessive penetration welding and partial penetration welding. The analysis method proposed in this paper could be used to monitor the keyhole stability in laser-MAG hybrid welding process.

Dynamics of solid-liquid interface and porosity formation determined through x-ray phase-contrast in laser welding of pure Al

Through X-ray phase contrast method, solid-liquid interface, keyhole, and porosity in laser welding of pure Al could be clearly observed, and the shapes could be quantitatively evaluated. Almost no molten metal occurred at the keyhole bottom, and the temperature gradient and cooling rate there were estimated to be enormously large as compared to other locations in the weld pool. The flow of molten metal around the keyhole bottom was about twice as fast as in the upper part of the weld pool, suggesting that the amount of heat transport around the bottom was larger than in the upper part. Porosity formation is determined by keyhole behavior. In the case of a stable keyhole, obtained with a fan to remove the laser-induced plume, the porosity rate was around 6%, and the gas in the voids consisted entirely of hydrogen. In the case of an unstable keyhole, obtained without fan usage, the porosity rate was around 18%, and the gas included a component of air (nitrogen) in addition to hydrogen.

Signal emitted from plasma during electron-beam welding with deflection oscillations of the beam

The oscillations of a signal, collected by a ring electrode with +50V potential, situated in the plasma over the welding zone during electron beam welding (EBW) with local beam deflection oscillations were studied. Samples of chrome-molybdenum steel were processed. The frequencies of the beam oscillations varied between 150 and 1400Hz and their trajectories were the simplest: along and across the joint. The position of the beam focus was changed. Fluctuations of the emitted plasma current by coherent accumulation of the signal amplitudes were analysed and the probability for instability excitations in the keyhole was estimated. This method allows the study of the random instability processes at the interaction of the beam with the keyhole wall and bottom. A time shift between the maxima of the function derived from the coherent accumulation and the extremes of the deflection coil current was found. This shift depends on the beam focus position. In addition, at longitudinal beam oscillations, and at variations of the focus position a difference of the extreme values of the studied signal was observed. At the lowest values of the current of the focusing electromagnetic lens, the differences between the maxima of the probabilities for excitation of the plasma signal oscillations in the moments at beam bombardment on the front and trailing walls of the keyhole in the molten pool were about 30%, while the beam over-focusing decreased these differences. For beams situated at the central part of the keyhole, the collected signal was minimal. From the observations of spikes in the weld root, at EBW with deflection oscillations, it was concluded that self-focusing of the beam in the bottom part of the keyhole in the cases of the most focusing positions was occurring.

Characteristics of weld pool behavior in laser welding with various power inputs

This paper investigates the numerical simulations of multi-kilowatt disk laser and fiber laser welding, ranging from 6 to 18 kW to study the behavior of molten pool in 20-mm-thick steel plate by using Volume-Of-Fluid (VOF) method and several mathematical models like Gaussian heat source, recoil pressure, Marangoni flow, buoyancy force, and additional shear stress and heat source due to the metallic vapor. Vortex flow pattern is observed for higher laser power except for 6-kW case, and the higher the laser power, the bigger the vortex flow pattern. Welding speed has an influence on molten pool in terms of depth of penetration and size of molten pool, but overall shape of molten pool remains the same. The reasons for the vortex flow pattern in high-power laser welding are the absorption of more energy at the bottom of keyhole, which promotes more liquid metal at the bottom, while for lower power with lower speed, the melt formation is more uniform in the thickness direction and most of the molten metal in the lower part of keyhole reaches the top of molten pool, and consequently, no vortex flow pattern is observed in the keyhole bottom.

Experimental observation and simulation of keyhole dynamics during laser drilling

Direct observation and simulation of keyhole dynamics is the key to the study of laser drilling. In this study, the author used a piece of transparent material GG17 glass to perform a direct observation of the dynamics in laser drilling. This technique enabled us to analyze the developmental process of elongated keyhole formation and the flow characteristics of the vapor and molten materials in the keyhole. Subsequently, the various features associated with laser drilling velocity and depth and the influence of laser power and defocus distance on the drilling velocity were investigated in experiments. In addition, to simulate the process of material vaporization removal during laser drilling process, a finite element model was established for laser drilling with Gaussian heating. To compensate for the multiple reflection of the laser on the keyhole wall and the energy loss during the drilling process, a convergence function, a coordination function and a factor function were introduced at the same time to model the changes of the laser energy and its intensity distribution as it reached the keyhole bottom.

Prediction of the laser-induced plasma characteristics in laser welding: a new modelling approach including a simplified keyhole model

The characteristics of the laser-induced plasma encountered in laser welding are investigated using a new three-dimensional modelling approach. A simplified keyhole model is employed to couple with our previous plasma plume model, and thus both the plasma inside a blind keyhole and the plasma plume issuing from the keyhole can be treated simultaneously. Investigations include the effects on the laser-induced plasma characteristics of many factors, including the velocity of metal vapour leaving from the keyhole bottom, the velocity of the shielding gas injected coaxially with the laser beam, the velocity and location of the assisting gas injected laterally with respect to the workpiece, and the energy absorption and radiation heat loss of the laser-induced plasma. Typical computed distributions of temperature, velocity and vapour concentration within the plasma are presented with the continuous-wave CO2 laser welding of iron workpiece as the calculation example. It is shown that the high-temperature core of the laser-induced plasma is mostly located inside the blind keyhole or near the keyhole top for the cases under study. The metal-vapour/shielding-gas momentum ratio plays an important role in determining the height of the plasma plume, and the plume height decreases with increasing shielding-gas velocity. The laterally injected assisting gas may also significantly affect the laser-induced plasma characteristics and thus can be used to control the unfavourable effect of the laser-induced plasma on the laser welding process. The predicted temperatures of the laser-induced plasma are reasonably consistent with corresponding experimental data.