Front Keyhole Wall Research Articles

Parametric study of the laser energy absorption in high-power laser beam welding

Laser energy absorption on the keyhole wall is decisive for the thermodynamic behavior and the resultant weld properties in the high-power laser beam welding process. However, its highly transient nature on a microsecond scale makes the quantitative analysis challenging. In this paper, the influence of the relevant welding parameters on laser energy absorption is studied statistically by utilizing multiphysical modeling, in which the three-dimensional transient keyhole dynamics and thermo-fluid flow are calculated. A dynamic mesh adaption technique and a localized level-set-based ray-tracing method are employed to improve the model accuracy further. The results show that the focus position has a remarkable effect on the time-averaged laser absorption, and in contrast, the laser energy distribution regime is only slightly influenced by the welding speed in the studied parameter range (1.5–3.0 m/min). The absorption ratio of the laser energy on the keyhole front wall decreases with increasing welding speed and increases with upward-moving focus positions. The comparison between the calculated results and the experimental measurements ensures the validity of the proposed model.

Analysis of the dynamic behaviors of molten pool and keyhole for the oscillating laser welding of dissimilar materials stake welded T-joints with a gap

The dynamic behaviors of molten pool and keyhole have the significant effects on weld quality and joint performance. To investigate the circular shaped oscillating laser welding (OLW) of dissimilar materials stake welded T-joints with a gap between the face plate (FP) and web plate (WP), a dynamic model of molten pool and keyhole for the OLW of dissimilar materials stake welded T-joints with a gap is developed and the dynamic behaviors of molten pool and keyhole during the weld formation process are analyzed and discussed in detail. Based on the calculated results, it is found that the laser beam mainly radiates on the front wall of keyhole and the hump is formed on the rear wall of keyhole relative to the moving direction of laser beam at the FP welding stage. At the gap penetration stage, the keyhole opening at the molten pool bottom of FP shows the trumpet-shaped characteristic and the molten pool is divided into the upper and lower parts by the gap. The formed gas cavity is separated and fused with the keyhole several times at the WP welding stage. The gap around the gas cavity is disappeared after the reintegration of the upper and lower parts of molten pool. The depth of molten pool shows the periodic fluctuation characteristic with time and the fluctuation period is nearly the same as the oscillation period of laser beam during the OLW process.

Temporal power modulation in high power laser beam welding of round bars

Temporal power modulation bears an enormous potential for high power laser beam welding of round bars since all its specific challenges are faced. They can be summarised as deviating welding conditions towards the round bar‘s centre. Accordingly, a tailored amount of energy would be provided to that area. The investigations are conducted on 30 mm diameter bars of 1.4301 stainless steel with the use of a 16 kW disk laser. The modulation parameters comprise 6/12/50/100/200 Hz modulation frequency and 0.30/0.47/0.73 modulation depth or power modulation ratio. Post analysis focuses on metallographic longitudinal sections and calculations. The modulation‘s outstanding capability is creating deeper and narrower weld seams due to higher peak power and more intense evaporation of protrusions along the keyhole front wall. Therefore, more laser power is provided to the weld root and despite applying lower average power, the same weld depth is achieved. Finally, more ecological and economical welding as well as welding of more temperature sensitive materials is enabled.

Frontal pyrometric snapshot measurements of the keyhole wall temperature in laser welding of pure aluminium

In deep penetration laser welding, the quality of the weld seam is strongly correlated with the process dynamics. Pronounced dynamics of the melt pool and the keyhole cause defects like pores and blow-outs which negatively affect the mechanical properties of the seam. The process dynamics are primarily governed by the fluctuations of the keyhole shape. Therefore, in order to avoid the occurrence of defects in the weld seam, it is necessary to gain control over the keyhole stability by understanding the underlying physical processes that contribute to its dynamics. Besides the intensity distribution of the laser beam on the keyhole walls, the keyhole wall temperature is a crucial influencing factor when it comes to the assessment of the local metal evaporation rate and the induced local recoil pressure acting on the keyhole wall. Using small high-melting tantalum probes as measuring channels, temperatures in the vicinity of the Knudsen layer at the keyhole front wall were measured at different depths below the sample surface by means of high-speed pyrometry. For this purpose, bead on plate welding was conducted at low process velocities in the Rosenthal regime using pure aluminium (EN AW-1050A) as substrate material. It is shown for different laser powers and process velocities that the melt temperature in the vicinity of the keyhole wall lies in the range of 3000 K ± 400 K, therefore intermittently exceeding the vaporisation temperature of the substrate material. Significant correlations between the keyhole wall temperature and the laser power as well as the measuring depth could not be identified.

A study of the magnetohydrodynamic effect on keyhole dynamics and defect mitigation in laser beam welding

In this paper, the highly transient keyhole dynamics, e.g., laser absorption, keyhole geometry, and fluctuation, etc., under a magnetic field are investigated using an experimental approach and multi-physical modeling. The model provides accurate predictions to the variation of penetration depth and weld pool profiles caused by the MHD effect, which is validated by the measurements of optical micrographs and in-situ metal/glass observation. The micro-X-ray computed tomography shows a remarkable reduction of keyhole-induced porosity with the magnetic field. The correlation between the porosity mitigation and the weld pool dynamics influenced by the magnetic field is built comprehensively. It is found that the magnetic field gives a direct impact on the laser energy absorption at the keyhole front wall by changing the protrusion movement. The porosity mitigation comes from multiple physical aspects, including keyhole stabilization, widening of the bubble floating channel, and the electromagnetic expulsive force. Their contributions vary according to the bubble size. The findings provide a deeper insight into the relationship between electromagnetic parameters, keyhole dynamics, and suppression of keyhole-relevant defects.

Influence of the wrinkle surface structures on the vapor flow and keyhole stability in 20 kW high power laser welding

The stability of the extremely narrow keyhole is still the biggest challenge for the 20 kW or higher-level laser welding. The underlying causes of keyhole instability, including the wrinkle structures-induced thermal instability and vapor-induced dynamic instability, are studied in this work. A multiphase flow model integrated with a high-efficient free surface reconstruction algorithm and a high-accuracy wrinkle surface structures observation platform are built to study the morphology evolution and thermodynamic behaviors on the extremely narrow keyhole. It is found that the wrinkle structures (humps) that exist on the front keyhole wall could significantly regulate the energy distribution, and change the pressure condition in keyhole civility. The excessive energy accumulation, which is aroused by the abnormal hump distribution, is the reason for the unstable keyhole wall fluctuation. By increasing the welding speed, the intermittent-contact behavior between the laser beam and the keyhole opening is suppressed, which helps reduce the hump generation. By increasing the laser power, the drilling effect on the solid-liquid interface could be avoided, which helps to improve the downward flow efficiency of the melting layer on the front keyhole wall. An optimizing strategy based on the welding speed and laser power adjusting is offered to regulate wrinkle morphology. The experiment results show that keyhole stability is significantly improved under the optimized wrinkle surface structures.

Effect of the laser-induced vapor in the keyhole on the weld surface roughness in fiber laser welding

Weld surface roughness is an important criterion for evaluating weld quality. The main factors that affect weld surface roughness in fiber laser keyhole welding are identified by an in situ observation of the dynamic behavior of a keyhole mouth and by analyzing the eruption characteristics of the laser-induced vapor on the front keyhole wall (FKW). In fiber laser keyhole welding, the weld surface roughness first decreases and then increases with the increase in welding speed. Both the distortion rate of the keyhole mouth and its range of fluctuation will increase as the welding speed increases. The fluctuation in the distortion rate of the keyhole mouth diameter can reflect the stability of fiber laser keyhole welding. The laser-induced vapor at the FKW can erupt to impact the rear keyhole wall, and this is the main factor that affects the distortion rate of keyhole mouth diameter and weld surface roughness in fiber laser keyhole welding. In fiber laser-arc hybrid welding or in CO2 laser keyhole welding, plasma can suppress the effect of laser-induced vapor impacting the keyhole and reduce the weld surface roughness.

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.

Numerical Analysis of the Partial Penetration High Power Laser Beam Welding of Thick Sheets at High Process Speeds

The present work is devoted to the numerical analysis of the high-power laser beam welding of thick sheets at different welding speeds. A three-dimensional transient multi-physics numerical model is developed, allowing for the prediction of the keyhole geometry and the final penetration depth. Two ray tracing algorithms are implemented and compared, namely a standard ray tracing approach and an approach using a virtual mesh refinement for a more accurate calculation of the reflection point. Both algorithms are found to provide sufficient accuracy for the prediction of the keyhole depth during laser beam welding with process speeds of up to 1.5mmin−1. However, with the standard algorithm, the penetration depth is underestimated by the model for a process speed of 2.5mmin−1 due to a trapping effect of the laser energy in the top region. In contrast, the virtually refined ray tracing approach results in high accuracy results for process speeds of both 1.5mmin−1 and 2.5mmin−1. A detailed study on the trapping effect is provided, accompanied by a benchmark including a predefined keyhole geometry with typical characteristics for the high-power laser beam welding of thick plates at high process speed, such as deep keyhole, inclined front keyhole wall, and a hump.

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.

Investigation on plume formation during fiber laser keyhole welding based on in-situ measurement of particles in plume

In this study, the particles in plume were in-situ measured based on the self-mixing effect of the semiconductor laser for revealing the plume formation during fiber laser keyhole welding. The plume consisted of two parts: the oscillating part at the bottom, and the slender part shaped like focused laser beam and with a decreasing ascending velocity. The diameters of particles in plume mainly distributed in the range of 0.3 μm∼0.65 μm (accounting for about 70 %). Along the laser beam direction, both the diameter and density of particles in plume decreased with the increase of the height. In the same horizontal measurement plane, the particle diameter distributed uniformly, and particle density decreased sharply when the monitoring plane further from the laser beam center. There are a great quantity of particles in the oscillating plume caused by eruption of the laser-induced vapor on the front keyhole wall, and that formation of the slender plume is related to the fact that when the oscillating plume is erupting along the laser beam direction, particles in it can enter the laser beam to move in the laser beam direction and to give out light after being heated by the laser beam.

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.

Correlation between the spatters and evaporation vapor on the front keyhole wall during fiber laser keyhole welding

Severe spattering is one of the common problems during fiber laser keyhole welding. In this study, the correlation between the spatter formation and laser-induced evaporation vapor on the front keyhole wall (FKW) was studied by changing the welding speed during fiber laser keyhole welding. The spatters were mainly generated in front of and behind the keyhole mouth, and the diameters of spatters were mainly between 50~100 μm at low welding speed, while the diameters of spatter particles were mainly between 100~150 μm at high welding speed. As the welding speed was increased, the tilt angle of the FKW decreased gradually, and the main spatter generation position shifted from in front of the keyhole mouth to behind the keyhole mouth. In low welding speed, the spatter velocity was higher than that in high welding speed. Further analysis shows that, the recoil pressure of laser-induced evaporation squeezed the molten metal layer in the FKW to generate spatters in front of the keyhole mouth in low welding speed. In high welding speed, the decrease of the molten metal layer in the FKW led to the decrease of spatters in front of the keyhole mouth; however, the laser-induced evaporation vapor from the FKW was more liable to impact the rear keyhole wall to generate spatters behind the keyhole mouth.

Defect structure process maps for laser powder bed fusion additive manufacturing

Accurate detection, characterization, and prediction of defects has great potential for immediate impact in the production of fully-dense and defect free metal additive manufacturing (AM) builds. Accordingly, this paper presents Defect Structure Process Maps (DSPMs) as a means of quantifying the role of porosity as an exemplary defect structure in powder bed printed materials. Synchrotron-based micro-computed tomography (μSXCT) was used to demonstrate that metal AM defects follow predictable trends within processing parameter space for laser powder bed fusion (LPBF) materials. Ti-6Al-4 V test blocks were fabricated on an EOS M290 utilizing variations in laser power, scan velocity, and hatch spacing. In general, characteristic under-melting or lack-of-fusion defects were discovered in the low laser power, high scan velocity region of process space via μSXCT. These defects were associated with insufficient overlap between adjacent melt tracks and can be avoided through the application of a lack-of-fusion criterion using melt pool geometric modeling. Large-scale keyhole defects were also successfully mitigated for estimated melt pool morphologies associated with shallow keyhole front wall angles. Process variable selections resulting in deep keyholes, i.e., high laser power and low scan velocity, exhibit a substantial increase of spherical porosity as compared to the nominal (manufacturer recommended) processing parameters for Ti-6Al-4 V. Defects within fully-dense process space were also discovered, and are associated with gas porosity transfer to the AM test blocks during the laser-powder interaction. Overall, this work points to the fact that large-scale defects in LPBF materials can be successfully predicted and thus mitigated/minimized via appropriate selection of processing parameters.

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.

Correlation between gas-dynamic behaviour of a vapour plume and oscillation of keyhole size during laser welding of 5083 Al-alloy

Abstract This paper concerns the dynamic variation in plume velocity and keyhole size during the laser welding process of 5083 Al-alloy. The keyhole size and plume velocity were measured from high-speed images through imaging process methods and the particle image velocimetry (PIV) method. The correlation between the oscillation in plume velocity and keyhole size was discussed based on the pressure balance inside the keyhole. Results show that the keyhole keeps opening and collapsing during the welding process. During the time the keyhole is open, the size of the keyhole continuously oscillates until it collapses. Plume velocity increases to a maximum at the initial stage of keyhole’s opening, and then varies in synchronisation with the oscillation of the keyhole size. It is considered that local evaporation occurs on the humps of the keyhole front wall, which results in the oscillation in plume velocity. The oscillation in plume velocity then leads to the variation in the pressure balance in the keyhole and drives the keyhole to expand or shrink accordingly.

Correlation between plume fluctuation and keyhole dynamics during fiber laser keyhole welding

In order to reveal the fluctuation mechanism of the plume induced during fiber laser keyhole welding, dynamic behaviors of both the plume and the keyhole were simultaneously observed by using the multiple-imaging method. The results show that the fluctuation period of the plume was the same as that of the laser-induced vapor moving down on the keyhole front wall (KFW). The tilt angle of the plume was the largest when the laser-induced vapor was close to the orifice. As the laser-induced vapor moved toward the bottom of the KFW, the tilt angle of the plume decreased gradually. The correlation between the plume fluctuation and the dynamics of the laser-induced vapor on the KFW was not affected by parameters such as welding speed, laser power, and focal length. Further analysis shows that the periodic drilling behavior of the laser spot on the KFW is the main reason for the periodic fluctuation of the plume.

Study of molten pool dynamics and porosity formation mechanism in full penetration fiber laser welding of Al-alloy

To study molten pool dynamics, root hump formation, periodic keyhole behavior in full penetration laser welding (FPLW) of aluminum alloy, a numerical simulation was carried out, in which volume of fluid (VOF) method and ray-tracing algorithm were adopted. A varied metallic vapor shear stress model was considered. Meanwhile a series of welding experiments on specimens being composed of aluminum alloy and quartz glass were conducted to analyze the porosity formation process. It was found that the over-heated sagging with low surface tension stagnated at the bottom side and solidified to form root hump. The dominant backward heat convection at the lower part can well explain the extreme spreading of molten pool at bottom surface. Periodic behavior of the keyhole was identified during quasi-steady stage. The porosity can be effectively suppressed in FPLW. Both the porosity ratio and average porosity number were reduced obviously. The main mechanism of porosity formation in FPLW is the collapse of the rear keyhole wall, mainly caused by bulges on the front keyhole wall. Bubble coalescence is responsible for large porosity size and coalescence efficiency depends on bubble size difference.

Dynamic interaction between a laser beam and keyhole front wall during fiber laser welding

A bright spot (laser beam spot-center-induced metal vapor) on the keyhole front wall (KFW) was observed during fiber laser keyhole welding of a ‘sandwich’ specimen to reveal the interaction between the laser beam and the KFW. The dynamic behavior of the bright spot shows that the keyhole formation is the result of periodical drilling of the laser beam spot center on the KFW during welding. Welding parameters have little effect on the drilling period. The drilling speed first increases and then decreases with the increase in the keyhole depth, and the next drilling process begins before the drilling speed reduces to 0 m s−1. The average drilling speed can be increased by increasing the line energy or power density of the laser.

Bulk-Explosion-Induced Metal Spattering During Laser Processing

Spattering has been a problem in metal processing involving high-power lasers, like laser welding, machining, and recently, additive manufacturing. Limited by the capabilities of in situ diagnostic techniques, typically imaging with visible light or laboratory x-ray sources, a comprehensive understanding of the laser-spattering phenomenon, particularly the extremely fast spatters, has not been achieved yet. Here, using MHz single-pulse synchrotron-x-ray imaging, we probe the spattering behavior of Ti-6Al-4V with micrometer spatial resolution and subnanosecond temporal resolution. Combining direct experimental observations, quantitative image analysis, as well as numerical simulations, our study unravels a novel mechanism of laser spattering: The bulk explosion of a tonguelike protrusion forming on the front keyhole wall leads to the ligamentation of molten metal at the keyhole rims and the subsequent spattering. Our study confirms the critical role of melt and vapor flow in the laser-spattering process and opens a door to manufacturing spatter- and defect-free metal parts via precise control of keyhole dynamics.Received 11 December 2018Revised 7 April 2019DOI:https://doi.org/10.1103/PhysRevX.9.021052Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDrop breakupEvaporationFlow instabilityInterfacial flowsCondensed Matter, Materials & Applied PhysicsFluid DynamicsInterdisciplinary Physics