Deep Penetration Laser Beam Welding Research Articles

Interdependence of metal vapor plume and melt pool dynamics during laser beam welding under vacuum

Deep penetration laser beam welding involves the emission of hot metal vapor and particles from the keyhole, which interact with the laser beam by means of scattering, absorption, and phase front deformation. The combination of scattering and absorption leads to an extinction of the laser beam, while the phase front deformation adversely affects the beam quality. The capillary formation depends on the local distribution of the absorbed irradiance of the laser beam, which in turn is related to the material process emissions. Additionally, the process atmosphere, in particular the oxygen content, and the working pressure influence the capillary fluctuations and cause variations in welding penetration. This study introduces a measurement setup using simultaneous high-speed observation of the melt pool and the keyhole while measuring the interaction mechanisms between the laser beam and the metal vapor plume during laser beam welding under vacuum of stainless steel. The obtained measurement provides a quantification of the various interaction mechanisms between the laser beam, vapor plume, keyhole, and interdepended melt pool dynamics. These quantitative high-frequency measurements at various working pressures and laser parameters provide useful insights that are crucial for understanding the formation of weld defects, resulting in process monitoring and validation of process-oriented welding simulations.

Process comparison of laser deep penetration welding in pure nickel using blue and infrared wavelengths

AbstractLaser sources with wavelengths in the visible blue spectrum are suitable for heat conduction mode welding of materials like copper and nickel due to the significantly increased Fresnel absorption compared to infrared laser radiation. Recently, blue laser sources with 445 nm wavelength have become available with sufficient power and beam parameters to exceed the intensity threshold for laser deep penetration welding. In laser beam deep penetration welding, the total absorption is significantly increased due to the multiple reflections in the keyhole compared to heat conduction mode welding. However, since the absorbed energy per reflection inside the keyhole is wavelength-dependent, it can be hypothesized that the choice of laser wavelength causes changes in the local energy distribution inside the keyhole, changing the keyhole dynamics. To investigate this, laser beam deep penetration welding experiments were carried out on 2.4068 pure nickel using an infrared laser source and a blue laser source with comparable beam properties. The experiments were monitored and compared by a multi-sensor setup and metallographic analyses. This setup included measurements of airborne acoustic emissions and two high-speed video cameras for spatter tracking and tracking of the keyhole area. The use of a blue laser beam led to a lower spatter quantity, an increase of porosity and a significant change of acoustic emissions, proving the hypothesis for pure nickel.

Numerical analysis of the effect of an oscillating metal vapor plume on the keyhole and molten pool behavior during deep penetration laser beam welding

The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.

Interpreting acoustic emissions to determine the weld depth during laser beam welding

The interpretation of sensor system data is critical for monitoring industrial welding processes and providing reliable information about the condition of the weld seam. Previous investigations have shown that acoustic emissions of frequencies up to several kilohertz during laser beam welding are parameter-dependent and contain valuable information about the process. A microphone was employed to record the acoustic emissions produced when performing deep penetration laser beam welding of copper. Experiments were conducted in which the laser power and the feed rate were varied so as to obtain acoustic data comprising frequencies of up to 1 MHz. The signals were preprocessed and features were extracted using Fourier and wavelet analysis as well as speech analysis techniques. The relationship between the features extracted from the acoustic signal and the weld depth was modeled using Gaussian process regression. The results showed that acoustic emissions during laser beam welding can be used to predict the weld depth without having to rely on process parameters, i.e., the laser power and the feed rate. Overall, 17 features were extracted from acoustic signals, with the zero-crossing rate displaying the highest significance for determining the weld depth. These investigations open up new possibilities of robust quality assurance for laser beam welding applications based on acoustic emissions.

Spatially tailored laser energy distribution using innovative optics for gas-tight welding of casted and wrought aluminum alloys in e-mobility

Electric mobility is undergoing a very rapid maturation process [A. Kampker, K. Kreisköther, P. Treichel, T. Möller, Y. Boelsen, and D. Neb, “Electromobility trends and challenges of future mass production,” in Handbook Industry 4.0, edited by W. Frenz (Springer, Berlin, 2022), D. Ziegler and N. Abdelkafi, “Business models for electric vehicles: Literature review and key insights,” J. Cleaner Prod. 330, 129803 (2022)]. While conventional vehicle design disciplines such as car body design are established, electromobility-specific disciplines are in the technological orientation and ramp-up phase. In particular, the demand for components like batteries, e-motors, and power electronics is growing continuously [A. Kampker, K. Kreisköther, P. Treichel, T. Möller, Y. Boelsen, and D. Neb, “Electromobility trends and challenges of future mass production,” in Handbook Industry 4.0, edited by W. Frenz (Springer, Berlin, 2022), D. Ziegler and N. Abdelkafi, “Business models for electric vehicles: Literature review and key insights,” J. Cleaner Prod. 330, 129803 (2022)]. One of the major materials chosen for these parts is aluminum alloys [C. Prieto, E. Vaamonde, D. Diego-Vallejo, J. Jimenez, B. Urbach, Y. Vidne, and E. Shekel, “Dynamic laser beam shaping for laser aluminium welding in e-mobility applications,” Procedia CIRP. 94, 596–600 (2020)]. Next to the material-specific challenges and mentioned requirements, the focus is on the gas-tight welding of aluminum alloys for parts like casted power electronics housings and heat exchangers made of sheet metal or extrusion profiles. Gas-tightness is a requirement, on the one hand, to shield electronic components from the influence of the surrounding environment and, on the other hand, to prevent leakage of the water-cooling circuit [C. Prieto, E. Vaamonde, D. Diego-Vallejo, J. Jimenez, B. Urbach, Y. Vidne, and E. Shekel, “Dynamic laser beam shaping for laser aluminium welding in e-mobility applications,” Procedia CIRP. 94, 596–600 (2020), A. Artinov, M. Bachmann, X. Meng, V. Karkhin, and M. Rethmeier, “On the relationship between the bulge effect and the hot cracking formation during deep penetration laser beam welding,” Procedia CIRP 94, 5–10 (2020)]. This paper offers insight into the requirements of these parts and an innovative optics approach with a novel MultiFocus solution. Material-specific challenges (e. g. porosity), especially, for helium-tight welding of aluminum casted housings with forging alloys are characterized. This analysis is conducted using gas-tightness measurements, CT-scans, micrographs, and high-speed recordings in order to elaborate on the fundamental laser-material-process interdependencies and the correlation between the process and resulting quality, in terms of tightness. Furthermore, high-speed synchrotron recordings are conducted at the DESY and based on that, a detailed evaluation of laser and material interaction is conducted. This allows an explanation of the interactions for the prevention of pore formation in aluminum alloys and, thus, the characterization of the boundary conditions for a reliable process of gas-tight welding on aluminum alloys [C. Prieto, E. Vaamonde, D. Diego-Vallejo, J. Jimenez, B. Urbach, Y. Vidne, and E. Shekel, “Dynamic laser beam shaping for laser aluminium welding in e-mobility applications,” Procedia CIRP. 94, 596–600 (2020)].

Evaluation of narrowed weld pool shapes and their effect on resulting potential defects during deep penetration laser beam welding

This study presents mechanisms of the evolution of a narrowed region in the weld pool center during deep penetration laser beam welding. In numerous numerical studies presented in this study, it was also found that the local reduction of the weld pool size can cause detrimental effects on the melt flow behavior and the resulting properties of the welds. A particularly large influence of this effect was identified in three aspects. First, the local variation of the solidification sequence of the weld pool causes an increase in the hot-cracking susceptibility due to a locally delayed solidification. Second, it was proven that a change in the local length and width of the weld pool is associated with an adverse impact on the potential flow routes of the molten material that induces stronger local variations of its solidification. Thus, the element mixing, e.g., during the welding with filler materials, is blocked. This leads to a nonhomogeneous chemical composition of the final weld and can cause undesired effects on the final material properties. Finally, another observed effect is related to the reduced ability of process pores to reach the top surface. As this type of porosity is usually produced around the keyhole tip, the change of the fluid flow regime above this area plays a significant role in determining the final path of the pores until the premature solidification in the middle of the weld pool captures them. This study summarizes mainly numerical results that were supported by selected experimental validation results.

Inline Weld Depth Evaluation and Control Based on OCT Keyhole Depth Measurement and Fuzzy Control

In an industrial joining process, exemplified by deep penetration laser beam welding, ensuring a high quality of welds requires a great effort. The quality cannot be fully established by testing, but can only be produced. The fundamental requirements for a high weld seam quality in laser beam welding are therefore already laid in the process, which makes the use of control systems essential in fully automated production. With the aid of process monitoring systems that can supply data inline to a production process, the foundation is laid for the efficient and cycle-time-neutral control of welding processes. In particular, if novel, direct measurement methods, such as Optical Coherence Tomography, are used for the acquisition of direct geometric quantities, e.g., the weld penetration depth, a significant control potential can be exploited. In this work, an inline weld depth control system based on an OCT keyhole depth measurement is presented. The system is capable of automatically executing an inline control of the deep penetration welding process based only on a specified target weld depth. The performance of the control system was demonstrated on various aluminum alloys and for different penetration depths. In addition, the ability of the control to respond to unforeseen external disturbances was tested. Within the scope of this work, it was thus possible to provide an outlook on future developments in the field of laser welding technology, which could develop in the direction of an intuitive manufacturing process. This objective should be accomplished through the use of intelligent algorithms and innovative measurement technology—following the example of laser beam cutting, where the processing systems themselves have been provided with the ability to select suitable process parameters for several years now.

An improved recoil pressure boundary condition for the simulation of deep penetration laser beam welding using the SPH method

Deep penetration laser beam welding is simulated using the mesh-free Lagrangian Smoothed Particle Hydrodynamics (SPH) method. The physical phenomena modeled are the elastic material behavior of the solid phase, the fluid dynamics of the liquid phase including temperature-dependent surface tension, heat transfer by heat conduction, and the phase transitions melting, solidification, and evaporation. The energy input of the laser beam is determined by a ray-tracing scheme that calculates the absorbed irradiance distribution in the capillary. Based on the locally absorbed irradiance, an improved recoil pressure model is introduced and compared with a temperature-dependent model frequently used in the literature. To transfer the momentum of the recoil pressure to the surrounding melt, two boundary conditions based on either dynamically detected boundary particles or surface forces are presented and validated using simple examples. The results show that the recoil pressure is a key factor for the formation of a deep capillary and the acceleration of the melt around the capillary. Comparison of capillary geometry and melt pool size from simulation and experiment shows good agreement for the irradiance-dependent recoil pressure model.

Laser beam welding setup for the coaxial combination of two laser beams to vary the intensity distribution

Deep-penetration laser beam welding is highly dynamic and affected by many parameters. Several investigations using differently sized laser spots, spot-in-spot laser systems, and multi-focus optics show that the intensity distribution is one of the most influential parameters; however, the targeted lateral and axial intensity design remains a major challenge. Therefore, a laser processing optic has been developed that coaxially combines two separate laser sources/beams with different beam characteristics and a measuring beam for optical coherence tomography (OCT). In comparison to current commercial spot-in-spot laser systems, this setup not only makes it possible to independently vary the powers of the two laser beams but also their focal planes, thus facilitating the investigation into the influence of specific energy densities along the beam axis. First investigations show that the weld penetration depth increases with increasing intensities in deeper focal positions until the reduced intensity at the sample surface, due to the deep focal position, is no longer sufficient to form a stable keyhole, causing the penetration depth to drop sharply.

Study on the transition behavior of the bulging effect during deep penetration laser beam welding

The present work is devoted to the study of the transition behavior of the recently confirmed widening of the weld pool, known as the bulging effect, during high-power deep penetration laser beam welding of thick unalloyed steel sheets. A three-dimensional transient multi-physics numerical model is developed, allowing for the prediction of the bulge formation and the study of its temporal behavior. The model is generalized to account automatically for the transition from partial to complete penetration. Several experimental measurements and observations, such as drilling period, weld pool length, temperature, efficiency, and metallographic cross-sections are used to verify the model and assure the plausibility of the numerical results. The analysis of the calculated temperature and velocity distributions, as well as the evolution of the keyhole geometry, shows that the formation of a bulging region strongly depends on the penetration depth of the weld. Based on the numerical results, the bulge is found to occur transiently, having its transition from a slight bulge to a fully developed bulging between penetration depths of 6 mm and 9 mm, respectively.

Root formation and metallurgical challenges in laser beam and laser-arc hybrid welding of thick structural steel

Single-pass laser beam welding (LBW) of steel components with wall thickness of > 10 mm is of high interest due to enhanced productivity. Deep penetration LBW provides excessive hardness and certain quality issues such as root humping in flat position, which is associated with disability of surface tension to sustain melt dropout. High hardness is associated with fast cooling rates and shortage of filler wire transportation to the root of the fusion zone. Use of laser-arc hybrid welding (LAHW) can promote acicular ferrite by adding filler metal and additional heat input from the arc. However, LAHW may promote humping and adjustment of many parameters is required hindering its application. In this work, a 16 kW disk laser was used in butt welding of 12 mm and 15 mm thick plates with different bevelling geometries. Root humping occurred within a wide range of process parameters providing narrow process window. Twelve millimeter thick plates were successfully welded with a single-pass technique providing good quality of root by using zero air gap regardless bevelling geometry. Welding of 15 mm plates was more challenging, and the process was sensitive even with a slight parameter change. Improved results were achieved with application of small air gap. Acceptable hardness in both weld metal and heat affected zone (< 290 HV) was achieved for both plate thicknesses providing good toughness of minimum 27 J at −50°C.

Elongated cavities during keyhole laser welding

During laser beam deep penetration welding, the characteristic keyhole occurs that enables efficient energy absorption due to multiple reflections. Since this welding mode usually shows high dynamic behaviour, often a high amount of weld imperfections like spattering or porosity occur. Elongated keyholes during buttonhole, a.k.a donut welding, have shown the possibility to create smooth welding tracks when controlled variations of energy input were induced, e.g. beam oscillation. It was observed that the elongated keyholes can maintain themselves due to the balancing effects of the melt pool surface tension, which can form a catenoid shape that does not require any additional forces to be stable. Basic theoretical descriptions of the catenoid shape keyholes were derived. However, the complex system requires a more detailed investigation of the effect of the metal plume and the melt flows in order to explain the opening and maintenance effects of the elongated keyholes.Therefore, this work examined different modes of elongated keyholes, comparing traditional non-oscillating keyholes to keyholes formed by different beam scanning strategies. Analysis was made mainly by observations with high-speed-imaging and theoretical considerations to explain the phenomena of elongated keyholes. The observations showed that extended keyholes are not necessarily related to a reduced occurrence of imperfections and not either to the formation of perfect catenoid shapes. The keyhole front is deformed where the laser beam impacts and the rear wall is impacted where the vapour plume strikes. The opening of an elongated keyhole is likely related to the keyhole vapour plume that pushes the keyhole rear wall rearwards, while the non-affected rear wall parts do not collapse the keyhole due to the balancing nature of the second curvature of the rear wall.

Porosity-grain growth relationships in the laser beam deep penetration welding of 6061 aluminum alloy

Porosity is one of the significant defects existing in the weld seam of aluminum alloy by laser. In the current research, the evolution of porosity and grain coupling growth in the weld seam is realized via a Cellular Automaton-Finite Element simulation method. A detailed calculating method of three phases was proposed by taking into account the liquid/solid phase transformation on the nucleation and growth of porosity. The results show that the porosity nuclei with larger size can grow preferentially, while the small porosity nuclei's development is inhibited. Because of the effect of gas/liquid surface tension, porosity increases spherically when it is enveloped by liquid. After touching with grain, the porosity's growth space is restricted by the complex dendrite network, and thus its shape becomes irregular. The coupling of grain and micro-porosities predicted by the simulation method is in good agreement with the results obtained by the experiments.

Influence of oscillating magnetic field on the keyhole stability in deep penetration laser beam welding

The stability of the keyhole decreases for deep penetrated high-power laser beam welding. The keyhole tends to collapse with increasing laser power and e.g. keyhole induced porosity can occur. This study deals with the observation of the keyhole during high-power laser beam welding in partial penetration mode by means of a high-speed camera. A butt configuration of 25 mm thick structural steel and transparent quartz glass was used for the experiments. An oscillating magnetic field was applied perpendicular to the welding direction on the root side of the steel plate. The keyhole was highlighted with a coaxial diode laser. It was ascertained that the stability of the keyhole and the weld penetration depth were increased by applying an oscillating magnetic field with an oscillating frequency of 1.2 kHz and a magnetic flux density of 50 mT.

On the relationship between the bulge effect and the hot cracking formation during deep penetration laser beam welding

Recent studies have confirmed the widening of the weld pool interface, known as a bulge effect, during deep penetration high power laser beam welding. The link between such geometric particularities of the weld pool shape and the hot cracking phenomena is significant. The present work seeks to extend the level of understanding by investigating their relationship. A coupled multiphysics, multiscale numerical framework is developed, comprising a series of subsequent analyses. The study examines the influences of the bulge on the three most dominant effects causing hot cracking, namely the thermal cycles, the mechanical loading, and the local microstructure. The bulge in the weld pool shape forms approximately in the middle of the plate, thus correlating with the location of hot cracking. It increases the hot cracking susceptibility by enhancing the three dominant effects. The numerical results are backed up by experimental data.

Grain growth and texture evolution of weld seam during solidification in laser beam deep penetration welding of 2219 aluminum alloy

Laser beam deep penetration welding (LBDPW) was performed to connect the 2219 aluminum alloy butt joint with different parameters. Then, the LBDPW process of 2219 aluminum alloy was modeled using a coupled FE-CA model, i.e., finite element couple with cellular automaton, which contained FE model that simulates temperature field and CA model that calculates microstructure evolution. Furthermore, the microstructure evolution in LBDPW of 2219 aluminum alloy was characterized under optical microscopy and a scanning electron microscopy (SEM). At the same time, the association between LBDPW processing thermal cycles and macrostructure/microstructure evolution is studied. The results show that the final microstructure of the welded joint included fine equiaxed grains in the equiaxed grains zone (EQZ), columnar in the fusion zone (FZ) and coarse equiaxed grains in the weld seam center zone. Moreover, it was indicated that the size of grains is associated with the temperature gradient of the welding process. Besides, the size of EQZ became narrow and uniform because of the undercooling increase, leading to the rise in the heat input.

Inline weld depth measurement for high brilliance laser beam sources using optical coherence tomography

As a result of the rapidly growing importance of applications in electro mobility that require a precisely defined laser weld depth, the demand for inline process monitoring and control is increasing. To overcome the challenges in process data acquisition, this paper proposes the application of a novel sensor concept for deep penetration laser beam welding with high brilliance laser sources. The experiments show that optical coherence tomography (OCT) can be used to measure the weld depth by comparing the distance to the material surface with the distance to the keyhole bottom measured by the sensor. Within the presented work, the measuring principle was used for the first time to observe a welding process with a highly focused laser beam source. First, a preliminary experimental study was carried out to evaluate the influence of the angle of incidence, the material, and the weld joint geometry on the quality of the sensor signal. When using a multimode fiber laser with a focus diameter of 320 μm, the measurements showed a distinct behavior for aluminum and copper. The findings about the measurement signal properties were then applied to laser beam welding with a single-mode fiber laser with a spot diameter of only 55 μm. The spot diameter of the OCT measuring beam was about 50 μm and thus only slightly smaller than that of the single-mode processing beam. A wide variety of tests were carried out to determine the limits of the measurement procedure. The results show that the application of OCT allows inline monitoring of the weld depth using both a multimode and a highly focused single-mode laser beam. In addition, various influences on the signal were identified, e.g., the material-specific melt pool dynamics as well as several characteristic reflection and absorption properties.

Reduction of pores by means of laser beam oscillation during remote welding of AlMgSi

The influence of a spatial beam oscillation on the dynamics of the capillary and the mechanism leading to an increased or reduced generation of process pores was investigated for deep penetration laser beam welding of the aluminum alloy AlMgSi. Welding with a feed rate of 4 m/min and a welding depth of 4 mm was examined with sinusoidal (longitudinal and lateral) and circular beam oscillation patterns at frequencies of 100 Hz and 200 Hz. The welding processes were analyzed by means of online X-ray imaging with a frame rate of 2 kHz, which provides the temporal and spatial resolution required to resolve the dynamics of the capillary. With conventional rectilinear welding, the weld seams are prone to the formation of process pores. By applying a circular beam oscillation, the weld seams were found to be virtually free from porosity. The mechanism leading to a reduced occurrence of process pores differed for the two investigated beam oscillation frequencies. At an oscillation frequency of 100 Hz bubbles are regularly formed in the melt pool but immediately removed at the subsequent pass of the laser beam by degassing into the vapor capillary. At 200 Hz the formation of process pores is completely avoided from the first. For a sinusoidal beam oscillation in longitudinal direction, the bubbles formed in the weld pool were found to be further inflated at each pass of the laser beam.

Fundamental investigations on the spiking mechanism by means of laser beam welding of ice

In order to gain further understanding on the mechanism of rapid seam depth variations, the so-called spiking, in deep penetration laser beam welding, this effect is analyzed and explained using the example of laser welding of ice. Laser welding of ice provides the advantage that the temporal behavior of the vapor capillary can be analyzed by means of common imaging methods in the visible spectral range. The occurrence of spiking and its frequency is found to depend on the feed rate but is not influenced by instabilities of the rear wall of the capillary. Also, the effect of downward moving structures at the capillary front can be shown to be independent from root spiking. Spiking is found to be governed by a periodically alternating irradiation of the capillary front wall by the incident laser beam in the lower part of the capillary. This is verified by ray-tracing calculations on capillary geometries extracted from high-speed videos. These findings are further substantiated by a transient smoothed particle hydrodynamics model to simulate the melt flow and evaporation at the capillary front that was coupled to the ray-tracing calculation to determine the local absorption of the laser radiation within the capillary.

Numerical assessment and experimental verification of the influence of the Hartmann effect in laser beam welding processes by steady magnetic fields

Controlling the dynamics in the weld pool is a highly demanding challenge in deep-penetration laser beam welding with modern high power laser systems in the multi kilowatt range. An approach to insert braking forces in the melt which is successfully used in large-scaled industrial applications like casting is the so-called Hartmann effect due to externally applied magnetic fields. Therefore, this study deals with its adaptation to a laser beam welding process of much smaller geometric and time scale. In this paper, the contactless mitigation of fluid dynamic processes in the melt by steady magnetic fields was investigated by numerical simulation for partial penetration welding of aluminium. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved based on temperature-dependent material properties up to evaporation temperature for two different penetration depths of the laser beam. The Marangoni convection in the surface region of the weld pool and the natural convection due to the gravitational forces were identified as main driving forces in the weld pool. Furthermore, the latent heat of solid–liquid phase transition was taken into account and the solidification was modelled by the Carman–Kozeny equation for porous medium morphology.The results show that a characteristic change of the flow pattern in the melt can be achieved by the applied steady magnetic fields depending on the ratio of magnetic induced and viscous drag. Consequently, the weld bead geometry was significantly influenced by the developing Lorentz forces. Welding experiments with a 16 kW disc laser with an applied magnetic flux density of around 500 mT support the numerical results by showing a dissipating effect on the weld pool dynamics.