Keyhole Surface Research Articles

Stability analysis and porosity inhibition mechanism of oscillating laser-arc hybrid welding process for medium-thick plate TC4 titanium alloy

The influence of oscillating laser frequency and oscillation diameter on the stability and porosity of laser-arc hybrid welding of titanium alloys were investigated. The microstructure and basic mechanical properties of oscillating laser-arc hybrid welded joints were revealed. The results show that the oscillating laser significantly improved the macroscopic morphology of the joints, reducing the porosity from 5.48 % to 0.12 %. In the conventional laser-arc hybrid welding process, the number of keyhole surface closures was more and the time of droplet transition was longer, resulting in poor process stability. When the frequency was 150 Hz and the diameter was 1.5 mm at oscillating laser conditions, the number of keyhole closures within 200 ms was significantly reduced to 5 times compared to the conventional laser-arc hybrid welding (23 times). The droplet transition was stable, which resulted in a more stable hybrid welding process. In addition, after one oscillation cycle, the forward distance of the laser was relatively short, which can reopen the smaller pores generated in the previous cycle, leading to a significant reduction in porosity. When the frequency and diameter of oscillation decreased, the fluctuation for keyhole morphology was greater and the droplet transition was instability, resulting in an increase in porosity; When the oscillation increased, the fluctuation of the keyhole outer diameter was larger and the penetration depth decreased, which was not conducive to the welding of medium-thick plates. In the oscillating laser-arc hybrid welding process, columnar crystals in the middle of the molten pool grew towards the surface, reducing the segregation of impurities in the weld center. As a result, the elongation percentage of the joints was improved by 36.58 % compared to the conventional laser joints.

Investigation on keyhole stability and surface quality in laser welding of TC4 titanium alloy with adjustable ring-mode (ARM) laser

Investigation on keyhole stability and surface quality in laser welding of TC4 titanium alloy with adjustable ring-mode (ARM) laser

Challenges in dynamic heat source modeling in high-power laser beam welding

The amount of absorbed energy in the keyhole as well as its spatial and temporal distribution is essential to model the laser beam welding process. The recoil pressure, which develops because of the evaporation process induced by the absorbed laser energy at the keyhole wall, is a key determining factor for the macroscopic flow of the molten metal in the weld pool during high-power laser beam welding. Consequently, a realistic implementation of the effect of laser radiation on the weld metal is crucial to obtain reliable and accurate simulation results. In this paper, we discuss manyfold different improvements on the laser-material interaction, namely, the ray tracing method, in the numerical simulation of the laser beam welding process. The first improvement relates to locating the exact reflection points in the ray tracing method using a so-called cosine condition in the determination algorithm for the intersection of reflected rays and the keyhole surface. A second correction refers to the numerical treatment of the Gaussian distribution of the laser beam, whose beam width is defined by a decay of the laser intensity by a factor of 1/e2, thus ignoring around 14% of the total laser beam energy. In the third step, the changes in the laser radiation distribution in the vertical direction were adapted by using different approximations for the converging and the diverging regions of the laser beam, thus mimicking the beam caustic. Finally, a virtual mesh refinement was adopted in the ray tracing routine. The obtained numerical results were validated with experimental measurements.

Keyhole stability and surface quality during novel adjustable-ring mode laser (ARM) welding of aluminum alloy

Aims at keyhole instability and poor surface-quality in welding of aluminum alloy, a novel laser technology has been studied on welding of Al 6061. The novel laser beam consists of a ring part and a center part. Different power ratio of ring laser beam to center laser beam and different kind of laser beam welding experiments have been conducted, respectively. The keyhole stability and surface quality were analyzed by using a high-speed camera and a confocal sensor accordingly. Additionally, the energy absorption rate was studied by analyzing the penetration and fusion area of cross-section. Experimental results showed that the ring laser beam provided great stability on the center laser-induced keyhole in high-speed dual-mode ring/center laser welding of aluminum. The stability effects can be improved by increasing the ring power in dual-mode ring/center laser welding of aluminum alloy, and the stabilized effect would contribute to the high-surface quality on welding of aluminum alloy. Furthermore, the combination of ring laser and center laser (Gaussian laser) greatly improved the welding efficiency due to its synergistic effects on process status in the ARM laser welding process. This work provided a basic research to both scientist and engineer on the mechanism of ARM laser welding aluminum alloy.

Heat transfer and melt flow of keyhole, transition and conduction modes in laser beam oscillating welding

Melting modes have significant effects on the heat transfer and fluid flow in laser beam oscillating welding (LBOW), which in turn affects the welding quality and solidification microstructure. In this paper, a computational fluid dynamics (CFD) model for LBOW of 2219 aluminum alloy is developed to study the melting/solidification, fluid flow pattern, keyhole dynamics and energy absorption under different welding modes: keyhole, transition and conduction mode. A hybrid heat source model is developed to describe the influence of the vapor heat and multiple reflections of the laser rays on the keyhole surface. The predicted molten pool dimensions and weld profiles are in good agreement with the experimental results. By increasing the oscillating frequency, the line energy density significantly decreases due to a higher scanning speed that results in a shallower keyhole with wider opening. The laser rays are more likely to leak out after 2-3 reflections which reduces the energy absorption. Thus, the melting mode will transfer from keyhole to conduction mode. With further increase of the oscillating frequency, the heat transfer will be dominated by heat conduction when the energy density is below the threshold for keyhole formation. Our work lay the foundations for optimizing the LBOW process to obtain sound joints.

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.

Effect of zinc vapor forces on spattering in partial penetration laser welding of zinc-coated steels

A three-dimensional thermal-fluid numerical model considering zinc vapor interaction with the molten pool was developed to study the occurrence of zinc vapor-induced spatter in partial penetration laser overlap welding of zinc-coated steels. The zinc vapor effect was represented by two forces: a jet pressure force acting on the keyhole rear wall as the vapor bursts into the keyhole and a drag force on the upper keyhole wall as the vapor escapes upwards. The numerical model was calibrated by comparing the predicted keyhole shape with the keyhole shape observed by high-speed X-ray imaging and applied for various weld schedules. The study showed that large jet pressure forces induced violent fluctuations of the keyhole rear wall, resulting in an unstable keyhole and turbulent melt flow. A large drag force pushed the melt adjacent to the keyhole surface upward and accelerated the movement of the melt whose velocities reached 1 m/s or even higher, potentially inducing spatter. Increased heat input facilitated the occurrence of large droplets of spatter, which agreed with experimental observations captured by high-speed camera.

Weld pool dynamics and joining mechanism in pulse wave laser beam welding of Ti-6Al-4V titanium alloy sheets assembled in butt joint with an air gap

A validated CFD model coupling the multiple phases and multiple physics was developed for the dynamic simulation of pulse wave laser welding (PWLBW) process of 1.2 mm-thick Ti-6Al-4V alloy sheets assembled in butt joint with a reserved air gap. A laser spot diameter of 700 μm was used to compensate for the energy loss with the clearance applied at 0.2 mm and therefore improve the gap bridging ability. The distributions and evolutions of temperature, velocity, phase interface and weld pool dimensions were characterized and discussed to reveal the joining mechanism and the heat and flow behaviors during welding. The results show that the liquid bridges firstly occur in the middle height of sheets as the welding begins, and rapidly extend to the whole thickness with the increasing heat input. The reduction on laser power causes the rebounding of keyhole surface and the backfilling of molten metal. The keyhole closes at root surface and rebounds completely in 2 microseconds, leading to the merging of liquid bridges, bottom-up moving of high temperature area, and temporarily enlargement on weld pool size at root surface. During the pulse interval, the weld pool volume shrinks significantly and the molten metal slows down from Marangoni vortexes to thermal buoyance flows. The proposed process parameters allow an air gap accounting for 16.67% of specimen thickness and result in a well-formed and defect-free weld bead.

Effects of sub-atmospheric pressure on keyhole dynamics and porosity in products fabricated by selective laser melting

A powder-scale multi-physics model is developed to investigate the keyhole dynamics and porosity in products fabricated by selective laser melting (SLM) under the sub-atmospheric pressure. The effects of the sub-atmospheric pressure on the plume attenuation of the laser beam, vaporization temperature and vapor recoil pressure are taken into account. The thermo-fluid behaviour in the melt pool under the atmospheric and sub-atmospheric ambient pressure is analysed in the SLM process. The simulation results suggest that the reduction of the sub-atmospheric pressure can increase the melt pool depth, whereas the effects are insignificant under the low sub-atmospheric pressure. As compared with the atmospheric pressure, the sub-atmospheric pressure leads to the decrease of the average temperature and the increase of the average recoil pressure on the keyhole surface, which is in favour of increasing keyhole stability and maintaining keyholes open. Consequently, the sub-atmospheric pressure can help to improve the keyhole stability and thus minimize pore defects in SLM-built products.

A more precise unified model to describe comprehensive multiphysics and multiphase phenomena in plasma arc welding

Plasma technology involves complex and coupled electric, magnetic, thermal and hydrodynamical interaction. It’s important to understand all the multiphysics mechanism for a wider industrial application. A 3D unified model was established, which includes a combination of hydrodynamical equations, heat transfer equation and electromagnetic equations, to comprehensively describe the electro-thermal conversion, electromagnetic effect, heat transfer, gas-liquid two-phase flow and solid-liquid phase change in plasma arc welding (PAW). Volume of Fluid method was used to track the gas-liquid interface of plasma arc and liquid metal pool, and an enthalpy-porosity technique was used to deal with the solid-liquid phase change during the welding process. The transient evolutions of temperature field, flow field and keyhole in the entire PAW process were predicted, and the distribution of electric potential and current density was obtained. It is found that there are high potential drop and current density near the electrode tip, which generates very high arc temperature and arc velocity. Plasma arc gradually melts the metal, then pushes the liquid metal to flow upwards along the keyhole surface, and finally penetrates through the workpiece, forming a “reversed bugle-like” configuration. Experiment was conducted with a plasma arc welding system, and it is found that both weld pool and keyhole geometry predicted by the model generally agree with experimental results. The calculated electric potential and full-penetration time also coincide with the experiment. It’s for the first time to give a relatively accurate prediction of electric potential drop and full-penetration time in unified PAW models. The research provides a deep and fundamental understanding of the comprehensive electric-magnetic-thermal-hydrodynamical multiphysics and gas-liquid-solid multiphase interaction in PAW process.

Metal flow of weld pool and keyhole evolution in gas focusing plasma arc welding

Establishing an open keyhole throughout work-piece is the key to penetrate work-piece fully in gas focusing plasma arc welding. In order to study heat transfer, metal flow and keyhole surface evolution in gas focusing plasma arc welding, the three dimensional mathematical model is developed. Arc heat model and plasma arc pressure model are used to describe heat and force action in gas focusing plasma arc welding. Temperature field, fluid flow in weld pool and keyhole evolution are simulated. It is found that metal in weld pool front flows bypass keyhole wall toward weld pool back. Center line of keyhole channel is crocked instead of going along z-axis. The calculated weld shape agrees with the experimental data basically. This work may enrich theoretical knowledge about the complicated thermal physical process and provide basic data to guide welding application in gas focusing plasma arc welding.

Blasting type penetrating characteristic in variable polarity plasma arc welding of aluminum alloy of type 5A06

Experiments were carried out in this paper first to clarify the characteristics of a digging process during aluminum alloy of type 5A06 variable polarity plasma arc welding (VPPAW). A special “Blasting type” penetrating phenomenon was observed through measuring the penetrating time by electrical and optical signals. A 3D model was established to numerically investigate heat transfer, fluid flow and surface shape during the digging process. An adaptive heat source was adopted to describe heat transfer process involving deformation of the keyhole tracked by volume-of-fluid (VOF) method. The plasma arc pressure attenuated in a parabolic form as the depth of the keyhole increases. It is found that the weld pool surface becomes concave after sufficient melting of metal. The keyhole depth exhibits responsive changes, which corresponds with the different stages of welding parameters. In the late stage, the depth increase rapidly to fully penetrating state, which shows a “blasting type” penetrating process. The keyhole formation was interpreted by considering energy accumulation and mass conservation of fluid flow. The maximum velocity during digging process always occurs on the keyhole surface, where a vortex in the opposite direction appears inside the weld pool. The calculated penetrating time, keyhole size and fusion line were basically in agreement with the experimental results.

Absorption peaks depending on topology of the keyhole front and wavelength

By high speed imaging, wavy patterns were observed at the keyhole front in Yb:fibre laser welding. Despite a regular appearance of downstreaming wave flow in the movies, deeper image analysis shows that the pattern structure is complex. The observed grayscale levels of the flow pattern are likely to correspond to a combination of temperature, topology, and emissivity of the keyhole front, in turn originating from variations of the beam absorption, temperature, boiling, ablation pressure, and melt acceleration across the keyhole surface. Bright domains can fluctuate on a time scale of 10–100 μs. For interpretation of the fundamental mechanisms, the evidence from high speed imaging analysis is accompanied by mathematical modeling of the absorption modulation, combining local absorptivity, and beam projection. The calculated results show that for a wide range of keyhole topology parameters, lasers with a wavelength of about 1 μm induce high absorption peaks at the wave shoulders while CO2-lasers have a more homogenizing behavior. Different regimes can be distinguished.

Laser Welding Process – A Review of Keyhole Welding Modelling

Laser welding is used in several industrial applications. It can be distinguished between conduction mode and keyhole mode welding, between pulsed wave and cw laser welding and between CO2-lasers with a wavelength of 10μm and various laser types of about 1μm wavelength. A deeper understanding of laser welding allows improving weld quality, process control and process efficiency. It requires a complementary combination of precise modelling and experimental investigations. The here presented review focuses on modelling of laser keyhole welding, for both wavelength regimes. First, the fundamentals of the laser welding process and its physics such as beam propagation, keyhole formation and melt pool dynamics are addressed. The main approaches for modeling energy transfer from laser beam to keyhole surface as well as fluid flow in the material are then discussed. The most relevant publications are systematically structured, particularly categorized with regard to the respective physical phenomena addressed. Finally some open questions are underlined.

Mechanisms of vapour plume formation in laser deep penetration welding

We analysed the dynamic shape of the metal vapour plume during deep penetration laser welding by means of high speed imaging. Our studies show that besides the inclination also the elongation of the vapour plume is determined by the evaporation processes inside the keyhole. When welding stainless steel sheets, the shape of the vapour plume as well as the shape of its origin, the keyhole, significantly vary with laser power and feed rate settings in a consistent way. This rather deterministic relationship allows for estimating how evaporation is distributed along the keyhole surface in different welding regimes. Based on the evaluation of the high speed sequences and corresponding microsections we propose a model of the vapour plume formation. The model indicates that, depending on the keyhole shape, the generation of the vapour plume is either governed by the vapour formed at the bottom of the keyhole or by the vapour formed at the inclined front keyhole surface, resulting in a very different shape of the plume in each case.

Finite-difference time-domain simulation of laser beam absorption in fully penetrated keyholes

Accurate predictions of laser beam absorptance and how laser beam energy is distributed on a keyhole surface are arguably the most important but challenging tasks in the study of laser keyhole welding. In this article, laser interaction with fully penetrated keyholes has been studied by solving the Maxwell equations for electrodynamics using the finite-difference time-domain method with the Drude model for metals. Based on the experimental observations of Fabbro et al. [J. Phys. D: Appl. Phys. 38, 1881–1887 (2005)], the keyhole is simplified as a tilted cylinder, and we have extensively investigated laser absorption phenomena considering three materials (Fe, Sn, and Al), three beam polarizations (two linear and circular), two laser beam wavelengths (1.06 μm and 10.6 μm), and six keyhole tilting angles (0°, 10°, 20°, 30°, 40°, and 45°). To the best of the authors' knowledge, this is the first electrodynamic simulation of a laser manufacturing process and reveals some interesting findings concerning laser beam absorption characteristics that can be only obtained by full electrodynamic simulations.

Role of surface-active elements during keyhole-mode laser welding

During high power density laser welding of mild steel, the keyhole depth, liquid metal flow, weld geometry and weld integrity are affected by base-metal sulfur content and oxygen (O2) present in the atmosphere or shielding gas. The role of these surface-active elements during keyhole-mode laser welding of steels is not well understood. In order to better understand their effects, welding of mild steel specimens containing various concentrations of oxygen and sulfur are examined. In addition, a numerical model is used to evaluate the influence of the surface-active elements on heat transfer and fluid flow in keyhole-mode laser welding. Increase in base-metal sulfur concentration or O2 content of shielding gas results in decreased weld widths. Sulfur results in a negligible increase in penetration depth whereas the presence of O2 in shielding gas significantly affects the weld penetration. It has earlier been proposed that oxygen, if present in the shielding gas, can get introduced into the weld pool resulting in formation of carbon monoxide (CO) at the keyhole surface and additional pressure from CO can result in increased penetration. Numerical modelling has been used in this work to understand the effects of formation of CO on the keyhole and weld geometries.

Numerical simulation of molten pool dynamics in high power disk laser welding

A single-phase problem is solved rather than a multiphase problem for numerical simplicity: and the solution is based on the assumption that the region of gas or plasma can be treated as a void because solid or liquid steel has a greater density level than gas or plasma. The volume-of-fluid method, which can calculate the free surface shape of the keyhole, is used in conjunction with a ray-tracing algorithm to estimate the multiple reflections. Fresnel's reflection model is simplified by the Hagen–Rubens relation for handling a laser beam interaction with materials. Factors considered in the simulations include buoyancy force, Marangoni force and recoil pressure; furthermore, pore generation is simulated by means of an adiabatic bubble model, which can also lead to the phenomenon of a keyhole collapse. Models of the shear stress on the keyhole surface and of the heat transfer to the molten pool via a plasma plume are introduced in simulations of the weld pool dynamics. Analysis of the temperature profile characteristics of the weld bead and molten pool flow in the molten pool is based on the results of the numerical simulations. The simulation results are used to estimate the weld fusion zone shape; and the results of the simulated fusion zone formation are compared with the results of the experimental fusion zone formation and found to be in good agreement. The effects of laser beam profile (Gaussian vs. measured), vapor shear stress, vapor heat source and sulfur content on the molten pool behavior and fusion zone shape are analyzed.

Fresnel absorption of 1 μm- and 10 μm-laser beams at the keyhole wall during laser beam welding: Comparison between smooth and wavy surfaces

The angle-dependent absorption of laser beams at metal surfaces is described by the Fresnel-equations. During keyhole laser welding the essential interaction takes place at very striping angles of incidence of the order of 1–8 degrees at the front of the vapour capillary, called the keyhole. For a smooth vapour capillary, laser beams with a wavelength of about 1μm operate in a Fresnel-regime where the absorptance increases with the angle of incidence at the wall, towards the weak Brewster-angle maximum. In contrast, for 10μm-lasers high absorptance around the more pronounced Brewster-angle peak takes place. From high speed imaging keyhole surface waves were observed. Mathematical modelling of the laser-keyhole interaction demonstrates that already relatively little waviness of the melt surface at the keyhole strongly modulates the angles of incidence and in turn the Fresnel-absorption due to varying angles of incidence, soon also leading to shadow zones. Due to this local variation of the angle of incidence the absorptance tends towards the angle-averaged value, with the consequence that for 1μm-lasers the direct absorptance and in turn the penetration depth increases, particularly at low welding speed, while for 10μm-lasers it generally decreases.

Coaxial hybrid process of hollow cathode TIG and YAG laser welding

To make progress in laser-arc hybrid welding, especially when applied to thick steel plate, it is essential to get the remarkable synergic effect of deep penetration. It is suggested that the effective process is combining hollow cathode TIG with high beam quality laser welding. Penetration of hollow TIG/YAG hybrid welding was investigated by numerical analysis. A keyhole surface model taking into account the laser beam parameter product (BPP) was developed. The calculated keyhole surface profiles of coaxial hybrid welding are wider and deeper than that of YAG laser welding alone. As a result, single-pass butt welding of 20 mm thickness steel plate was achieved under the conditions of BPP = 0.9 mm mrad at laser power 1.8 kW. To confirm numerical results, coaxial hybrid welding tests with a hollow cathode TIG was conducted. The welding penetration was about two times as deep as a YAG laser alone. It was found that coaxial hybrid welding with hollow TIG/YAG had a large synergic effect.