Keyhole Stability Research Articles

Assessment of keyhole stability in laser beam welding with external magnetic field using numerical simulation

The challenge of understanding the physical mechanisms behind porosity reduction by a magnetic field during laser beam welding (LBW) is partly due to the difficulty in quantitatively evaluating keyhole stability. The commonly used index, such as keyhole depth, is typically one-dimensional, which is insufficient to capture the dynamic and three-dimensional fluctuations of the keyhole. In this paper, by utilizing a 3D multiphysical model of LBW with magnetic field, a novel keyhole geometry reconstruction algorithm has been developed to describe the keyhole profile and its fluctuation in a statistical manner to evaluate keyhole stability quantitatively. An equivalent diameter is proposed in this algorithm to reduce the irregularity of the keyhole geometry. The calculation results indicate that the time-averaged keyhole shape over 300 ms in the LBW of steel is conical, regardless of the application of an external magnetic field, which provides a more representative shape. Meanwhile, it is observed from the statistical aspect that the keyhole diameter becomes smaller, except the top part, under the influence of the magnetic field. The standard deviation of the equivalent diameter can be used as a physical variable to assess the keyhole stability quantitatively. The application of an external magnetic field can produce a noticeable reduction of the standard deviation of the equivalent diameter, namely, stabilizing the keyhole during LBW of steel. However, the different contribution from the keyhole stability affected by a magnetic field in suppressing porosity is different with materials.

Study on asymmetric forming and performance strengthening mechanisms of 6061-T6 aluminum alloy joints fabricated by oscillating laser based on keyhole characteristics and molten pool dynamic behaviors

The study intends to unravel the processes behind the particular forming and performance improvement of 6061-T6 aluminum alloy joints by getting a thorough understanding of keyhole features and dynamical characteristics of molten pools during the laser welding with circular oscillation (C-OLW) process. The C-OLW experiments were carried out considering the pivotal parameters, including oscillation frequency (f) and amplitude (A), and their impacts on joint formation, microstructure, and performance were analyzed. Increasing f from 200 Hz to 300 Hz results in a more homogeneous and centered laser energy distribution, reducing weld breadth and the maximum depth. Raising A from 1.2 mm to 1.5 mm concentrated energy along the workpiece surface, increasing weld breadth but decreasing the maximum weld depth. A three-dimensional transient numerical model was established and verified. The simulation results reveal that increasing f considerably improves keyhole stability by virtue of the beam's mixing effect on energy, resulting in a more evenly distributed temperature within the molten pool. The velocity of the beam's motion intensifies, leading to elevated local temperature and subsequently causing an escalation in splattering. When A grows, keyhole stability reduces, as does the intensity of molten pool flow. The major effect of increased frequency is refining the grain size while increasing amplitude primarily promotes the columnar-to-equiaxed transition (CET) process. The extracted results of temperature distribution illustrate that an increase in f leads to an increase in the solidification rate (R), and temperature gradient values (G) are similar or higher along the same isotherm. Therefore, the G/R ratio rises, promoting the formation of finer microstructures. Although increasing A diminishes G, the reduced overall temperature and molten pool volume contribute to the rise in R. Consequently, the G/R ratio declines, fostering equiaxed grain growth, in line with experimental observations. A higher G value on the left side corresponds to finer grain structures, corroborating experimental findings via Electron Backscatter Diffraction (EBSD). The keyhole stability and microstructure characteristics explain why increasing f or A can enhance joint performance. The research reveals that strategically augmenting both oscillation frequency and amplitude enables the achievement of enhanced performance in 6061-T6 aluminum alloy joints.

Microstructural evolution in laser powder bed fusion of water-atomized high-carbon low-alloy steel: Analysis of melting mode effects

Medium/High‑carbon steels are considered susceptible to cracking during rapid cooling in laser powder bed fusion (LPBF) applications. Also, water-atomized (WA) steel powders, containing oxides, have been associated with porosity defects during this process. In this study, LPBF was employed to process WA high‑carbon low-alloy steel powders under three operational regimes – conduction, transition, and keyhole modes. Analyses based on the scaling law of keyhole stability, melt pool dimensions, and x-ray computed tomography (XCT) suggested that processing under the transition mode closely resembled a stable keyhole, enabling the successfully printing of water-atomized powders to a density of 99.93 %. Subsequent heat treatment, hardness, and residual stress assessments presented a prominent structural softening and relief of compressive stress under the transition mode. This was attributed to the intense in-situ tempering and re-austenitization that occurred within each layer. Under this condition, with no post heat treatment, an ultimate tensile strength of 1250 MPa with >2.6 % elongation was achieved. Finally, the ball-on-disk test showed that wear performance of the printed steels was primarily governed by the tribo-oxides, with a limited influence from the microstructural variation.

Keyhole critical failure criteria and variation rule under different thicknesses and multiple materials in K-TIG welding

The stability of keyhole was important to determine both the process and welding quality in keyhole tungsten inert gas (K-TIG) welding. In this study, the welding experiments were conducted on three kinds of materials (TC4 alloy, 304 stainless steel, Q235 carbon steel) with four kinds of thicknesses (2 mm, 4 mm, 6 mm and 8 mm). The keyhole critical failure criteria of K-TIG welding for different materials and thicknesses was established based on Gibbsian surface thermodynamics and regression analysis. Critical stability coefficient Kc was calculated by the values of upper width (Wf) of fusion zone and lower width (Wr) of fusion zone. When Kc>G/WfWr, the keyhole collapse failure would occur. Otherwise, a stable penetration could be obtained. On the other hand, the values of Kc increase with the increase of plate thickness, which meant the keyhole stable range would be smaller. The Kc values of TC4 alloy were smallest and the corresponding stable penetration range was largest. While the values and range of Q235 carbon steel were the largest. This was consistent with the rule reflected by the fitting curves of critical heat input. Furthermore, the difference in criteria variation rule of different materials and different thicknesses were discussed.

Free arc heating effect in double-layers hybrid arc source

Keyhole welding process is easily achieved with plasma arc heat source, but the welding speed for a stable keyhole is hard to increase because of the low-level heat input. Higher welding current will increase the heat input, but the induced heavy arc pressure acts to damage the keyhole stability. The narrow window of welding current creates limited usable range of heat input. Double-layers hybrid arc heat source is developed by compositing an outer layer of free arc into the center plasma arc jet. Set the plasma arc current level to create enough arc pressure to open a keyhole, the outer layer arc is used to control additional heat power into the weld pool but has little effect to influence the arc pressure. The free arc heating effect in the hybrid arc is figured out: (1) much faster welding speed can be achieved, it increases from 3 mm/s to 4.75 mm/s after additional 80 A/80 A free arcs are used, and (2) the increase of welding speed is highly related to the total heat input in the hybrid arc and is unrelated to the free arc number; (3) the calculated efficiency factor is selected as 0.75 for the added free arc. The developed hybrid arc is proved to be a robust method to increase the manufacturing efficiency.

Porosity suppression mechanism analysis in narrow-gap oscillating laser-MIG hybrid welding of aluminum alloys based on keyhole stability and molten pool flow behavior

In this study, the keyhole dynamic behavior and molten pool flow were investigated to elucidate the effects of oscillating laser on porosity in narrow-gap oscillating laser-MIG hybrid welding of aluminum alloys. A weld with a porosity of 0.08% was produced at the laser oscillating frequency of 600 Hz and diameter of 1 mm, while 2.27% without oscillating laser. Compared to non-oscillating laser, keyhole opening size increased by about 2.3 times and keyhole depth fluctuation was suppressed. The forces acted on the interface between keyhole wall and molten pool became more balanced. The increased vapor recoil pressure overcame the combined closure effect of gravity and molten pool pressure. While applying oscillating laser, the vortex in molten pool was eliminated and the liquid metal flow was more stable. The combined impact force caused by droplets transfer and liquid metal convection due to constraining effect of narrow gap was buffered, which had less effect on molten pool and keyhole stability. The liquid metal tended to flow from the molten pool inside to surface, which favored the escape of bubbles from the molten pool.

Influence of oscillation parameters on energy distributions and dynamic behaviors during laser welding of aluminum alloy T-joints assisted with solder patch

The laser energy distribution on base material (BM) surface plays an important role in the welding quality. To reduce pore and spatter welding defects during laser welding of aluminum alloy T-joints, a circular shaped oscillating laser beam welding (OLBW) method for aluminum alloy T-joints assisted with solder patch is proposed in this paper. An energy distribution model and a dynamic model for OLBW of aluminum alloy T-joints assisted with solder patch are established to analyze the influence of oscillation parameters on energy distributions of BM surface and dynamic behaviors of molten pool and keyhole. The energy distribution characteristics are obtained by the energy distribution model and the dynamic behaviors of molten pool and keyhole are calculated by the dynamic model for OLBW of aluminum alloy T-joints assisted with solder patch. The results show that the uniformity of energy distribution is improved with the increase in oscillation amplitude or frequency. The oscillation parameters can be adopted to adjust the keyhole stability and the temperature distributions, flow velocities and morphologies of molten pool, which is of great importance for aluminum alloy T-joints welding with high quality.

Effects of oscillating frequency on keyhole stability and porosity inhibition in high-power laser-arc hybrid welding of 10-mm-thick 6082 aluminum alloy

Keyhole instability and high porosity are prone to occur in laser-arc hybrid welding (LAHW) with an increase in aluminum alloy thickness. In this study, a high-power laser up to 8.0 kW at different oscillating frequencies (f) was used to achieve acceptable hybrid welding of a 10-mm-thick 6082 aluminum alloy. According to the observations of the laser keyhole from a high-speed camera, increasing f from 0 to 100 Hz increased the opening diameter from 1.3 to 1.6 mm and decreased the collapse frequency from 642 to 353 Hz. With the improved keyhole stability, the weld porosity decreased from 4.5% to 1.8%. In addition, beam oscillation refined the weld microstructure by 15% and transferred the tensile fracture from the weld center to the heat-affected zone (HAZ). Although the change in the ultimate tensile strength (UTS) was not obvious, the elongation increased by 25%. Beam oscillation promoted the formation of turbulent flow inside the molten pool, homogenized the melt flow, and reduced the necking tendency of the keyhole caused by the melt flow, thus greatly improving the keyhole stability.

Quantitative evaluation of keyhole stability during laser welding using optical coherence tomography

Online monitoring of keyhole during laser welding is of great importance to evaluate the stability of the live process and the final quality of the weld. Indirect methods based on visual imaging and thermal radiation are lack of strong correlations with keyhole behavior, resulting in a low accuracy. Optical coherence tomography (OCT) shoots beam coaxially with the processing beam into the bottom of the keyhole, enabling direct monitoring of keyhole behavior. However, current OCT keyhole monitoring systems suffer from the harsh noise and are limited to only evaluating the keyhole depth through statistical analysis. In this paper, an enhanced OCT system was built with a capacity of capturing keyhole open/collapse behavior, opening a way of quantitatively evaluating the keyhole behavior. First, the signal was denoised and filled given the nature of fluid dynamics in time domain. Then an algorithm was developed to automatically detect the keyhole open/collapse behavior, based on which the weld status (heat conduction mode and keyhole mode) is determined, and the metrics of interests are characterized quantitatively. The proposed method provides a solid method to quantitatively evaluate the keyhole stability, which is well suited for developing new laser welding process and monitoring laser involved high volume manufacturing.

A comparative study on welding characteristics and mechanical properties of Ti–6Al–4V laser welded joints under the sub-atmospheric pressure and beam oscillation

Sub-atmospheric pressure environment and oscillation laser are both excellent ways to improve welded joint quality. However, the differences in their effects and mechanisms are unclear. Therefore, the differences between sub-atmospheric pressure and oscillating laser in weld formation, porosity suppression and grain size refinement were analyzed in this study. The results revealed that penetration depth of welded joint was increased to 6.09 mm under sub-atmospheric pressure environment while it was decreased to 3.3 mm under oscillation laser. Furthermore, reducing fluctuating vapor pressure and suppressing vortex formation were the main means to improve the keyhole stability and promote the escape of bubbles in a sub-atmospheric pressure environment. The improvement of the oscillating laser in these two aspects was attributed to enlarging keyhole area, shortening escape path and oscillation keyhole absorption. Additionally, the average grain size in the upper weld was reduced from 134.6 μm to 118.6 μm and 114.1 μm under the sub-atmospheric pressure environment and laser oscillation, respectively. The reduction of the temperature gradient led to former grain refinement while the increase of heterogeneous nucleation rate was responsible for latter grain refinement. Correspondingly, the mechanical properties of welded joints were improved. The study offered valuable insights for the application of laser welding with oscillation laser and sub-atmospheric pressure in industrial production.

Investigations of weld profiling and intermetallic formation in laser welding of steel-to-aluminium: a multi-physics CFD approach using beam shaping

The mechanical properties of steel-to-aluminium laser welded joints are dependent on the interfacial intermetallic compounds (IMCs) and the weld interface area. Recent technological advances in laser beam shaping have shown improvements in keyhole stability, porosity formation, optimisation of weld profiles and reduced interfacial IMCs. This study investigated the effect of keyhole stability which is critical in the formation of IMCs and weld profiles. Multi-physics computational fluid dynamics (CFD) simulations and high-resolution microscopy were used to calibrate the model using various core/ring beam power ratios and study their effect on fluid-flow and microstructural evolution. The wider spot from the ring beam improved keyhole stability, whilst the narrower core beam created instability of the weld pool. This study revealed that the strength of the weld was improved by 16% with a ring-only versus core-only beam and total IMC thickness was reduced by up to 50%.

Investigating impact of zinc vapor jet on keyhole dynamics and liquid ejection from molten pool during remote laser welding of zinc-coated steel in zero-gap lap joint configuration

When Zn-coated steels are laser welded in a zero part-to-part gap lap joint configuration, highly pressurised Zn vapors are readily produced, generating spatters in the weld. The best practice is to set a non-zero gap to facilitate vapor degassing; however, this necessitates additional operations, escalating production costs. This paper aims to investigate the keyhole dynamics and the ejections from the molten pool due to the interaction between high-pressure Zn vapors and the liquid metal, during remote laser welding of Zn-coated DX57D+Z steel sheets at zero nominal part-to-part gap in lap joint configuration. This is achieved by complementing experimental trials with multi-physics Computational Fluid Dynamics (CFD) simulations. The findings showed that the interplay between heat-affected zone, Zn evaporation zone, size/shape of the laser beam are critical factors for controlling the effect of Zn vapors jet on keyhole stability and liquid ejections. Opportunities for laser beam shaping are discussed throughout the paper.

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

Identifying the keyhole stability and pore formation mechanisms in laser powder bed fusion additive manufacturing

Identifying the keyhole stability and pore formation mechanisms in laser powder bed fusion additive manufacturing

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.

Keyhole stability, arc behavior, and molten pool flow in narrow-gap oscillating laser-arc hybrid welding of titanium alloy

In the study, the keyhole stability, arc behavior and molten pool flow were investigated to clarify the welding process stability in narrow-gap oscillating laser-arc hybrid welding of titanium alloy. At an oscillating frequency of 300 Hz and amplitude of 1 mm, a concave weld with low porosity was produced. Compared with non-oscillating laser, the keyhole opening size increased about twofold, and the standard deviation of arc deflection angle differences decreased by approximately 87.6 %. Owing to the stirring effect of beam oscillation, a stable keyhole with larger opening size was formed, which facilitated the escape of bubbles from the keyhole. More excitation electrons were generated and erupted outward from the keyhole, and both plasma temperature and density increased by approximately 55.2 %. An enhanced electric channel was formed between the welding wire tip and groove center. As a result, the arc burnt stably at the groove bottom, which could melt the groove bottom synchronously, and suppress sharp corners and lack of fusion. The impact caused by droplets was cushioned as the liquid metal flowed towards the rear molten pool orderly with a circular path. Consequently, the liquid metal near the keyhole fluctuated slightly, indicating that the keyhole stability was enhanced. Besides, dendrites were fragmented and more equiaxed grains were produced due to the strong disturbance of dendrites by liquid metal in the whole mushy zone.

Effect of laser oscillation and beam incident angle on porosity in double-sided filler welding of 2219 aluminum alloy T joint

In the current research, the double-sided laser beam oscillation filler welding (DLBOW) with beam incident angle from 20° to 40° were carried out for 2219 Al-Cu alloy T joint welding. Double-sided laser beam filler welding (DLBW) was conducted as a reference group. 3D computed tomography (CT) scanning analysis method was performed to investigate the precise three-dimensional locations of the porosity defects in the T joints. High-speed photography was conducted to evaluate the keyhole stability. The dynamic behavior of keyhole of DLBW and DLBOW was simulated by Flow-3D to study the formation process of pores. The effects of beam oscillation and beam incident angle on the macrostructure, microstructure, porosity, and mechanical property of T joints were discussed. It was found that beam oscillation and smaller beam incident angle promoted porosity suppression. The mechanical properties of T joints were improved by laser oscillation. With the beam incident angle of 20°, the porosity of T joint welded by DLBOW was lowest (0.26 %). The hoop tensile strength and T-pull tensile strength of the T joint were 403 MPa and 302 MPa, approximately 89.5 % and 71.6 %, in contrast with that of the base metal.

Study on the influence of Al-Si welding wire on porosity sensitivity in laser welding and process optimization

Al-Si welding wire is commonly used in the aluminum alloy welding due to the excellent heat crack-resistance and corrosion-resistance, but the presence of large-sized pores hinders its widespread application in the field of laser welding. This study focuses on the cause of pores formation in the laser welding using Al-Si wires with different Si contents and investigates the dynamic behavior of laser keyhole by a high-speed camera-based molten pool monitoring system. The results showed that with the increase of wire Si content from 5% to 12% (wt.%), the keyhole stability decreased and the weld porosity increased. According to the thermodynamic calculations, the increased Si content enhanced the Reynolds number of molten pool, resulting in increased turbulence tendency. It caused the molten pool being more susceptible to external disturbances, which decreased the stability of keyhole. Furthermore, the reduced dynamic viscosity of molten pool increased both the depth-to-width ratio of keyhole and its opening and closing frequency. These combined effects ultimately contributed to the increased susceptibility to pores formation. Based on the above results, a circular oscillating laser was introduced to guide the regular flow of molten pool. The beam oscillation improved the resistance to fluctuations of molten pool and reduced the depth-to-width ratio of keyhole. Besides, the high-frequency rotating keyhole could capture and re-melt the formed pores. An optimal laser oscillating welding was determined using response surface methodology to control the porosity within the acceptable range.

A multi-physics CFD study to investigate the impact of laser beam shaping on metal mixing and molten pool dynamics during laser welding of copper to steel for battery terminal-to-casing connections

This study aims to investigate the impact of laser beam shaping on metal mixing and molten pool dynamics during laser beam welding of Cu-to-steel for battery terminal-to-casing connections. Four beam shapes were tested during LBW of 300 µm Cu to 300 µm nickel-plated steel. Both experiments and simulations were used to study the underlying physics. A CFD model was firstly calibrated against experiments and then deployed to explore the effect of the increasing ring-to-core diameter, as well as a tandem laser spot configuration. The study showed that metal mixing is influenced by the keyhole dynamics and collapse events, but also there is an intricate interplay between keyhole geometry, fluid dynamics via Marangoni forces and buoyancy forces. Notably, the buoyance forces due to the different densities of steel and Cu, along with the recoil pressure contribute to the upward flow of steel towards Cu, and hence impact meaningfully the material mixing. The study pointed-out that the selection of a custom ring-to-core diameter and ring-to-core power is a decision with a trade-off between the need of stabilising the keyhole dynamics and the need to reduce the mixing. Findings indicated that 350 µm ring and 90 µm core with 30% of ring power (weld configuration C3) resulted in more stable dynamics of the keyhole, with significant reduction of collapse events, and ultimately controlled migration of steel towards Cu. Additionally, the pre-heating approach with the tandem beam only led to local fusion of Cu and no significant improvement in keyhole stability was observed.

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.