Elucidating the effect of circular and tailing laser beam shapes on keyhole necking and porosity formation during laser beam welding of aluminum 1060 using a multiphysics computational fluid dynamics approach

The automotive industry shows a keen interest in utilizing aluminum and its alloys due to their high strength, excellent ductility, and low weight, which helps in meeting carbon emission targets. The 1xxx series, particularly AA1050 and AA1060, consisting primarily of pure aluminum, is used in battery pack manufacturing as an alternative to copper to reduce weight and material costs.^1,2 Laser beam welding has gained popularity in battery pack manufacturing, offering competitive advantages, such as low thermal deformation, high depth-to-width ratio, small heat-affected zone, high power density and welding speed, and good adaptability and flexibility.^3,4 One of the leading challenges in laser beam welding of the 1xxx series is the occurrence of weld porosity, which severely affects the reliability of the welding process and can have detrimental effects on the mechanical and electrical performance of the final products.² Porosity is attributed to gas diffusion⁵ and keyhole instability.⁶ While gas diffusion in the molten pool is controllable by the application of shielding gas to protect the molten metal from the environment, controlling pore formation due to keyhole instability is a remaining challenge. Keyhole-induced porosity is distinguishable from gas diffusion-induced porosity by the fact that clustered pores form toward the bottom of the fusion zone. Several studies have shown that optimizing welding process parameters, such as welding speed, power, and laser beam defocusing, and introducing new welding strategies, such as laser oscillation and wobbling, can help us to control pore size and distribution and reduce the overall pore volume in the fusion zone.^7–10 The fast oscillation of the laser beam has been found beneficial not only to reduce gas diffusion-induced porosity as experimentally validated by Zhang et al.,¹¹ but also helps significantly to control the keyhole-induced porosity due to the stirring effect and increase in keyhole size, as studied via simulations by Shi et al.¹² Ke et al.¹³ noticed that the back-and-forth swing of the keyhole in the molten pool during laser beam oscillation allows bubbles to merge with the keyhole or facilitate their outflow, thus reducing overall porosity in the fusion zone. Tao and Yang¹⁴ demonstrated that laser beam wobbling (i.e., oscillation of the laser beam in both transversal and travel directions) significantly reduces porosity in aluminum alloys and improves mechanical properties. Lin et al.¹⁵ studied the effect of laser power, speed, and inclination angle on keyhole-induced porosity and concluded that high beam inclinations led to instability of the front wall of the keyhole.

Opposed to laser beam oscillation, laser beam shaping makes use of a static beam and tailors the heat input in and around the fusion zone. This is achieved by adequate insertion of optical components (specially coated lenses of silica substrate) in the optical chain of the welding head or by electro-optical switching multiple laser beams generated in the laser source itself, where a single-mode fiber laser can be converted into multiple beams, enabling existing single-beam systems to adopt new tailored beam shapes.^16,17 The use of adjustable ring-mode (ARM) dual laser beams has been explored to study its effect on the stability of the keyhole, as suggested by Wang et al.¹⁸ Sun et al.¹⁹ studied that optimization of power ratio in ARM can substantially reduce the weld porosity area ratio in the fillet and lap laser welding configuration. ARM power ratio optimization also helps us to improve mechanical properties by improving grain morphology.²⁰ The dynamic laser beam shaping is an emerging technology in laser beam welding and is being investigated to control the material response to the heat input both spatially and temporally.²¹ 

Although the research has provided insights into porosity formation and possible ways to reduce porosity content by optimizing process parameters using circular spot beams or dual beams with outer rings and inner cores, the effect of beam shapes with noncircular power distributions is not well understood and more efforts are required to elucidate the impact of beam shape on keyhole behavior, molten pool dynamics, fluid velocity, and potential strategies to restrict or remove keyhole-induced porosity from the molten pool. Another complexity arises from the fact that laser beam shaping introduces additional parameters and optimizing the process can be expensive and time consuming since it may require dedicated equipment, expertise, and experimental setups. In this context, multiphysics computational fluid dynamics (CFD) enable simulations of the process to reproduce mechanisms that are difficult to observe with in situ investigations. With the raise of computational power and multicore computing on high performance clusters, advanced CFD simulation of laser beam shaping is now a close reality.²² 

This paper aims to studying the impact of both circular and tailing beam shapes on melt pool dynamics and the evolution of porosity due to the instability of the keyhole during laser welding of the AA1060 alloy in overlap configuration. This configuration is typically used in the busbar welding for battery pack manufacturing. A CFD model has been developed and calibrated using experimental data of a circular beam. The model has been then deployed to investigate the porosity formation. Simulations of tailing beam shapes show that there is the pronounced effect on the melt pool, fluid dynamics, and shape and size of the molten pool and keyhole which contribute to the reduced keyhole collapsing events, thereby reducing porosity formation. The physical phenomena during the melting, formation of the keyhole, and solidification are discussed throughout the paper, contributing to elucidate the impact of laser beam shaping on porosity control.

Aluminum AA1060 alloy sheets with dimensions of 50 mm (W) × 100 mm (L) were used for linear laser welding in overlap configuration as shown in Fig. 1. The choice of penetration depth is based on the requirement that the temperature at the lower end of the sheet remains below 473 K. This precautionary measure aims to prevent any potential damage to battery components. Additionally, to minimize the effect of weld geometry on the porosity formation a uniform depth of penetration is adopted across different beam shapes for comparative analysis. This selected range of penetration depth, specifically set at 3.0–3.2 mm, is predicated upon experimental investigations conducted using the circular beam.

The chemical composition of the base material is presented in Table I. Sheets were cleaned using acetone based commercial strain clearer to remove oil or any other contamination to avoid any possible effect during laser welding.

During welding experiments, no shielding gas nor filler wire was used. Laser welded samples were cut in transverse and longitudinal sections. They were hot mounted and polished using standard metallography methods and etched using 20% NaOH to expose weld pores. Sections were then imaged using Keyence VHX7000 optical microscope.

The welding parameters are given in Table II. The welding speed and laser power were selected in order to achieve all tested configurations and a penetration depth in the range of 3.0–3.2 mm. The laser beam was focused on the top surface of the workpiece and no beam oscillation/wobbling was used.

Three laser beam shapes were used in this study: (1) circular spot of 120 μm in diameter [Fig. 2(a)]; (2) elliptical spot (110 μm major axis and 40 μm minor axis) with tail of 3 mm [Fig. 2(b)]; and (3) square spot (20 × 20 μm²) with a tail of 3 mm [Fig. 2(c)]. Welding experiments were conducted using a 10 kW coherent laser having a beam parameter product of 3.5 mm mrad. The laser beam was delivered through an optical fiber of 100 μm diameter and coupled with the Scansonic ALO4-O welding head (Scansonic MI GmbH, Germany), which comes with 158 mm collimating length and focal length of 176 mm. The resulting Rayleigh length is 0.84 mm. First, beam shape (1) was used to calibrate the CFD model and then compared against beam shape (2) and (3) in terms of weld porosity formation. Beam shapes (2) and (3) were generated by PowerPhotonic Ltd UK.

The multiphysics model has been developed using the commercial CFD code FLOW-3D^® (solver version: 12, release: 7, update: 2) and its module FLOW-WELD (release: 7, update: 3.0.0.3.4). Temperature-dependent material properties were imported from the JMATPRO^® material database.

The following assumptions were considered for the model development: (1) air and vaporized metal are modeled as “void” type, with ambient temperature and pressure; (2) the computational fluid is assumed to be Newtonian and incompressible; and (3) any heat sinking effect of the clamp and bottom surface is neglected.

The conservation equations for momentum, energy, and material continuity are solved to calculate the melt velocity, temperature, and the pressure fields. The solution procedure involves (1) using the implicit scheme of momentum conservation to compute initial velocities; (2) iteratively adjusting pressure to satisfy mass continuity; (3) employing the VOF method for tracking void-fluid interfaces; and (4) solving the energy equation implicitly to determine temperature distribution and update thermal properties. Bubble pressure and surface tension models were also solved implicitly.

The simulated domain (Fig. 3) consists of three regions. Region-1 (20 × 5 × 5.45 mm³) is a fine mesh (fluid zone) that represents the section where phase change occurs during the laser welding process. Region-2 (20 × 10 × 5.5 mm³) was created with a coarse mesh (350 μm) for heat transfer, and it encloses the fine mesh region. Region-3 (20 × 10 × 2 mm³) is located above the top sheet and serves as a void region for tracking the free surface deformation. To simulate the transient evolution of fluid flow, keyhole formation, and heat transfer, coupled equations are solved with a small-time step, typically ranging from 1.0 × 10⁻⁶ to 5.0 × 10⁻⁷ s, determined by the mesh size in region 1.

To assess the ability of the CFD model to capture both the weld profile and porosity, a mesh sensitivity study was conducted by refining the mesh in a subregion (10 × 5.45 × 5 mm³) of region-1. Three different mesh sizes were utilized: 100, 75, and 50 μm. The comparison was carried out at a constant laser power of 2650 W and welding speed of 100 mm/s using beam shape (1). Simulations were then correlated to the weld profiles (both transversal and longitudinal sections). The results of the simulations in Fig. 4(a) revealed that the weld profile was not affected by the mesh size; however, the prediction of the porosity was noticeably influenced by the mesh size. Only 50 μm demonstrated a higher accuracy to capture porosity, as depicted in Fig. 4(b). Accuracy to capture porosity can be further improved by reducing mesh size; however, it would significantly increase computational time.

The generation of a keyhole is significantly influenced by recoil pressure. In the simulation, recoil pressure is adjusted through the calibration coefficients A and B, as indicated in Eq. (4). During the model calibration process, A was adjusted to 20 000 Pa, while B was set to 6.4.

The calibrated and experimentally validated model was subsequently employed to investigate the effect of the elliptical beam with tail and square beam with tail, on the size and shape of the molten pool and keyhole, as well as the stability of the keyhole and the evolution of porosity.

Figure 5 shows the weld seam x–z plane at the weld center line and depicts the evolution of the molten pool dynamics at different time steps—(a)–(e) show the solid fraction; (f)–(j) show the temperature field; and (k)–(o) show the pressure profile.

During aluminum welding, the Marangoni effect driven by the surface tension gradients plays a critical role in the fluid dynamics of the molten pool.²⁴ As the laser energy is applied, the intense heating causes the material to melt and evaporate, leading to the formation of a keyhole. Due to the negative surface tension gradient, the molten metal tends to move outward at the top surface of the keyhole as shown conceptually in Fig. 6. This outward motion extends slightly below the surface in the z direction until the molten metal comes in contact with the fluid that is moving backward and upward from the bottom of the keyhole as shown in Figs. 5(b) and 5(g). This fluid motion is primarily driven by the recoil pressure resulting from intense vaporization. The opposing movement of the fluid on the top surface and rear side of the keyhole creates a vortex within the molten pool. This vortex behavior is influenced in the first instance by the Marangoni effect, but then also by the gravity and the kinetic energy of the fluid as it interacts with solid interfaces. These factors collectively affect the shape of the vortex and the fluid pressure distribution within the molten pool. Additionally, the pressure variations in the molten pool are influenced by the fluid’s velocity, flow rate, and the shape and geometry of the boundaries between the molten pool and the solid interfaces. These hydrodynamic phenomena, combined with the effects of surface tension and recoil pressure, play a crucial role in determining the stability of the keyhole and the overall fluid dynamics within the molten pool.

The opposing movements of the fluid, both clockwise and counter-clockwise, at the top surface and rear side of the keyhole have an important consequence: they restrict the size of the molten pool. This restriction creates a high viscosity mushy layer, depicted in yellow in Figs. 5(a)–5(e). This mushy layer forms a barrier that limits the expansion of the molten pool. As a result, the high recoil pressure and the hydrodynamic forces within the confined molten pool lead to a significant increase in fluid velocity, which can ultimately result in the instability of the keyhole by pushing a rear keyhole wall closer to the front wall and filling the molten liquid in it, thereby raising the fluid surface closer to laser beam as shown in Figs. 5(c), 5(d), 5(h), 5(i), 5(m), and 5(n).

Closure or narrowing the top neck of the keyhole restricts the ejection of vapors out of the keyhole which leads to an increase in pressure within the keyhole and creates a high-pressure lob as shown in Fig. 5(m). As laser energy is restricted due to the closure of the keyhole, this leads to a drop in temperature as shown in Figs. 5(h) and 5(i) and ultimately results in the entrapment of the bubble in the solid front of the molten metal as shown in Figs. 5(d), 5(e), 5(i), 5(j), 5(n), and 5(o). Subsequently, a significant amount of laser radiation is absorbed by the neck of the keyhole, leading to the formation of an extremely high-temperature area as shown in Figs. 5(e) and 5(j). This intense heating generates large recoil pressures that exert a downward force on the keyhole bottom, causing it to open again and go deeper. This physical phenomenon along with all possible forces has been shown schematically in Fig. 6. It must also be noted that in deep and narrow keyhole regime, fluid pressure, recoil pressure, vapor pressure, and Marangoni effects (surface tension) play significant roles; whereas, the buoyancy force, that is dependent upon density which varies with temperature in molten pool, has similar effect on the fluid velocity irrespective of the shape of the keyhole, and this is especially true when welding similar metallic sheets.

The fluid dynamics within the molten pool, the shape of the keyhole, the thermal and pressure profile, and the solidification process, as observed in Figs. 5 and 6, highlight the importance of achieving a wide and deep keyhole with a large molten pool and relatively lower fluid velocities. Such configurations can help minimizing keyhole collapse events. To achieve these desired conditions, two tailing beam shapes have been investigated and discussed in Sec. V B.

Figures 7(a)–7(c) and 7(d)–7(f) provide detailed visualizations of the central plane of the welding seam in the x–z direction for both elliptical and square spots with a tail, respectively. Comparing the two laser beam shapes to the circular spot, it is observed that both the elliptical and square beam shapes result in a larger melt pool length in the travel direction of the laser beam. Specifically, the elliptical beam shape yields a melt pool length of 3.3 mm, while the square beam shape results in a length of 4.1 mm. In contrast, the circular spot laser beam generates a melt pool length of only 0.7 mm. The presence of the tail segment in the elliptical and square laser beam shapes significantly alters the size of the molten pool and the shape of the keyhole, leading to a more laminarlike flow with vortices eventually forming away from the keyhole wall [as shown in Fig. 7(d)]—this is opposite to the circular spot where vortices formed closer to the keyhole rear wall. In terms of keyhole shape, the rear wall of the keyhole takes the form of an arc for both the elliptical and square beam shapes as represented in Figs. 7(b) and 7(e) with the dotted blue line. However, the radial angle of the arc is steeper in the case of the square beam shape, while the rear wall of the keyhole remains parallel to the incident laser beam in the case of the circular spot. The tailing beam shape significantly alters the fluid dynamics of the liquid metal compared to a standard circular spot. Figure 7(b) illustrates that the liquid metal flows away from the rear wall of the keyhole, creating vortex flows at a distance behind the rear wall.

This complex flow pattern is crucial in maintaining the stability of the keyhole and reducing the likelihood of the keyhole collapsing and bubble formation. The arclike shape observed on the rear wall of the keyhole plays a crucial role in generating a molten pool with a large area of laminar flow that moves away from the rear keyhole wall. This is due to the shallow slope of the rear keyhole wall, which aligns the directions of gravity and recoil pressure. As a result, the resulting force drives the flow of the molten metal downward and backward.

The simulations conducted in this study also allowed for an examination of keyhole collapsing events in both circular spot and beam shaping techniques. Figure 8(a) presents the ratio of the keyhole’s average depth to its width, referred to as K-ratio, and establishes a correlation between this ratio and the occurrence of keyhole collapsing events along the unit weld length. It is evident that the higher K-ratio is associated with a greater number of collapsing events. In the case of the square spot with a tail, where the K-ratio drops to 3.1 (shallower keyhole), the occurrence of collapsing events is reduced to zero. This observation underscores the significance of keyhole geometry in mitigating porosity formation during the laser welding process which confirms the results in Refs. 25 and 26. An additional advantage of the tailing beam shaping is the creation of a smoother welding interface between the fusion zone and the base metal as shown in Fig. 8(b). In contrast, the fusion zone-base metal interface with the circular spot exhibits waviness and sharp edges. These irregularities can significantly impact the mechanical performance of the weld, particularly during tensile and fatigue testing, where stress concentrations may arise.

This paper investigated the effect of both circular and tailing beam shapes on melt pool dynamics and the evolution of porosity due to the instability of the keyhole. A combination of multiphysics CFD models and experiments have been presented to elucidate the mechanism of porosity formation during laser beam welding of AA1060 aluminum. Moreover, the CFD model provided information about temperature fields, fluid flow, and keyhole shape, all of which are difficult to measure directly via physical experiments. The main findings are summarized as follows.

1. When employing a circular spot, the concentrated heat leads to the formation of a deep and narrow keyhole. This leads to a narrow molten pool with high-velocity fluid flow, contributing to the instability to the necking of the keyhole and an increased occurrence of collapse events. Additionally, the high thermal conductivity of aluminum promotes rapid solidification, restricting the expansion of the molten pool and facilitating the entrapment of bubbles in the solidified material.

2. The study of the thermal distribution and fluid flow suggested that achieving a wider opening of the keyhole, accompanied by a larger molten pool and reduced fluid velocities, can help mitigate keyhole collapse events during laser beam welding. This was possible via a tailing beam shape with elliptical and square spots. By adjusting the laser power and speed, comparable weld penetration to the circular spot was maintained.

3. The geometry of the keyhole and the resulting welding interface are important factors in achieving welds with reduced porosity and improved mechanical performance.

4. Tailing beam shapes offer the advantage of creating a smoother welding interface between the fusion zone and the base metal, unlike the waviness and sharp edges typically observed with the circular spot. These irregularities can lead to stress concentrations and negatively impact the mechanical performance, especially during tensile and fatigue testing.

Despite the positive effect in terms of porosity control via reduction of keyhole necking, it is important to note that the tailing shapes require higher power input [as shown in Fig. 8(a) with the corresponding energy input] compared to the circular spot. Future work would focus on exploring the trade-off between the energy level and power distribution within the spot, considering the broader requirements of energy consumption, cost-effectiveness, and thermal distortions of the welded parts.

Results could be helpful in representing the distinctive benefits of the laser beam shaping to the laser-material processing community. This opens interesting opportunities in the broader area of laser beam shaping as a tool to create bespoke weld profiles/morphology, microstructures, and phase formation (even more relevant for dissimilar material welding). Although results are encouraging, compared to a single-beam laser, selecting the optimal set of parameters of a beam shaping system requires significant effort. Future work will take advantage of the CFD models and develop a database of welding scenarios and will validate and calibrate them with the experimental data.

This study was financially supported by WMG HVM Catapult and Jaguar Land Rover Research Agreement. We also kindly acknowledge the support of the EPSRC MSI (Research Centre for Smart, Collaborative Industrial Robots) (Grant No. EP/V062158/1).

The authors have no conflicts to disclose.

Qamar Hayat: Formal analysis (equal); Validation (equal); Visualization (equal); Writing – original draft (equal). Pasquale Franciosa: Conceptualization (equal); Funding acquisition (equal); Methodology (equal); Project administration (equal); Writing – review & editing (equal). Giovanni Chianese: Conceptualization (equal); Methodology (equal); Writing – review & editing (equal). Anand Mohan: Visualization (equal); Writing – review & editing (equal). Dariusz Ceglarek: Funding acquisition (equal); Project administration (equal). Alexander Griffiths: Conceptualization (equal). Christopher Harris: Conceptualization (equal); Funding acquisition (equal).