Keyhole Behavior Research Articles

Insight into the keyhole behaviour and its role on residual stress formation in vacuum electron beam welding of TC4 titanium alloy

Insight into the keyhole behaviour and its role on residual stress formation in vacuum electron beam welding of TC4 titanium alloy

Heat transfer and fluid flow modeling of steel-Inconel laser welding in an overlap configuration

This paper introduces a multiphysical model for laser welding in a lap joint configuration with dissimilar metals. Initially solved in a 2D axisymmetric configuration for static shots, the model is extended to 3D to simulate laser welding with a weld seam formation. Heat transfer, fluid flow, and species tracking are solved with the level set method to describe dynamically the keyhole and melt pool behavior. Validation against experimental data shows an accurate description of the main phenomena. The paper mainly highlights the need of introducing a thermal contact resistance to correctly predict the melted area dimensions. The study emphasizes the importance of considering imperfect material contact and proposes an effective approach for thermal contact resistance, a phenomenon poorly discussed in the literature.

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

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

Implementation of energy interaction between laser beam and keyhole wall based on algebraically reconstructing free surface: An experimentally validated numerical framework

The interaction of the laser beam with the material is critical for determining laser processing quality, with ray tracing methods mostly used in energy transfer research. These techniques generally rely on only the volume of fluid (VOF) method or level set (LS) function to track the molten pool free surface, resulting in either accuracy loss or difficulty in ensuring mass conservation of the interface. The few coupled algorithms are built on geometric notions that are difficult to understand while being limited to structural grids. In this paper, we have proposed a revised ray tracing framework, so-called ARCLS, to derive the exact intersections of the rays with the keyhole wall which is represented by a continuous LS surface algebraically rebuilt on top of the VOF interface. It is easy to code and convenient for unstructured meshes. This framework is tested through a V-groove case and a set of Ti6Al4V titanium alloy laser welding examples. The findings reveal that the reconstructed LS surface perfectly matches the VOF interface, and the predicted molten pool profile agrees with the actual data well. The keyhole morphology and molten flow patterns are also consistent with the physical truths. Spattering, porosity, necking, and swelling are observed in the developing keyhole, which are the possible reasons for the potential welding defects. Further investigation suggested that the ARCLS-based algorithm collected more laser rays at the keyhole bottom compared to the standard technique, achieving a deeper keyhole profile and a reflection pattern that was more optically logical. The work presents a reliable approach for estimating laser energy distribution, which can be effectively applied to the heat-fluid coupling simulation for laser manufacturing processes in order to forecast the dynamic behaviors of keyhole and molten pool.

Development of laser-arc hybrid welding process of butt joint for bogie pipe crossbeam components

High-speed railway vehicle bogies are the main load-bearing components, which are welded together from steel plates and tubes. Butt joint and T-joint are the main types and the joint thickness is between 8–20mm. The stability of laser-arc hybrid welding (LAHW) process correlated strongly with keyhole behaviour. In this paper, the common quality defects in full penetration root pass by LAHW were analysed. Controlling the root gap is adopted to stabilize the keyhole, to improve the stability of weld quality. A novel type of butt joint with micro structure based on Y-shaped grooves has been designed, to meet both the requirement of controlling root gap and industrial large-scale batch production. Two new structure parameters, 2 mm of the remaining blunt edge thickness and 0.3 mm of the half width of root opening, were proved by the flat plates welding test. Finally, the joint with micro structure and the root pass welding parameters were transferred to the welding process of tube crossbeam components. The LAHW process verification was completed, and the test results showed the process meets the requirements of ISO 15614.

Study on improvement of weld defect in oscillating laser welding of aluminum alloy T-joints assisted by solder patch

During oscillating laser welding of aluminum alloy T-joints assisted by solder patch, it is found that the pore defect can be reduced and the high quality welded T-joints are obtained. To understand the effect of oscillating laser on the formation process of T-joint welds, a dynamic model for laser welding of aluminum alloy T-joints assisted by solder patch is developed in this paper to analyze the dynamic behaviors of keyhole and molten pool. Based on the dynamic model, the morphology characteristics of keyhole and the evolution processes of molten pool temperature and flow fields during conventional laser welding and oscillating laser welding are calculated and compared. The simulation results are found to be kept consistent with the experimental results. It is demonstrated that the keyhole is characterized with better morphology and higher stability and the degassing condition of molten pool is improved during oscillating laser welding compared with conventional laser welding. The molten pool temperature and flow fields are much more uniform during oscillating laser welding. The obtained results are efficient for suppressing weld defects and achieving high quality welding of aluminum alloy T-joints.

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.

Arc and keyhole behavior in narrow-gap oscillating laser-MIG hybrid welding of thick aluminum alloys

Narrow-gap oscillating laser-MIG hybrid welding was developed to weld thick aluminum alloys. Weld formation and porosity defect with various oscillating laser parameters were systematically analyzed. Furthermore, the corresponding influencing factors, i.e., arc and keyhole dynamic behaviors were discussed. The difference between both sidewall penetrations was small while wire feeding speed was high and oscillating laser diameter was lower than 3 mm. When laser oscillating frequency was higher than 300 Hz, porosity was less than 0.5 %. Arc burnt stably and the deflection angles were small while laser oscillating frequency was 300–450 Hz and diameter was 1–2 mm. The small fluctuation of arc resulted in small difference between both sidewall penetrations. With the action of oscillating laser, the charged particles increased and diffused to the upper part of the groove. Therefore, arc was attracted by the charged particles and struck at the center bottom of groove. While the arc burnt stably, the difference between both sidewall penetrations was small. The pore formation was significantly inhibited by oscillating laser, as the keyhole opening size increased and fluctuation degree decreased. With the action of oscillating laser, the ratio of keyhole opening width to depth increased, the probability of Rayleigh jet instability on the keyhole wall was reduced.

A novel optimizing Pulsed Plasma Gas Variable Polarity Plasma Arc Welding (PPG-VPPAW) method for improving weld quality

To enhance the reliability of aluminum alloy welding quality, a novel Pulsed Plasma Gas Variable Polarity Plasma Arc Welding (PPG-VPPAW) system is proposed. This system enables independent control and rapid adjustment of the plasma gas, allowing for flexible modulation of arc pressure output under constant thermal conditions, thereby reducing porosity and refining grain structure. The research analyzed the impact of parameters of different plasma gas on arc characteristics, welding porosity, and welding microstructure. The results reveal that the periodic changes in pulsed plasma gas induce corresponding variations in arc temperature, arc shape, arc pressure, arc voltage, and the morphology of the weld pool keyhole. The magnitude of the difference between the base value and peak value of pulsed plasma gas determines the extent of arc pressure fluctuations and the behavior of the backside keyhole. The maximum oscillation amplitude of the backside keyhole can reach up to 4.5 mm². Additionally, the oscillation of pulsed plasma gas within the molten pool has been demonstrated to effectively reduce weld porosity and refine grain size. Under this process, the grain size of the weld metal can be reduced by up to approximately 25%. Current work provides an important direction for fabricating highly reliable aluminum alloy weldments.

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.

Blue diode lasers – Understanding and influencing melt pool dynamics in copper

Within the last years an increasing demand of copper components was observed, especially in the fields of energy transition, e-mobility, or digitalization. For joining those components, laser welding has become more and more popular. Infrared lasers are commonly used for metal welding. They are characterized by a low absorptivity in solid copper at room temperature. In contrast, lasers in the blue wavelength spectrum are characterized by a significantly higher absorptivity. This facilitates the initial energy input into the material and makes the welding process more efficient. Further, in the liquid state, the absorptivity change in copper is much smaller for the blue wavelength. This results in a continuous heating of the material, a more stable melt pool and a lower amount of spatter. One supplier of lasers in the visible wavelength spectrum is Laserline. In 2023, they launched a 4 kW blue diode laser, the currently highest available power for this wavelength. It makes accessible a broad variety of applications. But the scientific investigations for the blue wavelength at a power of several kilowatt are still limited. It is the paper’s purpose to extend them by analysing welding of copper and its alloys with a blue diode laser. One the one hand, synchrotron trials were performed allowing a look into the material during welding. A special focus was on the transition between heat conduction and deep penetration welding as well as the keyhole behaviour. On the other hand, conventional welding trials were performed analysing the impact of beam shaping on the melt pool dynamics.

Study on coupling characteristics of different welding modes in laser-double TIG hybrid welding

In order to study the low-power pulsed laser-induced double arc coupling discharge effect and improve the energy utilization rate of the hybrid heat source, this paper adopts different heat source arrangement modes (laser-guided, double arc-guided) and conducts welding experiments under different Dla (distance between laser beam and two tungsten electrodes). The high-speed camera and spectral diagnostic system are used to monitor the dynamic behavior of arc plasma information, molten pool, and keyhole in real-time and calculate the characteristics of electron temperature and electron density of the coupled plasma. The physical model is established to analyze and discuss the time characteristics and energy distribution of coupled discharge. The results show that according to the different Dla, the hybrid heat source has three different coupling effects: laser and double arc interfere with each other, laser and double arc good coupling, laser and double arc no significant effect. When the laser and the double arc have a good coupling state, the coupled plasma has a lower electron temperature and a higher electron density. At this time, the Marangoni force and arc pressure on the liquid metal in the laser keyhole is enough to overcome its own gravity. The keyhole has existed for a long time, which is conducive to the extension of the laser-arc coupling discharge effect and improves the energy utilization efficiency of the hybrid heat source. Compared with the laser-guided mode, the double arc-guided mode has a larger coupling discharge period and duty cycle, and the energy density of the conductive region in the center of the keyhole increases by 14.4 %, which is beneficial to improve the energy utilization efficiency of the hybrid heat source and obtain a larger penetration depth.

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.

Dynamic evolution of keyhole and weld pool throughout the thickness during keyhole plasma arc welding

Keyhole plasma arc welding (K-PAW) of medium-thick plates often faces instability in the initial keyhole penetration stage, so a thorough investigation of the keyhole and weld pool behavior in this stage is needed. In this paper, a special "sandwich visual sensing structure" was adopted to obtain images of weld pool and keyhole of the SUS304 stainless-steel plate during the plasma arc welding. This method allows direct observation of keyhole growth and weld pool flow throughout the thickness of the workpiece under different welding conditions. When the welding current is reduced, the plasma arc cannot melt through the metal, and the arc sweeps through the keyhole bottom like a “roller coaster”. The trends of keyhole dimensions along different directions during various welding processes were quantitatively investigated. As the weld proceeds to half of the plate thickness, the mutation zone of the rear keyhole wall becomes very common, and the rear keyhole wall's morphology frequently transforms between type I and type II. Weld pool flow characteristics during constant-speed welding were classified into four types. Last, the molten metal flow cycle and the molten metal cloud cluster have been observed by the new visual sensing method. This paper provides the industry with a more intuitive way to study the keyhole and the weld pool for K-PAW, as well as offers important guidance for the first normal penetration in the medium-thickness plate K-PAW process.

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.

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.

Investigation of keyhole behaviour and its impact on the performance of laser beam oscillating welding through imaging and acoustic signal analysis

As an emerging laser welding optimisation technique, beam oscillation still lacks detailed investigation into its underlying process mechanisms. This study investigates the microstructure and mechanical properties of 316LN welded joints under four oscillation modes, the results demonstrate that beam oscillation can suppress columnar grain growth and reduce porosity defects, thereby enhancing weld formation and mechanical properties. Building upon these findings, visual and acoustic sensing techniques are employed to analyse dynamic features of keyholes and energy distribution. The correlation between keyhole geometric features and porosity propensity across different oscillation modes is elucidated through visual information. Additionally, the time and frequency domain features of the acoustic signals effectively characterise the stability of the welding process and laser energy absorption efficiency.

Vacuum laser beam welding of AZ31 magnesium alloy: Weld formability, microstructure and mechanical properties

Laser welding of magnesium alloy is still challenging because of the frequently occurred defects. In this work, a promising vacuum laser welding technique was used to join AZ31 magnesium alloy. The effect of ambient pressure on weld surface appearances and weld fusion cross sections was symmetrically investigated. It was found that the penetration depth increased and the weld width became narrower with the descending of ambient pressure from 101 kPa to 10 kPa. However, the penetration depth did not increase and even decreased when the ambient pressure further decreased to 1 kPa and 0.1 kPa. In addition, hump defects occurred at lower ambient pressure of 1 kPa and 0.1 kPa. The weld pool and keyhole behaviors were in-site observed using a high-speed camera at various ambient pressures. The results showed that unstable keyhole and turbulent flow were found in the molten pool at 101 kPa. The keyhole became more stable as the ambient pressure decreased from 101 kPa to 10 kPa. When the ambient pressure further decreased to 1 kPa and 0.1 kPa, the liquid metal thickness between keyhole wall and weld pool periphery became very narrow, which sometimes resulted in unstable absorption of the laser energy. Finally, a 10 mm-thick bottom-locking weld was made under vacuum. The average grain diameter of the weld in vacuum is 28.4 μm, which is much lower than 46 μm in atmosphere. The tensile results showed that the average ultimate tensile strength and the elongation of the joints welded in vacuum at 10 kPa was 8.8 % and 44.0 % higher than that of the joints welded in atmosphere respectively. The enhanced mechanical properties were attributed to the reduced porosity and refined microstrustures of the weld joint made under vacuum.

Unified CNN-LSTM for keyhole status prediction in PAW based on spatial-temporal features

Despite the high efficiency of keyhole plasma arc welding (K-PAW), it still has several deficiencies, such as narrow welding parameter ranges, easily disturbed welding process and instability of welding quality, etc. It is a significant prerequisite to predict the keyhole/penetration status accurately for maintaining the welding process stability and improving the welding quality. Most researchers have focused on visual inspection techniques and convolutional neural networks (CNN), establishing a mathematical model of the correlation between weld pool images and penetration status. While CNN could extract the features of single weld pool images, it is difficult to predict the evolution trend of the incoming moments. In this paper, a novel model based on CNN and LSTM (long-short term memory) was developed to extract both the spatial and temporal features of topside weld pool images, and consequently the complex keyhole behaviors were predicted and described. The comparative study is carried out on different models, i.e. the single CNN model, the single LSTM model, and the CNN-LSTM model that integrates both spatial features of single images and temporal features of sequence. The keyhole initialization and establishing period, as the typical and critical welding scenario in full-penetration welding, is investigate and compared, resulting in a predicted value that is close to reality. Furthermore, ahead prediction of keyhole status was adopted, maintaining over 80% accuracy even when predicting keyhole behaviors 2 s into the future. Consequently, the unified CNN-LSTM model effectively improves the prediction accuracy of keyhole/penetration status, promising for intelligent K-PAW technology.

Dynamic vapor and keyhole behavior, and equiaxed dendrite formation in blue laser processing of copper

A coupled Cu vapor and keyhole numerical model including a simple virtual mesh refinement method was developed to investigated the Cu vapor and keyhole behavior in blue laser processing of a copper material. Electron backscatter diffraction (EBSD) was adopted to measure the crystal structure of the fusion zone. The Cu vapor mainly flowed upwards above the keyhole, while some downward flows were observed inside the keyhole. The recoil force and Marangoni stress caused the melt at the keyhole wall to flow downwards, and the Marangoni stress caused the melt at the pool edge to flow outwards. Owing to the multiple reflections and high absorption rate, the total blue laser energy efficiency can be as high as 83.1%. The EBSD, numerical and theoretical results indicated that the blue laser favored the fine equiaxed dendrite formation in the center of the fusion zone, which was attributed to the higher value of the cooling rate G × R, where G is the temperature gradient and R is the solidification rate, and the lower value of G/R at this location, unlike the previously studied infrared laser processing case.