As an advanced materials processing technology, laser welding technology has been widely applied in manufacturing industry. Keyhole effect is a major characteristic for deep penetration laser welding. At present, there are still many defects in deep penetration laser welding, such as weld depression, spatter and multi-hump, which hinder the further development of this technology. The occurrence of defects is closely related to the keyhole behavior caused by the melting, evaporation and flow of molten pool during the welding process. Therefore, it is of great significance to study the formation and maintenance mechanism of keyhole in deep penetration laser welding for optimizing the welding process and improving the weld quality.
In general, the keyhole inside the workpiece is not easy to be observed because of the opacity of the material. Many scholars have made achievements in keyhole observation and theoretical research on energy and mechanics in keyholes. Arata et al. (1985) tried to capture the side shape of the keyhole from the vertical direction of the welding speed using X-ray, and directly observed the fluctuation of the keyhole. Dowden et al. (1987, 1996, 2002), Postacioglu et al. (1987, 1997) have successively built theoretical models of open keyhole and blind keyhole, studied the energy balance and pressure balance in the keyhole during the formation and maintenance of keyhole, explored the generation of thermal stress during laser welding of thin plates, and determined mathematically that the viscous resistance related to steam movement was one of the factors causing the violent movement of keyhole and molten pool, and established a mathematical model to describe the axial movement of molten pool along the keyhole wall. Semak et al. (1995, 1997) took pictures of the keyhole and the molten pool with a high speed camera. It was calculated that the metal vapor induced by the laser was significantly larger than the surface tension and static pressure of the molten pool, thus promoting the flow deformation of the molten pool to generate keyhole. When the keyhole reached a certain size, it would gradually close under the action of surface tension and static pressure, and the process showed a certain periodicity. Kim et al. (2015) used a high speed camera to observe the influence of the zinc coating on the morphology of keyhole in the laser welding process. It was found that due to the existence of the zinc coating, under the strong evaporation of zinc, the bottom of the keyhole was basically open, while the bottom of the keyhole of the ungalvanized steel tended to close. This phenomenon shows that when considering the pressure balance in the keyhole, the influence caused by the evaporation of elements contained in the material needs to be carefully considered. In order to estimate the keyhole depth in the actual welding process, Lankalapalli et al. (1996) established a penetration model of two-dimensional conical keyhole and linked the penetration depth with the incident power. Solana and Negro (1997) established an axisymmetric model of multiple reflections inside the keyhole, and made the model take into account the inverse bremsstrahlung absorption. In Pecharapa's study (Pecharapa and Kar 1997), the phase transformation of materials in the welding process was taken into account, and a relatively good theoretical model was obtained. In the study of Strömbeck (Strömbeck and Kar 1998), the multiple reflection of laser in the keyhole was described as the self-focusing of the laser welding system, and a model with higher temperature at the bottom of the keyhole was obtained. Fabbro and Chouf (2000) investigated the uniform motion of keyhole along a straight line, considered the multiple reflection and inverse bremsstrahlung absorption inside the keyhole, and linked the drilling rate of laser beam with the moving speed of keyhole. Then, the high speed camera was used to observe the flow of the molten pool, and it was found that the interaction between the steam plume and the molten pool was the reason for the change of its flow characteristics (Fabbro et al. 2007, 2008, 2010). In the experiment, the welding speed was changed from slow to fast, it was found that the inverse of the keyhole depth and the welding speed were almost linear (Fabbro 2019).
With the in-depth study of keyhole, researchers can obtain a clear image of the keyhole in the laser welding process. The results obtained from the direct observation experiment of the keyhole become the basis for studying the characteristics of the keyhole. Jin’s team used the sandwich method to directly observe the keyhole (Li et al. 2002; Jin et al. 2003a, 2003b, 2006), and constructed a three-dimensional multi reflection model of the keyhole. It was found that most of the positions of the rear wall of the keyhole were not irradiated by the laser. In order to maintain the energy balance inside the keyhole, the energy required for the rear wall of the keyhole would be transmitted from the front wall to the rear wall by the molten pool flow (Jin and Li 2004, Jin 2008, Zhang et al. 2008). By using the sandwich method, Cheng et al. (2012), Jin et al. (2012) found that compared with Fresnel absorption, the inverse bremsstrahlung absorption of laser energy by keyhole plasma played a major role in absorbing laser energy, and the electron temperature inside the keyhole was uneven and distributed in the radial and depth directions. Li et al. (2014) observed that the steam flowing upward and downward inside the keyhole formed a steam vortex after meeting, and the fluctuation of steam flow and pressure were the key factors leading to the fluctuation of the keyhole. Zhang et al. (2018) used the modified sandwich method to observe the keyhole, and summarized the formation of the keyhole into three stages: the fast drilling stage, the slow drilling stage, and the quasi steady state stage. They believe that the key factor to make the width of the molten pool enter the quasi steady state was the balance between the rotation of the vortex and the lateral flow around the keyhole. Others believe that the steam recoil pressure generated by the energy reflected from the front wall of the keyhole to the rear wall is the main driving force for the deformation of the rear wall of the keyhole, and the rear wall collapses due to large surface tension and hydrostatic pressure during the oscillation of the keyhole (Zhang et al. 2019).
Numerical simulation is used to study the characteristics of keyholes. Wang et al. (2006) established a three-dimensional heat source model that composed of a rotating Gaussian volume heat source and a double ellipsoidal heat source to simulate the keyhole in the laser welding process. The numerical simulation results show that the eddy currents formed at the top and bottom of the weld pool are conducive to the overall heat transfer. Cho et al. (2009) simulated the flow of the molten pool at the initial stage of the keyhole formation, and found that in the initial stage, the flow direction in the center of the molten pool appeared axisymmetric oscillation, which was closely related to the recoil pressure, the cooling and the surface tension. Huang et al. (2017) theoretically studied the correlation among the surface area, the volume of the keyhole, the welding speed and the surface tension coefficient, believed that the surface tension controlled the oscillation period of the keyhole. Bedenko et al. (2010) conducted one-dimensional simulation research on the dynamics of keyhole plasma during laser welding, pointed out that the keyhole plasma has a periodic shielding effect on laser radiation, which makes the absorption of laser energy by workpiece materials alternately attenuate or stop, resulting in the pressure and temperature oscillations. Pang et al. (2016) proposed a mathematical model to describe the dynamic coupling behavior of keyhole and weld pool, pointed out that the surface tension has a great influence on the period of keyhole depth oscillation. The oscillation of the plume ejected from the keyhole is closely related to the instability of the keyhole, and the oscillation frequency is the same as the oscillation period of the keyhole in the depth direction. Li et al. (2019) simulated the laser welding of aluminum alloy under sub-atmospheric pressure. The numerical results show that compared with atmospheric pressure, the keyhole becomes wider and deeper, and the hump is smaller. With the decrease of environmental pressure, the eddy current on the rear wall of the keyhole will decrease or even disappear, which is conducive to improving the stability of deep penetration laser welding and suppressing the generation of defects. Cunningham, Mayi studied the transformation of the welding mode, found that there was a clear threshold for the sudden change from the conduction mode to the keyhole mode. During the transformation process, a semicircular pit appeared in the weld pool. Its depth and energy balance were determined by the effect of the recoil pressure on the weld pool (Cunningham et al. 2019; Mayi et al. 2021). Zou et al. (2017) believed that during the welding process only the front wall of the keyhole was exposed to the laser beam, and the absorbed energy at the rear wall was mainly absorbed by plasma radiation and multiple reflection of the laser. The depth of the keyhole is mainly determined by the drilling behavior caused by the first absorption of laser energy at the front wall of the keyhole. The molten pool flow around the keyhole and the behavior of the keyhole have been studied by some scholars (Seidgazov et al. 2019; Wu et al. 2019; Li et al. 2020), the surface tension is considered to be the main driving force for the molten pool flow. The reduction of the size at the entrance of the keyhole leads to the increase of the shear stress of the steam, which accelerates the formation of spatter. The bubbles are generated by the collapse of the front and rear walls of the keyhole. Based on the pressure balance of the keyhole, Huang et al. (2020) analyzed the relationship between the steam plume and the keyhole fluctuation, believed that the change of the total pressure led to the fluctuation of the keyhole size, and the fluctuation of the plume led to the fluctuation of the hydrodynamic pressure in the keyhole.
At present, many scholars have studied the keyhole and generally agree that the formation and maintenance of keyhole is the result of the combined effect of energy balance and pressure balance inside the keyhole. The study of energy balance in the keyhole is relatively comprehensive, due to the keyhole is hidden in the weld pool during deep penetration laser welding, and the pressure in the keyhole is difficult to measure, most of the scholars analyze and judge the pressure balance result in the keyhole on the basis of the energy balance, or conduct theoretical simulation research according to empirical formulas, so the conclusions obtained may lack the support of actual experimental data. Several experimental observation methods of keyholes have been summarized by the author (Hao et al. 2022), the numerical simulation research is mainly aimed at the parameters that are difficult to be detected in actual experiments, such as temperature field, pressure distribution, molten pool flow, etc., but the final results need to be confirmed by the experimental results. Limited by the existing technical means, it is difficult to experimentally analyze the influence of the pressure inside the keyhole and the physical properties of the material on the formation and maintenance of the keyhole. In the process of deep penetration laser welding, the generation of keyhole is mainly the result of the gas-liquid two-phase interaction between the metal vapor produced by the evaporation of the weld pool and the weld pool.
In this paper, an analogy method of keyhole formation is used to simulate the keyhole in deep penetration laser welding. Both the analogy welding and real deep penetration laser welding are carried out on liquid and modified sandwich structure respectively. In the analogy welding, liquid is used to simulate the molten pool, and gas jet is used to simulate the vapor. The behaviors of keyhole in analogy welding and real deep penetration laser welding are intuitively observed respectively, and the influences of relevant parameters on the behavior of the keyhole are explored.