Front Keyhole Wall Research Articles

Observation of the keyhole behavior, spatter, and keyhole-induced bubble formation in laser welding of a steel/glass sandwich

Laser welding of a steel/glass sandwich is carried out to clearly observe the keyhole behavior, spatter, and keyhole-induced bubble formation. The formation mechanisms of the spatter and keyhole-induced bubble are analyzed, and the relationship between the spatter and keyhole-induced bubble formation is also discussed. There are five necessary steps for the spatter formation around the keyhole: the formation of the bulges at the keyhole wall, the decrease of the keyhole entrance size caused by the upward flow of the bulges, the formation of the melt column around the keyhole, the increasing of the melt column size, and the spatter formation around the keyhole. The decrease of the keyhole entrance size contributes to the increase of the vapor shear stress, and thus accelerates the spatter formation. The large deformation of the top rear keyhole wall decreases the vertical component of the vapor shear stress, so few spatters are formed at the rear keyhole wall. Two types of the keyhole-induced bubble are formed by the whole keyhole collapse between the front keyhole wall and rear keyhole wall, and the rear keyhole wall collapse. The influence of the keyhole-induced bubble formation on the spatter formation is not obvious.

Dynamic keyhole behavior and keyhole instability in high power fiber laser welding of stainless steel

A three-dimensional numerical model, considering the real-time multiple reflections of a laser beam, adiabatic bubble model and shear stress, was developed to study the dynamic keyhole behavior and keyhole instability in fiber laser welding of stainless steel. The inner dynamic keyhole behavior and weld defect formation were directly observed in a high resolution assisted by transparent glass. The numerical and experimental results showed that the keyhole width reached the quasi-steady state earlier than the keyhole depth did during fiber laser welding of stainless steel. Due to the large recoil pressure at rear keyhole wall caused by the irradiation of laser energy reflected by the bulge at the front keyhole wall, the rear keyhole wall was severely deformed at keyhole bottom and keyhole middle. The rear keyhole wall was collapsed due to the high surface tension pressure and hydrostatic pressure. The whole keyhole collapse was attributable to the capillary instability of the keyhole associated with large depth/width ratio and the strong flow of the bulges at the keyhole wall. When the laser power was increased, the keyhole depth/width ratio was increased, so the keyhole was more capillary instable. The average inclined angle of the front keyhole wall was decreased.

Elucidation of keyhole induced bubble formation mechanism in fiber laser welding of low carbon steel

Laser welding experiment with the aid of glass, and numerical simulation are carried out to study the keyhole behavior and keyhole-induced bubble formation. Two mechanisms are responsible for keyhole-induced bubble formation. The first mechanism is the strong fluid flow inside the weld pool, and the capillary instability of the whole keyhole, causing the collapse between rear keyhole wall and front keyhole wall. This mechanism contributes to most of keyhole-induced bubble formation. The second mechanism is the instability of the rear keyhole wall caused by the increase absorption of laser energy reflected by the bulge at front keyhole wall. The breaking of molten bridge is analyzed based on static pressure balance. The molten bridge with large curvature and low temperature is difficult to be broken. Bubble coalescence can be clearly observed at the bottom of the weld pool. Large bubble and small bubble have high coalescence efficiency. The bubbles at keyhole bottom have more time to escape without broken by laser beam, and the bottom part of rear keyhole wall is more easily depressed, so bubbles are easily formed at the keyhole bottom.

Effect of magnesium content on keyhole-induced porosity formation and distribution in aluminum alloys laser welding

With the aid of transparent glass, the keyhole behavior and porosity formation are directly observed in laser welding of different aluminum alloys. A three-dimensional model is developed to investigate the recoil pressure change. The influences of Mg element on porosity formation and distribution are discussed based on both numerical and experimental results. During laser welding of aluminum alloys, the molten metal in the front keyhole wall is thick. However, with the increasing Mg content, the weld depth increases in the welding direction, and longer time is required to reach the quasi-steady state; besides, the keyhole is more stable, and less keyhole-induced porosity is formed. Based on pressure balance acting on the keyhole wall, the high recoil pressure contributes to the stability of the keyhole, resulting in the suppression of porosity. Higher density of porosity is formed at the middle and bottom of the weld due to the easier collapse of keyhole in the respective regions in laser welding of aluminum alloys with higher Mg content.

Interaction between the laser beam and keyhole wall during high power fiber laser keyhole welding

The crucial factor of laser welding is the laser energy conversion. For a better understanding of the process, the interaction process between the laser beam and keyhole wall was investigated by observing the keyhole wall evaporation during high-power fiber laser welding. The results show that the evaporation vapor, induced by the laser beam, discretely distributed on the keyhole wall. A tiny 'hollow' zone was observed at the spot center-action region on the FKW. The evaporation vapor induced by the spot center moved downward along the front keyhole wall (FKW) with a period of about 0.3~0.75 ms, which indicates that the keyhole formation is reminiscent of a periodical laser drilling process on the FKW. The evaporation vapor on the keyhole wall suggest the assumption that the laser energy coupling mode in the keyhole was multiple-reflection, and the keyhole depth was mainly determined by the drilling behavior induced by the first absorption on the FKW.

Effect of keyhole geometry and dynamics in zero-gap laser welding of zinc-coated steel sheets

Both zinc-coated and uncoated DP 590 steel sheets were used for laser welding experiment with a 2kW multi-mode fiber laser under the same experimental conditions. Using systematically obtained keyhole data, how the keyhole dynamically responds to an incident laser beam was studied by defining several key factors, such as keyhole expansion factor, keyhole motion range factor, average aperture diameters, mass loss fraction and melt pool volume size. The relative configuration of the keyhole and the laser beam was found to be the most influential factor for obtaining good welds, and when the beam was located away from the front keyhole wall a good weld was obtained. When the beam irradiates on the front keyhole wall, the zinc evaporation region between the two sheets can be directly heated by the beam, and the zinc vapor pressure can become extremely large, and therefore the melt pool is severely disturbed. The keyhole bottom aperture diameter and the keyhole expansion factor for the bottom aperture were both largely proportional to the mass loss for both zinc-coated and uncoated steels. Zero-gap laser welding of zinc-coated steels can be indeed successful and a theoretical basis is presented.

High-Speed X-Ray Transmission and Numerical Study of Melt Flows inside the Molten Pool during Laser Welding of Aluminum Alloy

By using the X-ray transmission imaging system, melt flows inside a molten pool were studied during laser welding of aluminum alloy at different welding speeds. Then, the correlation between temperature gradients along the direction of weld penetration and melt flows in the rear part of a molten pool was analyzed by using a three-dimensional numerical method. And the presented model was verified by experimental results. The corresponding investigation was carried out to further study the correlation between temperature gradient and melt flow behavior of the molten pool in the plate heated by preheating temperature. The results indicated that, in the rear part of the molten pool, the maximum flow velocity was located at the bottom of the molten pool. The melt metal in the rear molten pool caused by different welding speeds had significantly different flow trends. As the welding speed increased, the absorbed intensity on the keyhole front wall also increased as well as the recoil pressure that could maintain the keyhole opened. Consequently, the increase of the welding speed was more beneficial to improving the stability of the molten pool.

Distinct morphology of keyhole wall during high power fibre laser deep penetration welding

The keyhole wall is the interaction interface between the laser and the material during the laser deep penetration welding. Measuring the morphology of the keyhole wall is thus of significance for understanding the high power fibre laser deep penetration welding process. In this paper, the clear keyhole wall was reserved by suddenly closing laser during high power fibre laser welding of copper alloy. The results indicate that the keyhole can be divided into laser action region and metallic vapour pressure maintenance region. The laser action region is on the front wall of keyhole, and a series of concentric elliptical rings are observed in this region. In another region, its diameter is significantly larger than the spot diameter and the keyhole wall is basically smooth. The results are different from those generally accepted viewpoints, which clarify the flowing law of molten fluid in the keyhole and are thus of great guiding significance for optimising the welding technology.

A novel method for observing the micro-morphology of keyhole wall during high-power fiber laser welding

Most of the laser welding defects are likely to be related to the welding process transport phenomena induced by the energy conversion process in the keyhole. The keyhole wall is the interaction interface between the laser and welded material, and studying its micro-morphology is significant for understanding the energy conversion process. Because the current measuring methods are limited, observation of keyhole wall micro-morphology is still a challenge. In this paper, a novel method was designed. The keyhole wall was preserved as the high-power fiber laser was suddenly shut off during welding process, and then it was measured by using a scanning electron microscopy (SEM). The results show that the laser beam directly acted on the keyhole front wall during welding. The concentric elliptical rings, wrinkles, and ripples were found on the laser-action region. These micro-morphologies indicate that the energy coupling is mainly the absorption of the keyhole front wall, and the keyhole formation is similar to a laser drilling on the keyhole front wall during welding. The novel method can be used to investigate on the formation of the welding defects during high-power fiber laser welding in the future study.

A study of keyhole geometry in laser welding of zinc-coated and uncoated steels using a coaxial observation method

Experiments were conducted over a large process parameter space using a 2kW multi-mode fiber laser, and the effect of zinc-coating on the keyhole geometry was investigated using a high-speed coaxial video camera. From the top and bottom coaxial surface images of erratic keyhole motions that were obtained from separately conducted experiments, time-averaged keyhole structures were calculated using a physics-based semi-statistical frame analysis. For uncoated steel, the keyhole bottom aperture is located mostly within the laser beam area. In this case, because there is no zinc evaporation, the bottom aperture tends to be closed to absorb enough laser energy for sustaining a keyhole. For zinc-coated steel, the keyhole is mostly open at the bottom, and the front keyhole wall is tilted so that the laser beam is located on the front keyhole wall. The keyhole tilting angle is more important for zinc-coated steel and the beam interaction area is more relevant to uncoated steel.

Explanation of penetration depth variation during laser welding under variable ambient pressure

It has been observed that the penetration depth during laser welding (LW) under vacuum or reduced ambient pressure could be significantly greater than that during welding under atmospheric pressure. Previous explanations of this phenomenon usually limit to specific wavelength laser welding and have difficulties in explaining why the variation will disappear, as the welding speed increases. Here, we propose that this variation is caused by the temperature difference of keyhole wall under variable ambient pressure based on a correct physical description of related processes. A new surface pressure model, dependent on ambient pressure, is proposed for describing the evaporation process during laser material interaction under variable ambient pressure. For laser welding of a 304 stainless steel with 2.0 kW laser power and 3 m/min welding speed, it is shown that the average keyhole wall temperature is around 2900 K under atmospheric pressure, and only around 2300 K under vacuum, which results in significant penetration depth variations. Interestingly, it is also shown that as the welding speed increases, the average temperature of the front keyhole wall gradually rises due to the reduction of the mean incident angle of laser, and the magnitude of this increase is larger in welding under vacuum than under atmospheric pressure. It allows us to explain why the penetration depth improvement decreases to zero with the increase of welding speed.

Relationship between spatter formation and dynamic molten pool during high-power deep-penetration laser welding

The spatter and the molten pool behavior, which were the important phenomena concerned with the welding quality, were observed and studied by using the high-speed camera and the X-ray transmission imaging system during laser welding under different welding parameters. The formation mechanism of spatter and the corresponding relationships between the spatter and molten pool behavior were investigated. The increase of laser power could cause more intense evaporation and lead to more spatter. When the focal position of laser beam was changed, different forms of spatter were generated, as well as the flow trends of molten metal on the front keyhole wall and at the rear molten pool were changed. The results revealed that the behavior of molten pool, which could be affected by the absorbed energy distribution in the keyhole, was the key factor to determine the spatter formation during laser welding. The relatively sound weld seam could be obtained during laser welding with the focal position located inside the metal.

Experimental sensing of the keyhole exit deviation from the torch axis in plasma arc welding

Sensing the dynamic behavior of the keyhole is of great significance to control and ensure the process quality and the resultant weld quality in plasma arc welding (PAW). Usually, the keyhole size is used as the geometric features describing the keyhole status. In this study, it is first found that the keyhole exit is displaced in the direction opposite to the welding direction even at the low welding speed during the PAW process. To examine the correlation of the keyhole exit deviation and the process parameters, the developed cost-effective vision sensor is used to observe the keyhole exit at the backside of the stainless steel workpieces with medium thickness. Based on the pre-processing and calibration of the captured sequential images of the keyhole, both the keyhole geometry and its position relative to the torch axis are determined under different test conditions. It is found that the deviation distance of the keyhole exit from the torch axis takes a larger value at first, then drops, and finally gets a fixed level, as the keyhole undergoes the initial blind stage, the unstable stage, and the quasi-steady stage. The effects of the main process parameters (welding current, welding speed, and plasma gas flow rate) on the deviation distance and the sizes of the keyhole exit are quantitatively analyzed. It is found that the deviation distance of the keyhole exit varies more obviously than the keyhole size as the welding process parameters changes within the appropriate ranges. The distortion of the thermal field on the workpiece due to the motion of the plasma arc and the inclination of the front keyhole wall are the main reason of the deviation occurrence.

Experimental study of phenomena and multiple reflections during inclined laser irradiating

The phenomena during inclined laser irradiating on the metal surface were investigated to imitate the behaviour on the front keyhole wall in deep penetration laser welding. The results revealed that the velocity of molten layer increased when the laser power density increased. The molten layer could be torn off to form droplets owing to the high evaporating pressure induced by the high power density laser. The heating efficiency of laser induced vapour plume was lower than that of the molten metal. In reflection experiments, only when the incident angle was 88°, the absorption of reflected laser was greater than that of the first incident laser. The concaved surface of molten layer had a great effect to the reflected laser. During multiple reflections, the considerable absorption of laser energy mainly happened in the first two times. The calculated data were closer to the actual conditions when the influence of polarisation variation was avoided.

Dynamic keyhole profile during high-power deep-penetration laser welding

Based on a butt-joint configuration assembled from transparent glass and stainless steel, the dynamic keyhole profile was observed during 10-kW fiber laser welding. The “gauffers” and vapor flow on the front keyhole wall moved downwards to the bottom of the keyhole. The downward-flowing vapor met the upward-flowing vapor at the bottom of the keyhole, which resulted in a whirlpool of vapor. The suspended droplets were drove to move in a random fashion by the whirlpool. The fluctuating vapor flow and pressure were the key factors causing the fluctuating keyhole wall and resulting in a vapor-generated wave (VGW) on the rear keyhole wall. The dynamic VGW was a very important factor affecting the dynamic welding process. When the VGW broke at the keyhole outlet, it was accompanied by swellings, spatter, columns, a decrease of the diameter of keyhole inlet, and a change in the direction of plume.

Direct observation of keyhole characteristics in deep penetration laser welding with a 10 kW fiber laser

Keyhole formation is a prerequisite for deep penetration laser welding. Understanding of the keyhole dynamics is essential to improve the stability of the keyhole. Direct observation of the keyhole during deep penetration laser welding of a modified "sandwich" specimen with a 10 kW fiber laser is presented. A distinct keyhole wall and liquid motion along the wall are observed directly for the first time. The moving liquid "shelf" on the front keyhole wall and the accompanying hydrodynamic and vapor phenomena are observed simultaneously. Micro-droplets torn off the keyhole wall and the resultant bursts of vapor are also visualized. The hydrodynamics on the keyhole wall has a dominant effect on the weld defects. The emission spectrum inside the keyhole is captured accurately using a spectrometer to calculate the characteristics of the keyhole plasma plume.

Effect of gap on plasma and molten pool dynamics during laser lap welding for T-joints

The aim of the present research is to discuss the effect of gap on plasma plume, keyhole, and molten pool dynamics during laser lap welding for T-joints. The authors observe plasma plume, keyhole opening, and molten pool images by high-speed camera in different gaps during CO2 laser overlap welding of T-joints. The results show that gap causes beam energy fluctuations in the keyhole and leads to the instability of welding process. In laser spot welding, zero-gap and small gap greatly affect the stability of plasma and keyhole, which causes the formation of cavities in the weld metal, while a proper gap can help prevent porosity formation. In laser continuous welding, the disruption and closure of front keyhole wall at the gap periodically changes with the gap, which causes the formation of plenty of porosities at the gap. The instability of keyhole is closely related to dynamics of plume and molten pool, which gives an insight into the mechanism of porosity formation during laser overlap welding.

Observation of spatter formation mechanisms in high-power fiber laser welding of thick plate

This paper aims to present the dynamic behaviors of spatter formation, and to clarify the spatter formation mechanisms in the high-power fiber laser welding of a thick plate at low welding speeds. We used a modified “sandwich” specimen to directly observe the geometry of the longitudinal keyhole wall. The dynamic behaviors of the keyhole, vapor plume, and melt pool with the formation of spatters were observed using high-speed imaging. The mechanisms of the formation of the spatter ejected from the top and bottom surfaces were analyzed. The recoil momentum associated with the energized vapor plume jet acts on the tips of the gauffers on the front keyhole wall and micro-droplets inside the keyhole, thereby resulting in the formation of high-speed micro-spatter. At partial penetration, the spatter ejected from the keyhole inlet is influenced mainly by the upward melt flow above the keyhole, melt displacement around the keyhole, and the strong shear stream of the directed vapor plume force. Moreover, some spatter droplets are accelerated through the vapor plume outside the keyhole. At full penetration of the melt, spatters are generated when the downward momentum of the melt due to downward flow and gravity, or vapor burst with an open keyhole, exceeds the surface tension forces. At full penetration of the keyhole, the crucial driving force for spatter generation is the viscous friction drag associated with high-speed motion of the energized vapor plume through the open keyhole. The welding process evolves into almost a cutting process at a lower welding speed.

Study of dimensionless quantities to analyse front and rear wall of keyhole formed during laser beam welding

Fluid flow mechanisms present in Keyhole (KH) during Laser Beam Welding (LBW) process influence the associated heat and mass transfer. In an attempt to describe these complexities for eventual optimization of LBW parameters, a dimensionless analysis using Mach (Ma), Raleigh (Ra), Reynolds (Re) and Marangoni (Mg) numbers have been carried out. This analysis describes hydrodynamics of melt and vapour phase appearing in the front and rear wall of KH. The non-dimensional hydrodynamic quantities describe the mechanism behind flow pattern present in melt-vapour in terms of ratio of convection–conduction heat transfer occurring within KH. The analysis shows that the higher Marangoni number indicates stronger Marangoni convection in the KH causing relatively higher capillary flow in the melt pool. The laminar-turbulent flow of melt-vapour in KH medium is described in terms of ratio of Reynolds and Mach numbers (Re/Ma). The pressure distribution in the KH accounts for the melt-vapour ejection rate. A relationship between depth and radius of KH has been obtained as a function of delivered laser power.

The Evaporation Effect of Front Keyhole Wall in Penetration Welding with an Electron Beam

The process of vacuum electron beam welding is characterized by deep-penetration with the action of keyhole effect. The assumption of simple cylindrical physical model of keyhole is reasonable according to the thermal transfer of the keyhole effect during welding. There is an intensive evaporation arises from the front keyhole wall owing to the high energy density of electron beam. Therefore, an analysis model of heat transfer at the interface of vapor phase and front keyhole wall was proposed to the temperature calculation on the basis of heat transfer theory. The evaporation of the primary elements, which are Mg, Al, Zn and Mn in AZ series magnesium alloy, can be analyzed by the model, as well as the influence of keyhole radius varying on the temperature at vapor-solid interface offront keyhole wall. And dimensionless parameters are introduced to analyze the influence of the process condition on the thermal effect. The calculation results show that Mg and Zn are vulnerable to vaporize loss during the vacuum electron beam welding on AZ series magnesium alloy, and the evaporation of Mg occurs earlier than Zn. A longer electron beam acting duration and smaller keyhole size will increase the temperature of the front keyhole wall significantly, which has a considerable influence on the evaporation effect of the elements.