Rear Keyhole Wall Research Articles

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.

Mechanism analysis on suppressing porosity in laser-MIG hybrid welding of aluminum alloy via external magnetic field

An external magnetic field with a high magnetic flux density (≥300 mT) has been proven to reduce the porosity of aluminum alloy single laser welded joints. However, the additional welding current exists in laser-arc hybrid welding (LAHW) and its physical mechanism of interaction between the external magnetic field and the molten pool is different from magnetic-assisted laser welding. It was explored whether the porosity suppression of the external magnetic field works in LAHW using experimental observation and numerical simulation. The results suggested that the presence of an arc would induce the interaction between the magnetic field with a lower magnetic flux density and welding current, giving rise to a strong backward Lorentz force up to 1.44 × 105 N/m3, which differed from that in the magnetic-field-assisted laser welding. Although the presence of an arc heat source extended the solidification time of the rear weld pool. The impact of the droplet on the rear keyhole wall deteriorated the keyhole stability and caused severe keyhole collapse. Additionally, some vortices also deformed the keyhole rear wall and prevented the escape of bubbles in LAHW, leading to a greater porosity rate of 13.7 %. However, the impact of the droplet was weakened and vortices were reduced when an external magnetic field of 24 mT was used, which significantly decreased the porosity rate of LAHW to 2.0 %. This study will provide a more underlying understanding of magnetic-field-assisted welding and widen the industrial application of LAHW.

Numerical and experimental study of thermal fluid flow and keyhole dynamic in laser welding of aluminum alloy assisted by electromagnetic field

A steady electromagnetic field with high magnetic flux density was designed to improve weld appearance and suppress porosity defect of aluminum alloy laser welded joint. A comprehensive numerical model considering the multiple reflection of laser beam, the magnetohydrodynamics (MHD), and the vapor shear stress on free surface of keyhole was established to reveal the effect of electromagnetic field on thermal fluid flow and keyhole dynamic. A sufficient Lorentz force (∼1.255 × 106 N/m3) is induced when magnetic flux density reaches to 500 mT. The fluids flow downward, forward, and upward in the upper, middle, and lower region are inhibited by the Lorentz force, respectively. Therefore, less heat exists in the bottom and more heat accumulates in the middle edge of the weld pool, leading to the smaller weld depth, larger length at half depth, and disappeared necking when an electromagnetic field is used. Moreover, the impact of vortex on the rear keyhole wall is weakened by the Lorentz force, which decreases the oscillation and breakup of keyhole. This work can help improve the understanding concerning influence of the electromagnetic field on thermal fluid flow behavior in laser welding and provide guidelines for the suppression of porosity and splash defects.

Numerical Simulation of Preheating Temperature on Molten Pool Dynamics in Laser Deep-Penetration Welding

In this paper, a heat-flow coupling model of laser welding at preheating temperature was established by the FLUENT 19.0 software. The fluctuation of the keyhole wall and melt flow behavior in the molten pool under different preheating temperatures were analyzed, and the correlation between keyhole wall fluctuation and molten pool flow with spatters and bubbles was obtained. The results indicate that when the outer wall in the middle of the rear keyhole wall is convex, the inner wall is concave, which causes spatter or the bottom of the keyhole to collapse. When the metal layer in the middle of the rear keyhole wall turns into obliquely upward flow, welding spatter is generated. In contrast, the metal layer in the middle of the rear keyhole wall changes to flow into the keyhole, and the bottom of the keyhole collapses. When the preheating temperature is 300 K (ambient temperature), 400 K, and 500 K, the inner wall in the middle of the rear keyhole wall is concave. With the increase in the preheating temperature, the area of the concave gradually increases, and the size of the liquid column behind the keyhole opening gradually decreases. When the preheating temperature is 300 K, there are more spatters above the molten pool. In comparison, when the preheating temperature is 400 K or 500 K, there are less spatters, and the bottom of the keyhole collapses.

Effect of the laser-induced vapor in the keyhole on the weld surface roughness in fiber laser welding

Weld surface roughness is an important criterion for evaluating weld quality. The main factors that affect weld surface roughness in fiber laser keyhole welding are identified by an in situ observation of the dynamic behavior of a keyhole mouth and by analyzing the eruption characteristics of the laser-induced vapor on the front keyhole wall (FKW). In fiber laser keyhole welding, the weld surface roughness first decreases and then increases with the increase in welding speed. Both the distortion rate of the keyhole mouth and its range of fluctuation will increase as the welding speed increases. The fluctuation in the distortion rate of the keyhole mouth diameter can reflect the stability of fiber laser keyhole welding. The laser-induced vapor at the FKW can erupt to impact the rear keyhole wall, and this is the main factor that affects the distortion rate of keyhole mouth diameter and weld surface roughness in fiber laser keyhole welding. In fiber laser-arc hybrid welding or in CO2 laser keyhole welding, plasma can suppress the effect of laser-induced vapor impacting the keyhole and reduce the weld surface roughness.

Melt flow regularity and hump formation process during laser deep penetration welding

In this study, the relationship between melt flow and hump formation was directly revealed during laser deep penetration welding of transparentmaterial. In partial-penetration welding, the melt flowed obliquely upwards along the bottom of the keyhole to the rear surface of the molten pool. And its flow rate increased first and then decreased with the increase of the welding speed. There was a one-to-one correspondence between the humps formed on the weld surface and the fluctuation of the inclination angle of the front keyhole wall (FKW) near the Brewster angle. The maximum eruption rate of the laser-induced vapor on the FKW exceeded 30 m/s. In full-penetration welding, the melt flow direction changed and humping was suppressed. The laser-induced vapor on the FKW surface can act on the rear keyhole wall, and this is the main reason for melt flow. When the inclination angle of the FKW is close to the Brewster angle, the amount of laser-induced evaporation on the FKW will increase to form a high speed jet of vapor which will drive melt flow in the molten pool behind the keyhole, and this is the main reason for hump formation on the rear surface of the molten pool.

Simulation Study on Weld Formation in Full Penetration Laser + MIG Hybrid Welding of Copper Alloy.

Considering the coupling of a droplet, keyhole, and molten pool, a three-dimensional transient model for the full penetration laser + metal inert gas (MIG) hybrid welding of thin copper alloy plate was established, which is able to simulate the temperature and velocity fields, keyhole behavior, and generation of the welding defect. Based on the experimental and simulation results, the weld formation mechanism for the hybrid butt welding of a 2 mm-thick copper alloy plate was comparatively studied in terms of the fluid dynamic feature of the melt pool. For single laser welding, the dynamic behavior of liquid metal near the rear keyhole wall is complex, and the keyhole has a relatively drastic fluctuation. An obvious spattering phenomenon occurs at the workpiece backside. Meanwhile, the underfill (or undercut) defect is formed at both the top and bottom surfaces of the final weld bead, and the recoil pressure is identified as the main factor. In hybrid welding, a downward fluid flow is strengthened on the rear keyhole wall, and the stability of the keyhole root is enhanced greatly. There are large and small clockwise vortexes emerging in the upper and lower parts of the molten pool, respectively. A relatively stable metal bulge can be produced at the weld pool backside. The formation defects are suppressed effectively, increasing the reliability of full penetration butt welding of the thin copper alloy plate.

Correlation between the spatters and evaporation vapor on the front keyhole wall during fiber laser keyhole welding

Severe spattering is one of the common problems during fiber laser keyhole welding. In this study, the correlation between the spatter formation and laser-induced evaporation vapor on the front keyhole wall (FKW) was studied by changing the welding speed during fiber laser keyhole welding. The spatters were mainly generated in front of and behind the keyhole mouth, and the diameters of spatters were mainly between 50~100 μm at low welding speed, while the diameters of spatter particles were mainly between 100~150 μm at high welding speed. As the welding speed was increased, the tilt angle of the FKW decreased gradually, and the main spatter generation position shifted from in front of the keyhole mouth to behind the keyhole mouth. In low welding speed, the spatter velocity was higher than that in high welding speed. Further analysis shows that, the recoil pressure of laser-induced evaporation squeezed the molten metal layer in the FKW to generate spatters in front of the keyhole mouth in low welding speed. In high welding speed, the decrease of the molten metal layer in the FKW led to the decrease of spatters in front of the keyhole mouth; however, the laser-induced evaporation vapor from the FKW was more liable to impact the rear keyhole wall to generate spatters behind the keyhole mouth.

Study of molten pool dynamics and porosity formation mechanism in full penetration fiber laser welding of Al-alloy

To study molten pool dynamics, root hump formation, periodic keyhole behavior in full penetration laser welding (FPLW) of aluminum alloy, a numerical simulation was carried out, in which volume of fluid (VOF) method and ray-tracing algorithm were adopted. A varied metallic vapor shear stress model was considered. Meanwhile a series of welding experiments on specimens being composed of aluminum alloy and quartz glass were conducted to analyze the porosity formation process. It was found that the over-heated sagging with low surface tension stagnated at the bottom side and solidified to form root hump. The dominant backward heat convection at the lower part can well explain the extreme spreading of molten pool at bottom surface. Periodic behavior of the keyhole was identified during quasi-steady stage. The porosity can be effectively suppressed in FPLW. Both the porosity ratio and average porosity number were reduced obviously. The main mechanism of porosity formation in FPLW is the collapse of the rear keyhole wall, mainly caused by bulges on the front keyhole wall. Bubble coalescence is responsible for large porosity size and coalescence efficiency depends on bubble size difference.

A novel electrode-arc-weld pool model for studying the keyhole formation in the keyhole plasma arc welding process

A novel electrode-arc-weld pool model considering the influence of the backside keyhole deviation on the heat source distribution, the influence of the keyhole constraint on the arc pressure and the keyhole geometry dependent plasma shear stress, is developed to study the keyhole formation in the KPAW process. The numerical and experimental results show that: the keyhole depth increases very quickly at first, then smoothly, and then quickly again. In the blind keyhole stage, the arc pressure deepens the keyhole. The plasma shear stress facilitates the fluid flow along the rear keyhole wall and decreases the curvature of the keyhole. The process between the weld pool penetration and the keyhole penetration is defined as the blasting penetration stage, in which the molten bridge under the arc is quickly broken. The pressure balance and mass conservation mechanisms are proposed to be responsible for the blasting penetration. The low melting temperature, low surface tension and bottom surface deformation promote the blasting penetration, while the high thermal conductivity suppresses the blasting penetration. In the penetrated keyhole stage, two contrary convective eddies are found inside the weld pool. At the top surface of the weld pool, the molten metal flows upward and backward near the keyhole, but obviously flows forward at the rear part of the weld pool. At the bottom surface of the weld pool, the molten metal flows downward and backward.

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.

Numerical and experimental study on keyhole and melt flow dynamics during laser welding of aluminium alloys under subatmospheric pressures

Porosity defects was highly related to the keyhole and melt flow dynamic during laser welding process. In this paper, a novel 3D numerical model was developed to describe the keyhole dynamic and melt flow behaviors during laser welding of 5A06 aluminium alloy under subatmospheric pressures. The effect of ambient pressure on laser welding process was taken into consideration by optimizing the boiling point of aluminium alloy and recoil pressure of evaporated metallic vapor jets based on vapor–liquid equilibria calculation and Wilson equation. A moving hybrid heat source model was employed to describe the laser energy distribution under subatmospheric pressures. Numerical results indicated that a wider and deeper keyhole with less humps was produced under subatmospheric pressure comparing with that of atmospheric pressure. The vortices in the rear keyhole wall became unapparent or even disappeared with the decrease of ambient pressures. The melt flow velocity on the keyhole wall was larger under a lower pressure. A smaller difference between boiling point and melting point was produced and this led to the formation of a thinner keyhole wall and improved the stability of molten pool. Larger recoil pressure produced under subatmospheric pressure was responsible for the weakened vortices and enhanced melt flow velocity. Bigger keyhole opening size, larger melt flow velocity, thinner keyhole and the weakened vortices all resulted into the reduction of porosity defects during laser welding of aluminium alloys. Based on the simulation results, the plasma distribution, weld formation and porosity defects had been demonstrated. The compared results showed that the simulation results exhibited good agreements with the experimental ones.

Mechanisms for Improvement of Weld Appearance in Autogenous Fiber Laser Welding of Thick Stainless Steels

High-power fiber laser welding is an efficient and effective way to produce heavy section structures. However, there is a significant challenge in producing the welds with free of imperfections such as nail-head-shaped welds, spatters, and root sagging. This is partially due to a lack of understanding of the welding mechanism of high-power fiber laser. In this paper, we were especially interested in the mechanism to improve the appearance of welds, and we focused on the autogenous laser welding on thick stainless steel plates by a 10 kW fiber laser. To look into the relations of process parameters and the quality of welds, a high-speed imaging system was applied to observe the molten pool flow and vapor plume during the welding process. The appearances of welds subjected to different welding conditions were analyzed. The results showed that (1) nail-head-shaped welds were suppressed by using a gas jet during laser welding process. (2) In the forward welding, a gentle upwelling molten metal flow on the rear keyhole wall, a deeper weld pool and a weaker vapor plume resulted in no spatter. (3) The gravity affected the formation of underfills and root sagging significantly during autogenous laser welding of thick plates. (4) When the workpiece was placed vertically in the transverse position, the welding process was stable without an aggregation of molten melt at the back surface. Moreover, the mechanisms of forming root sagging and humps were different at the top surface.

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.

Experimental and numerical analysis of molten pool and keyhole profile during high-power deep-penetration laser welding

High power laser welding is as an ideal advanced technology for joining the thick steel plates with high efficiency in the industries. This paper proposes a 3D numerical simulation modeling method to investigate keyhole profile characteristics and the effect of keyhole evolution on the molten pool, and understand weld formation process and defect generation, including the welding phenomena such as swelling, column and spatter in the weld pool during high-power deep-penetration laser welding. To reveal the weld formation process, the gas layer, keyhole free surface, molten pool surface, and solid-liquid-vapor three-phase transformation are considered in the model. Based on the proposed model, the keyhole periodic variation profiles and molten pool evolution are calculated. The interaction between the keyhole and molten pool dynamic behavior and weld defects are also identified and discussed in details. The numerical simulation results are compared with the experimental and literature results and good agreements are achieved. It is found that the constriction and bulge formed on the rear keyhole wall is the key factor causing periodical change of keyhole inlet. The oscillating interface between the keyhole and molten pool is often accompanied by swellings and columns, which is the main factor for spatter formation.

Flow dynamics during single- and dual-spot laser welding with one common keyhole of 321 stainless steel

The keyhole is unstable with the presence of humps at the keyhole walls during single-spot laser welding (SSLW). The keyhole stability is improved and the presence of humps is reduced during dual-spot laser welding (DSLW). The vortices present at the rear keyhole wall during SSLW disappear during DSLW. There is an upward motion of the melt flow at the rear keyhole wall and the melt flow velocity is higher along the width of the molten pool during DSLW. A sandwich specimen is constructed to investigate the effect of keyhole and melt flow dynamics on the final shape of the weld seam. The results show that DSLW produces smoother, more regular weld beads due to higher stability of the keyhole and molten pool. DSLW produces a weld seam with fewer porosities and a more homogeneous nickel distribution due to the upward motion and absence of vortices in the melt flow. DSLW produces a wider molten pool due to the higher melt flow velocity along the width of the molten pool.

Understanding the effect of oxygen on weld pool and keyhole in laser beam welding

The deep penetration laser welding process involves complex physical phenomena and widely concerned areas. In this paper, a transient multiphase full coupled model rigorously incorporating the physical process and presenting the effect of oxygen on weld pool is proposed. The phenomena of inverse Bremsstrahlung absorption, scattering and plasma induced refraction are discussed to renew the heat source, which consists of incident laser energy and vapor plume. A series of CO2 and fiber laser welding experiments are executed to validate the model. The scattering phenomenon and the interaction between the scattering behavior and different oxygen environments are discussed. The effects of oxygen on the molten pool and keyhole are investigated with the proposed model. The investigation proves that oxygen presence will improve the laser energy focusing state by attenuating the Mie scattering phenomenon and laser's refraction, and suppress the flow intersection on the rear keyhole wall by changing the direction of Marangoni convection. The weld width will be reduced, weld penetration will be increased, and the porosity in the molten pool will be suppressed eventually.

Analysis of mechanisms of underfill in full penetration laser welding of thick stainless steel with a 10 kW fiber laser

With the aim to explore the formation mechanisms of surface underfill, full penetration laser welding of thick stainless steel was conducted, with the use of a 10kW fiber laser. A modified “sandwich” specimen was used, so as to directly observe the dynamic behaviors of the keyhole, vapor plume, and melt pool with the formation of underfills. On the basis of the experimental investigations, the formation mechanisms of the underfills at the top surface and bottom surface were analyzed. The results show that the downward flow of the molten metal caused by the recoil momentum is a crucial driver for formation of the underfill on the top surface. At full penetration of the melt, a deep underfill with a periodic wide–narrow–wide serrated pattern is formed on the top surface of the weld owing to the periodic fluctuation of the rear keyhole wall. At full penetration of the keyhole, the formation of a deep underfill on the top surface of the weld and undercut on the bottom surface of the weld is presented with massive direct melt loss from the weld pool.

高功率激光深熔焊接孔内气流与孔壁的耦合行为研究

激光深熔焊接过程中深熔小孔内金属蒸气与焊接熔池孔壁之间存在直接相互作用的气液流体动力学耦合行为，决定着焊接稳定性并影响最终的焊接质量。本文将不锈钢金属板材与玻璃组合夹紧制成异种材料对接接头，实现异种材料的激光深熔焊接。采用高速相机从玻璃一侧对小孔形态和孔壁波动特征进行观察研究，分析了不同焊接条件下蒸气与孔壁的相互力学耦合作用及其产生的规律。结果显示孔内蒸气流的作用促使小孔壁形成移动波，移动波在小孔开口处破碎时伴随着飞溅和液柱的形成。低速焊接时，小孔深、孔内蒸气波动大，后沿孔壁波动频繁。激光束离焦量的变化，使得小孔直径和孔内激光能量分布发生变化，导致孔内蒸气流的压力和流速发生变化，从而改变了孔内金属蒸气对孔壁的作用大小。熔透激光深熔焊接时，在气化反冲压力和蒸气流的驱动下，后沿孔壁上的液态金属主要为向上流，只有接近孔底的小部分金属熔液向下流。 The hydrodynamic coupling behavior of gas-liquid interaction between the metal vapor in the keyhole and the keyhole wall during deep penetration laser welding determines the welding sta-bility and affects the final welding quality. In this paper, stainless steel sheet metal and glass were clamped to make a dissimilar materials butt joint, and then the deep penetration laser welding of dissimilar materials was achieved. A high-speed camera was used to observe the characteristics of keyhole and the fluctuation of the keyhole wall from the glass side. The interaction behaviors between the vapor and the keyhole wall under different welding conditions were analyzed. The results showed that the effect of the vapor flow in the keyhole caused the keyhole wall to form a moving wave, which was accompanied by the formation of splash and liquid column when the moving wave crushed at the opening of the keyhole. During low speed welding process, the keyhole was deep, the keyhole vapor was fluctuating, and the keyhole wall was fluctuating frequently. The variation of laser beam defocusing distance effected the keyhole diameter and the laser energy distribution in the keyhole, and resulted in the change of the pressure and velocity of the vapor flow in the keyhole, thus changed the effect of the metal vapor on the hole wall in the keyhole. In deep penetration laser welding, the liquid metal on the rear keyhole wall was mainly moving upwards, which was driven by the gasification recoil pressure and vapor flow, and only a small part of the molten metal closing to the bottom of the keyhole was moving downwards.