Weld Pool Flow Research Articles

Metal flow of weld pool and keyhole evolution in gas focusing plasma arc welding

Establishing an open keyhole throughout work-piece is the key to penetrate work-piece fully in gas focusing plasma arc welding. In order to study heat transfer, metal flow and keyhole surface evolution in gas focusing plasma arc welding, the three dimensional mathematical model is developed. Arc heat model and plasma arc pressure model are used to describe heat and force action in gas focusing plasma arc welding. Temperature field, fluid flow in weld pool and keyhole evolution are simulated. It is found that metal in weld pool front flows bypass keyhole wall toward weld pool back. Center line of keyhole channel is crocked instead of going along z-axis. The calculated weld shape agrees with the experimental data basically. This work may enrich theoretical knowledge about the complicated thermal physical process and provide basic data to guide welding application in gas focusing plasma arc welding.

Study on Short-Circuiting GMAW Pool Behavior and Microstructure of the Weld with Different Waveform Control Methods

In order to study internal relation among the behavior of the weld pool, the microstructure of weld bead and the waveform of short-circuiting gas metal arc welding (S-GMAW), a high speed photograph-images analysis system was formed to extract characteristics of weld pool behavior. Three representative waveform control methods were used to provide partly and fully penetrated weld pools and beads. It was found that the behavior of the weld pool was related to the instantaneous power density of the liquid bridge at the break-up time. Weld pool oscillation was triggered by the explosion of the liquid bridge, the natural oscillation frequencies were derived by the continuous wavelet transform. The change of weld pool state caused the transition of oscillation mode, and it led to different nature oscillation frequencies between partial and full penetration. Slags flow pattern could be an indication of the weld pool flow. Compared with the scattered slags on fully penetrated weld pool, slag particles accumulated on partially penetrated weld pools. The oscillating promoted the convection of the welding pool and resulted in larger melting width and depth, the grain size, and the content of pro-eutectoid ferrite in the weld microstructure of S235JR increased, the content of acicular ferrite decreased.

Experimental investigation of weld pool flow under external DC magnetic field

Experimental investigation of weld pool flow under external DC magnetic field

Mitigation of porosity defects in fiber laser welding under low vacuum

Abstract Laser welding trials were conducted under low vacuum to reduce porosity defects. The effect of vacuum on the characteristics of porosity was quantitatively evaluated using 3D micro-CT. To explore the mechanisms of porosity formation at atmospheric pressure and porosity defects mitigation under vacuum, the weld pool and fluid flow behaviors were directly observed by placing a transparent glass plate on one side of the weld pool. The results showed that the vacuum condition was favorable for reducing porosity, especially process porosity. It was also found that the weld pool size and fluid flow were totally different at atmospheric pressure and under vacuum. At atmospheric pressure, bubbles were trapped in the vortex induced by two opposing fluid flows, failed to escape from the weld pool and finally formed pores. Under vacuum, suppression of vapor plume and reduction in the boiling point led to a deeper keyhole and a thinner weld pool, resulting in an upward flow in the rear weld pool. In this case, the bubbles could escape easily from the weld pool with the help of the upward flow.

Fabrication of curved overhanging thin-walled structure with robotic wire and arc additive manufacturing (RWAAM)

PurposeRobotic wire and arc additive manufacturing (RWAAM) is becoming more and more popular for its capability of fabricating metallic parts with complicated structure. To unlock the potential of 6-DOF industrial robots and improve the power of additive manufacturing, this paper aims to present a method to fabricate curved overhanging thin-walled parts free from turn table and support structures.Design/methodology/approachFive groups of straight inclined thin-walled parts with different angles were fabricated with the torch aligned with the inclination angle using RWAAM, and the angle precision was verified by recording the growth of each layer in both horizontal and vertical directions; furthermore, the experimental phenomena was explained with the force model of the molten pool and the forming characteristics was investigated. Based on the results above, an algorithm for fabricating curved overhanging thin-walled part was presented and validated.FindingsThe force model and forming characteristics during the RWAAM process were investigated. Based on the result, the influence of the torch orientation on the weld pool flow was used to control the pool flow, then a practical algorithm for fabricating curved overhanging thin-walled part was proposed and validated.Originality/valueRegarding the fabrication of curved overhanging thin-walled parts, given the influences of the torch angles on the deposited morphology, porosity formation rate and weld pool flow, the flexibility of 6-DOF industrial robot was fully used to realize instant adjustment of the torch angle. In this paper, the deposition point and torch orientation of each layer of a robotic fabrication path was determined by the contour equation of the curve surface. By adjusting the torch angle, the pool flow was controlled and better forming quality was acquired.

Investigation of humping defect formation in a lap joint at a high-speed hybrid laser-GMA welding

Abstract Productivity rise can be realized at high-speed welding, but in a practice, the constraints emanate from the tolerance welding speed limit beyond which generate humping defects and the likes. These defects are undesirable in welding as they undermined the weld bead integrity and decrease productivity. In this study, an investigation on a lap joint welding using a hybrid Laser-GMAW (L-GMAW) welding process on a carbon steel sheet was undertaken to examine the mechanisms involved in the arc behavior, the droplet transfer, and the weld pool flow dynamics in a hump defect formation at high-speed welding. A variation of the process parameters in the hybrid welding system such as the wire feed speed (WFS), the traveling speed (TS) and the oscillation of the laser beam were used as input signals. A high-speed video camera was used with different viewing positions to capture the sequence of events during the hybrid welding process. The high-speed images of the arc behavior, the weld pool flow pattern with the use of tracer particles in arriving at the root mechanisms were acquired. Based on the experimental resultant, the occurrence mechanisms of the weld defects were proposed. In this study, the key factors in the humping defect formation are the two dominant lateral flows toward the top and bottom sheets, the collision between the weld pool back- and rear-flow and the interruption of the weld pool flow channel. Also, the generation of copious spatter due to the two lateral flows and the swollen buildup towards the hump closure were responsible for the next hump initiation formation. The beam oscillation applied to wet the weld toe angle to delay the outset of the humping defect occurrence was insignificant at higher travel speed. Furthermore, the force of gravity and surface tension were the key forces in this study.

Effect of depositing torch angle on the first layer of wire arc additive manufacture using cold metal transfer (CMT)

Purpose This paper aims to mainly report the impact of torch angle on the dynamic behavior of the weld pool which is recorded and monitored in real time with the aid of a high-speed camera system. The influence of depositing torch angle on the fluctuation behavior of weld pool and the quality of weld formation are compared and analyzed. Design/methodology/approach The FANUC controlled robotic manufacturing system comprised a Fronius cold metal transfer (CMT) Advanced 4000R power source, FANUC robot, water cooling system, wire feeding system and a gas shielding system. An infrared laser was used to illuminate the weld pool for high-speed imaging at 1,000 frames per second with CR600X2 high-speed camera. The high-speed camera was set up a 35 ° angle with the deposition direction to investigate the weld pool flow patterns derived from high-speed video and the effect of torch angles on the first layer of wire additive manufacture-CMT. Findings The experimental results demonstrated that different torch angles significantly influence on the deposited morphology, porosity formation rate and weld pool flow. Originality/value With regard to the first layer of wire arc additive manufacture of aluminum alloys, the change of torch angle is critical. It is clear that different torch angles significantly influence on the weld morphology, porosity formation and weld pool flow. Furthermore, under different torch angles, the deposited beads will produce different defects. To get well deposited beads, 0-10° torch could be made away from the vertical position of the deposition direction, in which the formation of deposited beads were well and less porosity and other defects.

Effect of welding parameters on microstructure and mechanical properties of laser welded Al-Si coated 22MnB5 hot stamping steel

Laser lap welding of 22MnB5 with different laser power, welding speed and coating condition were presented. The effect of the welding parameters on microstructure and mechanical properties of the joint was investigated. The fusion zone was composed of lath martensite. Based on the location, the heat affected zone (HAZ) was divided into three regions: inner HAZ was composed of martensite, middle HAZ was composed of martensite and ferrite and outer HAZ was composed of tempered martensite which was the most soften area of the joint. Al-Fe intermetallic phases formed along the fusion boundary due to the agglomerating of the mixture of coating and base metal. The tensile shear load of the weld in coated condition was lower than that in non-coated condition at the same welding parameter due to the Al-Si coating agglomerating along the fusion boundary deteriorating the mechanical properties of the joint. Regardless of the excessive heat input, failure occurred directly at the fusion boundary with a quasi-cleavage pattern in coated condition but took place at the interface with a dimple pattern in non-coated condition. The tensile shear load was increased by increasing laser power or decreasing welding speed in the experimental parameters except the excessive heat input. For the diffusion of Al can be enhanced by the increased flow of weld pool, which results from wider bead width associated with higher laser power and lower welding speed.

Effect of keyhole mode on weld shape and mechanical properties of thin sheets of Al-Si coated 22MnB5 steel

The great change of plasma or plumes and weld pool flows existed between “blind” keyhole and open keyhole, especially for thin sheets. The keyhole mode could be changed by increasing the power density for deeper keyhole penetration. In this study, weld shape, aluminum content, δ ferrite fraction, and tensile strength were investigated in laser welding Al-Si coated steel. “Blind” keyhole led to “Y” weld shape, lower aluminum content, less δ ferrite, and higher strength at the optimized parameter. On the contrary, open keyhole led to “X” weld shape, higher aluminum content, more δ ferrites, and lower strength. The microstructure was more homogeneous in low speed “Y” weld. From the high speed images, the open keyhole was accompanied with fierce molten flow on the bottom side. The bottom coating was also important in the aluminum increase. In the suitable combination of welding parameters for “blind” keyhole, their strength could reach the level of decoated sample with no need for decoating preparation.

Microstructure and alloy element distribution of dissimilar joint 316L and EH36 in laser welding

ABSTRACTMicrostructure and alloy element distribution of dissimilar joint 316L and EH36 in laser welding were researched, especially the influence of misalignment on weld profile and alloy element distribution. Weld zone (WZ) consisted of austensite and ferrite (F), and heat affected zone (HAZ) of EH36 side consisted of martensite, F, pearlite and carbide precipitation. Positive misalignment increased both upper and bottom widths (BWs), while negative misalignment made BW wider and upper width narrower. HAZ in steel EH36 side was clear and was narrowed and homogenised by negative misalignment. Alloy element contents of Cr and Ni in WZ was much more in negative misalignment. The interface transition gradient of alloy element between 316L and WZ was sharpened in the +0.8 mm misalignment, but diminished in the −0.8 mm misalignment. The influence of misalignment on alloy element distributions can be attributed to the effect of gravity on transient flow of weld pool.

Erratum to: Mechanisms of weld pool flow and slag formation location in cold metal transfer (CMT) gas metal arc welding (GMAW)

This work investigates the weld pool flow behavior by observing the flow pattern of floating slag particles for cold metal transfer (CMT) gas metal arc welding (GMAW) using three different welding wires. These wires contain different amount of deoxidizers in the form of silicon and manganese. In situ high-speed videography shows that for the wire containing a high amount of deoxidizer, the weld pool flows from the center towards the toe. With the presence of a lower amount of deoxidizers, it flows from the toe towards the center. Besides, a higher amount of CO2 in the shielding gas changes the amount and location of slag formation. Chemical composition analysis of weld metals relates the weld pool flow behavior with the amount of dissolved oxygen (surface active element). The presence of adequate amount of oxygen in the weld pool can alter the weld pool flow pattern, which changes with the amount of deoxidizers present in the materials and the amount of oxygen supplied through the shielding gas. Based on this concept, the mechanisms involved in weld pool flow pattern and slag formation location for different composition of the consumables are disclosed to improve weld quality and productivity through the selection of the proper welding consumables in CMT-GMAW.

Mechanisms of weld pool flow and slag formation location in cold metal transfer (CMT) gas metal arc welding (GMAW)

Mechanisms of weld pool flow and slag formation location in cold metal transfer (CMT) gas metal arc welding (GMAW)

Investigation of heat transfer and fluid flow in activating TIG welding by numerical modeling

Heat transfer and fluid flow of arc plasma and weld pool in tungsten inert gas (TIG) welding and activated flux tungsten inert gas (A-TIG) welding of SUS 304 stainless steel are investigated comparatively though a 3D unified model. The model differs from the previous ones in that it considers the arc length more realistic for welding production. Tungsten electrode, anode (work piece) and arc plasma are all included. The effects of buoyance, plasma drag force, Lorentz force and Marangoni force on the weld pool flow are taken into account. By solving the conservation equations of mass, momentum, energy as well as Maxwell equations, the distributions of temperature and velocity of arc plasma and weld pool are obtained for TIG and A-TIG welding. The heat flux, current density and shear stress at the weld pool are presented. Dimensionless numbers are employed to compare the relative importance of the driven forces and that of convection and conduction in heat transfer of the weld pool. It is demonstrated that there is no significant difference in the heat flux at the weld pool, and total heat input to the anode and thermal efficiency is almost equal for TIG and A-TIG welding. The current density and the heat flux at the weld pool are more concentrated in more realistic welding condition. As a result, both of the temperature of the weld pool for TIG welding and A-TIG welding increases, while the latter is more significant. Marangoni force ranges from zero to 100Pa and dominant the weld pool flow. Compared with the conventional TIG welding, the reversion of the Marangoni force results in inward flow and thus causes inward heat convection in weld pool of A-TIG welding. Heat convection was the main mechanism of heat transfer for SUS 304 stainless steel, which results in directly both shallow weld pool shape in TIG welding and remarkably increased weld pool depth and slightly constricted weld pool width in A-TIG welding.

Suppression of humping bead in high speed GMAW with external magnetic field

In high speed gas metal arc welding, the higher momentum of backward flowing molten jet in a weld pool contributes to the occurrence of humping bead. In this study, the external magnetic field is exerted to control the fluid flow and its velocity distribution inside the weld pool to suppress the humping bead. The effects of the external magnetic field on the weld pool flow dynamics, arc column and the metal transfer are experimentally investigated. The required excitation current levels to suppress humping bead under different welding conditions are obtained. It is found that the deflection of arc column and liquid metal stream at wire tip can be offset by a suitable backward inclination angle of torch. With appropriate combination of external magnetic flux density and backward inclination angle of torch, a good quality weld bead without humping and spatter can be obtained in high speed GMAW.

NUMERICAL ANALYSIS OF FLUID FLOW IN LASER+GMAW HYBRID WELDING

Laser + gas metal arc welding(GMAW) hybrid welding fully combines the merits of both laser welding and GMAW, which can achieve high quality, high efficiency and comparatively low-cost welding of thin and thick plate, thus having great application prospect in manufacturing industry. However, compared with single heat source welding, hybrid welding involves more welding parameters and more complicated physical process,leading to difficult process optimization. When mismatching the process parameters, welding defects can still appear in high-speed welding, which affects the reliability of hybrid welding. Therefore, it is necessary to study the physical mechanism in hybrid welding deeply for suppressing welding defects and improving welding stability. In hybrid welding, fluid flow in weld pool has a critical influence on the weld formation. So, modeling and simulating the fluid flow is helpful for understanding the process mechanism completely. To date, however, there is only little study on velocity field in hybrid welding due to its complexity. In this work, with considering the effects of droplet and keyhole on weld pool, a three dimensional transient model is developed to numerically analyze fluid flow in weld pool of laser+GMAW hybrid welding based on FLUENT software. Arc heat input is modeled using an doubleellipsoid heat source; laser heat input is regarded as a hyperbolic curve- rotated heat source with changing peak power density, its distribution parameters being determined based on the simplified model for keyhole geometry and size. Droplet transfer is described as the process of high temperature liquid metal flowing into weld pool from the certain domain above the weld pool. Using the built model, the keyhole behavior, fluid flow and temperature distribution in laser+GMAW hybrid welding under different welding conditions are calculated. The features of velocity field in hybrid welding are analyzed and the effect of laser power on the weld pool dynamic behavior is discussed. The results show that, in the case of 1 m/min, weld bead hump is generated in single GMAW(laser power0 W); when laser power is 500 W, bead hump disappears in welding, but there is no keyhole emerging in hybrid weld pool and fluid flow pattern is close to that in GMAW. When increasing laser power to 2000 W, keyhole is formed, which makes the fluid flow in weld pool more complicated. The predicted weld geometries and dimensions for varied laser powers are compared with the measured data, which are in good agreement, thereby indicating accuracy and applicability of the established model.

Effect of plasma current on surface defects of plasma-MIG welding in cryogenic aluminum alloys

An asymmetric undercut and black-colored deposit (smut) are created on the surface of Plasma-MIG Hybrid (PMH) weld in Al alloys. For the purpose of defect formation control, the effect of plasma current on surface defects in the PMH weld was investigated through arc phenomena with high speed imaging and metallurgy analysis as compared with MIG weld. It was found that the asymmetric undercut is created near the left fusion line of PMH welds by the electromagnetic stirring force induced by high plasma currents over 200A. As the plasma current increased, three types of smut appear in the welding bead: Smut outside the welding Bead (SOB) on the MIG weld, Smut spots on the welding bead (SS) and Smut inside the welding Bead (SIB) on the PMH weld. Three types of smut formed changed from SOB, to SS, and finally to SIB. The smut formation area moved from the base metal onto the welding bead. The amount of smut formed on the PMH weld is less than that of MIG weld. Smut on MIG and PMH weld have an identical chemical composition including aluminum, magnesium, oxygen and carbon from the base metal and the weld metal. The carbon is from a graphite lubricant on the surface of Al 5183 wire. SOB is generated by the arc generated between the wire and edge of the weld pool after short circuit transfer, as well as droplet explosion transfer depending on MIG current. SS is formed by the combination of graphite layers remained on the droplet and the weld pool. SIB is generated by a combination of the MIG welding wire feeding point, the asymmetric undercut and metal vapor from a droplet explosion transfer under a strong plasma forces induced by a plasma arc with plasma currents over 200A. Consequently, the plasma current influences the weld pool flow and type of smut by changing the metal transfer mode and strong plasma arc forces.

Experimental characterization of the weld pool flow in a TIG configuration

Tungsten Inert Gas (TIG) welding process relies on heat transfer between plasma and work piece leading to a metallic weld pool. Combination of different forces produces movements on the molten pool surface. One of our aims is to determine the velocity on the weld pool surface. This provides a set of data that leads to a deeper comprehension of the flow behavior and allows us to validate numerical models used to study TIG parameters. In this paper, two diagnostic methods developed with high speed imaging for the determination of velocity of an AISI 304L stainless steel molten pool are presented. Application of the two methods to a metallic weld pool under helium with a current intensity of 100 A provides velocity values around 0.70 m/s which are in good agreement with literature works.

Effect of external magnetic field on weld pool flow conditions in high-speed gas metal arc welding

In high-speed gas metal arc welding process, weld defects like humping bead and undercutting appear and the weld bead quality worsens when welding speed exceeds a critical value. The previous investigations have proved that the backward flowing molten jet with high momentum in the weld pool is responsible for the formation of humping bead in high-speed gas metal arc welding. In this study, an external electromagnetic field generator is developed to change and control the flow condition in the weld pool and suppress the occurrence tendency of the humping bead. Bead-on-plate welding experiments are performed on mild steel Q235 test-pieces, and the influences of the exerted magnetic strength on the arc behavior, the fluid flow inside the weld pool, the weld bead shape and the weld penetration and width are investigated. The reason why the external electromagnetic field can suppress the humping bead is explained. The results show that external magnetic field can remarkably adjust the weld pool fluid flow field and decrease the momentum of the backward flowing molten jet. Therefore, weld defects like humping bead and undercutting can be effectively suppressed, the quality of weld bead is remarkably improved, and the critical welding speed is notably increased.

Investigation of keyhole plasma during 10 kW high power fiber laser welding

During high power fiber laser deep penetration welding, the laser induced plasma/vapor has a great effect on the weld pool flow and keyhole stability. In this paper, experiments on 10 kW fiber laser welding of stainless steel have been carried out. A long keyhole and keyhole plasma have been observed. The distinct spectral lines of the keyhole plasma induced by the high power fiber laser were measured and used to calculate the parameters of the plasma. The pressure of the plasma was investigated experimentally. The welding images showed a clear long keyhole and uneven distribution of the keyhole plasma. The data demonstrated that, compared with the results during CO2 laser welding, the electron temperature was lower during fiber laser welding, while the electron density remained at a similar level. The ionization degree was found to be low. The pressure of the plasma was found to be much higher than atmospheric pressure. The results revealed that the low fiber laser absorptivity of the plasma and the high atomic density of the vapor induced by the high power fiber laser were the factors that dominated the values of the plasma parameters. In particular, the high pressure plasma was ascribed to the high atomic density of the metallic vapor.

A three dimensional transient model for heat transfer and fluid flow of weld pool during electron beam freeform fabrication of Ti-6-Al-4-V alloy

A three-dimensional transient model is proposed to understand the heat transfer and fluid flow behaviors of weld pool during single track electron beam freeform fabrication (EBF3) of Ti-6-Al-4-V alloy. Fluid flows driven by recoil pressure, surface tension and thermal capillary effect are coupling considered in the model. Free surface evolutions of weld pool are incorporated with Level Set method. An improved heat source model is also proposed to simulate the energy interactions between high energy electrons and deposits. Results indicate that there are vigorous fluid flows in the front part of weld pool and thermal-capillary force is the main driven force of fluid flow. Effect of droplet impacting could periodically influence the heat transfer and fluid flow of weld pool. High frequency elliptical scanning of electron beam could decrease the temperature gradient and the magnitude of fluid flow of weld pool. However, beam scanning does not essentially change the flow patterns. A good agreement is obtained between the simulations and experiments.