Metal Transfer Behavior Research Articles

Arc and molten pool behavior on quality of hybrid cold metal transfer and tandem pulsed narrow gap gas metal arc welds: a study

ABSTRACT This research proposes a novel hybrid cold metal transfer (CMT) and tandem Pulsed gas metal arc welding (P-GMAW) process for narrow gap welding (NGW) of high strength low alloy (HSLA) steel plates. Investigating three sets of lead and trail wire feed speed (WFS) ratios [WFSL/T - 1.4, 1, 0.7], the study delves into the complex arc and metal transfer behavior within the narrow gap. Utilizing synchronized arc images and current-voltage waveforms, the research assesses their influence on fusion and mechanical properties. The root pass employs the single wire CMT process, while filling and closing passes leverage tandem P-GMAW, effectively minimizing incomplete fusion and reducing heat input. Results indicate that a lower WFSL/T (0.7) reduces the molten pool hump and overflow within the narrow gap while enabling direct weld arc engagement with the previously deposited layer, enhancing weld penetration. WFSL/T of 1.4 and 0.7 revealed higher and lower Charpy impact toughness.

Observation of Arc and Metal Transfer Behavior according to Shielding Gas in the WAAM of Ti–6Al–4V Alloy Using the Pulsed Gas Metal Arc Process

In arc welding and wire arc additive manufacturing (WAAM) of Ti alloys, pulsed gas metal arc (GMA) processes have a higher deposition than shortcircuiting GMA mode processes, such as cold metal transfer, surface tension transfer, and controlled short-circuit processes. In this study, pulsed GMA WAAM of Ti–6Al–4V alloy was conducted under Ar, Ar50%/He50% mixed, and He shielding gases. Owing to the thermionic emission of electrons from the Ti substrate, cathode jets were emitted from the high-temperature region of the weld pool, which interfered with droplet transfer into the weld pool. The arc shape surrounding the droplet varied according to the shielding gas, and the arc was established at the bottom of the hanging droplet under the He shielding gas, which disturbed droplet detachment. Two spatter generation modes of droplet ejection from the weld pool surface and inflight droplet repelling were observed, and droplet ejection was the most frequent spatter generation mechanism. The mixed shielding gas showed the best performance in terms of arc stability, wire melting, droplet transfer, and spatter suppression. The arc, cathode, and metal transfer characteristics were elucidated in this study, and a suitable gas composition for pulsed GMA WAAM of Ti alloys was proposed.

The influence of bypass current on arc energy density and metal transfer behaviour in bypass current compressed gas metal arc welding

A new welding method, bypass current compressed gas metal arc welding (BCC-GMAW), was developed to enhance the arc energy density and welding stability of GMAW. When the bypass current was applied, the arc energy density was increased with the constraining effect of the circumferential bypass electrode, which was studied by current density and arc pressure, resulting in an increase of 46% in weld depth and a decrease of 38% in weld width. As the coupled arc increased the melting speed of wire and forces of droplet downward transfer, the droplet transfer frequency increased by 185%, and the metal transfer mode was changed from short-circuit transfer to stable small globule transfer, obtaining a smooth and uniform weld appearance.

Individual Effects of Alkali Element and Wire Structure on Metal Transfer Process in Argon Metal-Cored Arc Welding.

This study aimed to clarify the effect of wire structure and alkaline elements in wire composition on metal transfer behavior in metal-cored arc welding (MCAW). A comparison of metal transfer in pure argon gas was carried out using a solid wire (wire 1), a metal-cored wire without an alkaline element (wire 2), and another metal-cored wire with 0.084 mass% of sodium (wire 3). The experiments were conducted under 280 and 320 A welding currents, observed by high-speed imaging techniques equipped with laser assistance and bandpass filters. At 280 A, wire 1 showed a streaming transfer mode, while the others showed a projected one. When the current was 320 A, the metal transfer of wire 2 changed to streaming, while wire 3 remained projected. As sodium has a lower ionization energy than iron, the mixing of sodium vapor into the iron plasma increases its electrical conductivity, raising the proportion of current flowing through metal vapor plasma. As a result, the current flows to the upper region of the molten metal on the wire tip, with the resulting electromagnetic force causing droplet detachment. Consequently, the metal transfer mode in wire 3 remained projected. Furthermore, weld bead formation is the best for wire 3.

Process Stability and Material Properties of TC4 Alloy Welded by Bypass Current Hot Wire Plasma Arc Welding (BC-PAW)

To improve welding efficiency, bypass current hot wire plasma arc welding (BC-PAW) was employed to weld Ti-6Al-4V alloy. The influence of process variables on metal transfer behaviour was explored using high-speed camera, and the material properties were investigated by means of different microscopes and mechanical test. The result shows that the weld seam has a good surface finish without welding defects. The main current, bypass current, and wire feeding speed have a significant influence on the metal transfer behaviour, which further influence the grains’ formation in the weld zone. The microstructure of the weld mainly comprises α martensite and the average tensile strength (UTS) of the as-received joint is 986 MPa, which is larger than that obtained using the conventional PAW method. The fracture occurs in the heat-affected zone (HAZ), which shows a typical ductile fracture surface.

Metal Transfer Behavior of Metal-Cored Arc Welding in Pure Argon Shielding Gas

The metal transfer behavior of gas metal arc welding in a pure argon shielding gas was evaluated through experiments using a standard solid wire and a metal-cored wire. The investigation was conducted using observation techniques based on recording images by a high-speed camera equipped with laser assistance and bandpass filters in a range of welding currents. It was observed that the metal transfer mode became a streaming transfer mode when the welding current increased in the solid wire. Meanwhile, in the metal-cored wire, the droplet transfer frequency increased, and the droplet diameter decreased without changing the metal transfer mode in the globular transfer mode. We surmised that the streaming transfer in the solid wire would be caused by the spread of argon plasma at the wire tip, which decreases the effect of the electromagnetic force on droplet detachment. Conversely, due to the presence of flux inside the metal-cored wire, the argon plasma could not spread and was attached close to the iron vapor plasma at the overhead of the droplet. Hence, the electromagnetic force acting on the side of the unmelted flux was ineffective at promoting droplet detachment, preventing the transition to a streaming transfer mode. Furthermore, weld bead formation in the metal-cored wire was better than that in a conventional solid wire.

Numerical simulation of arc and metal transfer behaviors in double-wire gas metal arc welding

Numerical simulation of arc and metal transfer behaviors in double-wire gas metal arc welding

A three-dimensional wire-feeding model for heat and metal transfer, fluid flow, and bead shape in wire plasma arc additive manufacturing

A three-dimensional wire-feeding model has been developed to study the transient coupling behaviour of heat and metal transfer, fluid flow, and solidified bead shape in wire plasma arc additive manufacturing (WPAAM). A novel surface heat source model considering the arc energy shading effect is proposed and adopted. An improved momentum source of the arc force considering the arc pressure shading effect is also developed and used. This model has been used to study the metal transfer dynamics, flow patterns, and bead shape of the WPAAM process with a wire-feeding speed (WFS) of 1–5 m/min. The simulated results agreed reasonably with the experimental data. As the WFS increased from 1 to 5 m/min, three different metal transfer modes were observed, which changed from globular droplet mode to droplet-liquid bridge mode and solid-liquid bridge mode. Detailed metal transfer information was analysed, including metal transfer position, shape, average temperature, and main driving force. The effects of the arc shading and metal transfer on the melt pool dynamics and bead shape were simulated and discussed. A periodic flow pattern of the melt pool produced by the metal transfer impact causes ripples or even humping defects. As the WFS increased, the melt pool depression gradually disappeared due to the arc pressure shading effect. When the WFS increased to 5 m/min, a temperature drop of about 140 K in the central melt pool, caused by the arc energy shading effect and cold metal transfer, weakened the lateral flow significantly, which explained the decrease of bead width at a large WFS. The results demonstrate that the developed wire-feeding model and findings could be used as a theoretical tool and basis to better understand the underlying physical mechanisms and achieve bead shape control in the WAAM process.

Dynamic formation characteristics and mechanism of hybrid laser arc welding surface layer by Ni-based filler metal based on rotating laser induction

This study used a nickel-based filler metal to fabricate a surfacing layer on a low-carbon steel substrate by a hybrid welding process with a rotating laser and arc. The surfacing layer's weld formation was studied using optical microscopic observation (OM). The microstructure of the layers was studied by using the scanning electron microscope (SEM), energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The arc, metal transfer, molten pool flow behaviors, and U–I characteristics during hybrid laser arc welding (HLAW) were analyzed using the arc analyzer and high-speed camera. The results show that a rotating laser improves the weld formation of gas metal arc welding (GMAW). The microstructure of the two surfacing layers consists of dendritic γ phase and a small amount of Laves and NbC phases. The high dilution rate of the HLAW surfacing layer results in relatively more Laves and fewer NbC phases. The arc length of HLAW is dynamically deflected with the rotation of the laser, which expands the arc action and heating area of the molten pool. The droplet transfer mode changed from globular transfer dominant in GMAW to the projected transfer dominant in HLAW. The location where the droplet transfer into the molten pool is dynamic changed, forming a multi-point impact on the molten pool. Combined with the deflected arc with the laser rotating laser, the HLAW has a broad and deep molten pool. Understanding the metal transfer behavior of Ni-based alloys in rotating laser-induced arc hybrid surfacing is of great significance for improving surfacing forming.

Metal transfer behavior during CMT-based Wire Arc Additive Manufacturing of Ti-6Al-4V alloy

CMT-Wire Arc Additive Manufacturing (CMT-WAAM) has the advantages of low heat input and high production efficiency. However, the surface geometric accuracy of the CMT- fabricated sample is inferior, especially for the deposition of titanium alloy. The focus of this study was to analyze the metal transfer behavior during CMT-WAAM of titanium alloys by a high-speed camera. The metal transfer and molten pool behaviors during the single-layer and multi-layer deposition in both CMT and CMT + P modes were compared to optimize the characteristic process parameters. The results showed that the hybrid metal transfer mode, including the short-circuit transition and the globular transition, occurred in the CMT + P mode. The effect of Ip1 on the droplet diameter was more significant than that of tp1. Under these two modes, the size and spread ability of the molten pool increased with the increase of Iboost, tp1 and Ip1. While in the CMT + P mode, Ip1 did not yield a significant influence compared with tp1. Simultaneously, noticeable oxide skin was observed on the surface of the flowing molten pool when Iboost reached 350 A in the CMT mode. Compared with the CMT mode, new spatter types were generated by arc force and droplet free-fall motion in the CMT + P mode. Ultimately, the maximum effective rate of 70.4 % was obtained in the CMT + P mode when the parameters were selected as Ip1 = 400 A and tp1 = 4 ms.

Effect of Flux Ratio on Droplet Transfer Behavior in Metal-Cored Arc Welding

The effect of flux ratio on metal transfer behavior during metal-cored arc welding was elucidated through investigation using a standard solid wire and three metal-cored wires with flux mass ratios of (2-2) 10%, 15%, and 20%. Investigation was performed using a shadowgraph technique based on images recorded with a high-speed camera equipped with back-laser illumination. We observed that the droplet transfer frequency increased with both the welding current and flux ratio, with the effect of flux ratio being more dominant at low currents. We surmise that this is because the wire sheath area decreases as the flux ratio is increased. Hence, when the welding current is the same, a reduction in the sheath area (i.e., an increase in flux content) leads to an increase in the current density in the sheath, which enlarges the electromagnetic force at the tip of the wire and aids droplet detachment. Conversely, Joule heating is higher at high welding currents than at low currents. This increased temperature shortens the flux column inside the wire, such that the current flow into the molten droplets is more uniform. Hence, the droplet transfer frequency does not increase significantly if the flux ratio is increased in the high current range.

Research on aluminum alloy welding process based on high frequency and low power pulsed Laser-MIG hybrid welding

Taking 2219 aluminum alloy as the welding research base metal, a high frequency and low power pulsed laser-MIG hybrid welding system was established. When the laser pulse frequency ranges from 500 Hz to 1500 Hz and the laser-wire distance ranges from 1 to 2 mm, the effects of high-frequency and low-power pulsed laser on the arc shape, metal transfer behavior, and weld formation in the hybrid welding process were compared and analyzed by using high-speed camera system and electrical signal acquisition system. The results show that as the pulse frequency increases, the arc is gradually compressed, and the compression reaches the maximum when the frequency is 1500 Hz. In the test, the limit distance of laser attracted arc is 2 mm and the limit distance of laser stabilized arc is 1.5 mm. From the collected electrical signal, it can be seen that the welding current is reduced by 5.8A after adding laser irradiation. Since the welding power source is a constant voltage source, the welding voltage is almost constant, so the heat input of the welding arc to the base metal and droplets is reduced to a certain extent. When the laser is biased laterally along the welding wire, the weld morphology also shifts from the center to the laser irradiation point, which further verifies the guiding effect of the laser on the arc. In addition, the addition of the laser effectively improves the stability of the welding process, the weld seam is formed uniformly, and the oscillation of the molten pool caused by the laser accelerates the discharge of gas in the molten metal, which effectively reduces the pore defects.

Effects of He content in shielding gases on high-efficient hybrid laser arc welding with C-276 filler metal

High-efficient welding technology with rotating arc assisted by laser under shielding gases with different He content was investigated for the application of 9% Ni steel in LNG storage. High-speed image and electrical signal acquisition were adopted to investigate arc stability and metal transfer behavior during hybrid laser arc welding. The results indicated that with increasing content of He, the weld formation became better first and then worse. Ar+30 %He improved the formation of welding seam. The He content had a significant influence on the electric signal waveforms. With the addition of 30 %He, the waveforms had the minimal fluctuation. It was rotating spray transfer with pure Ar, Ar+30 %He and Ar+50 %He. The difference was that with the He content increase, transfer mode shifted from unstable rotating spray transfer to stable rotating spray transfer, to irregular rotating spray transfer. With pure He, it was large droplet transfer, resulting in the formation of large spatters. The research of rotating spray transfer trajectory showed that the rotation velocity was relatively constant and one rotation time was 3.19 ms in one pulse period under Ar+30 %He. The trajectory equation was obtained for describing the rotating spray transfer. This indicated that the addition of 30 %He improved the coupling effect of laser and arc, enhanced the matching degree between rotating spray transfer and arc pulse period. It was meaningful to understand the effects of shielding gases on 9% Ni steel hybrid laser arc welding, providing theoretical basis for LNG storage efficient and reliable manufacturing.

Influence of metal transfer behavior under Ar and CO2 shielding gases on geometry and surface roughness of single and multilayer structures in GMAW-based wire arc additive manufacturing of mild steel

Wire arc additive manufacturing (WAAM) is advantageous for fabricating large-scale metallic components; however, a high geometric accuracy as that of other AM techniques cannot be achieved because of the deposition process with a large layer. This study focuses on the WAAM process based on gas metal arc welding (GMAW). To clarify the influence of shielding gas used to protect a molten metal during fabrication on the geometric accuracy of the built part obtained via the GMAW-based WAAM process, the influence of the metal transfer behavior on the geometry and surface roughness of the fabricated structures was investigated via visualization using a high-speed camera when single and multilayer depositions were performed under different heat inputs and gases. It is known that Ar gas is not suitable for welding steel because it cannot provide the desired arc stability and weld bead characteristics. The results reveal that the arc is stable in the multilayer deposition of the WAAM process, and the short-circuit of the metal transfer at the heat input of 1.17 kJ/cm enables smoothing of the fabricated surface with large irregularities. Better arc stability under Ar gas is achieved when the oxygen content of the fabricated surface is 22 wt%. Furthermore, the short-circuit between the metal droplet and the fabricated surface, where the molten pool is insufficiently formed, resulted in a hump formation. The results indicate that proper use of Ar gas can improve the surface quality when depositing on rough surfaces.

Metal transfer behaviour of CMT-based step-over deposition in fabricating slant features

The metal transfer behaviour of slant feature of thin-walls fabricated by cold metal transfer (CMT) process in step-over deposition mode was observed by high speed camera. A slanted short circuiting transfer (SSCT) was generated during deposition and then analysed by asymmetrical magnetic and force model, complicating metal transfer behaviour in comparison with conventional deposition process. The SSCT allowed the droplet to transfer obliquely into the molten pool on previous layer, owing to the pushing force generated by unevenly distributed electromagnetic field in asymmetrical deposition model. The surface tension between wire and liquid metal is the most important force that retarded droplet transfer. The retraction force and electromagnetic force are the main forces to promote droplet transition. The positive forces that promote droplet transfer process can be arranged as Fem > Fr > Fg. The wall width was mainly affected by wire feed speed and the inclined angle can be significantly increased with increasing step-over distance.

Effects of alkaline elements on the metal transfer behavior in metal cored arc welding

The aim of this study was to clarify the effects of adding a small amount of an alkaline element to the wire in metal cored arc welding (MCAW) on the metal transfer process. Sodium was selected as the alkaline element, and a standard wire without sodium (wire 1) and three wires with 0.028, 0.056, and 0.084 mass% sodium (wire 2, 3, and 4) were prototyped. They were observed using a high-speed video camera equipped with laser illumination, which clarified that the droplet transfer frequency tended to increase proportionally with the amount of sodium added. Subsequently, the line spectra of the iron atom (particularly the Fe I 537.1 nm line) and sodium atom (particularly the Na I 589.0 nm line) were primarily observed using a bandpass filter. Iron vapor evaporated from the droplet bottom, while sodium vapor mainly evaporated from the molten wire tip or the neck between the wire and the droplet because of its low boiling point. Because the sodium atom has a low ionization energy, we considered that a new current path of sodium plasma was directly formed from the tip or neck of the molten wire through the arc plasma to the molten pool, bypassing the inside of the droplet. The formation of this new current path was considered to reduce the current of the iron plasma flowing from the droplet bottom reducing the arc pressure and further enhance the electromagnetic force acting on the neck to promote droplet detachment, thereby increasing the metal transfer frequency.

Numerical simulation of coupled arc-droplet-weld pool behaviors during compound magnetic field assisted gas metal arc welding

A compound external magnetic field (EMF) can be used to prevent weld bead defects by controlling the metal transfer and arc behaviors during high-speed gas metal arc welding. A unified model containing the filler metal, arc, weld pool, and compound EMF was established to study the effect of the compound EMF on the welding process. The heat and mass transfer among the filler metal, arc, and weld pool under the action of the EMF was numerically analyzed. The simulation results showed that the high-temperature region in the molten pool was reduced and the temperature distribution was more uniform after the EMF was introduced. The speed of the forward liquid metal flow in the front of the molten pool increased due to the forward inclination of the filler metal and arc plasma, and a small metal bulge formed at the leading edge of the molten pool. Because of the periodic swing of the arc and the droplet, the transverse filling of the liquid metal strengthened. The generated additional electromagnetic force inside the weld pool was analyzed, and it had limited impact on the weld pool behavior.

Process control of pulsed laser enhanced metal transfer behavior in CO2 gas shielded welding

Spot pressure, as an arc repelling force, hinders the metal transfer process and has a negative effect on welding process stability in CO2 gas shielded welding. In addition, the direction of the spot pressure changes uncontrollably with the deflection of the droplet. Therefore, it is particularly critical to provide an additional force to improve droplet transfer characteristics without disturbing the arc stability. To this end, a new approach of the pulsed laser enhanced metal transfer (PLE-MT) process in CO2 gas shielded welding was proposed to control metal transfer behavior. The results demonstrate that: different from the uncontrollable deflection of droplets caused by laser incident on the solid–liquid interface (S-L-I) of the droplet, the irradiation of solid-state welding wire (S-W) can fundamentally solve the droplet deflection problem. The cut droplet can enter into the molten pool smoothly along the axis, and the change of the incident height does not significantly change the droplet's detachment velocity. The droplet’s detachment time at three different laser incident heights (0.5, 1, and 1.5 mm) remained almost unchanged, all of which were about 6–8 ms. When the wire feeding speed is 4 m/min and the voltage is 25 V, after laser irradiation is added, the metal transfer frequency increases from 5 to 25 Hz. The droplet transfer mode changes from SC transfer to spray transfer, and it avoids the short-circuit spatter. However, the metal evaporation mass produced by laser heating liquid droplet or welding wires is about 3.5% of the droplet mass. Under the condition of direct current electrode negative (DCEN), although the melting efficiency of the wire can be effectively improved, the droplet will be repelled until the arc is extinguished, and the stability of the arc will also deteriorate due to the rise of the arc. The irradiation of the pulsed laser can impede the climb of the droplet (cut off the droplet when its size is small before the arc climbs up), promote metal transfer, and improve the stability of the welding process in DCEN. While expanding the process interval of CO2 welding, an efficient spatter-free, low-pollution metal transfer process was obtained under a relatively small current through the PLE-CO2 welding process.

Arc Technological Characteristics and Metal Transfer Behavior of Twin Electrode GMAW Deposition

Experiments proved that the arc voltage influences its spatial form and electrode metal transfer behavior characteristics during twin electrode GMAW with a single power source. Two specific arc forms were revealed for two corresponding types of metal transfer. The V-shaped arc exists on the melt drop common to the two consumable wires at voltage rate 24-27 V. The columnar shaped arc is formed due to voltage increase up to 34-36 V, which results in increased mobility of the cathode spot in the weld pool surface. As a result, the arc travels between the ends of two electrode wires, and the metal is transferred in drops of small size. It was demonstrated that for the common drop formation the gas mixture of 82% Ar+18% CO2 is preferable to pure argon. It decreases the surface tension on the boundary between the melted electrode metal and the vapor-gas mixture, resulting in the increased volume of the common drop. It was found that a consistent common arc from two electrode wires decreases dilution is made up 43%, which is 1,65 times more and improves the deposited metal formation quality.

3D Numerical Study of External Axial Magnetic Field-Controlled High-Current GMAW Metal Transfer Behavior

For gas metal arc welding (GMAW), increasing the welding current is the most effective way to improve welding efficiency. However, much higher current decreases the welding quality as a result of metal rotating-spray transfer phenomena in the high-current GMAW process. In this work, the external axial magnetic field (EAMF) was applied to the high-current GMAW process to control the metal transfer and decrease the welding spatters. A unified arc-droplet coupled model for high-current GMAW using EAMFs was built to investigate the metal rotating-spray transfer behavior. The temperature fields, flow fields in the arc, and droplet were revealed. Considering all the heat transferred to the molten metal, the Joule heat was found to be the dominant factor affecting the droplet temperature rise, followed by the anode heat. The conductive heat from the arc contributed less than half the value of the other two. Considering the EAMFs of different alternating frequencies, the arc constricting effects and controlled metal transfer behaviors are discussed. The calculated results agree well with the experimental high-speed camera observations.