Droplet Transfer Behavior Research Articles

The Influence of Coaxial Ultrasound on the Droplet Transfer of High Nitrogen Steel GMAW Process

The nitrogen bubble bursting phenomenon during the welding process of high nitrogen steel (HNS) can lead to unstable droplet transfer and welding process, reducing the quality of weld formation. In this study, a novel approach, ultrasonic-assisted gas metal arc welding (U-GMAW), is proposed to suppress the escape of nitrogen gas during droplet transfer. This study investigates the influence of ultrasound on the metal transfer process during two distinct metal transfer modes: short-circuiting and droplet transfer. Ultrasound has a significant effect on the welding process; as ultrasonic power increases, both the arc length and droplet size decrease, while the droplet transfer frequency increases and the electrical signal stabilizes. Under the experimental conditions of this study, ultrasound has the most effective improvement on the metal transfer behavior when the ultrasonic power reaches 2 kW. Ultrasound enhances the stability of the droplet transfer process, making U-GMAW an effective and novel approach for controlling the droplet transfer behavior of high nitrogen steel.

A Novel Preparation Approach of Aluminum-Based Functionally Graded Material: Auxiliary Wire Filling Gas Metal Arc Directed Energy Deposition

The 2319-1100 aluminum-based functionally graded material (FGM) with continuous gradient transition interface was fabricated by auxiliary wire filling gas metal arc directed energy deposition (AWF-GMA-DED) process via changing the feeding speed of the auxiliary wire ER1100. The droplet transfer behavior, deposition morphology, microstructure and mechanical property of the FGM wall were analyzed. Results indicate that with increasing feeding speed of ER1100, the droplet transfer mode of the auxiliary wire changes in the manner of large droplet transfer—bridge transfer—continuous transfer, while the droplet transfer of the main wire ER2319 is always stable. As the filling ratio of ER1100 increases, the area of interlayer equiaxed grain band and equiaxed grain size increases gradually, the continuity of eutectic structure at the grain boundary is weakened, and the eutectic structure is refined. When the ER1100 filling ratio increases from 10% to 35%, the microhardness decreases by 13.4%, the tensile strength 7.8%, whereas the elongation and corrosion resistance of the FGM part both improve.

Arc behavior and droplet transfer characteristics during oscillating laser-arc hybrid welding of 2024 Al alloy based on Zr-core-Al-shell wire

A novel zirconium (Zr)-core-aluminum (Al)-shell wire (ZCASW) combined with oscillating laser-arc hybrid welding (O-LAHW) process was employed for the welding of aluminum alloy 2024 (AA2024). The arc stability and characteristics, droplet transfer behavior and weld formation were systematically investigated. The results show that electrical signal waveform determines wire feeding rate, which affects the welding stability and weld quality. The increment in wire feeding rate increases magnetic field intensity and narrows arc cross-sectional area, which leads to arc contraction. The oscillating laser increases arc width and arc area, thereby expanding the conductive channel and greatly improving conductive ability of hybrid plasma, which promotes interactions between plasmas and helps to droplet transfer. Meanwhile, owing to the melting points differences between Zr wire and Al wire in ZCASW filler material, the melting rate of outer Al wire is larger than that of central Zr wire during welding, thereby resulting in two kinds of droplet transfer modes, and electromagnetic force and metal vapor reaction force strongly influence droplet transfer behavior. The oscillating laser improves the weld quality, which is deteriorated due to the appearance of spatters as wire feeding rate increases. When 1 m/min wire feeding rate is selected, a perfect formation is obtained. The current study offers insight into the formation mechanism of high-quality weld and welding stability during O-LAHW of AA2024 combined with ZCASW filler material.

Study on droplet transfer behavior and spattering mechanism in Mg-Gd-Y-Zr alloy regulated short circuit (Fronius CMT) welding

The quality, performance, and morphology of Mg-Gd-Y-Zr alloy regulated short circuit (Fronius CMT) welding components is directly depend on whether the droplet transfer process is stable. However, due to the inherent properties of the material, Mg-Gd-Y-Zr alloy is highly sensitive to heat input, and prone to spattering in the process of droplet transfer. In this work, three types of spattering were observed through high-speed cameras, which are caused by internal burst of droplet, wire withdrawal, and repelled transfer. Causal relationship of spattering mechanism and process parameter was explored especially in the stage of wire melting and droplet growth, and a simplified numerical model was used as an auxiliary means to further reveal the spattering of repelled transfer. The result indicated that the recoil pressure of metal vapor is the vital factor which destabilize the shape and motion trajectory of droplet and eventually caused spattering. The range of the process parameter for achieving a stable droplet transfer and well-formed weld bead is relative narrow which the peak current is recommended between 150 A to 170 A, and peak duration around 7 ms.

Effect of distance between heat sources on droplet transfer behavior and weld formation of AH36 during laser and CWW GMAW arc hybrid welding

Effect of distance between heat sources on droplet transfer behavior and weld formation of AH36 during laser and CWW GMAW arc hybrid welding

Investigating arc and molten metal transport phenomena in gas metal arc welding with Ar–CO2 gas mixtures using a numerical method

This paper presents a numerical investigation of the transient transport phenomena of the arc and molten metal during gas metal arc welding (GMAW) using shielding gas mixtures ranging from 100% Ar + 0% CO2 to 80% Ar + 20% CO2. The thermophysical parameters of the Ar–CO2 mixtures, considering the presence of metal vapor, were calculated as a function for a temperature range of 1000–30 000 K. The influence of metal vapor content and CO2 proportion on the thermophysical properties of the mixed gas was discussed in detail. As the CO2 content increased from 0 to 20%, the shape of the arc changed from a bell to a cone due to the increase in mass density, specific heat, and thermal conductivity. The maximum arc temperature and velocity decreased with increasing CO2 content, resulting in larger droplets and a lower droplet transfer frequency. Although the change in electrical conductivity did not affect the arc shape, it did influence the arc temperature by altering the distribution of current density. Experiments of droplet transfer and arc behavior were carried out, and the results showed that the simulated droplet size, transfer frequency, and arc temperature distribution agreed well with the experimental values. These findings could serve as a theoretical tool for better understanding the underlying physical mechanisms of the GMAW process using different shielding gases, ultimately aiming to achieve high weld quality.

Improved formation accuracy and mechanical properties of laser-arc hybrid additive manufactured aluminum alloy through beam oscillation

High-frequency circular beam oscillation was employed for the laser-arc hybrid additive manufacturing of AA4047 aluminum alloy. Under the optimized oscillating frequency of 300 Hz, the effects of oscillating diameter (D) on the forming accuracy, microstructure and mechanical properties of deposited thin-walls were studied. It was found that the effective forming width reached a maximum of 88.9 %, and the porosity decreased from 0.1 % to 0.01 % when the D increased from 0 to 1.2 mm. Furthermore, the machining allowance was reduced from 1.51 to 1.25 mm, which indicated that the formation accuracy was improved by 17 %. The increasing D reduced the average size of grains from 55.9 to 44.8 μm, showing a refinement degree of 19.9 %. These improvements reduced the microhardness distribution variance of thin-wall from 31.82 to 13.37. The ultimate tensile strength increased from 219.1 to 227.3 MPa, and the elongation increased from 12.9 % to 14.6 %. Based on the high-speed camera observation, the study discussed the influence of laser beam oscillation on the droplet transfer and melt flow behavior, as well as the improvement mechanism of deposition quality.

Effect of energy parameters on droplet transfer behavior and weld formation in laser-arc hybrid welding with cable-type welding wire

This paper describes the droplet transfer behavior and weld formation of laser-arc hybrid welding with cable-type welding wire (CWW). The droplet transfer behavior was photographed in real time by using a high-speed camera. The effects of energy parameters on arc morphology, droplet transfer behavior and weld formation were studied through experimental and theoretical analyses. The results show that the arc is deflected toward the laser direction, and the deflection degree increases with increasing laser power. The interaction between the laser and arc is enhanced by rotating the arc. The rotating arc enhances the interaction between plasma, the arc is more compressed and the arc volume is smaller as the welding current increases. The droplet transfer frequency increased with increasing laser power and decreased after reaching the critical value. The reaction force of metal vapor and electromagnetic force affect the droplet transfer frequency, and the maximum frequency of droplet transfer is 180.8 Hz under a laser power of 2.5 kW. The rotational force promotes molten droplet transfer and maintains the axial transition, and the welding current has a strong effect on increasing the droplet transfer frequency. The influence of laser power on weld penetration is greater than that of welding current, the weld penetration increases significantly with increasing laser power, and the cross section of the weld seam exhibits laser-arc hybrid welding characteristics.

Effect of polarities and shielding gases in CWW GMAW

ABSTRACTThe effect of polarity and shielding gases on the droplet transfer behavior and formation trend of cable-type welding wire gas metal arc welding was studied under the same welding parameters. An acquisition system was used to observe welding parameters, droplet transfer and arc behavior. The results showed that the arc shapes with different polarities under the same shielding gas were different. The arc length and width in direct current electrode positive (DCEP) mode were smaller than those in direct current electrode negative (DCEN) mode. For the same polarity and different shielding gases, with increasing CO2 gas content, the arc length and width gradually decreased. In DCEP mode, droplet transfer was characterized by globular and projected droplet transfer modes. The droplet transfer modes with different polarities differed under different shielding gases. The droplet transfer mode was basically the same for the same polarity. With increasing CO2 content, the weld bead formation gradually improved.

Effects of Oscillation Width on Arc Characteristics and Droplet Transfer in Vertical Oscillation Arc Narrow-Gap P-GMAW of X80 Steel

In fields, such as oil and gas pipelines and nuclear power, narrow-gap welding has often been used for the connection of thick and medium-thick plates. During the welding process, a lack of fusion was prone to occur due to groove size limitations, seriously affecting the service safety of large structures. The vertical oscillation arc pulsed gas metal arc welding (P-GMAW) method was adopted for narrow-gap welding in this study. The influence of the oscillation width on arc morphology, droplet transfer behavior and weld formation during narrow-gap welding was studied. Oscillation widths from 0 to 4 mm were used to weld narrow-gap grooves with a bottom width of 6 mm. The results show that, in non-oscillation arc welding, the arc always presented a bell cover shape, and the droplet transfer was in the form of one droplet per pulse, while the sidewall penetration of the weld was relatively small, making it prone to a lack of fusion. With an increase in the oscillation width, the arc gradually shifted to the sidewall. The droplet transfer mode was a mixed transfer of large and small droplets, and the sidewall penetration continued to increase, which was conducive to the fusion of the sidewall. However, when the oscillation width was wider than 3 mm, it led to the phenomenon of the arc climbing to the sidewall, and the weld was prone to porosity, undercutting and other welding defects. The oscillation width has a major impact on the stability of the welding process in vertical oscillation arc narrow-gap welding.

Optimization of CMT Characteristic Parameters for Swing Arc Additive Manufacturing of AZ91 Magnesium Alloy Based on Process Stability Analysis.

The droplet transfer behavior and stability of the swing arc additive manufacturing process of AZ91 magnesium alloy based on the cold metal transfer (CMT) technique were studied by analyzing the electrical waveforms and high-speed droplet images as well as the forces on the droplet, and the Vilarinho regularity index for short-circuit transfer (IVSC) based on variation coefficients was used to characterize the stability of the swing arc deposition process. The effect of the CMT characteristic parameters on the process stability was investigated; then, the optimization of the CMT characteristic parameters was realized based on the process stability analysis. The results show that the arc shape changed during the swing arc deposition process; thus, a horizontal component of the arc force was generated, which significantly affected the stability of the droplet transition. The burn phase current I_sc_wait presented a linear function relation with IVSC, while the other three characteristic parameters, i.e., boost phase current I_boost, boost phase duration t_I_boost and short-circuiting current I_sc2, all had a quadratic correlation with IVSC. A relation model of the CMT characteristic parameters and IVSC was established based on the rotatable 3D central composite design; then, the optimization of the CMT characteristic parameters was realized using a multiple-response desirability function approach.

Effect of Process Parameters on Arc Shape, Macroscopic Features, and Microhardness in Pulsed GMA–Additive Manufacturing

Gas metal arc welding-based additive manufacturing (GMA–AM) is a promising, low-cost approach to fabricate large-scale and complex geometry components using layer-by-layer deposition of metals. However, the low forming accuracy of GMA–AM still limits its one-off industrial application due to the strong and nonlinear interactions between arc–droplet transfer and molten pool. To fully understand the influential mechanism of this inherent interaction in the GMA–AM process to precisely control the part accuracy, the arc–droplet transfer behavior in the GMA–AM process with different current waveforms was firstly studied experimentally. The phenomena of the arc swing and the differing droplet transfer with the increase in deposited height were interpreted. The thermal force status of the molten pool and its balance boundary conditions were also theoretically analyzed. Finally, the microstructure and the hardness of the AM parts with different cooling times were tested and analyzed. The experimental results demonstrate that using the spray droplet transfer mode can generate a stable AM process under direct current application conditions, but it easily ends the AM process at the third or fourth layer deposition owing to excessive heat input. A more highly accurate deposition morphology can be obtained in one droplet per pulse mode under pulsed current application conditions, which also indicates that the AM process with a constant current welding supply is stabler and easily produces better deposition than the process with a constant voltage welding supply. With the increase in cooling time, the microstructure evolved from fine ferrite to equiaxed ferrite and to columnar ferrite combined with acicular ferrite with a lower proportion of pearlite in the vertical direction of the part, and the average hardness changed to ~168 HV (bottom), ~175 HV (middle), and ~250 HV (top). The analysis indicates that the heat accumulation of the molten pool is a critical factor that affects the deposition accuracy. To this end, a novel strategy that uses the heat accumulation to compensate for the energy formed in the molten pool is proposed to further reduce the arc heat input and weaken the heat accumulation, and its feasibility is discussed.

Research Status of Stability in Dynamic Process of Laser-Arc Hybrid Welding Based on Droplet Transfer Behavior: A Review

With the synergistic effect of laser and arc heat sources, laser-arc hybrid welding (LAHW) technology can improve welding speed and penetration depth, and enhance gap-bridging ability. This review describes the fundamental concepts and characteristics of droplet transfer behavior in LAHW. Emphasis was placed on the physical interaction between the laser and arc and the effect of the combined laser/arc heat sources on the welding process. However, the physical understanding of these multivariable and complex interactions is still evolving. Through numerous research findings and summary, it is found that there are several critical factors, including the laser-to-arc distance, heat source leading mode, shielding gas composition, and laser power, affecting the droplet transfer characteristics. This review critically interprets the latest development in the basic understanding of LAHW. It lays great stress on the coupling effect of laser and arc in droplet transfer dynamic process of LAHW, and offers a direction for the future study and progress of LAHW. Significant fields for future research are also confirmed.

Influence Mechanism of Arc Characteristics on Droplet Transfer Behavior in CMT-based Additive Manufacturing

Influence Mechanism of Arc Characteristics on Droplet Transfer Behavior in CMT-based Additive Manufacturing

Study on the droplet transfer behavior and microstructure of wire arc additive manufactured Inconel 718 alloy assisted by an alternating magnetic field

• The alternating magnetic field changes arc morphology and reduces droplet size. • Appropriate excitation current effectively reduces surface roughness. • The alternating magnetic field can effectively refine the grains and deflect the grains growth direction. In this paper, the bead geometric characteristics, droplet transfer behavior and microstructure of wire arc additive manufactured Inconel 718 alloy using cold metal transfer process assisted by an alternating magnetic field (AMF) are investigated by varying the excitation current. The results show that the AMF reduces the droplet size from 1.7 mm to 1.4 mm, and changes the arc shape from bell shape to “broom shape”. With increasing excitation current, the surface roughness first drops from 0.14 at 0 A to 0.08 at 20 A and then rises to 0.25 at 40 A, finally reaching a maximum value of 0.39 at 60 A. The grain growth direction is deflected along the build direction by the AMF, and the average grain size decreases to 69.5 μm.

Study on droplet transfer behavior during medium and low power laser assisted pulsed MAG hybrid welding

This paper comparatively investigates the arc plasma morphology and the droplet transfer behavior and Characteristic of electrical signal between the pulsed MAG welding and the medium and low power laser assisted pulsed MAG hybrid Welding. Compared with the pulsed MAG welding, the assisted laser acts on the substrate before the arc to generate a stable keyhole and mass metallic vapor is generated near the keyhole leading the expansion of arc plasma morphology. The arc plasma appears obvious compression phenomenon under the action of medium power assisted laser. In addition, the keyhole has an attractive effect on the discharge channel and the height of the discharge channel becomes shorter, which makes the arc initiation stage more stable. The contact area between the arc root and the workpiece is expanded, in addition the laser has a compression effect on the arc, making the arc energy density higher and the energy use more efficient. The acting of assisted laser increased the droplet transfer frequency with the power increases, which essentially changes the electromagnetic force acting on the droplet. Others, the range of welding voltage distribution and the probability of high voltage distribution are reduced, and the transition is more stable.

Effect of pulse current on droplet transfer behavior and weld formation of 304 stainless steel in local dry underwater pulse MIG welding

Effect of pulse current on droplet transfer behavior and weld formation of 304 stainless steel in local dry underwater pulse MIG welding

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.

Electric current evaluation for process monitoring in electron beam directed energy deposition

Electrons, which carry abundant information, can be used for the in situ monitoring of electron beam additive manufacturing, such as in the emerging electron optics method for monitoring surface morphology. In electron beam directed energy deposition (EB-DED), droplet transfer, which affects the stability and quality of the deposition, is a unique and critical process. In this study, we developed a dual-channel absorbed current monitoring method based on absorbed electrons for the in situ monitoring of droplet transfer in the EB-DED process, which differs from the common vision-based monitoring technology. The ability to identify different droplet transfer modes and parameter changes was verified through a series of experiments. The results show that this method exhibited prominent signal characteristics corresponding to typical droplet transfer modes in EB-DED, accurately identified the droplet transfer mode during part fabrication, and provided a clear criterion for identifying the stability of the deposition process. It also distinguished changes in the droplet transfer distance and wire-feeding speed via a quantitative analysis of the signal variation caused by the corresponding droplet behavior. We also analyzed and explained the generation mechanisms of the signal characteristics. The dual-channel absorbed current monitoring method exhibited high reliability and efficiency during the in situ monitoring of the droplet transfer of EB-DED. The proposed method represents a new tool for information sensing in closed-loop control and facilitates the study of droplet transfer behavior in EB-DED.

Study on droplet transition behavior, bead geometric characteristics and formability of wire + arc additively manufactured Inconel 718 alloy by using CMT MIX+ Synchropulse process

In this paper, the droplet transfer behavior, geometric characteristics of weld bead and formability of CMT MIX + Synchropulse (CSP) wire + arc additively manufactured Inconel 718 alloy under high wire feeding speed are studied. The results show that a CSP cycle is composed of a CSP high-frequency pulse stage (CSP–HFP) and CSP-CMT stage, and the periods ratio is about 1.7:1. The high frequency and large average welding current of CSP-HFP increase the arc force, which further increases the droplet impact force on the molten pool. The molten pool is significantly deformed, resulting in the penetration depth of the CSP weld bead (1.8 mm) apparently higher than that of CMT (0.5 mm). The surface of the CMT-WAAM sample is more rough and irregular. The large penetration depth provides sufficient support for the molten pool, which promotes the overall stability of the molten pool. The increase of the droplet impact and arc forces enhance the convections in the molten pool, which finally suppresses the overflow phenomenon.