Molten Pool Behavior Research Articles

Non-equilibrium solidification behavior associated with powder characteristics during electron beam additive manufacturing

For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification, microsegregation and the formation of interdendritic phases significantly affects the material properties. Notably, the powder characteristics influence the heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of the powder feedstock on solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was fabricated using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, samples built using the two powders were characterized and compared. By performing multi-scale numerical simulations, melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that after appropriate size control, compared with the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate was facilitated, thereby suppressing microsegregation. These findings contribute to PBF knowledge related to feedstock, proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.

Simulation of the influence of welding parameters on weld pool behavior during a TIG-MIG hybrid welding process

TIG-MIG hybrid welding produces better weld formation than traditional MIG welding at higher welding speeds. The adaptive heat source and arc pressure models of inclined TIG-MIG arcs on curved surface were improved based on their distribution on flat surface. A 3D transient model of heat and mass transfer in a weld pool was established to analyze the influences of various welding parameters on a high-speed TIG-MIG hybrid welding process. The influence of the TIG current, distance between electrodes, and welding speed on this hybrid welding process were simulated to study the quantitative relationships between process parameters, molten pool behavior, and weld beam formation. The heat flux field, temperature field, and fluid flow on the upper surface and longitudinal sections of the weld pool were simulated and analyzed under various welding conditions. Experiments were performed; the simulation results agreed well with the experiments.

Laser-arc hybrid additive manufactured Al[sbnd]Cu alloy for T-shaped structure: Microstructure evaluation and molten pool behavior

T-shaped structure is widely used in the aeronautic and aerospace industries for the development of lightweight applications. However, poor metallurgical bonding and thermal defects by remelting at the intersection is prone to occur in the additive manufacturing of the structure. In this study, T-shaped structure of AlCu alloy was fabricated by laser-arc hybrid additive manufacturing (LAHAM), and the molten pool behavior of the process was analyzed. According to the remelting of intersection, it was divided into the top layer and the remaining layers. The remelting zone of intersection (RZI) was obvious in the top layer. During the solidification of molten pool from the boundary to the center, the ratio of temperature gradient G to solidification rate R (G/R) at the S/L interface decreased from 2.57 × 108 (K·s)/m2 to 9.27 × 107 (K·s)/m2, and promoted the transformation of equiaxed dendrites. In the remaining layers, the recoil pressure, much smaller G/R and larger G∙R made the grains as fine equiaxed along the z direction in the laser zone (LZ), while coarse columnar were observed in the arc zone (AZ). Due to the microstructure changes, the deposited T-shaped structure exhibited a yield strength of 146.5 ± 2.2 MPa and an ultimate tensile strength of 290.3 ± 5.9 MPa.

Effect of reverse-polarity hot wire on the tandem arc welding process

A novel three-dimensional two-step numerical model was developed and analyzed for a tandem process with a reverse-polarity hot wire. Magneto-hydrodynamic analysis was conducted and the free surface of the molten pool was tracked to investigate the arc and molten pool behaviors of the proposed process, respectively. The results indicated that a change in electromagnetic force by the hot wire reduced arc deflection, which increased the arc velocity in the co-axial direction of electrodes, thereby increasing the arc pressure and arc shear force acting on the molten pool. Consequently, the molten pool in the novel tandem process was wider and deeper than that in the conventional tandem process.

Dynamic behavior of keyhole and molten pool under different oscillation paths for galvanized steel laser welding

This study investigated the dynamic behavior of a keyhole and molten pool under circular and zig-zag oscillating paths for laser welding on galvanized steel. A large variation in the length of the keyhole and a bulge in the molten pool resulted in more spatters in the case of a circular oscillating path, whereas a little spatter was observed in the case of a zig-zag oscillating path because of the relatively stable keyhole and molten pool observed by a high-speed camera. Numerical simulation results showed that zinc vapor entered the keyhole with a velocity of 93 m/s because of a large amount of zinc evaporation in the first half of the circular oscillating path, leading to a rapid increase in the keyhole size. In this case, the fluid near the rear wall of the keyhole accelerated to 1.7 m/s and gushed out of the molten pool in a spatter. In the zig-zag oscillating path, the zinc vapor continually entered the keyhole at a lower velocity, leading to a relatively stable keyhole. In this case, the maximum liquid velocity of the molten pool was 0.9 m/s, not reached the critical spattering velocity (1.36 m/s), thus no spatter occurred. These numerical results sufficiently explain the dynamic behavior of a keyhole and molten pool induced by zinc vapor, and an effective spatter suppression method is proposed.

Study on the molten pool behavior and porosity formation mechanism in dual-beam laser welding of aluminum alloy

Based on the mechanism of dynamic coupling between molten pool and keyhole, a three-dimensional (3D) transient model for the dual-beam laser welding of aluminum alloy is established by considering the surface tension, Marangoni force, and recoil pressure. The morphology of the molten pool, porosity formation process, and the heat transfer mechanism during the process of laser welding under different parameters are analyzed. A double rotating 3D Gaussian heat source is used to represent the laser beam, and the volume of fluid method is used to track the gas-liquid free surface, and the gas-liquid interface force is transformed by the continuous surface force model. The results show that in the process of dual-beam welding, the interaction between the two keyholes enhances the fluid flow perpendicular to the spot line, and the shape of weld pool surface changes from elliptical to circular. Furthermore, welding speed and spot spacing have a significant effect on the shape of the molten pool. Furthermore, it is observed that dual-beam welding can improve the stability of the keyhole and reduce the maximum oscillation amplitude and the number of keyhole breakups. At a specific spot spacing, a unique process of separation and fusion is discovered in addition to the common stages of growth, maintenance, breakup, and shrinkage of the keyhole. The simulation results are in good agreement with the existing experimental results. Overall, this paper provides useful insights into the dynamics of the molten pool in dual-beam welding and reveals the molten pool behavior and porosity formation mechanism.

Multi-scale simulation of microstructure evolution during direct laser deposition of Inconel718

As a free forming additive manufacturing technology for high performance metal parts, laser deposition manufacturing has a broad application prospect in aerospace and other fields. The solidification behavior and microstructure of molten pool determine the mechanical properties of the deposited parts. A multi-scale Cellular Automaton (CA)- Finite Element (FE) model is developed to simulate the microstructure evolution with different process parameters during the Direct Laser Deposition (DLD) process. Temperature data and solidification parameters under different process parameters are obtained by finite element method, which establish connections between macroscopic temperature field and microstructure simulation. In addition, the effects of different process parameters on dendrite morphology, element segregation and grain structure are investigated. The primary dendrite arm spacing (PDAS), Nb element precipitation and grain size all increase with the increase of laser power, which are caused by the decrease of cooling rate. Moreover, the trend of the simulated results of microstructure under various process parameters are consistent with the experimental results.

Region-based path planning method with all horizontal welding position for robotic curved layer wire and arc additive manufacturing

For the curved layer deposition with wire and arc additive manufacturing on complex surface, the molten pool behavior and bead geometry are variant at different welding positions for the dragging effect of gravity, which lead to inconsistent layer appearance. A region-based path planning method for robotic curved layer wire and arc additive manufacturing (CL-WAAM) is developed, with which all horizontal welding position can be maintained at arbitrary region of the complex surface. The adverse effect of gravity is minimized and uniform bead appearance can be guaranteed. Firstly, with a two-stage clustering method , the complex surface was divided into a certain number of simple subregions . Then, the contour line across cluster centroid of each subregion was employed as initial path, and an adaptive bead overlapping model changed with local inclined angle was used to generate the offset curved path covering the subregion. Therefore, the overall welding paths were approximate to the contour line on the surface, and the horizontal welding position and consistent forming appearance can be achieved. Finally, three surface parts were designed to validate feasibility of the method, and the forming appearance was compared with traditional iso-parametric method and flat layer WAAM (FL-WAAM). The results suggested that the surface waviness and thickness were relatively consistent at arbitrary region of complex surface with the region-based method, and the stair-step effect appeared in FL-WAAM was significantly alleviated with the CL-WAAM. The proposed method helps achieve the uniform layer appearance deposited on complex surface, which could be widely used in the manufacturing or repairing of wear-resistant surface parts, such as the hot forging die and hydraulic turbine blade. • A framework of robotic curved layer wire and arc additive manufacturing (CL-WAAM) was proposed. • A region-based path planning method with all horizontal welding position for the CL-WAAM was implemented. • The complex surface was subdivided into simple patches with a two-stage clustering method. • The contour line across the cluster centroid was employed as the initial path, with which the all horizontal welding position could be ensured. • The uniform forming appearance was achieved through the proposed region-based path planning method.

Experimental study of stratification behavior of the steady two-liquid corium pool

The prototypic materials have been used to investigate the stratification behavior of the steady molten pool at 3100 K in the context of the IVR condition. Induction heating in a cold crucible was used for melting of the 2 kg-scale UO2-ZrO2-Zr-Fe. A post-test examination of SEM, EDS, and XRD has been performed to investigate the metallic and oxide layers for all ingots. The effects of MFe/M∑ (initial iron to corium ratio) and Cn (initial degree of zirconium oxidation) on the configuration of the molten pool were studied. The results indicate that structure of the molten pools consists of the top light metallic layer and oxide layer. Nevertheless, a layer inversion may occur when the density of the metallic layer increased. The synergistic effect of MFe/M∑ and Cn on redistribution of the corium components leads to a significant increase of the uranium concentration for the metallic phase, which can explain why the layer inversion of the molten pool occurred.

Effects of Electrode Negative Pulsing Ratio in Direct Energy Deposition via Variable-Polarity Cold Metal Transfer Process on the Deposition Behavior and Microstructural Characteristics

Interest in research on the application of variable-polarity cold metal transfer mode in wire-based direct energy deposition has been growing; particularly popular are investigations into the respective influences of polarity, amplitude of the arc current, and polarity variation sequence on the quality of the final product manufactured via additive manufacturing. The application of the electrode-negative phase is more capable of yielding relatively large droplets and increasing the weight of the deposited material. However, the proportions of the electrode positive phase are typically larger than those of the electrode-negative phase because it maintains arc stability and droplet transfer. This discrepancy has prevented the accurate evaluation of the effects of the polarity mode and polarity sequences on the deposition characteristics associated with variable-polarity cold metal transfer. In this study, variable-polarity cold metal transfer was performed using a tuned waveform, and the effects of the electrode-negative pulsing ratio and pulse repetition on the geometrical features and deposition rate were assessed. The weight tended to increase with decreasing welding speed and increasing electrode-negative pulsing ratio. The number of repetitions influenced molten pool behavior, and when sufficiently high, induced ripple formation via droplet accumulation below the electrode. In addition, the effects of the electrode-negative pulsing ratio and repetition on the microstructure formation were analyzed. It was revealed that the average grain size was related to the amount of supplied energy and polarity switching during grain formation.

Numerical simulation for dynamic behavior of molten pool in cable-type welding wire GAMW

The cable-type welding wire gas (CWW) metal arc welding (GMAW) is an innovative arc welding process with high efficiency and high welding quality. However, there is still a lack of deep research on the molten pool of CWW-GMAW. The behavior of molten pool of CWW-GMAW was studied by both numerical simulations and experiments. The temperature field and flow field in the longitudinal section and across cross-section under the current of 450 A in CWW-GMAW were simulated. The result shows that the isotherm in front of the molten pool is denser than that in the back on the longitudinal section. The arc rotation force causes the velocity of the molten pool in the direction of weld penetration becoming more significant than that of weld width. The rotating momentum of the droplet affects the temperature distribution of the weld width and penetration on the cross-section of the molten pool. The coupling of electromagnetic force and rotating arc force makes the flow field present a clockwise vortex shape. The simulated results of weld geometries of CWW GMAW are in good agreement with the experimental results, indicating the applicability and accuracy of the established model.

Analysis of Controlled Driving and Spreading Behavior of Molten Pool in Cold Metal Transfer

The controlled short-circuit transfer is used to control the heat input of the molten pool and the base metal in the cold metal transfer welding process, including droplet formation, droplet transfer and molten pool flow. Based on the influence of arc pressure on the surface of the molten pool, droplet impact and residual energy on the flow behavior of the molten pool, this research proposes the arc pressure driving model, the droplet impact driving model and the residual energy transfer model on the molten pool surface respectively, and on this basis, a theoretical model of controlled driving and spreading of the cold metal transition bath is proposed. The theoretical relationship between the surface shape of the keyhole and the driving force, and the relationship between the surface shape of the molten pool and the welding current are established. The model accurately predicts the formation width and contact angle of the molten pool in a specific interval, which can better control the welding process and geometry.

Improvement of welding stability and mechanical properties of galvanized DP800 steel lap joint by high-speed tandem beam laser

Dual beam laser welding has shown promise to improve the weldability of galvanized dual phase steels. In this study, the lap joints of galvanized dual phase steel were welded with tandem beam laser at speed up to 100 mm/s. The power ratio for the dual beams was set to 50%:50%, 40%:60%, 30%:70%, and 20%:80%, respectively. A high-speed camera was used to observe the dynamic process of zinc vapor, molten pool, and keyhole behaviors under different welding conditions. The corresponding metallographic and mechanical analysis were conducted to reveal the effects of variable welding thermal cycles on the microstructures and properties of the welded joints. The results indicate that the fusion zone (FZ) is composed of blocky-lath martensite, whereas the heat-affected zone (HAZ) is characterized by the lath microstructure containing tempered martensite (TM) and ferrite (F). The power ratio of dual beams significantly affects the microstructures and hardness profiles of welded joints. It also influences the zinc-vapor and molten pool dynamic behaviors, thus affecting the weld bead formation. The weight of spatter decreases with the decrease of leading beam power in tandem beam laser welding process. In addition, the mechanical properties of the welded lap joints are significantly affected by the high-speed tandem dual beam laser welding process parameters. The failure load of tensile-shear testing reaches the maximum value when the beam power ratio is 30%:70%.

Review on Vacuum Laser Beam Welding

Vacuum laser beam welding (VLBW) exhibits better welding characteristics, e.g., deep penetration, less pore, and spatter generation, compared with the conventional laser welding technique under atmospheric pressure. Low ambient pressure affects the vaporization temperature of the base material and the interaction between laser and plasma plume, which affects the behaviors of laser-induced plasma plume, keyhole, and weld pool flow. This review introduces improvements in welding characteristics such as penetration depth, pore, and spatter generation in the VLBW process. The effect of ambient pressure on welding characteristics was reviewed, and the behavior of laser-induced plume, keyhole, and molten pool under reduced ambient pressure was discussed to understand the mechanisms for enhanced weldability.

Investigation of Weld Root Defects in High-Power Full-Penetration Laser Welding of High-Strength Steel

The currently available high-power laser shows promising opportunities for the welding of thick plates in a single pass. However, weld-root defect frequently occurs when a high-power laser is used to join thick plates in a full-penetration mode, which has a significantly adverse effect on the serviceability of the weld joint. The purpose of this work is to understand the defect formation mechanism and reduce these defects through controlling welding parameters. In this study, the characteristics of weld root defects were investigated using a 10 kW fiber laser using a program of experiment and theoretical analysis. The corresponding defect formation mechanisms were discussed based on the bottom molten pool behaviors observed by the high-speed camera. The results showed that there were four types of weld-root appearances as follows with an increase of linear heat input from 300 J/mm to 1000 J/mm: weld-root humping (30 mm/s), sound weld (25 mm/s), weld sagging (20 mm/s) and excessive weld sagging. The remedies for reducing weld-root defects were also presented to obtain sound weld bead by optimizing welding parameters. Weld-root humping was formed due to the quasi-full-penetration keyhole. Weld sagging resulted from the imbalance of the hydrostatic pressure and surface tension in the condition of a through keyhole. It was also found that the sound weld was formed when a through keyhole and a proper molten pool size were obtained. Thus, the state of the keyhole and molten pool geometry were the major factors that affect weld-root defects.

Dynamic molten pool behavior of pulsed gas tungsten arc welding with filler wire in horizontal position and its characterization based on arc voltage

To explore the dynamic behavior of molten pool and to find the electric characteristic signals reflecting the weld penetration states and degree for the fixed-point wire-filled pulsed gas tungsten arc welding (GTAW-P) of thin pipes in 2G position, molten pool images from different directions and arc voltage were sensed. During a peak current period, critical penetration occurred, with the molten pool behavior that the topside molten pool height collapsed sharply and was approximately 0.5 mm, and then rose to approximately 1.5 mm during the next base current period. With the increase in backside molten pool width, the oscillation amplitude and frequency of topside molten pool during the base current period increased and decreased, respectively. The oscillation amplitude reached the maximum when over penetration occurred. Such molten pool behaviors could be extracted from the arc voltage signals. The change in average peak voltage (ΔU⁎) could reflect the occurrence of critical penetration. The average base voltage (Ub¯) and the fluctuation of base voltage (ΔUb) increased linearly with the increase in backside molten pool width, which are reliable characteristic signals for the weld penetration degree. The effects of wire filling and base current on the dynamic molten pool behavior and characteristic signals were investigated. Experimental results showed when the base current is 10 A ̶15 A, Ub¯ and ΔUb can effectively characterize the weld penetration degree.

Correlation of parameters and microstructure of 3D printing bulk medical titanium-based metallic glass

Metallic glass has developed over the past few decades. To fabricate the metallic glass, high cooling rates (105 K/s) is needed to form the disorder atomic structure [1]. Because metallic glass has no grain boundary, its characteristics are superior to traditional crystalline materials, such as good wear resistance, and excellent corrosion resistance [2]. In the past few decades, most amorphous alloy systems can only be manufactured to a millimeter size, causing the application difficulties [3]. Due to development of 3D printing technology nowadays, it’s expected to manufacture metallic glass in a larger size due to the localized melting process with a high cooling rate. Due to the breakthrough in size, it can be applied to medical equipment such as scalpels, dental implants, and various implants in the future. However, the relationship between the materials, 3D printing process parameter, and microstructure evolution is still needed to be explored. Thus, in study, such relationship of Ti-based metallic glass is studied through changing parameters of scan speed and laser power to vary cooling rates and microstructure by Selective Laser Melting (SLM). The samples were examined through the X-ray diffractometer (XRD) and the scanning electron microscope (SEM) with energy disthispersive spectrometry (EDS). The composition variation and microstructure evolution during the 3D printing process was discussed in detail from the point of view of 3D printing parameter, molten pool behavior, and heat of mixing. Furthermore, the mechanical properties of metallic glass were investigated by using micro-indentation, which results showed further that the microstructure indirect effects hardness and young’s modulus of the alloys.

Dynamic Behavior of Keyhole and Molten Pool Under Different Oscillation Paths for Galvanized Steel Laser Welding

Dynamic Behavior of Keyhole and Molten Pool Under Different Oscillation Paths for Galvanized Steel Laser Welding

Thermal Behavior of Molten Pool for Laser Directed Energy Deposition of 316L/Inconel 718 Multi-Materials

Thermal Behavior of Molten Pool for Laser Directed Energy Deposition of 316L/Inconel 718 Multi-Materials

Effect of Molten Pool Behaviors on Welding Defects in Tandem Ng-Gmaw Based on Cfd Simulation

Effect of Molten Pool Behaviors on Welding Defects in Tandem Ng-Gmaw Based on Cfd Simulation