Characteristics Of Molten Pool Research Articles

Metal Additive Manufacturing and Molten Pool Dynamic Characterization Monitoring: Advances in Machine Learning for Directed Energy Deposition

Directed energy deposition (DED) has progressively emerged as a highly promising technology for the rapid, cost-effective, and high-performance fabrication of hard-to-process metal components with shorter production cycles. Recognized as one of the most widely utilized metal additive manufacturing (AM) techniques, DED has found extensive applications in critical industrial sectors such as aerospace and aviation. Despite its potential, challenges such as inconsistent part quality and low process repeatability continue to restrict its broader adoption. The core issue underlying these challenges is the complex, dynamic nature of the DED process, which involves the coupling of multiple physical fields. Within this context, the molten pool plays a pivotal role, serving as a key carrier that encapsulates abundant process characteristic information. The dynamic characteristics of the molten pool are intrinsically linked to the final part quality and the repeatability of the process. Consequently, integrating machine learning (ML) methodologies into the monitoring framework can offer robust data-driven support for enhancing both product quality and process consistency. This paper provides a comprehensive review of the research advancements and prospective trends in the dynamic monitoring and control of molten pool characteristics within DED processes underpinned by machine learning techniques. The review is structured around five key areas: an overview and fundamental principles of DED technology, methods for process information sensing during part monitoring, approaches for dynamically monitoring molten pool characteristics, the primary challenges currently faced in intelligent monitoring systems, and the potential future directions for further research and development. Through this detailed examination, the paper aims to shed light on the pivotal role of intelligent monitoring systems in advancing DED technology, ultimately contributing to more reliable and repeatable additive manufacturing processes.

Experimental research on heat transfer characteristics of a two-layer corium pool based on a three-dimensional ellipsoidal lower plenum

To better understand the application of IVR (In-Vessel Retention), extensive experiments have been conducted on the heat transfer characteristics of molten pool. However, most research mainly focuses on hemispherical lower heads, and the research on ellipsoidal lower head suitable for small pressurized water reactors is relatively lacking. In order to provide some reference for the implementation of IVR with ellipsoidal lower head, this paper conducted the experimental study on the heat transfer characteristics of a three-dimensional double-layer molten pool based on the design of a small pressurized water reactor. The test section consists of two parts: ellipsoidal and cylindrical, with a span of 1150 mm and a total height of 700 mm. The molten material is simulated by nitrate mixture (20mol%NaNO3-80mol%KNO3), and electric heating wire is chosen to simulate the oxide layer decay heat. The effects of heating power, oxide layer height and layered partition thickness on heat transfer characteristics of two-layer molten pool under static conditions were studied. The results show that the thermal stratification phenomenon mainly occurs in the lower and middle regions of the oxide layer, with a smaller dimensionless temperature gradient in the upper region; with the similar volumetric power densities, the height of the oxide layer has little effect on the wall heat flux density; the layered partition increases the thermal resistance between the two layers and reduces the upward heat transfer to the ceramic pool. In addition, the heat transfer relationships in the oxide layer, both downward and upward, are fitted for the internal Rayleigh number range of 3.43 × 1012 to 1.54 × 1013.

Research on energy utilization of electron beam melting for silicon purification

As a key material for solar photovoltaic power generation, the purity of silicon plays a crucial role in the photovoltaic conversion efficiency and performance. The electron beam melting (EBM), which is irreplaceable metallurgical methods, has been widely used for silicon purification. However, high energy consumption result in relatively high cost, which limits its widespread application in melting industry. This study was centered on the energy distribution that proposes a new approach to reducing energy consumption during EBM. The scanning form of electron beam is most important factor concerning energy consumption. Different scanning forms can affect the characteristics of molten pool which is closely correlated with the purification process. In this study, efficient and energy-saving preparation processes of EBM are being explored through the numerical model and experimental. The relationship of energy distribution form and purification efficiency is studied. The research results indicate that electron beam scanning under uniform distribution can achieve more uniform temperature and flow field distribution in the molten pool, which has a higher energy utilization rate during silicon purification by EBM. The mathematical model established in this study can effectively predict the energy consumption level under different melting parameters, providing important reference for the optimization and energy conservation of EBM process for other materials. In addition, evaporation characteristics of volatile impurity, migration of the second phase particles in the melt, controlling of melt pool characteristics and the crystal growth control all can be conducted based on this study.

Surface roughness and pore evolutions in multi-layer laser powder bed fusion of extra-low interstitial Ti-5Al-2.5Sn powder: A numerical study

In this paper, the three-dimensional discrete element method (DEM) and computational fluid dynamics (CFD) coupled approach was used to numerically reproduce the whole process of laser powder-bed-fusion (L-PBF) additive manufacturing (AM) of extra-low interstitial (ELI) Ti-5Al-2.5Sn powder. The effects of key parameters such as scanning strategy and hatch spacing (h) on the surface roughness (Ra) and pores during multi-layer printing are systematically investigated by characterizing the molten pool characteristics and thermal behavior upon laser motion; and the melt volume in this duration is quantified by the volume of fluid (VOF) method to demonstrate inter-layer interactions. The results show that Ra can be categorized according to the scanning directions. Along the scanning direction, the Ra is affected by the heat accumulation effect and increases as the h decreases. In this case, the Ra caused by the Marangoni effect can be reduced by increasing the melt volume at the end of the track through the layer rotation. The Ra perpendicular to the scanning direction is caused by the ripple-like surface formed by track overlap and decreases as the h decreases. For defects, the pores formed by shrinkage due to insufficient melting or by lack of fusion (LoF) due to incomplete track overlap decrease with the decrease of h. The LoF pores caused by weak inter-layer metallurgical bonding are affected by the surface morphology of the previous layer, which is increased as the h increases. The layer rotation can also reduce such LoF pores. On this basis, a quality control chart suitable for actual production is established.

Microstructural evolution and mechanical properties of maraging steel by additive manufacturing

Laser powder bed fusion (LPBF) is a revolutionary and innovative technology that provides advanced technical support for the forming of parts with high precision and complex structures. In this work, a new type of Fe-Ni-Mo-Ti-Al-Y cobalt-free maraging steel was successfully fabricated by LPBF. The additive manufacturing parameters and the microstructural evolution of cobalt-free maraging steel under different heat treatment parameters were systematically studied. The results show that the parts without cracks and close to complete density can be fabricated under a wide LPBF process window. The dislocation cell with 500 nm in the printing state makes the material have good plasticity. After post-heat treatment, the preferred orientation of LPBF maraging steel is reduced, but the typical characteristics of molten pool and cellular structure in maraging steel disappeared, which is the result of local composition and microstructure homogenization. The elongation of LPBF maraging steel in solution annealed state is higher than that in as-printed state, which is due to the reduction of dislocation density and grain coarsening. However, after direct aging, due to the formation of nano-sized Ni3(Ti, Mo) precipitates in the matrix, the mechanical properties of the material were effectively enhanced, and the tensile strength increases from ∼1.1 GPa in as-printed state to ∼1.6 GPa in aging state. In contrast, the corrosion resistance of as-printed maraging steel is higher and comparable to that of traditional cobalt-containing maraging steel. However, the corrosion resistance of LPBF maraging steel after being aged is reduced, which is due to the existence of precipitates and dislocations. This study provides a theoretical reference for the composition design and strengthening mechanism of LPBF cobalt-free maraging steel.

Numerical simulation of keyhole-induced pores for TA15 in laser powder bed fusion (L-PBF)

In the complex laser powder bed fusion (L-PBF) process, the occurrence of pores is often triggered when localized temperatures of the materials exceed their boiling point, posing a significant impediment to the overall quality of manufactured parts. In this work, considering the thermophysical properties and latent heat of phase transformation of the material, the formation and evolution of keyholes and pores during the L-PBF single-track processing of TA15 titanium alloy was investigated using X-ray computed tomography (X-CT) and FLOW-3D finite volume analysis software coupled with multiphysics fields. The simulated molten pool characteristics generally agree with the experimental data, including internal pores The results showed that the evolution of keyholes is mainly attributed to surface tension and recoil pressure, the Marangoni flow and surface tension contributed to closed localized cooling regions on the keyhole walls to generate pores. Additionally, the Marangoni flow and buoyancy effects lead to pore motion within the molten pool. Furthermore, the laser energy density is inherently related to the total pore counts and the mean penetration depth of keyholes, respectively. Understanding the underlying physical mechanisms helps to optimize process parameters and reduce pores in L-PBF, ultimately enhancing the quality of the additively manufactured parts.

Finite element simulation analysis of flow heat transfer behavior and molten pool characteristics during 0Cr16Ni5Mo1 laser cladding

0Cr16Ni5Mo1 super martensitic stainless steel has excellent weldability and is an economical material with a high life cycle and low cost, but there are few researches and applications of this material in additive manufacturing. In order to realize the application of 0Cr16Ni5Mo1 large-area laser cladding technology, the multi-track lap cladding of the material was studied. In this work, a multi-physical finite element model of multi-layer multi-track laser cladding was established, taking into account thermodynamic processes such as phase transition, Marangoni convection, and buoyancy. The flow and heat transfer behavior of 0Cr16Ni5Mo1 in multi-layer multi-track laser cladding process and the characteristics of the molten pool are studied. The research focused on the flow heat transfer behavior and molten pool characteristics of 0Cr16Ni5Mo1 during multi-layer multi-track laser cladding. The discussed aspects include the evolution history of the temperature field within the molten pool, flow field characteristics, molten pool dimensions, and dilution ratio under various laser powers and overlap rates. The results show that with the increase of overlap rate, the morphology of molten pools in different clad tracks will also change, and the flow rate, dilution ratio, and length of molten pools in the second track are negatively correlated, while the temperature and height of molten pools are positively correlated. As the laser power increases, all the characteristic quantities are positively correlated with it.

Blue Laser Conduction Welding of Dissimilar Cu and Al Sheets

Joining Cu-Al with 0.6 mm is important for the high-power density battery in the new energy field. Low laser absorptivity is a challenge in Cu-Al welding with conventional infrared laser at around 1000 nm wavelength, where keyhole welding is necessary. It is difficult to control welding qualities in keyhole welding due to the intense flow and violently changing absorption rate. At 450 nm wavelength, Cu and Al have high laser absorptizzvity which has the potential to implement stable conduction welding. Therefore, this work adopted a blue laser welding system, and for the first time, realized the conduction welding of 0.6 mm Cu and 0.6 mm Al sheets. The welding process, surface appearance, mechanical properties, and electrical properties were investigated. The results showed that high-power (1950 W) could effectively realize stable Cu-Al conduction welding without spatters, and the welding speed could reach 40 mm/s. Compared with an infrared laser, the blue laser could weld Cu-Al using the form of Cu on top and Al on bottom, which was beneficial for a wide process window and stable welding process. A larger bead width and more consistent intermetallic compound thickness resulting from the blue laser were conducive to performance improvement. In addition, the relationship between welding parameters, molten pool characteristics, and process qualities was built. It provided a possibility to control the welding quality under the influence of strong heat accumulation and high thermal conductivity. This work demonstrated that blue laser has great potential in joining Cu-Al for new energy applications.

Research on the mixing characteristics of the bottom blowing molten pool based on flow characteristics and mixing uniformity

In this study, a new method of combining lance–liquid flow characteristics and mixing uniformity is proposed to evaluate the stirring characteristics in the bottom blowing copper molten pool. A fluid simulation model of a bottom blowing molten pool was established, water was used to simulate the melt environment, and an experimental platform was set up for verification. The effects of swirl, multi-channel, and straight pipe spray on the lance–liquid stirring characteristics of the bottom-blown copper molten pool are compared through quantifying the flow characteristics and mixing uniformity. In addition, digital image processing technologies, such as image entropy variance and eddy current map entropy increase, are introduced. Through numerical simulation research, it is found that the transverse velocity of the swirl spray lance is the largest, which makes the rise time of the bubble increase to the greatest extent. Compared with the straight pipe spray, the swirl spray reduces the liquid splash height by 0.054 m, and the degree of vortex flow is higher. The lance phase stability is increased by 37.87%, and the maximum turbulent kinetic energy can be increased by 8.73%. The spray effect of the multi-channel spray is between the two. It is shown that the swirling spray lance can improve the stability of gas in the molten pool, enhance the uniformity of gas–liquid mixing, and improve the operation cycle and the smelting efficiency of the molten pool.

Research on molten pool flow characteristic in all-position LP-LMH welding

ABSTRACTThe flow characteristics of keyhole and molten pool impact the real-time process stability and seam formation severely in all-position low-power laser MAG hybrid (LP-LMH) welding. To probe into the molten pool’s real-time dynamics at various spatial welding positions, the high-speed camera images of metal transfer and molten pool were sensed. Meanwhile, a novel spatial flow model of molten pool was developed, and the functions of spatial position and gravity on the molten pool flow were investigated. The results proved that as the spatial position increases, the location of where droplet striking molten pool changes accordingly, further disturbing the keyhole’s stability. Meanwhile, the spatial position seriously affects the molten pool’s flow. The driving forces of molten pool flow at various spatial welding positions are basically the same, and gravity affects the overall movement of molten pool, thereby affecting the vortexes and heat transmission inside the molten pool, as reflected in weld penetration.

Mechanism study of flow characteristics of molten pool and keyhole dynamic behavior of K-TIG welding

Mechanism study of flow characteristics of molten pool and keyhole dynamic behavior of K-TIG welding

Defects detection of GMAW process based on convolutional neural network algorithm

It is significant to predict welding quality during gas metal arc welding process. The welding defect detection algorithm has been developed based on convolutional neural network (CNN). The sensing system and image processing algorithm for molten pools has been developed. It overcomes the interference caused by the arc light to obtain clear images of the molten pool's boundaries. The molten pools images are used to build up training set and test set for training and testing the CNN model. The model is designed to extract the visual features of molten pool images to predict the penetration state, the welding crater, and slags. Through optimizing the network parameters such as kernel-size, batch-size and learning rate, the prediction accuracy is higher than 95%. Moreover, the model enhances additional focus on the welding crater based on the welder experience. The mechanisms between molten pool characteristics and welding defects were analyzed based on the welder experience and the visual features of the model. It is found that the model judges the occurrence of burn-through with the black hole in the middle zone of the molten pool. When the surface pores are generated, the model exhibits a strong response to circular voids in the semi-solid region at the trailing end of the molten pool. The size and shape of fusion holes exhibit a strong correlation with the molten state. When the shape of the crater does not appear concave, it often signifies excessive penetration. It contributes to enhancing the algorithm's robustness during various welding scenarios.

Numerical simulation of melt pool size and flow evolution for laser powder bed fusion of powder grade Ti6Al4V

The study of molten pool characteristics is a powerful means of determining the quality of laser additive manufacturing forming. In this paper, a three-dimensional transient thermal flow field numerical model of Ti6Al4V powder processed by LPBF is developed based on FLOW-3D software, and the dynamic evolution of the molten pool with fixed process parameters is quantitatively described using dimensionless numbers in computational fluid dynamics. It is shown that the main heat transfer mode of the molten pool is thermal convection; the evaporative back sitting pressure, surface tension and Marangoni shear force are the main driving forces for the evolution of the molten pool. Furthermore, the influence of key process parameters on the heat flow field of the molten pool was analyzed. When the laser power was 300 W, the depression depth and molten pool depth caused by the recoil pressure were significantly larger, and the isothermal density of the solidification region was more intensive; when the scanning speed was increased from 0.4 m/s to 0.8 m/s, the molten pool length was reduced from 524 μm to 410 μm, and the line energy density was reduced making the amount of melted powder decrease. The wettability and fluidity of the molten state metal becomes worse; when the scanning interval increases to 120 μm, a large number of incompletely melted powder tracks cannot lap correctly at the midline of the double track. The results of the above simulations achieve a high degree of agreement with the surface quality of the specimens during the experiments, which provides guidance for quantitative analysis of molten pool evolution and prediction of Ti6Al4V forming quality.

Numerical analysis of the behavior of molten pool and the suppression mechanism of undercut defect in TIG-MIG hybrid welding

In order to investigate the multi-coupling transport phenomena in TIG-MIG hybrid arc welding, a three-dimensional model including droplet and molten pool was established. An innovative distribution pattern of arc heat and force that adapted to the evolution of molten pool surface was proposed, achieving a unified distribution of "arc current density-arc pressure-electromagnetic force-arc heat". The effect of TIG arc on molten pool behavior and undercut defect was quantitatively analyzed by comparing TIG-MIG hybrid welding with traditional MIG welding. The results showed that the electromagnetic repulsion between the two arcs with opposite currents increased the aspect ratio of the hybrid arc heat and force distribution. It led to a long and narrow gouging region, where the promoting effect of inertial force on the undercut was weakened by arc pressure and hydrostatic pressure. The liquid metal with a lower cooling rate had sufficient time to fully fill the weld toe and suppressed undercut. Sensitivity and dimensional analysis illustrated that the TIG-MIG hybrid arc eliminated undercut by reducing the inertia force, adjusting the arc pressure distribution, and enhancing the hydrostatic pressure. The groove sizes was predicated based on molten pool characteristics, including the stress state of liquid metal, morphology, and fluid flow patterns. It revealed the quantitative relationships among welding parameters, behavior of molten pool, and weld bead formation, promoting the implementation of digital twinning technology in the manufacturing industries.

Stable conduction mode welding of conventional high-reflectivity metals with 2000 W blue laser

High-reflectivity metals, e.g., copper and aluminum, have been widely applied for various industrial products such as power battery, electrical machinery, et al. However, owing to the low absorption rate of copper and aluminum to the conventional infrared laser (900–1080 nm), the keyhole welding mode is required, which is extremely unstable. A blue laser (450 nm) can improve the absorption rate of copper and aluminum evidently, which has potential to weld copper and aluminum at the conduction mode. This work studies the bead-on-plate welding of copper and aluminum by the blue laser. At a blue laser power of 1950 W, the welding depths of copper and aluminum can acquire 1.20 and 1.83 mm. Then, this work exhibits the molten pool characteristics of copper and aluminum in high-power blue laser welding, and builds a relationship between the molten pool characteristics and the welding bead qualities. The unstable boundary of the middle gray-scale region in welding of copper and the ripple phenomenon in welding of aluminum indicate a small depth and rough appearance. This work validates that the 2000 W high-power blue laser can weld conventional high-reflectivity metals (e.g., copper and aluminum) at the conduction mode without spatters, while the keyhole mode is required for welding with the infrared laser.

The Analysis of Energy Distribution Characteristics of Molten Pool in Welding of Aluminum Alloy with Oscillating Laser

The energy distribution in molten pool is highly related to the quality of the weld formation in aluminum alloy welding. The forming defects in the weld are often improved by adjusting the molten pool dynamic behaviors affected by energy distribution. Therefore, the energy distribution characteristics in the molten pool during the welding of aluminum alloy with oscillating laser in the “∞” shaped path are explored in this paper. The pore defects are found to be greatly reduced in welds from the oscillating laser welding of aluminum alloy compared to those from non-oscillating laser welding. The corresponding distribution characteristics of energy in the molten pool of welding with oscillating laser are obtained and analyzed. The dynamic behaviors and forming process of welds under welding conditions of different distribution characteristics of energy in the molten pool are discussed. The obtained results are in good agreement with the experimental results. Therefore, the proposed method can provide the desired energy distribution for stable dynamic behaviors of molten pool to improve welding quality.

Study of thermal behavior and microstructure formation mechanism of CuCrZr alloy melted by laser powder bed fusion

The properties of CuCrZr alloy parts subjected to laser powder bed fusion (LPBF) are severely affected by the thermal behavior of the molten pool in relation to the solidified structure. Detailed understanding of the changes in the thermal behavior of the molten pool and the relationship between the thermal behavior and the solidified structure are required to achieve high-performance LPBF parts. In this study, a numerical simulation of temperature field changes was used to model the heat transfer and molten pool characteristics during the LPBF of CuCrZr alloys. The effect of linear energy density (LED) on the morphology and flow of the molten pool was studied using a finite element model (FEM), and the microstructure of the alloy was analyzed in combination with the temperature field. The results show that the shape of the molten pool obtained by the simulation is approximately circular, which differs from the typical comet-shaped molten pool for other alloys. The LED affects both the size of the molten pool and the flow of bubbles in the melt. When the LED is 800 J/m, the size of the molten pool is optimal, the bubbles in the melt can escape smoothly, and the highest relative density is achieved (98.23±1.4%). The simulation results are also in good agreement with the experimental data. The simulation obtained the difference in the temperature gradient along different directions as well. This anisotropy caused the solidified structure to form a large number of equiaxed crystals in the horizontal plane. At the same time, the circular shape of the molten pool caused a large deflection in the temperature gradient direction. The grain orientation of the horizontal plane formed an angle of 45° to the scanning direction. As a result of this larger temperature gradient in the vertical plane, a large number of columnar crystals are epitaxially grown.

Study on the safety characteristics of molten corium in-vessel retention of annular fuel core

In the process of reactor severe accidents, maintaining the integrity of reactor pressure vessel to achieve IVR (In-Vessel Retention) of molten corium plays an important role in accidents mitigation and suppression. The ERVC (External Reactor Vessel Cooling) is an important technology to realize IVR. As a new economical annular fuel, the increase of zirconium content and power density in the core may threaten the integrity of lower head under severe accidents. In this paper, the IVRASA (In-Vessel Retention Analysis code in Severe Accident) is used to analyze the heat transfer characteristics of lower head molten pool corresponding to annular fuel core. The components of two-layer and three-layer configurations of molten pool are determined based on the geometric structures of annular fuel and verified heavy metal layer model. In terms of the integrity of lower head, the power density of the annular fuel core can be increased to about 110%.

Investigation on the relationships among process parameters, molten pool characteristics, and fabrication quality of Ti-10V-2Fe-3Al by laser direct energy deposition

Ti-10 V-2Fe-3Al (Ti-1023) titanium alloy is a metastable high-strength β titanium alloy. Defects such as spatters, pores, and poor surface quality tend to occur during the laser direct energy deposition (LDED) of Ti-1023 titanium alloys, prohibiting its large-scale engineering application. Therefore, to obtain high-quality LDED of Ti-1023 titanium alloy, this study used a regression analysis model to investigate the relationship between key process parameters and the geometrical characteristics of single clad tracks to obtain the forming process map. In addition, an off-axis high-speed camera was used to monitor the forming process of the Ti-1023 titanium alloy single clad tracks, and the correlation between the forming quality of the three types of forming conditions—namely incomplete melting, normal melting, and over-melting—and the characteristics of the molten pool was analyzed. Finally, this study established a simplified relationship between the process parameters, the molten pool characteristics, and fabrication quality of LDED of Ti-1023 titanium alloy, which is expected to improve development for the online monitoring and increase the application of LDED Ti-1023 titanium alloy.

Stability enhancement of molten pool and keyhole for 2195 Al[sbnd]Li alloy using fiber-diode laser hybrid welding

Molten pool and keyhole stability are vital concerns in laser welding of 2195 AlLi alloy, potentially affecting the weld quality and porosity defect. Fiber-diode laser hybrid welding, which coaxially superimposes dual laser beams with different wavelengths and spot diameters, is superior in improving welding stability and quality. In this paper, the molten pool dynamic characteristics associated with the keyhole during fiber-diode laser hybrid welding were innovatively investigated to explore the impact of different laser energy on welding stability. The inner keyhole was observed by utilizing the “sandwich” structure with 2195 AlLi alloy and refractory glass. The experimental results indicate that during fiber-diode laser hybrid welding, a larger molten pool and plasma plume, deeper keyhole depth, and smoother keyhole rear wall are generated compared to single fiber laser welding. It is shown that the closing of the keyhole entrance and the swelling of the molten pool are effectively inhibited due to the superposition of the diode laser. The average keyhole depth during fiber-diode laser hybrid welding at the PD of 2500 W is substantially increased compared to that during single fiber laser welding. Based on the keyhole and molten pool characteristics, the mechanism of stability enhancement with the assistance of a diode laser is understood. In addition, the fiber-diode hybrid welding method can achieve a weld seam (WS) with high-quality surface morphology and a low porosity ratio of 0.7 %. This work provides the necessary theoretical basis and technical guidance for the fiber-diode laser hybrid welding technology of aluminum alloy.