Molten Pool Flow Research Articles

Study on heat transfer and flow of molten pool during the deposition process of laser wire additive manufacturing

In the laser wire additive manufacturing process, the molten pool acts as the critical link between the wire and the deposited part. The heat transfer and flow behavior within the molten pool predominantly determine the quality of the final deposition. Under laser power ranging from 2400 to 3000 W, traverse speeds between 0.01 and 0.04 m/s, and wire feeding speeds from 0.03 to 0.08 m/s, three distinct flow states—single-swirl, double-swirl, and no-swirl—were observed with increasing heat input. Under the optimum process parameters, the molten pool with stable temperature distribution and orderly flow was obtained. In multilayer deposition, the implementation of a laser decay strategy mitigates steep temperature gradients, diminishes the Marangoni effect within the molten pool, and effectively reduces both heat accumulation and lateral flow. Consequently, the flow mode transitions from no-swirl to swirl, and the maximum flow velocity decreases by 40%.

Microstructure and mechanical properties of aluminum alloy laser welded joint assisted by alternating magnetic field

Magnetic field-assisted laser welding is an effective method to improve the quality of laser-welded joints. In this study, laser welding was conducted on a 2 mm thick sheet of 6061-T6 aluminum alloy, with the assistance of an alternating magnetic field oriented perpendicular to the surface of the sheet. The effects of the alternating magnetic field on the macrostructure, molten pool flow, porosity, grain morphology, and microhardness of the weld seam were analyzed. The results indicate that applying the alternating magnetic field results in more uniform and finer fish scale patterns on the upper surface of the weld. The porosity of the weld is significantly reduced. The Lorentz force generated by the alternating magnetic field suppresses liquid metal flow from the center to the edges of the molten pool, thereby hindering the heat flow and transfer within the pool. This concentration of heat in the lower part of the molten pool increases the size of equiaxed grains at the weld center and promotes the formation of more branches in the columnar grains near the fusion line. Some columnar grains in the columnar grain zone transform into equiaxed grains. Moreover, the microhardness distribution of the weld becomes more uniform, and the hardness in the heat-affected zone increases. Tensile tests revealed that all samples fractured within the weld seam, with the tensile strength of the welded joints increasing by 12.7%. The fracture mode transitions from a mixed ductile-brittle fracture to a purely ductile fracture.

Weld Pool Flow Characteristics in Double-Wire Arc Welding of Aluminum Alloys: Research by Numerical Simulations

Double-wire arc welding involves simultaneously feeding two wires into a molten pool, improving the efficiency and flexibility of traditional welding techniques. However, the interactions between the two wires and the molten pools are complex, which increases the difficulties in process and composition control. This work focuses on the weld pool flow characteristics in double-wire TIG arc welding. A CFD model incorporating a liquid bridge transfer model was developed to simulate the fluid flow phenomenon. Results show that the bead-forming appearances and flow characteristics of double-wire arc welding show no significant differences from single-wire arc welding. Welding current and welding speed have significant effects on the weld bead dimensions, while only welding current has effects on the flow characteristics. Wire feed XOZ angles show no significant influences on weld bead forming appearances and molten pool flow characteristics. Wire feed XOY angles influence the symmetry of the weld bead and the fluid flow. In 5B71/7055 heterogeneous double-wire arc welding, achieving a uniform distribution of alloy elements is difficult due to the complex convection patterns within the molten pool.

Microstructure and mechanical properties of CP780 steel - 7075 aluminum alloy laser welded joint assisted by rotating magnetic field

Abstract This study uses a rotating magnetic field for laser welding on 1 mm thick CP780 high-strength steel and 1.5 mm thick 7075 aluminum alloy. The effects of different welding parameters (B = 0 mT, B = 65 mT with V = 0°/s, B = 65 mT with V = 10°/s) on the morphology, microstructure, and tensile properties of welded joints are analyzed. At B = 0 mT, the weld shape is V-shaped, with the intermetallic compounds primarily consisting of needle-like brittle Al-rich (Fe, Si)Al2 phase and fewer granular ductile Fe-rich (Fe, Si)Al phase, resulting in poor mechanical properties. With the application of the rotating magnetic field, the laser energy becomes more concentrated, forming a ‘T’ shape weld. The rotating magnetic field (B = 65 mT with V = 10°/s) generates a constantly changing Lorentz force, promoting molten pool flow and enhancing Fe diffusion within the weld. This process reduces needle-like brittle Al-rich (Fe, Si)Al2 phase and increases granular ductile Fe-rich (Fe, Si)Al phase. It also accelerates the weld cooling rate and inhibits the reaction time and grain growth of intermetallic compounds, thereby reducing the thickness and content of the intermediate transition layer and significantly improving mechanical properties. A comprehensive comparison shows that the best mechanical properties are achieved at B = 65 mT with V = 10°/s. This study offers new insights and a theoretical foundation for achieving cost-effective, high-performance welded joints in advanced high-strength steel and high-strength aluminum alloy for automobiles, thereby facilitating lightweight vehicle development.

Enhancing NiCrBSi coating by the spatial selectivity of in-situ shock wave assisted laser cladding

The composite energy field laser cladding (LC) can improve the mechanical properties of coatings. Based on the spatial selectivity of laser shock, in-situ shock wave assisted LC (in-situ SWALC) was proposed to fabricate NiCrBSi coatings. Firstly, based on the self-developed in-situ SWALC system, four NiCrBSi coatings were prepared on the 16Mn substrate by changing the centre-to-centre distance (CTCD) of two laser spots. Then, the differences of microstructure and element content between LC coating and in-situ SWALC coatings were analyzed. Vickers hardness tester and wear tester were used to compare the influence of CTCD. Finally, based on the theoretical calculation and propagation characteristics of the shock wave, combined with real-time monitoring of the molten pool during the in-situ SWALC process using high-speed imaging, the strengthening mechanism of the in-situ SWALC coatings was revealed. When the CTCD was 0.9 mm, the NiCrBSi coating had the highest hardness and wear properties, which was attributed to the effects of shock wave on the molten pool flow and solidification process. Therefore, the spatial selectivity of in-situ SWALC will play a important role in improving coating performance and energy efficiency, which contributes to the UN Sustainable Development Goal 9 (UN SDG 9).

Influence of laser absorptivity of CuCr0.8 substrate surface state on the characteristics of laser directed energy deposition inconel 718 single track

An effective method for fabricating copper-nickel bimetallic liquid rocket engine thrust chambers involves utilizing laser directed energy deposition (LDED) technology. However, the state of the substrate surface significantly impacts the LDED process. This study investigates the effects of various substrate treatments on LDED single tracks, using CuCr0.8 high-copper alloy as the substrate and Inconel 718 as the deposition material. The treatments include polishing, sandblasting, laser etching, and cold spraying. Substrate surface roughness, laser absorptivity, molten pool morphology, and microstructure were characterized, and the mechanisms of laser absorptivity change and the different LDED processes were analyzed. The results indicate that the laser-etched surface exhibits the worst surface roughness (Ra 15.20 ± 0.60 μm), the highest laser absorptivity(80.70% at 1080 nm wavelength), the largest deposition width (947.33 ± 29.85 μm), and the maximum number of fine grains among the four substrates. Additionally, the cold-sprayed surface shows the largest deposition depth (237.33 ± 39.04 μm), the minimum number of fine grains and a higher laser absorptivity (66.20% at 1080 nm wavelength). In situ observations of molten pool formation and flow during LDED was conducted using an in situ high-speed high-resolution imaging system. The mechanisms underlying the alteration in laser absorptivity primarily involve the "trapped light" effect and modifications to the surface material. This research is significant as it provides foundational insights for laser processing of highly reflective materials, offering important theoretical and practical implications for engineering applications.

The energy synergistic mechanism of coaxial heterogeneous-wavelength hybrid laser beam and its effect on welding molten pool dynamics for aluminum alloy

The coaxial heterogeneous-wavelength hybrid laser beam (HW-HLB) heat source featuring a “Gauss + Flat top” energy distribution, which integrates the 1080 nm and 915 nm laser beam, was employed in this paper to achieve excellent stability in the welding of aluminum alloy. The interaction mechanism between aluminum alloy and laser beams with different wavelengths was taken as a point cut to explore the synergistic enhancement mechanism of coaxial HW-HLB . A thermal-fluid coupling model for HW-HLB welding, considering the synergistic effect of dual lasers, was established according to the welding experiment results. The influence of fiber laser power (PF) and diode laser power (PD) on the molten pool flow behavior during welding process was investigated. The dynamic of the molten pool and the maintenance mechanism of the keyhole were elucidated through in-situ monitoring experiments and thermal-flow simulations. The results indicated that, in contrast to the single fiber laser beam (FLB), the power density of HW-HLB presents a significant increase, which results in the “avalanche-like” augmentation of the plasma plume, as well as deeper depth of the keyhole. The energy synergistic enhancement effect of coaxial HW-HLB with PF=2800 W and PD=2500 W performs stronger and thorough changes in the morphology and flow behavior of the molten pool. The introduction of 915 nm laser beam improves the multi-reflection behavior in the inner keyhole. The findings of our research provide a theoretical framework for improving the stability of the laser weld molten pool and the quality of welds in aluminum alloys through the implementation of HW-HLB.

Investigation of molten pool geometry and flow field based on powder-scale modeling in laser directed energy deposition

Investigation of molten pool geometry and flow field based on powder-scale modeling in laser directed energy deposition

Study on asymmetric forming and performance strengthening mechanisms of 6061-T6 aluminum alloy joints fabricated by oscillating laser based on keyhole characteristics and molten pool dynamic behaviors

The study intends to unravel the processes behind the particular forming and performance improvement of 6061-T6 aluminum alloy joints by getting a thorough understanding of keyhole features and dynamical characteristics of molten pools during the laser welding with circular oscillation (C-OLW) process. The C-OLW experiments were carried out considering the pivotal parameters, including oscillation frequency (f) and amplitude (A), and their impacts on joint formation, microstructure, and performance were analyzed. Increasing f from 200 Hz to 300 Hz results in a more homogeneous and centered laser energy distribution, reducing weld breadth and the maximum depth. Raising A from 1.2 mm to 1.5 mm concentrated energy along the workpiece surface, increasing weld breadth but decreasing the maximum weld depth. A three-dimensional transient numerical model was established and verified. The simulation results reveal that increasing f considerably improves keyhole stability by virtue of the beam's mixing effect on energy, resulting in a more evenly distributed temperature within the molten pool. The velocity of the beam's motion intensifies, leading to elevated local temperature and subsequently causing an escalation in splattering. When A grows, keyhole stability reduces, as does the intensity of molten pool flow. The major effect of increased frequency is refining the grain size while increasing amplitude primarily promotes the columnar-to-equiaxed transition (CET) process. The extracted results of temperature distribution illustrate that an increase in f leads to an increase in the solidification rate (R), and temperature gradient values (G) are similar or higher along the same isotherm. Therefore, the G/R ratio rises, promoting the formation of finer microstructures. Although increasing A diminishes G, the reduced overall temperature and molten pool volume contribute to the rise in R. Consequently, the G/R ratio declines, fostering equiaxed grain growth, in line with experimental observations. A higher G value on the left side corresponds to finer grain structures, corroborating experimental findings via Electron Backscatter Diffraction (EBSD). The keyhole stability and microstructure characteristics explain why increasing f or A can enhance joint performance. The research reveals that strategically augmenting both oscillation frequency and amplitude enables the achievement of enhanced performance in 6061-T6 aluminum alloy joints.

Study on molten pool flow and hot cracking of narrow gap laser welding of 316LN stainless steel for fusion reactor

In this paper, narrow-gap laser welding (NGLW) of 316LN stainless steel was studied. The results indicated that continuous laser welding with filler wire (CLWFW) and pulsed laser welding with filler wire (PLWFW) can obtain good weld formation under the transition of the wire bridge, and when the pulsed laser was a trapezoidal wave, the hot cracks of the filling layer can be greatly inhibited. The intermittent output of the pulsed laser reduced the temperature gradient of the molten pool and enhanced the morphology of the caudal region of the molten pool. Meanwhile, timely liquid backfilling effectively relieved the stress concentration of the weld, thus inhibiting the generation of hot cracks. Besides, the intermittent pulse energy input had a stirring effect on the molten pool, which disturbed the growth direction of the dendrite grain and refined the grain; consequently, the impurity elements were fully diffused, and the distribution of elements in the weld center area was more uniform.

Effect of heat treatment on microstructure and mechanical properties of oscillating laser-arc hybrid welded 12Cr13 thick plate

Heat treatment of oscillating laser-arc hybrid welded 12Cr13 martensitic stainless steel was carried out, the microstructure and mechanical properties were studied. The results showed that sound formation was obtained without crack, pores and lack of fusion. The microstructure of the weld was mainly columnar austenite with strong texture. After heat treatment, the microstructure was coarsened but the homogeneity was improved with the grain sizes in the weld center of about 250 μm. The fractions of the high angle grain boundaries decreased while the fractions of CSL boundaries increased. After heat treatment, the strength and hardness of the weld decreased. However, the elongation and section shrinkage rate are increased by 25 % and 20 %, respectively, and the impact energy of the heat affected zone is increased from 62 J to 88 J. Finally, the effect of the oscillating laser beam on the energy distribution and molten pool flow was discussed, the effect mechanism of heat treatment on microstructure and properties of weld was illustrated, and the relationship between the microstructure and properties of the weld was established.

Wire-arc additive manufacturing of TiB/Ti6Al4V composites using Ti-TiB2 cored wire: processing, microstructure and mechanical properties

ABSTRACT A novel wire-arc additive manufacturing (WAAM) process utilising titanium flux-cored wire (FCW) containing TiB2, Al60V40 and Ti6Al4V powders was proposed for fabricating TiB/Ti6Al4V composites. The FCW exhibited the capability in customising the composition of TiB/Ti6Al4V composites from hypoeutectic to hypereutectic by tailoring the proportion of TiB2 in blended powders. The comprehensive influence of TiB2 content on molten pool flow, compositional homogeneity and porosity of as-printed composites was attributed to the reduction of melt flowability, driven by viscosity escalation. It was unravelled the porosity was the intrinsic deficiency for the FCW-WAAM process, originating from the gases within FCW, while the interlayered TiB clusters were caused by the lagging melt of TiB2 powders. Due to directional solidification, TiB whiskers possessed a large aspect ratio and were partial to upward growth in hypoeutectic composites, which enhanced the load transfer strengthening. However, the porosity degraded the ductility of composites and caused the variation in mechanical performance.

Processing correlations of laser direct energy deposited Inconel718 based on multi-field numerical simulation

Laser direct energy deposited (LDED) Inconel718 always have problems of non-uniform temperature distribution because of the complex flow of molten pool and layer-by-layer fabrication characterization which directly affected the microstructure and mechanical properties of materials. In this work, Inconel718 revealed a refined grains and a weave structure exhibiting excellent mechanical properties which were superior to those of the conventional casting process were successfully fabricated by adopt a high-precision and intelligent coaxial powder feeding system during the LDED processing. Moreover, based on multi-field numerical simulation method, a two-phase flow model and a transient thermal mechanical deformation model were established to simulate the LDED processing, while takes into account the evaporated mass loss of the inconel718 alloy. The morphology prediction of laser directed energy deposition by multi-physics numerical simulation, the relationship among the laser process parameters, i.e., the laser power, scanning speed and the amount of powder feeding, the temperature filed and the microstructure and mechanical properties of Inconel718 alloy during LDED were investigated and established. The simulated results consistent well with the experimental results. Results shown that the relative density of sample was sensitive to the laser power, the precision and stability of the powder feeding rate. With increasing laser power, the grain size of Inconel718 is refined, and the microhardness and tensile properties of the materials increased. The optimized LDED processing parameters were determined. Evaluating the mechanical properties of the materials LDEDed at a laser power of 600W and a scanning speed of 8.5mms-1 exhibited a high tensile strength of 944±5 MPa and a good elongation of 29 ± 0.5 %. This work suggests that the processing parameters can be optimized to prepare densified materials with excellent mechanical properties.

Analysis and optimization of laser cladding Ti-B4C composite coatings based on the interaction and GABP-NSGAII algorithm

Laser cladding process parameters are important to the quality of cladding layer, but the interaction between parameters is not clear. In this work, the effects of parameters, especially interaction, on laser cladding of Ti-B4C coating were studied by orthogonal experiments. And the optimal process parameters were obtained by GABP-NSGAII multi-objective optimization algorithm. The results show that the interaction (B4C content and laser power) had the greatest effect on the coating aspect ratio, cladding angle, and dilution rate. The microhardness of the coatings prepared with the optimum parameters of 15 wt%, 2200 W, and 800 mm/min is 434 HV0.5, which consists of columnar, dendritic, and equiaxial structures from the interface to the surface, respectively. The interaction affects the molten pool flow and the distribution of unfused B4C particles, and influences the concentration and distribution of reinforcing phases in the coating, as well as the heat input and phase transitions in the heat affected zone. This paper provides a reference for the preparation of high-performance and high-quality titanium matrix composite coatings by laser cladding.

Porosity suppression mechanism analysis in narrow-gap oscillating laser-MIG hybrid welding of aluminum alloys based on keyhole stability and molten pool flow behavior

In this study, the keyhole dynamic behavior and molten pool flow were investigated to elucidate the effects of oscillating laser on porosity in narrow-gap oscillating laser-MIG hybrid welding of aluminum alloys. A weld with a porosity of 0.08% was produced at the laser oscillating frequency of 600 Hz and diameter of 1 mm, while 2.27% without oscillating laser. Compared to non-oscillating laser, keyhole opening size increased by about 2.3 times and keyhole depth fluctuation was suppressed. The forces acted on the interface between keyhole wall and molten pool became more balanced. The increased vapor recoil pressure overcame the combined closure effect of gravity and molten pool pressure. While applying oscillating laser, the vortex in molten pool was eliminated and the liquid metal flow was more stable. The combined impact force caused by droplets transfer and liquid metal convection due to constraining effect of narrow gap was buffered, which had less effect on molten pool and keyhole stability. The liquid metal tended to flow from the molten pool inside to surface, which favored the escape of bubbles from the molten pool.

Mantis shrimp appendages-inspired Ag-CuO contact: Synergistically enhancing the erosion and impact resistance via gradient-layered scatter-island/chain-skeleton structure

Materials degradation induced by repeated arc erosion and impact poses a considerable challenge for Ag-based contacts. Fortunately, nature has evolved efficient strategies to construct complex gradient-layered structure that exhibit exceptional erosion and impact resistance. Drawing inspiration by the mantis shrimp appendages, a chain-like CuO nanofibre-reinforced Ag-CuO contact material with a gradient-layered scatter-island/chain-skeleton structure was designed and fabricated. Employing a combined experimental and simulation approach, the evolution of the microstructure during the erosion process was investigated. Results indicated that the interaction between the mantis shrimp appendage-like structure and the chain-like CuO nanofibres effectively suppressed the formation of cracks and pores on the contact surface. Particularly, the scatter-island layer not only played a crucial role in maintaining the viscosity of the molten pool and establishing stable electrical and thermal pathways, but also functioned as a “supplement” to continuously supply a steady source of Ag onto the molten pool surface, effectively promoting CuO skeleton restructuring. Meanwhile, the CuO skeleton diffusive-continuous restructuring of the chain-skeleton layer restricted molten pool flow, significantly enhancing contact erosion resistance. Furthermore, the strong synergistic effect between the gradient-layered structures dispersed stress and deformation concentration on the contact surface. It effectively delayed the defects formation and suppressed interfacial debonding, thereby endowing the contact with excellent impact resistance. Our work presents a fascinated route for designing novel biomimetic Ag-based contacts.

Numerical simulation and morphological analysis of laser cladded 316L stainless steel on inclined substrates

Laser cladding has been mostly studied based on the horizontal substrate state, but the component surface to be repaired is often complex in actual restoration process. Inclined substrate repair is the basis for studying complex surface repair. In this study, 316L stainless steel powder was taken as the research object. The simulation and experimental verification combined method was adopted to study the morphology of single-track and multi-track overlap on the inclined substrate. The main research contents were as follows: Through the simulation model, the molten pool flow and final cladding layer morphology under different scanning paths of laser cladding on the inclined substrate were analyzed. The results showed that when the inclination angle was less than 90°, the change of the cladding layer morphology had regularity. When the inclination angle was greater than 90°, splashing and dripping occurred in the molten pool under several scanning paths, and the cladding layer morphology was highly unstable. Excessively large inclination angle was not suitable for coaxial powder feeding. Neither positive nor negative y-direction scanning overlap cladding layer was horizontal under inclined posture. The cladding layer morphology with x-positive scanning and y-negative overlap was the most stable. An inclined cladding model considering the inclination factor was established in this study, which provided theoretical guidance and predictive analysis for laser cladding on the inclined substrate. Contributions were made to process optimization, path optimization and improvement of cladding layer quality for inclined substrate cladding in the future.

Multi-layer multi-track molten pool flow and grain morphology evolution of Inconel 718 manufactured by laser powder bed fusion

Multi-layer multi-track molten pool flow and grain morphology evolution of Inconel 718 manufactured by laser powder bed fusion

Investigation of interfacial microstructure and connection mechanism of magnetic field-assisted magnesium/aluminum laser welding joint

Variable process experiments to prepare hybrid joints of AZ31 magnesium (Mg) and 6061 aluminum (Al) alloys by magnetic field-assisted laser welding were performed in an overlap Mg-on-Al configuration with the addition of titanium (Ti) foil. Temperature field and flow field of molten pool including chemical potential of each element were also calculated. It is shown that at the optimal welding parameters, the shear force of joint reaches 887.79 N, which is 83.4 % higher than that of no-added joint. In the case of external magnetic field, the improved mechanical properties of joints largely depend on the diffusional barrier effect of Ti foil, which benefits from the enhanced energy absorption and flow of molten pool leading to the enlargement of interface connection area and the formation of Ti-Al compounds such as TiAl3 at Mg/Al interface. Meanwhile, the grains refines due to the magnetic field, which reduces the strain at the joint and enhances the uniformity of the interface microstructure. However, under the condition of excessive magnetic field strength, the enhanced stirring effect of molten pool will cause Ti foil break and the direct interaction between Mg and Al, which deteriorates the joint properties.

Porosity suppression and mechanical property improvement of wire-arc directed energy deposited TiCp/Al-Cu alloys joint by oscillating pulsed wave laser beam welding

Hybrid manufacturing technology combining wire-arc directed energy deposition (WA-DED) and laser beam welding (LBW) presents a promising avenue for efficiently manufacturing large-size aluminum alloy components. The high tendency of porosity defects, particle decomposition, and fusion zone softening are outstanding challenges in the LBW welding of additively manufactured particle-reinforced aluminum alloys. Herein, a WA-DED-processed Al-6.3Cu alloy embedded with 0.6 wt% TiC particles was employed as weldment. Four different laser beam modes, including continuous wave (CW), pulsed wave (PW), oscillating continuous wave (OCW), and oscillating pulsed wave (OPW), were used to join TiCp/Al-6.3Cu alloy. The effects of different laser modes on the porosity defects, microstructure evolution, and mechanical properties of WA-DED particle-containing aluminum alloy joints were studied comparatively. Multi-scale characterization results showed that the enhanced molten pool flow via laser beam modulation effectively suppressed porosity defects and facilitated the uniform particle distribution. The OPW mode provides an optimal effect on porosity inhibition and weld microstructure refinement compared to the pulsed or oscillating modulation modes. The heat-treated LBW joint via OPW mode achieved excellent comprehensive mechanical properties, whose elongation and ultimate tensile strength reached 9.5% and 449 MPa, respectively. This enhanced performance can be attributed to the decreased porosity, refined weld microstructure, and high-density nano-precipitation. This study provides fundamental guidance for the welding of high-strength Al-Cu alloy fabricated by additive manufacturing.