Molten Pool Morphology Research Articles

Track trajectory, molten pool and defect characterization in NiTi single-track selective laser melting (SLM) experiments

In order to explore the law of selective laser melting (SLM) forming of Nitinol alloy, and obtain high quality additive manufacturing parameters of Nitinol alloy, a series of single-track SLM experiments on Nitinol alloy were conducted under different combined parameters of laser power and scanning speed. The effects of laser power, scanning speed and linear energy density on track forming, molten pool evolution and pore formation were systematically discussed by characterizing the track trajectory, molten pool morphology, dimensions and defects of the molten pool, and the ideal process parameters were obtained. The experimental results revealed that the average track width and width uniformity decreased with the increase of the scanning speed, the spheroidization phenomenon and height fluctuation became more pronounced, and the phenomena of necking, subsection and interruption increased significantly at the fixed laser power. Under the fixed scanning speed, the width, height and width uniformity of the molten track gradually increased with the increase of the laser power, and the phenomena of spheroidization, subsection and interruption gradually reduced. Moreover, the upper zone of the molten pool is shaped like an inverted bowl, and the lower zone is mainly divided into five categories, which are lack of fusion, shallow bowl shape, deep bowl shape, keyhole shape and deeper keyhole shape under different process parameters. The defects of the molten pool mainly include lack of fusion and trapped gas. An ideal molten pool without defects can be obtained under the parameter conditions of 100-150W laser power and 100-200mm/s spanning speed.

Size effects and optimization during laser directed energy deposition on high thermal conductivity copper alloys

High-copper alloy/nickel-based high-temperature alloy bimetallic structures are widely utilized in critical components, such as thrust chambers of liquid rocket engines, water cooling circuits for fusion reactor deflectors, and electromagnetic railguns. Among various methods for fabricating Cu/Ni bimetallic structures, laser directed energy deposition (LDED) is considered one of the most promising techniques for depositing nickel-based high-temperature alloys onto high-copper alloy surfaces. In the LDED fabrication process, substrates with uneven thicknesses are a common. The process is characterized by its multi-parameter, high-sensitivity, and non-equilibrium solidification characteristics. Consequently, variations in substrate size can significantly affect molten pool morphology and fabrication stability. This study examines the size effect of a single track of LDED Inconel 718 (In718) on a CuCr0.8 high-copper alloy substrate. The results indicate a pronounced size effect when using a high thermal conductivity CuCr0.8 substrate, particularly regarding substrate thickness. By reducing the thermal conductivity of the CuCr0.8 substrate, the size effect is mitigated, improving the process adaptability of LDED. These findings provide valuable data for the laser processing of surfaces with uneven thickness and high thermal conductivity. They are expected to support the fabrication of variable thrust liquid rocket engines and advance the field of interplanetary exploration.

Research on the evolution mechanism of surface morphology and solidification crystallization simulation of molten pool in electron beam polishing

Clarifying the flow and solidification process rules of a molten pool in scanning electron beam polishing remains challenging. This paper establishes a geometric model that closely resembles the original milling morphology, investigates the morphology evolution process, the flow law of the molten pool, and the change in driving force under the action of the electron beam, and solves the solidification and crystallization parameters of the solid-liquid interface. Results demonstrate that thermocapillary force is the dominant driving factor for horizontal motion inside the molten pool, which immediately leads to the emergence of velocity conversion sites on the surface of the molten pool, thus effecting surface fluctuation variations. Combined with polishing inspection, the average surface roughness error is controlled within ±7 %. Solid-liquid interface deflection significantly affects grain growth, and solidification and crystallization parameters follow similar patterns at different workpiece speeds. The grain structure of the melt zone can be predicted with the crystallization parameters at various depths of the melt layer.

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.

Deep Learning-Driven Prediction of Mechanical Properties of 316L Stainless Steel Metallographic by Laser Powder Bed Fusion.

This study developed a new metallography-property relationship neural network (MPR-Net) to predict the relationship between the microstructure and mechanical properties of 316L stainless steel built by laser powder bed fusion (LPBF). The accuracy R2 of MPR-Net was 0.96 and 0.91 for tensile strength and Vickers hardness predictions, respectively, based on optical metallurgy images. Feature visualisation methods, such as gradient-weighted class activation mapping (Grad-CAM) and clustering, were employed to interpret the abstract features within the MPR-Net, providing insights into the molten pool morphology and grain formation mechanisms during the LPBF process. Experimental results showed that the optimal process parameters-190 W laser power and 700 mm/s scanning speed-yielded a maximum tensile strength of 762.83 MPa and a Vickers hardness of 253.07 HV0.2 with nearly full densification (99.97%). The study marks the first application of a convolutional neural network (MPR-Net) to predict the mechanical properties of 316L stainless steel samples manufactured through laser powder bed fusion (LPBF) based on metallography. It innovatively employs techniques such as gradient-weighted class activation mapping (Grad-CAM), spatial coherence testing, and clustering to provide deeper insights into the workings of the machine learning model, enhancing the interpretability of complex neural network decisions in material science.

Influence of welding sequences and boundary conditions on residual stress and residual deformation in DH36 steel T-joint fillet welds

This paper aims to investigate the influence of welding sequences and boundary conditions on welding-induced residual stress and residual deformation of DH36 steel T-joint fillet welds through experimental and numerical methods. A series of typical experiments of successive and simultaneous double-sided welding conditions are conducted to explore both the magnitude and distribution of temperature field, residual stress and residual deformation. Meanwhile, thermal elastic-plastic finite element analysis based on a double ellipsoidal heat source model is performed to investigate the heat transfer and deformation mechanism during the welding process. The influencing factors are further explored to determine and quantify the residual stress and residual deformation induced by different welding sequences and boundary conditions. There is generally well agreement between the experimental and numerical results in terms of temperature field, residual stress, residual deformation and molten pool morphology. The results show that the welding sequences have an important influence on the magnitude and distribution of residual stress and residual deformation. Continuous simultaneous double-sided welding can significantly reduce residual stress and residual deformation to obtain better-quality welds. Boundary conditions have a significant effect on the distribution and magnitude of residual deformation and little effects on residual stress. Tightening the ends of the flange plate with bolts during the welding process can significantly reduce the deformation.

Role of layer arrangement in microstructure and corrosion behavior for 78-mm thick-plate 316L stainless steel fabricated via narrow gap laser wire filling welding

Controlling repeated thermal cycling is one of the key mechanisms to design microstructures in the interlayer regions for narrow gap laser wire filling welding (NGLWFW) of austenite stainless steels. Herein, the optimization of process parameters for individual layers is tailored to customize the molten pool morphology during the NGLWFW process, facilitating precise control over the microstructure and, consequently, enhancing both mechanical properties and corrosion resistance of the joint. The results showed that the molten pool morphology acquired by simulation are obviously diverse from the bottom zone (BZ), middle zone (MZ) to upper zone (UZ) of the welded joint ascribed to the high heat accumulation effect, leading to the variation of microstructure. At BZ, the combination of smaller grain size and reduced dendrite spacing reduce the number of corrosion sites and promote the formation of a denser and more uniform corrosion product layer. Additionally, the diverse grain orientation, characterized by lower texture strength, facilitated a more uniform distribution of elements. Furthermore, the significantly higher content of δ-ferrite showing smallish shape disrupted the directional growth of austenite columnar grains, contributing to the excellent corrosion resistance observed at the BZ.

Determination of residual stress field in laser cladding using finite element updating method driven by contour deformation

Laser cladding is a cutting-edge technology widely used in additive manufacturing and surface modification. This process entails rapid melting and solidification driven by laser heating, which induces residual stress within the material. This study presents a data-driven approach to refine the numerical model of laser cladding, simultaneously emphasizing both the temperature characteristics and residual stress distribution within the thermomechanical coupling process. After integrating the residual stress distribution data obtained through the contour method with the molten pool morphology shaped by the thermal effects of laser cladding, a Levenberg-Marquardt optimization algorithm based finite element model updating scheme is proposed to efficiently identify the parameters of double ellipsoid heat source model, which employs parallel computing to reduce the computational time for iterations. The effectiveness of the proposed method is validated using experimental data. Moreover, this approach is adaptable, unaffected by the specimen size and shape, and can accurately predict the residual stress distribution characteristics. Therefore, it offers a robust framework for simulating the mechanisms of residual stress generation in titanium alloy laser cladding.

Achieving Equiaxed Transition and Excellent Mechanical Properties in a Novel Near-β Titanium Alloy by Regulating the Volume Energy Density of Selective Laser Melting.

This research investigated the relationship between volume energy density and the microstructure, density, and mechanical properties of the Ti-5Al-5Mo-3V-1Cr-1Fe alloy fabricated via the SLM process. The results indicate that an increase in volume energy density can promote a transition from a columnar to an equiaxed grain structure and suppress the anisotropy of mechanical properties. Specifically, at a volume energy density of 83.33 J/mm3, the average aspect ratio of β grains reached 0.77, accompanied by the formation of numerous nano-precipitated phases. Furthermore, the relative density of the alloy initially increased and then decreased as the volume energy density increased. At a volume energy density of 83.33 J/mm3, the relative density reached 99.6%. It is noteworthy that an increase in volume energy density increases the β grain size. Consequently, with a volume energy density of 83.33 J/mm3, the alloy exhibited an average grain size of 63.92 μm, demonstrating optimal performance with a yield strength of 1003.06 MPa and an elongation of 18.16%. This is mainly attributable to the fact that an increase in volume energy density enhances thermal convection within the molten pool, leading to alterations in molten pool morphology and a reduction in temperature gradients within the alloy. The reduction in temperature gradients promotes equiaxed grain transformation and grain refinement by increasing constitutive supercooling at the leading edge of the solid-liquid interface. The evolution of molten pool morphology mainly inhibits columnar grain growth and refines grain by changing the grain growth direction. This study provided a straightforward method for inhibiting anisotropy and enhancing mechanical properties.

Characterization of microstructure and mechanical properties of Ta-10W alloy manufactured by laser powder bed fusion

Ta-10W alloy is an important high temperature structure material because of its high melting point and excellent high temperature performance. Laser powder bed fusion (LPBF) provide a new scheme to break limitation in producing Ta-10W complex structures. High-density Ta-10W samples manufactured by LPBF were first achieved in this work. The variation of molten pool morphology and interior defects with energy density was investigated, and excellent mechanical properties in room temperature were obtained.

Achievement of a Parameter Window for the Selective Laser Melting Formation of a GH3625 Alloy.

In the selective laser melting (SLM) process, adjusting process parameters contributes to achieving the desired molten pool morphology, thereby enhancing the mechanical properties and dimensional accuracy of manufactured components. The parameter window characterizing the relationship between molten pool morphology and process parameters serves as an effective tool to improve SLM's forming quality. This work established a mesoscale model of the SLM process for a GH3625 alloy based on the discrete element method (DEM) and computational fluid dynamics (CFD) to simulate the forming process of a single molten track. Subsequently, the formation mechanism and evolution process of the molten pool were revealed. The effects of laser power and scanning speed on the molten pool size and molten track morphology were analyzed. Finally, a parameter window was established from the simulation results. The results indicated that reducing the scanning speed and increasing the laser power would lead to an increase in molten pool depth and width, resulting in the formation of an uneven width in the molten track. Moreover, accelerating the scanning speed and decreasing the laser power cause a reduction in molten pool depth and width, causing narrow and discontinuous molten tracks. The accuracy of the simulation was validated by comparing experimental and simulated molten pool sizes.

Effects of process parameters on microstructure properties of WE43 magnesium alloy by selective laser melting

Magnesium alloys have broad application in aerospace, biomedicine and automobile, et al. Selective Laser Melting (SLM) additive manufacturing of magnesium alloy can fabricate complex components with acceptable properties. However, the low evaporation heat of magnesium alloy at ambient temperature leads to vaporization instead of melting during the SLM forming process, posing significant challenges for achieving successful SLM fabrication of magnesium alloys. Hence, the objective of this paper is to analyze the impact of various process parameters on the distribution of macroscopic defects and the relative density during SLM additive manufacturing test of WE43 magnesium alloy samples. Additionally, it investigates the effect of forming parameters on the microstructure by examining molten pool morphology, grain size, and material composition. Mechanical property tests are conducted to establish correlations between relative density, microhardness, strength, and energy density in printed samples. Consequently, optimal SLM manufacturing parameters for WE43 magnesium alloy are determined as follows: laser power 200 W and scanning speed 800 mm/s. This study successfully achieves high-performance additive manufacturing of magnesium alloy materials.

Texture tailored through combination of molten pool morphology and scanning path in laser additive manufacturing of alloy 718

Texture control is vital for the performance improvement of laser additive manufacturing parts. In the current manuscript, the effect of molten pool morphology and scanning path on texture have been studied for Inconel 718 sample fabricated by directed energy deposition (DED). The molten pool formed at a pulse period of 20 ms is shallower than that formed at a pulse period of 100 ms. The V-shaped grains (<110> // BD) are generated in the overlapping area for all samples. While Z-type grains (<110> // SD) are generated in the molten pool center for the sample with bidirectional scanning path at a pulse period of 20 ms. The unique texture with V-shaped grains (<110> // BD) in the overlapping area and Z-type grains (<110> // SD) in the molten pool center was firstly observed in the current manuscript. The value of Taylor factor of Z-type grains (<110> // SD) is larger than that of V-shaped grains (<110> // BD). Meanwhile, the effect of different heat treatments on the texture evolution was analyzed. This research provides a promising approach for texture customization, which can also increase the understanding of texture evolution.

Influence of scale effect on surface morphology in laser powder bed fusion technology

ABSTRACT Laser powder bed fusion (LPBF) of multi-scaled structures is a prominent direction pursued by innovative designs and engineering applications. However, understanding the impact of scale effect on the effectiveness of actual structures is a pressing challenge. This work investigates the influence of structure scale on surface morphology and mechanical property by fabricating tensile samples with gauge diameters ranging from 0.2 mm to 5.0 mm. For the hatching scanning strategy, the surface roughness drastically reduces by ∼51% as the diameter increases. This appears to be the coupling results of heat accumulation, molten pool morphology at the end of single-track, overlap between multi-tracks within a layer, and successive rotational buildup between layers. Moreover, decreasing the diameter substantially reduces the tensile strength. Fortunately, a hatching-contour scanning strategy is proposed to eliminate the scale effect of surface morphology and strength. These efforts provide essential guidance for the broader application of multi-scaled structures.

Microstructure and molten pool morphology of TiB2/ AlSi7Mg composites fabricated by infrared-blue hybrid laser powder bed fusion

A hybrid laser composed of infrared and blue laser is applied in fabricating TiB2/AlSi7Mg composites on AlSi7Mg substrate by LPBF. The effect on formability, molten pool morphology, molten pool size and microstructure under infrared, blue and hybrid laser were compared. It was confirmed that hybrid laser can make up for the unbalanced energy distribution of infrared laser and the low energy density of blue laser. The increased energy input improves the molten pool size and cellular dendrites size. Therefore, the hybrid laser can improve the formability and forming stability in the LPBF process of low absorption rate alloys.

Effects of electromagnetic compound field on the macroscopic morphology of laser cladding

To enhance the controllability of the molten pool morphology, a directional Lorentz force is produced within the molten pool by synchronously coupling a steady electric field and magnetic field during the laser cladding process. A 2D numerical model of the molten pool considering solid–liquid phase change, heat transfer, fluid flow, deformed geometry and electromagnetic induction is established. The experimental and simulation results demonstrate that the magnetic or electrostatic field has negligible effects on the molten pool shape. The compound electromagnetic field (simply as a compound field) that generates a downward Lorentz force significantly impacts the molten pool height, and wetting angle. As revealed by the observation of the molten pool forming process, it is found that the downward Lorentz force disrupts the force balance of the molten pool, which leads to the outward expansion of the fluid due to an increased pressure difference. As a consequence, the height of the molten pool decreases. Meanwhile, the employment of a magnetic field results in a significant increase in the viscous resistance at the solidification interface. The fluid is accumulated at the bottom of the molten pool, which increases the wetting angle. Additionally, since this approach can produce a steady volume force similar to gravity, it can be used in a weightless environment, such as in a space station, thus significantly improving the process stability.

Deposit characteristics, morphology and microstructure regulation of single-track nickel-based alloy using quasi-continuous-wave laser direct energy deposition

A quasi-continuous-wave laser direct energy deposition (QCW-DED) process was employed to regulate the deposit characteristics, molten pool morphology and microstructure of single-track nickel-based alloy. The variation of the deposit characteristics with laser pulse frequency was investigated through experiment and numerical simulation. The molten pool flow pattern and the dendrite growth mode were described based on the observations of the deposition morphology and microstructure in the cross- and longitudinal-sections. It was concluded that the QCW-generated single-track deposits decreased in width and depth, yet increased solely in height as the pulse frequency increased. A “U” shaped structure composed of substrate materials was observed in the molten pool at the low pulse frequency. The flow in the molten pool of QCW specimens was alternately dominated by the Marangoni force and impact force, whereas the fluid flow in the molten pool formed by continuous wave (CW) was entirely dominated by the Marangoni force. The disappearance of the “U” shaped structure and the rapid remelting effect of the molten pool underlie the epitaxial growth of dendrites at the high pulse frequency. In the CW specimen, disorganized dendrites were produced in the heat flow direction along the molten pool boundary.

Effects of Laser Power on Molten Pool Morphology, Microstructure, and Mechanical Properties of Al2O3‐ZrO2 Eutectic Ceramics Shaped by Laser Powder Bed Fusion

Effects of Laser Power on Molten Pool Morphology, Microstructure, and Mechanical Properties of Al2O3‐ZrO2 Eutectic Ceramics Shaped by Laser Powder Bed Fusion

Microstructures and mechanical properties of additively manufactured T91 steel and T91/316H bimetallic components by laser powder bed fusion

Precise deposition of multiple materials by additive manufacturing (AM) has considerable potential for controlling the properties of materials. In this work, T91 martensitic steel with various process parameters was deposited on 316H steel fabricated by laser powder bed fusion (L-PBF), and the microstructure, mechanical properties, and corresponding defect formation of the T91 material were investigated based on the volumetric energy density (VED). This work studied the interface characteristics and properties of T91/316H bimetallic components with various process parameters to improve the performance of joint between martensitic and austenitic steels by AM. The presented results showed that the tensile strength and uniform elongation (UE) appeared to increase with the VED of the T91 materials. The ultimate tensile strength and UE of the T91 materials significantly decreased when the VED of the T91 materials approached 110 J, which was determined by the dramatic growth in defect number and defect size. Additionally, these bimetallic components obtained favorable tensile properties, exceeding those of the 316H sections, ascribed to solid solution strengthening at the interface. The 316H grains (γ) and the T91 grains (α) near the interface follow the K–S orientation relationship: {111}γ//{110}α, <110>γ//<111>α. The process parameters of the T91 sections obviously affected the tensile strength of these bimetallic components, as well as the molten pool morphology and mechanical properties of the 316H section near the interface.

Thermo-fluid dynamics and morphology evolution of nickel-based superalloy in a multilayer laser directed energy deposition process

The complex thermo-fluid dynamics of the molten pool determine the dimension accuracy and mechanical properties of laser directed energy deposition (L-DED) components. In this work, a three-dimensional (3D) multiphase computational fluid dynamics (CFD) model based on the Volume of Fluid (VOF) method was proposed to establish the relationship between the thermo-fluid dynamics and the molten pool morphology evolution of René N5 nickel-based superalloy in L-DED. The single-track and ten-layer deposition processes under unidirectional and bidirectional scanning strategies were studied. Results show that the predicted deposition morphology and dimensions are in good agreement with the experimental samples. The heat accumulation in the multilayer deposition process increases the size of the molten pool, thereby increasing the width of the deposit. Different from the single-layer deposition driven by the Marangoni force, gravity is the key factor in the multilayer deposition process without the side support. The hump structure at the starting position under unidirectional scanning is attributed to the backward flow of the melt of the molten pool, while the decline structure at the end position is enhanced by the forward flow of the melt. At the starting position of each layer under bidirectional scanning, the expansion of the molten pool resulted from the heat accumulation and mass addition increases the width of the two ends and compensates for the deposition height. This work can provide a theoretical basis for understanding the uneven morphology characteristics and the dimensional accuracy control in L-DED.