Recoil Pressure Research Articles

Multi-physical field simulation to yield defect-free IN718 alloy fabricated by laser powder bed fusion

In this study, we used a model that combines thermal, mechanical, and fluid dynamics principles to analyze the melting and solidification process in laser powder bed fusion (LPBF). To ensure accurate simulation results, our computational fluid dynamics model takes into account various factors, such as heat transfer, radiation, molten metal flow, phase change, and the interaction between the laser and the material. We found that the combined effects of recoil pressure, Marangoni convection, and surface tension play a crucial role in the evolution of the melted IN 718 alloy. Excessive recoil force can cause powder spatters and keyholes, while inefficient Marangoni effect leads to non-uniform melting. Through our simulation-experimental investigation, we were able to achieve a high-density (99.96 %) IN 718 alloy at a scanning speed of 700 mm/s. This research demonstrates the ability of a multi-physical model to accurately predict the quality of melted materials and contribute to the development of new materials, not limited to nickel-based alloys in LPBF.

Study of melt pool dynamics and porosity forming mechanism of laser beam oscillation welding of titanium and aluminum

In this study, a numerical model of oscillation weld butt joint is developed to investigate the welding of titanium alloy with aluminum alloy. Three oscillation paths, namely, straight, sine, and circular, are used to study the distribution of force in the molten pool, the welding temperature field, and the formation and evolution of porosity within the weld. A 3D Gaussian heat source is used to represent the laser beam. The volume of the fluid method is employed to track the gas-liquid free surface, and the gas-liquid interface force is transformed by using the continuous surface force model. The mechanism of keyhole collapse and pore formation was examined along with the fluid flow, surface tension, and recoil pressure on the molten pool. The results confirmed that the highest welding quality is acquired by using a laser welding circular path. Notably, numerical simulation results are validated through experimental data, and circular oscillating laser welding significantly reduced weld seam porosity in the welding of Ti–Al dissimilar alloys. The circular oscillation path with an offset of 0.6 mm and an oscillation amplitude of 0.6 mm is identified as the optimal approach for suppressing pores in the weld joint. This research provides valuable insights into the fundamental mechanisms of keyhole collapse and pore formation in laser welding, which contributes to the advancement of effective welding strategies for dissimilar alloys.

Theoretical and experimental investigations in thermo-mechanical properties of fused silica with pulsed CO2 laser ablation.

Laser ablation is widely used as a flexible and non-contact processing technology for the fabrication of fused silica. However, the introduction of thermal stress inevitably leads to crack growth and reduces the lifetime of fused silica. Due to the complicated coupling interaction and properties of fused silica, the unclear thermal stress formation is the bottleneck restricting further development of laser ablation. In this article, a three-dimensional multi-physics thermo-mechanical model was developed to reveal the evolution mechanism, and experiments were performed to validate the simulated results. The surface morphology evolution was elaborated during process cycles, with recoil pressure identified as the key factor in determining surface morphology. Moreover, thermal stress was quantified utilizing optical retardance and stress birefringence, effectively distinguishing between non-thermal and thermal stress induced by laser ablation. The theoretical simulations fit well with experimental measurements. Meanwhile, stress distribution and evolution behaviors were revealed under different processing parameters by this model. This work not only contributes to a profound understanding of the laser ablation process but also establishes a theoretical foundation for achieving high surface quality and non-thermal stress laser ablation.

Effect of initial/tail of the pulse waveform on surface morphology of AZ31 magnesium alloy laser spot welding

Effects of initial/tail of the pulse waveform on coupling efficiency and surface morphology of AZ31 magnesium alloy laser spot welding is investigated. Research shows that initial ramp-up waveform can improve the laser spot welding absorptance, but with limited effect (0.88 → 0.9), and melting time of ramp-up waveform is longer than rectangular for equal pulsed energies, making the welding energy coupling efficient and diameter larger. The ramp-down waveform can extend duration (9 ms → 12 ms) of molten pool above melting point temperature (630 °C), increase Marangoni number (R1: 29.29, R2: 32.11, R3: 23.70, R4: 26.62), slow down cooling rate (3 × 106 °C/s → 1 °C/s) and recoil pressure (R3: 52.90 P0 → 12.16 P0, R4: 95.95 P0 → 20.58 P0) of instantaneous closure of rectangular waveform, reduce the sensitivity of welding defects such as overfill, undercut and spatter, and improve the surface morphology of laser welding spots.

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.

Development of a comprehensive model for predicting melt pool characteristics with dissimilar materials in selective laser melting processes

Selective laser melting (SLM) is a popular additive manufacturing (AM) technique for the fabrication of precise components. The success of SLM relies on accurate control of the melt pool, where the metal powder melts and solidifies. Fluid flow inside the melt pool is driven by forces like Marangoni force and recoil pressure. SLM process begins with the fabrication of support, which involves the process of joining dissimilar metals. A strong support-base substrate connection ensures the success of subsequent additive manufacturing. In this study a comprehensive model was developed to predict the dimensions and characteristics of the melt pool with a focus on the first layer of the SLM process. An initial numerical simulation was done to obtain transient melt pool temperature and velocity distribution during the SLM process. The results were then coupled with a theoretical analysis development of the proposed model. The effectiveness of the model was validated by experimental results and melt pool characteristics from previous publications. The model shows Root Mean Square Error (RMSE) of 0.53 and 0.49 for height-to-width ratio and normalized area, respectively. The Mean Absolute Percentage Error (MAPE) of normalized area is 15.7% for the majority of the data, which demonstrates a good accuracy of the model. In addition, the transition from conduction mode to keyhole mode can be identified with the model. This model can be used to rapidly and effectively predict the melt pool characteristics for SLM and other similar AM processes with a wide range of processing parameters.

Modelling and simulation of surface formation in electrical discharge machining based on thermo-hydraulic coupling

Surface formation in electrical discharge machining (EDM) involves melting, evaporation, re-solidification and crater overlapping. However, almost all existing models concerning surface formation in EDM only consider the thermal process and ignore hydrodynamic phenomena during machining. Herein, a novel three-dimensional thermo-hydraulic coupling model incorporating the arbitrary Lagrangian–Eulerian scheme was developed to address the above problem. The model comprehensively considered the surface deformation caused by melt flow, temperature-dependent material properties, phase changes and the effects of practical forces on melt pools to simulate surface formation on an anodic workpiece at a discharge current of 2 A. Simulation results demonstrate that during single-pulse discharge, molten material flowed radially to the crater periphery. The radial melt flow was driven by evaporation recoil pressure and thermocapillary force. The effect of evaporation recoil pressure on melt flow was gradually counteracted by that of surface tension after discharge ignition, and thermocapillary force became the dominant factor that maintained the existence of bulges around craters. During multiple discharges, the bulges generated by different discharges underwent superposition owing to the flow of molten material and subsequently formed surface peaks. The simulation results also provide the measurements of crater diameter, surface roughness and recast layer thickness, which are consistent with experimental results. The proposed modelling method can provide a useful reference for further investigations on surface integrity in EDM with minimal computing resource requirements.

A high-fidelity comprehensive framework for the additive manufacturing printability assessment

Additive manufacturing is capable of fabricating complex and customized components that cannot be easily and economically produced by other techniques. It is of great significance to determine the printability map for the extensive application of the fabricated parts, which has been hindered by the common defects such as interior pores and surface roughness. Here, a comprehensive framework including multiphysics model, physics-informed machine learning, and experimental data is proposed to predict the printability. The characteristics for different phenomena (lack of fusion, balling and keyhole) are analyzed by the mechanistic model considering high-fidelity powder-scale model, fluid flow, recoil pressure and Marangoni effect, which provides a more accurate thermal history, molten pool dynamics and surface morphology compared to the finite element model. Classification criterion is established by three mechanistic variables based on the molten pool morphology, which divides the process map into four regions. For the first time, the relationship between the solidified-track surface morphology and the interior quality is established, and the optimal surface morphology corresponding to defect-free printing is determined. The printability is predicted by mathematical machine learning classification models via 10-fold cross-validation method, which validates the classification criterion and the comprehensive framework to assess the printability. This high-fidelity comprehensive framework to assess the printability could potentially guide the alloy design and printing parameters selection towards mass additive manufacturing production.

Operando tomographic microscopy during laser-based powder bed fusion of alumina

Laser-based Powder Bed Fusion (LPBF) of oxide ceramics enables fabrication of objects with complex three-dimensional shapes. However, mechanical properties of dense LPBF-manufactured ceramics are poor due to large amount of structural defects. Here, we perform the operando tomographic microscopy during LPBF of a magnetite-modified alumina to gain a deeper understanding of the underlying mechanisms. The effect of the laser energy density on the surface roughness, powder denudation zone and porosity formation mechanisms are investigated. Increasing laser power results in significant increase of the melt pool width, but not its depth and no melt pool depression is observed. Forces due to the recoil pressure are not seen to significantly influence the melt pool dynamics. Increasing power allows to avoid fusion porosity but enhances formation of spherical porosity that is formed by either reaching boiling point of liquid alumina, or by introducing gas bubbles by injection of hollow powder particles into the liquid.

Influence of recoil pressure, mushy zone flow resistance and reflectivity on melt pool shape in laser powder bed fusion simulation

Melt pool dimensions (depth: D and width: W) are strongly correlated with defects generated in laser powder bed fusion. The melt pool dimensions have been evaluated by CFD simulation including solid–liquid–gas phase change and laser multiple reflection. Such a multi-physics simulation requires a large number of parameters. Preliminary simulations for parameter identification require a lot of time and effort. The parameter sensitivity to the melt pool dimensions is important; however, the systematic evaluations have hardly been found in the literature. In this study, we performed systematic parametric evaluation for three parameters related to recoil pressure, flow resistance in solid–liquid mushy zone and Fresnel laser reflection. As a result, the D values increased with increasing recoil pressure, but the W values did not. The flow resistance force influenced the velocities in the mushy zone but not the melt pool dimensions. The D increased with increasing reflectivity, but the W did not. The melt pool dimensions varying recoil pressure, flow resistance force and reflectivity were all inconsistent with the experimental melt pool dimensions. In order to agree with the experimental melt pool dimensions, the problems to be solved were discussed by comparing the previous studies.

Challenges in dynamic heat source modeling in high-power laser beam welding

The amount of absorbed energy in the keyhole as well as its spatial and temporal distribution is essential to model the laser beam welding process. The recoil pressure, which develops because of the evaporation process induced by the absorbed laser energy at the keyhole wall, is a key determining factor for the macroscopic flow of the molten metal in the weld pool during high-power laser beam welding. Consequently, a realistic implementation of the effect of laser radiation on the weld metal is crucial to obtain reliable and accurate simulation results. In this paper, we discuss manyfold different improvements on the laser-material interaction, namely, the ray tracing method, in the numerical simulation of the laser beam welding process. The first improvement relates to locating the exact reflection points in the ray tracing method using a so-called cosine condition in the determination algorithm for the intersection of reflected rays and the keyhole surface. A second correction refers to the numerical treatment of the Gaussian distribution of the laser beam, whose beam width is defined by a decay of the laser intensity by a factor of 1/e2, thus ignoring around 14% of the total laser beam energy. In the third step, the changes in the laser radiation distribution in the vertical direction were adapted by using different approximations for the converging and the diverging regions of the laser beam, thus mimicking the beam caustic. Finally, a virtual mesh refinement was adopted in the ray tracing routine. The obtained numerical results were validated with experimental measurements.

Direct joining of quartz glass and copper by nanosecond laser

In present study, quartz glass and copper, which are with a large difference of coefficient of thermal expansion by 38 times, were successfully welded without any interlayer by nanosecond (ns) laser under non-optical contact for the first time. By reasonably tuning the welding parameters, well-formed quartz glass/copper joints without any defects such as microcracks or voids can be achieved. The welded seam includes laser-unirradiated smooth region and laser-irradiated spindle melted region and both the region are composed of “Cu2O″-SiO2 system (Cu–O–Si system) and CuO. The formation of such a particular microstructure feature is attributed to the recoil pressure from ns laser ablation and the Marangoni convection at the interface. The parameters of ns laser welding including laser power, scanning velocity and the number of scans influence the laser energy input and thermal accumulation greatly during welding, thus affecting the interfacial feature and the mechanical property of the resultant joints. The shear strength of the joint can reach 33.4 MPa, which is about 80% of quartz glass shear strength. The metallurgical bonding from interfacial reaction and mechanical interlocking from physical spindle structures are the main reasons for achieving a high shear strength of the joint. We expect this study could provide a new idea for joining materials with large differences in thermophysical properties.

A numerical study on the generation and migration of bubbles in laser powder bed fusion IN 718 alloy

An integrated multiphysics numerical simulation for the single track Laser-Powder Bed Fusion (L-PBF) process was developed utilizing both computational fluid dynamics (CFD) and discrete element method (DEM) models. This meso-scale simulation comprehensively accounted for diverse phenomena, including melting/solidification/evaporation, radiation and heat transfer, multi-phase flow, recoil pressure force, and the Marangoni effect, among others. By employing various laser energy densities in the numerical simulations, the study unveiled the mechanism governing bubble formation and the subsequent migration process. The complex dynamics of the melt pool were driven by the interaction and competition of multiple forces, leading to the development of a clockwise vortex. The simulation results demonstrated that enclosed bubbles primarily formed at the bottom of the keyhole due to the vortex effect. While some bubbles escaped from the molten pool's top surface along the vortex, others remained trapped inside, resulting in the formation of near-spherical pore defects in the solidified metal.

The formation mechanism of metal-ceramic interlayer interface during laser powder bed fusion

ABSTRACT Experiments on Laser powder bed fusion (LPBF) of powdered Ti on Al2O3 substrate were conducted and the interface formation was studied using a multi-material fluid dynamics model. Results show that the melt pool is relatively shallow, with relatively flat interlayer interface under LPBF’s conduction mode. In this condition, a thin sheath of molten Al2O3 forms and acts as a lubricating film for the molten Ti, leading to Rayleigh instability due to high flow inertia. Keyhole formation penetrates the Al2O3 substrate, resulting in a wavy interlayer interface. The recoil pressure from the keyhole and overall melt inertia are suppressed by the highly viscous molten Al2O3, thereby improving single-track melt pool stability. However, the thermal expansion coefficient difference between Ti and Al2O3 led to the formation of transverse cracks. Achieving a defect-free metal-on-ceramic single track remains a challenge, despite this study serving as a guide for melt track and interface control.

Molten pool air-liquid interface in high-power laser manufacturing: Evolution law and energy absorption characteristics

During modern high-power laser manufacturing, the laser beam-heat energy conversion mainly occurs in the air-liquid interface of laser-induced molten pool evaporation. The molten pool air-liquid interface's evolution law and laser energy coupling characteristics were performed by using in-situ optical observations and numerical simulation in this study. They revealed the four stages of this evolution process: preheating, expansion, oscillation, and contraction. In the expansion stage of the molten pool, the molten metal will flow from the center of the molten pool to the periphery due to the joint action of surface tension and laser-induced evaporation recoil pressure, and the air-liquid interface exhibits a middle depression. When the laser is turned off, the evaporative recoil pressure drop causes the molten pool's oscillation, which amplitude and duration positively correlate with laser power. Due to the effects of the solid state and the air-liquid interface depression after melting, the absorptivity first drops and then grows with the increase of laser power. Eventually, the melting efficiency increases with laser power.

The essential role of initial powder bed state in selective laser melting of 316 L stainless steel

Understanding the essential role of initial powder bed states (e.g., structure and temperature) in the printing process is crucial to obtain high-quality components in powder-bed-based additive manufacturing. In this article, a 3D high-fidelity model which considered the Marangoni effects and recoil pressure was established based on the computational fluid dynamics (CFD) approach to simulate the printing process of 316 L stainless steel through selective laser melting (SLM). The influences of powder bed structure and temperature on the macro- and microscopic characteristics of the molten layer during printing were comprehensively investigated. On this basis, corresponding mechanisms were analyzed. Results show that the developed numerical model can vividly reproduce the SLM process of 316 L stainless steel. With the increase of powder bed thickness from 32.02 μm (Hspreading=2 D90 and Vspreading=0.2 m/s) to 40.65 μm (Hspreading=2 D90 and Vspreading=0.01 m/s), the height of the molten layer will increase from 16.37 to 32.86 μm, the depth will decrease from 21.69 μm to 10.52 μm, and the width will decrease from 100.67 to 94.94 μm. Meanwhile, this will make the fluctuation of the depth (CV) increase from 0.31 to 0.85. When the initial temperature increases from 300 K to 773.15 K, the height, depth and width of the molten layer increase by 3.88 μm, 12.68 μm and 17.37 μm, respectively. In this case, the CV is reduced from 0.34 to 0.23, implying the improvement of the molten layer quality. Under the current laser parameters, the defects such as warping, pores, faults, and balling induced by the insufficient melting in the thick powder bed can to a certain extent be reduced or even eliminated using the high initial temperature. The melting pool collapse will occur in a thin powder bed, the resultant entrained bubble defect in the molten layer can also be properly avoided by the high initial temperature. However, a special groove structure will appear due to excessive energy input. During multi-track melting, the rough surface, irregular molten layer and the pores between the molten tracks are likely to occur due to the insufficient energy input, these problems can be properly solved by employing a high initial temperature.

Heat and mass transfer in electron beam additive manufacturing

The electron beam wire-feed additive manufacturing (EBAM) technology utilizes a high-energy electron beam as a heat source to bombard a metal wire in the vacuum environment to form a molten pool. The stability and quality of the deposition mainly depend on the transition behavior of the droplet. To conduct this research, a high-fidelity thermo-fluid dynamic simulation model based on the local reconstruction of the 3D model was developed and verified. The model used the Monte-Carlo algorithm to update the scattering trajectories and residual energies of high-energy electrons on the irradiated surface of the wire and molten pool at each time step in the numerical calculation. Quantitatively analyzed the “electron-wire” instantaneous energy coupling mechanism in the droplet transition mode, insertion transition mode, and liquid bridge transition mode, and finally established an energy-matching criterion for maintaining a stable deposited track morphology. The results show that during the initial stage of wire melting, the droplet will climb along the wire feeding direction, grow under gravity, and eventually drop under the combined effects of gravity and recoil pressure. Different forms of molten droplets will form various solidification morphology, with the liquid bridge transition mode being the most desirable. To maintain a stable liquid bridge transition, it is necessary to control the droplet generation height within the range of 0–2.7 mm. The collapse of the deposited track, caused by heat accumulation during the multilayer deposition, would also alter the droplet generation height. Therefore, to maintain the liquid bridge transition mode in the multilayer multi-channel forming process, it is also necessary to ensure that the energy density of the deposited track is slightly greater than 1.16×1010 J/m3 and the energy density of the wire is slightly greater than 2.63×1010 J/m3.

Numerical Simulation of Selective Laser Melting of 304 L Stainless Steel

This paper used the smoothed particle hydrodynamics (SPH) method to construct a three-dimensional mathematical model of the selective laser melting (SLM) process of 304 L austenitic stainless steel. Important driving force models for the melt pool in the SLM process were developed, including a surface tension model, a boundary normal-modified wetting effect model, a Marangoni shear force model, and a recoil pressure model. Meanwhile, the virtual particle boundary method prevented particles from flying over the solid boundary. Artificial viscosity, artificial stress, and artificial heat were added to correct the SPH equation, which provided a guarantee for the accuracy and speed of the numerical simulation of the SLM process. Finally, the temperature field and velocity field in the SLM process were explored according to the constructed mathematical model. The evolution mechanism in the melting process was analyzed, and the influence of different laser powers on the shape of the molten pool was mainly analyzed, which provided a reference for optimizing the laser parameters to reduce the surface roughness of the formed specimen.

Numerical Simulations to Predict the Melt Pool Dynamics and Heat Transfer during Single-Track Laser Melting of Ni-Based Superalloy (CMSX-4)

Computational Fluid Dynamics (CFD) simulations are used in this work to study the dynamic behavior of the melt pool and heat transfer during the single-track laser melting process of a nickel-based superalloy (CMSX-4). To include the effects of powder inhomogeneities and obtain a realistic distribution of the powder layer on the bed chamber, the CFD model is coupled with a Discrete Element Method (DEM) solver. The coupled model is implemented in the open-source software package OpenFOAM. In the CFD model’s governing equations, some key physical mechanisms, such as the Marangoni effect and recoil pressure, are considered. With the help of the coupled CFD-DEM model, we have investigated the effect of key process parameters, such as laser power, scanning speed of the laser, powder size, and powder shape, on the size and homogeneity of the melt pool. From the simulation results, it was discovered that high laser power and slow scanning speed create a deep and narrow keyhole that leads to porosity. In contrast, balling defects are found to be caused by a small melt pool obtained from fast scanning speeds and inadequate laser power.

Temporally modulated laser power dynamic drilling strategy with comprehensive optimization of quality and efficiency

Motivated by the need to comprehensively optimize the quality and efficiency of laser drilling, a dynamic drilling strategy wherein laser power is temporally modulated is proposed, the laser power of which increases with the number of pulses to match the dynamics of the drilling process. Taking nanosecond laser drilling as the specific research process, the proposed dynamic drilling strategy is successfully applied. A continuous hydrodynamic model is first established to reveal the mechanism and driving force of material removal. Subsequently, linear and hybrid power growth profiles are proposed to improve the driving force of melt ejection and evaporation intensity during the drilling process. By comparing the variations in crater depth, ablation rate, recoil pressure, and recast layer thickness with the number of pulses, the effect of dynamic drilling modes on the drilling process and micro-holes performances was revealed. Moreover, the effect of power growth rate on the dynamic drilling process and final micro-hole quality is discussed in detail to provide a basis for formulation and optimization of the power growth profile. Finally, an experiment using an aerospace Ni-based superalloy verifies the reliability of the dynamic drilling strategy and the material removal rate, taper, and recast layer thickness of the micro-holes are comprehensively optimized. The proposed dynamic drilling strategy provides an effective solution to the universal problems of micro-hole quality and efficiency in the field of laser drilling.