Melt Pool Behavior Research Articles

An Empirical Model and Feedforward Control of Laser Powder Bed Fusion

Abstract Reliable process control for the laser powder bed fusion process, especially at the melt pool scale, remains an open challenge. One of the reasons for this is the lack of suitable control-oriented models and associated control design strategies. To address this issue, this paper (1) identifies an empirical control-oriented model of geometry-dependent melt pool behavior and (2) experimentally demonstrates melt pool regulation with a feedforward controller for laser power based on this model. First, the study establishes that the melt pool signature increases as the scan lines decrease in length. An empirical model of this behavior is developed and validated on different geometries at varying laser power levels. Second, the model is used to design a line-to-line feedforward controller that provides an optimal laser power sequence for a given geometry. Finally, this controller is validated experimentally and is demonstrated to suppress the in-layer geometry-related melt pool signal deviations for different test geometries.

Laser butt welding of thin stainless steel 316L sheets in asymmetric configurations: A numerical study

Laser butt welding of thin metal sheets is a widely used fusion-based joining technique in industrial manufacturing. A comprehensive understanding of the complex transport phenomena during the welding process is essential for achieving high-quality welds. In the present work, high-fidelity numerical simulations are employed to investigate the influence of various symmetric and asymmetric welding configurations on the melt-pool behaviour in conduction-mode laser butt welding of stainless steel sheets. The analysis focuses on the effects of laser power density, heat source misplacement and different welding scenarios, including plates with a root gap, high-low mismatches, and dissimilar thicknesses, on the molten metal flow and heat transfer. The results show that advection is the dominant mechanism for energy transfer in the melt pool, and its contribution increases with higher laser power. The non-uniform temperature distribution over the melt-pool surface induces Marangoni shear forces, driving the flow of molten metal and leading to the formation of vortices and periodic flow oscillations within the pool. The effects of various types of asymmetries on the thermal and molten metal flow fields, as well as the process stability, are thoroughly examined and compared with symmetrical welding configurations. These comprehensive simulations provide valuable insights into the fluid flow dynamics and thermal field evolution during laser butt welding of thin metal sheets. The knowledge gained from this study can facilitate process optimisation and guide the improvement of weld quality in practical applications.

Effect of layer thickness on the melt pool behavior and pore defects evolution of selective laser melting CuCrZr alloy

Despite the great potential of selective laser melting technology, the low build rate has become a major reason limiting its large-scale use. Thick layer thickness can significantly increase the build rate, but an unlimited increase in layer thickness can easily lead to defects which can seriously affect the mechanical properties of the part. To date, the correlation between powder layer thickness and melt pool dynamics behavior and its influence on defect evolution in the production of copper-based alloys by selective laser melting remains unclear. Therefore, in this paper, a high-fidelity computational fluid dynamics (CFD) model combined with a ray-tracing algorithm is developed to investigate the effect of layer thickness on melt pool dynamics and the evolution of pore defects. The results show that as the powder layer thickness increases, the melt track width and depth rise, reaching a maximum of 169.8 µm and 177.2 µm at 60 µm layer thickness, which is probably attributed to the growth of global absorptivity. Besides, the increased instability of melt flow under thick layer thickness resulted in undulations in the solidification track surface and accompanying slag phenomena. It is worth noting that the generation of pores was observed at 45 µm and 60 µm layer thickness. Both the number and dimension of pores increase with thicker layer thickness, which are caused by the multiple reflection behavior of the laser rays in the keyhole. In addition, the elimination of pore defects under thick layer thickness was also explored, and it was found that remelting could effectively reduce the pores generated by previous layers.

Investigation of the melt track geometry during selective laser melting of CuSn10

The CuSn10 alloy (bronze with 10% tin) has remarkable mechanical properties, including good elongation and medium hardness. Additive manufacturing of this powder compound is developing at a fast rate. In this study, optimization of the process parameters of the selective laser melting method was carried out to manufacture CuSn10 compounds. In addition, a numerical model for the simulation of the melt pool behavior was created by utilizing the Ansys 2021 R1 software, and a comparison was carried out between predicted numerical data and the obtained experimental results. The formation conditions of various melt traces were modeled, measured and validated for this aim. In the experimental stage, a constant laser power of 95 W was used, and the effect of the variation of the scanning speed was studied between 10 and 1500 mm/s. The results showed that the variation of the scanning speed is not enough, and optimization must be carried out by including other process parameters. This indicates that by adjusting the process parameters to have a 365 W power, the liquid phase can be achieved in the production process.

Surface morphology evolution of GTD-450 stainless steel during laser powder bed fusion

GTD-450 is a new type of maraging stainless steel. It has a wide application prospect in the additive manufacturing of complex components such as GTD-450 compressor blades using L-PBF technology. In this study, a multi-scale numerical model was used to simulate the whole process of L-PBF forming of GTD-450 stainless steel block. This numerical model can describe the evolution process of L-PBF forming temperature field, melt flow field and melt free surface. With the help of simulation results, the surface morphology of GTD-450 stainless steel formed under different layers is clarified. The experimental and numerical data of surface morphology are in good agreement, which verifies the reliability of the numerical model. Based on the experimental results, the influence mechanism of melt pool behaviour and thermal history on melt solidification behaviour was discussed, and the development process of L-PBF block surface morphology under the influence of melt pool behaviour was explained.

Prediction and validation of melt pool dimensions and geometric distortions of additively manufactured AlSi10Mg

A finite element–based thermomechanical modeling approach is developed in this study to provide a prediction of the mesoscale melt pool behavior and part-scale properties for AlSi10Mg alloy. On the mesoscale, the widely adopted Goldak heat source model is used to predict melt pool formed by laser during powder bed fusion process. This requires the determination of certain parameters as they control temperature distribution and, hence, melt pool boundaries. A systematic parametric approach is proposed to determine parameters, i.e., absorption coefficient and transient temperature evolution. The simulation results are compared in terms of morphology of melt pool with the literature results. Considering the part-scale domain, there is increasing demand for predicting geometric distortions and analyzing underlying residual stresses, which are highly influenced by the mesh size and initial temperature setup. This study aims to propose a strategy for evaluating the correlation between the mesh size and the initial temperature to provide correct residual stresses when increasing the scale of the model for efficiency. The outcomes revealed that the predicted melt pool error produced by optimal Goldak function parameters is between 5 and 12%. On the part-scale, the finite element model is less sensitive to mesh size for distortion prediction, and layer-lumping can be used to increase the speed of simulation. The effect of large time increments and layer lumping can be compensated by appropriate initial temperature value for AlSi10Mg. The study aids practitioners and researchers to establish and validate design for additive manufacturing within the scope of desired part quality metrics.

Size effect on the microstructure, phase transformation behavior, and mechanical properties of NiTi shape memory alloys fabricated by laser powder bed fusion

In this work, NiTi samples with different thicknesses (0.15–1.00 mm) were fabricated by laser powder bed fusion (LPBF) under variable scanning speeds (500–1200 mm s–1). The densification behavior, phase transformation behavior, and mechanical properties of the sample with different thicknesses are studied. The results indicate a strong size effect in the LPBF-fabricated NiTi alloy. The decrease of the sample thickness results in (i) the increase of porosity, (ii) the decrease of the number of adhered NiTi powder particles at the surface, (iii) the monotonous decrease of the martensitic transformation temperatures (MTTs), and (iv) the decrease of the shape recovery temperature. The influence of sample thickness on the melt-pool behavior, and thus the microstructure and performance of NiTi alloys are discussed. It is suggested that the melt-pool is deeper and narrower in the thin samples than in the thick samples. We conclude that, apart from the LPBF process conditions, the sample dimensions have also to be considered to fabricate NiTi structures with predictable properties.

Influence of processing parameters on AlSi10Mg lattice structure during selective laser melting: Manufacturing defects, thermal behavior and compression properties

Selective laser melting (SLM) is a kind of material forming technology which involves many processing parameters. The appropriate processing parameters are very important to the forming quality of manufactured samples. In this paper, the influence of laser power and scanning speed on the manufacturing defects, thermal behavior and compression properties of AlSi10Mg lattice structures during SLM were systematically investigated. By observing the macroscopic and microscopic morphology of manufactured samples, it was found that the processing parameters had a significant influence on the strut diameter and internal porosity but had little influence on the dendrite spacing and grain size. A physical model considering thermal loss and thermal conduction in temperature field was proposed to simulate the thermal behavior of melt pool under different processing parameters. The results showed that the thermal behavior was more sensitive to the laser power, and the manufacturing defects were related to the transient temperature and dimension of the melt pool. The digital image correlation (DIC) method was adopted to analyze the strain distribution of manufactured lattice during the compression process. Meanwhile, the relationship between processing parameters, thermal behavior and manufacturing defects was discussed in detail. Finally, based on the optimal processing parameters (250 W-1400 mm/s), the accuracy of manufactured lattice structure was verified by finite element method. The research results can provide a reference for the selection of processing parameters for manufacturing AlSi10Mg lattice structure.

Surface topography and melt pool behavior in rapid turnaround regions of laser powder bed fusion additive manufacturing of nickel superalloy 625

The complexity and vast number of influencing variables in laser powder bed fusion have hindered the development of correlations between surface topography and part performance or process variables. To address this, we investigated the melt pool behavior in regions of the part where a rapid back-and-forth scan strategy occurs. Analysis of surface topography and melt pool cross section geometry on nickel superalloy 625 samples helped identify their relationship. From this, a conceptual model was developed and the implication this has for the development of strong, process-informed correlations is discussed.

Method for the Calculation of Local Bead Volume in Multi-Axis Additive Manufacturing

In recent history, Multi-Axis Additive Manufacturing enjoys increasing attention for both industrial and research application. In comparison to traditional processes, the additional rotation axes enable parts to be printed with greater design flexibility, while reducing the need for support structures. The multi-axis motion additionally allows for optimization of structural properties of the parts. However, curved-layer printing requires significantly higher effort for planning and computation, as the layers exhibit curvature in their shape and feature local variations of the layer height and bead width. Consideration of these properties is often neglected in the development of path planning methods, although they pose a significant influence to the optical and structural quality of the printed parts. Local over- or under-extrusion can result in material accumulation or voids within the part and need to be avoided. Additionally, some Multi-Axis Additive Manufacturing processes like Directed Energy Deposition are especially prone to errors in practice, as the melt-pool behavior is hard to model and predict accurately. This paper introduces a method for the precise calculation and adjustment of local bead volume, as a possible solution to these problems. The method processes existing Cartesian paths and uses a model of the bead geometry to simulate the extrusion by rasterization. The method considers printing head orientation, as well as neighborhood information of adjacent beads and builds a volumetric map of the paths. Local extrusion volume is then determined for each waypoint of the path from this volume map and provided to the printer along with the G-Code, enabling adjusted volume deposition both in advance and during the process by iterative path-adaptation. The provided implementation is able to achieve favorable results for both use-cases but proves especially beneficial for the latter.

High-speed X-ray study of process dynamics caused by surface features during continuous-wave laser polishing

During high-speed X-ray imaging of laser surface polishing experiments of specimens of 316L stainless steel at Argonne National Lab's Advanced Photon Source, it was discovered that the induced keyhole changes shape and dimensions while crossing an engineered surface feature without altering process parameters. It was observed that the post-surface feature keyhole was deeper than that of the pre surface feature keyhole. This work reports on the first in-situ observation of the effect of localized surface geometry on underlying melt pool behavior. This has implications for defect formation mechanisms during laser melting processes that rely on melt pool geometry.

Thermo-hydraulic analysis of melt pool dynamics and material removal on anode in electrical discharge machining

The melt pool dynamics and material removal mechanism in electrical discharge machining (EDM) is not clear yet. In this study, a thermo-hydraulic coupling simulation model considering practical boundary conditions was proposed to investigate the melt pool evolution and material removal on the anode in EDM, with the simulated results directly verified by the high-speed observation of the discharge process. Discharge currents of 5–15 A, a discharge duration of 10 µs, and workpieces of zinc and steel were used in the model, with the forces acting on the melt pool comprehensively integrated and their effects on the melt pool dynamics and material removal analyzed. The simulation results show that on the anode the evaporation recoil pressure is one of the dominant factors that drive the melt flow and cause the molten material removal. The increase of discharge current results in larger evaporation recoil pressure, and thus larger volume of the molten material is removed. Under the effect of evaporation, obvious fluctuation of the melt pool will occur. Meanwhile, the evaporation recoil pressure oscillates with the melt pool fluctuation, which may induce a wave-like motion of the molten material and promote the generation of debris particles. Novel high-speed observation methods were proposed to observe the melt pool evolution and the material removal, with the observed melt pool behavior consistent with the simulated results. The observation also shows that the intermittent removal of the molten material occurs in the case of long discharge duration, which is considered to be caused by the intermittent increase of the evaporation recoil pressure during the melt pool fluctuation.

Acoustics for in-process melt pool monitoring during metal additive manufacturing

Intrinsic to most metal additive manufacturing (AM) processes are melt pools generated from directed energy sources like lasers. Melt pools are critical as they function to join powder layers to previous layers during the process. Their criticality extends deeper as most porosity in AM parts stems from melt pool behavior while melt pool solidification dictates the parts’ microstructure. Significant breakthroughs in metal AM are severely hindered by the lack of access to experimental tools to study melt pools and modeling that do not fully capture the complex physics. Thus, melt pool related defects are often difficult to predict in occurrence and location while determining optimal process parameters to eliminate defects is extremely challenging and costly. Furthermore, the many exciting opportunities such as realizing new AM alloys, developing gradient materials/structure, and tailoring microstructure to intended applications are possible only with further understanding of melt pool behavior. With these clear needs, in-process acoustics have been proposed as plausible experimental tools for studying melt pool behavior. This presentation will provide an overview of the current activity in this area in addition to the specific needs the acoustics community can potentially address.

Quantitative study on the dynamics of melt pool and keyhole and their controlling factors in metal laser melting

Recent direct visualization of keyhole morphology evolution by ultrahigh-speed synchrotron X-ray imaging provides fascinating insights and basis for further understanding and investigation of the melt pool (MP) behavior during laser melting. By combining the ray-tracing method, the phase field method and the lattice Boltzmann method, we simulate the laser melting process of Ti‐6Al‐4V plates quantitatively and predict five distinct regimes observed experimentally, and uncover the underlying dynamics and controlling factors for each regime. Remarkably, we also reveal four types of distinct oscillations of the MP and their underlying mechanisms. The 3D irregular oscillation of keyhole tip is identified to be responsible for bubble spill-over. These revealed dynamics and their underlying controlling factors provide a solid physical foundation for controlling laser-based 3D printing processes. • The ray-tracing, the phase-field, and lattice Boltzmann Method were coupled to model additive manufacturing at powder scale • Keyhole observed by ultrahigh-speed synchrotron X-ray imaging was simulated quantitatively and its dynamics was revealed and discussed • Four types of oscillation and their mechanisms were proposed and discussed based on the quantitative simulations.

Evaluation of spatter particles, metal vapour jets, and depressions considering influence of laser incident angle on melt pool behaviour

The building of practical parts involves the application of metal-based powder bed fusion using a laser beam (PBF-LB/M), owing to its high-precision manufacturing. However, the quality of built parts obtained via the PBF-LB/M process varies with the building conditions, and a thorough understanding of the building mechanism has not been achieved owing to the complex and interrelated process parameters involved. The incident angle of the laser beam, which changes on the platform during the laser beam scan owing to the designed three-dimensional data, is among the principle parameters that affect the building aspects. In this study, the melt pool in the single-track formation during the PBF-LB/M process was visualised using a high-speed camera, and the influence of the laser incident angle on the ejection characteristics of spatter particles formed around the laser-irradiated area was investigated. Consequently, the spatter particles and metal vapour jets behaviour varied with the laser incident angle. There was a reduction in number of spatter particles owing to the origin of the incident direction being from behind the laser irradiation area. Additionally, the laser incident angle affected the melt pool morphology because of the depression in the melting. Furthermore, the burial depth of the pores varied with the laser incident angle, and was related to the depth of the depression during the melt pool formation.

Numerical modeling of thermal behavior of melt pool in laser additive manufacturing of Ni-based diamond tools

In this paper the heat absorption, heat transfer and solidification evolution of melt pool in the laser additive manufacturing of Ni–Cr metal bonded diamond tools were studied. Based on the ray tracing theory, the influence of diamond and Ni–Cr alloy powder parameters on laser energy absorptivity was analyzed and the temperature model of mixed powder bed was constructed. Combined with the model and experiment, the impact of the diamond itself and its arrangement on the temperature distribution and shape of melt pool was explored and compared. The change rule of melt pool and microstructure in laser multi-track scanning was investigated. The evolution of melt pool solidification and microstructure with the influence of diamond was revealed. The results show that the powder thickness has a great influence on laser energy absorptivity. The diamond has obvious influence on the temperature distribution and geometry of melt pool, and the influence degree and variation rule are related to the diamond arrangement and position. In the process of laser multi-track scanning, the temperature increases and the geometry of melt pool enlarges gradually. Near the diamond, the diamond changes the direction of temperature gradient of metal melt, accelerates crystals solidification, makes the growth direction of crystals approximately perpendicular to diamond, and increases the endogenetic nucleation phenomenon of crystals near the diamond interface.

Understanding Stellite-6 coating prepared by laser cladding: Convection and columnar-to-equiaxed transition

In the Stellite-6 coating prepared by laser cladding technology, temperature gradient and solidification rate play a critical role in grain growth. The columnar-to-equiaxed transformation behavior significantly affects the properties of the coating. In this paper, a finite element model including the changes of convection and temperature field in the melt pool was established, and the effects of laser power on the geometric characteristics and columnar-to-equiaxed transformation behavior of the coating were analyzed. The results show that the laser power significantly improve the convection intensity of metal liquid, which increased the width, depth and dilution ratio of the coating. In the solidification process of melt pool, convection gradually changed from diffusion around the coating to reflux to the top of the coating. Lower temperature gradient and higher solidification rate were conducive to the growth of equiaxed crystals. The microhardness of equiaxed crystal region was higher than that of columnar crystal region. Reducing laser power could accelerate the CET behavior of melt pool and improve the mechanical properties of coating.

Uncertainties Induced by Processing Parameter Variation in Selective Laser Melting of Ti6Al4V Revealed by In-Situ X-ray Imaging.

Selective laser melting (SLM) additive manufacturing (AM) exhibits uncertainties, where variations in build quality are present despite utilizing the same optimized processing parameters. In this work, we identify the sources of uncertainty in SLM process by in-situ characterization of SLM dynamics induced by small variations in processing parameters. We show that variations in the laser beam size, laser power, laser scan speed, and powder layer thickness result in significant variations in the depression zone, melt pool, and spatter behavior. On average, a small deviation of only ~5% from the optimized/reference laser processing parameter resulted in a ~10% or greater change in the depression zone and melt pool geometries. For spatter dynamics, small variation (10 , 11%) of the laser beam size could lead to over 40% change in the overall volume of the spatter generated. The responses of the SLM dynamics to small variations of processing parameters revealed in this work are useful for understanding the process uncertainties in the SLM process.

Numerical evaluation of geometric variations in internally heated melt pool

In Vessel Retention (IVR) is considered as one of the best safety strategy for a hypothetical severe accident in light water nuclear reactors (LWR). Since the strategy involves heat transfer between the corium melt pool and surrounding, it is highly impacted by the natural convection behaviour of such internally heated melt pool. Extensive experiments and numerical study has been performed worldwide to understand the behaviour of corium melt pool. COPRA (COrium Pool Research Apparatus) is one such experiment which uses molten salts of non-eutectic binary mixture (20 mol% NaNO3–80 mol% KNO3) and eutectic binary mixture (50 mol% NaNO3–50 mol% KNO3) as simulant material. Geometric variation in COPRA experiment is investigated to get a better understanding of the natural convection behaviour of the melt pool. Although the influence of geometry is reported in literature, the increase in geometry beyond the conventional size is yet to be explored. Eutectic salt (50 mol% NaNO3–50 mol% KNO3) is used as a simulant material for this study. A commercial CFD code ANSYS FLUENT 20.2 is employed for the numerical modelling of the corium pool. K-epsilon (k-ε) turbulence model and solidification-melting model are used in this simulation. Experimental data is used for validating this model. The geometric effect is investigated by assuming the same height and radius for the pool. Also, the ratio of heating power and cooling length is maintained for all the variations of geometry. The temperature field, liquid fraction contour, and velocity distribution of the melt pool are calculated. This paper provides an extension to the existing data thus further enhances the understanding of the thermal–hydraulic characteristics of the corium melt pool.

Evaluating the thermal characteristics of laser powder bed fusion

• Dynamic temperature behaviour around the melt pool during the PBF-LB/M was evaluated. • Temperature distribution was influenced by morphological changes of metal powder. • Laser beam incident was characterized by direct heating and heat conduction. • Dynamic temperature behaviour provided a direct explanation for the cooling rate. This study investigates the dynamic temperature behaviour around a melt pool in metal-based powder bed fusion using a laser beam (PBF-LB/M) to clarify the influence of the associated morphological changes of the metal powder experimentally. Gas-atomized 18Ni (300-grade) maraging steel powders were processed by PBF-LB/M while high-speed photography with a two-colour radiometric thermal imaging system that was employed to correlate the temperature with melt pool behaviour. In addition, the cooling rate of the melt pool was measured directly using the dynamic temperature distribution. The temperature distribution of the melt pool was influenced by the morphological changes of the metal powder induced by physical and thermal interactions, and the melt pool exhibited an asymmetric temperature distribution in the direction parallel to the laser scan. The significant factors were droplet cohesion at low melt pool temperatures, remaining heat energy from previous laser beam irradiation, and the heat conduction inside the melt pool. The laser beam incident on the metal powder was primarily characterized by two modes: direct heating induced by laser beam irradiation and heat conduction through the single track, droplets, and substrate. In addition, the dynamic temperature behaviour provided a direct explanation for the cooling rate, the values of which ranged from 0.1 to 0.9 × 10 6 K/s owing to the self-cooling induced by PBF-LB/M.