Melt Pool Evolution Research Articles

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.

A steady-state semi-analytical approximation of melt pool evolution in pulsed laser surface melting

Pulsed laser surface melting (pLSM) is a technique that offers an efficient and effective way to modify the geometry surfaces without any addition or removal of material. The resultant surface geometry plays a critical role in several applications. This paper presents a steady-state thin-film approximation of the melt pool created by pLSM and the resulting semi-analytical solution for the evolved surface geometry. These predictions of the semi-analytical solution are then compared with a validated numerical solution. The comparison demonstrates a good match with errors ranging from ~4% to ~25% across several pulse durations. Larger errors are observed at comparatively lower and higher pulse duration. Smaller errors are observed for intermediate pulse duration values because of the deviation of non-dimensional numbers from their assumed orders. Overall, the thin-film solution is a reasonable and useful approximation of the evolved surface geometry through the pLSM process, thus saving significant computational costs.

Modeling layer-by-layer laser melting and solidification of binary alloy powder bed

A model for simulating layer-by-layer melting and solidification of a binary alloy powder bed due to a moving laser source is presented in this paper. The model uses a modified enthalpy-porosity approach to capture simultaneous melting and solidification of a powder bed. The effects of surface tension driven Marangoni convection and thermal and solutal buoyancy driven convection are incorporated in the model. Multiple layer formation is modeled by shifting the domain in the vertical direction to include the new layer. Simulations are performed for Al-Cu alloy to see the effect of different process parameters on the melt pool evolution, solute transport and segregation, and thermal transport with the primary focus on quantifying the nonhomogeneity in the final species distribution. Fixed melt pool results show that the effect of Marangoni convection is dominant resulting in considerably increased segregation in the solidified region. Simulation of layer-by-layer melting and solidification of the entire domain show that the solidified region can be divided into three zones based on solute distribution—initial low concentration zone, middle zone with slightly higher concentration, and the final zone with very high concentration. Subsequently, parametric studies are done that show that the nonuniformity in solute distribution can be reduced by reducing the laser power, increasing the laser spot radius, increasing the initial solute concentration, or decreasing the layer thickness.

Nondiffractive beam shaping for enhanced optothermal control in metal additive manufacturing.

High thermal gradients and complex melt pool instabilities involved in powder bed fusion–based metal additive manufacturing using focused Gaussian-shaped beams often lead to high porosity, poor morphological quality, and degraded mechanical performance. We show here that Bessel beams offer unprecedented control over the spatiotemporal evolution of the melt pool in stainless steel (SS 316L) in comparison to Gaussian beams. Notably, the nondiffractive nature of Bessel beams enables greater tolerance for focal plane positioning during 3D printing. We also demonstrate that Bessel beams significantly reduce the propensity for keyhole formation across a broad scan parameter space. High-speed imaging of the melt pool evolution and solidification dynamics reveals a unique mechanism where Bessel beams stabilize the melt pool turbulence and increase the time for melt pool solidification, owing to reduced thermal gradients. Consequently, we observe a distinctively improved combination of high density, reduced surface roughness, and robust tensile properties in 3D-printed test structures.

Feedback Control of Melt Pool Area in Selective Laser Melting Additive Manufacturing Process

Selective laser melting (SLM), a metal powder fusion additive manufacturing process, has the potential to manufacture complex components for aerospace and biomedical implants. Large-scale adaptation of these technologies is hampered due to the presence of defects such as porosity and part distortion. Nonuniform melt pool size is a major cause of these defects. The melt pool size changes due to heat from the previous powder bed tracks. In this work, the effect of heat sourced from neighbouring tracks was modelled and feedback control was designed. The objective of control is to regulate the melt pool cross-sectional area rejecting the effect of heat from neighbouring tracks within a layer of the powder bed. The SLM process’s thermal model was developed using the energy balance of lumped melt pool volume. The disturbing heat from neighbouring tracks was modelled as the initial temperature of the melt pool. Combining the thermal model with disturbance model resulted in a nonlinear model describing melt pool evolution. The PID, a classical feedback control approach, was used to minimize the effect of intertrack disturbance on the melt pool area. The controller was tuned for the desired melt pool area in a known environment. Simulation results revealed that the proposed controller regulated the desired melt pool area during the scan of multiple tracks of a powder layer within 16 milliseconds and within a length of 0.04 mm reducing laser power by 10% approximately in five tracks. This reduced the chance of pore formation. Hence, it enhances the quality of components manufactured using the SLM process, reducing defects.

On the role of the powder stream on the heat and fluid flow conditions during Directed Energy Deposition of maraging steel—Multiphysics modeling and experimental validation

The Directed Energy Deposition (DED) process of metals, has a broad range of applications in several industrial sectors. Surface modification, component repairing, production of functionally graded materials and more importantly, manufacturing of complex geometries are major DED’s applications. In this work, a multi-physics numerical model of the DED process of maraging steel is developed to study the influence of the powder stream specifications on the melt pool’s thermal and fluid dynamics conditions. The model is developed based on the Finite Volume Method (FVM) framework using the commercial software package Flow-3D. Different physical phenomena e.g. solidification, evaporation, the Marangoni effect and the recoil pressure are included in the model. As a new feature, the powder particles’ dynamics are modeled using a Lagrangian framework and their impact on the melt pool conditions is taken into account as well. In-situ and ex-situ experiments are carried out using a thermal camera and optical microscopy. The predicted track morphology is in good agreement with the experimental measurements. Besides, the predicted melt pool evolution follows the same trend as observed with the online thermal camera. Furthermore, a parametric study is carried out to investigate the effect of the powder particles incoming velocity on the track morphology. It is shown that the height-to-width ratio of tracks increases while using higher powder velocities. Moreover, it is shown that by tripling the powder particles velocity, the height-to-width ratio increases by 104% and the wettability of the track decreases by 24%.

Melt pool evolution on inclined NV E690 steel plates during laser direct metal deposition

The melt pool evolution during the laser direct metal deposition (DMD) is intricate. However, most of the research focused on a horizontal substrate, and only a little literature has reported the studies on an inclined substrate with a non-vertically irradiated laser beam. This investigation aims to investigate the melt pool evolution on a substrate with different inclination angles. A paraxial high-speed camera and a coaxial industrial camera were applied to obtain the instantaneous images of the melt pool. Besides, the image processing technique was implemented to extract the coaxial melt pool area. The results showed that several small melt droplets were generated in front of the melt pool on a horizontal substrate; however, a similar phenomenon was not observed on an inclined substrate. Besides, the melt droplets dynamics were driven by the surface tension, resisted by the inertia, and the spreading process was much faster than the solidification process. When the substrate was inclined, the laser energy distribution on the substrate was demonstrated. The “critical angle” was defined as the maximum inclination angle when the single track without any humps. In our experiments, the critical angle existed between 36° and 38°, and a periodic appearance of the humping formation was observed at the inclination angle of 40°. Our obtained results revealed the melt droplets dynamics on a horizontal substrate, the range of the critical angle, the laser energy distribution on an inclined substrate, and the humping formation mechanism.

Closed-Loop High-Fidelity Simulation Integrating Finite Element Modeling With Feedback Controls in Additive Manufacturing

Abstract A high-precision additive manufacturing (AM) process, powder bed fusion (PBF) has enabled unmatched agile manufacturing of a wide range of products from engine components to medical implants. While finite element modeling and closed-loop control have been identified key for predicting and engineering part qualities in PBF, existing results in each realm are developed in opposite computational architectures wildly different in time scale. This paper builds a first-instance closed-loop simulation framework by integrating high-fidelity finite element modeling with feedback controls originally developed for general mechatronics systems. By utilizing the output signals (e.g., melt pool width) retrieved from the finite element model (FEM) to update directly the control signals (e.g., laser power) sent to the model, the proposed closed-loop framework enables testing the limits of advanced controls in PBF and surveying the parameter space fully to generate more predictable part qualities. Along the course of formulating the framework, we verify the FEM by comparing its results with experimental and analytical solutions and then use the FEM to understand the melt-pool evolution induced by the in- and cross-layer thermomechanical interactions. From there, we build a repetitive control (RC) algorithm to attenuate variations of the melt pool width.

A conservative level set method on unstructured meshes for modeling multiphase thermo-fluid flow in additive manufacturing processes

Additive manufacturing (AM) is an emerging technology that fuses deposited powder materials together layer-by-layer using a localized heat source to create an arbitrarily complex geometry. This technology is an inherently multiphysics problem, involving the simultaneous evolution of fluid flow, heat transfer, free surface, laser-material interaction and material phases including solid, liquid and gas. Multiphysics and multiphase models are typically used to understand the dominant physics driving AM processes through high fidelity simulations resolving the individual powders as they melt onto the substrate. However, these types of high fidelity models have not been applicable to modeling directed energy deposition (DED) process, where powders are delivered through the ambient vapor onto the substrate by a nozzle and subsequently fused. This paper presents a novel multiphase thermo-fluid formulation for modeling DED processes. A diffuse level set formulation coupled with the Navier–Stokes, energy conservation and radiative transport equation allows us to model complex free surface, fluid flow, thermal and laser interaction evolution. In addition, our formulation enables us to resolve the vapor flow field and its effect on deposited material during AM processes. The accuracy of our proposed method is assessed by comparing with literature solutions and experimental benchmarks. Our proposed formulation is then used to model simple DED processes that enable us to visualize the entrainment of powder particles into the melt pool. This process elucidates the dominant physical forces that can drive powders into the melt pool as well as their effect on the melt pool evolution within DED processes.

A novel heat source model for analysis of melt Pool evolution in selective laser melting process

In this paper, a novel hybrid heat source model is developed considering the different absorption mechanisms for porous and dense state materials, and an effective absorptivity is adapted to the proposed model to analyze the melting mode transition. The proposed model can predict the melt pool characteristics including the melt pool dimensions and the melting modes in the selective laser melting (SLM) process. The problem is formulated using the heat transfer equation considering the phase transition and the degree of consolidation based on the phase-field approach. The single-track scans of 316 L stainless steel for both the non-powder case and the powder case are simulated to validate the model and the obtained results are in good agreement with the experimentally measured melt pool dimensions (mean error within 6%). Furthermore, the melting modes (conduction and keyhole) can be distinguished based on the predicted melt pool morphology and the degree of vaporization with the proposed model, which provides insight to define the optimized process boundary. It is also found that the keyhole mode melting is more sensitive to the change of the process parameter than the conduction mode melting.

Numerical study of the impact of vaporisation on melt pool dynamics in Laser Powder Bed Fusion - Application to IN718 and Ti–6Al–4V

A finite element model of Laser Powder Bed Fusion (LPBF) process applied to metallic alloys at a mesoscopic scale is presented. This Level-Set model allows to follow melt pool evolution and track development during building. A volume heat source model is used for laser/powder interaction considering the material absorption coefficients, while a surface heat source is used to consider the high laser energy absorption by dense metal alloys. An energy solver is applied considering convection and conductivity evolution in the system. Shrinkage during consolidation from powder to dense material is modelled by a compressible Newtonian constitutive law. An automatic remeshing strategy is also used to provide a good compromise between accuracy and computing time. Different cases are investigated to demonstrate the influence of the vaporisation phenomena, of material properties and of laser scan strategy on bead morphology.

In situ X-ray observations of transient states in arc weld pools

Metallic alloys coalesce via extremely rapid melting and subsequent solidification to form fusion welded joints. The melt pool evolution in melting and solidification sequences during the welding process determines the formation of the final weld joint shape, microstructure and defects. The scientific insight on weld pool evolution and related phenomena can be a key contribution to enhance structural integrity and resilience of the welded structures or components. However, inherent complexity with multi-physics phenomena, associated high temperatures and the rapidness of the processes make direct experimental investigation of welding is extremely demanding. Thus, internal flow behaviour during welding or other melt-pool-based metal processing such as additive manufacturing remains unclear and hinders progression to process optimisation. In this contribution we report the observation of melt pool dynamics that take place during electric arc welding, obtained through in situ synchrotron imaging at millisecond scale. The analysis flow patterns along with the quantified weld pool surface dynamics revealed us to how different contributing forces dictate the flow conditions over the distinct durations of the relatively short existence of the liquid phase. Our preliminary results suggest the existence of arc, surface tension and gravity dominant regimes during the evaluation of the weld pool. Further, we present our observations on how different welding parameters influence these regimes and develop into different transient conditions.

Control-Oriented Modeling and Repetitive Control in In-Layer and Cross-Layer Thermal Interactions in Selective Laser Sintering

Abstract Although laser-based additive manufacturing (AM) has enabled unprecedented fabrication of complex parts directly from digital models, broader adoption of the technology remains challenged by insufficient reliability and in-process variations. In pursuit of assuring quality in the selective laser sintering (SLS) AM, this paper builds a modeling and control framework of the key thermodynamic interactions between the laser source and the materials to be processed. First, we develop a three-dimensional finite element simulation to understand the important features of the melt pool evolution for designing sensing and feedback algorithms. We explore how the temperature field is affected by hatch spacing and thermal properties that are temperature-dependent. Based on high-performance computer simulation and experimentation, we then validate the existence and effect of periodic disturbances induced by the repetitive in- and cross-layer thermomechanical interactions. From there, we identify the system model from the laser power to the melt pool width and build a repetitive control algorithm to greatly attenuate variations of the melt pool geometry.

Hierarchical spatial-temporal modeling and monitoring of melt pool evolution in laser-based additive manufacturing

Melt pool dynamics reflect the formulation of microstructural defects in parts during Laser-Based Additive Manufacturing (LBAM). The thermal images of the melt pool collected during the LBAM process provide unique opportunities for modeling and monitoring its evolution. The recognized anomalies are evidence of part defects that are to be eliminated for higher product quality. A unique concern in analyzing thermal images is spatial-temporal correlations – the heat transfer within the melt pool causes spatial correlations among pixels in an image, and the evolution of the melt pool causes temporal correlations across images. The objective of this study is to develop a LBAM modeling-monitoring framework that incorporates spatial-temporal effects in characterizing and monitoring melt pool behavior. Spatial-Temporal Conditional Autoregressive (STCAR) models are explored. STCAR-AR is identified as the best candidate among the numerous STCAR variants. A novel two-level control chart is constructed on top of the STCAR-AR model to monitor the melt pool dynamics. A hierarchical structure underlies the two-level control chart in the sense that global anomalies recognized in Level II can be traced in Level I for further inspection. A comparison with other recently developed in-situ monitoring approaches shows that the proposed framework achieves the best detection power and false positive rate.

Lattice Boltzmann Simulation of Metallic Powder Melting during Additive Manufacturing

Additive manufacturing (AM) is a prevailing topic. Among all the processes during AM, the initiation and evolution of melt pool is of vital importance. Lots of studies are devoted to the fundamental mechanism of melt pool, but most of them only focus on the macroscopic level. However, when it comes to the mesoscopic level, it can be founded that the melt pool is the result of melting and fusion of powders. To better understand the melt pool, it is necessary to understand the melting and fusion characteristics of powders. This paper aims to provide a new kind of mesoscopic perspective to the melt pool through the investigation on single powder melting. Enthalpy based Lattice Boltzmann method, combined with Neumann boundary, is adopted to model the powder melting under fixed and moving laser beam. Results indicate that for the fixed laser beam, the melting front presents a symmetric convex. The increase of the laser power input can increase the energy absorbed by the powder, speed up the melting process and enhance the flow inside the powder. For the moving laser beam, the melting front loses its symmetry. The increase of the scanning speed can decrease the energy absorbed by the powder, the melting volume fraction and the inside flow. The fast moving speed could easily result in partially melted powder. All these results could help us better understand the powder melting under different laser inputs, and thus arranging the laser power reasonably to make the best use of it according to various occasions.

Basis of the Lattice Boltzmann Method for Additive Manufacturing

Additive manufacturing (or 3D printing) is constantly growing as an innovative process for the production of complex-shape components. Among the seven recognized 3D printing technologies, powder bed fusion (PBF) covers a very important role for the production of structurally functional components starting from different metal powder. However, being PBF a production process involving very high thermal gradients, non-negligible deformations and residual stresses may affect the 3D printed component. One of the characterizing aspects of PBF is the evolution of the melt pool and the heat exchange with the surrounding solid powder. In literature many attempts to simulate melt pool evolution have been carried out, however the only approaches leading to interesting results rely on the lattice Boltzmann method. In this work, starting from the Boltzmann’s equation, we derive the lattice Boltzmann equation and we introduce the needed assumptions in order to recover the lattice Boltzmann method. Finally, we apply the lattice Boltzmann method to study some interesting problems related to powder bed fusion process, including droplets wetting, thermal convection and solid–liquid phase change.

Thermo-mechanical simulation of track development in the Laser Beam Melting process - Effect of laser-metal interaction

RésuméInterest has recently emerged for the manufacture of aeronautical parts by Laser Beam Melting (LBM) additive process. This energy efficient process can for instance be used to build complex geometries, which cannot be made with traditional processes. However, complex phenomena occur during powder melting and track development : vaporisation phenomena influence laser-matter interaction by creating metal vapours that are responsible for the reduction of absorbed energy. The recoil pressure generated by the vaporisation counteracts the surface tension between the melt pool and the inert gas, also inducing liquid instabilities. The study of laser-matter interaction and induced phenomena can help understand the origin of defects such as porosities or cracks. In this approach, a level-set modelling of the LBM process at a mesoscopic scale is proposed to follow melt pool evolution and track development during build. A volume heat source model is used for laser/powder interaction considering the material absorption coefficient. A surface heat source is used to take into account the high laser energy absorption by dense metal alloys. An energy solver is coupled with thermodynamic database and pre-determined solidification path. Shrinkage during consolidation from powder to liquid and compact medium is modelled by a compressible Newtonian constitutive law. An automatic remeshing adaptation is also used to save time and avoid high computational cost. In the future, the computation of multiple beads or the build of a wall in a context of lattice structures will have to be considered.

Study and modeling of melt pool evolution in selective laser melting process of SS316L

Abstract

Numerical Study of Competition Effects of Laser Parameters on a Transient Melt Pool Evolution Process

The melt pool, formed by the superheated molten metal, is the initiation of solid part and the general process of laser additive manufacture process. The transient phenomena in the melt pool are of paramount interest in obtaining an optimum mechanical performance. In this paper, the different transient evolution phenomena of melt pool for the moving and fixed laser beam during the laser irradiation process are investigated, then the quantitative competition effects of the laser parameters on those transient phenomena are also studied. The results show that the evolution of melt pool gradually enters into the quasi-steady-state for the moving laser condition, the deformation of free surface almost has no change near the forward of laser beam. However, the fixed laser condition has different evolution process and the deformation of free surface becomes larger and larger even piercing the workpiece. The melt pool evolution becomes vigorous and prone to enter the quasi-steady-state with the increasing energy input per unit area which directly dominated by the laser parameters. Further investigation on the quantitative percentage of each parameter on the evolution shows that the laser scan speed is the most important factor for the evolution of melt pool.

Evolution phenomena and surface shrink of the melt pool in an additive manufacturing process under magnetic field

The control of evolution of flow and heat transfer in melt pool is a key concern and hot topic to improve the produce quality, e.g. fissure and roughness, during the additive manufacture process. Researchers focus on controlling the evolution of melt pool by changing energy balance in melt pool. However, the momentum drive is seldom presented in literatures but fully necessary to further improve the control efficiency. Therefore, a promising magnetic force source is introduced to investigate the controlling effect on melt pool evolution. Based on a developed and validated laser melt pool model, the evolution results of three different magnetic field arrangements and intensities are obtained and analysed compared with that without magnetic force. In addition, the surface shrink characteristics during quenching process are studied to further evaluate the benefit of the added source. Results show that the melt pool evolution is significantly affected by the additional magnetic force. The increasing downward y-component force promotes the transport in melt pool which makes the surface deformation larger, as well as the melt pool dimension. Inversely, with the increase of x-component force, both the surface deformation and the melt pool dimension gradually decrease due to the induced transport limitation. During quenching process, the surface roughness is continuously reduced with a certain increase of the magnetic intensity caused by the increased melt pool duration and slightly vigorous surface flow. However, the improvement effectiveness becomes bad as the magnetic force exceeds a critical value owing to the increased shrink instability.