Melt Pool Temperature Research Articles

Simulation based optimized beam velocity in additive manufacturing

Manufacturing good parts with additive technologies rely on melt pool dimension and temperature and are controlled by manufacturing strategies often decided on machine side. Strategies are built on beam path and variable energy input. Beam path are often a mix of contour and hatching strategies filling the contours at each slice. Energy input depend on beam intensity and speed and is determined from simple thermal models to control melt pool dimensions and temperature and ensure porosity free material. These models take into account variation in thermal environment such as overhanging surfaces or back and forth hatching path. However not all the situations are correctly handled and precision is limited. This paper proposes new method to determine energy input from full built chamber 3D thermal simulation. Using the results of the simulation, energy is modified to keep melt pool temperature in a predetermined range. The paper present first an experimental method to determine the optimal range of temperature. In a second part the method to optimize the beam speed from the simulation results is presented. Finally, the optimized beam path is tested in the EBM machine and built part are compared with part built with ordinary beam path.

Quality part production via multi-laser additive manufacturing

Multi-Laser Powder Bed Fusion (ML-PBF), a metals additive manufacturing (AM) method, provides an opportunity to produce more parts in less time. However, the effects of using multiple, moving energy sources on sintered part integrity, as well as the transferability between process parameters developed for L-PBF and ML-PBF, remains unknown. Via validated simulations, this study provides insight into how various single- and multi-laser PBF scan strategies can impact powder/part heat affected zones (HAZs), melt pool temperatures and local temperature gradient magnitudes, which affect microstructural properties, defect statistics, and mechanical behavior of fabricated parts.

The Role of Lug Preheating, Melt Pool Temperature, and Lug Entrance Delay on the Cast-on-Strap Joining Process

This work deals with effective parameters in the cast-on-strap (COS) process during which grid lugs of a lead-acid battery are joined together by a strap. The effects of lug preheating, melt pool temperature, and lug entrance delay on the quality of joints and casting defects were investigated. Lug preheating was found to propitiously reduce joint internal voids because of flux elimination. Its adverse effect on lowering lug wettability, however, made it unfavorable under the experimental conditions. The melt pool temperature also showed a two-sided effect depending on the process conditions. Raising the temperature increases the strap melt fluidity, which improves the joint contact area; however, it has a negative effect on lug wettability by flux evaporation. Besides, higher temperatures cause more lug back-melting and, hence, lower relative contact lengths. Therefore, an intermediate temperature of 683 K (410 °C) was found to make the most proper condition. Moreover, the case at which the lugs enter the mold coincident with its filling by the melt rendered the best joint quality. In this condition, the melt flows through the interlug spaces, which helps the voids to escape, resulting in the better joint interface. As the conclusion, the lug entrance time has the most effective role on joint quality, considering that lug preheating does not show any improving effect.

Melt-pool motion, temperature variation and dendritic morphology of Inconel 718 during pulsed- and continuous-wave laser additive manufacturing: A comparative study

Pulsed-wave laser additive manufacturing offers a number of advantages, such as a lower heat accumulation, a higher cooling rate, finer microstructures and improved mechanical properties over continuous-wave laser additive manufacturing. However, how pulsed laser acts on the melt pool motion, thermal field and hence microstructure is not clear. In this work, a three-dimensional transport model utilizing the level set method is developed to simulate the transient melt pool motion, heat/mass transfer and fluid flow for pulsed-wave laser additive manufacturing. A boundary restriction on the fluid velocity along the liquid/gas interface is employed to confine the liquid flow within the melt pool. The simulated melt pool geometry and temperature are compared with experimental measurements. Moreover, melt pool geometry/motion, temperature variation, and their influence on the microstructure of fabricated samples using both pulsed- and continuous-wave lasers are analyzed. It is found that pulsed-wave laser additive manufacturing features a rounder shaped melt pool, a periodically heartbeat-like motion of the melt pool and a doubled cooling rate. The higher tilt angle of the solidification front results in a dendrite growth direction more tilted to the laser scanning direction and the higher cooling rate results in finer columnar dendrites.

Numerical modeling of the thermal behavior and residual stress in the direct metal laser sintering process of titanium alloy products

In this paper the heat transfer and residual stress evolution in the direct metal laser sintering process of the additive manufacturing of titanium alloy products are studied. A numerical model is developed in a COMSOL multiphysics environment considering the temperature-dependent material properties of TiAl6V4. The thermo-mechanical coupled simulation is performed. 3-D simulation is used to study single-layer laser sintering. A 2-D model is used to study the multi-layer effects of additive manufacturing. The results reveal the behavior of the melt pool size, temperature history, and change of the residual stresses of a single layer and among the multiple layers of the effects of the change of the local base temperature and laser power etc. The result of the simulation provides a better understanding of the complex thermo-mechanical mechanisms of laser sintering additive manufacturing processes.

Impact of Laser Additive Enhanced Powder Feeding Methods on Melt Pool Temperature Field

The surface of Q345 steel are coated with the Fe62 alloy powder by the synchronous powder feeding method and the prepositioned powder feeding method. The impacts of the two methods on the laser pool forming rate and the temperature field are tested and analyzed with a Charge-Coupled Device (CCD) colorimetric thermometer. The results indicate that the laser pool has faster forming rate, more stable and homogeneous temperature field with the synchronous powder feeding method than the prepositioned powder feeding method under the same conditions. The outcomes are of great significant to optimize the process of the laser additive enhancement technology.

Optical diagnostics of selective laser melting and monitoring of single-track formation

The article presents the optical diagnostics results of the selective laser melting process of single-track production. The track defects detection (such as balling effect, powder free zone formation, sparking) was shown, as well as the visualization of the independent particles consolidation in a solid track. The metal evaporation and the formation of the melt pool specific gas dynamic conditions were considered as important physical phenomena. The velocities of the particle emission from the melt pool, the rate of their involvement, and the velocity of the gas flow were estimated. The results make it possible to evaluate the kinetics of mass transfer under selective laser melting process. The surface thermal field of the laser-irradiated zone strongly influences the material qualitative characteristics after selective laser melting. The results becomes the basis for the development of optical monitoring and diagnostic systems for laser additive manufacturing processes based on the melt pool temperature online controlling.

Prediction of microstructure in laser powder bed fusion process

Additive manufacturing (AM) processes are receiving widespread attention due to the ability to create or repair precision engineering components without use of any die or mold. Currently, the approach to obtain a specific user defined/as-desired or conformal/epitaxial microstructure is a challenging and expensive iterative process. Modeling and validation of solidification microstructure can be leveraged to reduce iteration cost in obtaining a desired microstructure. Numerical Volume-of-fluid based method incorporating Marangoni convection can accurately predict the resultant melt pool geometry and temperature distribution which can serve as an input in prediction of microstructure evolution in solidifying mushy region. Hence, in the present study, computational fluid dynamics (CFD) analysis is used to predict melt pool characteristics and phase field modeling is employed to simulate microstructure evolution in the as-deposited state for laser powder bed fusion (LPBF) process. Different features of LPBF microstructure such as segregation of secondary elements, dendrite sizes, dendritic orientation, dendritic morphology, and surface roughness are investigated and validated through comparison with experimental results. Phase-field model suggests strong dependency of dendrite orientation on surface roughness and scan speed and suggests potential of columnar flip or oriented-to-misoriented transition at higher scan speed. Segregation of the secondary elements is found to be the dominant factor in resultant dendrite width in the range of 1–3 μm. Furthermore, the developed method can easily be extended to predict the change in orientation of dendrites as new layers are built atop previous layers.

Improvement and evaluation of debris coolability analysis module in severe accident analysis code SAMPSON using LIVE experiment

The purpose of this work is to validate the debris coolability analysis (DCA) module in the severe accident analysis code SAMPSON by simulating the first steady stage of the LIVE-L4 test. The DCA module is used for debris cooling in the lower plenum and for predicting the safety margin of present reactor vessels during a severe accident. In the DCA module, the spreading and cooling of molten debris, gap cooling, heating of a three-dimensional reactor vessel, and natural convection heat transfer are all considered. The LIVE experiment is designed to investigate the formation and stability of melt pools in a reactor pressure vessel (RPV). By comparing the simulation results and experimental data in terms of the average melt pool temperature and the heat flux along the vessel wall, a bug is found in the code and the model for the heat transfer between the melt pool and RPV wall is modified. Based on the Asfia–Dhir and Jahn–Reineke correlations, the modified version of the DCA module is found to give reasonable results for the average melt pool temperature, crust thickness in the steady state, and crust growth rate.

Numerical simulation and designing artificial neural network for estimating melt pool geometry and temperature distribution in laser welding of Ti6Al4V alloy

Weld geometry is a critical factor for determining the quality of Ti6Al4V welded joints. The size of the weld cross section profile has been quantitatively investigated through experimental and numerical analysis. Due to the difficulties in temperature measuring of the molten pool region, the temperature distribution through numerical simulation was exerted as an indirect approach for estimating the size of the melt pool profile and HAZ region. Moreover, the numerical model was used for prediction of cooling rate in the melt pool and thereby characterization of fusion zone microstructure. To achieve an accurate prediction of the weld geometry at low time and cost, the process was simulated based on artificial neural network. Different ANNs were developed for progressive prediction of the weld pool temperature distribution and weld geometry. Two feed-forward back propagation neural network models with 11 and 14 neurons were developed to predict optimum process parameters. The proposed artificial neural network models perfectly predicted the process with mean square errors of 0.079 and 0.063. The results indicated that ANN outputs were in good agreement with the experimental and numerical data.

FARO tests corium-melt cooling in water pool: Roles of melt superheat and sintering in sediment

The FARO tests have aimed at understanding an important severe accident mitigation action in a light water reactor when the accident progresses from the reactor pressure vessel boundary. These tests have aimed to measure the coolability of a molten core material (corium) gravity dispersed as jet into a water pool, quantifying the loose particle diameter distribution and fraction converted to cake under range of initial melt superheat and pool temperature and depth. Under complete hydrodynamic breakup of corium and consequent sedimentation in the pool, the initially superheated corium can result in debris bed consisting of discrete solid particles (loose debris) and/or a solid cake at the bottom of the pool. The success of the debris bed coolability requires cooling of the cake, and this is controlled by the large internal resistance. We postulate that the corium cake forms when there is a remelting part in the sediment. We show that even though a solid shell forms around the melt particles transiting in the water pool due to film-boiling heat transfer, the superheated melt allows remelting of the large particles in the sediment (depending on the water temperature and the transit time) using the COOLAP (Coolability Analysis with Parametric fuel-cooant interaction models) code. With this remelting and its liquid-phase sintering of the non-remelted particles, we predict the fraction of the melt particles converting to a cake through liquid sintering. Our predictions are in good agreement with the existing results of the FARO experiments. We address only those experiments with pool depths sufficient/exceeding the length required for complete breakup of the molten jet. Our analysis of the fate of molten corium aimed at devising the effective scenarios for its safe cooling in the containment so that predicted the minimum pool depth for no cake formation as functions of the melt superheat and water (subcooled/saturation) temperatures.

Spectroscopic monitoring and melt pool temperature estimation during the laser metal deposition process

Laser metal deposition is an additive manufacturing process that allows the production of near net shape structures. Moreover, the process can also be applied for the addition of material to an existing component for repair. In order to obtain structures with reproducible and excellent material properties, it is necessary to understand the thermal behavior of the process better and to monitor and control the process. One of the critical parameters in this process is the measurement of the melt pool temperature and its distribution. The varying emissivity in space and time for the melt pool forms a fundamental physical problem. This also prevents a correct temperature measurement of the melt pool temperature distribution with a thermal camera. The usage of the spectral information within the emitted light of the melt pool can form a key enabling element in the estimation of the emissivity and as such reveal the temperature information. In future, this information can be used in a controlling system in order to prevent excessive heat transfer toward the substrate and to reduce the amount of the residual stress. Another key criterion for the additive manufacturing process is the prevention of oxidation of the deposited layers in order to eliminate the formation of brittle zones within the components. The appearance of specific discrete spectral lines can reveal essential information of the stability of the process such as oxidation. In this paper, an optical setup with an optical spectrometer will be described for the measurement of the radiated spectrum ranging from 400 up to 850 nm. The spectra were obtained during the laser metal deposition process of the stainless steel (316L). The measured spectroscopic data of different locations on the melt pool surface will be presented and explored in this paper. Based on the measured spectra different temperature estimation algorithms will be presented and evaluated. The appearance of specific discrete spectral lines will also be examined.

Data indicating temperature response of Ti-6Al-4V thin-walled structure during its additive manufacture via Laser Engineered Net Shaping.

An OPTOMEC Laser Engineered Net Shaping (LENS™) 750 system was retrofitted with a melt pool pyrometer and in-chamber infrared (IR) camera for nondestructive thermal inspection of the blown-powder, direct laser deposition (DLD) process. Data indicative of temperature and heat transfer within the melt pool and heat affected zone atop a thin-walled structure of Ti–6Al–4V during its additive manufacture are provided. Melt pool temperature data were collected via the dual-wavelength pyrometer while the dynamic, bulk part temperature distribution was collected using the IR camera. Such data are provided in Comma Separated Values (CSV) file format, containing a 752×480 matrix and a 320×240 matrix of temperatures corresponding to individual pixels of the pyrometer and IR camera, respectively. The IR camera and pyrometer temperature data are provided in blackbody-calibrated, raw forms. Provided thermal data can aid in generating and refining process-property-performance relationships between laser manufacturing and its fabricated materials.

Effect of real-time cooling rate on microstructure in Laser Additive Manufacturing

An infrared thermal imaging system is developed to compare the effects of cooling rate and melt pool temperature on the microstructure of 316L stainless steel produced by Laser Additive Manufacturing (LAM) with injected powder melting. The system captures the real-time cooling rate to provide online monitoring and control of the microstructure at the onset of solidification. Further study is conducted to evaluate cooling rate effects on grain size, phase transformations and hardness. Cooling rate changes are also studied with respect to the traveling speed, in order to identify a suitable controlling action for a microstructural controller. The study provides a strong experimental justification of the current theories in solidification and microstructural evolution during the LAM with injected powder melting process.

Understanding the Microstructure Formation of Ti-6Al-4V During Direct Laser Deposition via In-Situ Thermal Monitoring

Understanding the thermal phenomena associated with direct laser deposition (DLD) is an important step toward obtaining ‘process–property–performance’ relationships for various designed parts and materials, as well as achieving increased process control for meeting application constraints. In this study, a thermally monitored laser engineered net shaping (LENS™) system was used with time-invariant (uncontrolled) build parameters to construct Ti-6Al-4V cylinders. During fabrication, the part’s thermal history and melt pool temperature were recorded via an in-chamber infrared camera and a dual-wavelength pyrometer, respectively. These tools demonstrate the use of non-destructive thermographic inspection for ensuring target part quality and/or microstructure. For the chosen part geometry, the melt pool was found to be approximately 40%–50% superheated during DLD, reaching temperatures as high as 2500°C. Temperature gradients varied and peaked around 1000°C/mm along the diameter of the relatively small cylinders. Cooling rates within the melt pool were found to increase as maximum melt pool temperature increased, for instance, from 12,000°C/s to 25,000°C/s. The post-DLD Ti-6Al-4V microstructure was found to vary from columnar near the substrate, or substrate-affected zone, to equiaxed approximately 2–3 mm from the substrate. Bulk heating of the part due to successive layer deposits was shown to promote α″ to an α + β decomposition, while prior-β grains were observed near and far from the substrate.

Study of oxygen effect on the melting pool temperature during selective laser melting

The on-line optical diagnostics studies of selective laser melting are difficult to accomplish due to the need of closed chamber of inert gas protective atmosphere. In this term, this study was initiated to make evidence of oxygen influence on the signal from the melt pool. The results make clear the temperature growth with presence of oxygen and its effect on the metallurgical contact of the single-track to the substrate. It was shown that for both used materials, the oxidation leads to an overheating but the metallographic quality differs. The experiments highlighted that the optical diagnostic means such as IR thermography camera is suitable but not sufficient solution for process control and it needs adequate evaluation. Using the track width as an approximation of the melt pool width at melting point, it was possible with some assumptions to restore the true temperature of the studied zone.

The COPRA experiments on the in-vessel melt pool behavior in the RPV lower head

The COPRA (COrium Pool Research Apparatus) facility was designed to investigate the in-vessel molten corium pool behavior for the in-vessel corium retention during severe accidents in Chinese large-scale advanced PWRs. The main part of the COPRA facility is a two-dimensional 1/4 circular slice test section with an inner radius of 2.2m to simulate the lower plenum of the reactor vessel at 1:1 scale for the Chinese large-scale advanced PWR. Up to now, the COPRA experiments have been performed in different melt volumes, heat generation rates and multiple injection of the melt in the one test. At present, the well-known simulant material NaNO3–KNO3 in non-eutectic composition (20mol% NaNO3–80mol% KNO3) is used. This work presents the behavior of a large-scale homogenous melt pool regarding the melt pool temperature, heat flux distribution through the vessel wall and crust thickness in transient and steady state conditions. And the heat transfer towards the vessel wall of the COPRA test was compared with previous experiments and experimental correlations. It showed that the downward heat transfer from the COPRA experiments was lower than those from other experiments within the same range of Rayleigh numbers (1015–1017).

COPRA: A large scale experiment on natural convection heat transfer in corium pools with internal heating

During a severe accident in light water reactors, the core may melt and relocate into the lower plenum of the reactor pressure vessel. The decay heat will threaten the structural and thermal integrity of the reactor vessel if there is no effective cooling mechanism. Natural convection plays an important role in determining the thermal-hydraulic behaviors inside the debris pool, which is directly relevant to the problem of retention of molten corium inside the lower plenum. The COPRA (COrium Pool Research Apparatus) experiments were performed to study the natural convection heat transfer behavior in an internally heated melt pool with high Rayleigh numbers. The COPRA test facility is a two-dimensional 1/4 circular slice vessel with an inner radius of 2.2 m to simulate the lower plenum of reactor vessel for the Chinese advanced PWR in a full scale. The inner width of the slice is 20 cm and the curved vessel wall has a thickness of 30 mm. 20 electrical heating rods, each with a diameter of 16 mm but different lengths according to their locations, are uniformly distributed in the vessel to simulate homogenous internal decay heat. They can provide a maximum of 30 kW power to the melt pool. The outside of the curved wall is enclosed with a regulated external cooling path to keep the boundary temperature nearly isothermal. The top surface of the pool can be maintained insulated with an adiabatic lid. 79 thermocouples are installed in the melt pool to measure the melt pool temperature field and 48 in the curved wall to obtain local heat flux distribution along the curved wall. In the first series of experiments, water was employed as the simulant material. Due to the full scale geometry, the Rayleigh number within the pool could reach up to 1016, matching that in the prototypical situation for PWR. The heat transfer characteristic with volumetrically internal heat was investigated by using the full scale facility COPRA. Relations of pool temperature and heat flux distribution, as well as Nudn-Ra′ were developed. The results have been compared with the results and correlations from other experiments.

Natural convection heat transfer test for in-vessel retention at prototypic Rayleigh numbers – Results of COPRA experiments

Large-scale COPRA experiments were performed to investigate the natural convection heat transfer in melt pools for the in-vessel retention during severe accidents in Chinese large-scale advanced PWRs. Both water and binary mixture of 20 mol% NaNO3 – 80 mol% KNO3 were used as the melt simulant material in performed tests. Due to the full scale geometry of the COPRA test section, the Rayleigh numbers of the melt pool could reach up to the prototypic magnitude of 1016. Natural convection heat transfer tests at prototypic Rayleigh numbers have been performed to study the influence of the heat generation rate and melt simulant material on the melt pool temperature, heat flux distribution and heat transfer capability. The comparisons of the melt pool temperature and heat flux distribution from water experiments and molten binary salt experiments showed that the crust formation along the inner surface of the vessel wall could impact the heat transfer characteristics of the melt pool. And the heat flux distribution from COPRA water tests and molten salt tests were in good agreement with those from Jahn-Reineke water experiments and RASPLAV molten salt experiments, respectively. The heat transfer capability of the melt pool Nudn from COPRA molten salt tests were larger than those from water tests, but both were lower than those from ACOPO and BALI predictions within the same range of Rayleigh numbers (1015 – 1017).

Surface tension-powered build dimension control in laser additive manufacturing process

This research is aimed at providing insight into the process phenomena that determine the surface finish quality and dimensional accuracy of laser additive manufactured parts. It was found that melt pool fluid flow has a major effect on the sidewall dimensions of the final deposit. Thermocapillary gradient is the main driving force for melt pool flow, and the gradient can potentially be controlled by material composition. Thus, the effects of three characteristic thermocapillary gradients (positive, negative, mixed) on melt pool fluid flows and build geometry are investigated in this study. The work studied mechanisms responsible for sidewall nonuniformity and deposit bulge in detail. The results showed that the bulge in deposit width occurring at the start of the build is reduced by approximately 56 % by deposition of material having mixed thermocapillary gradient as compared to the bulge resulting from deposition of material with a negative thermocapillary gradient. Also, melt pool temperature distributions are visualized during laser deposition of five-layer single-track builds. More accurate build geometry and reduced deposit sidewall bulge can be expected to lead to lower total cost of additive manufactured parts.