Pool Aspect Ratio Research Articles

Physics guided heat source for quantitative prediction of IN718 laser additive manufacturing processes

Challenge 3 of the 2022 NIST additive manufacturing benchmark (AM Bench) experiments asked modelers to submit predictions for solid cooling rate, liquid cooling rate, time above melt, and melt pool geometry for single and multiple track laser powder bed fusion process using moving lasers. An in-house developed Additive Manufacturing Computational Fluid Dynamics code (AM-CFD) combined with a cylindrical heat source is implemented to accurately predict these experiments. Heuristic heat source calibration is proposed relating volumetric energy density (ψ) based on experiments available in the literature. The parameters of the heat source of the computational model are initially calibrated based on a Higher Order Proper Generalized Decomposition- (HOPGD) based surrogate model. The prediction using the calibrated heat source agrees quantitatively with NIST measurements for different process conditions (laser spot diameter, laser power, and scan speed). A scaling law based on keyhole formation is also utilized in calibrating the parameters of the cylindrical heat source and predicting the challenge experiments. In addition, an improvement on the heat source model is proposed to relate the Volumetric Energy Density (VEDσ) to the melt pool aspect ratio. The model shows further improvement in the prediction of the experimental measurements for the melt pool, including cases at higher VEDσ. Overall, it is concluded that the appropriate selection of laser heat source parameterization scheme along with the heat source model is crucial in the accurate prediction of melt pool geometry and thermal measurements while bypassing the expensive computational simulations that consider increased physics equations.

A model of the axial plume temperature profile of rectangular pool fires dominated by convection under different atmospheric pressures and aspect ratios

This paper experimentally studied the axial plume temperature profile of heptane pool fires dominated by convection under different atmospheric pressures and aspect ratios. Results indicate that McCaffrey's axial temperature model in the plume region is not applicable at low-pressure conditions. The Gaussian plume model proposed by Tang et al. was verified based on the experiments of 36 cm2 pool fires, which are dominated by conduction, for different atmospheric pressures and aspect ratios. By mathematical derivation of the plume mass flow rate of pool fires, the relationship between the air entrainment coefficient and atmospheric pressure was obtained, and the experimental results of the air entrainment coefficient with different pressures can be well correlated with the analytical model. Finally, by introducing the air entrainment coefficient and temperature decay factor, a new axial plume temperature model for pool fires dominated by convection was developed, considering different atmospheric pressures and pool aspect ratios, which shows a good agreement with experimental results. This work can provide some good references for the fire safety of pool fires related to liquid fuel leakage.

The influence of novel beam shapes on melt pool shape and mechanical properties of LPBF produced Al-alloy

New generation of multi-core fiber laser sources provide in-source dynamic beam shaping. Such sources can switch between Gaussian and ring beams, providing new irradiance profiles. The new irradiance profiles add up to the process flexibility for controlling the temperature fields generated in the laser powder bed fusion (LPBF) process. On the other hand, they bring further complexity as new process parameters should be defined and their influence on the mechanical properties unveiled. Accordingly, this work studies the use of seven different beam shapes provided by a multi-core industrial fiber laser source with beam diameters varying between 47 μm to 144 μm during the LPBF of AlSi7Mg0.6 alloy. The beam spatial profiles were measured and descriptive irradiance parameters were defined. The Al-alloy constituted a challenging material type due to its high reflectivity and low melting point, hence, more prone to lack-of-fusion and keyhole defects. LPBF experiments were conducted at fixed energy density and peak irradiance levels investigating the influence of the beam shapes on the melt pool geometry in prismatic samples. The results showed that the peak irradiance and the ring intensity had a direct impact on the melt pool aspect ratio (AR) and melt pool depth to layer thickness ratio (h/z). With all the conditions providing adequately dense parts (>99.5 %), the mechanical properties were found to be correlated to the melt pool geometry. The results confirm that the beam shape can be tuned between the central peak and the ring to further manipulate the material properties in LPBF.

Grain growth competition during melt pool solidification — Comparing phase-field and cellular automaton models

A broad range of computational models have been proposed to predict microstructure development during solidification processing but they have seldom been compared to each other on a quantitative and systematic basis. In this paper, we compare phase-field (PF) and cellular automaton (CA) simulations of polycrystalline growth in a two-dimensional melt pool under conditions relevant to additive manufacturing (powder-bed fusion). We compare the resulting grain structures using local (point-by-point) measurements, as well as averaged grain orientation distributions over several simulations. We explore the effect of the CA spatial discretization level and that of the melt pool aspect ratio upon the selected grain texture. Our simulations show that detailed microscopic features related to transient growth conditions and solid–liquid interface stability (e.g. the initial planar growth stage prior to its cellular/dendritic destabilization, or the early elimination of unfavorably oriented grains due to neighbor grain sidebranching) can only be captured by PF simulations. The resulting disagreement between PF and CA predictions can only be addressed partially by a refinement of the CA grid. However, overall grain distributions averaged over the entire melt pools of several simulations seem to lead to a notably better agreement between PF and CA, with some variability with the melt pool shape and CA grid. While further research remains required, in particular to identify the appropriate selection of CA spatial discretization and its link to characteristic microstructural length scales, this research provides a useful step forward in this direction by comparing both methods quantitatively at process-relevant length and time scales.

The influence of beam focus during laser powder bed fusion of a high reflectivity aluminium alloy — AlSi10Mg

LPBF of Al alloys is associated with numerous challenges when compared to other commonly used alloys due to their higher reflectivity and thermal conductivity. In this work, processing diagrams, temperature prediction models, XCT, and metallography are used for establishing criteria in process parameter optimization of high reflectivity Al alloys based on AlSi$_{10}$Mg response in using 57 different process parameter combinations - 21 using a focused Gaussian laser beam and 36 using divergent beams. For LPBF systems with focused beam diameters <100 {\mu}m, divergent beams obtained by defocusing to a position above the LPBF build plate primarily lead to conduction mode melt pools, while a focused beam leads to transition and keyhole mode melt pools. Conduction mode melting helps in avoiding keyhole mode defects, resulting in parts with densities >99.98%. Additionally, an analytical model-guided selection of laser power and velocity settings for a focused beam help in stabilizing melt pool and spatter dynamics in the transition melting mode thereby enabling a potential to obtain density values close to conduction mode densities (~99.98%). A dimensionless keyhole number (Ke) was derived in this work to identify distinct regions of conduction (Ke of 0-12), transition (Ke of 12-20), and keyhole (Ke > 20) mode melting during LPBF of AlSi$_{10}$. A melt pool aspect ratio (ratio of melt pool depth to width) of ~0.4 is observed to be the threshold between conduction and transition/keyhole mode melt pools for AlSi$_{10}$, different from the conventionally assumed 0.5. Lastly, inferred laser absorptivity values (from experimental melt pools) of transition/keyhole mode melt pools are observed to be >40% higher when compared to conduction mode melt pools. This work demonstrates a dimensionless-process map method to obtain near fully dense parts that can be generalized for LPBF of high reflectivity alloys.

High-throughput screening of surface roughness during additive manufacturing

Smooth surfaces in the printed parts are essential for long fatigue life and high dimensional accuracy. They are currently achieved by post-process machining and grinding of external surfaces that add extra costs. Mitigating the surface roughness of internal channels remains a challenge. Here we use a high-throughput screening approach that analyzes the value of a dimensionless index for many experiments and provides a pathway for reducing the surface roughness without the need for post-processing. The index is derived by dimensional analysis of causative variables that affect the roughness of the surface such as heat input, powder diameter, layer thickness, pool aspect ratio, Marangoni force, contact angle, and enthalpy of melting of alloys. Using the results of high-throughput screening, we develop easy-to-use process maps that are consistent with the experimental observations. Among the causative variables, heat input and the contact angle of the molten material with the substrate have the highest and lowest influence on the smoothness of printed surfaces. An aluminum alloy, AlSi10Mg is found to be the best choice for printing smooth surfaces among the four alloys considered here. These findings can improve the surface quality of additively manufactured parts that now significantly hinder their wider industrial adaptation.

A testbed for investigation of laser powder bed fusion at elevated atmospheric pressure

Metal additive manufacturing (AM) by laser powder bed fusion ( L -PBF) builds upon fundamentals established in the field of laser welding which include the influence of gas and plume dynamics on weld depth and quality. L -PBF demands a thorough investigation of the complex thermophysical phenomena that occur where the laser interacts with the metal powder bed. In particular, melt pool turbulence and evaporation are influenced by the ambient gas chemistry and pressure, yet typical L-PBF equipment lacks flexibility to alter the ambient pressure within the process chamber. This paper presents the design and validation of high pressure laser melting (HPLM) testbed. This system accommodates bare metal plate samples as well as manually-coated single powder layers, and operates at up to 300 psig. The open architecture of this testbed allows for full control of all relevant laser parameters in addition to ambient gas pressure and gas flow over the build area. Representative melt tracks and rasters on bare plate and powder are examined in order to validate system performance, and preliminary analysis concludes that pressure has a significant impact on melt pool aspect ratio. The HPLM system thus enables careful study of the effects of pressure on the outcomes of the L-PBF process, and can be applied in the future to materials that are challenging to process by L-PBF under ambient pressure, such as those with high vapor pressures.

Laser spot size and scaling laws for laser beam additive manufacturing

Laser powder bed fusion (L-PBF) additive manufacturing (AM) requires the careful selection of laser process parameters for each feedstock material and machine, which is a laborious process. Scaling laws based on the laser power, speed, and spot size; melt pool geometry; and thermophysical properties can potentially reduce this effort by transferring knowledge from one material and/or laser system to another. Laser spot size is one critical parameter that is less well studied for scaling laws compared to laser power and scan speed. Consequently, single track laser scans were generated with a spot size (D4σ) range of 50 μm to 322 μm and melt pool aspect ratio (depth over spot radius) range from 0.1 to 7.0. These were characterized by in-situ thermography, cross-sectioning, and optical microscopy. Scaling laws from literature were applied and evaluated based on melt pool depth predictions. Scaling laws that contain a minimum of three dimensionless parameters and account for changing absorption between conduction and keyhole mode provide the most accurate melt pool depth predictions (<35% difference from experiments), which is comparable to thermal simulation results from literature for a select number of cases.

Numerical and experimental investigation of complex surface topography evolution during laser surface modification with raster scan

This work investigates the complex surface topography evolution of an area during laser surface modification with raster scan. A nonlinear three dimensional transient numerical model is developed to capture the melt pool dynamics, solidification and resulting surface evolution, during laser surface modification with raster scanning. Further laser surface modification of Ti6Al4V is performed with different combinations of scanning speed and beam overlap. Surface profiles from laser modified samples are captured using optical profilometer. In order to study the surface features quantitatively and compare against the numerical predictions, three surface parameters are defined: crest-trough area ratio, trough spacing along overlap direction and mean feature aspect ratio. A modified non-dimensional heat input for raster scan is also introduced, to which Marangoni number is more sensitive, as compared against the traditional non-dimensional heat input for laser based processes. Surface parameters from numerical predictions compare well against the experimental results. Irrespective of beam overlap, slower scanning speeds led to lower crest-trough area ratio and mean feature aspect ratio. This is attributed to wider melt pool (lower melt pool aspect ratio), owing to higher heat input at lower scanning speeds, which promotes the surface tension driven forces and melt pool circulation. Numerical results indicated improved fluid circulation at higher beam overlaps as well. However, crest-trough area ratio increased at higher beam overlaps, as surface features from previous passes undergo re-melting. Numerical results reveal that higher beam overlaps lead to downward movement of molten metal towards the re-melted region, which leads to asymmetric melt pool and filling of trough regions (from previous passes). This reduces trough area upon solidification (higher crest-trough area ratio) and lowers average surface feature height as beam overlap increases.

Resolving the porosity-unmelted inclusion dilemma during in-situ alloying of Ti34Nb via laser powder bed fusion

A parametric experimental study on the role played by the power (P), velocity (VL) and profile (top-hat or Gaussian) of the laser on the porosity, inclusion content and microstructural evolution of in-situ alloyed laser powder bed fusion (LPBF) manufactured Ti34Nb was conducted. For this, alloys were printed with three sets of processing parameters, in which the above-mentioned parameters were varied but the energy density was held constant. A detailed tomographic and microstructural investigation of these alloys was followed by tensile tests. Observations of melt pools in the single tracks and single stripes were complemented by thermal finite element method (FEM) simulations that studied their evolution. Results show that a top-hat laser profile combined with high P (> 650 W), high VL (> 650 mm/s) and short stripe width scanning strategy forms a large single stripe melt pool that moves slowly and has a low aspect ratio. The large and slow melt pool allows Nb to melt efficiently and minimizes its unmelted content while the low melt pool aspect ratio prevents keyholing that can otherwise result in porosity. These melt pool attributes also favor the formation of a columnar β-Ti microstructure with a strong {100} texture, which has the ideal combination of strength and elastic modulus. The mechanisms of microstructural evolution were explained and the scope of this parameter optimization principle was discussed in the context of further improving the properties of this alloy.

Experimental study on vertical temperature distribution of symmetry axis of two identical rectangular pool fires with long sides parallel

The axial temperature distribution of fire plume is an important subject in the field of fire research. While the gas temperature distribution induced by multiple fires is still not clear. This paper presents an experimental study on axial temperature distribution of two identical rectangular pool fires with long sides parallel. The pool aspect ratio and spacing were varied. Results showed that corresponding to different flame merging states including merging from the base, merging above the base and no merging, the axial temperature distribution appears three different change laws. Regarding the case of merging from the base as a large area fire, correlation of axial temperatures from various pool aspect ratios and spacings is proposed by introducing a virtual origin characterizing the effect of fire geometry. For the cases of merging above the base, taking the merging point as the boundary and determining the fire geometry and heat release rate of the upper assumed single fire, a piecewise function for the axial temperature distribution upon the merging point is established. The axial temperature characterizing the flame merging is further determined as 930 °C for two rectangular heptane pool fires, which can be used to judge whether the flames merge or not.

High deposition rate approach of selective laser melting through defocused single bead experiments and thermal finite element analysis for Ti-6Al-4V

Selective laser melting (SLM) is a commonly used powder bed fusion metal additive manufacturing (AM) process. Although SLM is preferred due to its near-net-shape part commitment, the deposition rate of this process is slower compared with alternative metal processes. A higher deposition rate of SLM can be obtained by increasing the laser scanning velocity and laser power; however, this results in decreased part quality due to the SLM process’s physical limits. This study presents the conditions for a higher deposition rate for various process parameters with defocused beams to eliminate the void defects due to keyholing formed in the melt pool. Single bead experiments were conducted, and the thresholds of the process parameters resulting in voids were identified. A melt pool depth-to-width ratio of 0.85 was found to be a critical value for preventing voids in the process. The melt pool aspect ratio was related with the process parameters by using the normalized enthalpy and the volumetric energy density. The threshold values of the normalized enthalpy due to voids were independent from the beam diameters. Moreover, unstable single bead track thresholds were plotted as a function of the beam diameters. In addition to the experiments, a finite element analysis model was built with calibrated absorptivity and heat source parameters to predict the melt pool geometries for a wide range of process parameters (power = 100–370 W, velocity = 200–2000 mm/s, and beam diameter = 100–260 μm).

Competitive grain growth and dendrite morphology evolution in selective laser melting of Inconel 718 superalloy

Microstructure control is a great challenge for selective laser melting (SLM) mainly because of nonuniform and ultra-rapid heating and cooling in the process. It is related to the control of dendrite morphology evolution and competitive grain growth by adjusting the process parameters of SLM. In this study, a multiscale simulation approach, which combines the finite element method (FEM) and the phase field (PF) method, is performed to investigate the dendrite morphology and stray grain growth under various SLM parameters for Inconel 718 superalloy. The cooling characteristics of melt pool, including the cooling rate, thermal gradient/solidification ratio (G/R), and the slope of the solidification front are investigated. Meanwhile, experiment verification adopting the manufacturing parameters identical to the simulation is performed. It is shown that simultaneously increasing or decreasing the laser power and scan speed leads to a drastic change of melt pool aspect ratio (MPAR), and this in turn affects the competitive growth of stray grains. Overall, a larger MPAR is found to suppress the growth of stray grains more effectively. Also, the experimental results are found to agree well with the simulation results. Three mechanisms of stray grain growth are revealed from the simulation results, and their roles are analzyed. This study provides important insights into the microstructure control in the laser additive manufacturing processes.

Hydraulics of flow over rectangular labyrinth weirs

Laboratory experiments were carried out to study the hydraulics of flow over rectangular labyrinth weirs in both free and submerged flow conditions. In order to study the effects of approach velocity and rectangular pool dimensions, nine weir models with different pool lengths a, widths b, and discharge were tested. The length-to-width aspect ratio a/b was varied from 0.25 to 3. The discharge coefficients of rectangular labyrinth weirs were compared with the discharge coefficients of linear sharp-crested weirs of the same channel width B and weir height P. It was found that the discharge coefficients decreased with increasing the approach velocity and the pool aspect ratio. Semi-empirical formulations were developed to predict the discharge over rectangular labyrinth weirs in free flow condition. A critical approach velocity was introduced to evaluate the advantages of the rectangular labyrinth weirs over sharp-crested weirs. It was found that rectangular labyrinth weirs are hydraulically more efficient than sharp-crested weirs for ho/P ≤ 0.4 where ho is the water head over the crest of the weir in free flow condition. Performance of rectangular labyrinth weirs in submerged flow condition were investigated. Experimental data indicated that rectangular labyrinth weirs are more sensitive than the linear sharp-crested weirs in submerged flow condition. Having the same headwater in submerged flow condition, flow over rectangular labyrinth were 10% and 20% less than linear sharp-crested weirs for pool aspect ratios of a/b ≥ 1 and a/b < 1, respectively.

A Large Eddy Simulation of LNG Pool Fire on Board a Chemical/Oil Tanker

The air pollution from maritime transportation has become one of the major environmental concerns. The liquefied natural gas (LNG) has better environmental performance compared to conventional ship fuels. Therefore, the use of LNG as ship fuel has recently gained more attention in the maritime industry. On the other hand, LNG as a fuel can have high risks of explosion and fire on ships. Among the various LNG ship fires, the pool fire is the most common phenomenon. Therefore, this study focuses on the LNG pool fire on board a chemical/oil tanker with different pool aspect ratios. Also, the effects of wind speed on the flame characteristics are investigated. The LNG pool fire on board a chemical/oil tanker is studied by using large eddy simulation approach and Fire Dynamic Simulator (FDS) code, numerically. The results show that the flame characteristics are affected markedly by the pool aspect ratio and wind speed. While the aspect ratio increases, the mean flame height reduces, whereas the heat flux values increases. It is also found that the heat flux values increase with the wind speed.

Experimental Study on Flame‒Flame Interaction and Its Merging Features Induced by Double Rectangular Propane Diffusion Burners With Various Aspect Ratios

ABSTRACTAn experimental study is conducted to reveal the flame‒flame interaction (including completely continuous merging phenomenon, intermittent merging phenomenon, and non-merging phenomenon) of and the merging flame length of double propane diffusion flame with various aspect ratios, and its influence is measured in terms of the fire heat release rates, separate spaces between rectangular sources and different pool aspect ratios (1:1, 1:2, 1:4, and 1:8). The flames merging behaviors are recorded by a charge-coupled device (CCD) camera. The merging probability of double propane diffusion flame is found to increase with the increasing of the heat release rate and the pool aspect ratio. A correlation has been developed to estimate the merging probability of the double propane diffusion flame under different aspect ratios and separate spaces. The correlation is proposed to describe the merging flame length of two rectangular diffusion flames with various aspect ratios, which is an inverse proportional function of dimensionless heat release rate.

The burning rate of n-heptane pool fires with constraint effect beneath an arced ceiling

This paper focused on flame shape and burning rate of n-heptane pool fires beneath an arced ceiling. To investigate the influence of constraint effect, a sequence of scale-model experiments were conducted with varying transverse fire locations. Also, the different pool aspect ratios and layouts were also considered. The results showed that the flame front produced vortices beneath the arced ceiling owing to the restriction of the curved ceiling structure for the transverse flame propagation. As the transverse clearance was reduced, the air entrainment deviated the flame away from the vertical direction. Meanwhile, it was demonstrated that the fuel burning rate presented a non-monotone variation, which means the fire attached to the sidewall did not obtain the peak value, but rather that near the sidewall. In addition, the sidewall constraint accelerated the boiling occurrence of pool fires because a smaller clearance owns a stronger radiation heat feedback and heat transfer to fuel, thus resulting in a earlier boiling time. However, it was also found that the development of the wall fire was mainly restricted by the air entrainment so that its burning rate at steady state occurred a significant reduction.

An experimental study of temperature and heat flux in a channel with an asymmetric thermal plume

A set of small-scale experiments were conducted to investigate the temperature and heat flux characteristics in a channel with an asymmetric thermal plume led by the restriction effect of sidewall. Rectangle heptane pools with different aspect ratios were burned to produce the thermal plume, and distance between the pool and sidewall was varied systematically. The classical McCaffery equation was modified to characterize the vertical flame temperature distribution under the channel ceiling. The ceiling temperature contours were analyzed by which the correlation between ceiling temperature, incident heat flux below, and ceiling flame length were investigated involving the sidewall effects. The dimensionless longitudinal heat flux distribution was correlated to the dimensionless distance to the pool using an exponential equation, while the decay rate of longitudinal heat flux was also investigated taking into account the pool aspect ratio. The heat flux at the fuel level for both symmetrical and asymmetrical flames can be predicted well by introducing a dimensionless heat release rate.

Experimental study of sidewall effect on flame characteristics of heptane pool fires with different aspect ratios and orientations in a channel

A series of small scale tests was conducted to investigate the influence of sidewall on flame characteristics of heptane pool fires in a channel, considering pool shape (aspect ratio: 1, 2, 4 and 8) and pool orientation relative to sidewall. Distance between fire and sidewall was changed systematically. Both transverse flame development (along the direction of channel width) and longitudinal flame development (along the direction of channel length) were recorded by digital video. Results show that for a fixed fire location, the heat release rate increases with the increasing pool aspect ratio (namely a larger pool perimeter), which indicates more air entrainment and more intense combustion. In wall fire cases, when the long pool rim is perpendicular with channel sidewall, the flame can obtain more air entrainment with a weaker boundary restriction from the sidewall, compared to the case with the long pool rim being parallel with sidewall. Comparison of some previously established correlations based on various experimental conditions with our test results is made. Due to the fact without considering sidewall effect and fuel shape on the air entrainment of fire plume, the classic correlations need to be further improved. Therefore, an integral flame length model considering both sidewall effect and fuel shape is developed, which correlates well with all the data from cases with various pool positions, orientations and aspect ratios.

Integrated control of solidification microstructure and melt pool dimensions in electron beam wire feed additive manufacturing of Ti-6Al-4V

The ability to deposit a consistent and predictable solidification microstructure can greatly accelerate additive manufacturing (AM) process qualification. Process mapping is an approach that represents process outcomes in terms of process variables. In this work, a solidification microstructure process map was developed using finite element analysis for deposition of single beads of Ti-6Al-4V via electron beam wire feed AM processes. Process variable combinations yielding constant beta grain size and morphology were identified. Comparison with a previously developed process map for melt pool geometry shows that maintaining a constant melt pool cross sectional area will also yield a constant grain size. Additionally, the grain morphology boundaries are similar to curves of constant melt pool aspect ratio. Experimental results support the numerical predictions and identify a proportional size scaling between beta grain widths and melt pool widths. Results further demonstrate that in situ indirect control of solidification microstructure is possible through direct melt pool dimension control.