Melt Pool Geometry Research Articles

Ripple formations determine the heterogeneous microstructure of directed energy deposition (DED)-printed 316L components

This paper presents an experimental study to characterize the ripples and other morphological formations as well as their influence on the heterogeneous microstructure on the surfaces of the directed energy deposition (DED)-printed stainless steel (316L) components. While ripple formations in the welding literature have been studied extensively, they have not received much attention in the DED process. They are often wrongly conflated with the melt pool geometry. The experiments consisted of printing 10 mm cubic components on a hybrid machine tool (MTS 500 from Optomec) under nine different combinations of laser power, scan speed and dwell time. The ripple formations and the microstructure on the scan surface were subsequently observed and statistically characterized at four different magnifications on an optical microscope. The study establishes that the ripple traces are a main determinant of the dendritic structures, as well as the heterogeneous spatial distribution of the microstructure formations in DED-printed 316 components. The statistical characterization also highlights the limitations of the ripple waveform descriptors employed commonly in the welding literature and introduces spectral quantifiers to adequately capture the complex waveform patterns of ripples in DED.

A Detailed Analysis of the Microstructural Changes in the Vicinity of a Crack-Initiating Defect in Additively Manufactured AISI 316L

The fatigue life of metals manufactured via laser-based powder bed fusion (L-PBF) highly depends on process-induced defects. In this context, not only the size and geometry of the defect, but also the properties and the microstructure of the surrounding material volume must be considered. In the presented work, the microstructural changes in the vicinity of a crack-initiating defect in a fatigue specimen produced via L-PBF and made of AISI 316L were analyzed in detail. Xenon plasma focused ion beam (Xe-FIB) technique, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were used to investigate the phase distribution, local misorientations, and grain structure, including the crystallographic orientations. These analyses revealed a fine grain structure in the vicinity of the defect, which is arranged in accordance with the melt pool geometry. Besides pronounced cyclic plastic deformation, a deformation-induced transformation of the initial austenitic phase into α’-martensite was observed. The plastic deformation as well as the phase transformation were more pronounced near the border between the defect and the surrounding material volume. However, the extent of the plastic deformation and the deformation-induced phase transformation varies locally in this border region. Although a beneficial effect of certain grain orientations on the phase transformation and plastic deformability was observed, the microstructural changes found cannot solely be explained by the respective crystallographic orientation. These changes are assumed to further depend on the inhomogeneous distribution of the multiaxial stresses beneath the defect as well as the grain morphology.

Parametric study of melt pool geometry in hybrid plasma arc-laser melting process for additive manufacturing application

Wire-based hybrid arc-laser additive manufacturing is suitable for producing large metallic parts (metres in scale) with high deposition rates and near-net-shape. In this process, the surface quality and dimensional accuracy of the deposited parts are determined by the melt pool geometry. However, how to control the melt pool in the hybrid process is complex due to the multiple parameters that can be used. In this study, control of melt pool geometry by investigating different process parameters, including laser power, travel direction, arc-laser separation distance, laser beam size, and arc current in the hybrid plasma transferred arc (PTA)-laser process, was studied systematically. It was found that a larger melt pool was achieved with the PTA-leading configuration compared to that with the laser-leading configuration due to a higher laser absorption occurred with the former configuration. The melt pool was enlarged by either increasing the laser power or arc current due to the increased energy input. However, if the laser power density is high enough to determine the melt pool depth, the increasing arc current has very little effect on melt pool depth but only increases the melt pool width. In addition, the melt pool became shallower and wider when using a larger laser beam. The arc-laser separation distance had a minor effect on the melt pool geometry due to the fixed energy input used in the studied cases. The results of this study provide a reference for melt pool control in wire-based hybrid arc-laser additive manufacturing.

Increasing the build rate of high-strength aluminium alloys produced by laser powder bed fusion

Aluminium alloys from the 2xxx series represent important structural materials due to their optimal combination of strength and corrosion resistance. However, their production by means of laser powder bed fusion (PBF-LB/M) is hampered by the presence of detrimental pores and hot cracks. These defects can be mitigated with appropriate process parameters investigations, often at the expense of the build rate.In this study, a combination of modelling and experimental work is adopted to predict the consolidation behaviour of AA2024, a well-known high-strength Al alloy, in regimes characterised by a progressive increase of build rate. The melt pool geometries are predicted as a function of various process parameters using the Rosenthal solution and variable values of absorptivity. This leads to a facile production of near-full dense crack-free parts at an increasing build rate. The analysis of the specimens produced with such processing parameters reveals that the microstructural features and mechanical properties of AA2024 are largely preserved when build rate is increased.Our approach can be easily implemented in standard PBF-LB/M systems as it does not rely on machine adaptations. Therefore, our new proposed approach might result in a practical solution to increase the productivity of other high-strength Al-alloys in industrial settings.

Enhancing SLM Stainless Steel: Model-Based Parameter Optimization

SLM (selective laser melting) has faced challenges in ensuring consistent mechanical properties for various geometries. However, the authors have developed a system that solves this issue by calculating the optimal process parameters for each scan vector on each layer. This approach ensures consistent melt pool geometry, derived from the most robust parameters in conduction welding mode. The results from the single melt-line experiments and the presented thermal conduction solver demonstrate the effectiveness of this approach in achieving a stable melt pool geometry in stainless steel (1.4404 or 316L). Furthermore, a testing regimen has been developed to evaluate the system under different changing boundary conditions, to further demonstrate its usefulness. By utilizing this system, manufacturers will achieve consistent and high-quality mechanical properties which improves cost-effectiveness for small lot sizes, including spare and obsolescent parts.

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.

The Effects of Melt Pool Geometry and Scan Strategy on Microstructure Development During Additive Manufacturing

During the powder bed fusion additive manufacturing (AM) process, hundreds of grains may solidify simultaneously within the melt pool, depending upon the processing conditions and the grain size of the base plate and powder. Thus, a small variation in the shape of the melt pool or scan strategy may result in very different as-printed microstructures, since new grains may become more favourably aligned with the overall direction of growth. The microstructure affects the performance of the additively built component, and it is therefore critical to understand the interplay between the initial microstructure, material properties, heat source, and scan strategy on determining the as-printed microstructure. We examine columnar grain growth during AM using a phase-field model to perform simulations with single melt pools predicted by the cylindrically symmetric Rosenthal solution. Quantitative measures of the competitive grain growth are extracted from the simulation data. To aid in their understanding, we map these quantities onto visualizations of the microstructure and compare them to the inverse pole figure map, from which we find that the crystallographic directions of the grains that grow closest to the centre of the laser track may not be preferentially aligned with the scan direction.

Mathematical prediction of melt pool geometry and temperature profile for direct energy deposition of 15Cr5Ni SS alloy

Mathematical prediction of melt pool geometry and temperature profile for direct energy deposition of 15Cr5Ni SS alloy

Multimodal probabilistic modeling of melt pool geometry variations in additive manufacturing

Laser Powder Bed Fusion (LPBF) is commonly used to fabricate metal additive manufacturing (AM) parts. Advanced sensing empowers the collection of in-situ signals (e.g., melt pools) for real-time monitoring of AM processes. The variations of melt pool geometry are closely pertinent to mechanical and functional properties of final AM builds. Most of previous works are concerned about the extraction of melt pool features (e.g., geometric statistics, size, shape descriptors) for AM process monitoring. However, very little has been done to investigate the emission physics of photons that leads to the generation of melt pool images. To fill the gap, this work presents a novel approach to simulate the emission of photons for statistical estimation and modeling of the multimodal probability distribution function (PDF) of a melt pool. Specifically, the complex geometry of a melt pool is represented as a multimodal PDF. Photons emitted to generate melt pool images are treated as independent and identically distributed (iid) random samples drawn from the multimodal PDF. Further, we propose a new Gaussian mixture model to represent and estimate the multimodal PDF via the expectation–maximization procedures. In addition, we investigate how process conditions influence the geometric variations of melt pools. Experimental results show that the proposed methodology effectively builds the multimodal distribution model of melt pool geometric variations. Most importantly, the proposed methodology is flexible enough to take raw images as the input and shows great potentials to be generally applicable for different types of melt pool images and AM processes.

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.

Numerical Simulation of Temperature Field and Melt Pool Characteristics of CP-Ti Manufactured by Laser Powder Bed Fusion

A coupled heat source model that combined a Gauss surface heat source with a Gauss cylindrical volumetric heat source was introduced to simulate temperature field distribution and melt pool characteristics using a finite element simulation (FEM) method for the deep and narrow melt pools formed in laser powder bed fusion (L-PBF) aiming at commercial pure titanium (CP-Ti). For comparison, the same simulations using the Gauss surface heat source model and the double ellipsoid heat source model were also performed. The simulated melt pool geometries using the coupled heat source model match well with the measurements, with an average error of 1% for the melt pool depth and 7% for the width. Based on the single-track experimental results, it was found by comparing the simulated results from the three heat source models that the coupled heat source model had better accuracy than the other two. Then, the temperature field and the melt pool geometries of CP-Ti fabricated at different laser power levels from 300 W to 500 W and scanning speeds from 600 mm/s to 4000 mm/s were simulated. According to the simulated maximum temperature and geometries of the melt pool, a suitable process parameters map for CP-Ti was obtained. The reported experimental results agree well with the simulated map. The coupled heat source model is more accurate and applicable for the deep and narrow melt pools formed during L-PBF.

Optimization of processing parameters for pulsed laser welding of dissimilar metal interconnects

An experimental and numerical investigation is performed into the optimal processing parameters for the fabrication of aluminum and copper dissimilar lap joints using a pulsed laser welding (PLW) method with a wobble strategy. A circle packing design algorithm is first employed to select 43 representative combinations of the peak laser power and tangential welding speed. The selected parameters are then supplied to a computational fluidic dynamics (CFD) model of the PLW process to predict the melt pool geometry (i.e., interface width and penetration depth) and copper concentration. The simulation results are used to train three surrogate models to predict the melt pool geometry and copper concentration for any combination of the PLW parameters within the design space. Finally, the processing maps constructed using the surrogate models are filtered in accordance with three quality criteria to determine the PLW parameters that produce dissimilar joints with no cracks or pores in the fusion zone and enhanced mechanical and electrical properties. The validity of the proposed optimization approach is demonstrated by evaluating the shear strength, intermetallic compound (IMC) formation, and electrical contact resistance of experimental samples produced using the optimal welding parameters. The results confirm that the optimal parameters yield a high shear strength of 1209 N and a low electrical contact resistance of 86 µΩ. Moreover, the fusion zone is free of defects, such as cracks and pores.

A physics-based modeling framework to assess the cost scaling of additive manufacturing, with application to laser powder bed fusion

Purpose This study presents a framework to estimate throughput and cost of additive manufacturing (AM) as related to process parameters, material thermodynamic properties and machine specifications. Taking a 3D model of the part design as input, the model uses a parametrization of the rate-limiting physics of the AM build process – herein focusing on laser powder bed fusion (LPBF) and scaling of LPBF melt pool geometry – to estimate part- and material-specific build time. From this estimate, per-part cost is calculated using a quantity-dependent activity-based production model. Design/methodology/approach Analysis tools that assess how design variables and process parameters influence production cost increase our understanding of the economics of AM, thereby supporting its practical adoption. To this aim, our framework produces a representative scaling among process parameters, build rate and production cost. Findings For exemplary alloys and LPBF system specifications, predictions reveal the underlying tradeoff between production cost and machine capability, and look beyond the capability of currently commercially available equipment. As a proxy for build quality, the number of times each point in the build is re-melted is derived analytically as a function of process parameters, showcasing the tradeoff between print quality due to increased melting cycles, and throughput. Originality/value Typical cost models for AM only assess single operating points and are not coupled to models of the representative rate-limiting process physics. The present analysis of LPBF elucidates this important coupling, revealing tradeoffs between equipment capability and production cost, and looking beyond the limits of current commercially available equipment.

Enabling Part-Scale Scanwise process simulation for predicting melt pool variation in LPBF by combining GPU-based Matrix-free FEM and adaptive Remeshing

• A highly efficient GPU-based matrix-free finite element modeling with adaptive remeshing framework is developed to overcome the high computational expense of l -PBF thermal process modeling, making it feasible to do detailed scanwise thermal simulation for part-scale within days. • The effects of the scanning strategy, geometrical features, and proximity to the turning points on the melt pool size variability and lack of fusion porosity are investigated for two different parts. • The predicted variation in melt pool size and lack of fusion porosity has good correlation with experimental measurements. This work proposes to combine matrix-free finite element modeling (FEM), adaptive remeshing, and graphical processing unit (GPU) computing to enable, for the first time, scanwise process simulation of the Laser Powder Bed Fusion (L-PBF) process with temperature-dependent thermophysical properties at the part scale. Compared to the conventional FEM using the global stiffness approach and a uniform mesh running on 10 CPU cores, l -PBF process simulation based on the proposed methodology running on a GPU card with 5,120 Compute Unified Device Architecture (CUDA) cores enables a speedup of over 10,000x. This significant speedup facilitates detailed thermal history and melt pool geometry predictions at high resolution for centimeter-scale parts within days of computation time. Two parts consisting of various geometric features are simulated to reveal the effects of scan strategy and local geometry on melt pool size variation, which correlate well with melt pool and lack-of-fusion porosity measurements obtained via experiment.

Effects of melt-pool geometry on the oriented to misoriented transition in directed energy deposition of a single-crystal superalloy

The successful repair of single-crystal (SX) gas turbine blades using the directed energy deposition (DED) technique requires unidirectional dendrite growth without an oriented to misoriented transition (OMT). In this work, the effects of melt-pool geometrical parameters on OMT behavior during single-track deposition of SX superalloys were thoroughly studied. The variations in melt-pool length, width, height and depth on the solidified structure characteristics (e.g., the number of anisotropic dendrites and the area fraction of [001]-oriented dendrites) were quantitatively examined through experimental observations. A simple mathematical model was established to elucidate the correlations between these parameters and OMT behavior, and the errors in this model were analyzed in detail. The DED parameters were linked with the melt-pool geometrical parameters to find out the suitable processing conditions for SX repair. It is concluded that a melt pool with a relatively low depth-to-height ratio and a high length-to-height ratio is generally beneficial for unidirectional dendrite growth, decreasing the heat and mass input is conducive to obtain such melt-pool shape. The presented findings afford an in-depth understanding of the effects of melt-pool geometry on competitive dendrite growth and are instrumental for the repair of SX gas turbine blades.

Residual Heat Effect on the Melt Pool Geometry during the Laser Powder Bed Fusion Process

The continuous back-and-forth melting of the powder bed during the laser powder bed fusion (LPBF) process leads to the development of residual heat, which affects the melt pool geometry as the laser scan progresses. The magnitude of the residual heat depends on the scan length, hatch spacing, location on the track, etc. In this regard, back-and-forth raster scanning was performed to investigate the effect of the scan length and hatch spacing on the melt pool size at different locations along the laser travel direction. Multi-track specimens with different scan lengths (0.5 mm, 1 mm, and 1.5 mm) were fabricated using 195 W laser power, three scan speeds (375 mm/s, 750 mm/s, and 1500 mm/s), and two hatch spacings (80 µm and 120 µm). A white light interferometer was used to analyze the surface morphologies of the fabricated samples, and metallography was performed to observe the melt pool boundary. The melt pool boundary obtained at different locations revealed that the effect of the residual heat was maximal in the laser-turn region. In addition, a powder scale numerical model was developed to investigate the effect of temperature distribution on the melt pool geometry. The numerical results show that the laser-turn region was most affected by the residual heat, as the melt pool from the two tracks merged. The depth of the melt pool increased with increasing track numbers, while the track height decreased. The addition of a second layer of powder showed that the inherent surface variation in the first layer leads to the difference in the actual layer thickness of the second layer.

Calibrating uncertain parameters in melt pool simulations of additive manufacturing

Melt pool scale numerical modeling of additive manufacturing (AM) processes can provide predictive capabilities and theoretical insight into the process-property-structure-performance relationships for AM parts. Despite capabilities of numerical models to solve complex multi-physics problems, it is often important to consider a tradeoff between detailed physics and computational cost. Therefore, sources of uncertainty in both experimental conditions and the parameters needed for modeling require models to be validated against empirical evidence. Here, a method is proposed to calibrate uncertain parameters used in continuum-scale melt pool models for powder bed fusion (PBF) AM. Both a simplified heat transfer model and a heat transfer and fluid flow model were investigated. A surrogate model and Markov chain-based optimization algorithm calibrated melt pool geometry for models within experimental variation of the target melt pool width and depth from the NIST AM-Bench 2018-02 dataset. The melt pool temperature distributions, solidification parameters, and simulated multi-layer solidification microstructures were compared between the two models. Similar results from both models indicate that calibrated, lower fidelity numerical models may be used in place of higher fidelity models to generate melt pool solidification data. These calibrated models therefore enable lower computational cost melt pool simulations without a noticeable decrease in simulation accuracy for grain-scale microstructure simulations.

Melt pool geometry dependent microstructural evolution induced by inoculants in IN718 processed by selective laser melting

This preliminary study aims to clarify how Co, TaCr2 and TiB2 grain refinement inducing inoculants influence the melt pool physics and resulting grain structure in superalloy IN718 processed by laser powder bed fusion (L-PBF). It was demonstrated that the presence of solid inoculant particles could affect the melt pool convection leading to a noticeable reduction in the melt pool width and penetration depth as well as the balling phenomenon. Interestingly, it was found that the penetration depth of the melt pool in samples containing TiB2 appeared to be unchanged due to the presence of nano-particles. Characterization of the resulting grain structures of the various samples revealed that the difference in melt pool geometries contributed to a predominant columnar-grained structure with< 101 > texture in samples containing Co or TaCr2 inoculants while an equiaxed-grained structure with< 001 > texture in samples containing TiB2 inoculants. The effectiveness of inoculants was found to be closely related to their impact on the melt pool physics, in addition to the degree of lattice disregistry with the IN718 matrix.

Impact of the Power-Dependent Beam Diameter during Electron Beam Additive Manufacturing: A Case Study with γ-TiAl

The development of process parameters for electron beam powder bed fusion (PBF-EB) is usually made with simple geometries and uniform scan lengths. The transfer to complex parts with various scan lengths can be achieved by adapting beam parameters such as beam power and scan speed. Under ideal conditions, this adaption results in a constant energy input into the powder bed despite of the local scan length. However, numerous PBF-EB machines show deviations from the ideal situation because the beam diameter is subject to significant changes if the beam power is changed. This study aims to demonstrate typical scaling issues when applying process parameters to scan lengths up to 45 mm using a fourth generation γ-TiAl alloy. Line energy, area energy, return time, and lateral velocity are kept constant during the additive manufacturing process by adjusting beam power and beam velocity to various scan lengths. Samples produced in this way are examined by light microscopy regarding lateral melt pool extension, melt pool depth, porosity, and microstructure. The process-induced aluminum evaporation is measured by electron probe microanalysis. The experiments reveal undesired changes in melt pool geometry, gas porosity, and aluminum evaporation by increasing the beam power. In detail, beam widening is identified as the reason for the change in melt pool dimensions and microstructure. This finding is supported by numerical calculations from a semi-analytic heat conduction model. This study demonstrates that in-depth knowledge of the electron beam diameter is required to thoroughly control the PBF-EB process, especially when scaling process parameters from simply shaped geometries to complex parts with various scan lengths.

Towards 3-D texture control in a β titanium alloy via laser powder bed fusion and its implications on mechanical properties

Fusion-based additive manufacturing (AM) offers a new opportunity to design metallic materials with complex, direction-dependent mechanical properties by controlling the orientation distribution of the constituent grains—also known as texture. Texture control may be achieved site-specifically by varying the processing variables and tailoring the local melt pool geometry and solidification kinetics. However, the type and direction of textures currently achievable in AM alloys are limited by the melt pool geometry itself. In this work, we advance the capabilities of controlling crystallographic texture in laser powder bed fusion (LPBF) by producing specimens of β titanium-niobium alloy with textures aligned along three different directions: the laser scan direction (SD), the build direction (BD), and—for the first time—the direction perpendicular to both BD and SD (i.e., PD). We achieve this three-dimensional (3-D) texture control by fine tuning the keyhole melt pool geometry and amount of overlap throughout the build. We test the tensile properties of the three different specimens along their respective texture axes and elucidate the relationships between crystallographic orientation, mesostructure, and mechanical behavior. We find that the novel PD texture exhibits the best combination of strength, strain hardenability, and ductility. We ascribe these results to the unique mesostructure of this specimen. This work opens new opportunities for designing novel materials with directional properties by achieving three-dimensional (3-D) texture control during fusion-based AM.