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Title: Verification and validation of a rapid heat transfer calculation methodology for transient melt pool solidification conditions in powder bed metal additive manufacturing

Abstract

A fundamental understanding of spatial and temporal thermal distributions is crucial for predicting solidification and solid-state microstructural development in parts made by additive manufacturing. While sophisticated numerical techniques that are based on finite element or finite volume methods are useful for gaining insight into these phenomena at the length scale of the melt pool (100 - 500 µm), they are ill-suited for predicting engineering trends over full part cross-sections (> 10 x 10 cm) or many layers over long process times (> many days) due to the necessity of fully resolving the heat source characteristics. On the other hand, it is extremely difficult to resolve the highly dynamic nature of the process using purely in-situ characterization techniques. This article proposes a pragmatic alternative based on a semi-analytical approach to predicting the transient heat conduction during powder bed metal additive manufacturing process. The model calculations were theoretically verified for selective laser melting of AlSi10Mg and electron beam melting of IN718 powders for simple cross-sectional geometries and the transient results are compared to steady state predictions from the Rosenthal equation. It is shown that the transient effects of the scan strategy create significant variations in the melt pool geometry and solid-liquid interfacemore » velocity, especially as the thermal diffusivity of the material decreases and the pre-heat of the process increases. With positive verification of the strategy, the model was then experimentally validated to simulate two point-melt scan strategies during electron beam melting of IN718, one intended to produce a columnar and one an equiaxed grain structure. Lastly, through comparison of the solidification conditions (i.e. transient and spatial variations of thermal gradient and liquid-solid interface velocity) predicted by the model to phenomenological CET theory, the model accurately predicted the experimental grain structures.« less

Authors:
 [1];  [2];  [3]
  1. Univ. of Tennessee, Knoxville, TN (United States). Dept. of Mechanical,Aerospace and Biomedical Engineering
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Manufacturing Demonstration Facility; Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Materials Science & Technology Division
  3. Univ. of Tennessee, Knoxville, TN (United States). Dept. of Mechanical,Aerospace and Biomedical Engineering; Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Manufacturing Demonstration Facility; Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Energy and Transportation Science Division
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Advanced Manufacturing Office (EE-5A)
OSTI Identifier:
1408593
Grant/Contract Number:
AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Additive Manufacturing
Additional Journal Information:
Journal Volume: 18; Journal Issue: C; Journal ID: ISSN 2214-8604
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 42 ENGINEERING; 97 MATHEMATICS AND COMPUTING; Additive manufacturing; Heat transfer; Heat conduction; Modeling; Grain structure; Process-structure relationships; SLM; EBM

Citation Formats

Plotkowski, A., Kirka, M. M., and Babu, S. S.. Verification and validation of a rapid heat transfer calculation methodology for transient melt pool solidification conditions in powder bed metal additive manufacturing. United States: N. p., 2017. Web. doi:10.1016/j.addma.2017.10.017.
Plotkowski, A., Kirka, M. M., & Babu, S. S.. Verification and validation of a rapid heat transfer calculation methodology for transient melt pool solidification conditions in powder bed metal additive manufacturing. United States. doi:10.1016/j.addma.2017.10.017.
Plotkowski, A., Kirka, M. M., and Babu, S. S.. 2017. "Verification and validation of a rapid heat transfer calculation methodology for transient melt pool solidification conditions in powder bed metal additive manufacturing". United States. doi:10.1016/j.addma.2017.10.017.
@article{osti_1408593,
title = {Verification and validation of a rapid heat transfer calculation methodology for transient melt pool solidification conditions in powder bed metal additive manufacturing},
author = {Plotkowski, A. and Kirka, M. M. and Babu, S. S.},
abstractNote = {A fundamental understanding of spatial and temporal thermal distributions is crucial for predicting solidification and solid-state microstructural development in parts made by additive manufacturing. While sophisticated numerical techniques that are based on finite element or finite volume methods are useful for gaining insight into these phenomena at the length scale of the melt pool (100 - 500 µm), they are ill-suited for predicting engineering trends over full part cross-sections (> 10 x 10 cm) or many layers over long process times (> many days) due to the necessity of fully resolving the heat source characteristics. On the other hand, it is extremely difficult to resolve the highly dynamic nature of the process using purely in-situ characterization techniques. This article proposes a pragmatic alternative based on a semi-analytical approach to predicting the transient heat conduction during powder bed metal additive manufacturing process. The model calculations were theoretically verified for selective laser melting of AlSi10Mg and electron beam melting of IN718 powders for simple cross-sectional geometries and the transient results are compared to steady state predictions from the Rosenthal equation. It is shown that the transient effects of the scan strategy create significant variations in the melt pool geometry and solid-liquid interface velocity, especially as the thermal diffusivity of the material decreases and the pre-heat of the process increases. With positive verification of the strategy, the model was then experimentally validated to simulate two point-melt scan strategies during electron beam melting of IN718, one intended to produce a columnar and one an equiaxed grain structure. Lastly, through comparison of the solidification conditions (i.e. transient and spatial variations of thermal gradient and liquid-solid interface velocity) predicted by the model to phenomenological CET theory, the model accurately predicted the experimental grain structures.},
doi = {10.1016/j.addma.2017.10.017},
journal = {Additive Manufacturing},
number = C,
volume = 18,
place = {United States},
year = 2017,
month =
}

Journal Article:
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  • Our study demonstrates the significant effect of the recoil pressure and Marangoni convection in laser powder bed fusion (L-PBF) of 316L stainless steel. A three-dimensional high fidelity powder-scale model reveals how the strong dynamical melt flow generates pore defects, material spattering (sparking), and denudation zones. The melt track is divided into three sections: a topological depression, a transition and a tail region, each being the location of specific physical effects. The inclusion of laser ray-tracing energy deposition in the powder-scale model improves over traditional volumetric energy deposition. It enables partial particle melting, which impacts pore defects in the denudation zone.more » Different pore formation mechanisms are observed at the edge of a scan track, at the melt pool bottom (during collapse of the pool depression), and at the end of the melt track (during laser power ramp down). Finally, we discuss remedies to these undesirable pores are discussed. The results are validated against the experiments and the sensitivity to laser absorptivity.« less
  • In research and industrial environments, additive manufacturing (AM) of metals and alloys is becoming a pervasive technology, though significant challenges remain before widespread implementation of AM can be realized. In situ investigations of rapid alloy solidification with high spatial and temporal resolutions can provide unique experimental insight into microstructure evolution and kinetics that are relevant for AM processing. Hypoeutectic thin-film Al–Cu and Al–Si alloys were investigated using dynamic transmission electron microscopy to monitor pulsed-laser-induced rapid solidification across microsecond timescales. Solid–liquid interface velocities measured from time-resolved images revealed accelerating solidification fronts in both alloys. We observed microstructure evolution, solidification product, andmore » presence of a morphological instability at the solid–liquid interface in the Al–4 at.%Cu alloy are related to the measured interface velocities and small differences in composition that affect the thermophysical properties of the alloys. These time-resolved in situ measurements can inform and validate predictive modeling efforts for AM.« less
  • Additive manufacturing (AM) of metals and alloys is becoming a pervasive technology in both research and industrial environments, though significant challenges remain before widespread implementation of AM can be realized. In situ investigations of rapid alloy solidification with high spatial and temporal resolutions can provide unique experimental insight into microstructure evolution and kinetics that are relevant for AM processing. Hypoeutectic thin-film Al–Cu and Al–Si alloys were investigated using dynamic transmission electron microscopy to monitor pulsed-laser-induced rapid solidification across microsecond timescales. Solid–liquid interface velocities measured from time-resolved images revealed accelerating solidification fronts in both alloys. The observed microstructure evolution, solidification product,more » and presence of a morphological instability at the solid–liquid interface in the Al–4 at.%Cu alloy are related to the measured interface velocities and small differences in composition that affect the thermophysical properties of the alloys. As a result, these time-resolved in situ measurements can inform and validate predictive modeling efforts for AM.« less
  • Purpose: The construction of complex collimators with a high number of oblique pinholes is very labor intensive, expensive or is sometimes impossible with the current available techniques (drilling, milling or electric discharge machining). All these techniques are subtractive: one starts from solid plates and the material at the position of the pinholes is removed. The authors used a novel technique for collimator construction, called metal additive manufacturing. This process starts with a solid piece of tungsten on which a first layer of tungsten powder is melted. Each subsequent layer is then melted on the previous layer. This melting is donemore » by selective laser melting at the locations where the CAD design file defines solid material. Methods: A complex collimator with 20 loftholes with 500 {mu}m diameter pinhole opening was designed and produced (16 mm thick and 70 Multiplication-Sign 52 mm{sup 2} transverse size). The density was determined, the production accuracy was measured (GOM ATOS II Triple Scan, Nikon AZ100M microscope, Olympus IMT200 microscope). Point source measurements were done by mounting the collimator on a SPECT detector. Because there is increasing interest in dual-modality SPECT-MR imaging, the collimator was also positioned in a 7T MRI scanner (Bruker Pharmascan). A uniform phantom was acquired using T1, T2, and T2* sequences to check for artifacts or distortion of the phantom images due to the collimator presence. Additionally, three tungsten sample pieces (250, 500, and 750 {mu}m thick) were produced. The density, attenuation (140 keV beam), and uniformity (GE eXplore Locus SP micro-CT) of these samples were measured. Results: The density of the collimator was equal to 17.31 {+-} 0.10 g/cm{sup 3} (89.92% of pure tungsten). The production accuracy ranges from -260 to +650 {mu}m. The aperture positions have a mean deviation of 5 {mu}m, the maximum deviation was 174 {mu}m and the minimum deviation was -122 {mu}m. The mean aperture diameter is 464 {+-} 19 {mu}m. The calculated and measured sensitivity and resolution of point sources at different positions in the field-of-view agree well. The measured and expected attenuation of the three sample pieces are in a good agreement. There was no influence of the 7T magnetic field on the collimator (which is paramagnetic) and minimal distortion was noticed on the MR scan of the uniform phantom. Conclusions: Additive manufacturing is a very promising technique for the production of complex multipinhole collimators and may also be used for producing other complex collimators. The cost of this technique is only related to the amount of powder needed and the time it takes to have the collimator built. The timeframe from design to collimator production is significantly reduced.« less