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Title: Modulating laser intensity profile ellipticity for microstructural control during metal additive manufacturing

Abstract

Additively manufactured (AM) metals are often highly textured, containing large columnar grains that initiate epitaxially under steep temperature gradients and rapid solidification conditions. These unique microstructures partially account for the massive property disparity existing between AM and conventionally processed alloys. Although equiaxed grains are desirable for isotropic mechanical behavior, the columnar-to-equiaxed transition remains difficult to predict for conventional solidification processes, and much more so for AM. In this study, the effects of laser intensity profile ellipticity on melt track macrostructures and microstructures were studied in 316L stainless steel. Experimental results were supported by temperature gradients and melt velocities simulated using the ALE3D multi-physics code. As a general trend, columnar grains preferentially formed with increasing laser power and scan speed for all beam profiles. However, when conduction mode laser heating occurs, scan parameters that result in coarse columnar microstructures using Gaussian profiles produce equiaxed or mixed equiaxed-columnar microstructures using elliptical profiles. Furthermore, by modulating spatial laser intensity profiles on the fly, site-specific microstructures and properties can be directly engineered into additively manufactured parts.

Authors:
ORCiD logo [1];  [2];  [2];  [2];  [3];  [2];  [2]
  1. Univ. of the Pacific, Stockton, CA (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  3. Univ. of the Pacific, Stockton, CA (United States)
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Workforce Development for Teachers and Scientists (WDTS) (SC-27)
OSTI Identifier:
1393688
Alternate Identifier(s):
OSTI ID: 1357374
Report Number(s):
LLNL-JRNL-713205
Journal ID: ISSN 1359-6454
Grant/Contract Number:
AC52-07NA27344; 15-ERD-037; 15-ERD-006
Resource Type:
Journal Article: Published Article
Journal Name:
Acta Materialia
Additional Journal Information:
Journal Volume: 128; Journal Issue: C; Journal ID: ISSN 1359-6454
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 42 ENGINEERING; additive manufacturing; laser powder-bed fusion; microstructure control; laser modulation; beam shaping

Citation Formats

Roehling, Tien T., Wu, Sheldon S. Q., Khairallah, Saad A., Roehling, John D., Soezeri, S. Stefan, Crumb, Michael F., and Matthews, Manyalibo J. Modulating laser intensity profile ellipticity for microstructural control during metal additive manufacturing. United States: N. p., 2017. Web. doi:10.1016/j.actamat.2017.02.025.
Roehling, Tien T., Wu, Sheldon S. Q., Khairallah, Saad A., Roehling, John D., Soezeri, S. Stefan, Crumb, Michael F., & Matthews, Manyalibo J. Modulating laser intensity profile ellipticity for microstructural control during metal additive manufacturing. United States. doi:10.1016/j.actamat.2017.02.025.
Roehling, Tien T., Wu, Sheldon S. Q., Khairallah, Saad A., Roehling, John D., Soezeri, S. Stefan, Crumb, Michael F., and Matthews, Manyalibo J. Sun . "Modulating laser intensity profile ellipticity for microstructural control during metal additive manufacturing". United States. doi:10.1016/j.actamat.2017.02.025.
@article{osti_1393688,
title = {Modulating laser intensity profile ellipticity for microstructural control during metal additive manufacturing},
author = {Roehling, Tien T. and Wu, Sheldon S. Q. and Khairallah, Saad A. and Roehling, John D. and Soezeri, S. Stefan and Crumb, Michael F. and Matthews, Manyalibo J.},
abstractNote = {Additively manufactured (AM) metals are often highly textured, containing large columnar grains that initiate epitaxially under steep temperature gradients and rapid solidification conditions. These unique microstructures partially account for the massive property disparity existing between AM and conventionally processed alloys. Although equiaxed grains are desirable for isotropic mechanical behavior, the columnar-to-equiaxed transition remains difficult to predict for conventional solidification processes, and much more so for AM. In this study, the effects of laser intensity profile ellipticity on melt track macrostructures and microstructures were studied in 316L stainless steel. Experimental results were supported by temperature gradients and melt velocities simulated using the ALE3D multi-physics code. As a general trend, columnar grains preferentially formed with increasing laser power and scan speed for all beam profiles. However, when conduction mode laser heating occurs, scan parameters that result in coarse columnar microstructures using Gaussian profiles produce equiaxed or mixed equiaxed-columnar microstructures using elliptical profiles. Furthermore, by modulating spatial laser intensity profiles on the fly, site-specific microstructures and properties can be directly engineered into additively manufactured parts.},
doi = {10.1016/j.actamat.2017.02.025},
journal = {Acta Materialia},
number = C,
volume = 128,
place = {United States},
year = {Sun Feb 12 00:00:00 EST 2017},
month = {Sun Feb 12 00:00:00 EST 2017}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1016/j.actamat.2017.02.025

Citation Metrics:
Cited by: 2works
Citation information provided by
Web of Science

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  • Additively manufactured (AM) metals are often highly textured, containing large columnar grains that initiate epitaxially under steep temperature gradients and rapid solidification conditions. These unique microstructures partially account for the massive property disparity existing between AM and conventionally processed alloys. Although equiaxed grains are desirable for isotropic mechanical behavior, the columnar-to-equiaxed transition remains difficult to predict for conventional solidification processes, and much more so for AM. In this study, the effects of laser intensity profile ellipticity on melt track macrostructures and microstructures were studied in 316L stainless steel. Experimental results were supported by temperature gradients and melt velocities simulated usingmore » the ALE3D multi-physics code. As a general trend, columnar grains preferentially formed with increasing laser power and scan speed for all beam profiles. However, when conduction mode laser heating occurs, scan parameters that result in coarse columnar microstructures using Gaussian profiles produce equiaxed or mixed equiaxed-columnar microstructures using elliptical profiles. Furthermore, by modulating spatial laser intensity profiles on the fly, site-specific microstructures and properties can be directly engineered into additively manufactured parts.« less
  • A three-dimensional, transient, heat transfer, and fluid flow model is developed for the laser assisted multilayer additive manufacturing process with coaxially fed austenitic stainless steel powder. Heat transfer between the laser beam and the powder particles is considered both during their flight between the nozzle and the growth surface and after they deposit on the surface. The geometry of the build layer obtained from independent experiments is compared with that obtained from the model. The spatial variation of melt geometry, cooling rate, and peak temperatures is examined in various layers. The computed cooling rates and solidification parameters are used tomore » estimate the cell spacings and hardness in various layers of the structure. Good agreement is achieved between the computed geometry, cell spacings, and hardness with the corresponding independent experimental results.« less
  • Here, the effective absorptivity of continuous wave 1070 nm laser light has been studied for bare and metal powder-coated discs of 316L stainless steel as well as for aluminum alloy 1100 and tungsten by use of direct calorimetric measurements. After carefully validating the applicability of the method, the effective absorptivity is plotted as a function of incident laser power from 30 up to ≈540 W for scanning speeds of 100, 500 and 1500 mm s –1. The effective absorptivity versus power curves of the bulk materials typically show a slight change in effective absorptivity from 30 W until the onsetmore » of the formation of a recoil pressure-induced surface depression. As observed using high-speed video, this change in surface morphology leads to an increase in absorption of the laser light. At the higher powers beyond the keyhole transition, a saturation value is reached for both bare discs and powder-coated disks. For ≈100 μm thick powder layers, the measured absorptivity was found to be two times that of the bare polished discs for low-laser power. There is a sharp decrease when full melting of the powder tracks is achieved, followed by a keyhole-driven increase at higher powers, similar to the bare disc case. It is shown that, under conditions associated with laser powder-bed fusion additive manufacturing, absorptivity values can vary greatly, and differ from both powder-layer measurements and liquid metal estimates from the literature.« less
  • Here, the effective absorptivity of continuous wave 1070 nm laser light has been studied for bare and metal powder-coated discs of 316L stainless steel as well as for aluminum alloy 1100 and tungsten by use of direct calorimetric measurements. After carefully validating the applicability of the method, the effective absorptivity is plotted as a function of incident laser power from 30 up to ≈540 W for scanning speeds of 100, 500 and 1500 mm s –1. The effective absorptivity versus power curves of the bulk materials typically show a slight change in effective absorptivity from 30 W until the onsetmore » of the formation of a recoil pressure-induced surface depression. As observed using high-speed video, this change in surface morphology leads to an increase in absorption of the laser light. At the higher powers beyond the keyhole transition, a saturation value is reached for both bare discs and powder-coated disks. For ≈100 μm thick powder layers, the measured absorptivity was found to be two times that of the bare polished discs for low-laser power. There is a sharp decrease when full melting of the powder tracks is achieved, followed by a keyhole-driven increase at higher powers, similar to the bare disc case. It is shown that, under conditions associated with laser powder-bed fusion additive manufacturing, absorptivity values can vary greatly, and differ from both powder-layer measurements and liquid metal estimates from the literature.« less