11 research outputs found
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Sensing Defects during Directed-Energy Additive Manufacturing of Metal Parts using Optical Emissions Spectroscopy
Critical components produced via additive manufacturing must be free of unwanted defects.
While defects may be detectable after deposition using nondestructive testing techniques,
detecting defects during the deposition process offers many benefits: it may enable users to
interrupt deposition to repair the part, or to abort deposition to minimize further loss of time and
material. Here, we present a method for real-time defect detection during directed-energy
additive manufacturing of metals. The method utilized optical emission spectroscopy and a
custom-built data acquisition and control infrastructure. It was implemented on a LENS MR-7
machine, and employed during manufacturing of Ti-6Al-4V components in which defects were
intentionally introduced. Emission spectra were correlated with defect locations, determined via
computed tomography and metallographic cross-sectioning. Preliminary results indicated that
defect formation was correlated with atomic titanium (Ti I) and Vanadium (V I) emissions and
that measurement of the line-to-continuum ratio for line emissions could be used for defect
detection. Based on these findings, sensing strategies for defect detection and, potentially, in-situ-defect repair may be realizable.Mechanical Engineerin
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Process Monitoring of Directed-Energy Deposition of Inconel-718 via Plume Imaging
Laser-metal interactions typically results in vaporization and plume formation. These
phenomena are complex and depend upon the details of the laser-vapor-melt interactions. As such,
plume characteristics are sensitive to changes in process characteristics. Here, a spectroscopy-based imaging technique is presented for the monitoring of directed energy deposition of Inconel
718. Plume geometry is shown to be related to the processing parameters and geometry of single-bead deposits.Mechanical Engineerin
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Beyond Laser-by-Laser Additive Manufacturing - Voxel-Wise Directed Energy Deposition
Conventional additive manufacturing is a layer-by-layer process, reliant on the sequential
deposition of 2-1/2 D layers oriented along a build axis. During directed energy deposition a feedstock is directed into a continuous melt pool formed by a laser or electron beam. The ability to
produce overhangs is limited due to the gravitational, surface tensions, and fluid-flow force acting
on unsupported melt pools. Here, we present a novel, directed-energy-deposition technique where
vertical and overhanging structures are formed by laser power modulation and the motion of a
laser beam in three dimensional space along the build-up direction, rather than strictly in a single
layer. We demonstrate that highly-overhanging Ti-6Al-4V structure, i.e. in which the overhang
angle exceeds 45 degrees with respect to the x-y plane, can be deposited using the developed
technique. High-speed imaging is used to gain insight into the physics of the process. The use of
a pulsed or power-modulated beam is found to be critical to the formation of overhangs.Mechanical Engineerin
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A Survey of Sensing and Control Systems for Machine and Process Monitoring of Directed-energy, Metal-based Additive Manufacturing
Additive manufacturing of metal parts is, even in the simplest of cases, a complex undertaking.
Parts typically involve hundreds or thousands of individual laser or electron-beam deposits, each
of which involve a complex interaction between energy source, feedstock, and substrate. During
deposition, many of the independent process variables that contribute to overall build quality—
such as travel speed, feedstock flow pattern, energy distribution, gas pressure, etc.—are subject to
perturbations from systematic fluctuations (such as changing build geometry or growing global
temperature) and random external disturbances (such as spatter on a cover lens). Such process
variations affect final part quality, including dimensional tolerance, microstructure, and
properties. Researchers have utilized a wide variety of sensor data and analysis for quality
monitoring and real-time control of the component geometry, microstructure, and properties.
Process attributes that have been targeted for measurement and control include melt pool
geometry, temperature, and layer build-height; process parameters that have been utilized for
control include processing-head stand-off, substrate angle, travel speed, material feed-rate, and
beam power. Here, we survey many of these methods for laser-based, directed-energy
deposition, and briefly discuss recently-introduced methods for real-time, closed-loop control of
build-plan.Mechanical Engineerin
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A Proposed Digital Thread for Additive Manufacturing
Additive manufacturing (AM) has been explored by the automotive, aerospace and
medical industries for many years but has yet to achieve wide-spread acceptance. This is
partially due to the lack of standard frameworks for the exchange of data related to design,
modeling, build plan, monitoring, control, and verification. Here, a unified paradigm, built on
Extensible Markup language (XML) -based file formats and influenced by the ASTM F291
standard, is proposed, to record and transmit data at every stage of the AM process. This digital
thread contains all essential parameters, from design to testing of metal-based AM parts while
remaining accessible, traceable and extensible.Mechanical Engineerin
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Illumination and Image Processing for Real-Time Control of Directed Energy Deposition Additive Manufacturing
This paper describes the optical setup and image processing required to estimate melt-pool width
and build height for real-time control of melt-pool geometry in directed energy deposition additive
manufacturing. To overcome optical interference from plasma emissions and laser interactions,
the melt-zone is imaged using laser illumination. A single camera, fixed to the processing laser,
views the laser interaction zone and provides images for estimating melt-zone width and build
height. Using a bandpass filter and a single aspheric lens, the camera system provides sufficient
magnification and depth of field to achieve a 1-mil (25.4 µm) resolution. Maintaining melt-zone
geometry within desired tolerances requires an image acquisition and processing rate on the order
of 100 frames per second. This bandwidth is achieved by a Camera Link camera and field-programmable gate array that implements algorithms for estimating melt-pool width and build
height. The design and experimental verification of the camera, illumination, and processing
systems are discussed.Mechanical Engineerin
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Dimensional Accuracy of Titanium Direct Metal Laser Sintered Parts
To address concerns regarding quality of production parts created using the Additive
Manufacturing (AM) process, a study was conducted to quantify the dimensional accuracy of said
parts. Fourteen AM builds were manufactured in Ti-6Al-4V material across two EOS DMLS
machines (EOSINT M 280 and EOS M 290). In addition to studying the impact of machine-to-machine variability, other factors potentially impacting final dimensional accuracy were studied,
including: powder state (virgin or reused); post-processing steps (heat treatment and part removal
from substrate); location of part on substrate; and nominal part size. The results of the dimensional
analysis showed that the individual machine itself was the dominant factor impacting dimensional
accuracy. Also, a non-linear relationship between dimensional accuracy and nominal part size was
identified, which would require a more complex machine calibration technique to correct.Mechanical Engineerin
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System Identification and Feedback Control for Directed-Energy, Metal-Based Additive Manufacturing
Additive manufacturing of metal parts is a complex process where many variables
determine part quality. In addition to manipulated process variables, such as travel speed,
feedstock flow pattern, and energy distribution, other exogenous inputs also determine part quality.
For example, changing build geometry and a growing global temperature. In addition, there are
random external disturbances such as spatter on a cover lens. Both manipulated process variables
and exogenous inputs affect dimensional tolerance, microstructure, and other properties that
determine the final part quality. Our long term aim is to improve part quality through real-time
regulation of measurable process variables using vision-based feedback control. As a starting
point, we present a process model that relates scanning speed and laser power to build height and
melt pool width. These results demonstrate the necessity for using multi-input multi-output
feedback control techniques and provide information for refining the frame rate and spectral
sensitivity of the imaging system.Mechanical Engineerin
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Studies on Thin and Thick-Walled SS316L Overhanging Specimens Manufactured Using Powder-Fed Laser-Based Directed Energy
Effective Design for Additive Manufacturing (DfAM) requires accurate knowledge of a
machine’s capabilities and limitations. One important factor is the machine’s ability to deposit
unsupported overhanging features. A general method was devised to characterize the ability of a
powder-fed Laser-Based Directed Energy Deposition (LB-DED) system to deposit overhanging
features with various overhang angles, wall thicknesses, and scan strategies, which was then
investigated on a DMG Mori LASERTEC 65 3D Hybrid LB-DED system using 3-axis toolpaths.
To assess the accuracy of the manufactured overhang angle, for each specimen the angle of the
downward facing surface and upward facing surface were measured and compared to the target
overhang angle. It was observed that for the longitudinal scan strategy, overhang accuracy
substantially degraded as wall thickness increased. For the transverse and cross hatch patterns
there is no observable trend between the achievable overhang angle and the wall thickness. In
addition, it was observed that in general increasing the target overhang angle degrades the
accuracy of the manufactured overhang angle. The most accurate overhang angles overall were
achieved for the longitudinal 1 bead thick specimens. These results will help inform future
DfAM and path planning strategies. This research fills a gap in the literature by providing a
general method to assess overhang capabilities for LB-DED systems.Mechanical Engineerin
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Optical, Layerwise Monitoring of Powder Bed Fusion
In powder bed fusion additive manufacturing, pre-placed layers of powder are successively
fused to form three-dimensional components. During part build-up, flaws in the material or part
geometry can occur and lead to an unacceptable part quality. Common flaws include porosity, poor
surface finish, and thermal deformation. Here, a layer-wise imaging technique is presented for process
monitoring. The technique relies on collection and analyses of images taken under oblique
illuminations of fused and pre-placed powder layers. Results of three-dimensional reconstruction of
image data and identification of potential flaws are presented.Mechanical Engineerin