Tech Briefs Magazine - September 2021 - PIT-13

application. For a more detailed analysis
tailored for your application, contact
your optical component supplier.
They should have historical data that
will help better determine the effects
of coating stress for your specific
optics.
Lead
This article was written by Cory Boone,
Technical
Marketing
Engineer,
Edmund Optics (Barrington, NJ). For more
information, contact Mr. Boone at
cboone@edmundoptics.com, or visit http://
info.hotims.com/79417-222.
References
1. Oliver, J. B., Spaulding, J., & Charles, B.
(2019). Stress compensation by deposition
of a nonuniform corrective coating.
Applied Optics, 59(5), 54-57. https://
doi.org/10.1364/AO.59.000A54
2. (2020). Coating Impact on Flatness
Calculator. Edmund Optics. https://www.
edmundoptics.com/tools/coatingimpact
Monitoring Additive Manufacturing with an Optical Microphone
A
dditive manufacturing (AM) technologies
have seen remarkable adoption
in the industry over recent years. As
digitalization and on-demand manufacturing
shift the face of production sites
to fully automated operating lines, reliable
process monitoring methods are
becoming increasingly important.
In laser-based processing of metals it is
a well-known fact that optical and
acoustic emission during the process can
be analyzed in real-time and correlated
to the final part quality.[1][2][3] This
approach delivers significant value, as
the occurrence of pores, cracks or other
inhomogeneities created during the
production process greatly influence the
mechanical properties and increase the
risk of defective components. Im -
mediate reaction to warning process signals
can thus reduce scrap material and
save time.
Nowadays, optical systems such as
pyrometers, high-speed cameras, IRcameras
and photodiodes are widely
used to monitor laser-based AM processes
in academic as well as industrial contexts.[4]
Even though they yield important
insight into the laser-metal interaction
zone, they are blind to relevant
aspects happening after the laser has
a)
switched off, e.g. cracking due to thermal
stress.
Structure-borne acoustic sensor systems
have recently become commercially
available, for instance within Powder Bed
Fusion (PBF) processes.[5] However,
structure-borne sound detectors are challenged
when it comes to processes like
laser metal deposition (LMD) or wire arc
additive manufacturing (WAAM) in
a)
b)
which the structure-bound signal path
changes or can even get interrupted
throughout the build time. In such cases,
a fixed-distance air-coupled acoustic sensor
provides a capable solution.
A New Ultrasound Inspection
Technique
In contrast to conventional membrane
or piezo-based acoustic sensors,
~ 30 cm
Figure 1. The fiber-coupled ultrasound sensor (a) within the PBF build chamber (b) at 30 cm measurement
distance from the center of the build plate.
109
b)
In-process cracks
108
Postprocess
cracks
107
106
40kHz
-1MHz
350kHz -1MHz
Cracks
105
200
400
600
800
Time (s)
Figure 2. Typical measuring distance in an LMD process (2a). Ultrasound signals in the range of 350 kHz-1 MHz associated to cracks during and after a 20
min building process (2b).
Photonics & Imaging Technology, September 2021
Cov
ToC
13
1000
1200
1400
20 cm
Acoustic energy

http://info.hotims.com/79417-222 https://www.doi.org/10.1364/AO.59.000A54 http://www.doi.org/10.1364/AO.59.000A54 https://www.edmundoptics.com/tools/coatingimpact http://www.abpi.net/ntbpdfclicks/l.php?202109TBNAV

Tech Briefs Magazine - September 2021

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