Tech Briefs Magazine - September 2021 - PIT-15

eddy
current
testing,
described in reference [7].
Powder Bed Fusion (PBF)
To pick up the ultrasound emission
during a PBF process, the optical microphone
was positioned inside the build
chamber, approximately 30 cm from the
build plate, as shown in Figure 1b. A typical
acoustic process signature is depicted
in Figure 3, which also demonstrates
the advantage of high frequency ultrasound
analysis, namely the absence of
reverberation, which in the lower frequency
regime can 'smear' the signal
and, subsequently, the superior temporal
resolution of process events.
In contrast to the LMD setup, PBF
employs mirror galvanometers to direct
the process laser across the powder bed.
This procedure causes distance- and frequency-dependent
attenuation of the
airborne signal, which follows a known
relationship and thus can be taken into
account if the laser coordinates are
simultaneously tracked. In many cases it
is also not necessary to maintain a constant
signal ground level, e.g. for detection
of short-term spectral anomalies.
Two examples of these are presented in
Figure 4: Figure 4a shows a broadband
peak in the spectrogram originating
from a laser pass over a molten particle
on the powder and Figure 4b shows a
crack signal in the 3D-spectrogram view
(upper) and the respective 2D-energy
curve (lower) integrated over a frequency
band of 200-600 kHz.
Wire Arc Additive Manufacturing
(WAAM)
A similar setup as in LMD can be ap -
plied for WAAM pro cesses (Figure 5). The
signature of the a coustic signal differs
strongly, even though it is also broadband
up to 600 kHz. Main phenomena are the
droplet formation cycle (distinct blocks
appearing roughly every 10 ms), a tonal
signal related to the arc modulation rate
(here: 80 kHz) and broadband process
emission, related to metal evaporation.
Two of the most critical parameters
for the process are the stability of the
arc and the formation cycle. To monitor
the former, a narrow bandpass filter
may be applied to the modulation frequency
and its higher harmonics, and
for the latter, an FFT analysis may be
performed on the high-pass filtered
time signal, in order to calculate the
comparatively slow droplet rate free
from background noise.
Photonics & Imaging Technology, September 2021
Cov
ToC
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applications than any other precision
automation company.
Visit aerotech.com/laser.
©2021 Aerotech, Inc.
as
further
Conclusion
Through the availability of a membrane-free,
broadband microphone for
highest ultrasound frequencies, previously
inaccessible information can now
be harnessed and utilized to monitor
acoustic process phenomena in realtime.
Future developments involve the
application of triangulation so that
sound origins can be localized to further
facilitate operators, process engineers
and machine manufacturers to
spot potential defects. Moreover, the
inclusion of onboard AI methods that
lend themselves to acoustic data streams
and facilitate the advancement of the
technology towards more complex,
industrial monitoring applications, is
targeted.
This article was written by Martin
Ursprung, Application Engineer; Thomas
Heine, Head of R&D; Balthasar Fischer, CEO;
Wolfgang Rohringer, Development Engineer;
and Ryan Sommerhuber, Application Engineer,
The most trusted name in
precision laser processing.
For more than 40 years, Aerotech has set the standard
for long-term durability and accuracy in precision laser
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Tech Briefs Magazine - September 2021

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