SAMPE Journal - July/August 2022 - 41

In this initial study the ICAT process material
temperature, tool temperature, and placement
speed were varied to determine the resulting
consolidation quality and down-select processing
parameters for further test panel fabrication. The
resulting temperature data from the trials were
also utilized to calibrate thermal analysis models
under development at NASA to understand the
transfer of heat into and out of the ply boundary
welding region during the ICAT process. Once
validated, accurate thermal models are anticipated
to be effectively utilized to reduce trial-and-error
processing experiments to fully optimize the ICAT
process parameters as they relate to autohesion,
crystallization and, ultimately, laminate
performance.
The experimental temperature
data will be used to confirm the analytical results,
indicating that the substrate and incoming tape
surfaces are quickly heated above the polymer
melt temperatures at the incident laser regions
and then quickly cool as the material is compacted
at the nip-point zone under the center of the AFP
machine (head) compaction roller.
In-situ Thermoplastic AFP Background
Thermoplastic composites are being utilized
more in transport and mechanical applications
as they offer several advantages compared to
thermoset composites including recyclability, long
out-life and ambient storage, good mechanical
and chemical performance, low volatile organic
compound emission during the manufacturing
process and shorter processing times. Another
advantage is their ability to be joined by fusion
bonding which is useful to join two parts without
the disadvantages of mechanical fasteners.
During the heated head-automated
thermoplastic-tape placement (HH-ATP) process
previously developed at NASA Langley Research
Center
(LaRC), an incoming
carbon
fiber/
thermoplastic composite tape was fusion bonded,
or welded, to a previously placed and consolidated
layer under heat and pressure locally applied to the
interface (Figure 1). By placing additional layers in
different directions, a part with desired thickness
and properties can be fabricated3
. The placement
process of thermoplastic composite tapes is
understood to involve the following general steps:
1. The incoming and substrate thermoplastic
matrix tape plies are heated above the material
melt temperature, TM
, (for semi-crystalline
polymers) or the glass-transition temperature,
Tg
, (for amorphous thermoplastic polymers).
2. A compaction load is applied to the viscous
material to establish intimate contact at the
ply interface between incoming and substrate
tape plies. The amount and time of application
of the compaction step is also dependent on
the surface roughness of the supplied tape
material4-7
.
3. With the viscous plies in contact, the polymer
molecules reptate across the boundary in the
autohesion process; the plies must be in contact
but not necessarily under compaction load for
adequate autohesion, or fusion bonding6
.
4. The fusion-bonded plies cool below the
polymer Tg
until the next AFP pass.
Consequently, development
of
, freezing the laminate properties
residual
stresses is unavoidable due to disparate thermal
characteristics of matrix and fiber materials and
due to non-uniform heating and cooling. From the
product quality standpoint such as interlaminar
strength, and dimensional accuracy, these stresses
should be kept within allowable limits. As another
quality requirement, a laminate should be void free
and well consolidated for reliable use in a structure
as a load bearing part. Achievement of adequate
fusion bond, or autohesion between individual plies
is critical5
welding
for structural performance. Incomplete
results
in
high
void
content, which
seriously degrades the mechanical performance
of the composite. In addition, prolonged times at
excessively high process temperatures may lead
to thermal degradation and decomposition of
the thermoplastic matrix. Selection of optimized
processing parameters is required to deliver quality
composites by HH-ATP.
Figure 1. Simplified schematic of the HH-ATP process.
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SAMPE Journal - July/August 2022

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