JEC COMPOSITES MAGAZINE - EXPERT’S DIGEST - ADDITIVE MANUFACTURING - 2020 - 82

EXPERT'S DIGEST
ATL/AFP
After forming, a second trimming step was
performed to obtain the final shape of the spar.

Fig. 4: Contour of
the blank

used in this work has a minimum fiber length
of 100 mm. This makes near net shape blank
lay-up challenging in tight corners. Hence,
some pre-forming trimming scrap could not
be avoided for certain ply orientations.
Figure 4 shows the pre-forming trimming
scrap, post-forming trimming scrap and final
spar contour. Four gripper tabs were included
for gripping during forming.
Tooling design
Matched metal aluminium tooling was
designed to form the part. The tooling cavity
was based on an average ply thickness of 0.135
mm, which was determined based on the
final thickness of [0/90]4s AFP blanks after
stamping with flat tooling.
Engravings in the tooling allowed for checking
the alignment of the blank after stamping.

trimming step, the blanks were dried in a
convection oven for 3 hours at 250 °C. This
prevents void formation and delamination by
moisture expansion during stamp forming.
The blanks were stamp formed on a 200
tonnes forming press. First, the blanks were
aligned in the transfer frame using laser
guiding and suspended by springs. Then, the
blanks were heated in an IR- oven to a core
temperature of 390 °C in 180 s. Subsequently,
the blank was automatically transferred to the
forming station without considerable temperature loss and the press was closed, forming
the spar. The spar was held under a pressure of
100 bar (414 kN) for an additional 60 s while
cooling in the isothermal tooling at 250 °C.

Consolidation quality
Initial forming trials showed promising
results. A series of five spars was produced.
The consolidation quality of the spars was
checked by visual inspection, C-scan (web),
A-scan (flanges) and microscopy. The blanks
showed consistent C-scans with no defects,
as shown in Figure 5 (top). Cross-sections
of the spar showed good consolidation of all
zones, including ply-drop regions. An example is shown in Figure 5 (bottom). Rough
surface areas were observed in the center of
the web, as well as on the outer radii near the
joggles. However, cross-sections confirmed
no consolidation issues in these regions. A low
quantity (<< 1%) of intralaminar micro-voids
was still observed, as the intralaminar voids
that were present in the used prepreg were not
completely eliminated. However, previous
work has shown that these voids do not affect
flexural performance [2].
Defects
Although the five spars showed good consolidation quality, some defects were present.
Firstly, warpage was observed (Figure 6

Forming simulations
Forming simulations were performed using
AniForm. The simulations enable the prediction the forming behavior and the occurrence
of potential defects. The forming simulations
did not show critical issues during forming.
The forming simulations also aided the design
of the tooling and gripper system, as well as
designing a near-net shape blank contour.
Spar production
The blanks were laid up on flat aluminium
tooling with a single 1/4 " tow Coriolis
Composites AFP machine at a rate of 200
mm/s. Laser heating was used to heat the tape
up to a nip-point temperature of 450 °C. The
lay-up rate was limited by both the output
power of the laser (1 kW) and the accuracy of
starting and stopping tapes, which is especially
important for ply-drops. After a pre-forming
Expert's digest jec

Fig. 5: Top: C-scan of the web of the spar. Bottom: Cross-section of zone C

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