JEC COMPOSITES MAGAZINE - Issue #105 - June 2016 - 48

laser cutting

Fig. 3: Surface texturing (Spectra-Physics logo and square features at the top and
bottom of the sample) and hole drilling in CFRP without any visible fibre damage
using the Quasar UV laser

number of process conditions, and in some cases the HAZ was
effectively zero. Burst machining proved to be advantageous,
achieving approximately 20 to 50% higher cutting speeds for
the same average power. Through additional process development and optimization efforts, it was shown that Quasar can
cut 250-µm-thick CFRP plates at 70 mm/sec with an HAZ of
< 15 µm.
Using the optimized laser process parameters, the team was
also successful in cutting a thicker ~1-mm-thick CFRP plate,
as shown in figure 2.

CFRP drilling
The drilling of CFRP parts is also a very important process
since traditional riveting and other types of fastening techniques require drilling holes in the material. Mechanical and
waterjet processes can be detrimental to the part's strength
due to fibre damage during the drilling process, and fibre
damage can ultimately lead to a weaker structure. The process
parameters developed for cutting can be effectively used for
trepan drilling holes in CFRP, as shown in figure 3.

joining can be economical compared to laser drilling holes.
Moreover, adhesive joining provides more efficient load management, so the thickness and hence the weight of parts can
be reduced. Also, it is possible that bolting or riveting may not
be an option for some parts due to their design. For adhesive
joining, CFRP parts need to be cleaned of mould release agent
residues and debris imparted to the surface during the moulding process. A thorough cleaning and surface texturing of the
part without damaging the fibres is crucial to achieve higher
joint strength.
Painting of CFRP parts is also challenging due to a low surface
wettability and poor surface adhesion, both of which can be
improved with laser processing. A thorough cleaning and texturing of the surface prior to painting improves the wettability
and hence the adhesion of paint to the surface.
The commonly used CFRP surface cleaning and texturing
techniques are peel-plies and mechanical abrading (or grit
blasting). While the peel-ply technique can provide a reproducible surface roughness, it requires the lamination of
peel-plies onto the surface prior to moulding and hence adds
manufacturing steps and costs. The repeatability of the peelply process is questionable since the thickness of the top resin
layer after the peel-ply process tends to vary across the part
surface. Moreover, release agent residues can be transferred
from the peel-ply onto the part and hence affect the joint
strength.
The major disadvantages of mechanical abrading are its low
throughput speed and the use of wet chemicals. Hence, a subsequent rinsing and drying of the part is necessary, which adds
manufacturing steps and costs. Additionally, this technique
is usually performed manually, making it very time consuming and difficult to implement for large CFRP parts. It also
has a high process variability and the risk of damaging fibres.
The grit blasting technique tends to damage fibres and leaves
residues and dust on the part, thus implying a subsequent
cleaning and drying.

CFRP surface cleaning and texturing
A well-developed joining technique for CFRP parts is essential to fully utilize these materials' potential for the manufacture of large parts in the aerospace and automotive industries.
A robust adhesive joining process has to be developed to
enable design engineers to fully utilize the advantages of
CFRP materials. Traditional bolting and riveting techniques
can be made to work by using a laser, instead of mechanical
tools, to drill holes and avoid fibre damage. However, large
numbers of holes are needed to join large parts and adhesive

48 jec composites magazine / No105 June 2016

Fig. 4: Lap shear strength test data for untreated, reference-abraded and Quasar
UV laser cleaned and textured parts

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #105 - June 2016