JEC COMPOSITES MAGAZINE - Issue #101 - November/December 2015 - 40

welding

Quality-controlled continuous
induction welding of CFRP composites
A simulation-based user interface for automated,
continuous induction welding of thermoplastic
fibre-reinforced polymer composites allows endusers to optimize process parameters and ensure
quality control in an efficient way.

Fig. 2: Crucial parameters of the continuous induction welding process

Process design

By
stePhan BeckeR anD Dennis MauReR,
ReseaRch assistant, ManufactuRing science
DR. MiRo Duhovic,
ReseaRch ManageR, Design & analYsis
PRof. DR.-ing. PeteR Mitschang,
ReseaRch DiRectoR, ManufactuRing science
institut füR veRBunDweRkstoffe gMBh (ivw)

he Institut für Verbundwerkstoffe GmbH (IVW) has been
investigating the continuous induction welding of carbon
fibre-reinforced polymer composites (CFRPCs) for years. This
fusion welding technique has the advantage that heat is quickly and
inherently generated in the joining parts by induced eddy currents
and joule losses. Due to their electrical conductivity, thermoplastic
CFRPCs can be inherently heated. In addition, heat input is locally
concentrated and high bonding strength values are achieved.

Fig. 1: Schematic of the continuous induction welding process and temperature
development in the joining zone and on the laminate surface

The continuous induction welding of thermoplastic CFRPCs is a
three-step process, as shown in Fig. 1. Due to the inhomogeneous
intensity of the magnetic field over the cross section, the laminate
surface closest to the induction coil undergoes the highest energy
input. Thus, the highest temperature in the laminate is localized at
this position. However, this unwanted effect causes delamination or
degradation of the surface. To avoid this, an active surface cooling
device using an impinging air jet is used [1, 2].
Figure 1 shows the temperature development of the laminate surface
and joining area as a function of time. In the first step, two thermoplastic CFRPC joining partners are heated in the joining zone
above the melting temperature (T2) by means of induction. Thereby,
polymer chains have enough thermal energy to move freely. However, the temperature must not exceed the degradation temperature
(T1) of the matrix. The second step includes the consolidation of the
joining area by means of a cooled roller that applies the consolidation pressure. Consequently, the joining area is cooled down below
the crystallization temperature (T3) and delamination is avoided.
However, the temperature in the joining area still has to be above the
crystallization temperature until the consolidation pressure is applied. If the temperature drops below the crystallization temperature,
autohesion of the polymer matrix across the bonding line is impeded
because the polymer chains do not have enough thermal energy to
move freely. In the course of the third step, the adherends are cooled
down by the surrounding atmosphere. If the temperature rises again
above the crystallization temperature during this step, voids and
delamination will appear.
Fig. 2 shows the crucial parameters that influence induction welding.
The generator power, the coupling distance between the inductor
and the laminate surface, the distance between the inductor and
the contact point of the roller, the volume flow of the air jet and the

40 jec composites magazine / No101 November - December 2015

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #101 - November/December 2015