polymers
Article
Nickel-Aluminum Thermal Spray Coatings as Adhesion
Promoter and Susceptor for Inductively Joined
Polymer-Metal Hybrids
Erik Saborowski * , Axel Dittes, Thomas Lindner
and Thomas Lampke
Materials and Surface Engineering Group, Faculty of Mechanical Engineering, Chemnitz University of
Technology, Erfenschlager Straße 73, D-09125 Chemnitz, Germany; axel.dittes@mb.tu-chemnitz.de (A.D.);
th.lindner@mb.tu-chemnitz.de (T.L.); thomas.lampke@mb.tu-chemnitz.de (T.L.)
* Correspondence: erik.saborowski@mb.tu-chemnitz.de
Citation: Saborowski, E.; Dittes, A.;
Lindner, T.; Lampke, T.
Nickel-Aluminum Thermal Spray
Coatings as Adhesion Promoter and
Abstract: Hybrid joints of metal- and fiber-reinforced-polymer offer great potential for lightweight
applications. Thereby, a fast and reliable joining process is mandatory for mass-production applications. To this end, this study assesses inductive spot-joining in combination with prior thermal
spray coating of the metal adherent. A nickel–aluminum 95/5 coating was applied to achieve high
adhesion through mechanical interlocking and to act as susceptor for the inductive joining process.
The joint strength was assessed with lap shear specimens consisting of EN AW-6082 aluminum
alloy and glass fiber reinforced polyamide 6 or polypropylene, respectively. The joints were further
investigated in terms of heating time and hygrothermal cyclic loading. The results showed that
significant time savings for the joining process as well as strong adhesion were achieved due to the
coating. Moreover, the high strengths were even preserved under hygrothermal cyclic loading.
Keywords: mechanical interlocking; polymer-metal hybrid; fiber-reinforced polymer; inductive
joining; lap shear test; hygrothermal cyclic loading; thermal spraying
Susceptor for Inductively Joined
Polymer-Metal Hybrids. Polymers
2021, 13, 1320. https://doi.org/
10.3390/polym13081320
Academic Editors: Lianshe Fu and
Geoffrey R. Mitchell
Received: 25 March 2021
Accepted: 14 April 2021
Published: 17 April 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affiliations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
Thermoplastic polymer-metal hybrids offer great potential for automotive and aviation
applications due to their high strength/stiffness-to-weight ratio. In contrast to hybrid
structures containing thermoset polymers, no time-consuming curing process is necessary.
Thereby, key challenges are the development of a well adhering interface as well as a fast
joining technique. Common industrial joining methods are e.g., adhesive bonding [1],
riveting [2], or clinching [3]. Another promising approach is thermal joining. Hereby, the
polymer itself serves as hot-melt adhesive as it infiltrates and interlocks with the roughness
features of the metallic surface. Metal and polymer are put together under pressure,
while the contact area between both materials is heated until the polymer melts. The
necessary thermal energy can e.g., be generated by direct laser heating [4,5], indirect laser
heating [6–9], ultrasonic oscillations [10–12], friction [13,14], heat conduction [15–17] or
induction [8,18–20]. Depending on the respective design, all methods are able to generate
the necessary heat within the low-second or even sub-second range. Advantages of
inductive heat generation are joining without damaging the metal surface like friction
based or ultrasonic methods, easy integration into joining tools as well as low investment
costs [21]. The susceptor that is needed for absorbing the electromagnetic energy can be
integrated into the polymer [18,19,22,23]. Furthermore, the metal part itself can serve as
susceptor [8,20].
Metal surface topography has a major impact on the achievable strength of the joint.
The most widely used structuring method is mechanical blasting [6,17,24] since it offers
acceptable bonding strength and can easily be integrated into industrial applications.
However, high residual stresses arise in the near-surface area, leading to considerable
Polymers 2021, 13, 1320. https://doi.org/10.3390/polym13081320
https://www.mdpi.com/journal/polymers
Polymers 2021, 13, 1320
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deformation of the metallic adherent. Laser structuring processes offer the highest possible
bonding strength [7,17,25,26]. However, the processing times are very high. Thermal spray
coatings provide a rough and undercut surface. Hence, they can be used as adhesion
promoter for polymer-metal hybrids, reaching an interlaminar strength higher than mechanically blasted metallic surfaces [17,26,27]. When the coating is applied without prior
activation of the substrate, residual stresses due to mechanical blasting can be avoided.
Although thermal spray coatings also exhibit residual stresses, their distribution is limited to the thickness of the relatively thin coatings, whereas mechanical blasting usually
has a much higher penetration depth [28]. Consequently, excessive deformation of thin
substrates is avoided. Another advantage of thermally sprayed coatings is the ability to
use a wide range of materials. By using a ferromagnetic metal with high permeability,
the coating can also serve as susceptor in addition to its purpose as adhesion promoter
since joule heating caused by induced eddy currents as well as hysteresis losses lead to an
acceleration of the inductive heating process [21].
The utilization of a coating as both susceptor and adhesion promoter enables a fast
and reliable process chain for producing strong joints between metal and thermoplastic
polymer. To this end, this contribution investigates self-adhering, ferromagnetic nickel–
aluminum 95/5 coatings for inductive joining of EN AW-6082 aluminum alloy to glass fiber
reinforced polypropylene and polyamide 6. A parameter study for coating application by
wire arc spraying is conducted to achieve the highest possible joint strength. The joined
specimens are strength-tested by lap shear tests. Furthermore, the coatings are investigated
in terms of their surface roughness to assess the relation between surface structure and joint
strength. The heating rates, depending on coating thickness and induction coil distance, are
investigated by infrared camera images. Additionally, the joined specimens are investigated
for their resistance against hygrothermal cyclic loading by climate tests to investigate how
physical and chemical aging of the polymer affect the joint strength. Finally, the fracture
surfaces are analyzed to determine the occurring failure mechanisms.
2. Materials and Methods
Metal sheets made up from 100 × 25 × 3 mm3 EN AW-6082 T6 aluminum alloy
(Hans-Erich Gemmel & Co. GmbH, Berlin, Germany) as well as polymer sheets made
up from 80 × 25 × 1 mm3 Tepex® dynalite 102-RG600 (LANXESS Deutschland GmbH,
Cologne, Germany) glass fiber reinforced polyamide 6 (PA6GF47BD) and Tepex® dynalite
104-RG600 glass fiber reinforced polypropylene (PPGF47BD) were used. Table 1 shows
the supplier specifications of the used material. The lap shear tests were conducted in
accordance to DIN EN 1465 for adhesively bonded assemblies. The utilized specimen
geometry is illustrated in Figure 1a.
Table 1. Material properties of the used metal and polymer, supplier specifications.
(kg/m3 )
Density
Tensile modulus (MPa)
Poisson’s ratio (-)
Yield strength (MPa)
Ultimate strength (MPa)
Elongation to failure (%)
Melting temperature (◦ C)
Thermal expansion coefficient (10−6 /K)
Fiber
Weaving Style
Fiber content (vol.-%)
Thickness per layer (mm)
EN AW-6082 T6
PA6GF47BD (ISO 1110)
PPGF47BD
2700
70,000
0.34
240–320
300–350
8–14
660
23.4
-
1800
18,000
380
2.3
220
18
E-Glass
Twill 2/2
47
0.5
1680
20,000
430
2.7
165
11
E-Glass
Twill 2/2
47
0.5
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Polymers
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3 of410
(a)
(b)
Figure
1. Lap
shear
specimen
geometry,
spot-joined
specimen
with
PA6GF47BD.
Figure
1. Lap
shear
specimen
(a)(a)
geometry,
(b)(b)
spot-joined
specimen
with
PA6GF47BD.
® AS-756 Nickel–Aluminum 95/5 (NiAl5) cored wire (DURUM
Ø1.6
DURMAT
Themm
influence
of the
coating thickness on the heating rate was determined with a
Verschleißschutz
Willich,
Germany)
utilized Advanced
for coatingSystems
deposition
on the
VarioCAM® HDGmbH,
head 900
infrared
camera was
(JENOPTIK
GmbH,
Jena,
aluminum
substrate.
The
coatings
were
applied
by
wire
arc
spraying
unit
VISU
ARC
350
Germany). The coil was placed 1, 3 and 5 mm above the coating and the high frequency
with
Schub 5was
spraying
gun at
(Oerlikon
Metco,
Wohlen,
Switzerland).
For
determination
of a
converter
activated
50% power
setting
for slightly
over 5 s.
The
resulting temperasuitable
combination
of current
and voltage for
the value
wire arc
process,
a parameter
ture curve
represents
the time-dependent
mean
of aspraying
circle with
Ø10 mm
in the censtudy
with
V1–V6,
shown
in Table
2, was
was conducted.
The IRBIS
best parameter
set in
ter of
thethe
coilvalues
(Figure
2b). The
recorded
data
evaluated with
3.1 plus software
terms
of lapGmbH,
shear strength
was
further utilized
assessing
thesurface,
influence
the coating
(InfraTec
Dresden,
Germany).
For the for
oxidized
nickel
anof
emission
coeffithickness
on
the
inductive
spot-joining
process.
Therefore,
a
number
of
1–8
layers
(1L–8L)
cient of ε = 0.37 was assumed. The resulting ΔT represents the temperature difference bewas
investigated.
tween
start of the heating process and 5 s heating time.
Table 2. Wire arc spraying parameters for NiAl5 feedstock materials.
Variant
Current
(A)
Voltage
(V)
Atomising
Gas (bar)
Spraying Distance
(mm)
Offset
(mm)
Feed Speed
(m/s)
Layers
V1
V2
V3
V4
V5
V6
120
160
200
120
160
200
25
25
25
30
30
30
3.5
3.5
3.5
3.5
3.5
3.5
130
130
130
130
130
130
5
5
5
5
5
5
1
1
1
1
1
1
4
4
4
4
4
4
Prior to the joining process, the aluminum and polymer sheets were ultrasonically
cleaned
in ethanol. Additionally, the PA6GF47BD was dried
(a)
(b) at 70 ◦ C for three days to
ensure a low moisture content and avoid bubble formation during the joining process.
Figure 2. (a) Inductive joining device, (b) infrared camera image.
The lap shear specimens were manufactured with the joining device shown in Figure 2a.
Metal and polymer were pressed together with a spring-loaded stamp with Ø12 mm, using
The hygrothermal cyclic loading test was carried out in accordance to test standard
a constant force of 125 N. Since the aluminum sheet as well as the coating can serve as
BMW PR 308.2 described by [25] with a VC 4018 climate chamber (Vötsch Industrietechsusceptor, both variants were examined. In the initial experiments for determining the best
nik GmbH, Reiskirchen-Lindenstruth, Germany). The test cycle switched between 90 °C
coating parameters, induction coil and loading stamp were placed above the aluminum
at 80% humidity and −30 °C at ambient humidity. The dwell time at the maximum and
sheet. Thereby, an even stress distribution and wetting was achieved in the joining zone.
minimum temperatures was 4 h. The switching time in-between was 2 h. Overall, 20 cycles
Hence, the bearable lap shear strength can be calculated. Deviating from DIN EN 1465, a
were performed, resulting in a total test time of 240 h.
shortened overlap length of 5 mm was used to limit the maximum force during the lap
The cross-sectional images were recorded with GX51 optical microscope (Olympus
shear test and thus avoid fracture of the polymer sheet. In the following experiments
Europe SE & Co. KG, Hamburg, Germany). The coating thicknesses were determined with
investigating spot-joining, the induction coil was placed above the polymer and the coating
ImageJ 1.52a image evaluation software [29] from the mean value of 10 measurements
was used as susceptor. Since the polymer sheets were not stiff enough for distributing
between substrate and top of the coating each. The fractured surfaces were evaluated with
the stress evenly during the joining process, no complete wetting of the overlapping area
MVX10
optical
microscope
Europe
SE & Co.
Hamburg,
Germany).
was
realized.
Hence,
only the(Olympus
breaking force
is given.
AnKG,
increased
overlapping
length
The
roughness
measurements
were
carried
out
in
accordance
to
DIN
EN
ISO
of 15 mm was used to ensure full support of the spring-loaded stamp. Figure 1b shows4287,
a
® T8000 stylus profiler (JENOPTIK AG, Jena, Germany). Five
using a Hommel-Etamic
spot-joined
specimen.
measurements with an evaluation length of 12.5 mm were recorded and evaluated for
coating parameter sets V1–V6. Thereby, a stylus tip wit 2 µm radius and 60° tip angle was
used for capturing the highest possible amount of profile details. Average maximum pro-
Polymers 2021, 13, 1320
converter was activated at 50% power setting for slightly over 5 s. The resulting temperature curve represents the time-dependent mean value of a circle with Ø10 mm in the center of the coil (Figure 2b). The recorded data was evaluated with IRBIS 3.1 plus software
(InfraTec GmbH, Dresden, Germany). For the oxidized nickel surface, an emission coeffi4 of 10becient of ε = 0.37 was assumed. The resulting ΔT represents the temperature difference
tween start of the heating process and 5 s heating time.
(a)
(b)
Figure
2. (a)
Inductive
joining
device,
infrared
camera
image.
Figure
2. (a)
Inductive
joining
device,
(b)(b)
infrared
camera
image.
The
inductive
heatingcyclic
was performed
with
a HFL
02/5out
5 kW
high frequency
converter
The
hygrothermal
loading test
was
carried
in accordance
to test
standard
(Frisch
Pforzheim,
The
utilized
coil had(Vötsch
an outer
diameter
BMWGmbH,
PR 308.2
describedGermany).
by [25] with
a VC
4018 induction
climate chamber
Industrietechof nik
22 mm,
an inner
diameter of 18 mm and
a wall thickness
1 mm.
The coilbetween
was placed
GmbH,
Reiskirchen-Lindenstruth,
Germany).
The testofcycle
switched
90 °C
0.5atmm
above
the aluminum
andatdirectly
onhumidity.
the polymer,
A the
power
setting of
80%
humidity
and −30 °C
ambient
Therespectively.
dwell time at
maximum
and
75%
was usedtemperatures
for joining the
specimens.
The power
wasin-between
active untilwas
the 2temperature
in cycles
the
minimum
was
4 h. The switching
time
h. Overall, 20
◦
joining
reachedresulting
200 ◦ C for
PPGF47BD
were zone
performed,
in the
a total
test timespecimens
of 240 h. and 260 C for the PA6GF47BD
specimens.
temperatureimages
in the joining
zone was
observed
a HH507
thermometer
The The
cross-sectional
were recorded
with
GX51 with
optical
microscope
(Olympus
(OMEGA
Engineering,
Deckenpfronn,
Germany).
After
deactivating
the
high
frequency
Europe SE & Co. KG, Hamburg, Germany). The coating thicknesses were determined
with
converter,
the joined
specimens
cooled
down [29]
for 15
s before
removal.
Prior
testing,
ImageJ 1.52a
image
evaluation
software
from
the mean
value
of strength
10 measurements
thebetween
PA6GF47BD
specimens
were
conditioned
according
to DIN EN
ISO 1110
three days
substrate
and top
of the
coating each.
The fractured
surfaces
werefor
evaluated
with
to MVX10
ensure similar
moisture
content
and
mechanical
behavior
of
the
polyamide
6.
The
lap
optical microscope (Olympus Europe SE & Co. KG, Hamburg, Germany).
shear tests
carried
out with an Allround-Line
20 in
kNaccordance
testing machine
The were
roughness
measurements
were carried out
to DIN(ZwickRoell
EN ISO 4287,
GmbH
&
Co.
KG,
Ulm,
Germany),
using
a
crosshead
speed
of
1
mm/min.
lap shear
®
using a Hommel-Etamic T8000 stylus profiler (JENOPTIK AG, Jena,Three
Germany).
Five
specimens
were
tested
for
each
parameter
set.
measurements with an evaluation length of 12.5 mm were recorded and evaluated for
The influence
ofsets
theV1–V6.
coatingThereby,
thickness
on thetip
heating
rateradius
was determined
with was
a
coating
parameter
a stylus
wit 2 µm
and 60° tip angle
® HD head 900 infrared camera (JENOPTIK Advanced Systems GmbH, Jena,
VarioCAM
used for capturing the highest possible amount of profile details. Average maximum proGermany). The coil was placed 1, 3 and 5 mm above the coating and the high frequency
converter was activated at 50% power setting for slightly over 5 s. The resulting temperature
curve represents the time-dependent mean value of a circle with Ø10 mm in the center of
the coil (Figure 2b). The recorded data was evaluated with IRBIS 3.1 plus software (InfraTec
GmbH, Dresden, Germany). For the oxidized nickel surface, an emission coefficient of
ε = 0.37 was assumed. The resulting ∆T represents the temperature difference between
start of the heating process and 5 s heating time.
The hygrothermal cyclic loading test was carried out in accordance to test standard
BMW PR 308.2 described by [25] with a VC 4018 climate chamber (Vötsch Industrietechnik
GmbH, Reiskirchen-Lindenstruth, Germany). The test cycle switched between 90 ◦ C at 80%
humidity and −30 ◦ C at ambient humidity. The dwell time at the maximum and minimum
temperatures was 4 h. The switching time in-between was 2 h. Overall, 20 cycles were
performed, resulting in a total test time of 240 h.
The cross-sectional images were recorded with GX51 optical microscope (Olympus
Europe SE & Co. KG, Hamburg, Germany). The coating thicknesses were determined
with ImageJ 1.52a image evaluation software [29] from the mean value of 10 measurements
between substrate and top of the coating each. The fractured surfaces were evaluated with
MVX10 optical microscope (Olympus Europe SE & Co. KG, Hamburg, Germany).
The roughness measurements were carried out in accordance to DIN EN ISO 4287,
using a Hommel-Etamic® T8000 stylus profiler (JENOPTIK AG, Jena, Germany). Five measurements with an evaluation length of 12.5 mm were recorded and evaluated for coating
parameter sets V1–V6. Thereby, a stylus tip wit 2 µm radius and 60◦ tip angle was used
for capturing the highest possible amount of profile details. Average maximum profile
Polymers 2021, 13, x FOR PEER REVIEW
5 of 10
Polymers 2021, 13, x FOR PEER REVIEW
Polymers 2021, 13, 1320
5 of 10
5 of 10
used for capturing the highest possible amount of profile details. Average maximum
profile height Rz, arithmetical mean height Ra and root mean square slope RΔq were
file
height
arithmetical
height
Ra and
root
mean
square
evaluevaluated.
Especially
themean
slope
has
good
accordance
inslope
past
studies
since
it is
height
Rz, Rz,
arithmetical
mean
height
Rashown
and
root
mean
square
slope
R∆qRΔq
werewere
evaluated.
ated.
Especially
the
slope
has
shown
good
accordance
in
past
studies
since
it
is
directly
directly
related
to
the
density
and
the
aspect
ratio
of
the
surface
roughness
features
[17,30].
Especially the slope has shown good accordance in past studies since it is directly related
related
to the density
theratio
aspect
of the surface
roughness
to the density
and theand
aspect
of ratio
the surface
roughness
featuresfeatures
[17,30]. [17,30].
3. Results and Discussion
3. Results
Results
and3Discussion
Discussion
and
Figure
illustrates the thinnest (V4: 90.8 ± 38.7 µ m) as well as the thickest (V3: 180.3
Figure
3
illustrates
thethinnest
thinnest
(V4:
90.8
± 38.7
as well
asthickest
the thickest
(V3: a
Figure
3 illustrates
the
(V4:
90.8
± 38.7
µm)µm)
asparameter
well
as therange.
180.3
± 28.7
µ m)
coating within
the investigated
spraying
In (V3:
all cases,
180.3
±
28.7
µm)
coating
within
the
investigated
spraying
parameter
range.
In
all
cases,
a
± 28.7
µm) coating
within the
investigated
spraying
parameter
range.the
In individual
all cases, a coating
comcomplete
bond between
coating
and substrate,
as well
as between
complete
bond
between
coating
and
substrate,
as
well
as
between
the
individual
coating
plete
bond
between
coating
substrate,
as well
as between
layers,
was
achieved.
The and
electric
spraying
parameters
affectthe
theindividual
amount ofcoating
moltenlaywire
layers,
achieved.
electric
spraying
parameters
affect
the
of molten
ers,
waswas
achieved.
TheThe
electric
spraying
parameters
the amount
molten
wire wire
durduring
the
spraying
process
and
therefore
the affect
thickness
of amount
the ofcoating
(Figure
4a).
during
spraying
process
and
therefore
the
thickness
thecoating
coating
(Figure
4a).affected.
However,
ing
the the
spraying
process
and
therefore
the
thickness
ofofthe
(Figure
4a).
However,
However,
the coating
surface
structure
and
morphology
are only
slightly
The
the
coating surface
surface
structure revealed
and morphology
are
slightly
The
roughness
the
coating
structure
and
morphology
are only
only
slightly affected.
affected.
Thefor
roughness
roughness
measurements
rather small
differences,
especially
the most
measurements
revealed
rather
small
differences,
especially
for
the
most
meaningful
R∆q
measurements
revealed
small differences,
RΔq
meaningful RΔq
value.rather
The lowest
values wereespecially
measuredfor
forthe
V4most
(Rz =meaningful
104 ± 8 µ m,
Ra =
value.
The
lowest
values
were
measured
for
V4
(Rz
=
104
±
8
µm,
Ra
=
16.0
±
1.5
µm,± 6
value.
values
were
measured
for V4 (Rz
= 104were
± 8 µm,
Ra = 16.0
1.5(Rz
µm,
RΔq
16.0 The
± 1.5lowest
µm, RΔq
= 0.855
± 0.031),
the highest
values
measured
for ±V3
= 131
± 0.031),
the
measured
forstrength
V3
(Rz
131 Ra
± 6=coating
µm,
=
=R∆q
0.855
± 0.031),
the
highest
values
were
for V3the
(Rz
= 131
± 6=between
µm,
21.6 Ra
± 1.8
µ m,= 0.855
Ra
= 21.6
± 1.8
µm,highest
RΔq =values
0.943measured
±were
0.035).
Since
and
21.6
±
1.8
µm,
R∆q
=
0.943
±
0.035).
Since
the
strength
between
coating
and
polymer
µm,
RΔq = 0.943
± 0.035).
Sinceonthe
strength
coating
and polymer
depends in
polymer
mainly
depends
the
surfacebetween
structure,
the achieved
lap mainly
shear strength
mainly
depends
on
the
surface
structure,
the
achieved
lap
shear
strength
in
connection
onconnection
the surfacewith
structure,
the achieved
shear
strength
inrange
connection
PA6GF47BD
variedlap
only
within
a small
of 19.7with
± 0.4PA6GF47BD
MPa and 21.0
with
PA6GF47BD
varied
only
a small
of
±±0.4
MPa
and
21.0 4b).
± in
0.7
varied
only
a4b).
small
range
of 19.7
0.4 range
MPa and
0.7
MPa
(Figure
This
is
± 0.7
MPa within
(Figure
This
iswithin
roughly
in± the
range
of 19.7
2421.0
MPa
that
were
reported
aMPa
recent
(Figure
4b).
This
is
roughly
in
the
range
of
24
MPa
that
were
reported
in
a
recent
study
for
roughly
24 MPa that
were fiber-reinforced
reported in a recent
study for
strength
study in
forthe
therange
shearofstrength
between
polyamide
6 the
andshear
NiAl5
coatings
the
shearfiber-reinforced
strength between
fiber-reinforced
polyamide
6 and
NiAl5 coatings [27].
between
polyamide
6 and NiAl5
coatings
[27].
[27].
(a)(a)
(b)(b)
Coating thickness / µm
Figure
3.
images
of
the
NiAl5
coatings
on
EN
AW-6082
substrate
(a)(a)
V4V4
and
(b)(b)
V3.V3.
Figure
3.
Cross-sectional
images
NiAl5
coatings
AW-6082
substrate
and
Figure
3. Cross-sectional
Cross-sectional
images
of of
thethe
NiAl5
coatings
onon
ENEN
AW-6082
substrate
(a)
V4
and
(b)
V3.
250
25
200
20
150
15
100
10
50
5
0
V1
V2
V3
V4
(a)(a)
V5
V6
0
V1
V2
V3
V4
V5
V6
(b)(b)
Figure
4. Results
Results
forfor
thethe
investigated
electric
spraying
parameters
on
(a)
coating
thickness
and
(b)
laplap
shear
strength
to to
Figure
4.
Results
investigated
electric
spraying
parameters
coating
thickness
and
shear
strength
Figure
4.
for
the
investigated
electric
spraying
parameters
onon
(a)(a)
coating
thickness
and
(b)(b)
lap
shear
strength
to
PA6GF47BD;
mean
values
±
1
SD.
PA6GF47BD;
mean
values
±
1
SD.
PA6GF47BD; mean values ± 1 SD.
The
heating
experiments
revealed
a strong
influence
of of
thethe
The
heating
experiments
revealed
a strong
influence
coating
thickness
The
experiments
revealed
coating
thickness
onon
thethe
heating
rate.
Figure
5a
illustrates
the
temperature
increase
ΔT
after
55 ss5 of
heating
at at
50%
heating
rate.
Figure
illustrates
the
temperature
increase
ΔT
after
s of
heating
50%
heating
rate.
Figure
5a5a
illustrates
the
temperature
increase
∆T
after
heating
power
setting
in in
dependence
coil
distance
and
thethe
coating
thickness.
AA
thick
coating
power
setting
dependence
coil
distance
and
coating
thickness.
thick
coating
of of
thethe
enables
coating
and
thus
leads
to
enables more
more absorption
absorptionof
ofelectromagnetic
electromagneticenergy
energythen
thena athin
thin
coating
and
thus
leads
to higher heating rates. Moreover, increasing the coil distance drastically decreases the
Polymers 2021, 13, x FOR PEER REVIEW
6 of 10
Polymers 2021, 13, 1320
6 of 10
enables more absorption of electromagnetic energy then a thin coating and thus leads to
higher heating rates. Moreover, increasing the coil distance drastically decreases the
heating rate. This is also reported by [23] as a result of the decrease in magnetic field
heating rate. This is also reported by [23] as a result of the decrease in magnetic field
intensity with increasing coil distance. Figure 5b shows exemplary temperature curves for
intensity with increasing coil distance. Figure 5b shows exemplary temperature curves for
coating parameter set V2. Especially when the coil distance increases, a thick coating still
coatingreasonable
parameterheating
set V2. Especially
whenonly
the acoil
distancetemperature
increases, a thick
coating
still
enables
rates, whereas
negligible
increase
can be
enables
reasonable
heating
rates,
whereas
only
a
negligible
temperature
increase
can
achieved without coating (coil distance 5 mm, no coating: ∆T = 10.5 K, 180.3 µm coating:be
achieved
without coating (coil distance 5 mm, no coating: ΔT = 10.5 K, 180.3 µm coating:
∆T
= 73.5 K).
ΔT = 73.5 K).
(a)
(b)
Figure
Heating
behavior
the
coatings,
high
frequency
converter
50%
power
setting
temperature
increase
after
Figure
5. 5.
Heating
behavior
ofof
the
coatings,
high
frequency
converter
atat
50%
power
setting
(a)(a)
temperature
increase
after
5
s
heating
time
in
dependence
of
coating
thickness
and
coil
distance,
uncoated
aluminum
and
parameter
sets
V1–V6
and
5 s heating time in dependence of coating thickness and coil distance, uncoated aluminum and parameter sets V1–V6 and
(b) temperature–time charts in dependence of the coating thickness for the coating parameter set V2 (coating thickness
(b) temperature–time charts in dependence of the coating thickness for the coating parameter set V2 (coating thickness
151.9 µ m).
151.9 µm).
Thespraying
sprayingparameter
parameterset
set V2
V2 was
was chosen
offered
the
The
chosen for
forfurther
furtherinvestigations
investigationsasasit it
offered
highest
lap
shear
strength.
A
number
of
one
to
eight
layers
were
investigated
to
determine
the highest lap shear strength. A number of one to eight layers were investigated to
the influence
of the coating
thickness
the actual
process. Asprocess.
expected,As
the
determine
the influence
of the
coating on
thickness
on spot-joining
the actual spot-joining
number
of
layers
is
directly
connected
to
coating
thickness
(Figure
6a).
The
necessary
expected, the number of layers is directly connected to coating thickness (Figure 6a). The
heating time
shown
inshown
Figure in
6bFigure
decreased
with increasing
number ofnumber
layers (1L:
14.92 ±
necessary
heating
time
6b decreased
with increasing
of layers
1.87
s,
6L:
7.64
±
1.51
s).
However,
a
further
increase
in
the
number
of
layers
from
six to
(1L: 14.92 ± 1.87 s, 6L: 7.64 ± 1.51 s). However, a further increase in the number of layers
eight
did
not
lead
to
a
further
reduction
in
the
heating
time
(8L:
8.08
±
0.53
s).
This
is
from six to eight did not lead to a further reduction in the heating time (8L: 8.08 ± 0.53 due
s).
to the
complete
of the magnetic
by thefield
layer
a certain
thickness.
This
is due
to the absorption
complete absorption
of thefield
magnetic
byabove
the layer
above
a certainA
further increase
in layer
thickness
therefore
has no
positivehas
effect
the inductive
heating
thickness.
A further
increase
in layer
thickness
therefore
noon
positive
effect on
the
rate. Theheating
breaking
force
in Figure
6c varied
within 6c
a small
of a4158
± 203
N to
inductive
rate.
Theshown
breaking
force shown
in Figure
variedrange
within
small
range
Consequently,
significant no
influence
of the
layer of
number
onnumber
the joint
of4534
4158 ±±369
203 N.
N to
4534 ± 369 N. no
Consequently,
significant
influence
the layer
be detected.
onstrength
the jointcan
strength
can be detected.
Since
variant
6L
provided
fastest
heating
rate
as as
well
the highest
breaking
Since variant 6L provided
the the
fastest
heating
rate as
well
the as
highest
breaking
force,
force,
it
was
further
investigated
in
connection
with
PPGF47BD.
The
breaking
force
it was further investigated in connection with PPGF47BD. The breaking force achieved with
achieved with
± 172 N) lower
is considerably
lower than
theachieved
breakingwith
force
PPGF47BD
(3140PPGF47BD
± 172 N) is(3140
considerably
than the breaking
force
achieved with
PA6GF47BD
(4534
± 369
N) due
to the
strength of
polypropylene
PA6GF47BD
(4534
± 369 N) due
to the
lower
strength
of lower
polypropylene
matrix
material
inmatrix
comparison
to polyamide
6 matrix
material. 6Inmatrix
contrast
to the results
reported
byresults
[25]
material
in comparison
to polyamide
material.
In contrast
to the
◦ C the
for
the 90 ◦by
C/[25]
−30for
climate
test, there
was notest,
significant
decrease
in breaking
forcein
reported
90 °C/−30
°C climate
there was
no significant
decrease
for
both polymer
materials
(PA6GF47BD:
4.5%, PPGF47BD:
−0.8%).
This is
possibly
breaking
force for
both polymer
materials−(PA6GF47BD:
−4.5%,
PPGF47BD:
−0.8%).
This
because
[25]
used
unreinforced
and
short-fiber
reinforced
polymer,
which
is
subject
to is
is possibly because [25] used unreinforced and short-fiber reinforced polymer, which
greater
stresses
and stresses
thus higher
of the
interface
dueinterface
to greaterdue
differences
subjectresidual
to greater
residual
and damage
thus higher
damage
of the
to greater
indifferences
the thermalinexpansion
coefficients
comparisoninto
the metal. Furthermore,
polyamide
the thermal
expansionincoefficients
comparison
to the metal. Furthermore,
has
an increased
after
the climate
and test
thusand
lower
and
polyamide
has anmoisture
increasedcontent
moisture
content
after thetest
climate
thusstrength
lower strength
stiffness.
In
this
work,
all
polyamide
samples
were
brought
to
the
same
moisture
content
and stiffness. In this work, all polyamide samples were brought to the same moisture
before
testing
by conditioning
according toaccording
ISO 1110, to
while
makes
no [20]
statement
content
before
testing by conditioning
ISO[25]
1110,
while
makesonno
this.
Based
on
the
results
in
this
study,
a
good
resistance
of
the
joints
against
hygrothermal
statement on this. Based on the results in this study, a good resistance of the joints against
cyclic
loading can
be stated.
hygrothermal
cyclic
loading can be stated.
Polymers 2021, 13, x FOR PEER REVIEW
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2021, 13,
13, 1320
x FOR PEER REVIEW
Polymers
7 of 10
of 10
10
77 of
20
15
10
5
0
(a)
1L
(a)
2L
4L
(b)
(b)
6L
8L
5000
Reference
-30 °C / 90 °C
4000
3000
2000
1000
0
(c)
(c)
PA6GF47BD
(d)
(d)
PPGF47BD
FigureResults
6. Results for
spraying parameter
set V2
with 1–8
deposited layers
on coating
(a) coating thickness,
(b) heating
time for
Figure
Figure 6.
6. Results for
for spraying
spraying parameter
parameter set
set V2
V2 with
with 1–8
1–8 deposited
deposited layers
layers on
on (a)
(a) coating thickness,
thickness, (b)
(b) heating
heating time
time for
for
PA6GF47BD
at 75%
power
setting,
(c) breaking
force
for
PA6GF47BD
and
(d)
breaking
force
for
6LPA6GF47BD
of PA6GF47BD
and
PA6GF47BD
at
75%
power
setting,
(c)
breaking
force
for
PA6GF47BD
and
(d)
breaking
force
for
6L
of
PA6GF47BD at 75% power setting, (c) breaking force for PA6GF47BD and (d) breaking force for 6L of PA6GF47BD and
and
PPGF47BD
before
and
after
climate
test;
mean
values
±
1
SD.
PPGF47BD
SD.
PPGF47BD before
before and
and after
after climate
climate test;
test; mean
mean values
values ±
± 11 SD.
Sufficient
wetting
of
coating
surface
withpolymer
polymer iscrucial
crucialfor
for theformation
formation of
Sufficient
wetting
of the
thethe
coating
surface
with
Sufficient
wetting
of
coating
surface
with
polymer isiscrucial
for thethe
formation of
specific adhesion
as
well
asasmechanical
interlocking
and
thus
a high
jointjoint
strength.
Figure
of
specific
adhesion
as
well
mechanical
interlocking
and
thus
a
high
strength.
specific adhesion as well as mechanical interlocking and thus a high joint strength.
Figure
7
shows
cross-sectional
images
of
the
interfaces
of
the
spot-joined
PA6GF47BD
and
7 shows
cross-sectional
images
of the
interfaces
spot-joinedPA6GF47BD
PA6GF47BD and
7Figure
shows
cross-sectional
images
of the
interfaces
of of
thethespot-joined
and
PPGF47BD
specimens.
In
both
cases,
almost
complete
wetting
was
achieved.
The
surface
PPGF47BD specimens.
In both
both cases,
PPGF47BD
specimens. In
cases, almost
almost complete
complete wetting
wetting was
was achieved.
achieved. The
The surface
surface
microstructures
of coating
the coating
were completely
almost completely
surrounded
byFurtherpolymer.
microstructures
of
the
were
almost
surrounded
by
polymer.
microstructures of the coating were almost completely surrounded by polymer. FurtherFurthermore,
no delamination
due to
thermal contraction
wasPPGF47BD
observed. PPGF47BD
more,
no delamination
due to thermal
contraction
was observed.
showed
more, no delamination due to thermal contraction was observed. PPGF47BD showed
showed
slightly
more strength-reducing
than PA6GF47BD,
butinthis
was still in a
slightly
more
strength-reducing
cavities than cavities
PA6GF47BD,
but this was still
a reasonable
slightly more strength-reducing cavities than PA6GF47BD, but this was still in a reasonareasonable
Thus,
the
quality
of the joints
concluded
to be high for both
amount.
Thus, amount.
the quality
of the
joints
is concluded
to be is
high
for both polymers.
ble amount. Thus, the quality of the joints is concluded to be high for both polymers.
polymers.
(a)
(a)
(b)
(b)
Figure 7.
7. Interface
Interface of
of spot-joined
spot-joined specimens
specimens with
with coating
coating parameter
parameter set
set V2/6L
V2/6L (a) PA6GF47BD and (b) PPGF47BD.
Figure
Figure 7. Interface of spot-joined specimens with coating parameter set V2/6L (a) PA6GF47BD and (b) PPGF47BD.
In general, a cohesive-adhesive failure between coating and polymer occurred
occurred for
for all
In general, a cohesive-adhesive failure between coating and polymer occurred for all
spottested lap shear specimens. Figure 8 shows the fracture surfaces of a 6L/PA6GF47BD spottested
lap shear
specimens.
Figure 8 shows
the fracture
surfaces
of aside,
6L/PA6GF47BD
spotjoined
specimen
without
hygrothermal
cyclic
loading.
At
the
coating
a
high
amount
hygrothermal cyclic loading.
amount
Polymers2021,
2021,13,
13,1320
x FOR PEER REVIEW
Polymers
10
88ofof10
of
ofpolymer
polymerresidues
residues(Figure
(Figure8a)
8a)as
aswell
wellas
aspartially
partiallyripped
rippedout
outglass
glassfibers
fibers(Figure
(Figure8b)
8b)were
were
visible, showing that a large proportion of the interface between coating and polymer was
visible, showing that a large proportion of the interface between coating and polymer was
able to withstand the occurring shear forces. Moreover, the coating showed no areas of
able to withstand the occurring shear forces. Moreover, the coating showed no areas of
delamination from the aluminum substrate, indicating a strong metallurgical bonding.
delamination from the aluminum substrate, indicating a strong metallurgical bonding. At
At the polymer side, a low amount of cavities is visible (Figure 8c). Furthermore, small
the polymer side, a low amount of cavities is visible (Figure 8c). Furthermore, small pieces
pieces of the coating adhere to the polymer (Figure 8d). Cohesive fractures in the coating
of the coating adhere to the polymer (Figure 8d). Cohesive fractures in the coating is
is caused from the brittleness and reduced strength of coated material in comparison to
caused from the brittleness and reduced strength of coated material in comparison to bulk
bulk material [28] as well as partially filigree-formed roughness features that are not able
material [28] as well as partially filigree-formed roughness features that are not able to
to withstand the occurring shear forces.
withstand the occurring shear forces.
(a)
(b)
(c)
(d)
Figure8.8.Fracture
Fracturesurfaces
surfacesof
ofPA6GF47BD
PA6GF47BDspecimen
specimen(a,b)
(a,b)coating
coatingside
sideand
and(c,d)
(c,d)polymer
polymerside.
side.
Figure
4.Conclusions
4.
Conclusions
In the
the present
presentwork,
work,aatechnology
technologycombination
combinationof
ofthermal
thermalspraying
sprayingand
andinductive
inductive
In
thermaljoining
joiningwas
wasdeveloped
developedwith
withthe
theaim
aimof
ofrapidly
rapidlyjoining
joiningaluminum
aluminumto
tothermoplastic
thermoplastic
thermal
polymer.The
Thealuminum
aluminumwas
wascoated
coatedwith
withnickel–aluminum
nickel–aluminum95/5
95/5by
bywire
wirearc
arcspraying.
spraying.The
The
polymer.
coating
coatingafterwards
afterwardsserved
servedas
assusceptor
susceptorfor
forinductive
inductiveheating
heatingas
aswell
wellas
asadhesion
adhesionpromoter
promoter
between
Based
onon
thethe
experimental
results
andand
the performed
analyses,
betweenmetal
metaland
andpolymer.
polymer.
Based
experimental
results
the performed
analthe
following
conclusions
can becan
drawn:
yses,
the following
conclusions
be drawn:
••
••
••
Nickel–aluminum 95/5
95/5 thermal spray coatings offer a rough
rough and
and undercut
undercut surface,
surface,
Nickel–aluminum
suitable for
forstrong
strongmechanical
mechanicalinterlocking
interlockingadhesion
adhesionto
tothermoplastic
thermoplasticpolyamide
polyamide66
suitable
andpolypropylene.
polypropylene.
and
Variationsin
inthe
thecurrent
currentand
andvoltage
voltageparameters
parametersfor
forthe
thethermal
thermalspraying
sprayingprocess
process
Variations
had aa large
large impact
impact on the coating
in
had
coating thickness.
thickness.AAdecrease
decreaseininvoltage
voltageand
andananincrease
increase
in
current
increased
the
coating
thickness.
The
increase
coating
thickness
slightly
current
increased
the
coating
thickness.
The
increase
in in
coating
thickness
slightly
inincreased
the
surface
roughness
values,
a marginal
change
the
creased the
surface
roughness
values,
butbut
led led
onlyonly
to a to
marginal
change
in thein
bondbonding
strength
between
coating
and polymer.
ing strength
between
coating
and polymer.
AAdifferent
of coating
layers
does not
affect the
bonding
differentnumber
number
of coating
layers
doessignificantly
not significantly
affect
the strength
bonding
between
coatedthe
metal
andmetal
polymer
strength the
between
coated
andadherents.
polymer adherents.
Polymers 2021, 13, 1320
9 of 10
•
•
•
•
Depending on the coil distance and coating thickness, the coating accelerated the
inductive heating process up to 700% in comparison to uncoated aluminum. An
increase in coating thickness up to 170 µm leaded to higher heating rates. Since the
magnetic field is completely absorbed above a certain coating thickness, a further
increase did not lead to a decrease in joining time.
Almost complete wetting of the coating surface with polymer was achieved with the
spot-joining process.
Hygrothermal cyclic loading between 90 ◦ C at 80% humidity and −30 ◦ C at ambient
humidity did not significantly affect the bond strength. Thus, a good hygrothermal
stability of the joints can be stated.
Cohesive-adhesive failure with partially ripped-out glass fibers occurred due to the conducted lap shear tests. The coating showed no delamination from the aluminum substrate.
In future work, the developed technology combination will be transferred into an
industrial application. For this purpose, a welding tool for inductive spot-like joining
was developed. The tool can be attached to industrial robots and thus automatically join
structural components made up from metal and thermoplastic polymer.
Author Contributions: Conceptualization, E.S.; Data curation, E.S.; Funding acquisition, T.L. (Thomas
Lampke); Investigation, E.S.; Methodology, E.S.; Project administration, T.L. (Thomas Lampke);
Software, E.S.; Supervision, T.L. (Thomas Lindner) and T.L. (Thomas Lampke); Validation, E.S.;
Visualization, E.S.; Writing—original draft, E.S.; Writing—review & editing, A.D., T.L. (Thomas
Lindner) and T.L. (Thomas Lampke). All authors have read and agreed to the published version of
the manuscript.
Funding: The authors gratefully acknowledge the Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke” e.V. (AiF) for support of this work (AiF-No. ZF4131905FH8) with
funds from the German Federal Ministry for Economic Affairs and Energy. The publication of this
article was funded by Chemnitz University of Technology.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Acknowledgments: The support by Christoph Wollschläger, Paul Seidel, Christian Loos, Thomas
Senß, Torsten Rabes, Michael Zychski and Robert Glaßmann (all from the Institute of Materials
Science and Engineering) is gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
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