Thermal Joining of Thermoplastics to Metals: Surface Preparation Methods

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Procedia CIRP 00 (2018) 000–000
www.elsevier.com/locate/procedia
Procedia CIRP
00 (2017)
000–000
Procedia
CIRP 74
(2018) 500–505
www.elsevier.com/locate/procedia
10th CIRP
10th
CIRPConference
Conference on
on Photonic
PhotonicTechnologies
Technologies [LANE
[LANE 2018]
2018]
Thermal joining28th
of CIRP
thermoplastics
to metals:
Surface
preparation of steel
Design Conference,
May 2018,
Nantes, France
based on laser radiation and tungsten inert gas arc process
A new methodology to analyze the functional and physical architecture of
a
a
b
Marie-Luise
Kohla,*, Klaus
Schricker
, Jean oriented
Pierre Bergmann
, Martin
Lohseidentification
, Martin Hertelb,
existing
products
for an
assembly
product
family
b
a
Uwe Füssel
Paul
Stief
*,
Jean-Yves
Dantan,
Alain
Etienne,
Ali Siadat
Technische Universität Ilmenau, Department of Mechanical
Engineering,
Production
Technology
Group, Ilmenau, Germany
b
Dresden University of Technology, Institute of Manufacturing Technology, Dresden, Germany
École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France
* Corresponding author. Tel.: +49-3677-693923 ; fax: +49-3677-691660. E-mail address: info.fertigungstechnik@tu-ilmenau.de
* Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: paul.stief@ensam.eu
Abstract
Abstract
Laser-based joining is a potential key manufacturing process for realizing metal plastic hybrids. Therefore, surface preparation is essential to
achieve a mechanical form fit. A new approach for surface preparation is based on a tungsten inert gas (TIG) arc process with anodic polarity.
In today’s business environment, the trend towards more product variety and customization is unbroken. Due to this development, the need of
This TIG structure was characterized compared to fiber laser manufactured structures in cw and pw mode. The comparison of different
agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production
preparations was based on pure ultimate tensile strength tests on spot joints. Finally, a transfer to overlap joints was successfully carried out and
systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to
characterized by tensile shear tests using fiber reinforced plastics.
analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and
© 2018
2018 The
The Authors.
Authors. Published
Published by
by Elsevier
Elsevier Ltd.
Ltd. This
This is
is an
an open
open access
access article
article under the
the CC
CC BY-NC-ND
BY-NC-ND license
license
©
nature
of components.
This fact impedes
an efficient
comparison
and choiceunder
of appropriate
product family combinations for the production
(http://creativecommons.org/licenses/by-nc-nd/3.0/)
(https://creativecommons.org/licenses/by-nc-nd/4.0/)
system.
A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster
Peer-review under
under responsibility
responsibility of
of the
the Bayerisches
Bayerisches Laserzentrum
Laserzentrum GmbH.
GmbH.
Peer-review
these
products in new
assembly oriented
product families for the optimization
of existing assembly lines and the creation of future reconfigurable
assembly
systems.
Based
on
Datum
Flow
Chain,
the
physical
structure
of
the
products is analyzed. Functional subassemblies are identified, and
Keywords: laser joining; laser welding; dissimilar materials; metal-plastic-hybrid joints; lightweight design; TIG welding; surface preparation; structured steel
a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the
similarity between product families by providing design support to both, production system planners and product designers. An illustrative
example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of
thyssenkrupp
Prestaand
France
is then
carried
of for
the proposed
approach.
1. Introduction
state
of the
art out to give a first industrial evaluation
applied
thermoplastics
transparent in laser wavelength. In
© 2017 The Authors. Published by Elsevier B.V.
comparison to transmission joining, heat conduction joining
Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018.
In relation to the reduction of CO2 emissions, light weight
covers a wider range of materials, e. g. fiber reinforced
constructions
are
gaining
importance,
especially
for
the
plastics (FRP) with high fiber volume content. Thereby, the
Keywords: Assembly; Design method; Family identification
automotive industry. Based on material combinations as
laser beam is focused on the metal surface as upper joining
plastic metal hybrids, the reduction of weight involves high
partner [1, 3]. Consequently, the heat is transferred through
potential to find use cases in numerous different applications.
the metal sheet into the plastic. The thermoplastic material
1.ToIntroduction
themolten,
productpenetrates
range and the
characteristics
manufactured
and/or
create plastic metal hybrids, thermal joining is a potential of
gets
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metal surface.
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Due to
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The useis ofnow
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an DIN
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existing product
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1910-3,
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zone products
depends toon
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It can be
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thatmaterial
classical
existing
radiation. in today’s market environments: a continuing
interaction
thickness
properties
tendency
towards
of product
times and
product
families
are regrouped
in function
of clients
or features.
Compared
to reduction
competitive
thermal development
joining processes,
the
like density,
thermal
conductivity,
specific
heat capacity
and
shortened
product
In addition,
therea islocally
an increasing
However,
oriented
advantages
of thelifecycles.
laser-based
joining are
limited
absorptionassembly
coefficient
[4]. product families are hardly to find.
demand
of customization,
being at the
same time
in a global
On
the literature,
product family
level,
products proved
differ mainly
in two
energy input
and the independence
of specimen
geometry
[1].
In the
several
publications
this behavior
In general, laser-based
thermalalljoining
can world.
be divided
heat
competition
with competitors
over the
This intrend,
main
characteristics:
(i) the
numberBesides
of components
andblasting
(ii) the
for different
structuring
processes.
corundum
conduction
joining the
and development
laser transmission
For
which
is inducing
from joining
macro [2-3].
to micro
[2, 5]
chemical treatment
[6], especially
continuous
wave
type
of or
components
(e.g. mechanical,
electrical,
electronical).
transmission
joining,
the laser beam
is transmitted
through the
(cw)
laser processing
[3, considering
7-9] and pulsed
laser
markets,
results
in diminished
lot sizes
due to augmenting
Classical
methodologies
mainlywave
single(pw)
products
plastic varieties
joining partner.
This process
variation
can only[1].
be
processing
used forproduct
surface pretreatment.
product
(high-volume
to low-volume
production)
or
solitary, [10-13]
alreadyareexisting
families analyze the
To cope with this augmenting variety as well as to be able to
product structure on a physical level (components level) which
2212-8271 possible
© 2018 Theoptimization
Authors. Published
by Elsevier
is an opencauses
access article
under theregarding
CC BY-NC-ND
license
identify
potentials
in Ltd.
the This
existing
difficulties
an efficient
definition and
(http://creativecommons.org/licenses/by-nc-nd/3.0/)
production
system, it is important to have a precise knowledge
comparison of different product families. Addressing this
Peer-review under responsibility of the Bayerisches Laserzentrum GmbH.
2212-8271 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(https://creativecommons.org/licenses/by-nc-nd/4.0/)
2212-8271
© 2017 The Authors. Published by Elsevier B.V.
Peer-review
under
responsibility
of scientific
the Bayerisches
Laserzentrum
GmbH.
Peer-review
under
responsibility
of the
committee
of the 28th CIRP
Design Conference 2018.
10.1016/j.procir.2018.08.135
Marie-Luise Kohl et al. / Procedia CIRP 74 (2018) 500–505
Author name / Procedia CIRP 00 (2018) 000–000
2
Comparing the joint strength for different types of surface
preparation on an exemplary joint (1.4301-PA6), the pw laser
treatment enables a higher level of tensile strength, e. g.
19 MPa [8] in contrast to competitively mechanical
preparations, like corundum blasting 14 MPa [5]. In addition,
anodizing preparation shows a comparable shear strength of
over 35 MPa [6]. Although, no generally valid correlation
between different types of surface preparation and joint
strength is given in the state of the art.
In addition to high tensile strength, a high reproducibility
and short processing times characterize the laser surface
treatment. But, a high area output requires a high invest for an
appropriate laser beam source. In contrast to laser-based
surface treatment, the surface preparation using tungsten inert
gas (TIG) arc process represents a novel approach for
realizing a high area output. Additionally, a further advantage
is generated, because the TIG preparation can be performed
by using a conventional welding machine as well as standard
welding torches. As a result, the experimental setup enables a
surface cleaning simultaneous to a surface structuring.
This paper is focused on the mechanical properties of
metal-FRP hybrid joints regarding tensile strength and it is
correlation to different metal surface pretreatments. Hence,
TIG arc-based surface preparation as well as laser structuring
processes with cw and pw operation mode were used to
structure the metal surface. In preliminary investigations,
characteristic values of those different structuring processes
were analyzed by materialographic cross-sections, laser
scanning microscopy (LSM) and scanning electron
microscopy (SEM) [14]. According to those characteristic
features, information about structure design (stochastically
distributed or recurring geometries), structure topography
(structure depth and width) as well as the area of undercuts
can be identified. The ultimate tensile strength was evaluated
by pure pull-off-tests on spot joints and the tensile shear
strength was examined based on overlap joint configuration.
The influence of the polymer base material was characterized
by using fiber reinforced as well as unreinforced polyamide as
plastic join partner.
Nomenclature
cw
FRP
pw
continuous wave
fiber reinforced plastics
pulsed wave
501
in spot joints was transferred to overlap configuration. Fig. 1
shows the schematically view of spot (Fig. 1 a) and overlap
joints (Fig. 1 b) as well as the force direction regarding to the
ultimate tensile or tensile shear strength. For the experimental
procedure, as metallic materials, the steel (1.0330) and a
galvanized steel (1.0322 Z100) were utilized for the
experimental investigation. The metal surfaces were cleaned
twice with isopropyl before the beginning of the structuring
process. Especially for spot joints, the metal joining partner
has a dimension of 75 x 75 x 1.5 mm³. The sheet size of
200 x 75 x 1.5 mm³ was chosen for overlap joint tests.
As thermoplastic joining partner, Polyamide 6 (PA 6) was
chosen for spot and overlap joints. In contrast to overlap joints
with a thermoplastic dimension of 200 x 75 x 2mm³, the
matrix material PA 6 was used for spot joint investigations as
rod (Ø = 10 mm, length = 75 mm). In addition to PA 6, fiberreinforced plastics (PA6GF50, PA6CF50) were applied only
for overlap joint experiments to investigate their influence on
tensile shear strength due to the fiber reinforcement based on
the penetration of structures.
Fig. 1. Schematic view of a) overlap joints and b) spot joints.
2.2. Surface preparation
For the surface preparation by a tungsten inert gas (TIG)
arc process, a partially automated setup was used. The
experimental setup consisted of a 6-axis robot, a vacuum
clamping device (dimension: 600 x 800 mm²) with an
integrated cooling device, an anodic polarization welding
torch and a conventional welding machine (EWM Tetrix). In
contrast to the welding torch, the metal sheet is in cathodic
polarized configuration. Table 1 depicts the constant and
variable TIG process parameters. As a result of TIG
treatment, the surface shows stochastically distributed
structures.
Table 1. Parameters of TIG arc surface treatment.
2. Material and experimental setup
2.1. Materials and joint configurations
The following investigations are divided in two tests with
different hybrid bond geometries. First, spot joints (Ø10 mm)
were used to realize pure pull-off tests to examine the ultimate
tensile strength of plastic metal hybrid joints. The spot joints
serve the fundamental research regarding to plastic metal
hybrid joints due to a structured metal partner. On the other
hand, tensile shear tests were implemented on overlap joints
(overlap area: 200 x 12.5 mm²). Overlap joints are highly
interesting for applications. As a result, the knowledge gained
constant parameters
variable parameters
arc length
3 mm
current rating
10 A – 70 A
distance gas jet
4.5 mm
inert gas
electrode diameter
4 mm
electrode angle
120 º
Ar
Ar + 30 % HeAr +
5 % H2
distance gas diameter
15 mm
process rate
2 mm/s – 250 mm/s
inert gas flow rate
15 l/min
The surface preparation by laser is used as reference to the
TIG structure process. Two fiber lasers with cw as well as pw
operation mode were used. In contrast to the stochastically
distributed TIG structures, the laser process generates
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Marie-Luise Kohl et al. / Procedia CIRP 74 (2018) 500–505
Author name / Procedia CIRP 00 (2018) 000–000
periodically recurring grooves in line shape. The laser
structure geometry was determined to lines with a distance of
120 µm.
The surface preparation, using the single mode fiber laser
(Trumpf TruFiber 400, λ = 1070 nm, Plaser max = 400 W,
Øfocus = 16.5 µm), is generated due to a laser beam power up
to 400 W and the number of repetitions (i = 1…6). Using the
nanosecond laser PowerLine F20 (Rofin) in pw mode, the key
parameters for surface preparation are the laser power (4 W to
20 W) and the pulse frequency (20 kHz to 80 kHz). The
further setup parameters are wavelength (λ = 1064 nm), pulse
duration (tpulse = 100 ns), pulse energy (Epulse = 0.95 mJ) and
focal diameter (Øfocus = 30 µm).
All created structures were analyzed using metallography,
laser scanning microscopy (LSM) and scanning electron
microscopy (SEM). The cross-sections provide twodimensional information about the presence of undercuts as
well as the structure depth and width in micrometer range. In
contrast, the results of LSM and SEM analyzing shows
detailed three-dimensional inspections which evince further
characterization features. For the following investigations the
cw structure with 400 W and 4 repetitions as well as the pw
structure with 20 W and 70 kHz were used independently
from the metal joining partner. In contrast, the TIG structure
changes dependent on the used metal.
3
2.4. Experimental setup - mechanical testing
The investigations on ultimate tensile strength (spot joints)
as well as the tensile shear strength (overlap joints) were
carried out on a universal testing machine (v = 10 mm/min)
with adapted test equipment (Fig 3 b). Both test devices allow
the evaluation of different surface treatments on joint
performance for the subjected tensile load respectively tensile
shear load. Furthermore, a comparison of the results
depending on the structure type within the spot joints
respectively the overlap joints is possible. The values of
ultimate tensile strength as well as tensile shear strength is
given by the nominal contact area between the structured
metal and the plastic joining partner (Aspot = 78.54 mm²,
Aoverlap = 312.5 mm²).
2.3. Thermal joining process based on diode laser
Laser-based heat conduction joining was carried out as
joining process. As laser beam source, the diode laser
Laserline LDM 1000 with a wavelength of 980 nm, a
maximum laser beam power of 1000 W and a focal diameter
5.3 mm (circular spot) was used. Two special clamping
devices were utilized for the joining process. Fig. 2 shows the
joining device for spot joints and Fig. 3 a for overlap joints.
Both devices enable the heat conduction joining as a laserbased thermal joining process.
Fig. 3. Experimental setup for a) overlap joint processes and b) the
mechanical testing device for pull-off-tests.
3. Results and discuss
3.1. Surface preparation by TIG arc and laser processing
For the characterization and the comparison of different
surface pretreatments, microsections as well as SEM and
LSM inspections were used to provide information of the
structured area. The surface treatment by a TIG arc process is
given by the influence of selected current, travel speed and the
composition of shielding gas.
Fig. 2. Experimental setup for spot joint processes.
Both setups enable the correct alignment of the joining
partners to create a spot joint (Ø10 mm) respectively an
overlap joint (200 x 12.5 mm²). In addition, the experimental
setups (see also [15] for overlap joints) allow to vary the
joining pressure. A force is applied due to a specific mass
holder (Fig. 2) for spot joints and clamping jaws (Fig. 3 a) for
overlap joints which results in static pressure
(poverlap = 0.4 N/mm²).
Fig. 4. Examples of TIG structures dependent on the selected structuring
process parameter.
A variation of different process parameter combinations as
well as the used metals results in different surface
modifications (Fig. 4). In general, the TIG arc surface
preparation depicts a stochastic distribution of grooves in subµm range which results in a limited surface enlargement.
4
Marie-Luise Kohl et al. / Procedia CIRP 74 (2018) 500–505
Author name / Procedia CIRP 00 (2018) 000–000
Fig. 5 shows a comparison of TIG arc and laser structured
surfaces. In contrast to TIG arc structures, the laser structure
processing obtained a recurring pattern of line-shaped grooves
which shows a high homogeneity and reproducibility.
503
value of 180 mm²/s (400 W, i = 4). This illustrates the high
potential of a TIG structuring process for a cost-efficient
surface pretreatment. Even though, the ultimate strength of the
TIG processed specimen is lower compared to laser, which
will be explained afterwards.
3.2. Fracture behavior of spot joints
Fig. 5. Examples of microsections, scanning electron microscope (SEM) as
well as laser-scanning-microscopy (LSM) inspections of TIG and laser
structured surfaces.
When comparing different structures, pw structures appear
wide and flat without undercuts for low pulse rates and high
beam powers. In contrast, a high laser beam power and a high
pulse rate generates a high number of undercuts, based on a
melt pool ejection (Fig. 5). Whereas, cw structure processing
generates deep and slim line-shaped grooves. Especially for
cw laser structure, the structure depth was increased due to an
increase of repetitions. The number of repetitions for cw laser
structure was limited to six, because the grooves were closed
by ejected material due of the melt pool movement. This is
comparable to the literature [3, 16].
The cohesive fracture behavior was calculated by image
processing and reached about 85 % for laser structures (Fig.
7). As basic material, LSM sections were used. The sections
were binary coded in black and white segments. The black
segment corresponds to thermoplastic residues on the
structured surface as an indicator for cohesive fracture. The
white segments define the adhesive fracture. In these areas, no
residues of polyamide were identified. The given
microsections represent typical areas with adhesive and
cohesive fracture to provide a further visualization of the
failure mechanisms. TIG structured joints show a significantly
higher percentage of adhesive fracture about 90 % in total.
This fact is traced back to the structure pattern. Laser
structures generate undercuts, deep structure depths and
widths due to line-shape. Whereas, TIG preparation depicts a
stochastic structure pattern without undercuts.
Fig. 7. Analysis of the fracture behavior of hybrid spot joints (1.0330–PA 6).
Fig. 6. Ultimate tensile strength of spot joints compared to the area output of
TIG (10A, 6mm/s, Ar) and laser structures (cw: 400W, i = 4; pw: 20W,
70kHz) (1.0330–PA6)
The structured area per time represents a further
characterization point. This parameter gives information about
the efficiency and the cycle time. Fig. 6 depicts the results of
ultimate tensile strength due to pull-off tests compared to the
area output for best parameters in surface pretreatment. In
general, the ultimate tensile strength increases due to a surface
treatment up to factor 1.6 for TIG structure, 4.9 for cw
structure and 6.9 for pw structure. In contrast to surface
preparation by laser, the TIG structuring process shows a
higher area output. For example, the area output value of
1,800 mm²/s (TIG arc process) is 10 times higher, in
comparison to the used laser cw structure processing with a
3.3. Transfer of the results of spot joints and ultimate tensile
strength to overlap joints and tensile shear strength
The knowledge and results about structure characterization
in combination with ultimate tensile strength values, emerged
from the preliminary studies, were transferred to possible
applications. For these investigations, overlap joints were
produced, using PA 6 as well as fiber reinforced polyamide
(PA6GF50). In general, the surface treatment generates an
increase of tensile shear strength for all hybrid joints. Fig. 8
depicts a comparison of the strength results depending on the
structure type, based on spot and overlap joints.
The results of spot joint investigations, e. g. for the
material combination 1.0330–PA 6, show an increase of
ultimate tensile strength for TIG as well as laser structures,
compared to the unstructured reference (Fig. 8 a). The value
of TIG structured joint is up to 2.64 ± 0.28 MPa. In contrast,
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Marie-Luise Kohl et al. / Procedia CIRP 74 (2018) 500–505
Author name / Procedia CIRP 00 (2018) 000–000
the increase in ultimate tensile strength of cw and pw laser
structures is higher. The cw structure is about
7.87 ± 0.36 MPa and pw structure about 11.13 ± 0.46 MPa.
Compared to the unstructured reference, the tensile shear
strength of overlap joints increases only for pw and TIG
structures due to the material combination 1.0330–PA6GF50.
The resulting values of tensile shear strength for overlap joints
are 6.34 ± 2.44 MPa (cw), 16.09 ± 0.98 MPa (pw) and
3.25 ± 0.69 MPa (TIG). The standard deviation is higher for
all structure types for overlap joints compared to spot joints.
This is based on a larger contact area (312.5 mm²) and the use
of fiber reinforced polyamide. In general, the fiber orientation
effects the locational fracture behavior of the joint.
On the one hand, vertical fibers can prevent the penetration
of the matrix material PA 6 into the structure depth.
Especially for cw structure, the presence of fibers produced no
significant improvement due to slim and deep structure lines.
Independent from orientation, fibers cannot penetrate
relatively narrow cw structures.
On the other hand, parallel oriented fibers penetrate wide
structures and improve the tensile shear strength, e. g. for
laser structures (Fig. 10 b). In contrast to spot joints, the
tensile shear strength of pw structured overlap joints is 1.5
times higher due to the penetration of fibers into wide and flat
structures compared to the use of unreinforced PA 6 (Fig. 8).
The comparison of the TIG structure shows a low increase
of factor 1.2 with an increase of the standard deviation. The
high standard deviation is based on the stochastic structure
contact area. Because of missing deep grooves due to
nanoscale structure pattern, a penetration of fibers is not
possible.
Fig. 8. Ultimate tensile strength (spot joints, 1.0330–PA6) and tensile shear
strength (overlap joints, 1.0330–PA6GF50).
5
Fig. 9. SEM inspections of TIG structured surfaces (1.0322 Z100).
Fig. 10 a shows the results of tensile shear tests for the
material combination 1.0322 Z100–PA6 and 1.0322 Z100–
PA6GF50. For example, the tensile shear strength of
unstructured 1.0322 Z100 and PA 6 (2.06 ± 0.12 MPa) as well
as PA6GF50 (3.12 ± 0.31 MPa) increases up to
6.25 ± 0.6 MPa (1.0322 Z100–PA6) and 12.38 ± 0.85 MPa
(1.0322 Z100–PA6GF50), dependent on the TIG structure
with 20 A, 3 mm/s and argon as shielding gas. The cw
structure shows a value of 7.9 ± 0.23 MPa (1.0322 Z100–
PA6) and 17.39 ± 0.67 MPa (1.0322 Z100–PA6GF50). For
pw structures, a tensile shear strength of 7.82 ± 0.17 MPa
(1.0322 Z100–PA6) and 17.39 ± 0.81 MPa (1.0322 Z100–
PA6GF50) was measured. In this regard, all values of tensile
shear strength exhibit a negligibly small standard deviation
due to a recurring structure pattern (max. standard deviation:
± 0.81 MPa).
3.4. Tensile shear strength of overlap joints using fiber
reinforced as well as unreinforced polyamide
Fig. 10. a) Tensile shear strength of TIG and laser structured hybrid joints
(1.0322 Z100–PA6GF50), b) comparison of fiber penetration (cw: 400W,
i=4; pw: 20W, 70kHz; TIG: 20A, 3mm/s, Ar) (1.0322 Z100–PACF50) and c)
microsections of hybrid joints using fiber reinforced as well as unreinforced
polyamide (1.0330–PA 6 / 1.0322 Z100–PA6CF50).
Further investigations on overlap joints were implemented
using unreinforced PA 6 in contrast to PA6GF50 and
PA6CF50. Using the galvanized steel 1.0322 Z100, the
presence of a zinc oxide layer benefits the increase of tensile
shear strength according to [17]. In SEM investigations, the
surface shows a higher number of filigree undercuts and a
more rugged surface compared to uncoated steel (Fig. 9). This
implies a combined effect of the physio-chemical interactions
based on the oxide layer and the form fit by surface treatment.
Independently from the material combination, structures
are completely filled with polyamide for all investigated
surface pretreatments. Fig. 10 b depicts three detailed
structures and Fig. 10 c two examples of hybrid joints of
different material combinations (1.0330–PA 6 / 1.0322 Z100–
PACF50). In addition to the structure filling, the
microsections shows scattered bubble formation, using
unreinforced as well as fiber reinforced polyamide.
Furthermore, according to the thermal joining process, a
6
Marie-Luise Kohl et al. / Procedia CIRP 74 (2018) 500–505
Author name / Procedia CIRP 00 (2018) 000–000
damage of the zinc layer on the galvanized metal
(1.0322 Z100) by the laser beam was not observed within
metallographic investigations.
In contrast to joints between steel and unreinforced PA 6,
the use of steel-FRP must be differentiated in two parts –
vertical and parallel course of fibers. Parallel oriented fibers
support the hybrid joint due to a penetration into the structure
depth (Fig. 10 b, c). Consequently, FRP enable an increase of
tensile shear strength, e. g. up to factor 5.6 for laser and factor
4 for TIG structures. This corresponds for all structure types
to a value doubling, compared to the use of the matrix
material PA 6. However, the penetration of fibers depends on
the surface treatment. Structure processes with cw as well as
pw laser beam present wide grooves which facilitate the fiber
penetration.
4. Conclusions
This paper is focused on TIG- and laser-based surface
preparation for steel-PA 6 joints. The different surface
pretreatments were characterized by metallography, LSM and
SEM. A further comparison was based on spot joints
regarding ultimate tensile strength and fracture behavior. In
order to address engineering applications, the results were
transferred from spot joints to overlap joints.
An evaluation of tensile shear strength was carried out on
overlap joints with unreinforced as well as fiber reinforced
PA 6. Therefore, the results were compared based on the
structure type and the material combination. The use of FRP
increases the bonding strength significantly and is highly
supported by the penetration of fibers into wide structures.
In summary, the surface pretreatment generates an increase
of ultimate tensile strength as well as tensile shear strength.
The value of mechanical properties depends on the used
structuring process, the material combination and the joining
parameters. In contrast to TIG structures, laser manufactured
structures ensured a high level of ultimate tensile strength,
independent of the used metal.
In addition to the current TIG structuring process, different
shielding gases with increased oxygen percentage will be used
to support the formation of oxide layers to improve wetting
behavior and the firmly bonding between both joining
partners. Furthermore, an increased mechanical interlocking
could be given due to the nanoporous oxide surface [16].
Moreover, further investigations on tensile peel strength will
provide results for the comparison of different surface
pretreatments.
Acknowledgements
The investigations were carried out within the project "TIG
based surface texturing of steel and laser-based joining with
fibre reinforced plastics" from the research association for
steel application (FOSTA, Research Association for Steel
Application). The research project (IGF-Nr. 19042 BR) is
supported by the federal ministry of economic affairs and
energy within the German Federation of Industrial Research
505
Associations (AiF - Arbeitsgemeinschaft industrieller
Forschungsvereinigungen „Otto von Guericke“ e. V.) which is
based on a resolution of the German parliament. We would
like to thank all funding organizations as well as the project
support committee led by Mr. Dipl.-Ing. Rainer Salomon
(FOSTA).
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