Bulle tin of the Transilvania University of Brașov Vol. 10 (59) No. 1 – 2017 [602848]

Bulle tin of the Transilvania University of Brașov • Vol. 10 (59) No. 1 – 2017
Series I: Engineering Sciences

OVERVIEW OF JOINING DISSIMILAR
MATERIALS: METALS AND POLYMERS

E. MOLDOVAN1 M.H. TIEREAN1 E.M. STANCIU1

Abstract: The increasing demand in manufacturing of hybrid structures and
components for engineering applications has determined a growing interest in
joining of dissimilar materials. This literature overview could serve to further
understand the processes involved and how to optimize the techniques for
obtaining enhanced metal to -polymer hybrid joints. Using the laser welding
technology, different types of joints could be produced , that are difficult to be
achiev ed by conventional joining methods , such as adhesive -bonding,
mechanical fastening and thermal p ressing.

Key words: joining, dissimilar materials, polymers, metals, laser welding.

1. Introduction

The need to combine the toughness of the metal with the chemical resistance and
lightweight of the polymer requires the research of the dissimilar joining of metals with
polymers. The main advantage of this kind of dissimilar joining is the enhancing of th e
design flexibility, using efficient ly the main properties of each material [ 11]. These joining
methods are: mechanical fastening, adhesive bonding and welding [ 6].
The aim of this paper is to analyse and summarize the results from the reference literature
concerning different techniques for joining of dissimilar materials, with emphasis on
polymers and metals. This paper could serve to better understanding of actual processe s
involved in dissimilar joining between materials like stainless steel, aluminium,
magnesium and polymers.

2. Mechanical fastening and adhesive bonding

In a complex study, Sadowski T. et al. tested the aluminium -polyurethane bonding
fastened with rivets [14]. The experimental results were compared with ABAQUS
simulation. The n umber of rivets was varied between 1 and 9; the higher bearing capacity
was found for 5 rivets. The best results w ere obtained for hybrid joints, their tensile strength
being by 11% higher than analogous bonded joint and by 130% higher than mechanical
fastening (Fig. 1). The energy absorption during the failure process increased with 64%
compared with the adhesive joint, due to addition of the rivets to the adhesive bonded join t.
Another mechanical joining method was tested by Paul, H. et al. [12], respectively j oining
of polyamide reinforced with 40 % glass fibers (PA6.6 GF40) with HC420LA car body

1 Department of Materials Engineering and Welding , Transilvania University of Brașov.

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Fig. 1. Comparative tensile stress test results for different hybrid joining methods [14]

steel. Three different joining mechanisms were observ ed during testing : force fit, direct
adhesion and form fit. The results show that b lasting with corundum of the metal surface
and the form fit ( ϕ6 mm hole in the metal) increased the mechanical properties of the
joining.

3. Surface functionalisation

The surface treatment of metallic materials could improve adhesion and welding strength
of metal -polymers dissimilar joints. Shimamoto K. et al. [16] analysed the influence of
surface preparation on the results of double cantilever beam test of dissimilar welding
between glass fiber reinforced polypropylene and A6061 aluminium alloy. The samples
surfaces were prepared by sandblasting (#120 Al 2O3 grit) and chemical etching
(AMALPHA treatment, propri etary technology of MEC Co. Ltd., Hyogo, Japan). After
preparation, the welding process steps were as follows : compression with 0.1 MPa, heating
from the aluminium side, cooling in the hot-press machine , removing of pression. The
results show a critical energy release rate for the chemical etched specimens four times
higher than those of specimens prepared by sandblasting . The results were not significant
modified by increasing of the AMALPHA treatment depth from 2 m to 15 m.
Further improvement of the mechanical properties of metal –composite joints was
investigated by Nguyen A.T.T. et al. [ 10]. He made four types of hemi -spherical dimples
and semi -cylindrical grooves on the metal surface (Fig. 2) using Selective Laser Melting
(SLM) method. Titanium was used as metallic material , co-cured to carbon fib er reinforced
epoxy composite (T700 carbon/epoxy unidirectional prepreg plies). The investigation was
based on mode I fracture toughness test. It was observed that interface strength of the hybr id
structure is higher than interlaminar composite strength, the crack front being deflected
from the metal –composite interface into the composite material. The best results were
obtained for th e outward groove feature, which increased the fracture toughness of the joint

Moldovan , E., et al.: Overview of joining dissimilar materials: metals and polymers 3

Fig. 2. Surface maximization using macro -features: (a) outward dimple;
(b) inward dimple;(c) outward groove;(d) inward groove [10]

by up to 50%, compared with composite material . This enhancing is due to increasi ng of
the crack path length and shifting from pure mode I to mixed -mode crack.
With the same goal, Salstela, J. et al. [ 15] investigated the influence of the microscale
structures silanization and oxygen plasma treatment on the aluminium/epoxy adhesion. The
micro -mesh printing with a hydraulic press was used for microstructuring, creating on the
aluminium surfaces mesh sizes of 100 m, 200 m and 400 m. Hierarchical
microstructures were obtain ed onto aluminium using different combinations of micro -mesh
printing and sandblasting. The micro -mesh structures improved the shear strength by 33%
and the hierarchical micro -microstructures by 116%, compared to the untreated surface.
The 20% O 2 plasma improve d the shear strength with 24%, compared to the untreated
surface. The shear strength was not significant influenced by the oxygen contents of the
plasma. Silanization with 3 -glycidoxypropyltrimethoxysilane improv es adhesion up to
68%, the lo w silane content giving the highest strength.

4. Welding

Friction stir spot welding is a useful method for joining o f dissimilar materials [ 9].
Goushegir S.M. et al. [3] tested the influence of the friction spot welding parameters on
metal/composite joining. The metal (AA2024 -T3) samples were softly dry ground using
P1200 SiC paper and sandblasted prior to welding. Both the metal and the composite
samples were cleaned in pressurized air, washed in acetone ultrasonic bath and air dried.
The samples were welded using rotational speeds in the range of 1000 –2900 rpm , plunge

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depth of 0.5–0.8 mm and joining force between 6.8–13.8 kN . The microscopic inspection
reveals consolidated polymer in the aluminium surface pores and carbon fibers embedded
in the deformed aluminium . Increasing of the rotational speed determines the enlargement
of the joining area, which has as consequence the gr owth of the lap shear force and larger
displacement. Increasing of the tool plunge depth improves the micro -mechanical
interlocking of the dissimilar materials.
Liu F.C. et al. investigated the influences of friction lap welding parameters on bubbles
and s hear strength [ 8]. The tested materials were AA6061 aluminium alloy and MC Nylon –
6. Unlike friction stir welding, the friction lap welding does not have a stir pin and the main
function of the rotation is to press and heat up the metal part, not to flow materials around
the stir pin [8]. It was observed a linear relationship between the friction lap welding
parameters (rotation rate)2/welding speed and the thickness of melted nylon. The increasing
of the rotation rate bends the aluminium plate and increas es the thermal input, resulting in
decreasing of the bubbles volume and enhancement of the joint hear strength. The
increasing of the welding speed reduces the thermal input and also the bubbles volume.
Katayama & Kawahito developed a rapid laser technique for dissimilar joining of metal –
polymer without adhesives or glues [4]. The joints were realized between 2 mm thick PET
sheet, overlapped with 30 mm on AISI 304 stainless steel plate, thickness 3 mm (Fig. 3).
Because of the 90 % transparency of PET, the steel surface is heated by laser beam, that
melts and partly decomposes the PET. As consequence of the occurrence of these pyrolysis
gases, m any submillimetre bubbles are formed inside the plastic near the joint. In TEM
characterization of the samples revealed a tightly bonding at atomic , molecular and
nanostructural level through the superficial Cr oxide film of the stainless steel . This method
was named Laser Assisted Metal and Plastic joining method (LAMP) [ 5].

Fig. 3. Linear diode laser beam direct joining of metal and plastic and the formation of
bubbles [ 4]

The LAMP method was also used by Lambiase & Genna to join 1 mm thick AISI 304
stainless steel with 2 mm thick polycarbonate using a 200 W diode laser [7]. The main
joining defect, the tunnel defect (complete detachment of the materials), was obtained due
to coalescence of bubbles caused by gas expansion at the PC –steel interface when linear
energy was high. Increasing of the linear energy leads to increasing of the bonded area and
also growth of the bubbles dimension. Best joining shear strength (3.7 MPa) was obtained
for linear energies between 36 and 40 J/mm, that created temperatures of 200 – 230 °C.

Moldovan , E., et al.: Overview of joining dissimilar materials: metals and polymers 5

Fig. 4. Direct laser joining setup used for AISI 204/PA66 welding [1]

Another experiment al study was performed by Fortunato A. and team [1]. The chosen
materials for joining were stainless steel AISI 304 sheet, 1 mm thickness, in overlap with
3 different polymers, PA66, PA66 with 35% glass fiber reinforced and PA66 with 40%
carbon reinforced fiber, all the polymer thickness having 2 mm thickness. For testing was
used a 100 W diode laser assisted by inert gas (Fig. 4) . The results of the tensile test show
a good behaviour of the joining, the failure being in the heat affected zone. There were not
recorded any significant variations of the tensile stress modifying the welding speed for
non-reinforced opaque PA66. The tensile stress of the transparent PA66 is directly
influenced by the welding speed.
Wahba M. et al. used the LAMP technique to join AZ91D magnesium alloy with
PET using 3 kW diode laser [18]. The irradi ation was done from the metal or from the PET
side, using different laser powers. In the last case, the authors also studied the influence on
metal functionalizing, by melting the surface to create few pores near the surface and then
ground to open the pores. The authors concluded that metal -side laser irradiation is better
than plastic -side irradiation, in terms of joint strength, caused by different morphology:
discrete bubbles in the first case vs. wormhole in the second one. This wormhole network
was ex plained by the multiple reflection of the laser beam in the case of plastic -side
irradiation (Fig. 5). Surface functionalizing improves the join strength.

Fig.5. Mechanism of bubbles coalescence for the plastic -side irradiation [18]

Roesner A. et al. studied the effect of surface functionalisation on laser and induction
welding of AISI 304 stainless steel with different type of polymers [13]. The
microstructuring of the surface is the result of both sublimation and melting and the
evaporation pressure generated by laser beam eject s the surrounding melt towards the
specimen surface (Fig. 6) .

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Fig. 6. a) Principle of the surface microstructuring ; b) cross section of the joining [13]

A pulsed Nd:YAG disc laser was used f or surface microstructuring (P = 40 W, F = 22.5
kHz, S = 450 mm/s). The dimensions of the grows were approximately 40 m width and
50 m depth. The laser welding was done by metal -side irradiation, with a diode laser
system at 250 W a nd 800 mm/s speed. Three types of plastics were used : polycarbonate,
polyamide and glass fiber reinforced polyamide. The induction welding was performed
with an induction coil on the metal part, for stainless steel with two types of plastics:
polyamide and glass fiber reinforced polyamide. Using PA66, t he best results in terms of
shear stress were obtained for laser welding of the structures samples (16 -17 MPa), followed
by induction joining on the structured samples and induction joining of untreated samples.
For PA66 GF30, t he best results were obtained by induction joining of the structures samples
(20-23 MPa), followed by laser welding on the structured samples. The shear strength for the
laser welding of the PC structured samples was 14 MPa. For the laser transmissi on joining,
the shear strength is raised with the increasing of the structure density.
Fuchs A.N. et al. applied both surface functionalisation and friction press joining for
welding of 6082 aluminium alloy with glass fiber reinforced polyamide [2]. Fricti on press
joining method is similar with the friction lap welding. Macroscopic surface
functionalisation was done using the Surfi -Sculpt method. The laser beam generates a melt
pool at the surface. The pins are created based on the humping phenomenon, melt being
accelerated in the opposite direction of the laser motion (Fig. 7) .

Fig. 7. a) Principle of the Surfi -Sculpt method ; b) microscopy of the tapered pins [2]

Microscopic surface functionalisation was done using the Remote Ablation Cutting
(RAC) method. The laser bea m attacks with high velocity the surface, partially evaporating
the material. Because the vapor pressure ejects the surrounding melt, there are generated
superficial kerfs. For welding was use d a ϕ15 mm tool moved with 800 rpm a t 240 mm/min
feed rate. The axial force was between 1.5 kN and 2.0 kN. The test results show the
decreasing of the shear strength with increasing of the macroscopic pin height, that

Moldovan , E., et al.: Overview of joining dissimilar materials: metals and polymers 7
weakened the joint by the larger notch effect. Tapering of the pins increas es the sh ear
strength. By increasing the laser power for RAC were generated deeper kerfs and larger
undercuts, resulting in higher shear strengths. The best results were obtained for 900 W.
Van der Straeten K. et al. studied both surface functionalisation (con-like protrusion, Fig.
8) and laser joining of AISI 304 austenitic stainless steel with PA6 having 47% vol. glass
fiber reinforcement . For surface functionalisation through ablation was used a n ultrashort
pulsed laser (P = 50 W, F = 800 k Hz) and the welding process was carried out using a
continuous laser (P = 300 W).

Fig. 8. a) Steps of the laser based metal -polymer connection using surface
microstructuring with con -like protrusions ; b) SEM of the metal surface [17]

After melting , the polymer was pressed (4 bars) against the functionalised surface using
a pneumatic clamping device. The bonding strength was greater than 25 MPa, higher than
adhesive bonding and other laser welding methods.

5. Conclusion s

Nowadays, different methods are available for joining of dissimilar materials. Surface
functionalization combined with laser processing can produce high quality dissimilar joints
between metals and polymers strong bonded at the molecular level.
Using a laser welding system with different parameters provides the possibility to
improve the adhesion and joint strength of hybrid materials . Formations of bubbles or other
micro structuring on the metal part are powerful properties to performance a strong bond
between polymers and metals.

References

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