Friction Spot Joining of Aluminum AA2024 to Carbon-Fiber Reinforced Poly(phenylene sulfide)composite single lap joints--- microstructure and mechanical performance.pdf

Accepted Manuscript

Friction Spot Joining of Aluminum AA2024 / Carbon-Fiber Reinforced

Poly(phenylene sulfide)composite single lap joints: microstructure and me‐

chanical performance

S.M. Goushegir, J.F. dos Santos, S.T. Amancio-Filho

PII: S0261-3069(13)00778-4

DOI: http://dx.doi.org/10.1016/j.matdes.2013.08.034

Reference: JMAD 5757

To appear in: Materials and Design

Received Date: 12 June 2013

Accepted Date: 10 August 2013

Please cite this article as: Goushegir, S.M., dos Santos, J.F., Amancio-Filho, S.T., Friction Spot Joining of Aluminum

AA2024 / Carbon-Fiber Reinforced Poly(phenylene sulfide)composite single lap joints: microstructure and

mechanical performance, Materials and Design (2013), doi: http://dx.doi.org/10.1016/j.matdes.2013.08.034

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Friction Spot Joining of Aluminum AA2024 / Carbon-Fiber Reinforced

Poly(phenylene sulfide)composite single lap joints: microstructure

and mechanical performance

S.M. Goushegira,b, J.F. dos Santosa, and S.T. Amancio-Filhoa,b,*

a Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Materials Mechanics,

Solid State Joining Processes (WMP) – Max Planck Strasse 1, D-21502 Geesthacht,

Germany

b Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Materials Mechanics,

Advanced Polymer-Metal Hybrid Structures Group – Max Planck Strasse 1, D-21502

Geesthacht, Germany

* Corresponding author. Email address: [email protected]. Postal address:

Sergio de Traglia Amancio-Filho, Max-Planck Strasse 1, D-21502 Geesthacht,

Germany. Tel.: +49 4152 87 2066; fax: +49 4152 87 2033.

Abstract

Friction Spot Joining is a promising alternative joining technology for polymer-metal hybrid

structures. In this work, the feasibility of friction spot joining of aluminum AA2024-T3 (bare &

alclad) / carbon-fiber reinforced poly(phenylene sulfide) is reported. The process temperature

and the microstructure of the joints were investigated. Lap shear tensile strength as high as 27

MPa was achieved by using aluminum bare specimens. Sand blasting was also performed as

an effective mechanical surface pre-treatment on aluminum surfaces, which resulted in higher

surface roughness and accordingly improved mechanical performance for the selected

conditions. In addition, the alclad specimens exhibited promising mechanical performance (lap

shear strength of up to 43 MPa) that justifies further investigations. Finally, the bonding and

failure mechanisms of the joints are briefly discussed.

Keywords: Friction Spot Joining, Hybrid structure, Aluminum alloy, Polymer composite, Carbon-

fiber reinforced poly(phenylene sulfide), Surface pre-treatment.

2

1. Introduction

Current economic and environmental policies have urged manufacturers, especially those in

transportation industries, to design lightweight structures, with the goals of reducing CO2

emissions and fuel consumption. To fulfill these goals, fiber-reinforced plastics (FRP) and

lightweight alloys, such as aluminum, titanium and magnesium, are being used extensively in

the new generation of aircraft and automobiles [1]. For instance, in the new Airbus A350 XWB

passenger aircraft, over 50% of the total weight will be composed of polymer composites [2].

The design of such large complex engineering structures composed of a combination of

dissimilar materials requires advanced and alternative joining technologies. Amancio and dos

Santos [3] categorized different joining technologies for polymer-metal hybrid structures, ranging

from more conventional adhesive bonding and mechanical fastening to new welding-based

technologies, such as ultrasonic welding and induction welding. Technological limitations, such

as the requirements of extensive surface treatments and long curing times in the case of

adhesive bonding as well as the stress concentration and weight penalty related to mechanical

fastening, have motivated the recent investigations on new joining techniques.

Abibe et al. [4] investigated the mechanical and failure behavior of polymer-metal joints using

injection clinching joining, a technology based on staking and mechanical fastening. The

feasibility of joining glass-fiber-reinforced thermoplastic composites with titanium grade 2 using

friction riveting was investigated by Blaga et al. [5]. Balle et al. [6, 7] investigated the feasibility

and mechanical properties of ultrasonic spot welding of aluminum / carbon-fiber reinforced

polymers (CFRPs). They demonstrated the possibility of joining AA1050 (1 mm) as well as

AA5754 (1 mm) with CF-PA66 (2 mm). These authors used a statistical model to optimize the

process parameters for the ultrasonic spot welding of AA5754 / CF-PA66 and achieved a quasi-

static tensile shear strength of up to 33 MPa [8]. The authors also investigated the joinability of

AA2024 / CF-PA66 using different post heat treatments, which resulted in joints with a shear

strength as high as 58 MPa [9]. J.P. Bergmann and M. Stambke [10] investigated the joinability

of steel (DC01) and PA66 using a laser as the heating source and evaluated the influence of

process parameters and surface conditions on the mechanical performance of the joints. The

induction welding of metals / CFRPs was investigated and reported by Mitschang et al. [11].

These authors demonstrated the feasibility of induction welding of steel DC01 and AlMg3 with

CF-PA66 as well as the effect of different metal surface pre-treatments on the joint shear

strength.

Friction Spot Joining (FSpJ) is a new technology based on the Friction Spot Welding (FSpW) of

metals [12], developed at Helmholtz-Zentrum Geesthacht in Germany [13]; the technology is

suitable for joining polymer-metal hybrid structures. The feasibility of this technique was recently

demonstrated by Amancio et al. [14] for joining magnesium-fiber reinforced polymer composites.

In the present work, the feasibility of FSpJ of aluminum alloy AA2024 with a carbon-fiber-

reinforced polymer composite was studied. In this paper, the principles of FSpJ, selected results

on the microstructure features, temperature measurements and the mechanical performance of

the joints determined using the lap shear tensile test are presented.

3

2. Principles of the Friction Spot Joining (FSpJ) technique

In the FSpJ process, similar to FSpW of metals, a non-consumable tool consisting of three parts

is used to generate frictional heat. The tool includes a clamping ring, sleeve and pin, which are

mounted coaxially and can be moved independently of each other (Figure 1). The clamping ring

is the external component of the tool used to hold together the parts to be joined during process.

The pin and sleeve can rotate independently and produce the required heat as a result of the

friction between these parts and the workpiece.

[Figure 1]

First, the joining partners are clamped together in an overlapping configuration, and the tool

approaches the top sheet (in this case, a metal alloy). There are two possible variants for the

FSpJ technique: “Pin Plunge” and “Sleeve Plunge”. In the sleeve plunge variant, the rotating

sleeve plunges into the metallic sheet to a pre-defined depth, while the pin retracts upwards.

Due to the friction between the sleeve and the metal, the temperature rises locally to below the

melting point of the metal, thereby causing local softening and plasticizing of the alloy. The

plasticized metal alloy flows in the reservoir left behind by the retraction of the pin (Figure 2-1).

In the second step, the pin is pushed against the softened metal to refill the key-hole in the

metallic sheet (Figure 2-2). Finally, the tool is retracted and the joint consolidates under

pressure (Figure 2-3). Note that the tool plunges into the metal part to a shallow depth that does

not reach the composite interface to avoid any damage to the load bearing network of fibers.

[Figure 2]

During the joining process, the plasticized metal is deformed by the plunging motion of the tool

and creates a geometrical undercut in the form of a “metallic nub”. The metallic nub is slightly

inserted into the composite and increases the mechanical interlocking between the joining

partners. Simultaneously, frictional heat is transferred from the metal alloy to the composite

interface via conduction, generating a thin layer of the molten polymer in the spot area that

spreads throughout the overlap region. The molten layer is then consolidated under pressure

and induces adhesion forces between the metal and the composite. Therefore, “mechanical

interlocking” and “adhesion forces” are the primary bonding mechanisms in the Friction Spot

(FSp) joints.

There are two main differences between FSpJ and FSpW. First, in contrast to FSpW of metals,

the tool plunges shallow (up to maximum 40% of metal thickness) just in the metallic partner

and does not reach the polymer (or composite) in FSpJ. The reason is to avoid excessive

degradation of the polymer and damages to the network of the load bearing fibers. Second, in

FSpJ, adhesion forces, due to the creation of the molten layer, act as one of the main bonding

mechanisms. Moreover, in FSpJ, materials mixing does not happen due to the huge physico-

chemical differences of metal and polymer partners which usually lead to sharp interfaces.

4

3. Materials and Methods

3.1 Aluminum 2024-T3

Two-millimeter-thick rolled AA2024 aluminum bare sheets in condition T3 (supplied by

Constellium, France) were used in this work, due to their lightweight characteristic and good

mechanical properties. AA2024 is a major alloy in many parts used in airplanes, such as the

fuselage, due to its high strength (up to 435 MPa in T3 condition), good fatigue resistance and

fracture toughness [15]. This alloy belongs to the 2xxx precipitation hardening aluminum alloys

that consist of two major alloying elements. Copper is the primary element (in the range of 3 - 6

wt%) and provides high strength to the alloy, although it degrades the corrosion resistance

because it tends to precipitate in the grain boundaries. This alloy is susceptible to intergranular-

and stress-corrosion as well as pitting. Therefore, it requires barrier coatings, such as the use of

a cladding layer. Magnesium is often added to increase the tensile strength and the natural

aging [15]. Table 1 lists some of the primary mechanical and physical properties of the AA2024-

T3 aluminum used in this work.

In addition to the bare sheets, AA2024-T3 in alclad condition (supplied by Constellium, France)

was also preliminary tested to analyze the effect of the cladded layer on the joinability of this

alloy. Alclad aluminum consists of a core aluminum alloy to which, either on one or both sides,

is metallurgically bonded a sheet of pure aluminum or another aluminum alloy. The coating

thickness is approximately 5% of the nominal thickness of the sheet, and it should be anodic to

the core aluminum alloy, thereby protecting the core against corrosion. In the case of AA2024-

T3, the cladding layer is usually pure aluminum, which is primarily applied by the hot rolling

process [15]. The thickness of the cladded layer in the AA2024-T3 used in this work is

approximately 100 µm.

[Table 1]

3.2 Carbon-fiber reinforced poly(phenylene sulfide)

2.17-millimeter-thick carbon-fiber reinforced poly(phenylene sulfide), CF-PPS, consisting of 5

harness woven quasi-isotropic laminates (from Tencate, the Netherlands) with 50 vol% fibers in

the [(0,90)/(±45)]3/(0,90) sequence (7 plies) were used as the partner for the spot joints. Some

of the primary physical and mechanical properties of CF-PPS are listed in Table 2. This

composite was chosen as one of the base materials due to its outstanding fracture toughness

and chemical resistance, which enables its use in some typical applications in primary and

secondary aircraft structures [17], such as in the J-Nose wing substructure [18]. In addition,

studies have shown that CF-PPS is able to maintain its mechanical properties (such as flexural

strength) when exposed to an elevated temperature and relatively high humidity (80°C, 85%

R.H.), due to the structural relaxation and crystallization enhancement [19].

[Table 2]

5

3.3 Experimental procedure

Before joining the parts, the aluminum surfaces were softly dry ground using P1200 SiC paper

to remove the intrinsic oxide layer and any possible surface contamination, as well as to

increase the surface roughness. A fraction of the aluminum samples were sand blasted (SB)

prior to welding to evaluate the effect of the metal surface pre-treatment on the mechanical

performance of the joints. Two corundum (Al2O3) particle sizes in combination with three sand

blasting pressures were selected to attain the best sand blasting parameters. The chemical

composition of the corundum and the selected sand blasting parameters are presented in

Tables 3 and 4, respectively. Both the aluminum and the composite samples were then cleaned

using pressurized air, followed by acetone cleansing in an ultrasonic bath. Finally, the samples

were dried in air.

[Table 3]

[Table 4]

Surface roughness was used as the criterion to correlate the lap shear strength of the joints with

the surface treatments. Non-contact measurement using a laser microscope (VK-9700,

Keyence, Japan) was employed to obtain the surface roughness. For this correlation purpose,

the surface roughness was calculated from three different samples; five different regions were

selected on each sample, and six measurements were performed for each of the selected

regions. The average of the all 30 measurements was considered to be the surface roughness

of each sample.

After the joining process, the joints were cut in the middle of the spot area parallel to the rolling

direction across the thickness of the aluminum sheets. The cut samples were embedded in

epoxy resin, followed by the standard grinding and polishing procedures used in preparing

samples for cross-sectional imaging. The cross-section of the joints was analyzed by optical

microscopy under reflective light (DM IR microscope, Leica, Germany).

Furthermore, the process temperature on the aluminum surface was monitored in the range of

150-700°C using an infrared camera (VarioTHERM camera, Jenoptik, Germany) with a

resolution of 256 × 256 pixels at 50 Hz. The temperature at the interface for one set of joining

conditions was measured using a thermocouple (type K Cr-Ni, 0.5 mm diameter). Figure 3

schematically shows the set up for the temperature measurement, as well as an example of a

snapshot, in which the peak temperature was obtained by infrared thermography, comprising of

a side view of the tool, the aluminum sheet and the composite thickness. Two lines were used

to analyze the temperature evolution. The first line (L1) monitors the temperature in the middle

of the aluminum surface, which exhibited the highest temperature during the process; the

second line (L2) that is located at the composite - aluminum border line is visible by the infrared

camera.

[Figure 3]

6

Finally, the global mechanical strength of the joints was evaluated through the lap shear tensile

test according to standard ASTM: D3163 using a universal testing machine (Zwick Roell model

1478) with a load capacity of 100 KN and a traverse speed of 1.27 mm/min at room

temperature. Three specimens were used to obtain the average value of the ultimate lap shear

force. For lap shear tensile testing, specimens with dimension of 100×25.4 mm2 were machined.

The overlap area was 25.4 × 25.4 mm2, and the distance between the grips was 125 mm.

3.4 Joining procedure

Joints were produced using a displacement-controlled friction spot welding machine (RPS 100,

Harms&Wende, Germany). In the first step, the joining partners were placed in the overlap

configuration against a backing bar. The samples were clamped together (see Figure 4) during

and after the joining process to ensure firm contact between them, as well as to avoid

separation due to the large differences in the coefficient of thermal expansion between the two

materials [15, 20]. An experimental consolidation time – intentionally extended to times well

above the minimum time required for polymer consolidation to ensure the absence of differential

contraction - of three minutes was selected for all the joining conditions. Four controllable

parameters were changed one at a time to evaluate the effect of each factor on the strength of

the joints and the microstructure features. Table 5 lists the ranges of the joining parameters.

[Figure 4]

[Table 5]

4. Results and discussion

4.1 Process temperature

Figures 5(a-d) illustrate the maximum temperature along L1–line on the surface of the aluminum

and L2-line at the border line of the composite – aluminum during the process. Two set of joints

for the bare and the alclad composite – aluminum combinations were produced using different

heat inputs. Specimens B1 and A1 (B: bare; A: alclad) were produced using a lower heat input

(1900 rpm, 0.5 mm, 4.8 s, 8.5 kN) while B2 and A2 were produced using a higher heat input

(2900 rpm, 0.5 mm, 4.8 s, 8.5 kN ). The figures clearly indicate that increasing the rotational

speed from 1900 to 2900 rpm (Figures 5(a) & 5(c)) causes an approximately 40 – 50°C

temperature rise in the peak temperature of the aluminum from 350°C to 400°C (both bare and

alclad samples). However, no significant changes were observed at the border line for the bare

specimen (Figure 5(b)). For the alclad specimens, the temperature at the border line exhibited

an increase of approximately 15°C (Figure 5(d)), which might be due to the higher thermal

conductivity of the cladded layer. The process temperatures for both the bare and alclad

specimens are summarized in Table 6.

7

The temperature of approximately 400°C lies in the range of the dynamic recrystallization of

AA2024 [15], which also undergoes high shear rates at this temperature due to the tool

rotational speed of over 1900 rpm. Therefore, fine recrystallized grains are expected to be

present in the stir zone, as reported by Amancio et al. [22]. However, the microstructure of the

aluminum is not the focus of this paper.

[Figure 5]

[Table 6]

The highest detected temperature at the border line of bare and alclad aluminum – composite

joints was between 240 - 270°C, as measured by the infrared camera (line L2). Although it is

believed that the temperature is slightly higher in the center of the composite – aluminum

interface, where the metallic nub is created, it should not exceed 350 - 400°C, as was confirmed

by placing a thermocouple at a position along the interface that is near the nub area. Figure 6

illustrates the temperature evolution at the interface of the composite – aluminum bare (B1)

specimen (rotational speed of 1900 rpm) with a peak temperature of 345°C. Such a temperature

is sufficient to melt a thin layer of the PPS matrix (Tm= 280°C), but is far below the reported

extensive thermal degradation range of this polymer (thermal degradation begins at

approximately 500°C) [23, 24].

[Figure 6]

4.2 Microstructural features

Figure 7(a) illustrates a top view of a sound joint between AA2024-T3 and CF-PPS (joining

parameters: rotational speed 1900 rpm, plunge depth 0.5 mm, joining time 4.8 s and joining

force 8.5 kN). The cross-sectional view of the joint is also shown in Figure 7(b). The deformed

aluminum feature, i.e., the metallic nub, is visible in the cross-sectional image of the joint

(indicated in the figure with an ellipse). The slight insertion of the metallic nub into the composite

is believed to increase the mechanical interlocking, and hence the shear strength of the joint

[14].

[Figure 7]

Detailed analysis of the nub region was performed using optical and laser-scanning confocal

microscopy (Figure 8). Different features are present in the cross-sectional images in the

figures. As a result of the texture changes induced by plastic deformation [25] and of the surface

pre-treatment, pores and crevices were formed on the aluminum surface in contact with the

composite. The molten PPS matrix fills these pores/crevices (Figure 8(a,c)), which, after

consolidation, increases the micro-mechanical interlocking and the global shear strength of the

joint. Furthermore, another important phenomenon is observed: a portion of the carbon fibers

are entrapped by the plasticized aluminum, thereby creating a micro-mechanical interlocking

(Figure 8(b,d)). During the course of the joining process, the molten PPS matrix is squeezed out

of the nub and is partially displaced by the softened aluminum. Due to the applied axial force by

2 mm

8

the sleeve and the pin, the plasticized aluminum penetrates into the first plies of the composite

and either individual fibers or fiber bundles become embedded into the aluminum. It is believed

that these two phenomena of pore filling by molten PPS and fiber enclosure by the aluminum

(known as the bonding mechanisms) are largely responsible for the shear strength of the joints.

Further analysis must be conducted to better understand these bonding mechanisms.

In addition to the above phenomena, some voids were observed in the consolidated PPS

molten layer (region A, Figure 8(a)) as well as at the interface between the fibers and the PPS

matrix (Region B, Figure 8(a,c)). The voids in region A are thought to be due to the air

entrapment in the viscous molten polymer layer due to the matrix squeezing flow [14]. As the

maximum process temperature lies between 350 - 400°C, extensive PPS degradation is not

expected [23, 24]. The delamination voids in region B are thought to be formed because of the

difference between the thermal expansion of the fibers and the PPS polymer matrix. This void

formation phenomenon was also observed by Ageorges and Ye [26] in the resistance welding of

thermoplastic carbon reinforced composites and by Jung et al. [27] in the laser direct joining of

carbon fiber reinforced polyamide 6 with zinc-coated steel. From our current knowledge the

amount and extension of delamination formed during joint consolidation appears to be directly

related to the amount of generated frictional heat and applied clamping pressure at cooling.

Further studies must be performed to better understand the correlation between void formation

and joining process parameters in FSpJ.

[Figure 8]

In contrast with the bare specimens, the alclad specimens exhibited a more homogeneous

deformation of the surface in contact with the composite (under the same joining parameters)

because the ductility of the cladded layer is higher than that of the AA2024. In addition, the

number of generated pores and crevices are expected to be higher compared to that for the

bare specimen because of the larger susceptibility to plastic deformation. Therefore, a larger

amount of molten PPS is expected to fill the crevices and pores, thereby increasing the

adhesion forces and improving the global mechanical properties. Further studies are in progress

to evaluate the formation of texture-related pores and its influence on the joint mechanical

strength.

4.3 Mechanical performance of the joints

Lap shear tensile testing was performed to evaluate the mechanical performance of the joints.

The ultimate lap shear force was extracted from the load-displacement curves (Figure 9). To

assess the strength of the joints, the ultimate lap shear strength (ULSS) for each of the joints

was additionally calculated by considering the nominal area corresponding to the outer diameter

of the sleeve (9 mm). This calculation approach is a common practice used by other

researchers [8] to estimate the lap shear strength of spot joints by considering the nominal area

of the joining tool. The joints displacement was obtained from the machine traverse

displacement.

9

Figure 9 shows as an example the force-displacement curves of both bare (B1) and alclad (A1)

specimens. As it can be observed from the figure, FSp joints normally exhibit elastic behavior

before final failure.

[Figure 9]

Generally, by increasing the rotational speed (while keeping the other joining parameters

constant), the ultimate lap shear force was observed to continuously increase. It is obvious that

by increasing the rotational speed of the tool, the heat generation increases (see increase of

process temperature in Figure 5), which gives rise to an increase in the joining area (measured

graphically on the fractured specimens). For instance, changing the rotational speed from 1900

to 2900 rpm results in a 70% increase in the joining area (see Figures 10 and 11). The larger

the joining area, the more intimate contact between PPS matrix and carbon fibers with

aluminum (Figure 11) is created, and hence better mechanical performance is achieved, as

observed in Figure 10(a) (950.6 ± 43.8 N for 1900 rpm; 1254.1 ± 41.6 N for 2900 rpm). The

displacement at the peak load exhibits a similar behavior (Figure 10(b)). Up to the tool rotational

speed of 1900 rpm, the changes in the displacement are not very pronounced; however, from

1900 rpm to 2500 rpm and at 2900 rpm, a sharp increase in the displacement was observed

(0,6 mm for 1900 rpm; 1.0 ± 0,1 mm for 2900 rpm).

[Figure 10]

[Figure 11]

A similar trend was also observed when increasing the tool plunge depth and the joining time.

Figure 12, as an example, shows the influence of increasing the plunge depth on the lap shear

force and the displacement at the peak load of the joints (762.6 ± 182.7 N / 0.45 ± 0.08 mm for

a plunge depth of 0.5 mm and 1276 ± 181.5 N / 0.65 ± 0.05 mm for a plunge depth of 0.8 mm).

This trend is associated with the generation of a more pronounced “nub” and the increased

intimate contact at the interface of the composite / aluminum, which can increase the adhesion

forces by micromechanical interlocking. Figure 13 displays the fracture surfaces of the two

specimens in Figure 12, with respective plunge depths of 0.5 and 0.8 mm. More fibers remain

attached to the sample with 0.8 mm of plunge depth (black arrow in Figure 13(b)). The detailed

laser microscopy characterization of the fibers is shown in Figure 13 (c). The white arrow

indicates the fibers remain attached to the aluminum, while the black arrow indicates the

aluminum in the nub region.

[Figure 12]

[Figure 13]

The average joint lap shear strength of up to 27 ± 2.8 MPa was achieved in the first set of trials

using bare AA2024 specimens at the tool rotational speed of 3300 rpm (0.5 mm / 4.8 s / 8.5 kN)

[28]. This result is related to the formation of the larger joining area (as discussed previously)

and the larger amount of molten polymer, which increase the adhesion forces.

10

Two joining conditions were selected to join the AA2024 alclad alloy with the CF-PPS composite

laminate. For both sets of conditions, a significant increase in the ultimate lap shear force was

observed in comparison to the bare specimens. Figure 14 depicts a comparison between the

mechanical performance of the joints, which shows an increase in ULSS up to an average of

36.6 ± 1.7 MPa when the alclad specimen is used (B1: 15.3 MPa, B2: 20.2 MPa and A1: 32.6

MPa, A2: 36.6 MPa). From our current knowledge, this behavior may be explained by the higher

ductility of the cladded layer, which deforms more easily during the process and embeds a

higher volume of the polymer matrix relative to the bare specimens. In addition, the generation

of more pores and crevices on the faying surface increases the micro-mechanical interlocking

between the aluminum and composite in the nub area, as already explained. Finally, because of

the higher thermal conductivity of this layer, the produced molten polymer consolidates more

homogeneously, and therefore a more even stress distribution is expected.

[Figure 14]

As mentioned before, the force - displacement curves (see Figure 9) provide information on the

elongation at the peak load of the joints. The elongation of the alclad samples (0.9 mm) is larger

compared to the bare specimens (0.6 mm). This larger elongation could be due to the larger

joint area and the increased amount of polymer attached to the aluminum surface. However,

some local changes can occur, such as the changes in the polymer crystallization state and

chain re-orientation, which influences the strength and ductility of the joints [29].

A further indication of the assumptions regarding the improved bonding forces observed in the

alclad specimens is observed in the fracture surfaces of the bare and the alclad specimens in

Figure 15. The fracture surfaces of the joints described in Figure 14 (B1 & A1) have a more

homogeneous consolidated molten polymer layer (Figure 15(b)) in addition to an increased

amount of polymer matrix and carbon fibers that remain attached to the aluminum surface

(Figure 15(c)) when AA2024-alclad was used. The homogeneous distribution of the

consolidated polymer layer is expected to reduce the stress concentration, thereby resulting in a

higher joint strength.

[Figure 15]

Metal surface pre-treatment has been shown to significantly influence the mechanical

performance of adhesive bonded joints [30]. Because one of the primary bonding mechanisms

in FSp joints of the metal-composite is the adhesion forces due to the presence of a thin

consolidated molten polymer layer, metal surface pre-treatment can improve the lap shear

strength of the joints [14]. To evaluate the effect of surface pre-treatment on the mechanical

performance of the FSp joints of AA2024/CF-PPS, the surface of aluminum was sand blasted

(see Table 4 for the sand blasting conditions). Depending on the selected sand blasting

parameters, an average surface roughness of Ra=2 - 4.5 µm was achieved. These results are

much higher compared to the attained surface roughness by mechanical grinding using P1200

SiC paper (Ra=0.84 µm). Table 7 lists the complete results of the surface roughness

measurements according to the different surface treatments compared to the as-received

11

specimens. Furthermore, Figure 16 illustrates the laser microscope images as well as the

topographical view of the measured regions for the as-received, P1200 mechanical ground and

sand blasted specimens. For sand blasting, the pictures correspond to the corundum size of

100 - 150 µm and a blasting pressure of 6 bar.

[Table 7]

[Figure 16]

As expected, the average surface roughness increases from the use of mechanical grinding to

sand blasting. In the case of sand blasting, increasing the corundum size or blasting pressure

leads to a higher surface roughness value. Mechanical grinding leads to smooth grooves on the

surface of the sample that varies in height. On the other hand, sand blasting exhibits a sharp

edged surface roughness, with different crevice shapes, height and width (see Figure 16(c)).

The same behavior is reported by Schulze et al. [31] for the sand blasting of titanium.

[Figure 17]

Figure 17 depicts the influence of surface treatment on the force – displacement curve of the

joints for a selected joining condition for both bare and alclad specimens (B1 & A1). Note that

the figures exhibit, as an example, the average curve of three replicates for both bare and alclad

samples. Both bare and alclad specimens display a pronounced increase in the lap shear force

and displacement when sand blasting was performed on aluminum. From Figure 17(a), the

sand blasted bare specimens exhibiting an average lap shear force of slightly less than twice

that of the mechanically ground specimens (SB: 1908.8 ± 23.2 N, P1200: 1276 ± 181.5 N). The

displacement at the peak load was determined to increase from 0.6 mm to 1 mm. In the case of

the alclad specimens (Figure 17(b)) an average lap shear force of 2027.6 ± 229.2 N was

observed when the mechanical grinding (P1200) was performed, while the sand blasted

specimens exhibited an average lap shear force of 2685.4 ± 301 N. The sand blasted

specimens also exhibited a larger displacement at the peak load (1.1 mm) compared to the

mechanically ground samples (0.9 mm). Note that for the alclad specimens, fine grain corundum

(size 40 – 70 µm) was used as the sand blasting medium to avoid excessive reduction in the

thickness of the cladded layer. Sand blasting clearly creates more pores/crevices and

distributes them more homogeneously on the aluminum surface (see Figure 16). Furthermore,

the deeper indentions created by sand blasting can better accommodate the molten polymer

and increase the micro-mechanical interlocking. The larger displacement of the sand blasted

specimens is most likely due to the improved interlocking of the polymer into the aluminum

pores and crevices, thereby retarding final failure. Considering the nominal area of the tool (9

mm), the sand blasted joints exhibited an average lap shear strength of 31 MPa (bare

specimens) and 43 MPa (alclad specimens).

12

4.4 Failure mechanisms

Fracture surface analysis indicated that all the joints fail in shear mode with micro-mechanical

fracture through a mixed adhesive-cohesive failure. Such behavior was also reported by

Amancio et al. [14] in Mg/CFRP FSp joints and by Esteves et al. [32] for Al6181-T4/CFRP.

However, no detailed description of the failure mechanisms has been reported.

Here, we introduce a very simple model to discuss the failure mechanisms in FSpJ. As

schematically shown in Figure 18, three zones can be identified in the fractured surface of the

FSp joints. The first zone corresponds to the area in which the mechanically deformed nub is

present and the process temperature was high enough in the region to produce a thin molten

polymer layer. This first zone is called the Plastically Deformed Zone (PDZ). The PDZ is the

strongest part of the joint, as there is intimate contact between the polymer matrix and the

carbon fibers with the plasticized aluminum volume. Most of the failure mechanisms, such as

cohesive failure and partial fiber breakage, occur in this region. The outer region is the zone in

which the molten polymer matrix is consolidated and adhesion forces are responsible for

holding the specimens together. This outer region is called the adhesion zone (AZ). A transition

zone (TZ) is identified next to the internal border of the consolidated layer. Due to the lower

viscosity of the flowing molten polymer (related to the distance to the rubbing area that created

the higher temperatures) during the joining process, air bubbles remain entrapped in this layer,

thereby reducing the local strength of the transition zone.

[Figure 18]

These three zones are not homogeneously distributed around the nub (Figure 18(b)), which is

an indication of inhomogeneous heat distribution in the aluminum bare specimens. However,

comparing the fracture surfaces of the bare and alclad specimens (see Figure 15(a,b)), the heat

distribution is more efficient for the aluminum alclad specimens, which results in a more uniform

distribution of the mentioned zones around the nub (see Figure 15(b)). The consistent

distribution of these zones should result in improved uniformity in the stress distribution and

improved mechanical performance. Because the fracture behavior of such joints is very

complex, investigations are in progress to better understand the failure mechanisms.

From our current understandings, the FSp lap joints fail by having the cracks initiate at the

periphery of the AZ and propagate along the interface between this layer and the aluminum or

composite surface in a plane parallel with the applied load direction. Should the cracks reach

the beginning of the TZ, their path can shift from the interface into the composite and propagate

in the PDZ within a volume close to the metallic nub, in the first plies of the composite. This

crack propagation is associated with a more cohesive failure (in PDZ), in which a larger amount

of polymer matrix and carbon fibers remain attached to the aluminum surface. Figure 19

summarizes the crack propagation behavior of spot joints failing by shear. For simplicity, only

unidirectional fibers in the first two plies of the composite are sketched in this figure.

[Figure 19]

13

4.5 Further investigations

All of the above discussions and assumptions in this preliminary work, for instance better

mechanical performance of the alclad specimens due to the higher thermal conductivity and

formability, are being currently investigated. Qualitative and quantitative analysis of joint fracture

surfaces, for example, with the aid of scanning electron microscopy, microanalysis and detailed

microscopy are being applied for the determination of the distribution of pores and the polymer

filling characteristics. Moreover, the effect of aluminum surface pre-treatments on the joints

strength is under investigation by electron microscopy and different surface analysis methods.

The new findings will be published in separate documents.

5 Conclusions

The feasibility of FSpJ 2-mm-thick AA2024-T3 (both bare and alclad conditions) with CF-PPS

was successfully demonstrated in this work. Average peak temperatures varying between 350

and 400°C were measured during the joining process, which are well below the thermal

degradation range of the polymer in the CF-PPS. The increase in the rotational speed of the

FSpJ tool from 1900 rpm to 2900 rpm results in an increase of the peak temperature of

approximately 40°C to 50°C. The temperature at the interface is slightly higher for the alclad

samples due to the higher thermal conductivity of this layer. The microstructural features of the

joints exhibit entrapped consolidated polymer in the pores / crevices on the surface of the

aluminum. Furthermore, carbon fibers are embedded in the deformed aluminum in the nub

region. A correlation between the joining area and the lap shear strength of the joint was

demonstrated. The higher the rotational speed of the tool is, the larger is the joining area;

hence, a higher lap shear force and larger displacement can be achieved by increasing the

rotational speed of the tool. The same trend was observed when increasing the tool plunge

depth, which could be due to the formation of a deeper nub and consequently an improved

micro-mechanical interlocking. Comparing the mechanical performance of the joints using the

bare and alclad samples, the AA2024-T3 alclad sample exhibited better results, due to the

higher and more homogeneous deformation of the metallic nub as well as the more uniform

distribution of the consolidated molten polymer layer (because of the improved heat distribution

as a result of the higher thermal conductivity of the cladded layer). The lap shear strength of the

alclad specimens (36.6 ± 1.7 MPa) is much higher than that of the bare specimens (20.2 ± 0.7

MPa). The effectiveness of the aluminum surface pre-treatment was demonstrated. The surface

roughness of the aluminum increased from 0.68 µm (as-received) to 0.84 µm (mechanically

ground) and 4.5 µm (sand blasted). The lap shear strength was demonstrated to increase up to

31 MPa (bare) and 43 MPa (alclad) due to the increase in the wettability of the aluminum

surface and the micro-mechanical interlocking leading to higher adhesive forces at the interface.

Finally, a simple model was proposed to discuss the failure mechanisms of the joints that

involve mixed adhesive-cohesive failure. In this model, the joint area is divided in three zones.

The outer zone, the adhesion zone (AZ), is believed to be the weakest part of the joint area,

with the radial cracks initiating at the periphery of this zone. The inner zone, the plastically

14

deformed zone (PDZ), is considered to be the strongest region in the joint, in which the metallic

nub is created and direct contact between the aluminum and the carbon fibers contributes to the

final joint strength. A thin transition zone (TZ) exists between the PDZ and AZ, in which some

volumetric flaws are present and are associated with the air bubbles remaining entrapped,

thereby reducing the strength of this layer.

Acknowledgements

The authors would like to acknowledge the financial support of the Helmholtz Association

through the Young Investigator Group, “Advanced Polymer Metal Hybrid Structures” (grant no.

VH-NG-626).

References

[1] Mallick PK. Materials, design and manufacturing for lightweight vehicles. First ed.

Cambridge, UK: Woodhead Publishing Limited; 2010.

[2] Faivre V, Morteau E. Damage tolerant composite fuselage sizing, characterization of

accidental damage threat. Airbus Technical Magazine (FAST). August 2011;48:10-6.

[3] Amancio-Filho ST, dos Santos JF. Joining of Polymers and Polymer-Metal Hybrid structures:

Recent Developments and Trends. Poly Eng Sci 2009;49:1461-76.

[4] Abibe AB, Amancio-Filho ST, dos Santos JF, Hage Jr. E. Mechanical and failure behaviour

of hybrid polymer–metal staked joints. Mater Des. 2013;46:338-47.

[5] Blaga L, Bancila R, dos Santos JF, Amancio-Filho ST. Friction Riveting of glass–fibre-

reinforcedpolyetherimide composite and titanium grade 2hybrid joints. Mater Des. 2013;50:825-

9.

[6] Balle F, Wagner G, Eifler D. Ultrasonic spot welding of aluminum sheet/carbon fiber

reinforced polymer – joints. Mat-wiss u Werkstofftech. 2007;38:934-8.

[7] Balle F, Wagner G, Eifler D. Ultrasonic Metal Welding of Aliminium Sheets to Carbon

Reinforced Thermoplastic Composites. Adv Eng Mat. 2009;11:35-9.

[8] Balle F, Eifler D. Statistical test planning for ultrasonic welding of dissimilar materials using

the example of aluminum-carbon fiber reinforced polymers (CFRP) joints. Mat-wiss u

Werkstofftech. 2012;43:286-92.

[9] Balle F, Wagner G, Eifler D. Combined Process of Ultrasonic Welding and Precipitation

Hardening of Aluminium Alloy 2024/Carbon Fibre Reinforced Composites Structures. 12th

International Conference on Aluminium Alloys (ICAA-12). Yokohama, Japan2010. p. 856-61.

[10] Bergmann JP, Stambke M. Potential of laser-manufactured polymer-metal hybrid joints.

Physics Procedia. 2012;39:84-91.

[11] Mitschang P, Velthuis R, Didi M. Induction Spot Welding of Metal/CFRPC Hybrid Joints.

Adv Eng Mater. 2013.

[12] Schiling C, dos Santos JF. Method and device for linking at least two adjoining work pieces

by friction welding. International Patent Publication WO/2001/036144; 2005.

15

[13] Amancio-Filho ST, dos Santos JF. Method for joining metal and plastic workpieces.

European Patent No. EP2329905B1;2012.

[14] Amancio-Filho ST, Bueno C, dos Santos JF, Huber N, Hage Jr. E. On the feasibility of

friction spot joining in magnesium/fiber-reinforced polymer composite hybrid structures. Mater

Sci Eng A. 2011;528:3841-8.

[15] Davis JR. ASM International: Handbook of Aluminium & Aluminium Alloys. 3rd ed. Ohio,

USA1996.

[16] Aluminium AA2024-T3 technical datasheet. Constellium France; 2012.

[17] CETEX® - PPS Guide Lines. Netherlands: Ten Cate Advanced Composites.

[18] Mitschang P, Blinzler M, Wöginger A. Processing technologies for continuous fibre

reinforced thermoplastics with novel polymer blends. Compos Sci Tech. 2003;63:2099-110.

[19] Ma CCM, Lee CL, Chang MJ, Tai NH. Hygrothermal Behavior of Carbon Fiber-Reinforced

PoIy(etheretherketone) and Poly(phenylene sulfide) Composites. I. Poly Comp. 1992;13:448-53.

[20] CETEX® PPS. Technical datasheets, Tencate Advanced Composites. Netherlands2009.

[21] Corundum. Technical datasheet, WIWOX GmbH Surface Systems. 2009.

[22] S.T. Amancio-Filho, A.P.C. Camillo, L. Bergmann, J.F. dos Santos, S.E. Kury, N.G.A.

Machado. Preliminary Investigation of the Microstructure and Mechanical Behaviour of 2024

Aluminium Alloy Friction Spot Welds. Materials Transactions. 2010;52:985-91.

[23] Ma CCM, Hsia HC, Liu WL, Hu JT. Thermal and Rheological Properties of Poly(Phenylene

Sulfide) and Poly(Ether Etherketone) Resins and Composites. Poly Comp. 1987;8:256-64.

[24] Li XG, Huang MR, Bai H, Yang YL. High-Resolution Thermogravimetry of Polyphenylene

Sulfide Film under Four Atmospheres. J Appl Polym Sci 2002;83:2053-9.

[25] Wouters O, Vellinga WP, Van Tijum R, de Hosson JThM. On the evolution of surface

roughness during deformation of polycrystalline aluminum alloys. Acta Materialia.

2005;53:4043-50.

[26] Ageorges C, Ye L. Fusion Bonding of Polymer Composites; From Basic Mechanisms to

Process Optimization. London, Great Britain: Springer; 2002.

[27] Jung KW, Kawahito Y, Takahashi M, Katayama S. Laser direct joining of carbon fiber

reinforced plastic to zinc-coated steel. Mater Des. 2013;47:179-88.

[28] Goushegir SM, Amancio-Filho ST, dos Santos JF. Friction Spot Joining of AA2024-T3 / CF-

PPS Composite: A Feasibility Study. Materials Science and Engineering Congress (MSE

2012). Darmstadt, Germany2012.

[29] Lampman S. Characterization and Failure Analysis of Plastics. Ohio: ASM International;

2003. p. 482.

[30] Prolongo SG, Ureña A. Effect of surface pre-treatment on the adhesive strength of epoxy–

aluminium joints. Int J Adhes Adhes. 2009;29:23-31.

[31] Schulze K, Hausmann J, Wielage B. Properties of Thermoplastic Fibre Metal Laminates

(FML) Titanium-PEEK Interface. ECCM15 - 15TH European Conference on Composite

Materials. Venice, Italy2012.

[32] Esteves JV, Amancio-Filho ST, dos Santos JF, Canto LB, Hage Jr. E. Friction spot joining

of aluminum 6181-T4 and carbon fiber reinforced poly(phenylene sulfide). 70th Annual

Technical Conference of the Society of Plastics Engineers 2012, ANTEC 2012. Orlando,

FL2012. p. 1698-704.

16

Table captions

Table 1: Selected physical and mechanical properties of AA2024-T3 at room temperature [15,

16].

Table 2: Selected physical and mechanical properties of CF-PPS at room temperature [20].

Table 3: Chemical composition of the corundum [21].

Table 4: Sand blasting (SB) parameters.

Table 5: Range of joining parameters used in this work

Table 6: FSpJ process temperature for CF-PPS / AA2024 bare (B) and alclad (A) specimens.

Table 7: Surface roughness results based on different surface treatments.

Figure captions

Fig. 1: The FSpJ tool comprised of a pin, sleeve and clamping ring (dimensions are in mm).

Fig. 2: Schematic of the FSpJ technique (sleeve plunge variant). (1) The sleeve plunging

softens the metal alloy; (2) spot refilling; (3) joint consolidation.

Fig. 3: Schematic set up for thermometry using an infrared camera, and an example of a

snapshot that determined the maximum process temperature. The black box shows the

monitored area. Lines L1 and L2 were used to analyze the process temperature evolution on

the aluminum surface and the aluminum – composite border line, respectively.

Fig. 4: Schematic view of the sample holder and the clamping part used in this work.

Fig. 5: Process temperature evolution along the lines L1 and L2 measured by infrared

thermography: (a) and (b) bare specimens (B1: 1900 rpm and B2: 2900 rpm, respectively); (c) &

(d) alclad specimens (A1: 1900 rpm and A2: 2900 rpm, respectively). Joining parameters: 0.5

mm, 4.8 s, 8.5 kN.

Fig. 6: Measured temperature at the interface near the nub region in bare sample B1 (1900

rpm, 0.5 mm, 4.8 s, 8.5 kN) using a K-type thermocouple.

Fig. 7: (a) Top view of a sound FSp joint of AA2024-T3 (bare)/CF-PPS (1900 rpm, 0.5 mm, 4.8

s, 8.5 kN), (b) cross-sectional macrograph of the joint, with the ellipse indicating the nub region.

Fig. 8: (a) AA2024-bare pore/crevice filling by molten PPS (black arrow), region “A” shows a

void in the consolidated molten PPS, while region B illustrates delamination voids between

fibers and PPS matrix. (b) Carbon fibers embedded in aluminum at the interface of bare

aluminum - composite. (c) Nub area in the alclad specimen that exhibits a uniform deformation

17

as well as pores/crevices at the interface with the composite. (d) Carbon fibers covered by the

cladded layer.

Fig. 9: Example of the force-displacement curve of AA2024-bare/CF-PPS and AA2024-

alclad/CF-PPS (B1 and A1: 1900 rpm / 0.5 mm / 4.8 s / 8.5 kN).

Fig. 10: The increase in the joining area as well as (a) the lap shear force and (b) the

displacement at the peak load caused by increasing the rotational speed of the tool (joining

parameters: 0.5 mm / 4.8 s / 8.5 kN).

Fig. 11: The increase in the rotational speed of the tool results in a larger joining area (white

circles): (a) 1900 rpm / 0.5 mm / 4.8 s / 8.5 kN; (b) 2900 rpm / 0.5 mm / 4.8 s / 8.5 kN.

Fig. 12: The influence of the tool plunge depth on the lap shear force and the displacement at

the peak load of the FSp joints (joining parameters: 1600 rpm / 4.8 s / 8.5 kN).

Fig. 13: The fracture surface of the joints presented in Figure 12: improved mechanical

interlocking and a larger number of attached fibers to the aluminum nub were observed due to

the larger plunge depth: (a) 1600 rpm / 0.5 mm / 4.8 s / 8.5 kN; (b) 1600 rpm / 0.8 mm / 4.8 s /

8.5 kN; (c) details of the fibers attached to aluminum in (b).

Fig. 14: Comparison of the ultimate lap shear strength of AA2024-bare/CF-PPS and AA2024-

alclad/CF-PPS (B1 and A1: 1900 rpm / 0.5 mm / 4.8 s / 8.5 kN; B2 and A2: 2900 rpm / 0.5 mm /

4.8 s / 8.5 kN).

Fig. 15: Fracture surfaces of (a) AA2024-bare/CF-PPS and (b) AA2024-alclad/CF-PPS (1900

rpm / 0.5 mm / 4.8 s / 8.5 kN). The alclad specimen exhibits a more homogeneous consolidated

molten polymer layer (indicated by the black arrows). (c) Image from the laser microscope

showing the details of the fibers attached in (b).

Fig. 16: Laser microscope images of (a) as-received, (b) P1200 mechanical ground and (c)

sand blasted AA2024-T3, as well as their respective topographical views and surface roughness

values, before the joining process. The sand blasting treatment parameters: 100-150 µm, 6 bar,

10 s, 25 cm.

Fig. 17: The average force-displacement curves of the FSpJ of (a) bare specimens (B1) and (b)

alclad specimens (A1) showing the influence of surface treatment on the mechanical

performance of the joints. The joining parameters: 1900 rpm / 0.5 mm / 4.8 s / 8.5 kN. The sand

blasting (SB) parameters: (a) 100-150 µm, 6 bar, 10 s, 25 cm, and (b) 40-70 µm, 6 bar, 10 s, 25

cm.

Fig. 18: (a) Schematic fracture surface of an FSp joint and (b) real fracture surface of an FSp

AA2024-bare/CF-PPS joint. The circles indicate the plastically deformed zone (PDZ), transition

zone (TZ) and adhesion zone (AZ). The dashed circles indicate the nub area.

Fig. 19: Schematic representation of the proposed crack propagation mechanisms in friction spot joints under shear loading.

1

Table 1 Selected physical and mechanical properties of AA2024-T3 at room temperature [14, 15].

Tensile

strength

(direction TL)

[MPa]

Yield

strength

(direction TL)

[MPa]

Elongation

[%]

Melting

temperature

[°C]

Thermal

conductivity

[Wm-1

K-1

]

Coefficient of thermal

expansion within 20-

300°C [µm/m-°C] []

437 299 21 500 - 638 121 24.7

1

Table 2

Selected physical and mechanical properties of CF-PPS at room temperature [20].

Tensile

strength (warp)

[MPa]

In plane shear

strength [MPa]

Glass transition

temperature

[°C]

Melting

temperature

[°C]

Thermal

conductivity

[Wm-1

K-1

]

Coefficient of thermal

expansion within 23-300°C

[µm/m-°C]

758 119 120 280 0.19 52.2

1

Table 3 Chemical composition of the corundum [21]

Element Al2O3 Na2O Fe2O3 SiO2 Rest

Weight (%) 99.70 0.20 0.02 0.02 0.06

1

Table 4 Sand blasting (SB) parameters

Corundum grades [µm] Pressure [bar] Time [s] Distance to sample [cm]

40 - 70 / 100 - 150 2 / 4 / 6 10 25

1

Table 5 Range of joining parameters used in this work

Rotational speed [rpm] Plunge depth [mm] Joining time [s] Joining force [kN]

1000 - 2900 0.5 – 0.8 4 - 6.8 6.8 – 13.8

1

Table 6

FSpJ process temperature for CF-PPS / AA2024 bare (B) and alclad (A) specimens

Peak temperature, bare [°C] Peak temperature, alclad [°C]

B1 B2 A1 A2

Aluminum L1 354 400 360 397

Interface L2 241 245 255 270

1

Table 7 Surface roughness results based on different surface treatments

Surface treatment Ra [µm]

As-received 0.68 ± 0.06

P1200 0.84 ± 0.08

SB (100-150 µm, 6 bar) 4.5 ± 0.4

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Highlights

The feasibility of friction spot joining of AA2024-T3 with CF-PPS was demonstrated.

Average process temperatures were below the thermal degradation of the PPS matrix.

Joints using alclad specimens showed higher lap shear strength than bare specimens.

Aluminum Sand blasting pre-treatment enhances mechanical performance of the joints.

A simple model to describe failure mechanisms of the joints was proposed.