Effect of plasma electrolytic oxidation on joining of AA 5052 aluminium alloy to polypropylene

using friction stir spot welding

S. Aliasgharia, M. Ghorbania, P. Skeldonb, H. Karamia, M. Movahedia

aDepartment of Material Science and Engineering, Sharif University of Technology, P.O. Box

11365-9466. Azadi Avenue, 14588 Tehran, Iran.

bCorrosion and Protection Group, School of Materials, The University of Manchester, Oxford

Rd., Manchester M13 9PL, England, U.K.

1

Abstract

The effect of a plasma electrolytic oxidation (PEO) pre-treatment on joining of AA 5052

aluminium alloy and polypropylene by friction stir spot welding (FSSW) is investigated using

lap tensile shear tests. Two surface conditions of the AA 5052 alloy are compared, one with a

PEO pre-treatment in a silicate-based electrolyte, another without any pre-treatment. The

resultant specimens are examined by high resolution scanning electron microscopy and

attenuated total reflectance-infrared spectroscopy. The PEO treatment generated a thermally-

insulating, porous ceramic coating, which has a highly porous, rough surface that is favourable

for incorporating polypropylene melted by FSSW. The pre-treatment significantly increased the

lap tensile shear strength, by about a factor of three, in comparison with the untreated alloy,

suggesting that open pores in the coating filled by polypropylene provide strong

micromechanical interlocking and covalent bonding between the coated alloy and the polymer.

Keywords: aluminium, alloy, polypropylene, plasma electrolytic oxidation, friction stir spot

welding

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1. Introduction

One of the important ways of reducing CO2 emissions, especially in the transport sector, is to

make greater use of lightweight materials[1]. Furthermore, the ability to join dissimilar light

metals and of light metals to polymers enables the production of hybrid structures with greatly

improved performance for the aerospace and automobile industries [2-4]. The joining of

dissimilar materials often requires the use of advanced joining processes [5]. A promising and

environment-friendly method is friction stir welding (FSW) [6-8]. There is an extensive literature

on FSW that covers a wide range of processing parameters, resultant weld properties and

characterization methods [9, 10] . FSW is often applied to joining of metals, but can also be

employed with plastics [11].

In comparison with FSW, little work is available that investigates friction stir spot welding

(FSSW). Bakavos et al. studied the effect of surface features of tools on the penetration of the

plastic zone on the bottom sheet [12]. Badarinarayan et al. used two kinds of pin, namely

cylindrical and triangular, to evaluate the effect of tool geometry on the strength of FSSW joints

[13]; welding with a triangular pin resulted in a greater strength than a cylindrical pin due to

generation of a finer grain size. The use of a scroll groove on the shoulder surface has been

examined for improving the tensile-shear strength of AA 6061-T4 aluminium alloy joints [14]; a

higher strength was obtained in comparison with a conventional tool. Amanico-Filho et al.

studied joining of AZ31 magnesium alloy to glass fibre- and carbon fibre-reinforced

polyphenylene sulphide (PPS) to produce hybrid structures. They reported that the main bonding

is due to mechanical interlocking and interfacial chemical adhesion between the polymer and the

3

alloy [15]. Other work [16], used lap shear tensile tests to show that sandblasting of the

aluminium alloy improved the strength of aluminium alloy-carbon fibre-PPS joints; furthermore,

increasing the rotational speed of the tool was shown to result in an increase of the peak

temperature. Another study demonstrated that the plunge speed affected the formation of bubbles

in the polymer in aluminium alloy-polyethylene terephthalate joints [17]. The literature also

reports that thin oxide films on metals may generate covalent bond between the oxide and a

polymer, which leads to an improvement of the bond strength [18, 19].

Plasma electrolytic oxidation (PEO) is a method of producing ceramic coatings on the light

metals (aluminium, magnesium and titanium) and their alloys, with coating thicknesses in the

range from ~1 to 100 μm [20]. The method has been commonly used to provide corrosion

protection and wear resistance to the treated surface, but in recent years it has also been

investigated as a pre-treatment for adhesive bonding of magnesium alloys [21, 22]. The coatings

are formed by polarization the metal to the dielectric breakdown voltage, usually in an aqueous

electrolyte. The coatings are typically composed of oxides from oxidation of the substrate and

other compounds, the latter depending on the composition of the substrate and the electrolyte.

The coating material is generated at the sites of short-lived microdischarges under very high

temperatures and pressures that are sufficient to melt the coating material [23]. Gas generated

during the process may escape from the molten coating forming pores and pancake-like features

at the coating surface [24]. The pores typically range in size from the nanoscale to about 10 μm

[25]. Additionally, large cavities may be created by plastic deformation of the heated coating

[26]. The presence of pores and cracks at the coating surface, the latter formed on solidification

of the molten coating, enables the penetration of a melted polymer into the coating surface. A

4

PEO pre-treatment has been shown to be effective for joining magnesium to polyethylene using

friction lap welding, due to a combination of micromechanical interlocking and chemical

bonding [27]. Other work, on aluminium, was concerned with the effect of different coating

microstructures on adhesion bonding of polybutylene terephthalate ejected by injection moulding

[28]. In both studies, surface porosity tended to increase the shear strength.

Only the one study [28] appears to be available in the literature on the bonding of polymer to

PEO-treated aluminium. The particular study employed an injection moulding method of

bonding PEO-treated AA 5052 aluminium alloy to glass-fibre-filled polybutylene terephthalate,

which is a polyester. The PEO was carried out by a DC process at 500 V using a phosphate-

tungstate electrolyte. The treatment time of the alloy and the concentration of tungstate in the

electrolyte were varied to optimize the shear strength of the polymer/alloy joints. A maximum

shear strength of 8.1 MPa was achieved, with the fracture occurring at the boundary between the

PEO coating and the polymer.

In the present study, the influence of a DC PEO treatment on joining of AA 5052 aluminium

alloy and polypropylene (PP), which is an aliphatic polymer, using FSSW is considered. The

coating was formed at 300 V in a simple silicate electrolyte. The selected PEO process results in

a silicon-rich surface with numerous nodular agglomerations of coating material. The

combination of significant porosity, produced by dielectric breakdown and gas evolution during

PEO, and an abundance of nodular features was considered potentially beneficial to bonding

between the coating and the PP. The main interests of the study were the effect of the PEO-

coated aluminium alloy on the strength of the joints, which was investigated using a lap tensile

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shear test, the fracture morphology of the joined materials and the effect of the FSSW process on

the chemistry and structure of the polymer.

2. Experimental

Commercial PP and AA 5052 alloy (Si <0.25%, Fe <0.4%, Cu < 0.1%, Mn 0.15-0.35%, Mg 2.2-

2.8%, Cr <0.1%, Al Bal) sheets, both with a thickness of 2 mm, were used. The specimens with

dimensions of 70 x 30 mm were cut from the sheets. In the case of the AA 5052 alloy specimens,

a hole, of 5 mm diameter, was drilled in the middle of each specimen about 30 mm from one of

the short sides. The AA 5052 specimens were then masked with a plastic sealant, leaving a

working area of ~ 9.0 cm2 on one side of the specimen, which contained the hole that was

accessible to the electrolyte. The masked specimens were then rinsed thoroughly in running

water and dried in ambient air. The hole in the alloy was filled by the polymer during FSSW and

improved the strength of the joint between the PP and the PEO-treated AA 5052 alloy. Prior

testing of specimens with holes of various sizes showed that a hole of 5 mm diameter resulted in

the highest strength of the joint.

PEO treatment was carried out using a DC power supply source with a capacity of 600 V and 4

A. An aqueous electrolyte was prepared by dissolving 10.5 g dm -3 Na2SiO3 (specific gravity 1.5)

and 2.8 g dm-3 KOH (ACS grades) (pH 12.5) in deionised water. A double-walled glass cell was

employed to contain the electrolyte, which had a volume of 0.6 dm3. The electrolyte was stirred

with a magnetic stirrer during PEO. The temperature of the electrolyte was kept at 25 ºC by a

flow of cold water through the cell wall. A stainless steel (type 304) plate of dimensions 5 x 12

6

cm was used as a counter electrode. The applied voltage was 300 V; treatment time for formation

of the coating was 10 min. After forming a PEO coating at the working area on each specimen,

the lacquer mask was removed.

FSSW joints were prepared using a FSW facility. The PP and AA 5052 alloy specimens were

clamped together with an overlap of 3 x 3 cm that included the hole in the alloy. The

arrangement of the AA 5052 alloy and PP for the joining process is shown in Figure 1. The alloy

and the PP were held within a frame manufactured from AISI 304 stainless steel. A cover sheet

and a back sheet, manufactured from AA 5052 alloy were used to prevent any movement of the

parts to be joined during the FSSW process. The tool (AISI Type H13 hot work tool steel) of 20

mm diameter was applied to the AA 5052 alloy side of the overlapped region at a rotation rate of

1000 rev min-1 and plunged into the AA 5052 alloy at a rate of 20 mm min-1 for 4 s. Under this

condition, the tool did not reach the polymer. However, the resultant temperature rise of the

specimen melted the PP.

Lap-shear tests of the joints were carried out in a Hounsfield H10 KS 50 kN Universal Testing

Machine, with QMAT software. The joints prepared as shown in Figure 1 were employed for the

tests. A rectangular piece of the alloy was inserted into the grip holding the end of the polymer

that extended beyond the joined region. Similarly, a rectangular piece of the polymer was

inserted into the grip holding the end of the alloy that extended beyond the joined region. The

inserted pieces were used in order to ensure that the joints were subjected mainly to a shear

stress. Each specimen was similarly located in the tensile test machine, with the same distance

separating the grips. An extension rate of 2 mm min−1 was applied until the failure of the test

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specimen. The load versus extension was measured during the test. The extension during the test

was determined from the displacement of the crosshead and included the extension of the

specimen within the joined region and that of the non-joined polymer and alloy outside the

joined region. In addition to the tests of the PEO treated specimens, tests were carried out on

FSSW bonded joints of untreated AA 5052 alloy, i.e. with the as-rolled surface condition. The

latter specimens were ultrasonically degreased with acetone for 15 min, rinsed with deionised

water, and dried in air at 40 ºC. Triplicate tests were carried out for each type of joint.

PEO-treated AA 5052 alloy specimens before and after joining to PP, and also fracture parts

after lap-tensile shear tests, were examined in plan view and cross-section using a Tescan

MIRAJ field emission scanning electron microscope equipped with energy dispersive X-ray

spectroscopy (EDS) analysis facilities. Cross-sections were ground through successive grades of

SiC paper, followed by finishing with 1-μm diamond paste.

The phase composition of coatings was investigated by X-ray diffraction (XRD), using a Philips

X’Pert ProMPD (PANanalytical) instrument with copper Kα radiation, a step size of 0.02º per

min and a scan range from 5º to 85º (in 2θ). Phases were identified using the Expert database.

The crystallization of the PP in the as-received condition and following joining to the AA 5052

alloy both without and with the PEO pre-treatment was evaluated by differential scanning

calorimetry analysis (DSC), employing a TA DSC Q100 instrument. An 8 mg sample of PP was

taken from the polymer that had flowed into the hole in the alloy in the joined specimens. For the

samples taken from the joined specimens, the temperature was increased to 250 ºC and kept for 5

8

min to eliminate the effects of the previous thermal history of the polymer, which was different

for the polymer in the as-received condition and following heating under FSSW. The

measurements were then performed using a heating rate of 10 ºC/min from 0 to 250 ºC in

aluminium pans under a N2 atmosphere. The endothermic crystallization peak during the scan

was recorded. The specimens were then cooled at a constant rate of 10 ºC/min. The temperature

and energy readings were calibrated during continuous cooling and heating using high purity

zinc and aluminium samples.

Attenuated total reflectance-infrared spectroscopy (ATR-IR, Bruker, Vertex 80, ATR diamond)

was also employed to detect possible thermal degradation of the PP and chemical bonding

between the PEO-treated AA 5052 alloy and the PP after lap-tensile shear test. The resolution

and scanning range were 5 and 4000-400 cm-1 respectively.

3. Results and discussion

3.1 Morphology and composition of PEO-coated AA 5052 alloy before lap shear tests

Figure 2 (a) shows a scanning electron micrograph of the surface of a coating formed on the AA

5052 alloy in the silicate-based electrolyte for 10 min. The surface revealed many pores, with

sizes ranging from ~0.5 μm to ~ 10 μm. Furthermore, nodular agglomerations of coating material

are present at many regions that result in a relatively rough, convoluted surface. In cross-section,

a highly porous coating is evident, with a non-uniform thickness in the range ~ 10 to 30 μm

9

(Figure 2 (b)). EDS elemental area analysis of the coating surface revealed (in at.%) 63.9% O,

12.6% Al, 1.4% Na,1.2% K and 20.9% Si, indicating a silicon-rich outer region. Figure 3 shows

the result of XRD examination of the specimen. In addition to peaks originating from the

penetration on X-rays to the AA 5052 alloy substrate, α-Al2O3, γ-Al2O3 and amorphous material,

which gives rise to the broad peak between ~10º and 25º, were detected from the coating. α-

Al2O3 was the main crystalline phase in the coating. The absence of peaks for silicon-containing

compounds indicates that silicon is associated with the amorphous material, which probably

contains silica [25, 29].

3.2 Lap tensile shear tests and fracture observations

Figure 4 shows the load-extension curves of the tensile lap-shear tests for the AA 5052 alloy-PP

joints. The curves for the three PEO-treated joints show a good reproducibility, with the applied

load increasing at a gradually decreasing rate until failure of the joints. The three tests of the

joints prepared without the PEO treatment displayed poorer reproducibility compared with the

three tests of the PEO-treated specimens. For the latter joints, the maximum load was reached at

an extension in the range 0.5 to 0.6 mm, compared with an extension at failure in the range 1.5 to

2.3 mm for the PEO-treated joints. A failure load of 1228 ± 120 N was achieved in the three tests

using the PEO-treated AA 5052 alloy, compared with 306 ± 135 N without the PEO treatment.

Hence, the PEO treatment enhanced the joint strength by a factor of about three. The large

difference between the shapes of the curves for the PEO-treated and non-treated joints was due

to the different modes of failure. Visual inspection showed that the joints prepared without the

PEO treatment failed by separation of the PP and AA 5052 alloy across the whole of the joined

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area under the applied shear loading. In the absence of the PEO-treatment, the bonding between

the PP and the alloy was evidently relatively poor. Although the PP had penetrated the hole in

the alloy, the mechanical key was relatively ineffective due to the weak bonding with the alloy.

In contrast, the failure of the PEO-treated joints occurred by fracture of the PP close to the hole

in the AA 5052 alloy and a shear failure close to the interface between the PP and PEO coating.

The appearances of the three specimens that were tested were very similar. An example of a

failed specimen is shown in the photograph of Fig. 5 (a). The fracture of the PP occurred across

the specimen width in a direction transverse to the applied load. It was consistently located

adjacent to the edge of the hole in the alloy on the polymer side of the joint, as shown in Fig. 5

(a). The PP detached from the PEO-treated alloy on this side of the joint, but remained attached

to the alloy elsewhere on the joined area. The absence of failure of the joint in the latter area is

attributable to the transfer of the load in this part of the specimen to the alloy side of the joint due

to the mechanical key provided by the ingress of the PP into the hole in the alloy. The presence

of the PEO coating around the sides of the hole had provided a strong bond with the PP. Thus,

the failure of the joint appeared to be due to separation of the polymer from the PEO-treated

surface followed by fracture of the polymer due to localised concentration of the stress in the

vicinity of the hole. The results support previous findings regarding the importance of the

selection of an appropriate surface pre-treatment in order to optimize the joint strength [30, 31].

Figure 5 (b) and 5 (c) show low magnification scanning electron micrographs of the matching

fracture surfaces of the parts of a PEO-treated specimen in the vicinity of the fractured PP.

Figure 5 (b) reveals the PP, with a relatively dark appearance, in the lower part of the image and

the PEO-coated AA 5052 alloy in the upper part of the image. The surface in the latter region

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mainly comprises the PEO coating from which the PP has separated, as will be shown in later

scanning electron micrographs. The former region is the underside of the joined PP sheet. The

white arrow shows a circular depression in the PP, which had flowed into the hole in the AA

5052 alloy. The depression is suggested to have originated due to the displacement of the PP

under the applied load. The dark spots on the PEO-coating, with diameters of up to about 2 mm,

are regions of residual PP that were left attached to the PEO coating at locations of gas bubbles

within the PP. The presence of PP was indicated by detection of a large amount of carbon by

EDS. The bubbles were formed due to degradation of the PP during FSSW, as discussed later.

Their presence may have enhanced the stress concentration within the PP in this region. Figure 5

(c) shows the surface of the PP that has separated from the PEO coating. The PP surface is later

shown to comprise mainly a ductile failure mode. Dark spots due to bubble formation in the PP

match those of the residual PP on the PEO coating in Fig. 5 (b).

Scanning electron micrographs of the PP side of the fracture surface of a joint made with PEO-

treated AA 5052 alloy are shown in Figures 6 (a) and 6 (b). The fracture surfaces shown in Fig. 6

were typical of the whole of the joined area that had failed under the applied load. The label 1 in

Figure 5 (c) indicates the location of the particular images of Figures 6. The micrographs reveal

fibrils of polymer, with lengths of about 20 μm, and particles of coarser material, with a light

appearance, that are shown in more detail in Figure 6 (b). Tearing of the polymer is evident at the

ends of the fibrils. The fibrous appearance of the fracture surface in Figures 6 (a) and 6(b) is

similar to that observed in the plastically-deformed zone of a fractured lap shear joint between

AA 2024-T3 alloy and carbon fibre-reinforced poly (phenylene sulphide) that was prepared by

friction stir joining [32]. The morphology of the fracture surface and that of the present joint are

12

indicative of a ductile mode of failure. EDS point analysis (Table 1) of the region labelled A in

Figure 6 (b), which was located directly under the tool, revealed a high concentration of

aluminium, oxygen and silicon and a relatively low concentration of carbon, which shows that

the particles are remnants of the outer, silicon-rich layer of the PEO coating.

Scanning electron micrographs of the AA 5052 alloy side of the fracture (Figure 7 (a)) revealed

the PEO coating, which at high magnification (Figure 7 (b)) disclosed remnants of the plastically

deformed polymer, which had penetrated into the pores and cracks of the coating surface. The

location of the fracture surface shown in Figure 7 is indicated by the label 2 in Figure 5 (b).

Similar fracture surfaces were observed at other locations on the PP surface that had separated

from the alloy. EDS point analysis (Table 1) of the region labelled B in Figure 7 (a), which was

located directly under the tool, revealed a high concentration of carbon and low concentration of

coating constituents, which indicates that this region is composed mainly of polymer.

Figure 8 shows scanning electron micrographs of the fractured surfaces of a lap shear specimen

that had been prepared by FSSW from untreated AA5052 alloy (i.e with no PEO treatment) and

PP respectively. Figure 8 (a) displays the AA 5052 alloy side of the fractured specimen revealing

the morphology of the original rolled alloy. The alloy surface reveals rolling lines and tears,

which are typical of a rolled aluminium surface [16]. The surface morphology indicates that the

failure had occurred at the interface between the alloy and the polymer. Figure 8 (b) shows the

polymer side of the fractured specimen revealing grooves that are similar to the rolling lines of

the alloy, which indicates that the polymer had flowed into the rolling features on the alloy

surface during FSSW.

13

Evidently, the bonding between the polymer and the alloy is superior for the PEO-treated joint

compared with that of the untreated joint. The bonding in the PEO-coated specimens involved

mechanical interlocking by flow of molten polypropylene into open pores, cracks and cavities,

associated with cohesive-adhesive fracture, whereas the untreated joints revealed an adhesive

failure [15, 32, 33].

Contact of the tool with the AA 5052 alloy work piece during FSSW creates frictional heating

and softens the PP beneath the alloy [8] which creates a pool of molten PP. The molten polymer

matrix fills the cracks and pores on the PEO surface, which provides mechanical interlocking of

the two parts of the joint. The increased adhesion improves the mechanical properties of the joint

in comparison with the untreated AA 5052 alloy. In the FSSW process, there are three sources of

heat generation: (i) friction between the tool and the alloy; (ii) friction between the alloy and PP;

and (iii) plastic deformation of the alloy and PP [34]. The generated heat is distributed through

conduction and melting of the PP. However, heating of the PP above the melting temperature can

degrade the polymer, with bubble formation and gas generation, including water vapour, carbon

monoxide and carbon dioxide [18, 35].

Figure 9 (a) displays a scanning electron micrograph of the fracture surface on the PP side of the

PEO-treated AA 5052 alloy-PP joint at the location marked 3 in Figure 5 (c). EDS analysis

revealed high concentrations of carbon, consistent with regions of polymer. Figure 9 (b) reveals

the details of the surface at higher magnification, disclosing a relatively smooth surface with a

grain-like microstructure, which contrasts with the fibrous appearance of the main fracture

14

surface. The micrographs are attributable to bubble formation in the polymer, which may be

greater in the PP on the PEO-treated joint was greater than on the untreated joint, due to greater

degree of polymer degradation in the presence of the insulating PEO coating [36].

3.3 Effects of FSSW on chemistry and structure of polypropylene

Figure 10 (a) displays the ATR-IR spectrum of the alloy side of the fracture surface of the PEO-

treated AA 5052 alloy-PP joint. Absorption peaks of PP were observed including the bending

vibration of δ CH2 and anitisymmetric vibration of δa CH3 at wavenumber 1453 cm-1 [37].

Symmetric vibrations of δs CH3 at 1384 cm-1 and ρ CH3 at 1165 cm-1 are also evident; the latter

corresponds to the rocking vibration for PP. Furthermore, a sharp peak at 1733 cm-1 can be

assigned to the C=O bond. Vibrational peaks of ν C-C would be expected to occur at 998 and

973 cm-1; however, this region is dominated by large, broad peaks due to inorganic species that

prevents the observation of the C-C vibrational peaks. Figure 10 (b) shows a comparison the

ATR-IR spectra of the fracture surface of a PEO-treated AA 5052 alloy-PP joint examined from

the alloy side, the PEO coated AA 5052 alloy prior to joining and the fracture surface of a non-

treated AA 5052 alloy-PP joint examined from the alloy side. The carbonyl (C=O) bond is only

detected on the PEO-treated AA5052-PP fracture surface. The presence of the carbonyl bond

suggests that thermal degradation of the polymer has occurred due to heating during FSSW [38,

39].

Figure 11 presents the crystallization curves of PP determined by DTA. Figure 11 (a) shows the

curve for the as-received polymer. An endothermic peak is evident at 165.9 °C, from which the

15

crystallinity of the polymer was determined to be 29.7%, using the procedure described

previously by other investigators [40]. The percentage crystallinity was determined using the

following equation: % crystallinity = [Hm / Hm0] x 100, where Hm is the heats of melting the

polymer, determined from the area of the endothermic peak and Hm0 is the heat of melting if the

polymer were 100% crystalline (8.70 kJ/mol). Figures 11 (b) and 11 (c) show the curves for the

polymer extracted from the joints that included the untreated and PEO-treated alloy, respectively.

Three curves are displayed in Figs 11 (b) and 11 (c), which are labelled 1, 2 and 3. Curve 1 was

measured during heating of the polymer to 250 °C. Relatively noisy endothermic peaks occurred

at about 170 °C for the polymer taken from holes in the alloy in both the untreated and PEO-

treated conditions. These peaks are attributable to the crystalline to amorphous transition of the

PP. The noise possibly originates from release of gas bubbles that were formed by degradation of

the PP during FSSW. Due to the noise it was not possible to determine the crystallinity of the PP

after FSSW. Curve 2 was measured during cooling of the polymer after holding the temperature

at 250 °C for 5 min. An exothermic peak due to crystallization of the PP is evident at a

temperature of 115 °C and 110 °C for the polymer taken from the joints without and with PEO

treatment, respectively. Curve 3 was measured during subsequent heating of the polymer to 250

°C. Endothermic peaks are present at a temperature 164.9 °C and 158.1 °C for the polymer of the

joints of the untreated and PEO-treated alloy, respectively. The peaks are smoother than those in

the curves of the initial heating cycle (labelled 1). From the peaks, the crystallinity of the

polymer was measured to be 26.6% and 17.5% in the PP sampled from the joints prepared

without and with a PEO treatment, respectively. Hence, sthe crystallinity was approximately 0.9

and 0.6 times that in the as-received polymer. The difference in the crystallinity is possibly to the

difference in the composition of the PP and also of the cooling rate of the PP in the joints formed

16

with and without the PEO treatment. It is suggested that the greater reduction in the crystallinity

for the joints prepared with the PEO pre-treated alloy is associated with the thermal insulating

property of the PEO coating, which led to a higher temperature and hence greater degradation of

the polymer. In contrast, the crystallinity was only reduced by ~ 10% when PEO was not

employed, which allowed better heat transfer from the PP and reducing the temperature rise of

the PP during FSSW.

4. Conclusions

1. Lap tensile shear strengths of the AA 5052 alloy-PP joints, with PEO pre-treatment of the

alloy in a silicate-based electrolyte, demonstrated remarkably strong joints that failed at a load of

about 1228 ± 120 N compared with 306 ± 135 N for joints prepared without pre-treatment of

the alloy.

2. The joints prepared with the PEO treated alloy revealed a fibrous ductile failure of the PP

close to the interface with the PEO coating. In contrast, an adhesive failure was associated with

the joints prepared using the untreated alloy.

3. SEM and ATR-IR showed that molten PP was incorporated into the highly porous and silica-

rich outer region of the PEO coatings providing micromechanical interlocking and, possibly,

chemical bonding between the polymer and the coating. Thermal degradation of the polymer was

indicated by the presence of carbonyl species in joints prepared using the PEO-coated alloy.

17

4. DSC measurements showed a decrease of ~ 41 % in the crystallinity of the PP in joints made

using the pre-treated alloy compared with ~ 10% for the untreated alloy. The greater reduction

following a PEO pre-treatment is due to the thermal insulating property of the PEO coating,

which results in an increased temperature of the polymer.

Acknowledgements

The authors are grateful to Iran’s National Elites Foundation (BMN) for support of this work

through a postdoctoral fellowship.

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Table

Table 1. Results of EDS analyses of points labelled A and B in Figures 6 and 7.

Figure captions

Figure 1. The arrangement of the AA 5052 alloy and PP for the joining process

.

Figure 2. (a) Scanning electron micrographs of (a) the surface and (b) the cross-section of AA

5052 alloy following PEO for 10 min in the silicate-based electrolyte.

22

Figure 3. XRD pattern for AA 5052 alloy following PEO for 10 min in silicate-based electrolyte.

Figure 4. Results of lap tensile shear test of FSSW joints of AA 5052 alloy to polypropylene (1,

2, 3) without and (4, 5, 6) with PEO coatings.

Figure 5. (a) Photograph of the failed joint from a lap-shear test of a specimen prepared using

PEO-treated aluminium. (b) Scanning electron micrograph of the AA 5052 alloy side of the

fracture surface. (c) Scanning electron micrograph of the polypropylene side of the fracture

surface.

Figure 6. Scanning electron micrographs showing the details of the fracture morphology on the

polypropylene side of a PEO-treated AA 5052-polypropylene FSSW joint. (a) Low

magnification. (b) High magnification.

Figure 7. Scanning electron micrographs showing the details of the fracture morphology on the

alloy side of a PEO-treated AA 5052 alloy-polypropylene FSSW joint. (a) Low magnification.

(b) High magnification.

Figure 8. Scanning electron micrographs showing the details of the fracture morphology on (a)

the alloy side and (b) the polymer side of a non-treated AA 5052 alloy-polypropylene FSSW

joint.

23

Figure 9. (a) Scanning electron micrograph of the polypropylene side of a PEO-treated AA 5052

alloy-polypropylene FSSW joint showing evidence of a bubble formed in the polymer. (b) Detail

of the bubble surface.

Figure 10. (a) ATR-IR spectrum of the fracture surface of PEO-treated AA 5052 alloy. (b)

Comparison of spectra for (i) non-treated AA 5052 alloy (ii) PEO-treated alloy before joining

(iii) PEO-treated AA 5052 alloy after joining.

Figure 11. Results of differential scanning calorimetry (DSC) of the polypropylene: (a) as-

received; (b) from untreated AA-5052 alloy-polypropylene FSSW joint; (c) from PEO-treated

AA 5052 alloy-polypropylene joint.

24