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Effect of plasma electrolytic oxidation on joining of AA 5052 aluminium alloy to polypropylene using friction stir spot welding
Surface & Coatings Technology 313 (2017) 274–281
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Effect of plasma electrolytic oxidation on joining of AA 5052 aluminium alloy to polypropylene using friction stir spot welding S. Aliasghari a, M. Ghorbani a,⁎, P. Skeldon b, H. Karami a, M. Movahedi a a b
Department of Material Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran Corrosion and Protection Group, School of Materials, The University of Manchester, Oxford Rd., Manchester M13 9PL, England, UK
a r t i c l e
i n f o
Article history: Received 5 November 2016 Revised 11 January 2017 Accepted in revised form 21 January 2017 Available online 28 January 2017 Keywords: Aluminium Alloy Polypropylene Plasma electrolytic oxidation Friction stir spot welding
a b s t r a c t 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, thermogravimetric analysis 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. © 2017 Elsevier B.V. All rights reserved.
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
⁎ Corresponding author. E-mail address: [email protected] (M. Ghorbani).
http://dx.doi.org/10.1016/j.surfcoat.2017.01.084 0257-8972/© 2017 Elsevier B.V. All rights reserved.
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 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 bonding 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 of 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
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morphology of the joined materials and the effect of the FSSW process on the chemistry and structure of the polymer.
2. Experimental
Fig. 1. The arrangement of the AA 5052 alloy and PP for the joining process.
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 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 shear test, the fracture
Commercial PP and AA 5052 alloy (Si b 0.25%, Fe b 0.4%, Cu b 0.1%, Mn 0.15–0.35%, Mg 2.2–2.8%, Cr b 0.1%, Al Bal) sheets, both with a thickness of 2 mm, were used. The specimens with dimensions of 70 × 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. The working area 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 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 in deionised water. The chemicals were of analytical grade. The pH of the electrolyte was 12.5. 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 °C by a flow of cold water through the cell wall. A stainless steel (type 304) plate of dimensions 5 × 12 cm was used as a counter electrode. The applied voltage was 300 V; the 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 × 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 Fig. 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 Fig. 1 were employed for the tests. A rectangular
Fig. 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.
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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 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 asrolled surface condition. The latter specimens were ultrasonically degreased with acetone for 15 min, rinsed with deionised water, and dried in air at 40 °C. 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 0.02° per min and a scan range from 5° to 85° (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 pretreatment 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 °C and kept for 5 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 °C/min from 0 to 250 °C in aluminium pans under a N2 atmosphere. The specimens were then cooled at a constant rate of 10 °C/min. The temperature and energy readings were calibrated during continuous cooling and heating using high purity zinc and aluminium samples.
Fig. 3. XRD pattern for AA 5052 alloy following PEO for 10 min in the silicate-based electrolyte.
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 Fig. 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 (Fig. 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. Fig. 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 ~ 10° and 25°, 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 Fig. 4 shows the load-extension curves of the lap tensile 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
Fig. 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.
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Fig. 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.
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 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]. Fig. 5 (b) and (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. Fig. 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 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. Fig. 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).
Fig. 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.
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Table 1 Results of EDS analyses of points labelled A and B in Figs. 6 and 7. at% region
C
O
Na
Mg
Al
Si
K
Au
A B
18.6 77.7
32.2 10.5
1.4 0.4
0.3 0.1
15.2 5.9
30.7 3.3
1.1 0.8
0.5 1.3
Scanning electron micrographs of the PP side of the fracture surface of a joint made with PEO-treated AA 5052 alloy are shown in Fig. 6 (a) and (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 Fig. 5 (c) indicates the location of the particular images of Fig. 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 Fig. 6 (b). Tearing of the polymer is evident at the ends of the fibrils. The fibrous appearance of the fracture surface in Fig. 6 (a) and (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 indicative of a ductile mode of failure. EDS point analysis (Table 1) of the region labelled A in Fig. 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 (Fig. 7 (a)) revealed the PEO coating, which at high magnification (Fig. 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 Fig. 7 is indicated by the label 2 in Fig. 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 Fig. 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. Fig. 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. Fig. 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. Fig. 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. 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]. Fig. 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 Fig. 5 (c). EDS analysis revealed high concentrations of carbon, consistent with regions of polymer. Fig. 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 surface. The micrographs are attributable to bubble formation in the polymer, which may be greater in the PP on the PEO-treated joint 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 the chemistry and structure of polypropylene Fig. 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 antisymmetric 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
Fig. 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.
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Fig. 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.
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 bonds 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. Fig. 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]. Fig. 11 presents the crystallization curves of PP determined by DTA. Fig. 11 (a) shows the curve for the as-received polymer. An endothermic peak is evident at 165.9 °C, from which the crystallinity of the polymer was determined to be 29.7%, using the procedure described previously by other investigators [40]. The percentage crystallinity was calculated using the following equation: % crystallinity = [Hm / Hm0] × 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). Fig. 11 (b) and (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 Fig. 11 (b) and (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, the crystallinity was approximately 0.9 and 0.6 times that in the as-received polymer. The difference in the crystallinity is possibly due to the difference in the composition of the PP and also of the cooling rate of the PP in the joints formed 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 reduced the temperature rise of the PP during FSSW.
Fig. 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.
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Fig. 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.
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. 4. DSC measurements showed a decrease of ~40% 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 (G11/ 70586)(BMN) for support of this work through a postdoctoral fellowship and Sharif University of Technology for funding a grant (G940306).
Fig. 11. Results of differential scanning calorimetry (DSC) of the polypropylene: (a) asreceived; (b) from untreated AA-5052 alloy-polypropylene FSSW joint; (c) from PEOtreated AA 5052 alloy-polypropylene joint.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of plasma electrolytic oxidation on joining of AA 5052 aluminium alloy to polypropylene using friction stir spot welding S. Aliasghari a, M. Ghorbani a,⁎, P. Skeldon b, H. Karami a, M. Movahedi a a b
Department of Material Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran Corrosion and Protection Group, School of Materials, The University of Manchester, Oxford Rd., Manchester M13 9PL, England, UK
a r t i c l e
i n f o
Article history: Received 5 November 2016 Revised 11 January 2017 Accepted in revised form 21 January 2017 Available online 28 January 2017 Keywords: Aluminium Alloy Polypropylene Plasma electrolytic oxidation Friction stir spot welding
a b s t r a c t 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, thermogravimetric analysis 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. © 2017 Elsevier B.V. All rights reserved.
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
⁎ Corresponding author. E-mail address: [email protected] (M. Ghorbani).
http://dx.doi.org/10.1016/j.surfcoat.2017.01.084 0257-8972/© 2017 Elsevier B.V. All rights reserved.
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 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 bonding 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 of 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
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morphology of the joined materials and the effect of the FSSW process on the chemistry and structure of the polymer.
2. Experimental
Fig. 1. The arrangement of the AA 5052 alloy and PP for the joining process.
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 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 shear test, the fracture
Commercial PP and AA 5052 alloy (Si b 0.25%, Fe b 0.4%, Cu b 0.1%, Mn 0.15–0.35%, Mg 2.2–2.8%, Cr b 0.1%, Al Bal) sheets, both with a thickness of 2 mm, were used. The specimens with dimensions of 70 × 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. The working area 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 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 in deionised water. The chemicals were of analytical grade. The pH of the electrolyte was 12.5. 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 °C by a flow of cold water through the cell wall. A stainless steel (type 304) plate of dimensions 5 × 12 cm was used as a counter electrode. The applied voltage was 300 V; the 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 × 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 Fig. 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 Fig. 1 were employed for the tests. A rectangular
Fig. 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.
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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 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 asrolled surface condition. The latter specimens were ultrasonically degreased with acetone for 15 min, rinsed with deionised water, and dried in air at 40 °C. 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 0.02° per min and a scan range from 5° to 85° (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 pretreatment 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 °C and kept for 5 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 °C/min from 0 to 250 °C in aluminium pans under a N2 atmosphere. The specimens were then cooled at a constant rate of 10 °C/min. The temperature and energy readings were calibrated during continuous cooling and heating using high purity zinc and aluminium samples.
Fig. 3. XRD pattern for AA 5052 alloy following PEO for 10 min in the silicate-based electrolyte.
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 Fig. 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 (Fig. 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. Fig. 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 ~ 10° and 25°, 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 Fig. 4 shows the load-extension curves of the lap tensile 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
Fig. 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.
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Fig. 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.
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 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]. Fig. 5 (b) and (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. Fig. 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 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. Fig. 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).
Fig. 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.
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Table 1 Results of EDS analyses of points labelled A and B in Figs. 6 and 7. at% region
C
O
Na
Mg
Al
Si
K
Au
A B
18.6 77.7
32.2 10.5
1.4 0.4
0.3 0.1
15.2 5.9
30.7 3.3
1.1 0.8
0.5 1.3
Scanning electron micrographs of the PP side of the fracture surface of a joint made with PEO-treated AA 5052 alloy are shown in Fig. 6 (a) and (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 Fig. 5 (c) indicates the location of the particular images of Fig. 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 Fig. 6 (b). Tearing of the polymer is evident at the ends of the fibrils. The fibrous appearance of the fracture surface in Fig. 6 (a) and (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 indicative of a ductile mode of failure. EDS point analysis (Table 1) of the region labelled A in Fig. 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 (Fig. 7 (a)) revealed the PEO coating, which at high magnification (Fig. 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 Fig. 7 is indicated by the label 2 in Fig. 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 Fig. 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. Fig. 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. Fig. 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. Fig. 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. 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]. Fig. 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 Fig. 5 (c). EDS analysis revealed high concentrations of carbon, consistent with regions of polymer. Fig. 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 surface. The micrographs are attributable to bubble formation in the polymer, which may be greater in the PP on the PEO-treated joint 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 the chemistry and structure of polypropylene Fig. 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 antisymmetric 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
Fig. 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.
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Fig. 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.
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 bonds 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. Fig. 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]. Fig. 11 presents the crystallization curves of PP determined by DTA. Fig. 11 (a) shows the curve for the as-received polymer. An endothermic peak is evident at 165.9 °C, from which the crystallinity of the polymer was determined to be 29.7%, using the procedure described previously by other investigators [40]. The percentage crystallinity was calculated using the following equation: % crystallinity = [Hm / Hm0] × 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). Fig. 11 (b) and (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 Fig. 11 (b) and (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, the crystallinity was approximately 0.9 and 0.6 times that in the as-received polymer. The difference in the crystallinity is possibly due to the difference in the composition of the PP and also of the cooling rate of the PP in the joints formed 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 reduced the temperature rise of the PP during FSSW.
Fig. 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.
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Fig. 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.
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. 4. DSC measurements showed a decrease of ~40% 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 (G11/ 70586)(BMN) for support of this work through a postdoctoral fellowship and Sharif University of Technology for funding a grant (G940306).
Fig. 11. Results of differential scanning calorimetry (DSC) of the polypropylene: (a) asreceived; (b) from untreated AA-5052 alloy-polypropylene FSSW joint; (c) from PEOtreated AA 5052 alloy-polypropylene joint.
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