Effect of an anodizing pre-treatment on AA 5052 alloy/polypropylene joining by friction stir spot welding

S. Aliasgharia,c, P. Skeldonb, X. Zhoub, T. Hashimotob,

a ASTeC, STFC Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK.

bCorrosion and Protection Group, School of Materials, The University of Manchester, Oxford Rd., Manchester M13 9PL, England, U.K.

cDepartment of Material Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466. Azadi Avenue, 14588 Tehran, Iran.

Abstract

A study has been carried out of the effect of an anodizing pre-treatment in a sulphuric acid electrolyte on the strength of AA 5052 alloy/polypropylene joints prepared using friction stir spot welding. Lap-shear tests were used to determine the joint strength. Comparisons were made with joints pre-treated using sand blasting. The failed specimens were examined by scanning and transmission electron microscopy. Anodizing improved the strength of the joints by a factor of about 6 compared with sandblasting. For the anodizing pre-treated joints, melted polymer infiltrated deeply within the nanoporous anodic film, forming a strong polymer-film bond. Joint failure occurred by ductile tearing of the polymer at or near the film surface. In contrast, sandblasted joints failed at the alloy/polymer interface.

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

1. Introduction

Joining of polymers, light metals and composites in multi-material components and structures is of interest in automobile and aircraft design for weight reduction [1, 2]. Various methods for joining dissimilar material have been investigated, such as adhesive bonding [3-6], mechanical fastening [7, 8] and welding-based methods [9-11], including friction stir spot welding (FSSW), which can be employed to join metals to fibre-reinforced polymer composites [12-15].

The present work examines the effect of an anodizing pre-treatment in a sulphuric acid electrolyte on the strength of AA 5052 aluminium alloy/propylene joints prepared by FSSW. The joints were subjected to lap-shear tests, followed by fractographic examination. As a reference condition, joints were prepared by sandblasting. Anodizing under the selected conditions produces nanoporous amorphous alumina films [16-19]. It is widely used in industry to assist bonding with paint systems, for corrosion protection [20], and with adhesives [21], for joining of aluminium parts. However, no studies of the influence of anodizing on FSSW aluminium alloy/polymer joining appear to have been reported in the literature.

2. Experimental

AA 5052 alloy (Mg 2.34, Cr 0.2, Fe 0.23, Si 0.06, Mn 0.006 bal. Al (wt.%)) and polypropylene (C30S grade – tensile strength 32 MPa, hardness 81 HV) sheets, with a thickness of 2.0 and 2.8 mm respectively, were employed for the study. The alloy composition according to optical emission spectroscopy (OES) is given in Table 1. Specimens with dimensions of 70 x 30 mm were cut from the alloy and polypropylene sheets.

Sandblasting of the alloy employed 180 μm SiO2 particles, applied with an air pressure of 8 bar, a flow rate of 2.17 m min-1, and an impact angle of 90°. The nozzle was positioned 10 cm from the alloy surface. The treatment time was 45 s. A compressed air jet was used to remove any SiO2 particles and debris from the surface.

Prior to anodizing, specimens were etched in 20 g l-1 NaOH solution for 120 s at 60 °C, then desmutted in 30 vol.% nitric acid (70 vol.%) for 30 s at room temperature. Following rinsing in deionized water, the specimens were anodized at a constant voltage of 16 V in stirred 2.0 M sulphuric acid at 23 °C for 10 min. The anodic film was formed on a 30 x 30 mm region that was subsequently joined to the polypropylene. A two-electrode cell was employed containing 1 dm3 of electrolyte. The cathode was a sheet of type 304 stainless steel of size 10 x 5 cm. After anodizing, the specimens were rinsed in deionized water and dried in a cool air stream. As shown later, the anodizing treatment resulted in a porous film about 0.75 μm thick

FSSW joints were prepared using a friction stir welding facility. The polypropylene and AA 5052 alloy specimens were clamped together with an overlap of 30 x 30 mm. The details of the arrangement of the specimens for the joining process and the conditions of FSSW have been published previously [22].

Lap-shear testing of the joints were carried out in a Hounsfield H10 KS 50 kN Universal Testing Machine, with QMAT software. A rectangular piece of the alloy was inserted into the grip holding the end of the polymer. Similarly, a rectangular piece of the polymer was inserted into the grip holding the end of the alloy. A length of 1 cm of the alloy and polymer was present between the joined region and the grip. The inserted pieces were used in order to ensure that the joints were subjected mainly to a shear stress. An extension rate of 0.5 mm min−1 was applied until joint failure and the load versus extension was recorded. The extension was determined from the displacement of the crosshead and included the extension in the alloy and polymer outside the joined region. Triplicate tests were carried out for each type of joint.

Alloy specimens prior to bonding were examined using Zeiss Evo 50 and Zeiss Ultra 55 scanning electron microscopes. Fracture parts after lap-tensile shear tests were examined using a Tescan MIRAJ field emission scanning electron microscope equipped with energy-dispersive X-ray (EDX) analysis facilities. The fractured specimens were gold-coated to avoid charging during examination. EDX analyses were made in rectangular areas of dimensions 500 x 300 μm.

Electron-transparent sections of nominal thickness 20 nm were prepared for tansmission electron microscopy (TEM) using a Leica Ultracut microtome, with initial trimming of specimens using a glass knife and final sectioning using a Micro-Star type SU diamond knife. The sections were examined using aTECNAI F30 G2 instrument, operated at 300 kV, with a Gatan imaging filter (GIF2001).

3. Results and discussion

3.1 Surface morphology of the pre-treated AA 5052 alloy.

Figure 1 presents scanning electron micrographs of the sand-blasted and anodized alloy surfaces. Sandblasting produced a cratered surface due to the erosion and plastic deformation caused by the particle impact (Fig. 1 (b)). Deep cavities, with sizes up to about 100 μm, and fragments of eroded material, produced by ploughing, cutting and plastic deformation, were also present. The anodized alloy displayed a uniform fibrous appearance, with fine porosity evident within the anodic film structure (Fig. 1 (c)). As shown later, the pores were of the order of 10 nm in diameter. Anodizing in sulphuric acid produces a porous film comprising roughly hexagonal cell of amorphous alumina each containing a central roughly cylindrical pore that extends from a thin barrier layer next to the substrate to the film surface [16, 23]. The cell and pore diameters are determined by the anodizing voltage [16, 23]. The films also contain sulphate ions derived from the electrolyte [19]. The fibrous appearance of the film results from thinning of the cell walls due to chemical dissolution by the sulphuric acid during the anodizing process [24], which causes cells to collapse and coalesce following removal from the electrolyte.

Figure 1. Scanning electron micrographs (secondary electrons) of the surface of the AA 5052 alloy: (a) sandblasted; (b) anodized.

3.2 Lap-shear tensile tests and fractography of joints

Figure 2 compares the load-extension curves from the lap tensile shear tests of joints prepared with sandblasting and anodizing. The average strength of the anodized joints was1792 kN, compared with 292 kN for the ones sandblasted. Anodizing also provided greater extensions at failure, about 1.3 mm, compared with <0.3 mm.

Figure 2. Force-extension curves from lap-shear tests of AA 5052 alloy/polypropylene joints with the alloy in the sandblasted and anodized conditions.

3.3 Lap-shear tensile tests and fractography of joints prepared using the anodized alloy

Figure 3 (a) presents optical images of the alloy and polypropylene parts of a fractured joint prepared using anodizing. The light triangular material at the bottom left-hand corner of the joined region of the alloy part consists of polypropylene, which has broken from the polypropylene (corresponding to the black area at the top left-hand corner of the joined region of the polypropylene part) and remained firmly attached to the anodized alloy. Dendritic features due to gas bubbles formed by degradation of the polymer during FSSW [22] are evident on both sides of the joint. The gas bubbles form close to the alloy side of the joint. Thermal degradation of the polymer can generate hydrogen and aliphatic and unsaturated hydrocarbons [25, 26], and in the presence of oxygen CO, CO2 and H2O [27]. Following joint failure, a thin layer of polymer is left attached to the alloy immediately beneath the bubbles. As discussed previously [22], the bubbles shield the underlying alloy/polymer interface from the applied shear stress, transferring the load to the bubble-free regions of the interface.

The scanning electron micrograph of Fig. 3 (b-1) shows a bubble boundary on the aluminium part of the fractured specimen. The grain-like appearance of re-solidified polymer is evident in the region labelled A, where EDX analysis detected mainly carbon (98 at.%) and a small amount of oxygen (2 at.%). The morphology of region B resembles the anodized alloy prior to bonding. Here EDX analysis revealed aluminium (72 at.%), oxygen (27 at.%) and sulphur (1 at.%) from the anodic film and the underlying alloy. Figure 3 (b-2) shows the ductile fracture of the polymer at the boundary between the two regions (boxed region in Fig. 3 (b-1)).

Figure 3 (c-1) presents the boundary of a gas bubble on the polymer side of the fractured specimen. The morphology of the polypropylene in the top half of the micrograph replicates the anodized alloy surface. The bottom half of the micrograph reveals the grain-like appearance usual for the interior of a gas bubble. EDX analysis revealed mainly carbon at both regions, confirming the presence of polypropylene. The higher magnification image of Fig. 3 (c-2) shows details of the polypropylene in the boxed region of Fig. 3 (c-1). Relatively large fibres are present at the boundary between the two regions (see arrow) produced by ductile failure of the polymer.

Figure 3 (d-1) presents a scanning backscattered electron micrograph of an ultramicrotomed cross-section of the fractured polypropylene/anodized alloy joint. The cross-section was prepared by cutting the specimen with a diamond knife. The section originates from a bubble-free region of the joint. The anodic film has an average thickness of about 0.75 μm. The film appears to be adherent to the alloy at all regions of the alloy/film interface. The film surface is irregular due to the chemical dissolution that occurred during anodizing, which results in the more open film porosity in the outer half of the film compared with the underlying region. The grading of the porosity originates from the time of exposure of the film to the anodizing electrolyte, which decreases from the film surface to the alloy/film interface, since the film material at the surface was first formed.

Figure 3 (d-2, d-3) presents transmission electron micrographs of ultramicrotomed sections from the polypropylene/alloy interface. The lower magnification micrograph of Fig. 3 (d-2) shows the anodic film, of thickness about 0.45 μm, attached to the alloy. The film is thinner than observed by SEM, which is due to detachment of the more porous outer part of the film under the stress of producing the thin sections. The film reveals cells of anodic alumina of about 25 nm diameter, with pores of about 10 nm diameter (see inset). The inset region shown extends to a depth of about 0.15 μm into the film. The pores are filled by melted and re-solidified polypropylene throughout the region, indicating that melted polypropylene had infiltrated deeply into the pores. The results indicate that failure of the joint in the bubble-free regions involved ductile tearing of the polymer at or close to the anodic film/polymer interface. The crack then propagated through the bubbles, leaving the thin layer of polymer attached to the anodic film.

Figure 3. Optical (a) scanning electron (b,c, d-1) and transmission electron (d-2) micrographs of an anodized AA 5052 alloy/polypropylene joint.: (a) fracture surfaces; (b 1-2) alloy side; (c 1-2) polypropylene side. The arrows in (a) show the locations of images in (b 1-3) and (c 1-3). The boxes in (b-1) and (c-1) show the locations of images in (b-2) and (c-2). (d-1, d-2) Ultramicrotomed cross-sections. (d-2).

The strengths of the anodized joints were similar to those achieved previously when the alloy was pre-treated using plasma electrolytic oxidation (PEO) to form a porous coating [22]. The anodizing pre-treatment has potential advantages over PEO in offering a lower cost, lower energy consumption and widely available process. Further improvement in the bond strength may be obtained from modification of FSSW parameters to reduce the thermal degradation of the polymer and of the anodizing conditions to further facilitate infiltration of polymer into the porosity and improve chemical bonding with the oxide. In contrast, the sandblasting resulted in failure at or close to the alloy/film interface, with weak bonding between the alloy and polypropylene. The alloy surface appeared similar to that of the original sandblasted alloy, although the fragmented debris observed on the latter was less in evidence, possible due to coverage by residual polymer. The polymer side of the fracture revealed a grain-like appearance of the re-solidified polypropylene in the bubble-region, and replicated the sand-blasted surface in the adjacent region, where the polymer revealed fibres and tears due to a plastic mode of failure.

4. Conclusions

1. Lap tensile shear tests of the AA 5052 alloy-polypropylene FSSW joints revealed a large improvement in the joint strength, by about a factor of six, using an anodizing pre-treatment of the alloy in sulphuric acid compared with a sandblasting pre-treatment.

2. The anodizing condition selected resulted in a 0.75μm thick nanoporous film. The porous film facilitated infiltration of porosity by melted polypropylene and provided strong adhesion between the polypropylene and the anodized alloy.

3. Failure of joints prepared using anodizing involved ductile tearing of the polymer near the anodic film surface. In contrast, sandblasting led to failure at or close to, the alloy/polypropylene interface, with relatively weak bonding between the alloy and polymer.

Acknowledgements

Authors acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 665593 awarded to UKRI Science and Technology Facilities Council (STFC). The authors also are grateful to Iran’s National Elites Foundation (BMN) for support of this work through a postdoctoral fellowship.

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