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Study of bending strength for aluminum reinforced with epoxy composite.

ABSTRACT

Fiber reinforced composite-metal hybrids are of great interest in automotive industry because of their excellent mechanical properties and light-weighting potential. Adhesive bonding is a preferred joining technique for manufacturing fiber reinforced composite metal hybrids. Metal surface due to the presence of process contaminants (oil, dirt, oxide layer etc.) is difficult to bond. The surface prior to adhesive application needs to be cleaned or modified using surface pretreatment methods. Especially for adhesive bonding of multimaterial joining, surface treatment is a very important step. In this paper, 7xxx series extruded aluminum alloy surface was treated using different conventional surface treatment methods and bonded with glass fiber reinforced epoxy tape/mat (FRP). The intent is to stiffen aluminum with lightweight FRP for energy absorbing part application.

The treated surface was characterized using water contact angle measurement for surface energy, optical profilometry for surface roughness, SEM for surface features and Auger electron spectroscopy for chemical analysis of surface. The adhesion strength between Aluminum and continuous glass fiber reinforced epoxy mat was evaluated by lap shear test. The bending strength of aluminum reinforced with epoxy composite was also evaluated to understand the relation between bonding mechanism and mechanical properties. We observed that surface treatment improves adhesion between aluminum and the epoxy composite and thereby results in higher mechanical properties and energy absorption.

CITATION: Khan, S., Sarang, S., and Hiratsuka, I., "Study of Bending Strength for Aluminum Reinforced with Epoxy Composite," SAE Int. J. Mater. Manf. 9(3):2016, doi:10.4271/2016-01-0516.

INTRODUCTION

7xxx aluminum alloys are relatively new in automotive industry. They are extensively used in aerospace because of high strength to weight benefit. Automotive industry is also getting more and more interested to utilize 7xxx series aluminum alloys to meet the increasing demand of vehicle light-weighting. Fiber reinforced plastic (FRP) is another avenue of light-weighting that is becoming popular in automotive. An exciting opportunity to design efficient and lightweight vehicle part is to utilize local reinforcement of FRP in metal part or vice versa. This paper presents a study involving a hybrid laminate of 7xxx aluminum alloy and glass fabric reinforced epoxy tape. Five different types of common surface treatment were applied on aluminum surface prior to tape application. The effect of surface treatment on adhesion strength and bending strength of the hybrid laminate were studied.

Fiber reinforced plastic and metal hybrids are used in aerospace industry for decades [1,2]. Automotive industry also uses FRP tape materials to locally stiffen a metal part, especially to reinforce thin metal sections. On the other hand, thin metal sheets are sometimes used to co-mold or bond with plastic parts for providing enhanced strength and crash resistance. The typical applications of these metal/FRP hybrid materials are in body parts, such as door module, roof module, front end carrier module etc. The performance of the part depends on the proper bonding between the metal and the plastic composite. Since, most of the commercially available fiber reinforced tape materials are developed to adhere onto steel, it is worth investigating their bond performance with extruded aluminum specially 7xxx aluminum alloy that is utilized for crash absorbing components in vehicles such as bumper beam, door beam and so on [3].

Surface preparation is an important prerequisite for adhesive bonding. The purpose of surface preparation is either or a combination of the following:

1. to clean the surface from contamination that is detrimental to adhesion,

2. to abrade or roughen the surface to create sites for mechanical interlocking with the adhesive,

3. to raise the surface-free energy of the substrate to enhance wetting of the surface,

4. to change surface chemical composition in order to facilitate molecular (chemical) attachment of adhesive to the surface

Aerospace industry have developed and optimized a wide variety of surface treatment methods for aluminum bonding [4,5,6]. The surface treatment methods (Table 1) utilized for this paper were selected based on extensive literature review and ease of adoption in automotive industry [7,8,9].

MATERIALS AND METHOD

Materials

Test coupons of dimensions shown in figures 1 and 2 were sectioned from extruded aluminum part. Composite tapes of similar dimensions were then applied on aluminum surface. The test pieces were cured at 165[degrees] C for 35 minutes. The aluminum parts used for this study are produced by Aisin Light Metals. The alloy is similar to 7003-T5 in composition. The composite tape is a magnetic, heat curing epoxy sealant laminated to a 0-90 woven fiberglass reinforcing layer from Henkel Corporation (Terocore 16005[TM]) available in 1-2 mm thickness.

Methods

Surface Pretreatment

The common aluminum surface pre-treatments classified as [4,5]:

1. Mechanical: Grit blasting, sanding

2. Etching: Alkaline, acid, other

3. Anodizing: DC, AC etc.

4. Coupling: saline, sol gel, conversion coating

5. Physical: Plasma, laser etc.

After extensive literature review the following surface treatment methods (Table 1) were selected based on two criteria: 1) Proven method for aluminum adhesion bonding, and 2) Feasible and low cost process for high volume automotive application.

Surface Characterization

Aluminum surface after each surface treatment was characterized to measure water contact angle and surface roughness. The oxide layer chemical composition was studied using Auger Electron Spectroscopy (AES). Plasma treated surface was evaluated within 3 hours of treatment.

Contact Angle Measurement

Contact angle is a measure of wettability of the surface. Water contact angle was measured on each treated aluminum surface and the untreated surface as the control using a Rame-hart 200-F1goniometer.

Surface Roughness

Surface roughness was measured using an Olympus LEXT OLS4000 laser microscope. An arithmetic average height roughness (Ra) was measured over an area of 640umx640 um.

Auger Electron Spectroscopy

Auger electron spectroscopy data were collected on PHI 680 Auger nanoprobe equipped with a field emission electron gun and a cylindrical mirror kinetic energy analyzer. All samples were measured using a 5kV10nA beam with a beam cross section of about 35 nm on the sample surface under ultra-high vacuum better than 1X[10.sup.-9] torr. Typically 25 scans were averaged to achieve good signal-to-noise ratio. The surface elemental compositions were calculated from the first order derivative of the averaged data by using the sensitivity factors in the MultiPak software V.9.6.0.15.

Scanning Electron Microscopy

Scanning electron microscopy was performed to observe differences in surface profile and cross-section observation. A Joel JSM-6480LV system was used for SEM measurements.

Mechanical Testing

Lap Shear Test

Lap Shear Test was performed according to ASTM D5868 with some modifications in the sample preparation as shown in Figure 1.

The cured glass epoxy mat was bonded with another piece of aluminum coupon using a structural adhesive. Emery cloth was used to protect the epoxy mat in the grip area during lap shear test.

3-Point Bend Test

3-point bend test was performed according to ASTM D7264 with some modifications in the sample preparation as shown in Figure 2.

RESULTS

Surface Characterization

The most important variables that affect bonding are: wettability, surface micro-roughness, oxide-layer chemistry, and chemical groups present on the surface [4-5].

Aluminum has a thick native oxide layer that prevents corrosion. However, the native oxide layer may be contaminated, too thick or loosely bound to the bulk material [12-13]. Therefore, surface treatment attempt to remove the native oxide layer and form a new and clean oxide layer with controlled thickness, physical and chemical structure [4,13,14]. The following sections provide the surface characterization results.

Contact Angle Measurement

Water contact angle measurement helps to determine the relative surface energy of surfaces with different pretreatment. The method measures the contact angle between a droplet of water and the surface. If the surface has higher energy than the surface tension of water (~72dyne/cm), the droplet will wet the surface. As a result, the contact angle will be lower than 90[degrees]. Similarly, if the surface energy is low, the water droplet will not spread or wet the surface. As a result, the contact angle will be higher than 90[degrees]. By measuring the contact angle, it is possible to determine if the pretreatment helped to increase the surface energy and provide better wetting. Better wetting of the surface by the adhesive is required for better adhesion. However, water contact angle is not always an accurate prediction of adhesive wetting or adhesion strength. There are other factors such as polarity and roughness of the surface that also affect the wetting. Also, the surface energy of epoxy adhesive is between 39-46.8 dyne/cm at room temperature which is much lower than the surface tension of water [11]. Therefore, epoxy should be able to wet a surface with surface energy higher than ~47 dyne/cm. It is also found that during curing at high temperature, the surface tension of epoxy gets as low as 0.4 dyne/cm [10,11]. Table 2 presents the contact angle data on the pretreated aluminum surfaces with different treatment methods. It is observed that only plasma treatment increase the surface energy and make the aluminum surface hydrophilic. Since, the surface treatment methods were developed to address durability of the bonded joints in corrosive environment, surface hydrophobicity will play an effective role in long term durability of the joints [15].

Surface Roughness

Surface roughness helps adhesion by providing larger surface area and sites for mechanical interlocking between the adhesive and the metal surface. Therefore, certain degree of roughness is desirable. However, the benefit of surface roughness on adhesion always depends on the specific surface and adhesive type. Table 3 lists the roughness values for different surface treatments on aluminum.

Figure 3 presents the surface profilometry pictures of Aluminum surface treated with different methods showing comparison of 3D profile taken by laser profilometer.

From the data it is found that except coating, all other pretreatment methods changed the surface roughness of Aluminum. Grit blasting and anodizing increased the roughness, on the other hand, acid etch and plasma decreased the surface roughness.

Auger Electron Spectroscopy

Auger Electron Spectroscopy (AES) is a surface sensitive technique. It is capable to detect the elements present in a few nm thin layer of outermost surface that play very important role in forming adhesive bonding.

Figure 4 shows the ratio of Oxygen to Aluminum (Top) and the ratio of Magnesium (Mg) to Aluminum (Bottom). It has been reported previously that the native oxide layer of Aluminum alloy consists of high concentration of Mg. The structure of the oxide layer at the air-oxide boundary is in the form of MgxAlyOz. It was also reported that presence of Mg in the oxide layer is not favorable for good adhesion and corrosion resistance [13].

It is observed that on untreated aluminum the outer oxide layer is rich with Mg consistent with the literature. Grit blasted samples also have considerable amount of Mg in the oxide layer. Since grit blasting is a mechanical abrasion method, the treatment may not be uniform on the surface in the micro-scale. P2 Etch is able to remove the Mg oxide layer most effectively. The conversion coating cleaned and covered the outer layer of aluminum surface. Therefore, Mg spectrum was not visible on coated surface.

Scanning Electron Microscopy

Scanning electron microscopy showed different surface topography due to different surface treatment. Figure 5 present the SEM pictures with side by side surface and cross-section observation.

The SEM observation supports the observation by profilometer. The anodized and grit blasted surfaces show higher roughness than the other surfaces. The cross section views show the wetting of the adhesive on surfaces. At 10,000x magnification, all surfaces seem to have good wetting. However, following are some observation that may be relevant to adhesion. The control has a thin uniform interface between the adhesive and aluminum (Figure 5(a)). This interface may be due to the native oxide layer. The grit blasted surface showed some crack formation on the aluminum surface creating areas where adhesive may be bonded with loose aluminum particles (Figure 5(b)). The etched surface (Figure 5(c)) shows pitting or preferential etching on the alloy surface. The adhesive and anodized aluminum interface looks fuzzy indicating fine nano-structure on the surface (Figure 5(d)). The coated and plasma treated surfaces show good wetting (Figures 5(e) and (f)) and thinner interface than the control.

Mechanical Testing

Lap Shear Test

Lap shear test shows significant difference in adhesion strength between treated and untreated samples (Control). The surface with the conversion coating showed the strongest bond (Figure 6).

Figure 7 presents the lap shear values and surface roughness in the same graph. No correlation is found for control and coated surface.

As previously discussed, due to presence of loosely bound oxide layer and contaminations, adhesion to non-treated (control) aluminum may be poor. On the other hand, the conversion coating forms strong chemical bond with the substrate and creates chemical bonding sites on the surface for the adhesive to attach. The coating is a Zr-Ti Oxide based conversion coating commonly used for corrosion protection of metal [16]. With embedded functional groups within the coating, it also works as an adhesion promoter by facilitating strong chemical bonding between the surface and the adhesive. Plasma treatment was also able to produce strong bonding. Therefore, adhesion mechanism in control (non-treated surface) and surface treated with conversion coating are not governed by roughness. Plasma pretreats the surface by cleaning contaminants, etching/polishing and increasing surface energy/wettability [17]. Atmospheric plasma is also able activate aluminum surface by creating reactive OH groups on the surface [18]. Anodizing and grit blasting show lower adhesion performance than the other methods. As seen by the cross sectional view, grit blasting created weak spots on the aluminum surface structure resulting in weak adhesion. Also, sulfuric acid anodizing may create fine porous structure on aluminum that may cause air trapped in the pores [19]. Since the adhesive is not in liquid form, it is possible that the adhesive was not able to wet the inner pores of the anodized surfaces resulting in weak adhesion.

3-Point Bend Test

3-point bend test measures the flexural strength and stiffness of the hybrid laminate of 7xxx series aluminum and glass fiber reinforced epoxy. Bare aluminum test pieces were included in the 3 point bend test to compare the mechanical properties with the hybrid laminates. Figure 8 represents the load vs. displacement behavior. The control sample (un-treated aluminum) gave similar curve as the bare aluminum. On the other hand the surface treated samples showed higher slope and maximum load value indicating higher stiffness and strength. Figure 9 and 10 present the maximum load and average energy absorption. Conversion coating and Plasma treatment shows highest load and energy absorption capability with least sample variation. The bend test result shows conformance with lap shear results suggesting adhesion improves mechanical properties.

From the data it is observed that coating and plasma were able to increase the energy absorption about 30-31% compared to bare aluminum.

SUMMARY/CONCLUSIONS

7xxx series extruded aluminum alloy was bonded with a 0/90 glass fabric reinforced epoxy tape to create a hybrid laminate. The aluminum surface was prepared using five different surface treatment methods: Grit blasting, P2 etching, DC sulfuric acid anodizing, plasma and conversion coating. Plasma increased the surface energy of aluminum; the other methods decreased the surface energy and made the surface hydrophobic. AES measurement showed that the native oxide layer on aluminum is rich in Mg. It was also observed by AES that there was variation in the ability of the surface treatment methods to clean the native oxide layer. Lap shear test and 3 point bend test showed significant improvement of adhesion and flexural properties in surface treated test pieces when compared with untreated test pieces. Plasma treatment and conversion coating showed 30-31% increase in energy absorption compared to bare aluminum. These results suggest that enhanced adhesion between aluminum and composite improves mechanical properties. When compared with bare aluminum and non-surface treated samples, hybrid laminates with surface treatment showed higher stiffness, strength and energy absorption. Adhesively bonded aluminum and FRP hybrid with optimum surface treatment can be a promising material for high performance lightweight automotive application.

REFERENCES

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[2.] Botelho, E. C., Silva, R. A., Pardini, L. C., and Rezende, M. C., "A review on the development and properties of continuous fiber/epoxy/aluminum hybrid composites for aircraft structures," Materials Research 9(3):247-256, 2006

[3.] Aiura, T., Yamashita, H., Oka, T., and Hashimoto, N., "Anti-collision extrusion parts made of aluminum alloy for automobiles," Research and Development-Kobe-, 52(3): 83-86, 2002

[4.] Park, S. Y., Choi, W. J., Choi, H. S., Kwon, H., and Kim, S. H., "Recent trends in surface treatment technologies for airframe adhesive bonding processing: a review (1995-2008)," The Journal of Adhesion, 86(2): 192-221, 2010

[5.] Critchlow, G. W., and Brewis, D. M, "Review of surface pretreatments for aluminium alloys," International Journal of Adhesion and Adhesives, 16(4): 255-275, 1996

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[10.] Abbott, J. R., and Higgins, B. G., "Surface tension of a curing epoxy," Journal of Polymer Science Part A: Polymer Chemistry, 26(7): 1985-1988, 1998

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[13.] Wilkinson, T., "The Effect of Aluminum Oxide Structure on Adhesive Bonding," SAE Technical Paper 831391, 1983, doi:10.4271/831391.

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[16.] Guan, Y., Liu, J. G., and Yan, C. W., "Novel Ti/Zr Based Non-Chromium Chemical Conversion Coating for the Corrosion Protection of Electrogalvanized Steel," International Journal of Electrochemical Science, 6: 4853-67, 2011

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CONTACT INFORMATION

Saida Khan

skhan@aisintca.com

ACKNOWLEDGMENTS

The authors would like to appreciate the support of Henkel Corporation surface treatment of the test samples.

Saida Khan, Santosh Kumar Sarang, and Ichiro Hiratsuka Aisin Technical Center of America

Table 1. A summary of the pretreatment steps studied in this paper.

Pre-treatment  Treatment     Description
               Type

Grit           Mechanical    Pressure: 0.4 MPa; Approach angle:
Blasting                     45[degrees]; Distance: 60 mm; Grit:
                             180 mesh Aluminum oxide
Sulfoferric    Acid Etching  * Degreasing: Bonderite C-AK 4335
etching                      (2min @ RT) followed by DI Rinse
(P2)                         * P2 solution etch[1]: 8min
                             @ 145[degrees] F
                             * DI water rinse: 2-3 min @RT
                             * Oven dry: 30min @ 60[degrees]C
Sulphuric      DC            * Cleaning: Bonderite C-AK 298 (10 min
acid           anodizing     @ 140[degrees] F) followed by DI Rinse
anodizing                    * Etch: Bonderite C-AK ALUM ETCH 35 (5 min
                             @ 143[degrees] F) followed by DI Rinse
                             * De-smut: SC-592 (10.5/to visual de-smut
                             @ RT) followed by DI Rinse
                             * Anodizing: Bonderite M-ED 730 Additive
                             to Sulfuric Acid (15wt%/12V/12A/15 min
                             @ 68[degrees] F) followed by DI Rinse
Plasma         Ablation      PlasmaTreat Openair plasma;
               /Oxidation    Distance: 6mm and Speed: 2m/min
Adhesion       Chemical      * Degreasing: Bonderite C-AK 4335 (2min
promoter       coupling      @ RT) followed by DI Rinse
                             * De-oxidizing: Bonderite HX-357 (1 min
                             @ RT) followed by DI Rinse
                             * Coating: Bonderite M-NT 5200 (1.5 min
                             @ RT) followed by DI Rinse

Table 2. Results for contact angle measurement on surface treated
aluminum.

Pretreatment              Water Contact Angle

No Treatment (Control)                 82.9
Grit                                  114.7
Etch                                  112.1
Anodize                                98.0
Coating                                94.5
Plasma                                 64.5

Table 3. Results for surface roughness measurement on surface treated
aluminum.

Pretreatment    Average Roughness, um

Control                0.43
Grit                   0.79
Etch                   0.36
Anodize                0.65
Coating                0.44
Plasma                 0.34
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Author:Khan, Saida; Sarang, Santosh Kumar; Hiratsuka, Ichiro
Publication:SAE International Journal of Materials and Manufacturing
Article Type:Report
Date:Aug 1, 2016
Words:3509
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