EFFECT OF ADHEREND SURFACE PREPARATION
ON THE MECHANICAL PROPERTIES OF
ALUMINIUM BONDED JOINTS

M. DOS REIS, M.D. Banea, Sandip Budhe, Silvio DE BARROS

► To cite this version:

M. DOS REIS, M.D. Banea, Sandip Budhe, Silvio DE BARROS. EFFECT OF ADHEREND SURFACE PREPARATION ON THE MECHANICAL PROPERTIES OF ALUMINIUM BONDED JOINTS. Annals of the "Dunarea de Jos" University of Galati. Fascicle II. Mathematics, Physics, Theoretical Mechanics, 2017. hal-04302806

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Submitted on 23 Nov 2023

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EFFECT OF ADHEREND SURFACE PREPARATION ON THE
MECHANICAL PROPERTIES OF ALUMINIUM BONDED JOINTS

M. dos Reis<fnref n="1" />, M. D. Banea<fnref n="1" />*, S. Budhe<fnref n="1" />, S. de Barros<fnref n="1" />

ABSTRACT

The surface treatments play an important role in improving the adhesive bonded joint strength and durability. However, each adhesive and adherend material will require different types of surface treatment to make the right balance between the joint strength and fracture modes.

In this study, the effect of adherend surface preparation on the mechanical properties of aluminium adherends bonded with a modern tough adhesive used in the automotive industry was investigated. Aluminium adherends were treated with three different surface preparation: solvent degreasing, sand abrasion and grit blasting with low and high blast pressure. The treated surfaces were analysed using a portable surface roughness tester and the effect of surface roughness on the joint strength was examined by testing single lap-joints (SLJs). Results showed an increase in bond strength with increasing surface roughness and the maximum strength was obtained for the grit blasted specimens. The higher surface roughness leads to better mechanical interlocking between the adhesive material and the aluminium surface. The fracture surface of the joints reveals a progressive increase in cohesive failure mode as the surface roughness increase.

KEY WORDS: single lap-shear tests; surface treatment; aluminium; fracture surface;

# 1. INTRODUCTION

Adhesively bonded aluminium joints are increasingly being used in structural application, especially in automotive and aerospace sectors, because of high strength to weight ratio, design flexibility, damage tolerance, fatigue resistance etc [1], [2]. However, the performance of bonded joints depends on many parameters such as geometrical (bond line thickness, surface roughness, overlap length, adhesive thickness, adherend thickness and fillet) and material (adhesive and adherend) [3], [4]. Surface preparation of the adherends is one of the key factors to enhance the bonding strength. Adherend pre-treatment is also important in order to eliminate dust, oil and foreign particles from the surface [5], [6]. In addition to this, there are many surface treatments used to improve the bond surface area, such as: grit blasting, low pressure plasma, sand blasting, laser ablation, chemical etching, etc. The aim of these surface methods is to ensure a smooth and rough surface, good surface activation, surface wettability, mechanical interlocking,

removal of thin layer of oxides, and removal of contaminants etc. [7, 9].

Mechanical abrasion is one of the most widely used surface treatment, because it is easy to perform and different surface pattern and surface roughness can be obtained. Grit blasting is one of the most effective mechanical abrasion methods, which can control the desired level of surface roughness by applying different blast pressure, spray distance, etc. [8], [10]. However, an increase in the surface area by mechanical abrasion methods is limited and also it may be difficult to generate an uniform surface roughness on the adherend surface [7]. The laser process generates a micro-column array which is more uniform and increase more the surface area compared to mechanical abrasion techniques [7]. On the other hand, the laser method produces short-time heat on the adherend during the process, which may cause the adherend melting and hence this method is more suitable for hard materials with a high melting temperature (i.e. titanium, carbon steel, etc.) [7], [11]. Lower plasma process overcomes the issue of heat, as this method generates very low heat or no heat, but for this process inert gas is required [12].

<footnote n="1">¹Federal Center of Technological Education in Rio de Janeiro, Brasil</footnote>

<footnote>*Corresponding author's e-mail address: [mdbanea@gmail.com](mailto:mdbanea@gmail.com)</footnote>
To summarize, there are many surface preparation methods available, however the challenge is to find an effective, simple, safe and economical method which ensures the desired level of surface roughness and surface pattern without change in substrate chemistry for maximum bond strength and long term durability.

The objective of this study is to investigate the effect of adherend surface preparation on the mechanical properties of aluminium adherends bonded with a modern tough adhesive used in the automotive industry. Three different preparation methods: solvent degreasing, sand abrasion and grit blasting with low and high blast pressure are used and the bond strength was evaluated using SLJ tests.

## 2. MATERIALS AND METHODS

### 2.1. Materials

Aluminium 1200 H14 was used as the adherend material (mechanical properties are presented in Table 1). Aluminium 1200 H14 plates were cut at required dimension of 107.5 x 25 x 1.6 mm. A two-component structural epoxy adhesive, Betamate™2096, supplied by Dow (Dow, Brasil) was selected as the adhesive material and cured for 2h at 60°C. The basic mechanical properties of the adhesive as per supplier are summarized in Table 2.

<table><caption>**Table 1.** Mechanical properties of aluminium 1200 H14</caption><thead><tr><td>Tensile strength (MPa)</td><td>Yield strength (MPa)</td><td>Elongation (%)</td><td>Hardness (BHN)</td></tr></thead><tbody><tr><td>145</td><td>95</td><td>3%</td><td>32</td></tr></tbody></table>

<table><caption>**Table 2.** Tensile data of Betamate™2096 adhesive.</caption><thead><tr><td>Young's modulus [GPa]</td><td>Tensile strength [MPa]</td><td>Tensile strain [%]</td></tr></thead><tbody><tr><td>1.7</td><td>29</td><td>9 %</td></tr></tbody></table>

### 2.2. Adherend surface preparation

All the adherend surfaces were initially degreased with acetone to remove the dust and to assure a clean surface. The adherends were subjected to different types of surface treatments as follows:

1) Solvent degreasing (G1): Solvent degreasing was conducted by wiping adherend surface using a cotton cloth soaked with acetone and then drying at room temperature. This surface treatment is denoted as G1.

2) Sand abrasion (G2): In this surface treatment, the adherend surfaces were abraded using sand paper with particle size of 60, followed by degreasing with acetone solvent to remove the dust particles. The surface roughness pattern applied to the adherend was ±45° orientation

(relative to loading direction). This surface treatment is denoted as G2.

3) Grit blasting at low pressure (G3): The grit blasting treatment was performed using angular sintered aluminium oxide grit with granulometry between 20 and 40, at a blasting pressure of 300 Kpa (3 bar), at a right angle to the surface. This surface treatment is denoted as G3.

4) Grit blasting at high pressure (G4): The adherend surface was prepared in the same manner as G3, except the difference in blast pressure. The treatment was performed at a pressure of 600 Kpa (6 bar), at a right angle to the surface. This surface treatment is denoted as G4.

### 2.3. Surface roughness measurement

To better understand and evaluate the influence of the surface preparation, the surface roughness parameters of the adherends were measured after surface treatments. The following roughness parameters: the average surface roughness (Ra), root mean squared deviation (Rq), total height of the profile (Rt), maximum height of profile (Rz), the mean height of the profile element (Rc) and mean spacing of the profile elements (RSm) were used to evaluate the surface quality of the specimens. Surface roughness values were measured using a roughness testing equipment (TESA RUGOSURF, 10G, Fig. 1) for all four surface treatment samples. The surface roughness measurements were performed in different areas. The measured surface roughness values (Ra, Rq, Rt, Rz, Rc and RSm) of aluminium adherends are given in Table 3.

<figure><img src="image_2.png" /><figcaption>**Fig.1.** Surface roughness testing equipment.</figcaption></figure>

### 2.4. Specimens fabrication

The geometry of the SLJs specimens is shown in Fig. 2. A mould with spacers for correct alignment of the substrates was used [13]. The bondline thickness was 0.4 mm and the length of the overlap was 12.5 mm. The bondline thickness was controlled using packing shims. Tabs were glued at the specimen edges for a correct alignment of the specimens in the testing machine. The bonding surface area was cleaned with acetone before the application of the adhesive in order to remove the particle during the surface treatments. The adhesive was applied on the adherend surface and spread over it with a spatula. The joints were cured using a hot plate hydraulic press (model SL-12/20). The curing time of 2 hours at a temperature of 60°C was used as recommended by the adhesive manufacturer.
<table><caption>**Table 3.** Surface roughness measurements values of aluminium adherend after surface treatment</caption><thead><tr><th>Surface treatment</th><th>R<sub>a</sub> (μm)</th><th>R<sub>q</sub> (μm)</th><th>R<sub>t</sub> (μm)</th><th>R<sub>z</sub> (μm)</th><th>R<sub>c</sub> (μm)</th><th>RS<sub>m</sub> (μm)</th></tr></thead><tbody><tr><td>G1</td><td>0.25 ± 0.15</td><td>0.34 ± 0.19</td><td>3.23 ± 1.13</td><td>1.98 ± 0.97</td><td>0.74 ± 0.38</td><td>90.8 ± 33.75</td></tr><tr><td>G2</td><td>1.91 ± 0.21</td><td>2.37 ± 0.26</td><td>14.67 ± 1.39</td><td>11.28 ± 1.07</td><td>5.42 ± 0.52</td><td>102.40 ± 14.81</td></tr><tr><td>G3</td><td>15.27 ± 0.66</td><td>19.52 ± 1.06</td><td>131.11 ± 18.59</td><td>101.82 ± 8.45</td><td>41.74 ± 1.81</td><td>212.4 ± 12.18</td></tr><tr><td>G4</td><td>18.57 ± 1.64</td><td>23.23 ± 2.18</td><td>142.43 ± 20.78</td><td>112.86 ± 8.29</td><td>50.16 ± 4.50</td><td>249.20 ± 21.99</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** SLJs specimen geometry (dimensions in mm).</figcaption></figure>

<table><caption>**Table.4.** Shear strength with respect to surface roughness value of aluminium SLJs.</caption><thead><tr><th>Surface treatment</th><th>Ra (μm)</th><th>Shear strength τ&#773; (MPa)</th><th>Increase in shear strength (%)</th></tr></thead><tbody><tr><td>G1</td><td>0.25 ± 0.15</td><td>9.97 ± 0.55</td><td>-</td></tr><tr><td>G2</td><td>1.91 ± 0.21</td><td>11.16 ± 0.17</td><td>11.93</td></tr><tr><td>G3</td><td>15.27 ± 0.66</td><td>11.60 ± 0.32</td><td>16.34</td></tr><tr><td>G4</td><td>18.57 ± 1.64</td><td>12.10 ± 0.45</td><td>21.36</td></tr></tbody></table>

## 2.5. Test method

The SLJ tests were performed at room temperature with a cross-head speed of 1 mm/min using a testing machine (INSTRON® model 5966) with a capacity of 10 kN. Four specimens were tested for each surface preparation condition. Load-displacement curves were recorded during the test. The SLJ specimen test set-up is shown in Fig. 3.

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** SLJ specimen test set-up.</figcaption></figure>

# 3. RESULTS AND DISCUSSION

## 3.1. Failure modes

The failure mode of the SLJs specimens was visually examined after testing. Figure 4 shows the failure surface of the joints as a function of surface treatment. It can be seen that (Fig. 4a), for the joints with no surface treatments (solvent degreasing), there was a mixed adhesive failure with a small region of adhesive failure. A small portion of adhesive failure, imply a good adhesion between the adhesive-adherend combinations studied here, even without any surface treatment. The percentage of cohesive failure in adhesive increases for the joints treated (Fig. 4b-d) and the maximum area of cohesive failure was observed for the joint treated with grit blast at 6 bar pressure (Fig. 4d).

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 4** Failure surface of SLJs as a function of surface treatment.</figcaption></figure>

## 3.2. Effect of surface treatment on the SLJs joint strength

Figure 5 shows representative load-displacement curves as a function of surface treatment. It can be seen that the maximum load is obtained for the grit blasting treatment at a pressure of 6 bar. The displacement increased significantly, almost 50%, for the grit blasting specimens compared with the solvent degreasing treatment specimens.

Table 4 presents the shear strength as a function of surface roughness values of aluminium adherends obtained by different surface treatments. Aluminium adherend having a surface roughness value of 0.25 ± 0.15µm (solvent degreasing) and their corresponding

bond strength 9.97 ± 0.55 N/mm² was taken as reference value for the comparison.

<figure><img src="image_9.png" /><figcaption>**Fig. 5** Representative load-displacement curves as a function of surface treatment method.</figcaption></figure>

From Table 4, it can be seen that the shear strength increases with increasing surface roughness of the aluminium adherend compared to the non-abraded adherend surface (solvent degreasing). A maximum of 21.36% increment in shear strength was observed when the adherend surface roughness was in the range of 18.57 ± 1.64 µm obtained by grit blasting as compared to the non-abraded adherend surface (0.25 ± 0.15). Previous studies [8, 14] also found that the grit blasting treatment shows higher bond strength than the other mechanical abrasion treatments. Grit blast pressure proves to be an effective parameter in the grit blasting treatment process, as it makes a significant difference in shear strength. Recently, Khan et al. [15] also noticed an influence of grit blast pressure on the strength, thus this parameter need to be carefully considered while using the grit blasting treatment for surface preparation.

Fig. 6 shows the increasing trend of shear strength with increasing the surface roughness. These results are in agreement with previous research from the literature [5]. Increase of bonding area, mechanical interlocking, surface modification, change in surface chemistry and micro-columns of the adherends are possible reasons for the improvement in strength at primary level. However, the large surface roughness value achieved by the grit blasting, did not result in a proportional increase in strength, when compared to the untreated specimens. It implies that there is no direct relationship between the surface roughness value and the strength and that there are other parameters which are also responsible for the change in shear strength (i.e. contact angle, wettability and surface energy among others). For example, it is well known that adding surface roughness will enhance the wettability caused by the chemistry of the surface. However, it was shown in the literature that increasing the roughness can decrease the surface energy and wettability, which lead to lower shear strength even though a larger contact area was achieved [6], [8], [16]. Hence, it is important to have a balance between the surface energy, surface roughness and wettability for a maximum bond strength. Nevertheless, a simple correlation with the surface roughness is not sufficient to predict the joint performance, as there are
other parameters responsible for the increment in
strength [17].

<figure><img src="image_10.png" /><figcaption>Fig. 6 Shear stress with respect to the surface roughness obtained by different surface treatment.</figcaption></figure>

To summaries, the shear strength associated with
adherend surface roughness obtained by different
surface treatments, cannot be explained only by the
increase in surface roughness. There are other factors
such as surface wettability, contact angle, surface
energy among others also responsible for the bond
strength. Therefore, it is important to have a balance
of all these parameters for an optimum bond strength.
Also, a careful selection of surface preparation
parameters such as grit blast pressure, spray distance,
roughness pattern, grid particle size, etc. is essential,
which ultimately makes a contribution for an
optimum bond strength.

4. CONCLUSIONS

In this study, the effect of adherend surface
preparation on the mechanical properties of
aluminium substrates bonded with a modern tough
adhesive used in the automotive industry was
investigated. The following conclusions can be
drawn:

* The surface preparation treatments affect the strength of the adhesive joints. Thus, to obtain the optimum performance from an adhesive joint a pre-treatment is required;

* Maximum strength and deformation of the adhesive joints studied here occurs for the grit blasted specimens;

* An increase in bond strength with increasing surface roughness was found. Higher surface roughness leads to better mechanical interlocking between the adhesive material and the aluminium surface. However, the increase in strength was not proportional to the increase in roughness, which indicates that there are other factors such as surface wettability,

contact angle, surface energy among others also
responsible for the bond strength.

* The grit blast pressure affects the surface roughness and consequently the joint strength (6 bar pressure provided higher value of roughness compared to 3 bar). However, applying a high pressure can damage the surface or induce plastic deformation of the adherends.

* By analysing the fracture surface of the joints, it can be concluded that the adhesive used in this study has a very good adhesion with aluminium adherends, since even for minimum surface pre-treatment (solvent degreasing) showed a reduced region of adhesive failure.

* The fracture surface of the joints reveals a progressive increase in cohesive failure mode as the surface roughness increase.

ACKNOWLEDGEMENTS

Authors would like to acknowledge the support of the
Brazilian Research Agencies CNPq, FAPERJ and Dow
Brasil for supplying the adhesive.

REFERENCES

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[2]. Budhe S., Banea M.D., de Barros S., da Silva L.F.M., An updated review of adhesively bonded joints in composite materials, International Journal of Adhesion & Adhesives, vol. 72, 2017, pp. 30-42.

[3]. da Silva L.F.M., Carbas R.J.C., Critchlow G.W., Figueiredo M.A.V., Brown K., *Effect of material, geometry, surface treatment and environment on the shear strength of single lap joints*, International Journal of Adhesion & Adhesives, vol. 29, 2009, pp. 621-632.

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