AITEM2011 Adhesives

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Adhesive joining of Aluminum A7075: effect of surface

treatmentsDi Bella G.1, Galtieri G. 2, Pollicino E.2, Borsellino C.31

CNR ITAE, Messina, [email protected]

University of Messina, Dep. of Industrial Chemistry and Engineering of Materials,Messina, [email protected].

3University of Messina, Dip. Ingegneria Civile, Messina,

[email protected].

Keywords: Adhesive bonding, Aluminum, Single Lap Joint

Extended Abstract

To optimise the adhesive joint performances it is necessary to pre-treat the metalsurface, not only to remove contaminants, but also to provide the intimate contactneeded for the adhesive to successfully bond with the adherent surface [1,2].Pre-treatment is necessary to remove such contaminants, as lubricants and oils, andto provide the intimate contact needed for the adhesive to bond successfully with theadherent surface. The mere cleaning of aluminum surfaces is not suitable for theirbonding due to the oxide layer that naturally occurs on exposure to air so we need toapply a more effective treatment (mechanical or chemical etching) to increase the

adhesion capability of the substrates [3].

a)

activated Ti

counter-electrode

standard

calomelan

electrode

A7075sheet

working

electrode

30 mm

30 mm

solution

b)

0

1

2

0 2 4

Strain

Stress

[MPa]

Fig. 1: a) Electrochemical etching; b) Typical Stress/Strain curve of the MSpolymer based adhesive


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In this work the effects of the substrate surface conditions on single lap aluminum jointproperties were evaluated. Three different kinds of treatment were performed:

- mechanical treatment [4]In order to remove either weak adhering or contaminated outer layers from the

substrate surface, thereby, exposing the freshly oxidised bulk material directlyto the adhesive, we performed a mechanical abrasion, enabling the formationof a suitable surface layer. This treatment is also useful in order to evaluatethe effect of roughness on the joint resistance. Roughening using abrasivepapers is common in many industries. The surface was prepared with threedifferent grinding papers, identified by P80, P120 and P180. The 2.5% of thethickness was removed in order to obtain a regular surface. The residualparticles remaining after mechanical abrasion were cleaned with pressurizedair.

- chemical etching [5]

Two different chemical etching were realized on the aluminum sheets; analkaline treatment and an acidic one, respectively with the followingprocedures:

a. FPL etch pre-treatment: degreasing in acetone, hot water rinse,immersion in FPL etch solution (16.3 vol% H2SO4, 6 wt% Na2Cr2O7)for 20 min at 60C, 20 min rinse under tap water, drying with hot airstream;

b. degreasing in acetone, then etching in 10 wt% NaOH solution for 50 sat 60C, rinsing under distilled water, and air drying.

- electrochemical etching.Two different electrochemical attacks were realized on the aluminum sheetsusing the previous solutions. Fig. 1a reports the scheme of theelectrochemical corrosion process. For the acidic solution a potential of 2 V(i.e. respect to the equilibrium potential) for 80 s was applied. For the alkalineone a potential of 1 V (i.e. respect to the equilibrium potential) for 50 s wasapplied. Both tests were carried out at room temperature.

Aluminum A7075 sheets were used as a substrate. This very tough and durable alloy,made primarily from Zinc, Magnesium and Aluminum, is commonly used in aircraftand aerospace applications. The behaviour of this material was characterised by theexperimental tests. Tab. 1 reports some properties. The sheet thickness was 4 mm.Two different adhesives were used: a MS polymer based adhesive (Fig. 1b shows the

typical stress/strain curve) and an epoxy resin.Properties ValueYield stress [MPa] 470Max stress [MPa] 530Hardness [HV] 190

Grain diameter [m] 100-120

Tab. 1: Properties of Aluminum A7075.


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The single-lap joint tests, in according with standard ASTM D1002, were carried out tomeasure the resistance of the joints. Finally a data statistical analysis was performedto identify whether the surface treatment and the kind of adhesive are effective.The results show that the adhesive influence the behaviour of the joint. Whereas the

influence of the treatment depends on the kind of adhesive that has been used.

References

[1] Wheeler M.J., Sheasby P.G., Kewley D., 1987. Aluminum structured vehicletechnology - a comprehensive approach to vehicle design and manufacturing inaluminum, SAE Technical Paper 870146.

[2] Bishopp J.A., 1988. The Adhesively Bonded Aluminium Joint: The Effect ofPretreatment on Durability, Journal of Adhesion 26(2-3), pp. 237-263.

[3] Dinnie L., 1995. Sticking with it, Engineering (Vehicle Engineering and Design)235(10), pp. 17-18.

[4] Borsellino C., Di Bella G., Ruisi V.F., 2007. Effect of chemical etching onadhesively bonded Aluminum AA6082, Key Engineering Materials 344, pp. 669-676.

[5] Borsellino C., Di Bella G., Ruisi V.F., 2009. Adhesive joining of aluminium AA6082:The effects of resin and surface treatment International Journal of Adhesion andAdhesives 29(1), pp. 36-44.


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Adhesive joining of Aluminum A7075: effect of thesurface treatments

Di Bella G.1

, Galtieri G.2

, Pollicino E.2

, Borsellino C.3

1 CNR ITAE, Messina, [email protected]

University of Messina, Dep. of Chimica Industriale e Ingegneria dei Materiali,Messina, [email protected].

3University of Messina, Dep. of Ingegneria Civile, Messina,

[email protected].

Abstract

In adhesive joining, pre-treatment of surfaces is necessary to remove a variety ofcontaminants, such as lubricants and oils providing an improved contact between theadhesive and adherents surface. Rather a specific surface treatment is required than

a mere cleaning of aluminum surfaces for their bonding due to the oxide layer thatnaturally occurs on exposure to air. In this work the aluminum sheets were subject toseveral treatment of mechanical, chemical, and electrochemical nature. Specimen,like single lap joints were prepared applying two different adhesives: a MS polymerbased and an epoxy resin. The impact on both substrates surface condition, producedby the treatments, and the kind of adhesive on single lap joint resistance wereanalysed. A design of experiments was defined in order to quantify the effect of theconsidered factors and their correlation.

Keywords: Adhesive bonding, Aluminum, Single Lap Joint

1 INTRODUCTION

When an effective bonding technology with a relatively high strength/weight ratio andreduced cost is required the adhesive joining is being applied more and more,specially in aerospace and automotive industry. The choice of single lap joint withaluminium alloys and adhesive bonding connects light weight, simplicity and efficiencyof realization.The adhesive bonding behaviour depends on many factors such as: physical andmechanical properties of both adherend and adhesive, geometrical parameters of the joints (like the overlap length, the overall length of sample, the thickness of thesubstrates and adhesive), the surface finish. The latter is one of the factors thatinfluences the joint strength because the roughness leads to an increased contactarea between the two substrates and increases the adhesion by mechanicalinterlocking [1]. Naturally we could expect that an increase in the surface roughness in

the overlap area leads to an increase in the strength, regardless of the nature of theadhesive and of the adherends. However, research has confirmed that the relationship


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between the joint strength and the substrate roughness is not only function ofroughness but depends on other factors. For instance, to generate roughness someform of substrate pre-treatment induces physical-chemical changes affecting surfaceenergy of adherends and then the wettability.[2-3]. The same surface treatments that

improve adhesion have the effect of roughening the surface involved. Treatments,such as abrasion and etching, also remove weak boundary layers, increase thereactivity of the surface, and improve its wetting behaviour [4].In this work the impact on both the substrate surface conditions and the type ofadhesive on single lap aluminium joint properties was investigated.Three different kinds of treatments were performed: mechanical treatment, with threevarious grit blasting [5]; two different chemical etching, an alkaline treatment and anacid one [6]; electrochemical etching. The single-lap joint test was chosen to measurethe joints resistance.A statistical data analysis was performed to determine whether the surface treatmentis effective and if it depends on the adhesive choice.

2 EXPERIMENTAL SETUP

2.1 Materials

AdherentsAluminum A7075 sheets were used as substrates. This very tough and durable alloy,made primarily from Zinc, Magnesium and Aluminum, is commonly used in aircraftand aerospace applications. The behaviour of this material was characterised by theexperimental tests. Table 1 reports some properties. The sheet thickness was 4mm.

Properties ValueYield stress [MPa] 470Max stress [MPa] 530

Hardness [HV] 190Grain diameter [m] 100-120

Table 1. Properties of Aluminum A7075.

AdhesivesTwo different adhesives were used:a) a MS (Modified Silane) polymer based adhesive, suited to bonding applications inthe manufacture of boats, was used. To identify its characteristics, tensileexperimental tests were carried out using an Universal Testing Machine model Lloyd,equipped with a 10 kN load cell. According to the ASTM 638M3 standard, three dogbone samples were tested applying a rate of 10 mm/min. Figure 1 shows the typicalstress/strain curve. The trend is elastic-plastic. Table 2 reports the mean values of the

mechanical properties.


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formation of a suitable surface layer. Roughening using abrasive papers is common inmany industries. The surface was prepared with three different grinding papers,identified by P80, P120 and P180. The 2.5% of the thickness was removed in order toobtain a regular surface. The residual particles remaining after mechanical abrasion

were cleaned with pressurized air.

Chemical etchingTwo different chemical etching were realized on the aluminum sheets; an acidictreatment (in the next A) and an alkaline one (in the next B), respectively with thefollowing procedures:

a) FPL etch pre-treatment: degreasing in acetone, hot water rinse, immersion inetch solution (16.3 vol% H2SO4, 6 wt% Na2Cr2O7) for 20 min at 60C, 20 minrinse under tap water, drying with hot air stream;

b) degreasing in acetone, then etching in 10 wt% NaOH solution for 50 s at60C, rinsing under distilled water, and air drying.

Electrochemical etchingTwo different electrochemical attacks were realized on the aluminum sheets using the

previous solutions A and B.Figure 2 reports the scheme of the electrochemical corrosion process. For the acidicsolution a potential of 2 V (i.e. respect to the equilibrium potential) for 80 s wasapplied. For the alkaline one a potential of 1 V (i.e. respect to the equilibrium potential)for 50 s was applied. Both tests were carried out at room temperature. In the next thesamples electrochemically treated will be named EA and EB, respectively.

activated Ticounter-electrode

standardcalomelanelectrode

A7075sheet

workingelectrode

30 mm

30 mm

solution

Figure 2: Electrochemical etching


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2.3 Roughness Measurements

A surface profiler, a RTH Form Talysurf, was used to determine the roughnessparameters, both before and after the surface treatment. The profiler moves along asingle direction with a scanning length of 2.5 mm. Measurements were performed indifferent areas of the same surface, along two different directions (i.e. parallel andorthogonal to the sheet axis), in order to verify the treatment uniformity; the averageroughness values Ra were calculated. The experimental results showed smallspreads, i.e. the variability in the measured Ra values was always lower than 10%.

2.4 Single Lap Joint Tests

A wide variety of joint configurations are possible when bonding structures. Thesingle-lap and double-lap configurations are the most commonly found in practice andare applicable for joining relatively thin adherents (i.e. 4 mm); on the other hand, themore advanced stepped lap and scarf configurations are used to transfer high loads in joint with thicker adherents. For these last the manufacturing is more complicated,requiring low machining tolerances. Since the single-lap joint is generally the simplest

and cheapest of all joints to manufacture due to its simple design and easy assembly,it was chosen for the test according to the standard ASTM D1002 for thedetermination of joint shear strength. The mean shear stress is evaluated by theformula:

A

P=

(1)

where P is the tensile load and A is the joint overlap area. Figure 3 reports the jointconfiguration. The sheets sizes were: length of 100 mm and width of 25 mm. The areaof overlap zone was 25x25 mm

2. The adhesive thickness was 0.1 mm.

Overlap zone

Constrained from all displacements

Figure 3: Joint configuration

The procedure for the samples preparation was the following: firstly, a thin layer ofadhesive was applied on the surface of the adherents using a brush, then thesubstrates were aligned and a pressure was applied with clips, to squeeze out theextra adhesive, until the standardised thickness was obtained. The thickness wasmeasured with a 1/100mm precision calliper. Moreover, the samples were realisedunder extractor fan, at constant and controlled temperature, and then tested when

curing time of both adhesives was fully completed. Universal Testing Machine modelLloyd, equipped with a 10 kN load cell and with tensile clamps, was used.


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2.5 Statistical Analysis

To verify if the surface treatment is a significant factor that influence the mechanicalproperties of the joint, a statistical analysis (ANOVA) was performed using Minitabsoftware.

3 RESULTS AND DISCUSSION

3.1 Roughness Measurements

The measured roughness values are reported in Table 4. Obviously the mechanicaltreatments induce the higher roughness, but also chemical etching is no less. Thedifferent values, observed between parallel and orthogonal, are due to themanufacturing process of aluminum sheet (i.e. rolling).

Ra [m]Treatment

parallel orthogonal

No treatment 0.095 0.178Mechanical P180 0.68 0.83Mechanical P120 1.07 1.11Mechanical P80 2.06 3.73

Acidic 1.21 1.29Alkaline 1.21 2.35

Electro-acidic 0.25 0.41Electro-alkaline 0.173 0.26

Table 4. Ra mean values

3.2 SINGLE LAP JOINT TESTS

A typical stress/displacement curve for a bonded sample mechanically treated with the

grinding paper shows, after an initial settlement, a normal elastic-linear trend. Whenthe stress continues to increase, the trend is not linear because the adhesive starts toplastically deform. After the reaching of the maximum value, the opening of the jointoccurs.Figure 4 e Figure 5 resumes the results obtained for all the tested joints with bothadhesives. The electrochemical etching does not affect strongly the mechanicalproperties of joints, it is comparable to no treatment.


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0

1

2

3

4

NT P180 P120 P80 A B EA EB

Stres

s[MPa]

mechanical chemical electro

Figure 4: Max Stress of adhesively MS polymer bonded joints

0

2

4

6

8

10

12

14

NT P180 P120 P80 A B EA EB

Stress[MPa]

mechanical chemical electro

Figure 5: Max Stress values of epoxy bonded joints

In the set of bonded samples with the MS polymer based adhesive, the mechanicaltreatments induce an improvement of the mechanical properties higher than bothchemical and electrochemical etching. The values of max stress for all surfacetreatments are within same range between 2 3 MPa. It shows that the joint strengthis influenced by the elastic-plastic behavior of the type of adhesive and it increaseswith the mechanical treatment.The group of adhesively epoxy resin bonded joints shows stress higher than previousset, 4 10 MPa. Also here the mechanical surface roughness enhances the adhesion.

It is possible to observe that the in alkaline solution etched samples present the


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highest stress value. This treatment dissolves the natural alumina layer, activates thealuminum surface and helps in generation of a new porous alumina layer that will welladhere [7]. In the latter oxide film the presence of MgO, thermodynamically passive inalkaline environment, interacting with the resin/alkaline hardener system, produces

better adhesive properties than mechanical treated joints.Figure 6 shows some typical failure of the tested samples. Figure 6a e Figure 6c showthe debonding surfaces observed for the mechanically treated samples. The failuredoes not occur within the adhesive, but interests also the interface adherent/adhesive.The better properties are due to the higher roughness of the adherents. In fact for thepresence of the peaks, induced on the surface by the grinding papers, the thickness islocally reduced. This reduction influences the joint resistance that increases [9]. Thisbehaviour has been observed also in previous works studying a commercialaluminum, mechanically treated [8].Figure 6b e Figure 6d show the failure that characterises the chemically treatedsamples. In Figure 6b the failure is fully cohesive and for this fact the properties arehigher than the not treated specimens. The presence of an oxide layer, porous andwell adhered, on the aluminium alloy is necessary to enhance the adhesive strength.In Figure 6d the failure occurs mainly at the interface in adhesive way, but notcohesive due to the brittleness of epoxy resin. In the mechanical treatment the surfaceroughness strengthens the joint, but in the alkaline process the change of oxide layerimproves the adhesion although the roughness is low.

a) c)

b) d)

Figure 6: Failure modes obtained with MS polymer (left) and epoxy resin (right)adhesives: a)/c) mechanical; b)/d)chemical.


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3.2 STATISTICAL ANALYSIS

Table 5 reports the results of the variance analysis that allows to verify thesignificance of the parameters (i.e. treatment and adhesive). It is possible to observethat both the parameters are significant factors (P


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5 CONCLUSIONS

The experimental results, analysed statistically, show that the treatment is a significantparameter, that influences the resistance of a single lap joint, as a function of the used

adhesive. In particular it is possible to draw out that:- Aluminum AA7075 joints bonded with MS polymer are characterised by: low

mechanical resistance, cohesive fracture and their resistance is not influencedby the surface treatments;

- on the converse when the joints are bonded with epoxy resin they arecharacterised by: high mechanical resistance with different failure modes as afunction of the treatment. In particular mechanical treatment and alkalineetching induce the better properties.

REFERENCES

[1] Petrie, E.M., 2000. Handbook of adhesives and sealants, New York: McGraw-

Hill, New York.[2] Reina, J.M.A., Prieto, J.J.N., Garca, C.A., 2009. Influence of the surfacefinish on the shear strength of structural adhesive joints and applicationcriteria in manufacturing processes, Journal of Adhesion, 85, pp. 324-340.

[3] Da Silva, L.F.M., Ferreira, N.M.A.J., Richter-Trummer, V., Marques, E.A.S.,2010. Effect of grooves on the strength of adhesively bonded joints,International Journal of Adhesion & Adhesives, 30, pp.735-743.

[4] Packham, D.E., 2003. Surface energy,surface topography and adhesion,International Journal of Adhesion & Adhesives, 23, pp.437-448.

[5] Borsellino, C., Di Bella, G., Ruisi, V.F., 2007. Effect of chemical etching onadhesively bonded Aluminum AA6082, Key Engineering Materials, 344, pp.669-676.

[6] Borsellino, C., Di Bella, G., Ruisi, V.F., 2009. Adhesive joining of Aluminum

AA6082: The effect of resin and surface treatment, International Journal ofAdhesion and Adhesives, 29, pp. 36-44.[7] Prolongo, S.G., Urena, A., 2009. Effect of surface pre-treatment on the

adhesive strength of epoxy-aluminium joints, International Journal of Adhesionand Adhesives, 29, pp. 23-31.

[8] Da Silva, L.F.M., Carbas, R.J.C., Critchlow, G.W., Figueiredo, M.A.V., BrownK., 2009. Effect of material, geometry, surface treatment and environment onthe shear strength of single lap joint, International Journal of Adhesion andAdhesives, 29, pp. 621-632.

[9] Di Bella, G., 2009. Experimental and numerical study of bonded joints withpolyurethane and MS polymer based adhesives subjected to in-plane and out-of-plane loads, Proceedings IX AITEM, Torino.

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