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hybrid composite/aluminium structures
R. Agogué, S. Mercier, D. Gomes, P. Beauchêne, B. Lamboul
To cite this version:
R. Agogué, S. Mercier, D. Gomes, P. Beauchêne, B. Lamboul. Assessment of a manufacturing process
of co-cured hybrid composite/aluminium structures. Euro Hybrid Materials and Structures 2014, Apr
2014, STADE, Germany. �hal-01082020�
* Corresponding author: 29 av. de la Division Leclerc, 92322 Châtillon, France, [email protected]
ASSESSMENT OF A MANUFACTURING PROCESS OF CO-CURED HYBRID COMPOSITE/ALUMINIUM STRUCTURES
Romain Agogué
1*, Sébastien Mercier
1, Denis Gomes
1, Pierre Beauchêne
1, Benjamin Lamboul
11
ONERA, 29 av. de la Division Leclerc, 92322 Châtillon, France
ABSTRACT:
A process of co-curing of composite/aluminium structures is presented. Several surface treatments from chromate conversion to several anodizing processes have been tested to improve the interface properties between the polymer matrix of the composite and the aluminum substrate during the process. Methods for monitoring and characterizing surface treatments are presented. To assess the interface properties, single lap joint testing has been performed. The ultimate force at breaking has been found to be highly dependant of the treatment. The highest ultimate failure force has been obtained with treatments based on phosphoric acid anodizing. The manufacturing of hybrid materials (composite / aluminum) has been performed, showing that it is possible to co-cure composite/aluminium structures. Non destructive evaluation has been used to monitor the delamination areas between composite and aluminium.KEYWORDS:
Composite, aluminium 2024, co-curing, interface properties, non destructive inspection1 INTRODUCTION
Hybrid composite/metal structures already exist for aerospace applications. Those hybrid structures are mainly made of composite for their good perform- ance to weight ratio. Metal inserts are generally used so as to fill the lack of composite perform- ances, such as conductivity and joining.
In such hybrid structures, where metal and com- posite are closely in contact, the strength of the interface between metal and composite is funda- mental to obtain a good junction. It is well known that metal pre-treatments like chromate conversion or anodizing is beneficial to the adherence of polymers on aluminium [1, 2, 3]. These processes are for example commonly used in painted alumin- ium structures or aluminium adhesive bonding in aerospace industry.
It is therefore of great interest to study the influ- ence of such aluminium treatments in increasing interface properties in hybrid aluminium/composite structure.
The classical industrial process is to manufacture the composite and metal components separately, prepared them before performing the assembly step to get the final hybrid structure. This process is time consuming and some operations, such as drill- ing of the composite material, could drastically reduce its performances. To overcome this situa- tion, this study focuses on co-cured processes for the production of hybrid structures.
This paper aims at presenting the first results obtained during an on-going project on co-cured hybrid composite/metal structures for aerospace applications.
Several treatments have been applied on a 2024 aluminium and characterised. Al/epoxy/Al single lap joints (SLJ) samples have been manufactured in order to simulate an Al/composite matrix interface, and SLJ shear tests have been carried out in order to compare the bonding strength obtained with the different treatments. Finally, two different ways of co-curing using an aluminium treatment are pre- sented and discussed.
2 EXPERIMENTAL METHODS
The material used in this study is a 2024 Alumin- ium alloy (in wt.%: 4.4% Cu, 1.50% Mg, 0.6%
Mn, Al bal. )
90*24*1.6 mm coupons were machined in 1.6 mm 2024 foil and used for both aluminium treatments and single lap shear tests. 80*70*1.6 mm foil was used for hybrid composite/Al co-curing tests.
Two carbon/epoxy composites materials were considered in this study: a quasi-isotropic unidirec- tional laminate (T700GC/M21, Hexcel Compos- ites) and a twill 2/2 laminate (carbon fibre G0986, Hexcel Composites) / epoxy resin (RTM6, Hexcel Composites).
An epoxy based film adhesive FM300-2 (Cytec) was used for the single lap joint samples.
2.1 Aluminium surface treatments before join- ing and co-curing
Six surface treatments have been tested in order to compare their adherence in single lap shear test.
The aim of these surface treatments is to build a thin alumina scale on the surface of the 2024 alloy in order to improve the adhesion between the alu- minium and the resin of the composite. The differ- ent surface preparations were as follows:
1- Aluminium coupons without any treatment after machining,
2- wet sand-blasting followed by degreasing in 10% NaOH for 5min. at 60°C, then rinsing in de- ionised water and etching in 30% nitric acid solu- tion for 2 min. at room temperature,
3- 2 + dichromate etching in a 5g/L CrO3 + 4.5g/L Na2Cr2O7 + 0.5g/L KF solution during 5 min. at 33°C,
4- 2 + sulfo-tartric anodizing in a 200g/L H2SO4 + 80g/L C4H6O6 solution at 34°C with 15V polarisa- tion during 15 min.,
5- 2 + phosphoric anodizing in a 100g/L H3PO4
solution at 25°C with 10V during 25 min,
6- 2 + sulfo-phosphoric anodizing in a 100g/L H2SO4 + 100g/L H3PO4 solution at 27°C with 20V during 15 min.
After each surface treatment, the samples have been rinsed in deionised water and dried at 80°C during 1h. Samples have then been treated either with adhesive joining or co-curing with 24h after surface preparation.
The dichromate etching is well known to improve adhesion between aluminium and polymers in adhesive joining or painting systems. But, due to its CrVI compounds contain, it has only been cho- sen as a reference in this study to be compared with other treatment containing no CrVI compounds, and identified as potential substitutes in aerospace industry.
2.2 Al/epoxy/Al joining before single lap shear test
Single lap joint samples were prepared to assess the mechanical properties of the aluminium/epoxy interface. Each lap was made of aluminium 2024 after the surface treatment. The 2 laps were bonded with a FM300-2 adhesive. The dimension of each aluminium lap was 90x24x1.6 mm3. The overlap- ping surface was 24x24 mm².
A special care was taken to limit the storage time between the aluminium treatment and the curing of the adhesive to avoid surface pollution. The adhe- sive was cured 90 min at 120°C, at 2.5 bar pres- sure. A sample holder was designed to keep a uni- form adhesive thickness of 0.2 mm. Three SLJ samples were prepared during the same fabrication (i.e. using exactly the same curing cycle) to reduce experimental dispersions.
3 RESULT AND DISCUSSION
3.1 CHARACTERISATION OF SURFACE TREATEMENTS
SEM has been carried out on 2024 alloy after sur- face treatment for both dichromate etching and anodizing processes.
According to the very thin alumina scale obtained after dichromate etching, it has not been possible to characterise the morphology of the layer with SEM. However, Energy Dispersive Spectroscopy of the surface (Fig. 1) shows a high oxygen pike on the treated surface confirming the formation of an alumina layer, whereas only a very low pike can be seen on untreated area. It can be noted that no chromium signal could be identified.
Fig. 1 EDS spectrum of untreated (red) and ano- dized surface (black) of 2024 alloy.
Fig. 2 a) shows the morphology of the surface of 2024 alloy after anodizing. Contrary to the dichro- mate etching case, the oxide layer can be observed by SEM. It can be seen that a porous alumina scale has been formed as it was expected. Here also, the EDS analysis of the surface of the anodized surface confirms that the layer is composed of pure alu- mina.
The thickness of the alumina layer obtained after phosphoric anodizing has been determined by SEM cross section and found to be about 3µ m thick.
a) b)
Fig. 2 SEM micrographs of a) the surface of 2024 alloy after sulfo-phosphoric treatment and b) the cross-section of the alumina scale formed on 2024 alloy after sulfo-
phosphoric treatment
3.2 MECHANICAL PROPERTIES OF THE ALUMINIUM/EPOXY INTERFACE The single lap joint tests were carried out at a con- stant strain velocity of 1mm/s.
The lap-shear ultimate loads of the samples treated by different treatments before bonding are shown in Fig. 3. The maximum ultimate load of the bonded joint was influenced by the surface treat- ment. The highest interface properties were ob- tained for the phosphoric anodizing and the sulfo- phosphoric anodizing. The results obtained with sulfo-phosphoric anodizing were slightly higher than phosphoric anodizing but not significantly different according to experimental dispersion.
Error bars in Fig. 3 correspond to the standard deviation of the three measurements.
Results obtained for sand-blasting and sand- blasting + chromate conversion were nearly the same. It seems that chromate conversion didn't increase the properties of the aluminium/epoxy interface.
Fractured surfaces of SLJ samples were also stud- ied (Fig. 4). An adhesive failure mode was clearly observed for untreated aluminium, sand-blasting and chromate conversion treatments (Fig. 4a, 4b, 4c respectively). The failure mainly occurred at the substrate/adhesive interface with large delamina- tion areas. This behaviour is relevant of poor inter- face properties which is confirmed by the low lap- shear ultimate loads obtained in SLJ shear test.
The morphology of the fracture surfaces on ano- dized samples is more difficult to analyse, but it seems that the failure is mixed with both adhesive and cohesive failure. Delamination areas are also smaller than with untreated, sand-blasted and chromated samples showing better interface prop- erties which is in good agreement with SLJ shear test results.
-2 0 2 4 6 8 10 12 14 16
1
Load at breaking point (kN)
aluminium sand-blasting chromate conversion sulfo-tartic anodizing phosphoric anodizing sulfo-phosphoric anodizing
Fig. 3 Effect of aluminium pre-treatments on the ultimate loads using single lap joint testing
Fig. 4 Fractured surfaces of specimen treated by different processes. a. untreated alumin- ium; b. sand-blasted; c. chromate conver- sion; d. sulfo-tartric anodizing; e. phospho- ric anodizing; f. sulfo-phosphoric anodizing
3.3 MANUFACTURING OF CO-CURED COMPOSITE/ALUMINIUM
MATERIALS
Two processes were considered to co-cure compos- ite/aluminium materials: Resin Transfer Moulding (or RTM) and prepreg manufacturing.
Concerning RTM process, metallic inserts were put into a mould with dry carbon fibres. Then a resin was injected though the fibre preform and was used both as a matrix for the composite and an adhesive between the composite and the metal, after curing.
The main objective is to perform one shot injection and to obtain nearly net shape parts after releasing the whole hybrid structure. For prepreg manufac- turing, prepreg and aluminium inserts were put into the mould. The pressure applied during the process make the resin of the prepreg flow to the alumin- ium interface and then formed the compos- ite/aluminium interface.
Hybrid materials were manufactured as following:
the RTM6 resin, used for RTM process, was cured 75 min at 160°C then post-cured 90 min at 180°C;
the T700/M21 prepreg was cured 2h at 180°C.
This results shows that it is possible to co-cure composite/aluminium structures.
Fig. 5 Hybrid composite/aluminium material manufactured
Two surface treatments were tested for the manu- facturing of co-cured hybrid materials: untreated aluminium and chromated conversion.
Samples were examined using standard ultrasonic immersion inspection (C-scan) at 2.25 MHz from the aluminium side. The temporal gate window was centred around the metal/composite interface.
Local delaminated areas are suspected from the C- scans obtained with untreated aluminium samples (Fig. 6).
This suspected delamination areas were not ob- served on samples with chromated aluminium.
Fig. 6 Ultrasonic inspection of a hybrid material untreated aluminium. The suspected de- laminated areas appear in red/yellow in the scan image.
The ultrasonic inspection (Fig. 6) has to be con- firmed by micrograph observations of the delami- nation zone and of the bonded compos- ite/aluminium interface.
4 CONCLUSIONS
A process of co-curing of composite/aluminium structures was presented. Several surface treatments were tested to improve the interface properties between the polymer matrix of the composite and the aluminum substrate. The interface properties were assessed using the SLJ test and by examinating the factured surfaces. The best interface properties were obtained for the phosphoric anodizing and the sulfo-phosphoric anodizing.
Hybrid materials (composite / aluminum) were manufactured with different treatments. The effect of the surface treatment on delamination between composite and aluminium layers was observed using ultrasonic inspection. No delaminations were observed for sample with chromated aluminium.
According to the results of the SLJ tests, a better composite/aluminium interface properties would be obtained with phosphoric anodizing and the sulfo- phosphoric anodizing.
REFERENCES
[1] Higgins H. : Adhesive bonding if aircraft structures. International Journal of Adhesion and Adhesives, 20: 367-376, 2000
[2] Petrie E. M.: Adhesion and bonding, adhesive bonding of aluminium alloys. Metal Finish- ing, 49-56, September 2007
[3] Zhang J.-S. et al.: The bonding strength and corrosion resistance of aluminium alloy by anodizing treatment in a phosphoric acid modified boric acid/sulphuric acid bath. Sur- face and Coating Technilogy, 202: 3149- 3156, 2008
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