Silane based field level surface treatment methods for aluminium, titanium and steel bonding
Introduction
Grit blast silane (GBS) and grit blast AC-130 sol–gel (GBSG) surface treatment methods have been used widely in the structural bonding of aircraft grade 7075 aluminium [1], [2], [3], [4], [5]. The bonding is usually performed on unclad aluminium alloy. In the case of clad aluminium alloys, the cladding layer has to be removed before the surface treatment and bonding. With the GBS or the GBSG treatments, primers must be used in most applications and they affect the complexity of the surface preparation (GBS primer and GBSG primer). The primers usually contain chromates, and thus, they increase the health risks.
The GBS and GBSG methods have also been tested with titanium and stainless steel, with some successful results [4], [5], [6]. With titanium and steel, the processes may need slight modification. The GBS method has also been used with a specific type of silane on naval grade aluminium for composite repairs [7].
The objective of this study was to investigate the performance of the silane based surface treatment methods (GBS and GBSG) without primers on unclad and clad aircraft aluminium, on unclad naval grade aluminium and on titanium and stainless steel. We searched for procedures suitable for field level applications. The preferred surface treatment was GBSG without a primer, since it is slightly more robust than the GBS method. If a primer is not needed, then the treatment proceeds more quickly and is also more suitable to field level conditions.
We studied the performance of the treatments using lap shear and wedge tests. An immersion wedge test was included in the test programme to assess whether or not the GBSG method can be applied to immersed bonded aluminium repairs. Titanium was only tested at this stage with wedge tests, since this test is mostly used to compare the durability of the surface treatments. Stainless steel plates were bonded with the same adhesive film as the aluminium and titanium plates in order to obtain comparable results for the wedge tests. A possible application for the GBSG treatment is also the surface preparation of steel inserts potted in sandwich panels. Therefore, two adhesives were used in single lap shear tests of steel.
In a case study done for this investigation we used the GBSG treatment on naval grade 5083 aluminium. Fibreglass reinforcements were used to stop corrosion at the bottom of an aluminium naval vessel.
Section snippets
Metals
We tested aircraft grade 7075-T76 unclad and 7075-T6 clad aluminium. The different tempers of the alloys and the clad layer led to slightly different yield and ultimate strengths between the 7075 aluminiums. The composition and strength values of the materials given in Table 1 are based on the handbooks [8], [9], [10], [11], [12].
During the testing, the clad layer was normally not removed from the specimens prior to the surface treatment and bonding, which makes the surface preparation simpler.
Testing methods
The performance of the surface treatments was studied using single lap shear (SLS) testing, since it is a fast and widely used method in surface preparation and adhesive quality control testing. An environmental exposure is required to study the durability of the joints.
Additionally, double lap shear (DLS) specimens were used because they are more representative of the actual bonded joints. DLS specimens can provide a more usable estimate of the bonded joint strength than the SLS specimens with
Single lap shear tests
We treated unclad 7075-T76 single lap shear specimens using the GBS and RDS methods. We treated 5083 aluminium specimens using the GBSG method. The average SLS strength (force divided by joint area) values are shown in Fig. 1.
The failure modes of the 7075 grade specimens were 100% cohesive. The failure modes of the 5083 aluminium were also acceptable in all cases. The specimens either failed cohesively from the adhesive or between the nonwoven fibreglass layers. The failure modes of the
Repair problem
The Finnish Navy discovered corrosion pits on the bottom of a 48.5 m long aluminium naval vessel hull during routine maintenance. The corrosion should be stopped before it penetrates through the bottom plates of the hull. A welded repair is not an option because of the size of the corroded areas and the expected residual thermal stresses.
We proposed a solution for repairing the corrosion pits using fibreglass reinforcements cured at room temperature. For this, we should apply the GBSG treatment.
Single lap shear specimens
We performed all aluminium surface treatments of the SLS specimens without a primer. The silane treatments conducted using both deoxidation methods (GBS and RDS) resulted in SLS strengths of close to 30 MPa for the 7075-T76 unclad aluminium. The failure modes were 100% cohesive. This is considered a good result for the test specimen configuration and materials used. The adhesive film manufacturer has reported 37 MPa SLS strength values at RTD when positive pressure is used during the curing
Conclusions
According to this study, the GBS and RDS methods provide a durable bond for unclad and clad 7075 aluminium without a primer during a hot/wet exposure. The results also suggest that the GBSG and RDSG methods provide durable bonding for clad 7075-T6 aluminium without a primer and without cladding removal during a hot/wet exposure.
The GBSG method did not provide acceptable results without a primer for unclad aluminium. This behaviour may depend on the low pH level of the solution, but further
Acknowledgements
This research was partly sponsored by the Finnish Air Force. The authors acknowledge FINAF Air Materiel Command for providing us with the opportunity to conduct this study. The support of Patria Aviation and the Finnish Navy are also greatly appreciated.
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