Determination of adhesive bonding properties of different metals in hybrid composites

In the light of studies in the past years, high strength materials have brought the incremental increase in composite material usage and production. Composite materials are generally used in a combination with the other type of materials such as metals or polymers in structural designs. The metal-composite structures that are mostly being used in aviation and automotive sectors are called as hybrid composites. The operation temperature, required strength, production capabilities, the type of material couple and coupling type are the fundamental parameters for a hybrid structure design. Mechanical bonding, welding, adhesive bonding and mechanic-adhesive hybrid bonding are the types of bonding methods. The type and design of bonding area should also be selected by the stress type such as tensile, compressive, shear, split and peel stresses. Adhesive bonding is the main topic of this study. Surface properties resulting from mechanical or chemical applications such as cleaning, etching and roughing methods determines the bonding capability of the metals with the effect of metal type. These properties affect the suface energy and wettability relatively. The other main point is the type of adhesive; curing time and temperature, the pressure applied on bonding area identifies cohesive properties of adhesives. The adhesive is selected as (Loctite) Hysol EA9394 Epoxy Paste Adhesive made by Henkel. The selection of adhesive is completed after several trials with different brand and types. Hysol EA9394 has a proper viskosity level that can be applied easily that neither flows too easily nor resists smearing. Hysol EA9394 is mostly used in aviation sector due to good gap filling capability, outstanding mechanical properties, long pot life and room temperature curing ability. It is consist of two component; resin and hardener. The curing process after mixing the components can be completed at room temperature in 24 hours or at elevated temperatures in shorter time. The accelarated curing process is set to 1.5 hour heat treatment in the oven at 85 °C and furnace cooling in 24 hours. The thickness of the adhesive needs to be around 1 mm in order to obtain considerable shear strength. However, some amount of pressure needs to be applied to the bonding area while holding during curing process. 0,5-1 mm diameter tempered glass beads are use in the experiments to prevent pouring out of the adhesive. Different metal types with various surface treatments are adhesively bonded to one type of composite within this study. Aluminum 2024 T6, AISI 304 Stainless Steel and Titanium Ti6Al4V are the metal types that are coupled with polyamide composite material. The composite material is 35% Glass Reinforced PA 66 supplied by Farplas Inc. Metals are 2 mm thick sheet metals as ASTM D1002 states. The thickness of the PA 66 is 2.5 mm on average. Single lap shear tests of various metal-composite hybrid structures are performed during the process. The samples and tests are prepared and applied according to ASTM 1002 in Istanbul Technical University Mechanical Engineering Faculty. The sample preparation stage is completed in Machine Shop and Metallography and Heat Treatments Laboratory. The single lap shear tests are applied in Mechanical Experiments Laboratory. Preventing meeting the surfaces of composite and metal samples and providing the eccentirity and equal heat cycles of the samples are the key parameters for a successful curing. A fixture is designed within this study in order to provide stable and identical tests. The fixture is milled from an Aluminum bullet in the machine shop in ITU. Two of this part is used as a couple and assembled to each other keeping the sample in the middle. The identical parts at two sides of bonded surfaces provides equal heating and cooling curves to the sufaces. The fixture can be put into the oven without using any other equipment. Various chemical and mechanical processes are applied on the surfaces to evaluate their effects on adhesive bonding. Two different etching solutions are applied to Aluminum surface and the results are reported. The tests with Aluminum are all performed with the selected etching solution after this test. In order to maintain different roughness values; sand papering with different roughness, blasting applied to metal and composite surfaces; also laser texturing is applied onto the metal surfaces. Laser texturing application helps to both mechanical and chemical bonding. The texture which is made of grits at 1 mm intervals, 45° to stress direction has 0.5mm depth that makes adhesive penetration possible. At the same time, laser melting process forms oxide layers on the metal surface. Oxide layers increases surface energy of the metal, higher surface energy results with better wettability property. To give a brief information on this topic; the high surface energy of a solid surface requires to lower its energy by getting wet by a liquid. This means; the higher surface energy of a solid surface, the better wettability. On the other hand, the wetting ability of the liquid material is described with the term "surface stress". The liquid drop with high surface stress tends to reduce its stress by soaking the solid surface In order to identify the surfaces after several treatments, roughness measurements and contact angle tests are completed. Contact angle test, wettability test or drop test in other words, is performed in ITU Global Textile Design Center. The wettability test is applied to metal and composite surfaces. The aim of this test is to report the effect of different surface treatments on aluminum, titanium, stainless steel and polyamide surfaces to water drop contact angle. By using this relation, identifying the effect of wettability performance on adhesive bonding. It is seen that, roughness increases at all surfaces incrementally as a result of the processes from sand blasting to laser texturing. The wettability of the surfaces which is measured by contact angle is increased by the roughing operations on the surface. Unexpectedly, blasting operation shows an inverse effect due to surface friction on wettability even it creates high roughness on the surface. The inverse effect of blasting shows that, the type of roughness of the surface plays role on wettability. It is not possible to set a direct correlation between roughness and wettability. Titanium surfaces creates lower angle in the contact angle test comparing to the other two type of metals. The laser textured titanium surface is able to create an extremely low angle of contact which is desirable in adhesive bonding. Aluminum surfaces creates relatively higher angle of contact that shows the material is tent to be hydrofobic. The relationship between wettability, roughness and shear strength on the aluminum, stainless steel, titanium and polyamide is shown by the contact angle tests within this study. Increased roughness evaluates the shear strength as recorded in all hybrid samples. For all types of metals, the laser textured surfaces reach the maximum shear strength. Al 2024- PA 66 couple has the highest average shear strength with 8 MPa. Laser textured Al 2024 is also the metal type with the highest roughness of 15Ra. The maximum shear strength values belong to Ti 64- PA 66 with 7.5 MPa and AISI 304- PA 66 with 6.7 MPa. The highest ductility of the hybrid bonds is seen at Ti 64- PA 66 couple even this couple does not show the best shear strength. Al 2024 and AISI 304 follows te ductility property relatively. Also, the effect of cold working is tested on Aluminum surfaces. The bonding area is increased by cold deformation. The surface area is increased 35% by the half spheres made by centre bit. When hybrid structure tested, 45% increase in shear strength is recorded. Adhesive bonded metal-metal stuctures are also tested to provide an approach for changing the designs from metal-metal structures to metal-composite hybird structures. It is seen that the strength of the metal-composite sturcture is always lower than the metal-metal structure. Approximately 15% decrease in shear strength is seen in the hybrid structures of aluminum. This drop is seen as 7-15% in hybrid structures of AISI 304. In this study; the hybrid stucture of the differently roughed surfaces of Aluminum, Stainless Steel and Titanium metal metarials and PA 66 are tested by single lap shear tests. The shear strength values of these adhesive bonded couples are reported in association with their roughness and wettability properties in this thesis. For further studies, the fracture surfaces may be examined in detail in order to understand the failure mechanism of adhesive bonds. The fracture of laser treated metal surfaces and polyamide couples shows that the composite surface needs to be prepared just as metal surface. Also, SEM microscopy can be used for roughened and fractured surface in future studies.