Characterisation of surface pretreatments for aluminium-epoxy joints

Abstract

This study investigated a modified phosphoric acid anodising (PAA) process and grit blasting as surface pretreatments to increase the strength and integrity of aluminium bonded with SpaBond 345 epoxy by Gurit. SpaBond 345 as an adhesive for aluminium is not widely documented but was shown in this study to have a high bond strength and durability. Anodising and grit blasting have historically been used to successfully increase the strength of aluminium bonded with epoxy. Anodising is an electrochemical process that creates micropores on non-ferrous metal surfaces such as aluminium. Grit blasting mechanically cleans and prepares metal surfaces through a blasting medium, such as glass beads. Testing was done based on the ASTM-D1002 standard, which uses single-lap joints to test the bonds' strength and durability. The overlap length of the joint was modified from 12.7 mm to 25 mm, to account for the difference in thickness of the aluminium used, while the crosshead speed was kept at 1.3 mm/minute per the standard. The single-lap joint samples were tested to failure in an MTS universal testing machine, and the maximum shear strengths were recorded. The testing conditions were unaged, 300 hours aged, and 600 hours aged in a 3% NaCl solution at 60 °C. Test results showed that anodising produced bonds that were on average stronger for all ageing conditions than grit blasted bonds. After testing, all fractured samples were photographed, and a few were chosen for analysis by scanning electron microscope (SEM). The optical results showed cohesive and adhesive failure in the anodised samples, but only adhesive failure for the grit-blasted samples. Moisture ingress was more prevalent in the grit blasting samples than in the anodised samples, which is a deciding factor in the durability of a bond. SEM analysis confirmed that the structures attained by PAA and grit blasting were consistent with what is found in the literature in terms of surface morphology and also how the epoxy was able to better adhere to the anodised samples even after ageing. Anodising produced bonds with a higher lap shear strength in all ageing conditions and prevented moisture ingress to a greater extent than the grit-blasted joints. Grit-blasted joints are sufficient for non-critical applications to reduce costs, and anodising should be reserved for bonding critical components.