Development of Surface Pretreatments of Aluminum Alloy EN AW-6061 for Metal-Polymer Hybrid Joints

Abstract

Surface pretreatments of aluminum alloys are known to be of great importance for stronger and environmentally stable adhesively bonded joints. Aluminum alloy adhesive joints offer a wide range of potential applications in the automotive and aerospace industries, thus detailed research of their performance is necessary. Better surface treatment of the substrates is necessary to offer good surface characteristics that will increase the strength and stability of adhesive joints. In this work surface pretreatments like degreasing, grit blasting, and/or anodizing are done on aluminum alloy EN-AW-6061. Surface roughness profile was studied though optical profilometry. Surface morphological studies were done through scanning electron microscopy (SEM) and compositional analysis through energy dispersive X-ray spectroscopy (EDX). The surface pretreated substrates were adhesively bonded using polyamide or nylon-6 (PA-6) in a convection oven. The bond line thickness was measured using optical microscopy. Single lap shear testing of adhesively bonded joints was performed at tensile testing machine. The grit blasted samples showed a good roughness value Ra of around 14 μm and 15 μm as compared to the degreased ones which showed Ra of 0.5 μm. The grit blasted and anodized samples showed good lap shear strengths of 19 MPa and 16 MPa, respectively. The control samples, i.e., as received, or degreased ones demonstrated a very low lap shear strength of 9 MPa and 13 MPa, respectively. After hydrothermal aging, the lap shear strengths of grit blasted samples were dropped to a 5 MPa, whereas the samples received other pretreatments showed almost zero lap shear strengths after this hydrothermal treatment. The fracture analysis revealed the polymer residues on grit blasted surfaces of and no such residues were observed at fracture surfaces of decrease or as received samples. This observation explained the better performance of decreased samples compared to that of as received or degreased ones.