Investigation of the effect of surface treatments on the shear performance of CFRP/Ti–6Al–4V single-lap adhesive joints

Carbon fiber-reinforced polymer (CFRP)/titanium alloy joints offer exceptional lightness, superior fatigue behavior, and impact resistance for aerospace applications. This study aims to experimentally investigate the bonding of CFRP and Ti–6Al–4V titanium alloy, which are critically important in aerospace and defense industry applications, using an epoxy-based adhesive film, and the effects of different surface preparation processes on adhesion performance. Ten-layered CFRP and Ti–6Al–4V plates were bonded together using an epoxy-based adhesive film. Titanium surfaces were applied to five different surface treatment configurations: untreated Ti surface, HNO3 etched, cross-hatches by CNC milling, sandblasted with silica sand, and surface coated with colorless anodizing after glass sandblasting. After these processes, roughness of Ra = 0.52–2.36 µm (Rz = 3.5–15.6 µm) was obtained on the Ti alloy specimen surfaces. Failure mechanisms were investigated using optical microscopy and scanning electron microscopy (SEM) / X-Ray Fluorescence (XRF) analyses. Single-lap shear tests performed according to ASTM D1002 standards showed that the highest shear strength was obtained in anodized titanium samples with an average of 13.20 MPa and in untreated titanium samples with an average of 12.6 MPa. The lowest shear strength was recorded in samples with cross-hatched surfaces, averaging 7.35 MPa. Metallographic examinations and SEM-XRF analyses have shown that cross-marks created by CNC machining and the etching process negatively affect adhesion performance. However, it was determined that the anodizing process after sandblasting created a stronger interfacial bond between the CFRP and Ti alloy adhesive.