Investigations on pre-treatment processes for structural bonding of Titanium with Composites

Titanium and Titanium alloys are widely used in aerospace industries. Recent generations of aircraft features a high amount of carbon fiber reinforced materials. To ensure galvanic compatibility between the composite materials and the metallic structures Titanium alloys are applied. Titanium alloys feature attractive properties for a number of reasons. However, it is well known in the aerospace industry that Titanium materials exhibit issues in regard to long-term stable adhesion. Rivet rash on Titanium rivet heads or delamination of fiber metal laminates are just some examples of this issue. Due to the increasing amount of Titanium and the increase in hybrid joints with composite structures new titanium are under development do ensure long-term stable adhesion. Within this study a new anodizing process is used as a pre-treatment for structural bonding of the titanium alloy Ti6Al4V. For reference, an alkaline etching process (Turco 5578) and a second anodizing procedure (NaTESi-process) are employed to evaluate the nanostructure obtained by the new process. The influence of nano- and microstructure on bonding strength was investigated using key bonding tests. The fracture surfaces of the titanium oxide layers were analyzed via scanning electron microscopy (SEM). The mechanical tests show that the area enlargement due to the anodizing process enhances the bonding durability in hot/wet conditions. The results obtained, confirm the correlation between the morphology and the bond strength. Also the failure loci vary due to surface structure. The nanostructured surfaces exhibits a predominant cohesive bonding failure. The investigations have demonstrated that the new anodizing process for titanium is a promising treatment to enhance the long-term durability of bonded titanium joints. This new treatment could be an enabler for Titanium/Composite hybrid joints.