Adhesive bonding of Titanium
Q : What surface preparation is required to form strong joints in titanium using readily available adhesives such as epoxy resins or polyurethanes?
A : Adhesive bonding technology is going to play an essential role in the development and growth of titanium structures. The ability of a structural joint to maintain satisfactory long-term performance, often in severe environments, is an important requirement of a structural adhesive joint, as the joint should be able to support design loads, under service conditions, for the planned life time of the application.
A number of factors determining the durability of structural adhesive joints have been identified and can be grouped in three categories: environment, materials and stresses. The environment is dominated by temperature and moisture. The materials category includes the adherent, the adhesive, and the inter-phase between them both. The last category refers to the stresses to which the bond is subjected during or after exposure to service environment, affecting both longevity and residual strength.
The principal challenge for the adhesive bonding in titanium is the titanium oxide passive layer that is created on the surface of titanium pieces due to the high reactivity of titanium with oxygen from the air. The nature of the oxide on the surface of titanium and the Ti-6Al-4V or TI3AL2.5V alloys, which are the most used in titanium applications, was studied intensively after a number of different treatments. No evidence for material other than titania in its rutile form was obtained even though anatase and fluoride have been reported.
The first operation that is needed prior to bonding is for the surfaces of titanium sheets is the mechanically polishing by wire brushing. Then the most efficient surface for adhesive bonding is a rough surface of the black oxide such as is produced by treatment of the metal in alkaline hydrogen peroxide.
Other non critical operations that have the potential of greatly increasing the strength of the bond are the ultrasonic cleaning by acetone and the modification of surfaces by plasma ion implantation using plasma nitriding. However, these are most likely, to technological to be used other that in high level titanium applications.
It is also worth nothing that acrylic adhesive providing better wetting and an increase in the contact area between oxide and adhesive. The difficulty of wetting the surface with epoxy adhesives was eased by using a primer or by preheating the surface before bonding.