TWI has developed a solid state bonding procedure for aluminium lithium alloys.
The successful development of aluminium alloys containing up to 3wt% Li has led to a new range of materials offering low density, high stiffness and high strength. The potential of these materials lies in areas where weight saving is important, and their properties are exploited by the aerospace industries. One other advantage of the AI Li alloys is that they can be processed to achieve a very fine grain size, which imparts superplasticity to their range of properties. This opens up a wide range of structures that can be formed using existing superplastic forming technology. The challenge faced was to combine this technology with diffusion bonding to enable complicated multi sheet structures to be developed as is possible with titanium structures.
The inherent difficulty with diffusion bonding aluminium alloys is the stable, tenacious oxide covering all aluminium surfaces. Many means of removing or displacing this oxide have been examined, the most successful being a liquid phase bonding to break up the surface oxide.
However, this technique, which uses eutectic reaction between a dissimilar coating or foil interlayer and the parent material, is restricted to certain alloys and bonding temperatures. The latter restriction is particularly important for superplastic aluminium alloys because of the need to prevent significant grain growth.
TWI has addressed this problem, and a solid state bonding procedure has been developed for the 8090 AI Li alloy, based on an aluminium foil interlayer containing little or no lithium. This foil distorts during the bonding process; the oxide film thus mechanically disrupts, and lithium diffuses from the parent alloy into the foil.
The procedure requires little sophistication as the parts are simply mechanically abraded and degreased before assembly in the bonding apparatus. Bonding parameters of 530◦C, 8 10MPa pressure and 20 60min bonding time have been successfully applied, giving joint shear strengths of about 80MPa.
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