Characterisation of roll-bonded interfaces of Al-Li alloys

Replacement of conventional aluminium alloys is being attempted in aircraft, space and missile applications. Constant effort for reducing the net weight of aircraft structural components has triggered the development of light weight, high elastic modulus aluminium alloys. Lithium additions to aluminium can accomplish such properties. Every one weight per cent of lithium added to aluminium reduces the density by 3% and increases elastic modulus by 6%. Other elements such as Cu, Mg, and Zr are added to improve overall properties of Al-Li based alloys. Typical semi-finished products are: sheets, plates, extrusions and forging stock. These light weight alloys are therefore of considerable interest to aerospace industries. Large surfaces of Al-Li cannot be welded by conventional methods. Different bonding techniques are invariably used in joining Al-Li alloys in construction of aircraft and aerospace applications. Roll bonding is a simple and versatile process that has great potential in joining of similar and dissimilar metals with and without interlayers. In the present paper, preparation of Al-Li alloys for roll bonding by sandwiching of Al-Li plates with copper rivets and the resulting interfaces/joint properties have been studied. Al-Li alloy sheets of 3.6 mm thickness were closely held together by copper rivets and the assembly was rolled at temperature of 525 °C with 50% reduction in a single pass. Four sets were rolled for observing the repeatability. In roll bonding at high homologus temperature, the tenacious surface oxide scale is expected to break followed by emergence of nascent surface as well as a limited diffusion in the available time. However, from metallographic analysis it is apparent that the bondings were not at all perfect for the four sets of sheets. The shear strength of the interface was about 25% of the base alloy. The sample peeled off at 30°, 50° and 60° bend angles. The physical bondings between the sheet were adequate and the shear strength also is considered to be adequate for certain applications, but it can be concluded that for perfect bonding ofAl alloys a higher deformation to break the oxide film for efficient bonding is necessary. This aspect has been studied by imposing different total reduction percentage through different roll pass sequences.