Weldability of Li-bearing aluminium alloys

Abstract

Lithium bearing aluminium alloys constitute a relatively new generation of high performance, lightweight aviation alloys that are being considered for a variety of applications requiring welded construction. As with other aluminium alloys, there are a number of weldability issues associated with these alloys, including resistance to defect formation during fabrication, mechanical property degradation, and service performance. This report reviews the pertinent literature regarding the welding characteristics, properties, and weldability of a number of commercial alloys. The review is divided into the following major sections: (1) development and physical metallurgy of Al–Li–X alloys, (2) microstructure evolution, (3) mechanical properties, (4) weld cracking susceptibility, (5) porosity formation and prevention, and (6) corrosion behaviour. The commercial Al–Li–X alloys are welded using a variety of processes, including arc welding, high energy density welding, and solid state welding. The strength of welds in these alloys varies widely, depending on the welding process, filler metal selection, and post-weld heat treatment. In general, these alloys have low joint efficiency (ratio of weld strength to base metal strength) in the as welded condition and require post-weld aging to achieve efficiencies substantially above 50%. Weld porosity has been a particular problem with these alloys in part due to the hygroscopic nature of Li-containing aluminium oxides. This problem can be controlled if proper surface preparation and cleaning procedures are used. The Al–Li–X alloys tend to be more susceptible to weld solidification cracking than comparable alloys without Li additions. Basic weld solidification theory is used to explain this increase in susceptibility. Some of these alloys exhibit an unusual fusion boundary cracking phenomenon that is associated with an equiaxed grain zone that forms via a solidification mechanism in alloys containing Li and Zr.