Investigation of the Microstructure of High-Strength Laser Welded Joints of Aluminum-Lithium Aeronautical Alloys

Introduction. The development of the aeronautical and aerospace industry is associated with up-to-date highstrength aluminum alloys used in constructions and parts. Aluminum-lithium alloys of the Al-Mg-Li, Al-Cu-MgLi, Al-Cu-Li systems are preferable due to its low density in response to the lithium addition. Civil wide-body aircrafts are made with the aid of two technologies: part riveting that includes about a million of rivets, and the progressive laser welding technology. It should be noted that today, the riveting technology is worse than the laser one, because the latter provides higher effi ciency and performance, full automation, multi-purpose character and environmental friendliness. However the strength of the weld joints without post-processing is still low. The aim of the work is to perform the experimental comparison of the laser welding of high-strength aluminum-lithium alloys (the Al-Cu-Li and Al-Mg-Li systems) followed by the postheat treatment (quenching and quenching with ageing), in order to get the high-strength weld joint. The performance of the effect of alloying elements in the aluminum alloys on the microstructure and mechanical characteristics is also the aim of the work. Results And Discussion. The microstructural analysis is carried out, the chemical composition of the weld joints in the aluminumlithium alloys is determined. It is shown that the properties of the weld joint change essentially when Mg or Cu are added into the alloy. For the alloys 1420 and 1424 (the Al-Mg-Li system), heat treatment results in the well-defined dendritic structure, and the aggregates exist both inside the dendrite, and on the borders of dendritic grains (localized evidently). For the alloys 1441 and 1469, which feature is copper, such localization is not observed. Two types of aggregates with fundamentally different chemical compositions are found by means of the electronic microscopy. The first type, with relatively low concentration and size below 10 mu m, is characterized by the presence of essentially excessive rare-earth elements, Zr and Sc above all. The chemical composition of the second type of aggregates, most part of which is localized on the dendrite grain boundaries (alloys 1420 and 1424), is close to the solid solution composition, whereas in the alloys 1441 and 1469 (with Cu), there are dark aggregates with increased amount of copper. It is found that utilization of the complex method of fixed joining which includes both laser welding and postheat treatment of samples, made it possible to reveal for the first time a fundamental difference in the processes of crystallization of the Al-Mg-Li and Al-Cu-Li systems. The strength of the laser-weld joints after the full heat treatment reaches 0.91 of the basic alloy 1420 (Al-Mg-Li) strength (the alloy, 0.95 for 1424 (Al-Mg-Li), 0.94 for 1441 (Al-Cu-Mg-Li), 0.8 for 1469 (Al-Cu-Li).