A serious number of critical aspects have to be considered when aluminium and its alloys, especially if heat treated, are welded. These drawbacks, to date, limit the application of the welded joints in the aeronautical sector. The possibility of overcoming these limitations is focused in current paper by studying the mechanical performance of two types of dissimilar laser beam welds in T-joint configuration. The dissimilar joints, planned for aeronautical applications, combine the same skin, made of AA 6156 alloy, with two different base materials for the stringers, namely the AA 2139 and the non-standard alloy PA765. The alloys under investigation are characterized by enhanced weldability if compared to the standard AA 2024, widely used in aeronautic. Due to the contemporary presence of dissimilar heat treatable aluminium alloys, both joints have been subjected to dedicated multi-step heat treatments.

Metallurgical and microstructural analysis were carried out in order to highlight the influence of both chemical composition and thermal cycles experienced on the final microstructures of different zones of the joints, governing the mechanical properties of the joint itself.

Vickers micro-hardness measurements were performed to fully characterize the heterogeneity in mechanical properties across the different zones of the dissimilar welds. Strain data acquisition by Digital Image Correlation (DIC) allowed to assess the influence of local weld properties on joints strength in tensile loading. The important role of the temper conditions of the parent materials, resulting from the different joint pre and post-welding heat treatment options, on the tensile behaviour of the skin and, in this way, on the global strength of the connection was showed. The dramatic role played by the Heat Affected Zone and the influence of the distribution of the strengthening elements on examined joints mechanical properties has been put in evidence, according to mechanical test results.