Microstruttura e difettosità in giunti laser AA5754/Ti-6Al-4V

This study aims to characterize butt joints between AA5754 and Ti-6AI-4V alloy processed through laser welding. The joints were welded by focusing the laser beam on the titanium alloy at two different offset distances of 0.75mm and 1mm. The line energy (ratio between laser power and feed rate) was changed on three levels (30.0 J / mm, 36 J / mm and 72 J / mm). The welds microstructure and defects were analysed. All the joints exhibited a complete keyhole penetration. The fusion zone (FZ) of the titanium alloy extends up to the aluminium interface leading to melt the AA5754 alloy. The heat-affected zones (HAZ) of the two alloys were clearly distinguished. The FZ microstructure of the Ti alloy was mainly martensitic while that of the Titanium HAZ was only partially martensitic. As a consequence, the highest hardness values of the joints were found in the titanium FZ. Then the hardness gradually decreases in the Titanium HAZ as the distance from the fusion line increases. A thin layer of intermetallic compounds (IMC) of different stoichiometry was found at the interfaces between Aluminium and Titanium FZ. Particularly TiAI3, TiAI and Ti3AI compounds were identified. The layer morphology changes with process parameters becoming highly irregular and curved at the highest line energy and lowest offset distance. All the welding seams exhibited uneven thickness. The inhomogeneity increases with line energy.