Influence of cryogenic treatment on mechanical and ballistic properties of AA5754 alloy friction stir welded joints

In the present study, the mechanical and ballistic properties of friction stir welded (FSW) aluminum alloy (AA5754) samples were investigated, both untreated and cryogenically treated, when impacted by a 7.62 mm armour-piercing (AP) bullet at an impact velocity of 682±20 m/s. The FSW technique was used to prepare the welded samples for AA5754, with an axial force of 7 kN, a feed rate of 20 mm/min, and a speed of 1200 rpm. The cryogenic treatments performed after welding, including deep cryogenic treatment (DCT) at −196 °C and shallow cryogenic treatment (SCT) at −80 °C, for 6 and 72 h, respectively. The microstructure and mechanical characteristics of cryogenically treated and untreated joints were examined. The cryogenic treatment refined the grain size (1.05 μm) and enhanced the microhardness (93 Hv). Moreover, DCT-FSW significantly improved the tensile strength (13.93%) and impact strength (8.45%) compared to untreated FSW sample. Additionally, in untreated FSW samples, the fracture behaviour varied: the impact fracture mode primarily exhibited ductile failure, while the tensile fracture exhibited a mixed fracture mode. In contrast, the tensile and impact fracture modes of the DCT-FSW were dominated by a ductile failure mode. The DCT-FSW target demonstrated a lower depth of penetration (DOP) of 31 mm compared to the SCT-FSW and untreated FSW targets. Post-ballistic SEM analysis in the crater region of all three zones revealed the formation of frictional grooves, small cracks, and adiabatic shear bands (ASBs).