Influence of Post Weld Cryogenic Treatment on the Ballistic Penetration Resistance of Friction Stir-Welded AZ31B Magnesium Alloy

Abstract

Influence of post weld cryogenic treatment conditions (deep and shallow) on the ballistic resistance of friction stir welded (FSW) AZ31B magnesium alloy has been investigated. AZ31B welded joints which were prepared by using friction stir welding process were subjected to two types of cryogenic treatment conditions. The microstructural investigation of cryogenic treated joints showed the formation of Mg₁₇Al₁₂ precipitates and conversion of coarse eutectic phase into thin discontinues phase along with halos. In addition, superior microhardness values were noticed for the cryogenically treated joints compared with non-treated joints and base metal. High velocity ballistic experiments were conducted on cryogenically treated and non-treated weld targets using 7.62 mm deformable projectiles at an initial velocity of 450 ± 10 m/s. At the stir zone, the depth of penetration (DOP) of the projectile into the deep cryogenic treated (DCT) target was observed to be 11% and 34 % lesser than the shallow cryogenic treated (SCT) and FSW targets. Thus increase in the ballistic resistance was observed for the post-weld cryogenic treated FSW joints compared to non-treated FSW joints. SEM investigation of the projectile impacted DCT targets showed the formation of cracks, dimples and micro voids at the crater region. It is also noticed that, the Stir Zone (SZ) and Heat Affected Zones (HAZ) of the DCT target has effectively absorbed the ballistic impact energy of the projectile by a ductile fracture and hence DCT can be used to improve the ballistic penetration resistance of the FSW joints at the SZ and HAZ regions.