Investigation of microstructural evolution in a hybrid additively manufactured steel bead

Abstract Motivated by the beneficial effects of friction stir processing (FSP) for microstructural grain refinement, equiaxed grain production, and minimizing metallurgical defects, additive bead (AB) produced by the gas metal arc welding-wire arc additive manufacturing (GMAW-WAAM) technique was subjected to FSP. This was because deposited additive bead often develops defects such as shrinkage, voids, solidification cracking, during liquid to solid transformation. In this study, a low carbon steel double pass additive bead with 32 % lateral overlap was fabricated by the GMAW-WAAM technique followed by hybridization through FSP in the overlapped region (OR). The peak temperature estimation during bead deposition and FSP on bead was done through modeling by using ABAQUS. The microstructural analysis was carried out by using optical microscopy, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. The microstructure of OR of deposited additive bead is dominated by a combination of ferrite and bainite while that of hybrid additive bead (HAB) is dominated by a combination of ferrite and martensite. Further, the analysis revealed the effects of FSP on the OR in the form of grain refinement from 5.56 µm to 3.50 µm and a decrease in the low angle grain boundaries from 35.4 % to 10.6 %. The continuous dynamic recrystallization is observed since the bainitic fraction in the overlapped region decreased along with an increase in the fraction of martensite in the friction stir processed zone. The kernel average misorientation is observed to decrease after FSP from 1.001 of AB to 0.608. The microhardness test reveals the decrease in the hardness after FSP.