Mechanical properties and microstructure of resistance spot welded severely deformed low carbon steel

Abstract

► Resistance spot welding is successfully used for severely deformed steel sheet. ► Microstructures of FZ and HAZ are refined to lower sizes at higher pass number. ► Mechanical properties in FZ and HAZ are increased with increasing the pass number. ► Electrode dip and nugget diameter are increased with increasing the pass number. The welding of nanostructured low carbon steel sheets produced by severe plastic deformation (SPD) has been considered in the present paper. Constrained groove pressing (CGP) method is used for imposing the severe plastic deformation to the steel sheets as a large pre-strain. The SPDed sheets are joined using resistance spot welding (RSW) process. The results show that severe plastic deformation can effectively increase the electrical resistivity of steel sheets; therefore it can affect the microstructure and mechanical properties of spot welds. Microstructure and mechanical properties of fusion zone, heat affected zone (HAZ), recrystallized zone and base metal of SPDed sheets are investigated and the results are compared with those of as-received specimens. The results show that with increasing the large pre-strain in sheets, at constant welding parameters (welding current and time), the fusion zone size, electrode indentation and nugget diameter are increased. Thus, peak load and hardness in fusion zone and HAZ are increased with increasing the CGP pass number. Also, the microstructures of fusion zone and HAZ are refined to lower sizes for larger pre-strained specimens.