Investigating local strain rate sensitivity of the individual weld zone in the friction stir welded DP 780 steel

Abstract

In this study, the effect of variable transverse speed (200 mm/min and 400 mm/min) on the microstructure and mechanical properties of FSWed DP 780 steel was systematically investigated. A detailed microstructural characterization was conducted using OM, SEM and EBSD to reveal the microstructural evolution in specific weld zone and was subsequently explained in context of variable transverse speed. Moreover, nanoindentation-based strain-rate jump tests (SRJT) were implemented to critically investigate the local characteristics with respect to the strain rate sensitivity (m-value) in distinct regions of FSW joint. Indentation hardness contour maps were characterized and analyzed in conjunction with the microstructural observations using EBSD. Refined grain size was observed in the SZ of FSWed-44 sample as compared to FSW-42, which was attributed to the increased dynamic recrystallization. The m-values and local hardness from SRJT for individual weld zones confirmed the rate dependent deformation behavior, which was further explained in conjunction to microstructure-process-property relationship.

Keywords

• Friction stir welding (FSW)
• Dual phase (DP) steel
• Scanning electron microscopy (SEM)
• Mechanical performance
• Strain rate jump test