The influence of laser-induced tempering on the microstructure and mechanical properties of 1500 MPa martensitic steel friction stir welded joints

A comprehensive comparison illustrates the role of laser technology in altering the macro morphology and microstructure of martensitic steel joints produced by friction stir welding (FSW). In this study, laser-induced tempering (LIT) was introduced to the FSW of 1500 MPa martensitic steel. The mechanism by which LIT enhanced the mechanical properties of the FSW joints was elucidated by comparing joints produced with and without LIT. Experimental findings revealed that LIT effectively eradicated tunnel defects under the same FSW parameters. Even with an increase in travelling speed from 60 mm/min to 120 mm/min, flawless welds were achieved under LIT conditions. At 500 rpm and 60 mm/min, the bottom of the stir zone (SZ) in conventional FSW (C-FSW) joint exhibited a ferrite and martensite dual-phase microstructure. In contrast, the SZ of the LIT-FSW joint had a fully martensitic structure. Moreover, LIT resulted in significant variations in the martensitic hierarchical structure within the SZ. LIT reduced ferrite distribution in the dual-phase zone, caused recrystallized ferrite to appear further from the SZ. The stress concentration in the dual-phase zone attributed to joint fracture. The improvement in the mechanical properties of martensitic steel FSW joints was explained by analyzing the correlation between the stress concentration factor and phase fraction. Additionally, the increase in traveling speed narrowed the dual-phase zone, triggered a constraint effect that led to a maximum ultimate tensile strength (UTS) increase from 986 MPa at 60 mm/min joint to 1142 MPa at 120 mm/min joint.