Biaxial deformation behavior of friction stir processed TRIP steel sheets

Abstract

In this study, effects of Friction Stir Processing (FSP) on the biaxial deformation behavior of 1.95 mm thick TRIP 780 steel sheets were investigated. FSP induced large plastic shear strains imposed at elevated temperature of about 945°C have drastically changed both microstructure and flow behavior of the steel. For these reason, after the FSP, significant changes in the microstructural and mechanical properties were obtained. After FSP, initial microstructure of the TRIP-steel transformed into a microstructure that mainly dominated by martensite grains.  This transformation resulted with nearly two-fold hardness increase in stir zone. Similarly, lath martensite dominated microstructure elevated the FSPed condition into an ultra-high strength level with expense of room temperature ductility.  After FSP, yield strength and UTS increased from 415MPa and 829 MPa to about 1280 MPa and 1475 MPa. Uniform elongation and elongation to failure decreased from 23% and 11% to 34% and 22% respectively.  In accordance to decreased ductility, Erichsen index (EI) of the steel decreased from 9.16 mm to 4.90 mm under biaxial stretching conditions In contrast to strength enhancement punch force at EI of TRIP-780 also decreased from 80.6 kN to 45.4 kN respectively.  This simultaneous decrease in both Ei and FEi attributed to increase in cracking tendency of the FSP induced microstructure. 

Keywords

• TRIP steel; AHSS; friction stir processing; formability; Erichsen test

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