Transient Microstructure Evolutions and Local Properties of Dual-Phase 980 MPa Grade Steel Via Friction Stir Spot Processing

Abstract

Friction stir processing is a novel solid-state process to modify microstructures and their properties by intense, localized plastic deformation. However, little research has been reported for microstructure evolutions of advanced high-strength steels during the process. The present work focuses on the study of transient microstructure changes and local mechanical properties for friction stir spot processed dual-phase (DP) 980 MPa grade steel (DP980) under different peak temperatures. A pinless silicon nitride ceramic tool was used to produce relatively simple material deformation and flow near the tool. Friction stir spot processed steel samples were characterized by optical and electron microscopies. Furthermore, Vickers microhardness and nano-indentation measurements were used to study local mechanical properties for correlation with microstructures. A swallow layer of refined grains (<0.6 µm) was obtained with a low peak temperature (under 400 °C), whereas higher peak temperatures (>Ac1) led to a change in grain size with different microstructures (fine-grained DP or martensite). Electron back-scattered diffraction characterizations revealed a large deformation in the as-received microstructures (mixture of ferrite and tempered martensite) induced by friction stir spot processing, leading to recrystallization and grain refinement around the stirred zone. Also, nano-indentation measurements showed a higher hardness than the hardness of the as-received DP980. Friction stir processing with different process conditions effectively changed microstructures and local mechanical properties.

Fatigue and Impact Behavior of Friction Stir Processed Dual-Phase (DP600) Steel Sheets

“…Figure 3e demonstrates that the volume fraction of lath martensite significantly decreased in the TMAZ. In the TMAZ, the undissolved ferrite stayed the same, and the carbon-rich martensite phase transformed into austenite at the maximum processing temperature in the A c1 -A c3 range [14,16]. At this temperature range, the ferrite phase may not transform into austenite, leading to the ferrite-based microstructure remaining unchanged (Figure 3f,g).…”

“…Lee et al stated that the strength deficiency in the heat-affected zone of DP590 steel following FSW significantly reduced the formability [15]. Taniguchi et al subjected DP980 steel to friction stir spot processing to reveal transient microstructural changes and local mechanical properties at different peak temperatures [16]. Gotawala et al developed a coupled 3D thermomechanical and phase transformation model predicting the temperature history, strains, and phase transformation during the process [17].…”

Transient Strain Monitoring of Weldments Using Distributed Fiber Optic System

“…The integration of plastic strain rates during welding leads to a residual strain field that generates RS in the weldment, thus affecting the durability of the joint [1]. The use of in situ sensors capable of accurately measuring the real-time total and thermal strain during the welding process may provide industry partners with an understanding of the transient strain formation which is known to affect the microstructure of the material [2]. In this study, the primary focus is the use of novel instrumentation techniques and how they help in the validation process of transient computational welding mechanic (CWM) models, versus the actual physics that leads to the formation of RS or microstructural changes within the weldment.…”