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    Deformation behavior and formability of friction stir processed DP600 steel
    (De Gruyter Poland Sp Z O O, 2022) Yilmaz, Imren Ozturk; Yilmaz, Mumin; Saray, Onur
    The effect of friction stir processing (FSP) on the formability of DP600 steel was experimentally investigated and the basic relationships between biaxial deformation behavior and FSP-induced evolutions in microstructural and mechanical properties were established. FSP formed a microstructure that consists of lath martensite with increased volume fraction compared to as-received (AR) microstructure that mainly consisted of well-distributed fine martensite particles in a ferrite matrix. Consequently, AR yield strength (301 MPa) and ultimate tensile strength (621 MPa) increased to about 811 and 1054 MPa, respectively. This strength enhancement achieved accompanied by adequate uniform elongation and elongation to failure values of 6.3 and 13.0%, respectively. Under biaxial loading conditions, good strain hardenability of the AR DP600 steel brought about a large membrane stretching regime leading to high punch force for biaxial flow. After FSP, both punch displacements within the membrane stretching regime decreased due to the increased volume fraction of lath martensite leading to higher cracking tendency. In result, cup depth and peak punch force of FSPed DP600 decreased from 8.7 mm and 33.2 kN to 7.1 mm and 28.1 kN, respectively. The obtained results simply indicate that FSP can be employed to enhance the strength of dual-phase steels with a reasonable level of formability.
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    EFFECT OF FRICTION STIR PROCESSING ON FATIGUE BEHAVIOR OF THIN DUAL PHASE (DP600) STEEL SHEETS
    (Tanger Ltd, 2018) Saray, Onur; Yilmaz, Mumin; Ozturk Yilmaz, Imren
    In this study, effects of Friction Stir Processing (FSP) on the deformation behavior of Dual Phase (DP600) steel sheets under static and cyclic loading were investigated. Fatigue tests were performed at a frequency of 15 Hz during repeated tension at a cycle asymmetry R = 0 and 10(6) loading cycles. DP600 steel reflected yield strength (sigma(y)) of 301 MPa and ultimate tensile strength (sigma(UTS)) of 621 MPa with uniform elongation of 21.3% and fractured after a total elongation of 34.7% in its as-received condition. After FSP, it was observed that the yield strength increased to 811 MPa and the ultimate tensile strength reached to 1054 MPa. This effective strength enhancement brought an acceptable decrease in ductility of the DP600 steel resulting in uniform elongation and elongation to failure of 6.3% and 13.0%, respectively. Based on obtained ductility values, it can be considered that, FSPed DP600 shows a deformation behavior that mostly dominated by the strain hardening. Static strength enhancement obtained by FSP of DP600 steel also yielded a favorable effect on the fatigue behavior and stress level leading to transition to the infinite life. As a result of the fatigue tests, it was determined that the fatigue limit of the as-received DP600 steel increased from 350 MPa to 480 MPa after the applied FSP. Experimental results obtained in the study mainly indicate that, FSP is an easy to apply and practical procedure which provides significant enhancement on the mechanical performance of DP600 steel under both static and cyclic loading conditions.
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    Effect of friction stir processing on fatigue behavior of thin Dual Phase (DP600) steel sheets
    (TANGER Ltd., 2018) Saray, Onur; Yilmaz, Mumin; Yilmaz, Imren Ozturk
    In this study, effects of Friction Stir Processing (FSP) on the deformation behavior of Dual Phase (DP600) steel sheets under static and cyclic loading were investigated. Fatigue tests were performed at a frequency of 15 Hz during repeated tension at a cycle asymmetry R = 0 and 10 6 loading cycles. DP600 steel reflected yield strength (? y ) of 301 MPa and ultimate tensile strength (? UTS ) of 621 MPa with uniform elongation of 21.3% and fractured after a total elongation of 34.7% in its as-received condition. After FSP, it was observed that the yield strength increased to 811 MPa and the ultimate tensile strength reached to 1054 MPa. This effective strength enhancement brought an acceptable decrease in ductility of the DP600 steel resulting in uniform elongation and elongation to failure of 6.3% and 13.0%, respectively. Based on obtained ductility values, it can be considered that, FSPed DP600 shows a deformation behavior that mostly dominated by the strain hardening. Static strength enhancement obtained by FSP of DP600 steel also yielded a favorable effect on the fatigue behavior and stress level leading to transition to the infinite life. As a result of the fatigue tests, it was determined that the fatigue limit of the as-received DP600 steel increased from 350 MPa to 480 MPa after the applied FSP. Experimental results obtained in the study mainly indicate that, FSP is an easy to apply and practical procedure which provides significant enhancement on the mechanical performance of DP600 steel under both static and cyclic loading conditions. © 2018 TANGER Ltd., Ostrava.
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    Fatigue and Impact Behavior of Friction Stir Processed Dual-Phase (DP600) Steel Sheets
    (Mdpi, 2024) Yilmaz, Mumin; Ozturk Yilmaz, Imren; Saray, Onur
    This study investigates the impact of friction stir processing (FSP) on the deformation behavior of 1.1 mm-thick DP600 steel sheets under both static and dynamic loading scenarios, with a focus on the automotive applications of the material. During the process, the large plastic shear strains imposed by FSP resulted in a maximum temperature of 915 degrees C, leading to a morphological transformation of the martensite phase from well-dispersed fine particles into lath martensite and grain refinement of the ferrite phase. DP600 steel showed an almost two-fold increase in static strength parameters such as the hardness value, yield strength, and ultimate tensile strength. As-received and processed DP600 steel exhibited a plastic deformation behavior governed by strain hardening. However, uniform elongation and elongation to failure after FSP took lower values compared to those of the as-received counterpart. Following the improvement in the static strength of the steel, the fatigue strength of the steel increased from 360 MPa to 440 MPa after the FSP. The finite-life fatigue fracture surfaces of the as-received samples were characterized by the formation of fine bulges due to the variation in the crack propagation path in the vicinity of the martensite particles/clusters. After FSP, the transformation of the martensite particles into coarser lath martensite also transformed the fracture surface into a step-like morphology. The microstructural evolution after FSP caused a decrease in the absorbed impact energy and maximum striker reaction force from 239 J and 37.6 kN down to 183 J and 33.6 kN, respectively. However, the energy absorption capacity of the processed steel up to failure was higher than the absorbed energy value of the as-received steel at the same impact displacement. The simultaneous decrease in both impact energy and reaction force is attributed to the higher cracking tendency of the processed microstructure due to the lower volume fraction of the ferrite phase. The experimental results reported in this study mainly show that FSP is an easy-to-apply and functional solution to significantly improve the static and cyclic strength of DP600 steel. However, it is clear that the reduced total impact energy absorption capacity after FSP may be taken into account in design strategies.