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Article Citation - WoS: 6Citation - Scopus: 6Excessive Damage Increase in Dual Phase Steels Under High Strain Rates and Temperatures(Sage Publications Ltd, 2021) Cobanoglu, Merve; Ertan, Rasim K.; Simsir, Caner; Efe, MertDamage formation in dual phase steels is a complex process and it may be sensitive to the deformation conditions and mechanisms. In this study, the damage parameter is measured and compared under quasi-static and industrial forming conditions (temperatures: 25 vs 200, 300 degrees C and strain rates: 10(-3)vs 10 s(-1)) for DP590 and DP800 steels. Resonance frequency and ultrasonic sound velocity techniques are utilized for the measurements to test the effectiveness and validity of each technique. At a given strain, the damage values can be up to 700% higher at industrial forming conditions, under which dynamic strain aging (DSA) controls the deformation behavior. DSA results in lower formability and is the likely mechanism responsible from the abnormal damage evolution. Measured damage parameters are also confirmed with the void fraction characterization by microscopy, which also provided details on the void shape and distribution with respect to the deformation conditions.Article Citation - WoS: 37Citation - Scopus: 40Dynamic strain aging in DP steels at forming relevant strain rates and temperatures(Elsevier Science Sa, 2017) Bayramin, Berkay; Simsir, Caner; Efe, MertMechanical testing of dual phase (DP) steels at low strain rates (10(-3) s(-1)) have shown that they are susceptible to dynamic strain aging (DSA) between 100 degrees C-400 degrees C. During industrial forming processes at intermediate strain rates (1-10(2) s(-1)), the local temperatures may rise to the DSA range due to deformation heating which may disturb the exceptional formability of these steels. In this study, two grades of DP steel (DP590 and DP800) are tested at thermomechanical conditions relevant to forming and the effects of DSA on the formability are established. Test results show that the DSA controls the deformation between 200 degrees C-300 degrees C through serrations in the stress-strain curves of both grades. With increasing strain rates (up to 1 s(-1)) and temperatures, DSA intensifies and results in severe drops in uniform and total ductility with negative strain rate sensitivity, indicating poor formability at these conditions. A detailed analysis of the serrations coupled with dislocation density measurements by x-ray analysis suggests that the serrations can be linked to a periodic microstructural feature.Article Citation - WoS: 8Citation - Scopus: 9A Comprehensive Study of the Effect of Scanning Strategy on In939 Fabricated by Powder Bed Fusion-Laser Beam(Elsevier, 2024) Dogu, Merve Nur; Ozer, Seren; Yalcin, Mustafa Alp; Davut, Kemal; Obeidi, Muhannad Ahmed; Simsir, Caner; Brabazon, DermotThis study provides a comprehensive investigation into the effects of different scanning strategies on the material properties of IN939 fabricated using the PBF-LB process. The scanning strategies examined included alternating bi-directional scanning with rotation angles of 0 degrees, 45 degrees, 67 degrees, and 90 degrees between adjacent layers (named as shown), as well as alternating chessboard scanning with rotation angles of 67 degrees and 90 degrees (named as Q67 degrees and Q90 degrees). The results revealed that the 45 degrees and 67 degrees samples had the highest relative density, while the 0 degrees and Q67 degrees samples showed the highest average porosity. Moreover, various types of cracks, including solidification, solid-state, and oxide-induced cracks, were observed. Among the bi-directional scan samples, the 0 degrees sample displayed the most extensive cracking and the highest sigma max residual stress values in both XZ and XY planes. Conversely, the 45 degrees and 67 degrees samples exhibited fewer cracks. Notably, the lowest sigma max residual stress in the XZ planes among the bidirectional scan samples was observed in the 67 degrees sample. Additionally, microstructural analyses indicated differences in grain size and morphology, among the samples. Texture analysis indicated that the 0 degrees and 90 degrees samples exhibited strong cube textures, whereas the texture intensity weakened for the 45 degrees and 67 degrees samples. Moreover, the alternating chessboard scanning strategy led to rougher surfaces (higher Sa and Sz values) compared to the alternating bi-directional scanning strategy, regardless of the rotation angles. Furthermore, the microhardness values among the samples showed minimal variance, ranging between 321 + 14 HV and 356+ 7 HV.
