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Öğe Biaxial deformation behavior and formability of precipitation hardened ultra-fine grained (UFG) Cu-Cr-Zr alloy(Elsevier Science Sa, 2016) Saray, OnurThe combined effects of ultrafine-grained (UFG) microstructure and precipitation on the formability and biaxial deformation behavior of a Cu-Cr-Zr alloy were investigated. The UFG microstructure formation results in good formability with an Erichsen index (Ei) of 4.05 mm compared to that of peak-aged coarse grained (CG) alloy (3.95 mm). Aging heat treatments increase strength and formability of the UFG alloy simultaneously. Biaxial deformation behavior is found to be dependent on the strain hardenability. Excellent strain hardenability of the CG alloy brought about higher punch displacement within the membrane stretching regime. However, deformation localization with the early onset of necking is evident in the UFG alloy. Subsequent aging treatments decrease deformation localization behavior of UFG alloy with increasing aging durations. Results also show that both grain refinement and aging increased the punch load due to enhanced strength. A linear relationship is generated based on punch load vs. punch displacement curve slope to predict ultimate tensile strength (UTS) with high accuracy. It is concluded that synergetic effect of UFG microstructure formation and subsequent aging provides a simple and effective procedure to produce Cu-Cr-Zr alloy for applications where balance of strength and formability are needed. (C) 2016 Elsevier B.V. All rights reserved.Öğe Biaxial deformation behavior of friction stir processed TRIP steel sheets(2019) Öztürk Yılmaz, İmren; Saray, Onur; Yılmaz, MüminIn 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.Öğe EFFECT OF FRICTION STIR PROCESSING ON FATIGUE BEHAVIOR OF THIN DUAL PHASE (DP600) STEEL SHEETS(Tanger Ltd, 2018) Saray, Onur; Yilmaz, Mumin; Ozturk Yilmaz, ImrenIn 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.Öğe Effect of precipitation on mechanical and wear properties of ultrafine-grained Cu-Cr-Zr alloy(Elsevier Science Sa, 2014) Purcek, G.; Yanar, H.; Saray, Onur; Kraman, I.; Maier, H. J.The effect of an ultrafine-grained (UFG) microstructure and subsequent aging treatment on the mechanical and wear behavior of a Cu-Cr-Zr alloy was investigated. The results indicate that the precipitates dispersed within the UFG matrix significantly enhance the strain hardening, resulting in improvement of hardness, strength and wear resistance of the alloy, without notably sacrificing the elongation to failure and electrical conductivity, due to the combined effect of grain refinement and precipitation. The wear behavior of Cu-Cr-Zr alloy was found to be strongly dependent on its strength and hardness. The minimum weight loss (or the highest wear resistance) was obtained when the sample was processed by equal channel angular extrusion (ECAE) through an additional aging treatment, as this resulted in ultra-high strength and hardness. The wear results also indicate that the wear behavior of Cu-Cr-Zr alloy in all processing conditions is consistent with the Archard approach. Complex wear mechanisms such as adhesive, oxidative and abrasive wear, and delamination were found to be operative in the differently processed Cu-Cr-Zr alloys. It is to be concluded that the use of a two-step process, the first resulting in an UFG microstructure and a subsequent aging treatment provides a simple and effective procedure for extraordinary increase in strength, hardness and wear resistance of Cu-Cr-Zr alloys without modification of the chemical composition. (C) 2014 Elsevier B.V. All rights reserved.Öğe Effect of two-pass friction stir processing on the microstructure and mechanical properties of as-cast binary Al-12Si alloy(Elsevier Science Sa, 2015) Aktarer, S. M.; Sekban, D. M.; Saray, Onur; Kucukomeroglu, T.; Ma, Z. Y.; Purcek, G.The effect of two-pass friction stir processing (FSP) on the microstructural evolution, mechanical properties and impact toughness of as-cast Al-12Si alloy was investigated systematically. Severe plastic deformation imposed by FSP resulted in a considerable fragmentation of the needle-shaped eutectic silicon particles into the smaller ones. The length of eutectic Si particles decreased from 27 +/- 23 mu m to about 2.6 +/- 2.4 mu m. The average aspect ratio of 6.1 +/- 5.1 for eutectic Si particles in the as-cast state decreased to about 2.6 +/- 1.0 after FSP with a corresponding increase in their roundness. The hardness, strength, ductility and impact toughness of the alloy increased simultaneously after two-pass FSP. The increase in the yield and tensile sirength values after FSP was about 20% and 29%, respectively. The FSPed alloy exhibited 25% elongation to failure and 15% uniform elongation which were almost seven times and five times higher, respectively, than those of the as-cast alloy. The hardness of the alloy increased from 58 Hv0.5 for the as-cast state to about 67 Hv0.5 after FSP. The absorbed energy during impact test increased to about 8.3 J/cm(2) after FSP, which is about seven times higher than that of the as-cast alloy. Improvements in all mechanical properties were mainly attributed to the radical changes of the shape, size arid distribution of the eutectic silicon particles along with the breakage and refined of the large alpha-Al grains during two-pass FSP. (C) 2015 Elsevier B.V. All rights reserved.Öğe Formability of friction stir processed low carbon steels used in shipbuilding(Journal Mater Sci Technol, 2018) Sekban, D. M.; Akterer, S. M.; Saray, Onur; Ma, Z. Y.; Purcek, G.The stretch formability of a low carbon steel processed by friction stir processing (FSP) was studied under biaxial loading condition applied by a miniaturized Erichsen test. One-pass FSP decreased the ferritic grain size in the processed zone from 25 mu m to about 3 mu m, which also caused a remarkable increase in strength values without considerable decrease in formability under uniaxial loading. A coarse-grained (CG) sample before FSP reflected a moderate formability with an Erichsen index (EI) of 2.73 mm. FSP slightly decreased the stretch formability of the sample to 2.66 mm. However, FSP increased the required punch load (F-EI) due to the increased strength by grain refinement. FSP reduced considerably the roughness of the free surface of the biaxial stretched samples with reduced orange peel effect. The average roughness value (Ra) decreased from 2.90 in the CG sample down to about 0.65 mu m in fine-grained (FG) sample after FSP. It can be concluded that the FG microstructure in low carbon steels sheets or plates used generally in shipbuilding provides a good balance between strength and formability. (C) 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.Öğe Improvement of formability of ultrafine-grained materials by post-SPD annealing(Elsevier Science Sa, 2014) Saray, Onur; Purcek, G.; Karaman, I.; Maier, H. J.Ultrafine-grained (UFG) IF-steel, as an example of an essentially single phase UFG microstructure, was annealed at different temperatures and time intervals in order to improve its formability by achieving a good strength-ductility-formability balance. In general, annealing increased the ductility and formability of UFG steel. Annealing at temperatures inside the recovery region caused a limited improvement in the formability of UFG steel due to the relief of internal energy without considerable grain coarsening. As the grain size increased to above 4 mu m by annealing at temperatures inside the partially recrystallization region, the formability of UFG steel in the uniform region increased considerably, and localized deformation with early necking changed to a homogeneous mode as revealed by increased uniform thinning and enlargement of the membrane straining regime. Further grain coarsening resulted in a slight increase in uniform elongation both in uniaxial and biaxial tests. The UFG microstructure reduced the roughness of the free surface of biaxially stretched samples by decreasing the non-uniform grain flow, which leads to the so-called orange peel effect. Annealing of UFG microstructure did not degrade this positive effect due to the formation of sharp recrystallization textures although the annealed microstructures have relatively coarse grains. It can be concluded that a good balance between strength and uniform formability without an orange-peel effect can be achieved in UFG microstructures by well-design annealing processes. (C) 2014 Elsevier B.V. All rights reserved.Öğe Microstructure, mechanical properties and formability of friction stir processed interstitial-free steel(Elsevier Science Sa, 2015) Sekban, D. M.; Saray, Onur; Aktarer, S. M.; Purcek, G.; Ma, Z. Y.The microstructure, mechanical properties and stretch formability of fine-grained (FG) interstitial-free steel (IF-steel) formed by friction stir processing (FSP) was investigated systematically. One-pass FSP drastically refined the microstructure with aid of dynamic recrystallization (DRX) mechanism during processing and formed volumetric defect free basin-like processed region (PR) with a mean grain size of 5 mu m (initial grain size was 40 mu m). This microstructural evolution brought about a considerable increase in both hardness and strength values of IF-steel without considerable decrease in ductility values. Also, strain hardening dominated deformation behavior was obtained with the FSPed samples as an essential property for the engineering application. Coarse-grained (CG) IF-steel demonstrated high formability with an Erichsen index (El) of 2.88 mm. Grain refinement by FSP yielded very close El value of 2.80 mm with increasing punch load (F-El). Force-displacement curves obtained in each process conditions reflected a similar membrane straining regimes where samples uniformly thinned under biaxial tension loads with aid of strain hardening capability. The formation of FG microstructure by FSP reduced the roughness (orange peel effect) of the free surface of biaxial stretched sample by decreasing the nonuniform grain flow leading to the so-called orange peel effect. It is concluded that a good balance of strength, ductility and strain hardenability along with equivalence formability to CG condition can be achieved by FSP as a single step practical procedure. (C) 2015 Elsevier B.V. All rights reserved.Öğe THE RELATIONSHIP BETWEEN FORCE-EXTENSION DIAGRAM AND VISUAL INSPECTION ON THE EDGE CRACKING TENDENCY OF DUAL PHASE STEELS(Tanger Ltd, 2018) Ardali, Refiye; Yildirim, Omer; Sahin, Kubilay; Saray, OnurLight-weighting targets of the automotive sector requires to use of advanced high-strength steels (AHSS) with higher strength more extensively. However, as a common behavior, increasing strength levels of AHSS accompanied with decreasing ductility/formability levels. This behavior become more pronounced during secondary deformation of a blanked edge. To examine this concept, hole expanding (HE) test, that simulates the local formability of the edges after blanking operations, has been started to be widely used. In this study, deformation stages of the HE tests were examined by means of main characteristics and variations of F (force) - X (extension) curves and derivative of F with respect to the X (dF/dx) during the punch travel until cracking. In order to correlate variation in curve characteristics with deformation stages, high-resolution camera images of the tested samples concurrently collected during the whole test and main relationships between them were determined. Based on these relationships a new approach to predict "stop stroke" of the HE tests is proposed. Therefore, elimination of the visual inspection and decision making of an operator or an in-situ image processing software is aimed. Proposed approach was developed using DP600 steel with a thickness of 1.2 mm as a common steel grade that widely used in automotive industry. Also, validation of developed "stop stroke" criteria was tried in HE testing of thinner and thicker DP600 samples. Results showed that a good prediction of stop stroke can be achieved by using proposed approach.Öğe Room temperature superplasticity in fine/ultrafine-grained Zn-Al alloys with different phase compositions(Trans Tech Publications Ltd, 2018) Demirtas, M.; Yanar, H.; Saray, Onur; Purcek, G.Three Zn-Al alloys, namely Zn-22Al, Zn-5Al and Zn-0.3Al, were subjected to equal-channel angular pressing (ECAP), and the effect of ECAP on their microstructure and room temperature (RT) superplastic behavior were investigated in detail referring to previous studies reported by the authors of the current study. ECAP remarkably refined the microstructures of three alloys as compared to their pre-processed conditions. While the lowest grain size was achieved in Zn-22Al alloy as 200 nm, the grain sizes of Zn-5Al and Zn-0.3Al alloys were ~540 nm and 2 µm, respectively, after ECAP. After the formation of fine/ultrafine-grained (F/UFG) microstructures, all Zn-Al alloys exhibited superplastic behavior at RT and high strain rates. The maximum superplastic elongations were 400%, 520% and 1000% for Zn-22Al, Zn-5Al and Zn-0.3Al alloys, respectively. It is interesting to point out that the highest RT superplastic elongation was obtained in Zn-0.3Al alloy with the largest grain size, while Zn-22Al alloy having the lowest grain size showed the minimum superplastic elongation. This paradox was attributed to the different phase compositions of these alloys. The formation of Al-rich ?/? phase boundaries, where grain boundary sliding is minimum comparing to Zn-rich ?/? and ?/? phase boundaries of Zn-Al alloys, is the lowest level in Zn-0.3Al alloy among all the alloys. Therefore, it can be concluded that if it is desired to achieve high superplastic elongation in Zn-Al alloys at RT, keeping Al content at a possibly minimum level seems to be the most suitable way. © 2018 Trans Tech Publications, Switzerland
