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Öğe Detailed deformation behaviour analysis of DP steels at warm forming temperatures via warm tensile tests(Taylor & Francis Ltd, 2022) Sen, Nuri; Civek, TolgahanDP advanced high strength steels (AHSSs) have an important place in automotive industry for producing parts like B-pillars, door rings, etc. However, the excellent strength of AHSSs increases the issues related to their formability. Therefore, it is necessary to implement new forming processes to fully utilize the excellent properties of DP steels in more complex parts. In this study, the formability of DP steels (DP600, DP800) and their deformation behaviour at warm forming temperatures have been investigated by a series of tensile tests conducted at elevated temperatures (75, 175, 275, 375 degrees C) and two different strain rates (0.05, 0.005 s(-1)). To understand the deformation behaviour thoroughly, both instantaneous and average strain hardening - n and strain rate sensitivity - m values, the change in the hardening capacity and the microstructure have been analyzed. The results have shown that the formability of DP steels at 375 degrees C is more favourable as compared at room temperature.Öğe Effect of Different Conical Punch Angle Geometries and the Initial Hole Diameters on the Hole Expansion Ratio of DP steels(Society of Automotive Engineers Turkey, 2022) Şen, Nuri; Civek, Tolgahan; Bektaş, N.In the recent decade, advanced high strength steels (AHSS) have gained a great popularity in the automotive manufacturing industries due to their high strength to weight ratio, which significantly improves the safely of the manufactured automobiles while reducing the weight and thus, enabling to improve the fuel efficiency. However, it is known that some types of AHSSs, especially DP steels, are highly susceptible to edge cracking behaviour during the forming operations. Edge cracking behaviour is generally investigated with a 600 conical punch as suggested by the ISO 16630 standard. However, in mis study, to observe the behaviour of edge cracking ability of DP steels under different conical punch angles for different initial hole diameters, hole expansion tests have been performed with conical punches with three different angles (300 ,600 and 900) for three different initial hole diameters (14, 16 and 18 mm). The results have shown that the hole expansion ratio (HER) does not differ considerably with the variation of the conical punch angle and the initial hole diameter due to low fracture strain of DP steels observed after hole expansion tests. The major factor for the edge stretching ability of DP steels have been observed to be microstructure rather than geometrical factors such as conical punch angles. © 2023. Multidisciplinary Science Journal. All rights reserved.Öğe Experimental, analytical and parametric evaluation of the springback behavior of MART1400 sheets(Springer Heidelberg, 2022) Sen, Nuri; Civek, Tolgahan; Seçgin, ÖmerDue to high amount of springback behavior of MART steels, a large amount of time is wasted during the manufacturing of correct die setup for the intended products. Therefore, many sheet metal forming industries rely on the predicting ability of finite element analysis to reduce their forming costs. In this study, the effects of bending parameters on the springback behavior of MART1400 steel have been investigated by conducting V-bending tests with various die angles (30 degrees, 60 degrees, 90 degrees, and 120 degrees), punch radiuses (2 mm, 4 mm and 6 mm) and force holding times (0 s, 10 s). Furthermore, the predicting ability of different isotropic hardening models (Hollomon, Ghosh, Hocket-Sherby, Swift and Voce) coupled with the Von-Misses yield criterion on the springback behavior of MART1400 steels has been investigated. Additionally, the effect of applying a local heating around the bending area of MART steel on the springback behavior has been parametrically investigated. It has been found that increasing of die angle, and punch radius have resulted in an increase of springback, while a force holding time of 10 s has decreased the springback. Application of heat at 375 degrees C and 475 degrees C around the bending area of MART1400 has resulted in 40.18% and 55.13% reduction of springback due to the lowering of strain hardening.Öğe Investigation of the effects of lubrication type, blankholder force and force holding time on the spring-back behaviour for hat-shaped MART steels(Taylor & Francis Ltd, 2023) Sen, Nuri; Civek, Tolgahan; Yalcin, IremIn this study, hat-shaped bending experiments have been conducted on MART1200 and MART1400 Ultra-High-Strength Steels (UHSS). To investigate the influence of different lubrication conditions, on the spring-back, three different lubricant types (Graphite, Teflon film and Teflon film + Graphite) have been used in the experiments. A constant 6.5 tons of bottoming force has been applied at the end of the bending stroke and the force has been held on the samples for 0, 10s and 60s to observe the effect of force holding time on the spring-back. In addition, the effect of blankholder force on the spring-back has also been investigated for MART1400 steel. It has been found that the decrease of blankholder force and the friction between the die and sheet interfaces has considerably reduced the spring-back in the sidewalls and flange regions and also lowered the side wall curl. It has been revealed through finite element analysis that the sheet bottom has suffered a larger deformation in lower friction conditions and blankholder forces and caused in the reduction of spring-back.Öğe Investigation of tribological performance of hydrothermal carbon by pin-on-disc test and warm deep drawing process(Iop Publishing Ltd, 2024) Yurt, Ozgur Erdem; Sen, Nuri; Simsir, Hamza; Kucuk, Yilmaz; Altas, Emre; Gok, M. Sabri; Civek, TolgahanIn this study, the synthesis of hydrothermal carbon (HTC) lubricant and its usability as a lubricant under hot industrial conditions were investigated. In this context, the characterization of HTC produced from organic sources at low cost and in a short time was performed, and its tribological performance was analysed in detail. HTC produced by the hydrothermal carbonization method was characterized through SEM images and EDS analysis. To determine the effect of HTC on friction at different temperatures, HTC was subjected to a pin-on-disc wear test with AA5754 material. The effect of various lubricants, temperatures, blank holder pressure, and forming speed parameters on the forming force for the analysis of the tribological effect of HTC on deep drawing processes were statistically analysed. The performance of HTC was compared with Teflon, fullerenes, graphene, and carbon nanotube (CNT) materials. According to the results obtained from wear tests, the lowest friction coefficient value was achieved in the presence of fullerenes as the lubricant, and as the temperature increased, the friction coefficient decreased. It was observed that HTC exhibited lower performance in the wear test compared to fullerenes due to oxidation. When the effect of deep drawing parameters on the forming force was analysed, it was concluded that the most effective parameters were temperature (72.32%) and lubricant (20.89%). According to the S/N analysis results, the minimum forming force was obtained under the conditions of solid Teflon lubricant, 250 degrees C temperature, 15-bar blank holder pressure, and 2 mm/s forming speed. The tribological performance difference between HTC and Teflon is at the 1% level. The results demonstrate the potential industrial usability of HTC as a lubricant.Öğe A new lubrication approach in the SPIF process: Evaluation of the applicability and tribological performance of MQL(Elsevier Sci Ltd, 2022) Sen, Nuri; Şirin, Şenol; Kivak, Turgay; Civek, Tolgahan; Seçgin, ÖmerThe abrupt changes in the customer demands and the need to achieve versatility in production for satisfying the different needs of consumers have resulted in the development of rapid prototyping processes. Single Point Incremental Forming (SPIF) is a forming method which doesn't require dies for production and thus enables to greatly reduce the manufacturing costs for low to medium batch productions while enabling versatile manufacturability. One of the major disadvantages of this process is the poor surface quality of formed parts due to friction between tool and sheet interfaces. For this reason, suitable lubrication techniques and the use of lubricants are of great importance in obtaining the desired surface quality. In this study, the Minimum Quantity Lubrication (MQL) technique, which provides significant improvements in metal cutting processes with its effective lubricating feature, has been applied for SPIF process to investigate its effectiveness in SPIF processes. For this purpose, a series of experiments have been carried out by forming 7128 sheet metals with SPIF process in a CNC vertical machining center. In the experiments, SPIF parameters (feed rate, tool stepdown, etc.) have been kept constant and the effects of MQL parameters on surface roughness/topography, thickness distribution, roundness deviation, angular deviation have been focused. Three different pressures (4, 6, and 8 bar), three different flow rates (50, 75, and 100 mL/h) and three different vegetable-based oils with different viscosities have been selected as MQL parameters. In addition, 7128 sheet metal has been formed with the SPIF process by using a paste lubricant for the comparison of paste lubricated and MQL assisted SPIF process. Finally, friction tests have been carried out in a ball-on-disk device to determine the friction coefficients of the oils. Experimental results have shown that the increase in pressure has greatly increased the surface quality and dimensional accuracy, while the increased flow rate level improved the surface quality and dimensional accuracy up to a certain value (75 mL/h). It has been concluded that the surface quality can be improved by 14.60% with the MQL assisted SPIF process over the paste lubricated SPIF process, which has proved the applicability and superiority of MQL technique for SPIF processes.Öğe Prediction of Flow Behavior and Deformation Analysis of AA5754 Sheet Metal at Warm and Hot Temperatures(Springer, 2024) Sen, Nuri; Civek, Tolgahan; Ilhan, Omer; Yurt, Ozgur Erdem; Cetin, M. Huseyin; Simsir, HamzaThe utilization of lightweight materials such as AA5754 aluminum alloys in the inner body panel parts of vehicles has been significantly important for automotive manufacturers to minimize the high fuel consumption by reducing the overall weight. In this study, the flow behavior of AA5754 sheet metal has been discussed by conducting uniaxial tensile tests at five different temperatures (RT, 200, 250, 300, 350 degrees C) and three strain rates (0.001, 0.01, 0.05 s(-1)). Additionally, the capability of Fields and Backofen (F&B) and Voce hardening rules in describing the flow behavior of AA5754 at different temperatures and strain rates has been investigated by conducting uniaxial tensile tests in finite element analysis. It has been found that the main deformation mechanisms for the AA5754 are the strain hardening mechanism up to 250 degrees C, strain hardening and dynamic recovery mechanisms at 300 degrees C, dynamic recrystallization and strain hardening mechanisms at 350 degrees C. While the F&B hardening rule has been able to successfully capture the flow behavior of AA5754 up to 250 degrees C with a 14.36% error, its capability has significantly reduced after 250 degrees C due to its incapability of describing the effects of dynamic recovery and recrystallization. Voce hardening model has been better able to describe the flow behavior of AA5754 at all the temperature levels than F&B model due to its saturation behavior.Öğe Prevention of Tearing Failure during Forming of Lower Control Arm via TRIZ Methodology(Springer, 2021) Sen, Nuri; Baykal, Yakup; Civek, TolgahanMany forming failures such as tearing and wrinkling are encountered during sheet metal forming operations, and it is important for manufacturers to find a quick solution for these failures that hinder the production of a desired part. The theory of inventive problem-solving (TRIZ) is a problem-solving method that considers the patents issued in the past that have been useful in solving problems across a variety of industries. In this paper, TRIZ methodology was utilized to solve the tearing problem occurring during the forming of the lower control arm (automotive suspension part) without altering the desired shape and material of the part. For this purpose, a contradiction matrix was developed and the four different TRIZ principles of segmentation, taking out, the other way around, and parameter change were implemented in four different experiments. Of the conducted experiments, the experiment in which the TRIZ principles of segmentation, taking out, and parameter change were implemented together resolved the problem in a way that was acceptable and feasible for mass production of the lower control arm.Öğe Yüksek ve ultra yüksek mukavemetli çeliklerin ılık şekillendirme sıcaklıklarındaki deformasyon davranışlarının karakterize edilmesi(Düzce Üniversitesi, 2022) Civek, Tolgahan; Şen, NuriGünümüzde gerek fosil yakıt kaynaklarının azalması ve gerekse de bunların çevreye vermiş olduğu zararlı etkenlerden dolayı özellikle yeni nesil araçlarda hafif ve yüksek mukavemetli malzemeler tercih edilmeye başlanmıştır. Bu bağlamda, yüksek ve ultra yüksek mukavemetli çelikler, düşük alaşımlı çeliklere göre sahip oldukları yüksek mukavemetleri sayesinde otomotiv parçalarının daha ince üretilebilmesine olanak sağlamış ve otomotiv sektörünün geleceğinin şekillenmesinde önemli bir yer edinmiştir. Buna karşın yüksek ve ultra yüksek mukavemetli çeliklerin göstermiş olduğu yüksek mukavemet değerleri, bu malzemelerin şekillendirilebilmesi açısından zorlukları da beraberinde getirmekte ve daha karmaşık geometriye sahip parçaların şekillendirilmesindeki kullanımlarını kısıtlamaktadır. Ilık şekillendirme yöntemiyle birlikte, bu çeliklerin şekillendirilebilme kabiliyetleri bir miktar daha arttırılabilmektedir. Fakat öncelikle bu çeliklerin ılık sıcaklıklar etkisi altındaki deformasyon davranışının karakterize edilip, ılık sıcaklıkların bu çeliklerin deformasyonunda nasıl bir etkene sahip olduğunun anlaşılması gerekmektedir. Bu sebeple, bu çalışmada DP600 ve DP800 yüksek mukavemetli çelikler oda sıcaklığı (OS), 75 °C, 175 °C, 275 °C ve 375 °C, MART1200 ve MART1400 ultra yüksek mukavemetli çelikler ise OS, 175 °C, 275 °C, 375 °C ve 475 °C sıcaklıklar altında ve iki farklı gerinim hızı değerlerinde (0,05 s-1 ve 0,005 s-1) çekme testlerine tabi tutulmuştur. Çekme deneylerinden elde edilen verilerle birlikte çeliklerin anlık deformasyon yaşlanması ve anlık deformasyon hız hassasiyetlerindeki değişimleri, sertleşme ve uzama kapasitelerindeki değişimleri, ve mikroyapıda oluşan değişiklikler araştırılmıştır. Deney sonuçlarına göre her iki malzeme içinde 175 °C ve 275 °C sıcaklık değerlerinde dinamik deformasyon yaşlanması (DSA) belirtilerine rastlanmış ve bu sıcaklıklar altında gerçekleştirilecek olan deformasyon işlemlerinin şekillendirilebilirlik açısından avantajlı olmayacağı sonucuna varılmıştır. Yüksek mukavemetli çeliklerde 75 °C ve 375 °C sıcaklıklar altında gerçekleştirilen çekme deneylerinde çeliklerin uzama kapasitelerinde önemli bir artış görülürken, ultra yüksek mukavemetli çeliklerde bu artış 375 °C ve 475 °C sıcaklıklarda gözlemlenmiştir. Yüksek mukavemetli çeliklerde gözlemlenen bu artışın malzemenin mikroyapısında bulunan ferrit ve martenzit taneleri arasındaki sertlik farkındaki düşüşten dolayı meydana gelebileceği, ultra yüksek mukavemetli çeliklerde ise martenzit yapısının temperlenmesinden dolayı meydana gelebileceği sonucuna varılmıştır.
