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Bending fatigue lives of resistance spot welded (RSW) joints using commercial DP600 and DP1000 advanced high strength automotive sheets were compared in this study. Advanced high strength steels, especially DP600 and DP1000 steels which are used in this study, are the most important metal materials in which today's automotive body and attachment parts are produced. The materials used for the manufacture of the vehicle under dynamic conditions need to be identified of their all features are vey important. The literature has mainly focused on axial fatigue. Fatigue lives of base material and resistance spot welded joints obtained from different welding parameters were investigated by using two different high strength dual phase steel. It was seen that as the welding current increased and the electrode pressure increased up to a certain value as in the axial fatigue, the bending fatigue life was increased from the results of this study. Direct proportional relationship was obtained between the weld nugget size and fatigue life. 7 kA-4 bar (DP600) and 7 kA-3 bar (DP1000) welding parameters yielded better fatigue life. DP1000 and DP600 steel sheet samples have similar fatigue life values in low cycle fatigue. It was also seen that DP600 steel had better fatigue life than DP1000 in high cycle fatigue.

International Iron & Steel Symposium
UDCS

Muhammed Elitaş Bilge Demir

In this study, a dual phase steel DP600 which is frequently used for automotive industry because of lightness, durability performance and economical attainability, was joined by resistance spot welding (RSW) method that are used frequently in automobile industry, and then exposed to cyclic bending load with different amplitude values. In addition, fatigue test was repeated with additional tensile force that is applied with bending forces and results that obtained from bending fatigue tests and bending-tensile fatigue tests are evaluated and compared with each other. As a result of the fatigue tests that performed to the samples, it was observed that the axial tensile force decreased the fatigue life. In literature frequently axial forces were applied to resistance spot welded specimens for fatigue tests in most research. In real condition, fusion zone can be exposed to different axial forces. It is necessary to know and investigate the fatigue life due to tensile and bending resultant force as well as axial force, which is very important for vehicle stiffness and reliability.

International Iron & Steel Symposium
UDCS

Mustafa Göktaş Muhammed Elitaş Bilge Demir

The microstructure investigation of springback formed after sheet metal forming by the drawing mold is the basis of this study. In this study, microstructure properties after springback were investigated in two different metal groups such as high-strength steel and Al. The estimation of the amount of springback, the ways of avoidance and microstructure investigations are of great importance in producing parts with high commercial value. Specially designed deep drawing mold was used in experimental studies. The staple radius in the drawing die was used as a die factor and kept constant in all experiments. Four different materials and four different thicknesses were used as material factors. The microstructure was examined after the forming, the amount of springback was calculated as angular. The results showed that the increase in sheet metal thickness increased the amount of springback and the difference in chemical composition was more effective than the resistance.

International Iron & Steel Symposium
UDCS

Abdülsamet ÖZDEN Bilge Demir

— In this study research, the RSW process was applied at various welding currents and in different electrode pressures in electrical spot welding of commercial DP600 steel sheets which has 1 mm thickness that was investigated and calculated the relationship between the effects of the electrode force and that of the welding current has an effect on the weld quality and the diameter of the weld nugget. The current and electrode force been adjusted through the increment of 2 kA from 4 to 8 kA and 1 Bar from 3,5 to 5,5 respectively. The results of the experimental reveal that the tensile shear load and the nugget diameter are increases as the current increases and then it decreases as the electrode pressure increases.

International Iron & Steel Symposium
UDCS

Abdulkarim Alzahougi Muhammed Elitaş Bilge Demir

It is well known that the homogeneous incorporation of a small concentration of carbon nanotubes (CNTs) in epoxy matrices can enhance the mechanical properties of the composites. However, CNT has a tendency to aggregate during mixing with epoxy resin due to the high van derWaals interaction energy between CNT particles. In order to overcome this problem surfactants are used during sonication to promote the homogenous dispersion of CNT in suspension.

International Symposium on Light Alloys and Composite Materials
UHAKS

Hatice YAKUT PEKTÜRK Fatih ÖZ Sina Narimani Gharajeh Nuri ERSOY Bilge Demir

This article presents an experimental investigation on the thrust force and surface roughness in the drilling of (Mg/0.25wt.%GNPs) Metal Matrix Composite (MMC). The composite material was produced by the powder metallurgy method. The powders were mixed via mechanical alloying for two hours with speed of 250 rpm. The stainless-steel mold of 25 mm internal diameter was used for the cold pressing process. The powders were pressed by applying pressure of 500 MPa at the room temperature. This process was produced the green samples with a diameter of 25 mm and thickness of 10 mm. The samples were sintered for 1.5 hours at a temperature level of 550 Cº under the controlled atmosphere of high purity Argon gas to prevent the oxidation. The experiments were carried out with uncoated, PVD and CVD coated tungsten carbide drill bits (the diameter of 5 mm) at three levels of cutting speeds (30, 40 and 50 m/min), and three different levels of feed rate (0.10, 0.15 and 0.2 mm /rev) under dry machining conditions. The contribution of drilling process parameters to the thrust force and surface roughness was determined by S/N ratio analysis using the Taguchi’s technique throughout the Minitab17 software. The microstructure of specimen was investigated using Scanning Electron Microscope (SEM) before and after drilling. The micro-structural results indicate that the microstructure is free of porosities as shown in Figure 1.When the mapping images of the composite are evaluated, GNPs scattered homogenously in the Mg matrix. There exists significant bonding among GNPs and Mg particles [1-3]. Figure 2 shows the variation of the thrust force with feed rates at different cutting speeds for (Mg/GNPs) MMC when using uncoated, PVD and CVD coated drill bits. In almost of experiments, the thrust forces are lower when drilling (Mg/GNPs). This result can be recognized to the fact that GNPs particles decrease friction on the tool-material interface and as well between the tool and chip produced due to their natural solid lubricant property. The figure shows that the thrust force is highly dependent on the feed rate, cutting speed and type of drill bits [4,5]. The experimental results in the figures indicated that the thrust force is increased by increasing the feed rate, while thrust force decreases with increasing of cutting speed. The surface roughness (Ra) values of drilled surfaces are lower during the drilling of (Mg/GNPs) while using coated drill bits. This is due to the hard coating over the drill bit surface, which reduced the BUE formation during drilling.

International Symposium on Light Alloys and Composite Materials
UHAKS

Mustafa M. ABDULGADIR Bilge Demir Muhammet Emre Turan

Demiryolları batı ülkelerinin 19 ve 20 yüzyılda gelişmesinde çok önemli bir rol oynamıştır. Özellikle demiryolu ray çeliği bu gelişimde önemli bir yere sahiptir. İlk ticari demiryolu rayı Dünyadaki ilk çelik demiryolu 1850'lerin ortalarında İngiltere'de yapıldı ve 1862'de bir demiryolunda deneme kurulumu yapıldı. 1867 de ise Amerika’da Bessemer çeliği olarak üretildi ve kullanılmaya başlandı. 20. Yüzyıla gelindiğinde çelik üretimi ve proseslerindeki gelişmeler demiryolu raylarında da önemli gelişmeleri beraberinde getirdi. Bu gelişmeler kimyasal kompozisyon ve mikroyapıya bağlı olarak: yüksek dayanım, güvenlik, aşınma direnci, katar yüklerinin artması şeklinde özetlenebilir. Bu gelişmedeki anahtar rol ötektoit ince perlit yapısından ileri gelmekteydi. Günümüzde özellikler daha da optimize edilerek demiryolu raylarının servis ömrü ve kapasiteleri oldukça arttırılmıştır. Yeni olarak perlitik raylar yerine beynitik rayların özellikle yüksek mukavemet istenen yerlerde kullanılması önemli bir alandır. Bu çalışmada dünyadaki ve demiryollarındaki bu gelişmeler kritize edilmiştir. Railways played a very important role in the development of western countries in the 19th and 20th centuries. Especially railway rail steel has an important place in this development. The first commercial railroad rail The world's first steel rail was built in the mid-1850s in the UK, and in 1862, a railroad was set up. In 1867, Bessemer steel was produced in America and started to be used. In the 20th century, the developments in steel production and processes brought about important developments in railway tracks. Depending on the chemical composition and microstructure, these developments can be summarized as: high strength, safety, abrasion resistance and increased string loads. The key role in this development was the structure of the autoctoite fine perlite. Nowadays, the features are further optimized, and the service life and capacity of the railway rails have been greatly increased. In this study, these developments in the world and railroads are categorized

International Symposium on Railway System Engineering
ISERSE

Bilge Demir

Bu çalışmada, ticari ray üretimi proses sıcaklığının ve kademeli soğutma işleminin ray mekanik özellikleri üzerine etkisi araştırılmıştır. Esasen ray çelikleri gibi kritik bir malzemenin özelliklerine etkiyen her türlü parametre çok elzemdir. Bu faktörlerinden en önemlilerinden bir tanesi önceki östenit tane boyutudur (PAG). Sıcak haddeleme sırasındaki sıcaklık ve deformasyon kombinasyonu ile sağlanan PAG üretim ve servis şartlarında oluşabilecek hasarlar açısından önemi Ray çelikleri için oldukça fazladır. Bununla birlikte yeterince somut bilgiler mevcut değildir. Bu çalışmanın araştırma içeriği, deneysel çalışmalarda oluşturulan yüksek sıcaklık tavlamaları ile haddeleme tavlama işlemlerinin benzeri model yapıların oluşturulmasını sağlayan ısıl işlemler ve devamında bütün ray çelik numunelerinin karakterizasyonu, yani ısıl işlemler, metalografi ve mekanik testlerden oluşmaktadır. Deneysel çalışmalarda, farklı östenitleme sıcaklıklarında tavlamanın (A3 sıcaklığı üzerinde 900°C -1200°C) ardından çekme test numuneleri hadde sonrası soğuma şartlarını benzeştirmek için havada ve PAG analizlerinin yapılacağı numuneler suda soğutulmuştur. İlave olarak, PAG numuneleri, önceki-tavlama sırasındaki östenit yapılarının ortaya çıkarılması ve analizi için temperleme işlemine tabi tutulmuştur. Deneysel çalışma sonuçları, östenit tane boyutunun bu çalışmada tercih edilen ısıl işlem-kimyasal dağlama kombinasyonu ile sağlıklı bir şekilde ortaya çıkarılabildiğini ve sıcaklığa göre östenit tane boyutunun ve östenit tane boyuna bağlı olarak da çekme özelliklerinin önemli oranda değiştiğini göstermektedir. In this study, the effect of commercial rail production process temperature and step cooling process on rail mechanical properties is investigated. Essentially, any kind of parameter that affects the properties of a critical material such as rail steels is very important. One of the most important of these factors is the austenite grain size (PAG). Owing to the combination of temperature and deformation during hot rolling, the importance of PAG in terms of damage to production and service conditions is quite high for rail steels. However, not enough information is available in literature. The research content of this study consists of heat treatments, which provide the formation of similar model structures for high temperature annealing and rolling annealing processes in experimental studies, followed by characterization of all rail steel samples, ie heat treatment, metallography and mechanical tests. In experimental studies, the samples of annealing at different austenitizing temperatures (900 ° C -1200 ° C above A3 temperature) and in the air and PAG analyzes to simulate the post-roll cooling conditions were cooled in water. In addition, PAG samples were subjected to tempering to reveal and analyze austenitic structures during prior-annealing. The results of the experimental study show that the austenite grain size can be revealed in a healthy way with the preferred heat treatment-chemical etching combination in this study and that the austenite grain size and the austenite grain size vary significantly according to the temperature

International Symposium on Railway System Engineering
ISERSE

Bilge Demir