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    Dynamic Strain Aging Behavior of Cobalt-Based Haynes188 Superalloy
    (Springer, 2024) Nohutcu, Samet; Kacar, Ramazan
    Superalloys have been developed to meet the need for materials that will serve at high operating temperatures without losing their properties. The three major classes of superalloys are nickel-, iron-, and cobalt-based alloys. Cobalt-based superalloys are used between 650 and 1100 degrees C temperatures due to their wear, creep, oxidation, heat, and corrosion resistance. Haynes 188, an essential family member, is mainly used in gas turbine engines such as combustion chambers, air transfer ducts, fixed blades, exhaust nozzles, and flame holder components. Strengthening is achieved by solid solution or carbide and intermetallic formed in their structures. None of the cobalt-based superalloys are fully solid solution alloys. Almost all contain carbide or intermetallic compounds. This can lead to dynamic strain aging at different service temperatures in the alloy under load, resulting in a reduction in ductility. Considering the temperatures in service conditions, the dynamic strain ageing behavior of Haynes 188 alloy at 25, 250, 400, 550, 700, 850, and 1000 degrees C was thoroughly examined for the first time in this study. The unsteady yielding with different type's serration verified that the dynamic strain ageing is formed in Haynes 188 alloy. Therefore, ductility of alloy decreased in the temperature range of 250-700 degrees C, particularly at 550 degrees C.
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    Mechanical properties of plasma arc welded AISI304LN austenitic stainless steel at various temperatures
    (Taylor & Francis Ltd, 2023) Kacar, Ramazan; Emre, Hayriye Ertek; Nohutcu, Samet
    AISI 304 grade austenitic stainless steels are widely used in a wide variety of fields such as chemical, automobile, food industries, etc. due to their good mechanical properties and corrosion resistivity. In this study, AISI 304LN austenitic stainless-steel couples were joined with plasma arc welding. The effect of temperature on the mechanical properties, formability and fracture morphologies of joints was evaluated. For this purpose, tensile testing of the joints was carried out in the temperature range of -50 degrees C, and 25 degrees C-850 degrees C. Thus, the ultimate tensile strength, (UTS), yield strength, (YS) and elongation (EL%) amounts of welded joints were determined. The fracture morphologies of the tensile test samples were examined. It was observed that the rupture (except 850 degrees C) occurred from the weld metal. The highest formability was obtained in the samples tested at 850 degrees C. In addition, the existence range of dynamic strain aging (DSA) of the plasma arc welded samples was also investigated. The unsteady plastic flow between the yield and the ultimate tensile of the sample subjected to the tensile test at 250-550 degrees C are evaluated as signs of DSA.
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    Single and multi-objective optimization for cutting force and surface roughness in peripheral milling of Ti6Al4V using fixed and variable helix angle tools
    (Elsevier Sci Ltd, 2022) Sur, Gokhan; Motorcu, Ali Riza; Nohutcu, Samet
    Ti6Al4V titanium (Ti) alloy is a frequently used engineering material in industrial applications due to its superior properties. In this work, single-objective and multi-objective methods were used to optimize control factors (cutting speed, feed rate, and cutting tool helix angle) for minimal cutting force (Fc) and surface roughness (Ra) in peripheral milling of Ti6Al4V. Machinability experiments were performed using carbide end mill cutting tools with fixed and variable helix angles. Following the machinability tests performed using the L18 orthogonal array, the Taguchi technique was used as a single-objective optimization for each of the cutting forces and surface roughness. The Entropy-weighted TOPSIS approach was used in the second step to provide the optimum levels of the control factor, which minimizes both cutting force and surface roughness. The effects of control factors and interactions on thrust force and surface roughness were determined by Analysis of Variance (ANOVA) and contour plots. As a result of this study, it was determined that the helix angle of the cutting tool (84.91 % contribution) was the most effective parameter on the cutting force and the feed rate (22.71 % contribution) on the surface roughness. The cutting force and surface roughness values decreased in peripheral milling at high cutting speed, whereas the cutting forces increased, and the surface quality of the workpiece deteriorated in machining at high feed rates. The helix angle of the cutting tool has a favorable influence on the cutting force and surface roughness when milling with fixed helix tools. Cutting forces are minimized, and workpiece surface quality is improved using cutting tools with a larger helix angle. Variable helix tools with a large helix angle performed best in cutting force and surface roughness. Variable helix end mills with large helix angle (HA = 35 degrees/ 38 degrees), high cutting speed (Vc:108 m/min), and a low feed rate (f:0.04 mm/tooth) should be used in peripheral milling of Ti6Al4V Ti alloy to achieve both minimum cutting force and best surface quality. The mean cutting force and surface roughness values were obtained as 628.69 N and 0.363 mu m, respectively, in the peripheral milling process performed at different levels of control factors. The cutting force was reduced to 227.52 N (34.71 % improvement) and the surface roughness value to 0.237 mu m (276.32 % improvement) using the Entropyweighted TOPSIS multi-objective optimization approach.
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    Weldability of haynes 188 cobalt based superalloy and AISI 316L austenitic stainless steel
    (Gazi Univ, 2024) Nohutcu, Samet; Kacar, Ramazan; Ertek Emre, Hayriye
    Because of their high temperature and oxidation resistance, similar and dissimilar metal welding is needed in producing gas turbine and rocket engine parts, in which cobalt-based superalloys are also used. The fusion welding methods such as gas tungsten arc welding (TIG), Laser welding (LW), Electron beam welding (EBW), and Plasma arc welding (PAW) are widely used for dissimilar metals welding applications. Weld discontinuity such as solidification cracks and liquefaction cracks can occur in the weldment. One way to overcome these problems is to use a low heat input in the welding process. Force TIG welding machine, which has been developed in recent years, allows automatic welding with its integrated systems. Thus, it provides the opportunity to obtain the heat input homogeneously and to perform autogenous welding with its control of the welding parameters. The study aims to investigate the weldability of the Haynes 188 - AISI 316L couple, as supplied, automatically with the Force TIG welding machine, without opening the weld groove, and without using any filler metal (autogenously). For this purpose, the strength and hardness of the dissimilar welded sample were determined, and its microstructure was evaluated in detail.