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Öğe Design and construction of a thixo forming unit and production of Al-Si alloys(Elsevier Ltd, 2008) Özyürek, D.; Aktar, N.; Aztekin, H.In this study, a laboratory scale thixo forming unit was designed and constructed in order to carry out semi-solid process. Al-Si alloys were produced from the powders ground mechanically from ingots containing 5% Si on this unit. The powders were then cold pressed in order to produce cylindrical parts and thixo forming was applied to these cylindrical parts. Semi-solid processes were carried out at three different temperatures (580, 590 and 600 °C) for 1 and 2 min under 100 kN squeezing force. The results showed that spheroidisation tendency of the grains increased with increasing temperature. The grain size was also found to increase with increasing temperature. In addition, the period pressure applied was found to be an effective parameter in microstructural alterations. Thixo forming resulted in better microstructure with increasing pressure application period. © 2007 Elsevier Ltd. All rights reserved.Öğe Effect of ageing treatment on wear properties and electrical conductivity of Cu-Cr-Zr alloy(Indian Academy of Sciences, 2011) Saglam, I.; Özyürek, D.; Çetinkaya, K.In this study, the effect of ageing processes on the wear behaviour and electrical conductivity was investigated. Prior to solid solution heat treatment at 920°C and ageing at 470°C, 500°C and 530°C for 1 h, 2 h and 3 h, respectively, the prepared samples were homogenized at 920°C for 1 h. After the ageing processes, all samples were characterized in terms of electrical conductivity, scanning electron microscope (with energy dispersive X-ray spectrum (EDS)) and hardness (HV5). In wear tests, pin-on-disc type standard wearing unit was used. As a result, starting from 1 h aged specimens, orderly increase of electrical conductivity was defined. From EDS analyses it was observed that Cr rate increases as precipitates grow. With increase of Cr rate there was also a defined rise of electrical conductivity. From the wear tests, it was observed that the least wear loss was in Cu-Cr-Zr alloy aged at 500°C for 2 h and the most wear loss was in specimens aged at 530°C for 2 h. Furthermore, it was observed that the friction coefficient values resulting from wear rate were overlapped with hardness results and there is a decrease tendency of friction coefficient as wear distance increases. © Indian Academy of Sciences.Öğe The effect of diffuser and non-diffuser runner systems on the mechanical properties of A356 alloy(2013) Tunçay, T.; Tekeli, S.; Özyürek, D.In this study, runner systems which have equal cross-sectional area and different forms of cross-sectional diffuser and non-diffuser runner system's effects on mechanical properties of A356 aluminium alloy, depending on the movement of the liquid metal, were investigated. The aim of the study is to reduce casting defects due to inclusion caused by runner systems such as gas or shrinkage porosity. Therefore, the horizontal runners have been extended by using diffusers having cross-sectional area equivalent with a different cross-sectional shape (square, rectangular and circular outlet). Tensile test specimens were prepared from the plates of casting A356 alloy and tensile tests were performed. Obtained results were examined Weibull statistical analysis. As a result, one hand, it is observed that the turbulence effect on liquid metal has been decreased the highest tensile results and Weibull modulus has been appeared on the samples which have horizontal runners produced by using a square section diffuser in designed horizontal diffuser. On the other hand, according to Scanning Electron Microscope (SEM) analysis on fracture surfaces, fracture has been observed in the area of intermetallic secondary phases (oxides etc.) and in area with gas or shrinkage porosity.Öğe Effect of repeated quenching heat treatment on microstructure and dry sliding wear behavior of low carbon PM steel(Trans Tech Publications Ltd, 2007) Güral, A.; Tekeli, S.; Özyürek, D.; Gürü, M.The effect of repeated quenching heat treatment on microstructure and dry sliding wear behavior of low carbon PM steel was investigated. For this purpose, atomized iron powder was mixed with 0.3 % graphite and 1 % Ni powders. The mixed powders were cold pressed and sintered at 1200°C for 30 min under pure Ar gas atmosphere. Some of the sintered specimens were intercritically annealed at 760°C and quenched in water (single quenching). The other sintered specimens were first fully austenized at 890°C and water quenched. These specimens were then intercritically annealed at 760°C and re-quenched in water. The martensite volume fraction in the double quenched specimens was higher than that of the single quenched specimen. Wear tests were carried out on the single and double quenched specimens under dry sliding wear condition using a pin-on-disk type machine at constant load and speed. The experimental results showed that the wear coefficient effectively decreased in the double quenched specimen.Öğe The effect of semi-solid processing parameters on microstructure in Al-7wt.%Si alloy(2011) Özyürek, D.In the present work, the effect of temperature and pressing time on the microstructure of a Al-7wt.%Si alloy was studied. Al-Si specimens were prepared by pre-shaping powder obtained with mechanical processing from Al-7wt.%Si alloy produced with ingot metallurgy under 800 MPa pressure. The prepared specimens were subjected to semi-solid processing at 100 MPa pressure and at different temperatures (580, 590 and 600°C) for various pressing durations (1 and 2 min). The specimens produced with semi-solid processing were analysed with optical and scanning electron microscope (SEM) and energy dispersive spectroscope (EDS). The results indicated that grain growth increased with the increasing temperature. Fine global grains were obtained at 590°C for 2-min pressing time. © 2011 Academic Journals.Öğe An effect of weld current and weld atmosphere on the resistance spot weldability of 304L austenitic stainless steel(Elsevier Ltd, 2008) Özyürek, D.Because of their superior mechanical and corrosion properties, 304L austenitic stainless steel is used widely in industry. Resistance spot weld is most widely used as a joining process for sheet materials. The influence of the primary welding parameters affecting the heat input such as; weld peak current, on the weld quality such as; surface appearances, weld nugget size, weld penetration, weld internal discontinuities, strength and ductility was determined for 304L resistance spot welded materials. An effect of different weld atmosphere on the resistance spot weld quality of 304L stainless steel was also investigated in detail. Therefore, microstructure, microhardness, and tensile shear load bearing capacity of welded materials were determined for all conditions. An optimum weld quality was obtained by using 9 kA peak weld current in nitrogen atmosphere. © 2007 Elsevier Ltd. All rights reserved.Öğe The effects of retrogression parameters in RRA treatment on tensile strength of 7075 aluminium alloys(2012) Özyürek, D.; Yilmaz, R.; Kibar, E.In this study retrogression and re-aging (RRA) treatments has been applied on 7075 aluminium alloys after T6 treatment. The alloys were retrogressed at various temperatures and times in the retrogression stage of RRA treatment. The effects of the parameters mentioned above on tensile strengths and % elongations were investigated. Besides fracture surfaces were examined by scanning electron microscopy (SEM) for determination of the effects of retrogression temperatures and times on fracture mechanism. The alloy retrogressed at 220°C for 60 minutes showed the highest tensile strength values, on the other hand, the lowest elongation values. Strong relationship has been seen between hardness and tensile strength; however, no effective relationship for elongation was available. Retrogression temperatures and times affects on microstructure of the alloys, therefore, it has also great influence on the hardness and tensile strength, elongation and fracture mechanism.Öğe The effects of retrogression parameters on hardness and wear behaviours of 7075 aluminium alloys(2012) Yilmaz, R.; Özyürek, D.; Kibar, E.In this study, retrogression and re-aging (RRA) treatments on 7075 aluminium alloys has been carried out. 7075 aluminium alloys that were already T6 temper applied were retrogressed at the range of 180-260°C for durations between 15-75 minutes and later re-aged in T6 aging condition of 120°C for 24 hours. The effects of retrogression temperature and times on the hardness and wear properties were investigated. Wear tests were carried out by using machine that is for pin on disc technique. Worn surfaces of the samples after tests were also examined by scanning electron microscopy (SEM) for better understanding of wear mechanism. Experimental studies show that retrogression treatments parameters such as temperatures and times have great effects on wear resistance of 7075 aluminium alloys. Close relationship between the hardness and wear resistance values were seen in those alloys. Variations in hardness values were all depend on retrogression temperatures and times. The alloy retrogressed at 220°C for 60 minutes showed the highest hardness values and wear resistance.Öğe The effects on microstructure and mechanical properties of filtration in Al-Si-Mg alloys(Gazi Universitesi Muhendislik-Mimarlik, 2014) Tunçay, T.; Özyürek, D.In this study, on microstructure and mechanical properties of Al-Si-Mg alloy was investigated effects of ceramic foam filters having different density of cell. Ceramic foam filters is used to minimizing bifilm, inclusion, sand wash away from of the mould wall, gas or shrinkage porosity such as casting defects. As a result of the tensile test results were evaluated with Weibull distribution and the quality index. Mechanical properties and castability were examined by ceramic foam filters using in Al-Si-Mg alloy. Experimental studies were used on horizontal runner a ceramic foam filter having different density of cell (20 and 30 ppi). According to Tensile test results, due to the increased density of ceramic foam filter cell gap could be controlled flow behaviour of the liquid metal in the running system. Improving mechanical properties of Al-Si-Mg alloy has been demonstrated by increasing Weibull modulus and quality index. In addition to increasing the amount of cell gap in ceramic foam filter, quality index is also increasing. Tensile fracture surfaces of samples was investigated Scanning Electron Microscope (SEM), to fracture occurs secondary intermetallic phases, inclusion and bifilm defect concentration regions.Öğe An investigation of sintering characteristics of reinforced with 15% TiB2 Titanium Composites(2010) Özyürek, D.; Tuncay, T.In this study, titanium matrix composite materials reinforced with 15% TiB2 were produced using mechanical alloying/milling (MA/MM) method. MA process carried out used an attritor. Titanium-composite powders were cold pressed at 630 MPa and then sintered at different temperatures for different time. Produced titanium-matrix composite (TMC) samples have been characterized by optical and scanning electron microscopy (SEM),X-ray diffraction (XRD), hardness, densification after sintering. The results showed that the amount of the reaction synthesized TiB increased with sintering temperature. However, transformation of the TiB2 ceramic particulates in the titanium matrix was not completed. The reinforced with 15 vol.% TiB2 titanium composite sintered at 1200°C has maximum values of density and hardness. © 2010.Öğe An investigation on wear resistance of SiCp-reinforced aluminium composites produced by mechanical alloying method(2010) Özyürek, D.; Tekeli, S.In this study, Al-SiCp composites were produced by a mechanical alloying method using SiCp powders up to 20 voI% as a reinforcement. The produced compositions were cold pressed at 630 MPa with a single action and sintered at 560°C for minutes under Ar gas atmosphere. The experimental result showed that the reinforcement phase SiCp was homogeneously dispersed in the A1 matrix and the interface between A1 matrix and SiC p was well formed. Wear tests were carried out under dry sliding conditions using a pin-on-disk type testing machine at three different loads of ION, 25N and 50N and at a sliding speed of 2.08 m/s. Total sliding distance was selected as 400, 800, 1200 and 1600 m. After wear tests, the wear amount was determined by measuring the weight and dimension of specimens. It was seen that the wear rate increased with increasing the applied load from 10N to 50N, sliding distance and decreased with increasing SiCp amount. Microstructures and worn surface of Al-SiCp composites were investigated by SEM.
