Browsing by Author "Kahraman, AD"

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    Effect of the substrate surface and coating powder hardness on the formation of a cold sprayed composite layer
    Kahraman, AD; Kahraman, F; Culha, O; Yagci, T
    In this study, the powder hardness and substrate surface hardness on the coating formation in the cold spray process was investigated. The AA6082 aluminum alloy hardened by the shot-peening process was used as the base material. Two different metallic powders and a ceramic particle powder were used as powder materials with different hardness. Thus, the powder particles from different materials were sprayed onto the surface under the same spraying process conditions. In order to obtain a workpiece surface with different hardness values, shot-peening treatment was applied to the substrate material at different treatment times. According to the microstructural examination, the harder metallic coating powder has accumulated more and the lower hardness metallic coating powder has accumulated less when the substrate material hardness increases. Al2O3 particles in the layer formed were distributed close to homogeneous. Furthermore, the size of Al2O3 particles near the contact surface has become much finer especially in the long-term shot peened samples due to their higher hardness. From the indentation experiments, the elastic behavior and recovery amount of the composite coating layer increased due to the increase of the substrate surface hardness, and the stress distributions were performed less after the load was removed.
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    Effect of substrate surface preparation on cold sprayed Al-Zn-Al2O3 composite coating properties
    Kahraman, AD; Kahraman, F; Çulha, O
    In this study, a powder mixture containing Al-Zn and Al2O3 particles on AA6083 aluminum alloy was coated by the cold spray method. In order to investigate the effect of the substrate surface properties on the composite coating structure obtained by the cold spray process, the substrate surface was prepared by grit blasting and shot peening. It was found that Al and Zn particles in the coatings of all samples flattened near the substrate surface regions due to the deformation effect. But, Zn particles, because of their low hardness in contrast to the shot peened surface fragmented near the material surface regions through the impact. Similarly, the size of the Al2O3 particles decreased incrementally from the coated surface to the substrate. Particularly in the shot peened samples, the size of the Al2O3 particles decreased by striking the hard substrate surface and fracturing more. Moreover, the amount of Al2O3 particles in the substrate surface of all samples was lower as the particles rebounded. It was also found that pore formation in the coating layer of all samples was very limited. Pore formation in the shot peened samples was by contrast lower than in the grit blasted samples. In addition, a very low oxide layer was found between the particles in the upper region of both the grit blasted and shot peened samples' coating layer.
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    Wear behavior of ultrasonic impact treated S235JR steel
    Kahraman, F; Kahraman, H; Yolcu, C; Kahraman, AD
    In this study, wear behavior of ultrasonic impact treated S235JR steel was investigated. Samples were peened for different times (5, 15 and 30 minutes) by ultrasonic impact treatment. It was found that triple layers (nanocrystallization, plastic deformation and residual stress) formed on the treated surface. The thickness of the layers treated samples increased with increasing treatment time. It was also determined that surface roughness of treated samples decreased with increasing treatment time. All treated samples have significant surface hardness. Nanocrystallization layer on top of the treated surface has the highest hardness values in comparison with the hardness of the other layers. The treatment increases not only surface hardness, but also wear resistance of all treated samples. The maximum wear resistance was observed for the 30 min treated samples due to the high hardness and thickness of the nanocrystallization layers.
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    LOW-TEMPERATURE NITRIDING BEHAVIOR OF COMPRESSIVE DEFORMED AISI 316Ti AUSTENITIC STAINLESS STEELS
    Kahraman, F; Gençer, GM; Kahraman, AD; Yolcu, C; Kahraman, H
    The effects of compressive cold deformation under the quasi-static loads on the nitride formation, nitride layer growth and surface hardness properties were researched in this study. Martensite structure did not form in AISI 316Ti stainless steel as a result of quasi-static deformation. Diffusion layer did not form in all nitrided samples. Both the deformed and undeformed samples have only compound layer on the surfaces at the low-temperature nitriding conditions (400 degrees C, 7h). According to the X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and electron probe microanalysis (EPMA) results, S-phase and chromium nitride (CrN) were formed in the compound layers of the deformed samples. However, CrN did not form in the compound layer of the undeformed sample. The optical microscope (OM) results showed that the compressive cold deformation increased the nitrogen diffusion rate and led to thicker nitrided layer than the undeformed sample under the same plasma-nitriding conditions. All nitrided layers presented higher microhardness values ( similar to 1200 HV) when compared with the untreated sample hardness. It was also verified that the deformation amount did not affect significantly the nitrided layer hardness.
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    Effect of the loads on the wear resistance of the Cr2O3-TiO2 coated AA 6082 alloy under unlubricated conditions
    Kahraman, AD; Yolcu, C; Kahraman, F
    In the present study, AA 6082 was coated with Cr2O3-TiO2 powders by using flame spraying technique and wear behavior determined in a block on disc configuration under dry sliding conditions at different loads (20, 30 and 40 N). The selected initial load was kept constant for the rest of the sliding. The optical microscope and SEM were used to examine for both of the microstructure of the layers and the wear tracks. The phases in the coating were analyzed with x-ray diffraction and the hardness value of the layer was also obtained with a Vickers microhardness tester. The average hardness values of the coating was 925 HV. Due to the higher hardness values of the coating, the weight loss is very low especially lower loads like as 20 N. Increasing the load from 20 N to 30 N, weight loss is increased and the abrasive wear mechanism becomes effective. Surprisingly after the 40 N loads mass loss was decrease due to the plastic deformation. This observation shows that the plastic deformation of the coating due to a critical load reduces abrasive wear by preventing the ceramic particles from breaking away.
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    Surface nanocrystallization of SAE 1008 steel by single and multipass thread rolling process
    Kahraman, F; Gençer, GM; Kahraman, AD
    In this study, the layers are investigated which are obtained on the surfaces of the single and multi-pass thread rolled screws. It is observed that deformation is more homogenous depending on the pass number and thread laps failure do not form on the 3-passed threads. A nanocrystallized layer that thicknesses and size depends on deformation amount is determined on the surfaces of the thread rolled SAE 1008 steel. While top of this layer has finer grain sizes, depending on the deformation amount grains coursed and oriented by the deformation direction. The finer grains are observed in the single pass thread rolled sample as size of 136 nm at the 15 mm inside from the surface of the root of the thread. Compared with bulk material, significant hardness increase is observed on the surfaces of the all thread rolled samples. The effect of pass number on the hardness is very low, increases of the pass number, hardness increases slightly. But thicknesses of layer are enhanced to increase the pass number.