Papers by Author: Bekir Sami Yilbas

Abstract: nanostructured coating materials become demanding, since it improves the mechanical properties, such as hardness, through grain refinement. One of the methods of producing nanostructured coatings is to use an arc spray coating process using nanostructured wires. Although the arc spraying process is well developed, the influence of nanoparticles on the fracture toughness of the coating surface has not been examined in details. Consequently, in the present study, arc spraying of nanostructured wires on carbon steel surface is carried out. The influence of coating thickness on the microstructure and fracture toughness of the coating is investigated. It is found that the self-annealing due to large coating thicknesses has a noticeable effect on the microstructure and fracture toughness.

Abstract: In the present study, two layer HVOF coatings of carbon steel is carried out. The microstructural changes in the coating layers and fracture toughness of the coated surface are examined. The micro indentation method is incorporated for the fracture toughness measurements, while optical and scanning electron microscopy are used for the microstructural analysis. It is found that the fracture toughness of the coating surface produced by the tungsten carbide blended powders is less than that of the coating produced by corrosion resistance powders. Key words: HVOF, Diamalloy 2002, Diamalloy 4010, Fracture toughness.

Abstract: In the present study laser nitriding of titanium alloy surface is carried out and fracture toughness of the resulting surface is measured using the micro-indentation method. The fracture toughness is then related to the microstructure of the laser treated surface. It is found that laser gas assisted nitriding lowers the fracture toughness of the surface due to the micro-stress formed at the surface region during the high cooling rates.

Abstract: There has been a vast interest in the general coupled field analysis of thermopiezoelectro-magnetic materials under which smart piezoelectric, thermopiezoelectric and magnetostrictive materials can be studied. The smart materials are often bonded as thin films on host structures for the purpose of sensing and/or actuation. It is well-known that the placement of sensors and actuators is important in order to obtain the appropriate sensor input and to provide the adequate actuation power. This study aims at modeling the important phenomenon of thermopiezoelectro-magnetism suitable for beam and/or plate type-host structures. The thermopiezoelectro-magnetic materials are modeled using the finite element method and the resulting equations are used for decision making on the best placement of the smart actuators on various host structures.

Abstract: Parameters on a CO₂ laser machine effects the variations on hole diameters. In this study, a CO₂ laser machine is used for drilling processes. Processing parameters are selected between 1.2-4 mm workpiece thickness, 2500-4000 W laser output power, -4 to 2 laser focus setting, 8-14 bar assisted gas pressure and 500-1200 Hz laser frequency. After the drilling process, with the diameter measurements, pictures were taken with an optic microscope then the effect of the processing parameters on hole diameter variations were investigated.

Abstract: Laser cutting of aluminum-silicon sheet is carried out. The influence of laser output power on the kerf width is examined in details. The lump parameter analysis is introduced to predict the kerf width size. The percentage of kerf width size variation due to different laser output power levels is formulated. It is found that the kerf width size predicted agrees well with the experimental data. The influence of laser output power on the kerf width size is more pronounced for power levels 350 W. Keywords: laser cutting, aluminum-silicon, kerf width

Abstract: Laser cutting of ceramics offers considerable advantages over the conventional cutting methods. Some of these advantages include precision of operation, high speed processing and low cost. In addition, cutting process is independent of the mechanical properties of the ceramic material such as hardness and fracture toughness. Alumina is one of the widely used ceramics in industry due to their resistance to harsh environments. Laser cutting of alumina requires high laser power due to high latent heat of melting and elevated melting temperature. In efficient laser cutting process, the low laser power is required. Consequently, for optimum laser cutting conditions of alumina tiles, the thermal efficiency of the cutting process may not be a maximum. The optimum laser cutting parameters provide crack free and parallel edge cuts. Therefore, investigation into laser cutting of alumina ties and the efficiency analysis becomes essential. In the present study, laser cutting of alumina tiles with 3 mm thickness is carried out and thermodynamic analysis associated with the efficiency analysis is introduced. The lump parameter method is incorporated in the thermodynamic analysis. The optimum cutting conditions are then related to the first and second law efficiencies.

Abstract: Flexural motion of a cantilever bar can be used to monitor the taper angle measurements of the bar without incorporating any geometric measurement. This provides non-destructive taper measurement with fast processing in manufacturing line. The laser ablation of a free end of the cantilever bar enables to generate a recoil pressure at the irradiated surface while resulting in external forcing function for the initiation of the flexural motion. Consequently, in the present study, laser induced ablation of free end tapered cantilever bar and resulting displacement and frequency of the flexural motion is examined. The study is extended to include influence of positive and negative bar taper angles on the flexural characteristics. In addition, deviation of displacement and frequency of the flexural motion due to tapered bar from the parallel bar is examined. It is found that the flexural characteristics of the tapered bar enable to determine taper angle of the bar with high accuracy.

Abstract: Laser gas assisted nitriding and TiN coating of Ti-6Al-4V alloy is carried out. The microstructures developed in the laser irradiated region are examined using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD). The laser nitrided surfaces are coated with TiN using the PVD facility to enhance the tribological properties. The heating model incorporating the lump parameter analysis is used to predict the melt layer thickness during the laser treatment process. It is found that laser nitrided surfaces posses δ-TiN+ε-TiN phases and TiN coating of laser nitrided surfaces improves the microhardness at the surface.

Abstract: HVOF coating finds application in industry to protect the surface from the harsh environments such as high temperature, corrosion, and abrasion. In the present study, HVOF coating consisting of Inconel 625 powders blended with WC particles and sprayed on to 304 steel is carried out. The mechanical properties, such as elastic modules and fracture toughness, of the resulting coating are determined using the indentation tests. The influence of the mass fraction of WC on the fracture toughness and elastic modulus of the coating are also examined. It is found that addition of WC particles in Inconel 625 powder enhances the fracture toughness of the resulting surface. This is attributed to increased elastic modules and hardness.