Browsing by Author "Çulha, O"

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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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    Aluminum Alloy Development for Wheel Production by Low Pressure Die Casting with New Generation Computational Materials Engineering Approaches
    Yagci, T; Cöcen, Ü; Çulha, O
    Computational Materials Engineering (CME) is a high technological approach used to design and develop new materials including the physical, thermal and mechanical properties by combining materials models at multiple techniques. With the recent advances in technology, the importance of microstructural design in CME environments and the contribution that such an approach can make in the estimation of material properties in simulations are frequently discussed in scientific, academic, and industrial platforms. Determination of the raw material characteristics that can be modeled in a virtual environment at an atomic scale by means of simulation programs plays a big role in combining experimental and virtual worlds and creating digital twins of the production chain and the products. In this study, a new generation, alternative and effective approach that could be used to the development of Al-Si based wheel casting alloys is proposed. This approach is based on the procedure of optimizing the physical and thermodynamic alloy properties developed in a computer environment with the CME technique before the casting phase. This article demonstrates the applicability of this approach in alloy development studies to produce Al-Si alloy wheels using the low pressure die casting (LPDC) method. With this study, an alternative and economical way is presented to the alloy development studies by trial and error in the aluminum casting industry. In other respects, since the study is directly related to the automotive industry, the reduction in fuel consumption in vehicles is an expected effect, as the new alloy aims to reduce the weight of the wheels. In addition to conserving energy, reducing carbon emissions also highlights the environmental aspects of this study.
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    Investigation of metallurgical properties of Al-Si-Mg casting alloys with integrated computational materials engineering for wheel production
    Yagci, T; Cöcen, Ü; Çulha, O; Armakan, E
    In this study, integrated computational materials engineering, which is one of the new generation approaches in materials science, was used in the production of aluminum alloy wheels by low pressure die casting method. In casting alloys, the efficiency of grain refinement provided by master alloys added to the melt decreases with increasing silicon content of the alloy. In this context, as-cast properties of silicon reduced (Si: 5.0 wt.%) alloys with different Mg ratios (Mg: 3.0, 5.0, 7.0 wt.%) are discussed using integrated computational materials engineering approaches. It has been evaluated whether the examined alloys can be an alternative to the AlSi7Mg0.3 alloy, which is currently used traditionally in the production of aluminum-based wheels, with their microstructural and mechanical properties. The study consists of three stages which are computer-aided production, pilot production, testing and characterization studies. In computer-aided production, original sub-eutectic compositions were determined in types and amounts of alloying elements, alloy designs were realized and a database was created with a computational materials engineering software. Then, low pressure die casting analysis were performed in a virtual environment by transferring these data directly to the casting simulation software. Thus, the microstructural and mechanical properties of the wheel were obtained computationally on the basis of the varying alloy composition. In the second stage, the virtually designed alloy compositions were prepared and sample wheels were manufactured by the low pressure die casting method on an industrial scale. In the testing and characterization phase, spectral analyses, macro and microstructural examinations, hardness measurements and tensile tests were carried out. As a result of this study, it was determined that the studied alloys could be used in the production of wheels by the low pressure die casting method considering the metallurgical properties expected from the wheel. In addition, it is thought that the mathematical design of the material with integrated computational materials engineering approaches before casting simulations will play an active role in the competitiveness and sustainability of the aluminum industry in technological conditions.
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    Microshear Bond Strength and Finite Element Analysis of Resin Composite Adhesion to Press-on-Metal Ceramic for Repair Actions after Various Conditioning Methods
    Kanat, B; Çömlekoglu, ME; Çömlekoglu, MD; Çulha, O; Özcan, M; Güngör, MA
    Purpose: This study evaluated the repair bond strength of differently surface-conditioned press-on-metal ceramic to repair composites and determined the location of the accumulated stresses by finite element analysis. Materials and Methods: Press-on-metal ceramic disks (IPS InLine PoM, lvoclar Vivadent) (N = 45, diameter: 3 mm, height: 2 mm) were randomly divided into 3 groups (n = 15 per group) and conditioned with one of the following methods: 9.5% hydrofluoric acid (HF) (Porcelain etch), tribochemical silica coating (TS) (CoJet), and an unconditioned group acted as the control (C). Each group was divided into three subgroups depending on the repair composite resins: a) Arabesk Top (V, a microhybrid; VOCO), b) Filtek Z250 (F, a hybrid;3M ESPE); c) Tetric Evo-Ceram (T, a nanohybrid; lvoclar Vivadent) (n = 5 per subgroup). Repair composites disks (diameter: 1 mm, height: 1 mm) were photopolymerized on each ceramic block. Microshear bond strength (MSB) tests were performed (1 mm/min) and the obtained data were statistically analyzed using 2-way ANOVA and Tukey's post-hoc test (alpha = 0.05). Failure types were analyzed under SEM. Vickers indentation hardness, Young's modulus, and finite element analysis (FEA) were performed complementary to MSB tests to determine stress accumulation areas. Results: MSB results were significantly affected by the surface conditioning methods (p = 0.0001), whereas the repair composite types did not show a significant effect (p = 0.108). The interaction terms between the repair composite and surface conditioning method were also statistically significant (p = 0.0001). The lowest MSB values (MPa +/- SD) were obtained in the control group (V = 4 +/- 0.8; F = 3.9 +/- 0.7; T = 4.1 +/- 0.7) (p < 0.05). While the group treated with T composite resulted in significantly lower MSB values for the HF group (T = 4.1 +/- 0.8) compared to those of other composites (V = 8.1 +/- 2.6; F = 7.6 +/- 2.2) (p < 0.05), there were no significant differences when TS was used as a conditioning method (V = 5 +/- 1.7; F = 4.7 +/- 1; T = 6.2 +/- 0.8) (p > 0.05). The control group presented exclusively adhesive failures. Cohesive failures in composite followed by mixed failure types were more common in HF and TS conditioned groups. Elasticity modulus of the composites were 22.9, 12.09, and 10.41 GPa for F, T, and V, respectively. Vickers hardness of the composites were 223, 232, and 375 HV for V, T, and F, respectively. Von Mises stresses in the FEA analysis for the V and T composites spread over a large area due to the low elastic modulus of the composite, whereas the F composite material accumulated more stresses at the bonded interface. Conclusion: Press-on-metal ceramic could best be repaired using tribochemical silica coating followed by silanization, regardless of the repair composite type in combination with their corresponding adhesive resins, providing that no cohesive ceramic failure was observed.
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    Effect of Chip Amount on Microstructural and Mechanical Properties of A356 Aluminum Casting Alloy
    Kaya, AY; Ozaydin, Ö; Yagci, T; Korkmaz, A; Armakan, E; Çulha, O
    Aluminum casting alloys are widely used in especially automotive, aerospace, and other industrial applications due to providing desired mechanical characteristics and their high specific strength properties. Along with the increase of application areas, the importance of recycling in aluminum alloys is also increasing. The amount of energy required for producing primary ingots is about ten times the amount of energy required for the production of recycled ingots. The large energy savings achieved by using the recycled ingots results in a significant reduction in the amount of greenhouse gas released to nature compared to primary ingot production. Production can be made by adding a certain amount of recycled ingot to the primary ingot so that the desired mechanical properties remain within the boundary conditions. In this study, by using the A356 alloy and chips with five different quantities (100% primary ingots, 30% recycled ingots + 70% primary ingots, 50% recycled ingots + 50% primary ingots, 70% recycled ingots + 30% primary ingots, 100% recycled ingots), the effect on mechanical properties has been examined and the maximum amount of chips that can be used in production has been determined. T6 heat treatment was applied to the samples obtained by the gravity casting method and the mechanical properties were compared depending on the amount of chips. Besides, microstructural examinations were carried out with optical microscopy techniques. As a result, it has been observed that while producing from primary ingots, adding 30% recycled ingot to the alloy composition improves the mechanical properties of the alloy such as yield strength and tensile strength to a certain extent. However, generally a downward pattern was observed with increasing recycled ingot amount.
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    Failure analysis of cardan shaft?s flange yoke fracture occurred during torsional fatigue test
    Akkas, O; Isik, E; Çulha, O
    In this study, the analysis and characterization of the failure that occurred during the torsional fatigue test of the flange yoke unit part of the cardan shaft used in heavy commercial vehicles were carried out. In order to determine the root cause of the failure according to the production, structure, property and performance relationship in materials science, chemical analysis of the steel raw material used in the production of flange yoke, hardness measurement, grain size analysis, decarburization measurement, inclusion analysis and raw material characterization by scanning electron microscopy (SEM) were carried out. As a result of the studies, it was determined that the values obtained for 41Cr4 steel raw material were conformant according to the TS EN ISO 683-2 standard, but in the SEM examination and EDS analysis, elongated MnS inclusions in the microstructure and cracks were detected at the interface of the metal matrix and MnS inclusions. In this context, as a result of the characterization of the flange yoke unit part, it was determined that the MnS ratio was 2-4 mu m x 898-1.181 mu m according to the ASTM E45 standard, at the same level as the raw material and equivalent to the A-type value of 3.0-3.5. It was determined that MnS inclusion formations in the subsurface region formed crack propagation zones.
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    Investigation of microstructural and mechanical properties of hot forged 31Mn4 dual phase steel with computer-aided simulations
    Kilerci, I; Çulha, O; Yagci, T
    In this study, it is aimed to realise the hot forging process design and prototype production of the steel yielding support connection clamp, the critical safety element of underground mining ground support systems, by using 31Mn4 dual phase steel. In this context, at the end of the forging and deburring processes, air cooling was designed, simulated and cooling rates were obtained. The cooling curve was integrated into the continuous cooling transformation diagram and phase formation was predicted. The distribution of pearlitic, ferritic, bainitic structures and formation rates, hardness distributions, strength data were also obtained. The hardness and tensile strength calculations, microstructural examinations and SEM analysis were carried out for validation. It has been determined that pearlitic-ferritic microstructures are formed in regions where the cooling rate is slow. In thinner sections where the cooling rate is around 1.2 degrees C/sec, ferritic structures become smaller and bainite phase was observed. As a result, the average tensile strength in 1st and 2nd regions were recorded as 684.5, 690 MPa in simulation, while these values were recorded as 688.45 and 694.15 MPa in tensile tests. It has been determined that this result corresponds to the 99.4% accuracy rate of the prototype obtained by simulation-supported and real productions.