Browsing by Author "Davut, Kemal"
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Article A comprehensive characterization of the effect of spatter powder on IN939 parts fabricated by laser powder bed fusion(Materials and Design, 2023-10-15) Doğu, Merve Nur; Özer, Seren; Mussatto, Andre; Yalçın, Mustafa Alp; Davut, Kemal; Obeidi, Muhannad Ahmed; Kumar, Ajay; Hudson, Sarah; O'Neill, Darragh; O'Connor, Robert; Gu, Hengfeng; Brabazon, DermotThis study is focused on a comprehensive characterization of virgin and spatter IN939 powders and the effects of a certain amount of spatter powder on the part quality of IN939 fabricated by the L-PBF process. A brown tint coloration formed Al2O3 oxide, pores, a 124.4% increase in the average particle size, a 10.2% decrease in the powder circularity, and a 7.5% decrease in the powder aspect ratio were observed in the spatter powder. Additionally, higher average grain size and lower nanohardness were obtained for the spatter powder. In order to understand the effect of a certain amount of spatter powder on the part quality, 10 wt.% spatter powder was mixed with the virgin powder. This addition was found to decrease the flowability of the powder. Moreover, this addition decreased relative density by around 0.3% and increased surface roughness by around 80.8% in the fabricated samples (termed as V and SV). On the other hand, there was no considerable microstructural, texture, microhardness, and nanohardness difference between V and SV samples, although the spatter powder addition caused a 30.2% increase in the average grain size of SV. The overall texture for both V and SV samples exhibit (001)//BD.Item COATING OF TITANIUM (TiAl6V4) ALLOY BY ELECTROSPUN POLY (ε CAPROLACTONE) (PCL)(2022-01-24) Algelal, Hasan Mohammed Ali Abdullah; Şaşmazel, Hilal; Davut, KemalThe goal of this study is to fabricate a biomaterial implant composed of Titanium alloy TiAl6V4 coated with poly (ε caprolactone) (PCL) using electrospinning mechanism. Moreover, the effect of surface-treatment for the titanium alloy on the coating is also studied, whereas coating thickness is determined using a micrometer. Characterization of specimens’ morphology is executed using Scanning Electron Microscopy (SEM). The wettability of the surface is performed using the contact angle (CA) and the optical micrographs of the Titanium alloy microstructure are carried out using optical microscopy. The detection of composition and elements in the substrates Energy dispersive spectroscopy (EDS) is also performed. For more quantitative analysis of surface morphology and to evaluate the roughness at the surface of implants, AFM and roughness measurements are used. To determine the adhesion of the PCL to the TiAl6V4 , adhesion test is conducted. EDS shows that the obtained specimens are free from any other elements after being treated. The optical microscopy is used to observe the changes in the microstructure through optical micrographs, after pre-treating the alloys. Through SEM, the change in the surface morphology after grinding, polishing, and etching is also recorded. The roughness measurements indicate the mean values of roughness quantitatively, which are gradients from 0.005 micron in 3 micron polishing TiAl6V4 sample to 0.56 micron in the 120 grinding TiAl6V4 sample. After measurement, the thickness of the coating is determined as 0.01 mm. The contact angle measurement shows that the best sample in hydrophilicity is the etched alloy after coating it with PCL, which is 58.63 °. The adhesion test also proves that the best two samples for coating adhesion are the etched and 1200 grinding: more than 99% of the coating do not stick out of the coating after the removal of the tape.Article Effect of solution heat treatment on the microstructure and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam(Journal of Materials Research and Technology, 2023-05-19) Doğu, Merve Nur; Özer, Seren; Yalçın, Mustafa Alp; Davut, Kemal; Bilgin, Mert Güney; Obeidi, Muhannad Ahmed; Brodin, Håkan; Gu, Hengfeng; Brabazon, DermotThe effect of various solution heat treatment temperatures (i.e., 1120, 1160, 1200 and 1240 °C) on the microstructure, grain morphology and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam (PBF-LB) was investigated. Microstructural analyses showed that the high-temperature gradient and rapid solidification of the PBF-LB processing caused different resulting microstructures compared to conventionally produced counterparts. The melt pool morphologies and laser scanning paths were examined in the as-fabricated samples in the XZ- and XY-planes, respectively. After the application of solution heat treatment at 1120 °C, the as-fabricated PBF-LB initial microstructure was still apparent. For solution heat treatments of 1200 °C and above, the melt pool and scanning path morphologies disappeared and converted into a mixture of columnar grains in the XZ-plane and equiaxed grains in the XY-plane. On the other hand, large equiaxed grains were observed when the samples were solutionized at 1240 °C. Additionally, γ' phase precipitated within the matrix after all solution heat treatment conditions, which led to increase in the microhardness values. According to electron backscatter diffraction (EBSD) analyses, both as-fabricated and solution heat-treated samples had intense texture with {001} plane normal parallel to the building direction. The first recrystallized grains began to appear when the samples were subjected to the solution heat treatment at 1160 °C and the fraction of the recrystallized grains increased with increasing temperature, as supported by kernel average misorientation (KAM) and grain spread orientation (GOS) analyses.Item INVESTIGATION OF CARBURIZED QUENCHING PROCESS OF SHAFTS USING COMPUTER SIMULATIONS(2022-02-28) Yazır Terzi, Büşra; Şimşir, Caner; Davut, KemalCarburizing and subsequent quenching heat treatments are commonly used industrial processes to improve the properties and performance of steel shafts. Variation of the case depth related to these treatments are the most frequent reason for product rejection. Therefore, the major aim of this study is to identify the suitable control parameters for subsequent optimization of carburized quenching DIN 16MnCr5 (1.7131) steel shafts. Another aim is to estimate inevitable experimental variations and determine the accuracy of computer simulations in presence of experimental uncertainties in the process parameters. This study is performed on both experiments and simulations. Experimental study is used to verify the simulations. In order to determine the control parameters of the process, a local sensitivity analysis is conducted using computer simulations in which each control parameter is perturbed around its reference state and the associated dimensional changes were correlated to those perturbations by a dimensionless sensitivity index. Additionally, for the purpose of identifying the effective parameters, a virtual design of experiments is performed on finite element method software SYSWELD®.Item INVESTIGATION OF CARBURIZED QUENCHING PROCESS OF SHAFTS USING DESIGN OF EXPERIMENTS(2022-03-01) Yılmaz, Hasan; Şimşir, Caner; Davut, KemalCarburizing is a widely-used thermochemical heat treatment process in shaft manufacturing. Insufficient case-depth, inadequate surface/core hardness, and distortion are the common reasons for scrapped products after carburizing. Thus, understanding, control and optimization of the process parameters in carburizing is crucial to avoid losses. In this study, carburizing parameters were investigated and optimized by Design of Experiments (DoE) using the Taguchi method. The target of the optimization is the minimization of the variability in the production due to the use of different furnaces for the process. Experiments were carried out on as-turned shafts (plain and stepped) made of two different grades of steel (16MnCr5 and 20NiCrMo2-2) using two different industrial gas carburizing furnaces. After the carburizing experiments, dimensional changes were measured using a Coordinate Measurement Machine (CMM), while carbon and hardness profiles were determined using Optical Emission Spectroscopy (OES) and Vickers indentation techniques. The results indicated that carbon potential of the carburizing gas is the most important parameter associated both with the variability and the mean effect for the case-depth and the diameter change.Item MATERIAL CHARACTERIZATION STUDIES FOR CARBURIZED QUENCHING PROCESS DESIGN AND QUALITY ASSESSMENT(2022-02-25) Yıldız, Seçil; Davut, Kemal; Şimşir, CanerCarburized quenching is a surface hardening process applied to low and medium carbon steel parts to improve fatigue and wear resistance. Distortion, cracking, inadequate case depth and surface/core hardness are the most frequently encountered problems during this process. During the last two decades, computer simulations became popular to predict and avoid those problems instead of conventional analytical and trial-and-error approaches. Aside from troubleshooting, heat treatment simulations enable determination of optimal process parameters yielding the desired microstructure and residual stress distribution to improve the performance of the part. Primary aim of this study is to complement computational materials engineering methods to develop a material data set for the carburized quenching simulation of DIN 22NiCrMo2-2 (SAE 8620H) steel. For that purpose, first, the raw material is characterized chemically and microstructurally to provide necessary input to computational techniques. Then, the kinetics of austenite growth, which has strong impact on phase transformations during succeeding quenching step, is investigated. Finally, critical temperatures and transformation kinetics are determined and presented in the form of TTT and CCT diagrams. Raw material characterization results indicate that the billets are qualified for the process and the validation study as the billets are free of macro-segregation and exhibit a homogeneous and mildly banded equiaxed ferritic/pearlitic grain structure. Moreover, austenite grain growth study revealed that the grain growth in 22NiCrMo2- 2 can be expressed by an ideal grain growth law. Finally, computationally and experimentally determined CCT diagrams are in a good agreement. The largest differences between computationally and experimentally determined TTT diagrams are observed in the bainitic transformation range which is more sensitive to hard to control factors such as local chemical composition and local prior austenite grain size. Secondary aim of this study is to conduct microstructural investigations for the quality assessment and validation of computer simulations conducted in complementary studies within the scope of the same project. In one of those complementary studies, an experimental Design of Experiments (DoE) using the Taguchi method is conducted on steel shafts made of DIN 22NiCrMo2-2 and 16MnCr5 steels to minimize the variability of the industrial process. In the other study, the same DoE is investigated using computer simulations. This thesis complements those studies with determination of microstructure and hardness distributions. The experimental results indicate an agreement with the simulations results and the agreement can be improved with better characterization of bainite transformation kinetics including its dependence on stress, local grain size and chemical composition.Item NONDESTRUCTIVE CHARACTERIZATION OF BEARING STEELS BY MAGNETIC BARKHAUSEN NOISE TECHNIQUE(2017-06-02) Arslan, Ebru; Davut, Kemal; Şimşir, CanerOne of the most popular type of bearing steels is 100Cr6. The strength, toughness, hardness, fatigue life, wear resistance of it can be improved by heat treatment applications which also changes the microstructure. For controlling the microstructure, traditional metallographic and XRD- based methods, that involve taking representative specimens from lots, are used. Magnetic Barkhausen noise (MBN) technique may provide an alternative nondestructive, fast and practical measurement method to those traditional techniques. This study aims at investigating the possibility of nondestructive characterization of microstructure of heat treated 100Cr6 bearing steels by using the MBN technique. For that purpose, 100Cr6 steel specimens were heat treated in a quenching dilatometer under different austenitization conditions and then quenched to room temperature and to -130°C, in order to generate variations in their microstructures. After heat treatment applications, microstructural properties of the specimens including the fraction and distribution of carbides, amount of retained austenite and also hardness were determined by metallographic and XRD analysis. Moreover, MBN measurements were performed and then the results were correlated to the microstructural parameters. MBN signals correlate with the microstructure variations in the 100Cr6 steel samples via simple linear relations; however, the prediction bands were quite wide and the MBN technique was not sufficiently sensitive, for direct characterization of hardness, retained austenite and carbide fraction of the specimens that were quenched to room temperature. In order to characterize these interdependent microstructural parameters via MBN technique, non-linear relations based on carbide dissolution kinetics are needed. On the other hand, the MBN measurement results of the sub-zero treated specimens showed that; elimination of retained austenite significantly improved the goodness of fit of on those linear relations. Although coefficient of determinations of both carbide fractions and hardness were acceptable, it can be improved by developing a newer non-linear model. Nevertheless, all of the results were promising for the future applications of MBN technique on nondestructive characterization of microstructure variations in 100Cr6 steels.