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    Cryogenic machining of carbon fiber reinforced plastic (CFRP) composites and the effects of cryogenic treatment on tensile properties: A comparative study
    (Elsevier Sci Ltd, 2018) Morkavuk, Sezer; Koklu, Ugur; Bagci, Mehmet; Gemi, Lokman
    Carbon fiber reinforced plastics (CFRPs) are prone to damage locally during machining due to the applied cutting forced and generated heat. Cryogenic machining can reduce the heat generated damages of CFRPs by utilizing cryogenic liquids instead of conventional cutting fluids. The goal of this study is to investigate milling performance of CFRPs in cryogenic medium. For this, a new cryogenic machining approach was adopted to slot milling of CFRPs by submerging the workpiece within a cryogenic liquid. The CFRPs were fabricated via vacuum assisted resin transfer method by using woven carbon fiber fabric as a reinforcement and epoxy as a matrix. Machining performance was evaluated based on the resulting cutting force, delamination factor, surface roughness, and surface damage. Moreover, the influences of cryogenic coolant on the tensile properties, fracture surface microstructure, and machined surface of the CFRP laminates were analyzed with scanning electron microscopy (SEM). SEM analysis revealed that combination of different damage modes such as debonding, micro matrix crack, fiber pull out, and bundle pull out, delamination, and fiber breakage were observed. The results showed that cryogenic machining approach provided less damage formation on the machined surface, reduced delamination factor and surface roughness but increased resulting cutting force during machining of the CFRPs. On the other hand, there was a slight improvement (about 3%) of the tensile properties for the CFRPs exposed to cryogenic coolant due to matrix hardening and increasing in the fiber strength and shear strength.
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    The effects of stacking sequence on drilling machinability of filament wound hybrid composite pipes: Part-1 mechanical characterization and drilling tests
    (Elsevier Sci Ltd, 2020) Gemi, Lokman; Koklu, Ugur; Yazman, Sakir; Morkavuk, Sezer
    In the first part of this two-part comprehensive study, mechanical properties and machinability characteristic of filament wound hybrid composite pipes with various stacking sequences of glass and carbon fibers (Glass-Carbon-Glass (GCG), Carbon-Glass-Glass (CGG), and Glass-Glass-Carbon (GGC)) were investigated experimentally. In order to determine the mechanical properties of the pipes, hardness test (Shore D), ring tensile test (ASTM D2290), and burst test (ASTM D1599) were carried out. Machinability tests were performed at various feed rates (50, 150, 250 and 350 nun/min) and spindle speeds (796, 1592, 2388 and 3184 rpm) using with and without a back-up. The results showed that stacking of the carbon layer between two glass layers (GCG) presented better performance in terms of mechanical properties and machinability characteristic. The maximum ring tensile stress of GCG specimen is 27% and 19% higher than those of GGC and CGG specimens, respectively. On the other hand, the lowest thrust forces measured during the drilling of GCG specimen while the GGC represented highest values. In addition, the use of back-up led to an increase in thrust force. The highest increase was observed in GGC sample. In GGC sample, a change in a spindle speed increased thrust force by 18-35%, while a change in feed rate increased thrust force by 20-30%.
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    The effects of stacking sequence on drilling machinability of filament wound hybrid composite pipes: Part-2 damage analysis and surface quality
    (Elsevier Sci Ltd, 2020) Gemi, Lokman; Morkavuk, Sezer; Koklu, Ugur; Yazman, Sakir
    In the first part of this two-part study, filament wound hybrid composite pipes with various stacking sequences were manufactured and mechanical properties such as hardness, ring tensile strength, and burst strength were experimentally investigated. After determining mechanical properties, drilling tests were performed to research machinability characteristics. The second part of the study consists damage analysis and surface quality examination including ring test damage analysis, push-out delamination analysis, borehole damage examination and borehole surface quality. The experimental data suggested that cutting parameters, stacking sequence, and the use of back-up were impactful on the formation and propagation of various types of damages. Especially, the effect of stacking sequence was remarkable. A larger delamination area was formed in Glass-Glass-Carbon (GGC) sample after the ring tensile tests compared to Glass-Carbon-Glass (GCG) and Carbon-Glass-Glass (CGG) samples. In all cases, the utilization of back-up lead to decrease of delamination with 9-40% reduction in surface roughness. When the back-up is not used during drilling, an excessive push-out delamination occurred in all drilling tests. Moreover, CGG samples represented lower push out delamination. In addition, position of the hole depending on the winding angle plays a key role on damage formation and surface quality.
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    Experimental study on the effects of cold chamber die casting parameters on high-speed drilling machinability of casted AZ91 alloy
    (Elsevier Sci Ltd, 2020) Yazman, Sakir; Koklu, Ugur; Urtekin, Levent; Morkavuk, Sezer; Gemi, Lokman
    In this study, the effects of the cold chamber die casting parameters on high-speed drilling machinability of AZ91 Magnesium alloys were experimentally investigated. The influence of different casting parameters (casting temperature, molding pressure, and gate speed) on microstructure, mechanical properties and machinability characteristics (thrust force, tool wear, built-up edge, built-up layer formation, surface topography, chip morphology, and burr formation) were examined. The experimental results showed that the grain size of the conventional casting sample was around 50 microns, while in other cold chamber die casting tests, it varied depending on temperature, pressure, and gate speed. It was observed that the tensile strength values of the samples produced with 1000 bar mold pressure were higher than those of other samples. In the formation of thrust force, the feed rate is more effective than the cutting speed. The least tool wear occurred in the drilling of the As-cast sample, while the highest tool wear occurred in the drilling of the sample which was produced with low pressure and low gate speed combination. As a result of the drilling tests, depending on casting and cutting parameters, three different types of chips were formed: fan, spiral cone, and long ribbon type. Furthermore, uniform and transient burrs in different sizes were observed.
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    An experimental study on the effects of various drill types on drilling performance of GFRP composite pipes and damage formation
    (Elsevier Sci Ltd, 2019) Gemi, Lokman; Morkavuk, Sezer; Koklu, Ugur; Gemi, Dilek Soylu
    Drilling is the most commonly used machining operation among others for assembly applications. The necessity of this operation has come to the forefront in composite materials that are becoming widespread nowadays. In this study, drilling performance of glass fiber reinforced plastic (GFRP) composite pipes used in many sectors such as natural gas transmission lines, pressurized sewer lines, industrial waste transmission, defense industry, and construction industry was investigated. The GFRP pipe was produced by filament winding method with a winding angle of +/-55 degrees. Different drill types (conventional twist drill, brad and spur drill, and brad center drill) with a drill diameter of 4 mm were utilized for drilling the pipe and the effect of drill type on the drilling performance was investigated. Drilling tests were performed at a constant 5000 rpm spindle speed and six different feed rate parameters (25, 75, 125, 175, 225, and 275 mm/min). Thrust forces generated during drilling were recorded and after the drilling operations, hole exit surface damage, and borehole surface damage were examined by a digital microscope and scanning electron microscope (SEM). Results showed that the brad center drill produced lower thrust forces while the twist drill generated higher thrust forces. The severity of damages could vary depending on the tool geometry and feed rate. Especially, at lower feed rates, conventional twist drill results with increased delamination and uncut fibers as compared with other drills. The brad center drill presents better performance since it generates fewer damages. Also, it is observed that these damages formed in the winding angle direction (orientation).
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    Optimization and prediction of thrust force, vibration and delamination in drilling of functionally graded composite using Taguchi, ANOVA and ANN analysis
    (Iop Publishing Ltd, 2019) Koklu, Ugur; Mayda, Murat; Morkavuk, Sezer; Avci, Ahmet; Demir, Okan
    Composite materials offers many advantages over traditional materials in terms of weight, strength and design flexibility. However, machining of these materials leads to some critical problems such as delamination, inferior surface finish and excessive tool wear due to their anisotropic and inhomogeneity structure. In particular, in manufacturing of the functionally graded composite (FGC) materials, this situation becomes more complicated since these composites have been made with different stacking sequences. In this work, drilling performance of FGC, depending on delamination, thrust force and vibration, has been investigated experimentally by using different cutting parameters, which are feed rate, spindle speed and material directions (carbon/epoxy and glass/epoxy). From the results of experiments, the material direction has deeply affected the delamination (89.5%) but has rarely affected the thrust force (0.1%) and vibration (8.4%). Feed rate is the most impactful factor on thrust force and vibration generation. Whereas the feed rate has a direct proportion to the thrust force and delamination, the spindle speed has an inverse proportion to these responses. When the results evaluated generally, In addition to that, a surrogate model is created through ANN to estimate the responses or cutting parameters on the drilling process in a wider range.