Investigation of the Relationship between Vibration, Cutting Forces and Tool Wear During the Processing of Inconel 718 Super Alloy under Coolant Pressure Jet-Assisted Turning with Variance Analysis

Bu calismada, sogutma sivisi basinci altindaki tornalama kosullarinda kesici titresimleri, asinma kosullari ve kesici kuvvetleri izlenmistir. Is malzemesi olarak Inconel 718 kullanilmistir. Kesme aninda olusan veriler bilgisayara kayde-dilmis. Sogutma sivisi basinci ile titresim, takim asinmasi ve kesici kuvvetler arasindaki iliski Varyans (ANOVA) analizi yontemi ile incelenmistir. Varyans analizinde titresim_RMS (Root Mean Sequare)’i, kesici kuvvetlerin RMS degerleri ve takim asinmasi bagimli degisken, sogutma sivisi basinci ise bagimsiz degisken (faktor) olarak secil-mistir. Analiz sonucunda sogutma sivisi basincinin titresim, kesici kuvvetler ve takim asinmasi uzerine etkisinin cok onemli oldugu tespit edilmistir (P<0.01). Varyans analizinden sonra Duncan coklu karsilastirma testi uygulanmistir. Duncan testi sonuclarina gore siniflama yapildiginda ise 100 bar’lik kesme deneylerinin en uygun sogutma sivisi basinci altinda yapilan deneyler oldugu gorulmustur.


INTRODUCTION
Machining is one of the most important methods of today's manufacturing technologies and remains current. One of the most important raw materials of machining is Nickel-based super alloys. Nickel-based superalloys have had a great use recently in the aviation industry. It is also used in industrial gas turbines, spacecraft, rocket engines, nuclear reactors, submarines, steam generating plants, petrochemical devices and other heat resistant applications. In these respects, it is important to know the most suitable machining parameters for a super alloy such as Inconel 718. In this study, it is aimed to determine suitable parameters for Inconel 718 super alloy.
Hard-to-cut materials are widely used in many engineering applications. These materials are preferred in automotive and aviation designs due to their properties. One of these materials is Inconel 718. Hegab and Kishawy (2018) studied the determination of the appropriate values of the cooling and lubrication processes during the turning process of Inconel 718 material. It is observed that the processing performance of the Inconel 718 material is enhanced by nanofluids. Alvarez et al. (2017) measured the temperature values with a sensor during the processing of the Inconel 718 material. The temperature value measured in their study is in the range of 250-1200 centigrade degrees. These values are then estimated by developing a mathematical model. Chuangwen et al. (2018) and Prasad and Babu (2017) reviews the importance of relationship between vibration, tool wear, cutting forces and surface roughness during material processing. Correlations among these are tried to be calculate by processing different materials and measuring result values. For estimation of these results, Prasad and Babu (2017) performs some simulation studies. Kindi et al. (2018) and Yılmaz et al. (2014) generates these studies in both jet-assisted turning and milling operations. They use variance analysis methods to calculate the appropriate parameters in milling operations. Lotfi et al. (2018) utilize the ultrasonic data that is one of the measurements used in variance analysis. The data obtained by ultrasonic measurements are interpreted by digital processing techniques. Güngör (2011) and Bhuiyan and Choudhury (2015) reports that RMS calculation can be used in the evaluation and interpretation of obtained signals, because RMS calculation is a reliable method for interpreting sinus amplitude signals.
The following steps are applied to calculate the RMS value of a signal as discrete (digital): -The amplitude values are taken at a specific sampling time for a period of signal -Sum of the squares of these values are calculated. This result is divided by the number of samples taken -The square root of this section is taken.
While the RMS value of a signal is continuously (analogue) calculated, measurement of the effective value (Ueff) of the voltage (u(t)) in the alternative and optional form of electrical signals has great importance in automation and control technologies. Germer (2001) calculates Xrms as in equation 2 in the effective value measurement systems.
There is a discrete-time signal processing in the calculation method. A sampling frequency (fs) can be mentioned in discrete time signal processing. This sampling frequency should be at least twice the input frequency (fi) according to Nyquist's theorem (equation 3). Petrović (2015) points out that sampling frequency should be kept as high as possible to make the measurement results more precise.

fs ≥ 2fi
(3) In this study, equation 1 is used to calculate the digital equivalent at each point. Paul et al. (2016) specifies that modified hard materials are difficult to process in a pure, dry environment during material processing with a jet-assisted turning. For this reason, Yan et al. (2016) and Revankara et al. (2014) aims to improve the cutting process by using high pressure coolant during material processing. The purpose of using coolant is to reduce cutting temperatures and prevent surface corrosion. Güngör (2011) and Kamruzzaman and Dhar (2009) (2007) indicates that the sharp edges during material processing at high speeds could cause temperatures and stresses by Finite Element Method (FEM). Güllü et al. (2008) demonstrates the great importance of the processing parameters due to the hardness of the Inconel 718 inside the super alloy materials. Xavior et al. (2017) performs variance analyzes for cutting edge selection and cutting force parameters during processing of this super alloy.

MATERIAL AND METHOD
The data presented in this study are obtained at the cutting speed of 50 m/min, the feed rate of 0.15 mm/cycle and the cutting depth of 2.5 mm in CNC jet-assisted turning processing of Inconel 718 super alloy material. Keeping these parameters constant, three different parameters are used by the coolant pressure. These are coolant pressure values of 6, 100 and 300 bar. The CNC jet-assisted turning which the tests are performed is shown in Figure 1.
Values are recorded to the computer with Cut-Pro program and interpreted in Matlab program. PCB Piezotronic model 353B31 (Figure 2) is used as a vibration sensor.
A "Kistler 9257 A" model dynamometer ( Figure 3) is used to measure the cutting force signals.  "SECO Jet Stream" is preferred as cutting tool. The reason for preferring tools from the Jet Stream 26 system in this study is their ability to respond to maximum pressure of 350 bar. The cutting edge used in this study is shown in Figure 4 and geometric dimensions of cutting edge is shown in Table 1.   Figure 5. In addition, the chemical composition of the material is given in Table 2 and its mechanical properties are shown in Table 3.   T is tool life (minute), V is cutting speed (m/min) and n is the tool base and it depends on factors such as tool material, workpiece material, machining conditions, feed amount or depth of cut, tool geometry and coolant. Thus; C is the cutting speed corresponding to one-minute life, which is dependent on the tool and the part material. As the cutting speed is increases in equation (4), it is seen that tool life decreases.
It is analyzed whether there is a relationship between coolant pressure and vibration, cutting forces (vertical, horizontal) and tool wear with variance analysis (using SPSS statistical program). In variance analysis, vibra-tion_RMS, RMS values of cutting forces and tool wear are chosen as dependent variables, and coolant pressure is chosen as independent variable. It is found that the effect of coolant pressure on vibration is very important (P<0.01). Duncan multiple comparison test is applied after analysis of variance.
The results of the variance analysis between vibration and coolant pressure are shown in Table 4. According to Duncan test results in table 4, the lowest mean vibration_RMS is 1.67 m/s 2 as a result of the 100 bar cutting tests, while it is calculated as 4.72 m/s 2 and 5.13 m/s 2 with 4 and 6 bar cutting tests respectively and the difference between these vibration_RMS' are found to be insignificant. The error margin is calculated 0.593. In the light of these data; it can be said that there is no difference in vibration which is observed during cutting of the material under 6 or 300 bar pressure. It is determined that the most ideal cutting process is under the pressure of 100 bar coolant.
The results of variance analysis between vertical cutting force and coolant pressure are shown in Table 5. The results of variance analysis between horizontal cutting force and coolant pressure are shown in Table 6. The results of variance analysis between tool wear and coolant pressure are shown in Table 7.

RESULTS
In the results of 32 cutting experiments under different coolant pressure; according to the analysis of variance between coolant pressure and vibration, it is observed that the minimum vibration is at 100 bar pressure. According to this result, it can be said that 6 bar coolant pressure does not cool the vibration at the cutting end sufficiently, and 300 bar coolant pressure creates additional vibration at the cutting end. When evaluated in terms of shear forces, it is seen that the pressure applied with the least force is 100 bar with the coolant pressure of 300 bar. It is observed that more force is applied in order to take place the cutting operation at 6 bar coolant pressure. When examined in terms of tool wear, it is observed that the minimum tool wear is during the coolant pressure of 100 bar. Here it is observed that a similar result is produced by vibration. According to this result, it can be said that there is a direct relationship between vibration and tool wear. In the light of all these data, 100 bar pressure value is determined as the most ideal coolant pressure between 6, 100 and 300 bar coolant pressures during the processing of Inconel 718 super alloy material. Other cutting parameters (cutting speed, feed rate, cutting depth) are selected from the parameters accepted in the literature and kept constant.