İnconel 718 süper alaşımının alın frezeleme sonrası yüzey pürüzlülüğü ve sertliğinin incelenerek optimum parametrelerin belirlenmesi
Determination of optimum parameters by investigating the surface roughness and hardness of inconel 718 super alloy after face milling
Yazar: AHMET UĞUR BATUK
Danışman: PROF. DR. ADNAN DİKİCİOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: CNC frezeleme = CNC milling ; Süper alaşımlar = Superalloys ; Yüzey pürüzlülüğü = Surface roughness ; Yüzey sertliği = Surface hardness

Özet:
Yapılan bu çalışmanın amacı, nikel bazlı bir süper alaşım olan inconel 718 malzemesinin ısıl işlemsiz halinin düşük kesme hızlarındaki optimum işleme parametrelerini tespit etmektir. Bu amaçla, yüksek sıcaklıklarda dahi mukavemetini koruyan bu malzemeden oluşan 20 mm çapında 15 mm yüksekliğindeki silindirik numunelerden CNC dik işleme merkezinde bor yağlı soğutma sıvısı da kullanılarak 0.5 mm talaş kaldırılmıştır. Kesici takım olarak da 16 mm çapında TiSiN kaplamalı karbür parmak freze kullanılmıştır. 
Parametreler; 30 m/dak, 40 m/dak, 50 m/dak, 60 m/dak ve 70 m/dak olmak üzere toplam 5 farklı kesme hızı; 0,1 mm/dev, 0,05 mm/dev ve 0,0025 mm/dev olmak üzere de toplam 3 farklı ilerleme hızından oluşmaktadır. Bu nedenle toplamda 15 farklı numune kullanılmıştır. Kesme derinliği tüm numunelerde sabit tutulmuştur. 
İşlenen her bir numunenin yüzey pürüzlülüğü profilometre cihazı ile detaylı bir şekilde incelenmiştir. Elde edilen veriler ile birlikte, öncelikle sabit kesme hızı - değişken ilerleme hızı grafikleri, sonrasında da sabit ilerleme hızı - değişken kesme hızı grafikleri çizilerek pürüzlülük üzerinde hangi parametrenin daha belirleyici olduğu tespit edilmeye çalışılmıştır. 
Yüzey pürüzlülüğü analizlerinden sonra, her bir numuneden mikrovickers sertlik ölçüm cihazında 5'er adet ölçüm alınmıştır. Bu ölçümlerin ortalaması alınarak her bir numunenin ortalama sertlik değerleri hesaplanmaya çalışılmıştır. Sonrasında, yüzey pürüzlülüğü analizlerinde yapıldığı şekilde önce sabit kesme hızı - değişken ilerleme hızı grafikleri, sonra da sabit ilerleme hızı - değişken kesme hızı grafikleri çizilerek sertlik üzerinde hangi parametrenin daha belirleyici olduğu tespit edilmeye çalışılmıştır.
Bu çalışmalar sonucunda nikel bazlı süper alaşım inconel 718 malzemesinin freze tezgâhında düşük kesme hızlarında işlenmesinde, istenen kalitede bir yüzey elde edebilmek için kullanılması gereken optimum parametreler tespit edilerek genel bir değerlendirme yapılmıştır. Değerlendirmeler sonucunda da düşük kesme hızlarında, ilerleme hızının hem sertlik hem de yüzey kalitesi üzerinde daha etkili olduğu tespit edilmiştir.
Sonuç olarak da, yüksek ilerleme hızları ile işlenen numunelerin sertlik değeri daha düşük, pürüzlülüğü ise daha yüksektir, yani yüzey kalitesi düşüktür. Diğer bir deyişle, düşük ilerleme hızları ile işlenen numunelerin sertlik değeri daha yüksek, pürüzlülük değerleri ise daha düşüktür ve daha kaliteli yüzeyler elde edilmiştir.

Abstract:
The aim of this study is to determine the optimum processing parameters of a nickel-based superalloy, inconel 718 which is without any heat treatment, at some low cutting speeds. For this purpose, the workpiece samples had been prepared. The samples have 20 mm diameter and 15 mm height. In order to prepare these samples, a 20 mm diameter cylindrical rod inconel 718 had been used. In total, 18 samples had been examined. 
For milling operations, 16 mm diameter TiSiN coated carbide end mill is used as cutting tool. There are many reasons for using a TiSiN coated cutting tool. Compared to TiAlN and TiN coatings, about % 7-14 Si added TiAlN and TiN coatings have higher strength properties. Especially Si added TiN coating has a huge increase in strenth because of Si addition. Also, Si addition provides high toughness against impacts. In many researches, the best cutting performance and the longest tool life were optained in TiSiN coated cutting tool as a result of the tests carried out at both room and high temperatures.
After the preparation of the samples and choosing the cutting tool, face milling operations had been applied by using boron oil coolant in CNC vertical machining center. Cylindrical samples which have 20 mm diameter and 15 mm height had been face milled in CNC vertical machining center with 0.5 mm depth of cut which is contant for all the samples.
As cutting parameter, different cutting speeds and feedrates had been used. The samples were milled with 6 different cutting speeds which are 20 m / min, 30 m / min, 40 m / min, 50 m / min, 60 m / min and 70 m / min and with 3 different feedrates which are 0.1 mm / rev, 0.05 mm / rev and 0.0025 mm / rev. Therefore, 18 different samples were used during the experiments and measurements in total.
The surface roughness of each processed sample was examined in detail with the profilometer. The surface roughness had been measured for all the samples from 6 different locations on the surface. After getting 6 different surface roughness value, their average values were taken for comparison. With the obtained data, firstly the constant cutting speed - variable feedrate graphs and then the constant feedrate - variable cutting speed graphs were drawn to determine which parameter was more determinant on surface roughness.
After the measurement of the surface roughness values for all the samples, the sketched curves were compared to each other in order to get some opinions about the effect of the parameters on the surface quality. 
At constant cutting speeds, surface roughness increases significantly as feed rate increases. As shown in the graphs, the surface quality of the samples with a cutting speed of 70 m / min was better. In cases where the cutting speed is less than 60 m / min, it is difficult to compare the surface roughness. In other words, if the cutting speed is less than 60 m / min, the surface roughness values of the samples with the same feedrate and different cutting speeds are very close to each other. As the feed rate increases for all cutting speeds, the surface roughness increases, and at cutting speeds of less than 60 m / min, the effect of the cutting speed decreases and the feed rate becomes more important. At low cutting speeds, feed rate and surface roughness are proportional to each other.
In addition, the surface roughness of the samples processed with a higher feedrate than the samples at the same cutting speeds. Values up to a cutting speed of 50 m / min are almost identical for samples processed with the same feedrate. When the cutting speed exceeds 50 m / min, the surface roughness value decreases. In other words, surface quality increases. One of the main reasons for this is the increase of the cutting speed with the increase in temperature on the surface of the material and the material becoming more ductile. Due to the ductility of the material, the surfaces through which the cutting tool passes become less rough. When the cutting speed is greater than 50 m / min, the cutting speed is directly proportional to the surface quality, i.e. the higher the cutting speed, the higher the surface quality.
After surface roughness analysis, the surface hardness values of each sample had been measured from 5 different locations on the surface bu using microvickers hardness tester. The average hardness values of each sample were calculated by taking the average of these measurements. Then, as the surface roughness analysis was done, firstly the constant cutting speed - variable feedrate graphs and then the constant feedrate - variable cutting speed graphs were drawn to determine which parameter was more determinant on hardness.
According to the surface roughness values it was seen that at constant cutting speeds, the hardness of the material surface decreases as the feed rate increases. The main reason for the decrease in hardness is the increase in the feedrate and the increase in temperature on the material surface. The highest hardness values were obtained at a cutting speed of 30 m / min. At higher cutting speeds, the feedrate is inversely proportional to the hardness, but the constant feedrates and hardness values of the samples at different cutting speeds are very close to each other. For these reasons, when the cutting speed is greater than 30 m / min, the cutting speed is no longer the decisive parameter and the feedrate is more important. In other words, the feed rate starts to play a decisive role on surface hardness.
In addition, where the cutting speed is less than 40 m / min at constant feed rates, the hardness is inversely proportional to the cutting speed. That is, the hardness decreases as the cutting speed increases. The main reason for this is the heating of the surface of the material as previously mentioned. In cases where the cutting speed is greater than 40 m / min, the constant feedrate and hardness values of the samples at different cutting speeds are very close to each other and the cutting speed in this range is no longer the determining parameter. For all cutting speeds, the hardness of the material surface increases at low feed rates. At constant cutting speeds, the hardness values decrease as the feed rate increases.
Based on these results, slower feed rates should be used for better surface quality at low cutting speeds; up to 50 m / min cutting speed, the surface quality values for the same feedrate are almost the same and the preferences in this speed range are optional; for values higher than 50 m / min cutting speed, achieving better surface quality depends on increasing the cutting speed, reducing the feed rate, or both; the best surface quality can be achieved with high cutting speed and low feedrate values; for values higher than 40 m / min cutting speed, the surface hardness depends only on the feed rate, i.e. surface hardness increases as the feed rate decreases; at constant cutting speeds, the feedrate must be reduced to increase surface hardness; at values below 40 m / min cutting speed, surface hardness increases as the cutting speed decreases; in the same range, a low cutting speed, a low feedrate, or both should be used to obtain higher hardness values.
As a result, samples processed with high feed rates have a lower hardness and a higher roughness, i.e. lower surface quality. In other words, the hardness values of the samples processed with low feed rates were higher, the roughness values were lower and better quality surfaces were obtained.