Karbür kesici takımların süper alaşım talaşlı imalat performanslarına borlama işleminin etkisi
The effect of boriding process on super alloy machining performance of carbide cutting tools
Yazar: RİFAT YILMAZ
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Bu çalışmada; Inconel 718 uçak motoru parçalarının torna operasyonlarında kullanılan WC-Co kesici takımlar farklı borlama parametreleri altında pasta borlama işlemine tabii tutulmuş, borlanan takımlar üzerinde yapısal-mekanik karakterizasyonlar gerçekleştirilmiş ve saha testleri yapılarak takım performansındaki değişimler tespit edilmeye çalışılmıştır. Bu amaca yönelik olarak öncelikle literatür araştırması yapılmış ve WC-Co kesici takımlar üzerine yapılan difüzyon prosesleri incelenmiştir. Literatür çalışmalarından yola çıkılarak yapılan ön borlama deneyleri sonucu farklı sıcaklık ve sürelerden oluşan bir deney planı hazırlanmıştır. Buna göre 800, 850, 900, 1000 ve 1100 °C olmak üzere 5 farklı sıcaklık ile 4 ve 8 saat olmak üzere iki farklı sürede borlama işlemi yapılmıştır. 
Borlama işlemi % 94 WC, % 6 Co içeren V yataklı  bilenmiş yuvarlak kesici takım üzerine pasta uygulamak suretiyle argon koruyucu atmosferi altında tüp fırın içerisinde gerçekleştirilmiştir. Borlanmış takımlarda borür tabaka kalınlığının ölçülmesi ve mikroyapı incelemesi için SEM kullanılmış; EDX modülü ile çizgi ve nokta taramalar yapılarak bor difüzyonu araştırılmıştır. Buna göre borür tabaka kalınlıklarının 6 ile 32 μm arasında değiştiği ve borun  Co matris içerisine difüze ettiği anlaşılmıştır. Borür tabakasında oluşan fazların tespiti için kesici takımların XRD analizleri yapılmıştır. XRD sonuçları incelendiğinde kesici takım yüzeyinde oluşturulan borür tabakasında CoW2B2, CoW3B3, CoB ve W2B fazlarının bulunduğu ve sertlik artışının da bu fazlar sayesinde meydana geldiği anlaşılmıştır. Oluşan fazların literatürdeki diğer borlama çalışmalarında elde edilen fazlarla uyumlu olduğu görülmüştür. 
Elde edilen borür tabakalarının Vickers sertlik cihazında 1 ve 2 kg yük altında sertlik incelemeleri gerçekleştirilmiştir. Borlama sonucu altlık malzemesine göre sertlikte maksimum 511 ile 630 HV arasında artış sağlanmıştır. Borlanan kesici takımlarda borür tabakasının yapışma kabiliyeti ve gevrekliği, Rockwell adhezyon testi ile belirlenmiştir. Bu testte borür tabakası üzerinde 60 ve 150 kg yük altında Rockwell sertlik cihazı ile batma izi oluşturulmuştur. Bu izin etrafında meydana gelen çatlaklar ve dökülmeler incelenerek standard hasar görüntüleri ile karşılaştırılmıştır. Buna göre 60 kg yükte 1000 °C - 8 saat, 1100 °C - 4  saat, ve 1100 °C - 8 saat borlanmış numunelerin HF6 tipi hasara uğradığı görülmüştür.
Takımların aşınma testleri Tusaş Motor Sanayi bünyesindeki makina parkında yer alan HNK marka dik torna tezgahında, Inconel 718 süper alaşımından imal bir uçak motoru şaft parçasının tornalama operasyonunda gerçekleştirilmiştir. Aynı kesme parametreleri altında, borlanmış takımların birbirlerine ve borlanmamış takıma göre serbest yüzey aşınmalarının kıyaslaması yapılmıştır. Aşınma sonuçlarına göre borlanmamış takımdaki ortalama serbest yüzey aşınması 231 μm olmuştur. En az aşınan takım ise 800 °C sıcaklıkta 8 saat borlanmış takım olup ortalama aşınma miktarı 118  μm olarak ölçülmüştür. 
Sonuç olarak pasta borlama işlemi WC-Co esaslı kesici takımlara başarı ile uygulanarak süper alaşım talaşlı imalat performanslarında % 48 iyileşme sağlanabilmiştir. Çalışma; dünya literatüründe çok az çalışılan borlanmış WC-Co kesici takımların süper alaşım işlemedeki performansının saha testleriyle araştırılması bakımından özgün bir özelliğe sahip olup, literatüre önemli bir katkı sağlaması ümit edilmektedir.

Abstract:
Today, modern aircraft engines are designed by manufacturers considering low fuel consumption and emission. In order to achieve these requirements for high efficient aeroengines; turbine temperatures are increased by designers and engine manufacturers. So high temperature materials which exhibit excellent strenght, creep resistance etc. must be used for hot section parts of aeroengines. Nickel based super alloys are commonly used for manufacturing of turbine blades, discs, combustion chamber and they have unique high temperature properties compare to other engineering materials. 
Super alloy parts are generally manufactured by machining operations (turning, milling, grinding etc.) from forged raw materials. So, due to the mentioned mechanical properties of super alloys, cutting tool consumption is very high. In aviation industry, generally PVD or CVD overlay coated cemented carbide cutting tools are utilized for super alloy machining. Also, there are some academic studies on the diffusional coating of cutting tools like boriding & nitriding and good performance results could be obtained by researchers. Boriding, which is thermo chemical surface hardening method, provides attractive performance development for WC-Co cutting tools. In the literature, there are several boriding methods like pack, molten salt (with and without electrolysis), paste, gas and plasma boriding could be used by users. By the means of boride layers, treated surfaces woud have high hardness, good tribological properties and anti-corrosion resistance.  
In this study, WC-Co cutting tools used in turning operations of Inconel 718 superalloy aero engine components, were borided with different process parameters and then with the help of structural-mechanical inspections (SEM, EDX, XRD analysis, Vickers hardness test, Rockwell adhesion test) and field tests (turning); change in the tool performances were investigated. For this purpose, firstly a literature survey was conducted and informations about diffusion processes of WC-Co cutting tools were gathered. Under the light of literature informations, pre-boriding experiments were realized and boriding parameters which consist of different temperatures and time intervals were planned. As a result, five different temperatures namely 800, 850, 900, 1000 and 1100 °C and two different time interval (4 and 8 hrs) have been used for boriding of cutting tools.
In the experiments, paste boriding method was used and experimental setup was prepared. Ekabor II paste (EkaGermany®) was used as boron agent and boriding process was realized by using grinded round inserts which are made from % 94 WC and % 6 Co under Argon (Ar) inert atmosphere in tube furnace. Before the boriding experiments, cutting tools were cleaned with the aceton and distilled water. After cleaning, cutting tools and boron paste were put into the plain carbon steeel ring container and then this experimental setup was dried in the oven for 15 minutes. 
For the inspection of borided cutting tools in order to find boride layer thickness, optical microscope was firstly used. But, because of sample polishing problems (edge rounding), layer thicknesses were determined by using SEM which can compensate sample preparation problems. As a result of SEM examinations, it is observed that the thickness of the boride layers were changing between 6 μm (800 °C - 8 hrs) and 32 μm  (1100 °C - 8 hrs). According to these results boride layer thicknesses were risen with increasing temperature and time, because boriding is a diffusion controlled process and increase in temperature enhances diffusion capability/rate of the boron atom. Also boriding time was less effective on the boride layer thickness than temperature, because for same temperature, when time was increased, boride layer thickness haven't changed remarkably.
After determination of layer thicknesses, XRD analysis of boride layers were conducted. As aresult of XRD patterns; for original cutting tool, only WC phase peaks were seen and there was no any Co phase peak observed due to the low amount of Co matrix (% 6). When the XRD pattern of borided cutting tools were considered, boride layer was consist of CoW2B2, CoW3B3, CoB and W2B phases depending on boriding temperature and time. For borided cutting tools under 800 °C - 8 hrs, 850 °C - 4 hrs, 900 °C - 4 hrs parameter, there were not observed any boride phase peak except WC phase peak. It was not mean that there were not any boride phases in these cutting tools. Due to low amount of boride phases, XRD equipment could not detect any boride peaks for these cutting tools. On the other hand, cutting tool borided at 1100 °C - 8 hrs had CoB and W2B phases, apart from others. According to XRD patterns, it is found that WC phase peak intensity decreased with incerasing boriding temperature. On the contrary, peak intensities of CoW2B2 and CoW3B3 phases  decreased with increasing boride temperature and time for borided cutting tools. In the literature, similar boride phases could be found by several researchers.
The detailed boron diffusion was investigated by EDX point and line elemental analysis.  According to point EDX results, it was understood that boron diffuse into the Co matrix and formed boride phases between WC grains. In the point EDX analysis, carbon and boron EDX peaks were not distinguished exactly by equipment due to the atomic number similarity, so boron + carbon rich regions were accepted as boron diffusion regions in this thesis. In addition, line EDX results showed that there were two different region in the boride layer; first one was boron rich layer located near the outer surface of cutting tool and second one has low boron content which is called as diffusion layer close to substrate. We obtained that, boron concentration increased gradually from substrate to outermost surface of the borided cutting tools. These results were also compatible with literature.   
Hardness of borided layers were tested in Vickers hardness device under 1 and 2 kg load. Borided cutting tools were not polished before hardness tests because of preventation boride layer removal. Although we haven't polished borided cutting tools, indentation was easily seen. During the hardness tests, six indivudual hardness results were realized for each borided cutting tools. Then average of these six indivudual hardness values for each load were calculated. After boriding, compared to substrate material; 511 to 630 HV harder boride layer was achieved. It is found that, increase in the hardness was caused by hard ternary boride phases (CoW2B2 and CoW3B3) which have orthorhombic crystall structure and high hardness (20 - 45 GPa). But decrease in the hardness at borided cutting tool under 1100 oC - 8 hrs came from low hardness of CoB phase (11 GPa) and fully consumption of tough Co matrix around the WC particles via boron diffusion. Also hardness values of borided inserts went up with increasing boriding temperature and time.
The adhesion capability of the boride layers to substrate were inspected by using Rockwell adhesion test. Firstly, standart load (150 kg) was applied to borided inserts and indentation images were inspected by optical microscope with 100x magnification. Results of this test showed that all borided cutting tools had micro cracks and delaminations. According to the standard failure scale (VDI 3198) HF5 and HF6 type failures were obtained in Rockwell C test. Due to the high hardness and brittleness of boride layer, we also used 60 kg load (Rockwell A) for adhesion test and according to microscopic examinations, cracks and delaminations (HF6 type failure) around the indentation were observed in borided cutting tools under 1000 °C - 8 hrs, 1100 °C - 4  hrs and 1100 °C - 8 hrs parameters. So other borided cutting tools apart from mentioned ones exhibited good adhesion behaviour in Rockwell A adhesion test which is generally used for hard coatings (like diamond) in the literature. Crack and delaminations increased with boriding temperature and time. In the high boriding temperatures, boride layer thickness is higher and due to brittle nature, hertz stress formed microcracks and spallations near the diamond cone indentor during adhesion tests.
After structural and mechanical characterization, the wear performance test of borided cutting tools were conducted on the HNK vertical turning machine placed in machine park of Tusaş Engine Industry and cutting tests were realized on the turning operation of shaft part made from Inconel 718 super alloy. Under the same cutting parameters, flank wear of the borided and original cutting tools were compared to each other. Flank wear values were measured in stereo microscope under 80x magnification. Each borided inserts were tested by using two side of inserts. Average flank wear of these two side in original cutting tool was measured 231  μm. Minimum amount of flank wear was found in borided cutting tool under 800 °C for 8 hrs and measured 120.4  μm. Maximum flank wear was found in borided cutting tool under 1100 °C for 8 hrs and measured 539 μm. Flank wear increase with increasing boriding temperature and time. Because thicker boride layer rises brittleness and causes high amount of abrasive wear. Also borided sample under 1100 °C for 8 hrs parameter, exhibited worst performance, because hardness of this tool is lowest among others. Also borided cutting tools under 1000 °C - 8 hrs, 1100 °C - 4  hrs and 1100 °C - 8 hrs exhibited worse adhesion capability to the substrate, had worse performance than original cutting tool. It is obvious that adhesion and hardness tests had a good agreement with wear test results.
All of these characterization and cutting test results showed that, for WC - Co inserts (6 % Co), optimum boriding parameters should be below:
-	Boriding temperatures: 800-900 °C
-	Boriding time interval: 2-6 hrs
-	Boride layer thickness: < 10 μm
As a result, paste boriding method was applied succesfully on WC-Co cutting tools  in this study. So super alloy machining performance of borided cutting tools were improved by % 48. This study has unique property which is investigation of wear performance of borided WC-Co cutting tools on the Inconel 718 super alloy via field tests, so we hope that it will significantly contribute to the literature.