Küresel grafitli dökme demir (GGG50) malzemeye bakır ihtivasının yorulma dayanımına etkisinin incelenmesi
The influence of copper addition on fatigue properties of GGG50 grade  spheroidal graphite cast iron
Yazar: MERTCAN KIZGIN
Danışman: PROF. DR. MAHMUT ERCAN AÇMA
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Bu çalışmada küresel grafitli dökme demirlerin alaşımlı ve alaşımsız olarak mekanik özellikleri incelenmiştir. Ardından ise; alaşımsız ve farklı oranlarda bakır ihtiva edilen küresel grafitli dökme demirin dönmeli-eğilmeli yorulma testleri ile kırılmaya kadar geçen çevrim sayısına göre performansları incelenmiştir. Yapılan mekanik testler sonucunda çıkan gerilme-uzama grafiklerine göre tokluk değerleri incelenmiş olup, ayrıca bakırın perlit yapıcı etkisinin anlaşılması için ferrit ve perlit oranları kayıt altına alınmıştır. 
5 farklı bakır kompozisyona sahip numuneye çekme testi, sertlik testi, mikro yapı incelemesi, spektral analiz ve yorulma testi yapılarak değişimler gözlemlenmiştir.
Yapılan deneysel çalışmalarda küresel grafitli dökme demire ihtiva edilen bakırın, malzemenin mekanik özelliklerine olumlu yönde etki ettiği gözlemlenmiştir. Bunun yanı sıra, düşük frekanslı yorulma testinde bakır ihtivasının %1,49 oranına kadar  yorulma dayanımına da arttırıcı yönde etki ettiği görülmüştür. Bunun sebebi ise mikro yapıda bulunan perlit miktarının artması olarak gösterilmektedir. Yapılan mikro yapıdaki faz değişiklikleri incelendiğinde bakırın perlit ve grafit oluşumunu desteklediği sonucuna ulaşılmıştır. Ayrıca sertlik ve çekme dayanımlarının artışına bağlı olarak uzama değerlerindeki düşüş gözlemlenmiştir. Buna paralel olarak yapılan tokluk hesabında, beklendiği üzere azalma meydana gelmiştir. 
Tüm bu sonuçlar ışığında alaşımsız küresel grafitli dökme demirin özellikle yorulma kuvvetlerine maruz kaldığı uygulama alanlarında ve doğal yorulma dayanımının yeterli olmayacağı öngörüsünde bulunulduğunda, bakır ilavesi ile malzemenin yorulma dayanımının arttırılabileceği bilgisine ulaşılmıştır. Yorulma dayanımını etkileyen başlıca faktörlerin literatür araştırmalarına göre, sıcaklık, korozyon, yüzey pürüzlülüğü, çentik etkisi, malzeme çapı, test frekansı gibi parametreler olduğu görülmüştür. Bunların etkisini en düşük seviyeye çekmek adına deney numunelerinin aktif çapı 6mm, sıcaklık oda sıcaklığı, yüzey pürüzlülüğü 0,8 mikron, çalışma frekansı 25 Hz ve nemli olmayan, korozyon etkisi oluşmayacak şekilde testler gerçekleştirilmiştir. Test sonuçlarının yorumlanabilir istatistik vermesi için ise her bir kompozisyondan üçer adet numuneye bahsedilen testler yapılıp, sonuçların ortalama değerleri kullanılmıştır.
Deney verilerinin sonucunda, yapılan çalışmada %1,49'a kadar bakır ilavelerinin küresel grafitli dökme demirin hem mekanik özelliklerini iyileştirdiği hem sertlik değerlerini arttırdığı hem de yorulma dayanımını iyileştirdiği ortaya konmuştur.

Abstract:
Spheroidal graphite cast irons have better mechanical properties when they are compared to alternative cast irons and are obtained as a result of the reaction in the nodulation melting pot with the help of a spheroidizer (Mg or Ce) added to the molten cast iron in small amounts. 
Spheroidal graphite cast irons show similar mechanical properties to steel. With the castability and low production cost of ductile graphite cast iron, which provides steel's high strength, ductility, high fracture toughness and wear resistance properties, it has enabled designers to produce superior machine parts at low cost.
Spheroidal graphite cast irons have ferritic, pearlitic, austenitic, bainitic or martensitic internal structure. The addition of copper to  Spheroidal graphite cast irons promotes the formation of perlitic structure.
Spheroidal graphite cast irons are iron alloys that contain 3.4 - 3.9% carbon and 1.8 - 3.1% silicon and have a spheroidal graphite structure. Numerous studies have been conducted on the production of materials with optimum properties in the process from the years when metal materials were used to the present day. Spheroidal graphite cast irons, one of the cast iron types, have more uses than other types of cast iron in terms of both production and mechanical properties.
Spheroidal graphite cast irons; It is also known as nodular, sphero, ductile. These types of cast irons have the same composition as gray cast irons, the only difference being the shape of graphite. While graphite is lamellar in gray cast iron, it is spheroidal in spheroidal graphite cast irons. Ce or Mg added to molten cast iron enables graphite to form spherically. Spheroidal graphite cast irons have high strength compared to other types of cast iron, as well as low melting temperature, good fluidity and pourability. In this respect, it has engineering advantages of both gray cast iron and steel. In addition to all these advantages, desired mechanical properties can also be obtained by processes similar to heat treatments applied to steel.
Spheroidal graphite cast irons, whose production has increased since 1970, create a very useful structure by combining many advantages of steel and the economical machining of cast irons. Thanks to these superior properties, it has gained a more common usage rate by preventing gray cast iron, tempered cast iron and steel casting. It has a wide range of use, especially where high strength and good wear resistance are required.
Some properties of spheroidal graphite cast irons are as follows. 
* Spheroidal graphite cast irons are 10% lighter than steels. Gears made of spheroidal graphite cast iron work quieter than steels.
* Graphite in the structure increases the damping capacity by 40%.
* Spheroidal graphite cast irons have less notch sensitivity than steel. For this reason, surface treatments do not need to be made as sensitive as with steels.
* Spheroidal graphite cast iron as raw material is cheaper than steels
* Gears made of spheroidal graphite cast iron can be cast to give their final shape. For example, threads can be created during the casting process.
* Machinability of spheroidal graphite cast irons is much better than annealed steel. Therefore, the consumption of expensive processing tools is also less.
* The energy used in the production of spheroidal graphite cast iron gears is less than the energy used in the production of steel gears. In this way, energy savings can be increased up to 50%.
* Spheroidal graphite cast irons exhibit a lower friction coefficient and lower wear rate compared to steels, thanks to the free graphite contained within.
Due to these superior properties mentioned above, ductile graphite cast irons have many uses. Automotive and architectural applications have the largest share in these areas of use. Crankshafts, front wheel support arms, complex shapes of steering linkages, brake discs, engine connecting rods, free arms, wheel hubs, power transmission connections, turbo housings and manifolds for high temperature applications and high safety valves are manufactured from spheroidal graphite cast iron. Besides that, the spheroidal graphite cast iron pipe industry constitutes the other biggest usage area. In addition, it is also used in mining and metallurgy industry, crushing bodies, hot rolling mills, molds, melting and slag pots.
Spheroidal graphites can be of different sizes which are up to the section thickness of the casting and the number of cores required for graphite precipitation. Generally thinner sections contain more cores. Consequently, the numbers of graphite spheres in thin sections are higher, but smaller in size. In spheroidal graphite cast irons, the number and distribution of graphites is a very good and precise indicator of metallurgical quality. The number of spheres of 150 - 250 pieces / mm2 is preferred as an average value. The structure of spheroidal graphite cast irons consists of a main phase and spherical graphites within this main phase. It is preferred that graphites constitute 8 - 10% of the structure by volume. The carbides that can be found in the structure are not preferred except the austenitic ductile graphite cast irons.
The basic elements in the composition of spheroidal graphite cast irons are carbon and silicon. The most suitable carbon composition is 3.5 - 3.8%, for silicon it is 2.0 - 2.8%. The amount of Mg required for the formation of spheroidal graphites is usually between 0.04% and 0.06%. If the sulfur content is below 0.015%, 0.035 - 0.04% Mg amount is sufficient. If the amount of Mg is below the specified values, it causes insufficient spheronization and consequently the formation of vermicular graphite in the form of popcorn. The amount of sulfur is preferred to be below 0.02%. The sulfur amount in the composition after casting is approximately 0.015%. If the spheronization process is carried out in the mold, the sulfur amount should be below 0.01%.
In order to prevent carbide formation in spheroidal graphite cast iron, the manganese ratio must be controlled. Since the manganese carbide forming effect is limited to a maximum value that depends on the% Si and the cross section range. In thin castings with a section thickness of 25 mm or more, the carbide forming effect of manganese is compensated by the increase in the amount of silicon. For a casting piece with a section thickness of 12 mm, an increase in the silicon ratio from 2.5% to 3.0% allows the manganese ratio to increase from 0.25% to 0.35%. This kind of manganese increase occurs due to segregation in casting parts with thick sections. Manganese preferentially segregates in the liquid. As the solidification rate decreases, this segregation occurs clearly. It has been proved that in the alloy containing 0.4% Mn on average, the Mg content in the final solidifying liquid can be 2.5% or more. This segregation cannot be prevented by the silicon with the increase of the section thickness, because the silicon segregation is quite high in the first solidification.
In this study, the mechanical properties of spheroidal graphite cast iron with and without alloy were investigated. Then; the strength of spheroidal graphite cast iron, which is unalloyed and containing copper in different proportions, was investigated by rotational-bending fatigue tests.
In experimental studies, it was observed that copper addition to spheroidal graphite cast iron had a positive effect on the mechanical properties of the material such as tensile strength and hardness. In addition, it was found that copper content in GGG50 has an increasing effect on high cycle fatigue strength up to 1.49% as a result of the experiments.
In this respect, it is obtained that the fatigue strength of the material can be increased by adding copper in the application areas where unalloyed spheroidal graphite cast iron is subjected to fatigue forces and it is predicted that the natural fatigue strength will not be sufficient.
As a result of the experimental data, it has been shown that copper additions up to 1.49% improve both the mechanical properties of spheroidal graphite cast iron and increase fatigue strength.