Genleştirilmiş cam takviyeli sintaktik köpük malzemenin üretimi, içyapı ve mekanik özelliklerinin incelenmesi
Production and characterġzation of syntactik metalfoam
Yazar: BÜŞRA ŞAHİN
Danışman: DOÇ. DR. ALİ GÖKŞENLİ
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

Özet:
Köpük metaller içerisinde gözenekli yapı barındıran malzemelerdir. Hacimlerinin yaklaşık %65-90'ı gözeneklerden oluşmaktadır. Gözenek yüzeylerinin birbiri ile bağlantılı olduğu malzemelere açık hücreli köpük metal denirken, gözenek yüzeylerinin bağlantılı olmadığı malzemelere de kapalı hücreli köpük metaller denmektedir. Bu özelliklerinden dolayı açık hücreli köpük metaller ses emilimi, ısı akış transferi gibi fonksiyonel uygulamalarda kullanılırken, kapalı hücreli köpük metaller darbe dayanımı gerektiren yapısal uygulamalarda sıkça karşımıza çıkmaktadır.
Günümüzün ihtiyaçları da göz önünde bulundurulduğunda, köpük metallerin hafif olması, enerji ve darbe emiliminin iyi olması, basmaya dayanıklı olması gibi pek çok rekabetçi özellikleri bu malzemeyi ön plana çıkartmaktadır. Geniş alanlara hitap eden bu yenilikçi malzemenin uçak-uzay, demiryolu, inşaat, otomotiv vb gibi pek çok alanda uygulamalarının olduğunu görmekteyiz.
Köpük metaller çok farklı malzemeler ve çok farklı teknikler ile üretilebilmektedir. Günümüz dünyasının sıkça değişen koşullarına ayak uydurabilmek için pek çok farklı üretim yöntemleri denenmekte ve geliştirilmektedir. Her bir yöntemin kendine has avantajları ve dezavantajları bulunkatadır. Üretim yaparken bunlar göz önünde bulundurularak en uygun yöntem seçilir ve köpük metal üretimi gerçekleştirilir. Hala geliştirilmekte olan sintaktik köpük metal üretim yönteminde organik granül, inorganik granül ve içi boş kürelerin ikame malzeme olarak metal alaşımlarının da matris malzeme olarak kullanılmaktadır. İnfiltirasyon ve döküm yöntemleri ile üretim gerçekleştirilir. İnfiltrasyon yönteminde gözeneklerin homojen dağılması ve bu yöntemin yüksek hacimli parça üretme olanağını sağlaması bu yöntemin avantajları arasında yer almaktadır. Kolay uygulanabilirlik ve düşük bütçeli olması da döküm yönteminin avantajlarıdır. Genel olarak çalışmalarda matris malzeme olarak alüminyum, ikame malzeme olarak ta seramik küreler kullanılmıştır. Elde edilen ürünün mekanik özellikleri incelenmiştir.
Bu çalışmada amaç yapısal uygulamalarda sıkça kullanılan 7075 alüminyum alaşımı ve ikame malzeme olarak genleştirilmiş cam küreler kullanılarak sintaktik köpük metal üretimi yapılmaktır. Genleştirilmiş cam kürelerin içerisine eritilmiş alüminyum dökülüp ve ardından üzerine basınç uygulayarak üretim sağlanacaktır. Üretilen ürünlerin mekanik özelliklerini incelemek için basma deneyleri yapılacaktır. Çalışmada proses parametrelerinin incelenmesi ve optimize edilmesi hedeflenmektedir.

Abstract:
Foam metals are materials that contain porous structure. This porous structure is consisted of approximately the volume of 65-90%.
If the pore surfaces are connected to each other, the materials are called open-cell foam metal. When the pore surfaces are not connected, the materials are called closed-cell foam metals. Because of these properties open-cell foam metal are used in functional applications such as sound absorption, heat flow transfer, while closed cell foam metals are frequently used in structural applications requiring impact resistance.
To take into consideration today's needs, foam metals come to the forefront because of their competitive properties as light weight, good energy and impact absorption, good compressive behavior etc.
It is seen that this innovative material has applications in many fields such as aircraft-space, railway, construction, automotive, biomedical, ship etc.
Metal foams can be produced with plenty techniques with using different types of materials. Still lots of production method are tried to develop in order to keep up with the frequently changing conditions of today's world.
Each production method has its own advantages and disadvantages. The most suitable production method is selected by taking these advantages and disadvantages into consideration.
In syntactic foam metal production technique that still, continuous to progress, organic granules, inorganic granules and hollow spheres are used as spaceholder materials and metallic alloys are used as metal matrix.
There are two different type of production; infiltration technique and casting technique. Both two technique have some advantages. In filtration technique, the main advanteages is homogeneous distribution of pores. Other advantages is that this technique gives an opportunity to produce parts that have high volume. When infiltration technique is compared that casting technique is more applicable and cheaper way for production.
Besides these; the most important advantages of syntactic production way is that pore sizes can be controlled with spaceholder materials. The percentage of matrix volume can be controlled with the size of spaceholder materials.. In addition to these,
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syntactic production way is very flexible to produce parts that have different geometrical shapes.
Generally, in studies aluminum is used as matric material, ceramic spheres are used as spaceholder material, and mechanical properties are investigated.
When syntactic foam metals are compared with other types of foam metals, it is seen that has a few superior properties such as high yielding point, more homogenous mechanical properties and high-energy absorption related with plateu.
Plateau is most important property for foam metals.Plateau stress is that the horizontal region in the strain strain curve. It represent that how much energy can be absorbed by the material. When the compressive stress is applied, the material shows plateu because of the pores structure.
The purpose of this study is producing syntactic metal foam from 7075 aluminum that is mostly used material in structural applications and expanded glass spheres.
Expanded glass spheres that are supplied by Omnis Kompozit are produced by recyclable glass with foaming technique. Expanded glass spheres have good compressive strength, good thermal conductivity coefficient and humidity resistance. 7075 aluminum alloy has high stiffness, high strength and good machinability.
Melt aluminum is poured into expanded glass spheres then pressure is applied. Compression tests will be done to investigate mechanical properties of final product. In this study, it is aimed that to investigate and optimize process parameters such as sphere sizes, amount of spheres and sequences of process.
As a result of this study, syntactic foam metals were produced using with expanded glass spheres in successfully. The most proper process parameters are defined with an optimization during production.
Totally five specimen are investigated to define mechanical properties. The data obtained are as follow:
The lowest density that is obtained is 1,33 g/cm^3 and the highest density is 1,68 gr/cm^3. When the density of the prodeced syntactic foam metals and the density of 7075 aluminum alloy were compared, it was found that syntactic foam metal has much lower value than 7075 aluminum alloys. It is decreased from 2.80 g / cm ^ 3 to 1.33 g / cm ^ 3.
It is seen that all produced parts show plateau behaviour. The material that has highest density shows lowest plateau stress and lowest energy absorption.
According to microstructure analysis, expanded glass spheres are distrubuted homogeneously. Even it is seen that porous structure in spheres are preserved.
Syntactic foam metal production is an open area to investigate. As future studies, this research can be widen with lots of new researches. For example, it can be produced with using different sized spheres to investigate the effect of sphere sizes on mechanical properties. Different types of particles such as alumina, pumice stone, ash etc. can be studied. New production techniques such as 3D printing can be developed.
Beside development of production techniques, produced parts can be heat treated and it can be examined how heat treatment effects mechanical properties. Also machinability properties can be investigated. Thereby optimized the machining parameters, a huge area can be discovered.
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As a result, pores structure of metal foams gives a big opportunity in lots of industry. Eventhough there are lots of production techniques, it is continued that new production techniques are still developed. It is an open subject to develop in lots of ways.