Polimer emdirme yöntemi ile alüminyum 6063 alaşımı esaslı açık hücreli köpük üretimi ve karakterizasyonu
Production and characterization of open cell aluminum foams by polymer replication method
Yazar: CEREN YAĞŞİ
Danışman: PROF. DR. ÖZGÜL KELEŞ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Günümüzde otomotiv, uzay – uçak, savunma gibi sektörler incelendiğinde, en çok istenen malzeme özellikleri hafif, yüksek mukavemet, yüksek darbe direnci, yüksek yük taşıma kapasitesi gibi özellikler olduğu görülmektedir. Bu bağlamda, yaygın olarak kullanılan malzemeler incelendiğinde bu özellikleri karşılayan malzeme gurubunun metalik köpükler olduğu görülmektedir. Metalik köpükler %75 ile %90 arasında gözeneklerden oluşan, rijit metal veya alaşımlardır. Gözenekler birbirine bağlantılı bir şekilde bulunuyorsa açık hücreli, gözenekler duvarlar ile birbirlerinden ayrılıyorsa kapalı hücreli köpük olarak adlandırılmaktadır. Mukavemet/ yoğunluk oranının yüksek olmasından dolayı açık hücreli köpükler tercih edilmektedir. Açık hücreli köpükler polimer köpük ile hassas döküm, metal enjeksiyon kalıplama, boşluk tutucular etrafına döküm, toz boşluk tutucu ve polimer emdirme yöntemleri ile üretilmektedir. Prosesin kolay olması, hammaddelerin ucuz olması ve gözenek boyutunun ayarlanabilir olmasından dolayı polimer emdirme yöntemi üzerine çalışmalar yapılmaktadır. 
Alüminyum ve alaşımları köpürtülebilir metal gurupları arasından düşük yoğunluk, yüksek mukavemet, enerji absorplama kabiliyeti gibi özellikleri ile ön plana çıkmaktadır. Yüksek mukavemet ve hafifliğin ön planda olduğu sektörlerde saf alüminyum yeterli olmadığından çeşitli alüminyum alaşımlarından köpük üretilmesi gerekli hale gelmektedir. Alüminyum alaşımları incelendiğinde 6xxx serisi alaşımların sektörler tarafından talep edilen özellikleri karşıladığı görülmektedir. Isıl işlemler uygulanarak bu alaşımların mukavemet ve sertlikleri arttırılabilmektedir. 
Köpük üretim işleminde en kritik aşama sinterleme prosesi olmaktadır.  Bu proseste model malzeme (polimer sünger) yapıdan uzaklaşması ve alüminyumun sinterlenerek köpüğün mukavemetlenmesi sağlanmaktadır. Geleneksel sinterleme yöntemleri incelendiğinde sinterleme sürelerinin uzun saatler sürdüğü görülmektedir. Sinterleme süresini azaltmak amacıyla indüksiyon sinterleme yöntemi alternatif olarak ortaya çıkmaktadır.
Bu çalışmada, polimer emdirme yöntemi ile 6063 alüminyum alaşımından açık hücreli köpük üretimi gerçekleştirilmiştir. İlk olarak belirli boyutlarda kesilen poliüretan süngerler hazırlanan çamur karışımına daldırılmış ve polimerin çamurla kaplanması sağlanmıştır. Üretilen bu süngerler ilk olarak 24 saat oda sıcaklığında kurutulmuş daha sonra 120oC, 6 saat vakumlu fırında bekletilmiştir. Üretilen köpüklere mukavemet kazandırılması için sinterleme işlemleri gerçekleştirilmiş olup önce yapıdan model malzeme giderilmiş daha sonrasında ise sinterleme işlemi gerçekleştirilmiştir. Bu proses indüksiyon sinterleme yöntemi ile gerçekleştirilmiş olup kısa sürede mukavemeti ve sertliği yüksek köpükler elde edilmesi amaçlanmıştır.
Deneylerde kullanılan alüminyum tozun SEM ve EDS analizi,  partikül boyut dağılımı, XRD analizi, DSC analizi yapılmıştır. XRD analizinde tozun 6063 alüminyum alaşımı olduğu görülmüş olup, SEM görüntülerinden tanelerin yuvarlak şekilli olduğu gözlemlenmiştir. Üretilen köpüklerin XRD, SEM ve EDS analizleri yapılmış, oluşan fazlar ve oksit miktarları incelenmiştir. XRD analizinde ısıl işlemler sonucu oluşan fazlar gözlenmiştir. Sertlik ve basma testleri malzemelerin mekanik özelliklerinin belirlenmesi amacıyla gerçekleştirilmiştir.

Abstract:
In the nature many materials are porous forms. Even in human body porous structure material which are bones carries the whole weight of the body. This structure provides materials light weight especially high strength / density ratio. Lately, humankind is realized the importance of this structure and started to use it widely. Compared to dense solid materials porous materials have superior properties, among them most important one is high strength with less material.   Nowadays, automotive, aerospace, defense and other sectors demand some material features such as light weight, high strength, high impact resistance and high load carrying capacity. In this context, when the commonly used materials are examined, it is seen that the material group meeting these properties is metallic foams. 
Metallic foams are stiff metals or alloys which contain porosities between 75% and 90%. If the pores are interconnected, they are called open-celled, and if the pores are separated from each other by walls, they are called closed-cell foam. Closed cell structures form when the gaseous phase is kept inside the pores so a continuous solid phase and a discontinuous gaseous phase form. On the other hand, open cell structure is characterized by a continuously dispersed gas phase into a solid matrix. Open cell foams are mostly preferred due to their high strength/density ratio. These two types of foams are used in different applications due to their physical and mechanical properties. Closed cell foams are mostly used in structural applications such as sport equipments, automotive and aircraft parts, because their strength and load carrying capacities are higher than the open cell foams. Open cell foams are used in filters and heat exchangers because their open cells provides them high surface area. Also, in these applications high strength is not required. 
Open cell metallic foams are produced by investment casting with polymer foams, casting around space holder, metal injection molding, powder space holder technique and polymer impregnation methods. Studies on polymer impregnation method has been done extensively due to easy process, cheap precursors and adjustable pore size. The strength of these foams is directly related with the pore size that is why adjusting pore size is essential. The strength of the foam is easily arranged by changing the pore size of the polymer model material. 
When the steps of this process are preparation of the slurry, coating polymer model with the slurry homogeneously, drying and sintering. Even though it is simple, the difficulty of this polymer impregnation method is arranging the rheological properties of the slurry. It is desired to keep the prepared sludge mixture uniformly on the walls of the model material and leave a dense structure behind after sintering. Slurry mixtures containing 50-70% of solids are generally preferred in order to achieve this desired state. In less viscous mixture there are problems in holding the mixture to the model material. If the viscosity is higher than this, it prevents the mixture from penetrating homogeneously to every point of the model material. Coating of the model uniformly is important because studies show that viscosity of the slurry has direct effect on strength. In order to find optimum slurry composition, it is necessary to make a few trials. 
Aluminum and its alloys stands forward among the foaming metal groups with their low density, high strength, ability to absorb energy. In sectors where high strength and lightweight are preliminary, pure aluminum cannot meet the requirements and it becomes necessary to produce foam from various aluminum alloys. When aluminum alloys are examined, 6xxx series alloys appear to meet the specifications demanded by the sectors. By applying heat treatments, strength and hardness of these alloys can be increased. In the 6xxx series, 6063 alloy is widely used due to easy production process and especially high strength. In order to increase strength and hardnes of this alloy heat treatments applied. 
This heat treatment involves the stages of solutioning, quenching and aging. During the dissolving process, the alloy is heated to a temperature below the eutectic temperature and is held at this temperature for a period of time sufficient for all the phases to dissolve in the matrix, thereby forming a homogeneous phase. The quenching operation is to rapidly supercool the single phased alloy to become saturated. When the cooling is done fast, there is no phase transformation and the solid solution becomes oversaturated without phase transformation. The aging process is applied to stabilize the oversaturated unstable phase. When this heat treatment is applied to the 6063 aluminum alloy, the alloy elements (Mg and Si) incorporated in the structure become resistant due to precipitation in the aluminum matrix and preventing dislocation movements.
The most critical stage in the foam production process is the sintering. In this stage, model material (polymer sponge) is removed from the structure and the aluminum is sintered to provide strength of foam.. After the drying process is performed, it is ensured that the organic materials (dispersant, binder and solvents) and polymeric model material added to the slurry  are removed from the system by burning. Depending on the reactivity of the metal powder, the treatment can be carried out in an open atmosphere, in a suitable gaseous atmosphere or under vacuum. The heating rate is a major factor of the removal of the model from the structure. The heating rate must be well controlled and kept low to avoid collapsing during the evaporation of the polymer, residual stress and cracking. After this step, hollow beams are formed and normal sintering is required. The parameters of the sintering process vary depending on the type of metal powder used. Since the bonding of the powders to each other in the polymer impregnation method is achieved only by sintering, the mechanical properties of the material obtained by this method are closely related to the sintering parameters.
The sintering furnaces in which conventional sintering processes are performed allow the use of the sintering atmosphere created according to desired characteristics while controlling the temperature and time during the sintering process. However, the long process times and high dust consumption of conventional sintering processes in electric or gas furnaces are disadvantageous for industrial use. In order to prevent this disadvantage, sintering methods with different heating methods such as sintering, plasma sintering, laser sintering and induction sintering have been developed by means of microwaves and process times have been shortened considerably. The technique of sintering metallic powders by induction current, which is a very fast and effective method than these sintering techniques, is a new process and there are not many studies on it yet. In the induction sintering process, unlike conventional methods, it is the principle of heating the material directly without the need for additional heat sources. This principle draws more attention to daylight as a new variable in powder metallurgy research and industry. This interest in the induction sintering process is due to the fact that the sintering process can be carried out at high temperatures, the adjustability of the heating and cooling rates, the process control capability and the high energy efficiency.
In this study, open cell foam production from 6063 aluminum alloy was performed by polymer impregnation method. First, the polyurethane sponges cut in certain sizes were immersed in the prepared slurry mixture and the polymer was covered with it. In the slurry production, 6063 aluminum powder, pure water used as solvent, PVA (polyvinyl alcohol) used as a binder with great effect in slurry viscosity, and Dolapix (polycarboxylic acid) used as a dispersant to prevent precipitation and clumping. These sponges were first dried at room temperature for 24 hours and then dried in a vacuum oven at 120 ° C for 6 hours. The sintering process was carried out in order to give strength to the produced foams, first the model material was removed and then the sintering process was performed. This process has been realized by induction sintering method and it is aimed to obtain foam with high strength and hardness in a short time. Produced aluminum foams were heat treated by using the same induction furnace. 
The aluminum powder used in the experiments SEM and EDS analysis, particle size distribution, XRD, DSC analysis were performed. XRD analysis showed that the powder was 6063 aluminum alloy, and SEM images showed that the particles were rounded. The produced foam XRD, SEM and EDS analyzes performed, the resulting phases and oxide content were examined. Also, optical microscope images were examined to observe phases. In XRD analysis, phases were observed which resulted from heat treatment. Hardness and compression tests were performed to determine the mechanical properties of the materials.