AT 33 magnezyum alaşımına Ti ve Nd elementleri ilavesinin özelliklere etkisinin incelenmesi = Investigation of the effect of addition of Ti and Nd elements to AT 33 magnesium alloy on properties

Abstract

Yüksek özgül mukavemeti ve hafiflik özelliklerinden dolay magnezyum alaşımları, başta savunma sanayi ve taşımacılık sektörü olmak üzere otomotiv, bilgisayar, havacılık gibi birçok sektörde tercih edilmektedir. Magnezyumun iyi dökülebilirliği, yüksek sönümlenmesi, iyi işlenebilmesi, yüksek ısıl iletkenliği gibi bütün bu özellikleri magnezyumu kullanıma uygun bir metal yapmaktadır. Fakat magnezyum, alaşımsız olarak düşük mukavemet ve kırılma tokluğu değerlerine sahip olmasından dolayı genellikle alaşımlandırılarak kullanılmaktadır. Bu çalışmada AT 33 Magnezyum Alüminyum ve Kalay alaşımına Titanyum (Ti) ve Nadir Toprak Elementi Neodyumun (Nd) ilave edilerek alaşımın mikroyapısındaki ve mekanik özelliklerindeki değişimin incelenmesi amaçlanmıştır. Alaşımların üretiminde Mg (99,85), Al (%99,90), Sn (%99,90), Al-6Ti ve Mg-20Nd malzemeleri kullanılarak kokil döküm yöntemi ile üretimler gerçekleştirilmiştir. Döküm işlemi sırasında koruyucu gaz olarak CO2 +%2SF6 kullanılmıştır. AT 33 alaşımına ayrı ayrı ağırlıkça %0,05-0,1-0,2 oranlarında titanyum ve neodyum elementi ilaveleri yapılmış ve üretilen tüm alaşımlar üzerinde mikroyapı ve çekme testleri uygulanmıştır. Elde edilen her bir alaşım için çekme testleri uygulanmıştır. Yapılan deneyler sonucu alaşımların mikroyapısal özellikleri, taramalı elektron mikroskobu (SEM) ve optik mikroskobu kullanılarak incelenmiş ve sonuçları yorumlanmıştır. Yapılan çalışma sonucunda, Mg3Sn3Al alaşımının mikroyapısı; alaşımın α-Mg dendiritlerinden, ötektik ağdan ve Mg2Sn intermetalik fazlarından oluştuğu tespit edilmiştir. Mg3Sn3Al alaşımının tane boyutu analizi ile α-Mg dendiritlerin ortalama tane boyutu 50-80 μm olarak hesaplanmıştır. Titanyum ilavesinin AT 33 alaşımı mikroyapısını modifiye ettiği, α-Mg dendiritlerinin boyutlarında azalmalara neden olduğu ve çekme mukavemeti değerlerini artırdığı tespit edilmiştir. Bununla birlikte, Neodyum ilavesi ile de AT 33 alaşımında mikroyapının incelmesi ile birlikte yapıda intermetalikler oluşturarak çekme mukavemetinde artışlar neden olduğu bulunmuştur. Mg3Sn3Al alaşımının çekme mukavemeti değeri 150 MPa olarak bulunmuştur. Mg3Sn3Al alaşımına ağırlıkça %0,1'e kadar titanyum ve neodyum ilavesinin, alaşımın çekme mukavemeti değerlerini artırmıştır. Çekme deneyi sonucu kırılma yüzeyleri incelendiğinde, Ti ve Nd katkısının Mg3Sn3Al alaşımının gevrek kırılmadan yarı sünek- gevrek kırılma davranışına uğradığı görülmüştür. Ayrıca yapılan çekme testleri sonucunda Ti ve Nd ilavesinin Mg3Sn3Al alaşımının % uzama değerlerinin arttığı tespit edilmiştir. Yapısal bileşenlerin hafif olması, maliyetleri düşürmek, çevresel etkiyi ve enerji tüketimini azaltmak için oldukça önemlidir. Bunu başarmak için çelik gibi malzemelerin alüminyum veya magnezyum gibi daha az yoğunluğa sahip malzemelerle değiştirirek yapısal bileşenleri hafifletmek en popüler yöntemlerden birisidir. Bununla birlikte, bu gibi daha hafif malzemelerin her zaman yapısal uygulamalarda mukavemet gereksinimlerini karşılayamaz ve bu nedenle mekanik özelliklerinin iyileştirilmesi gerekmektedir. Bu nedenle mekanik özellikleri iyileştirmek için daha fazla malzeme geliştirmeye ihtiyaç duyulmaktadır. Ayrıca bu tez, kokil kalıba döküm yöntemi ile üretilen magnezyum alaşımlarının çökelme sertleşmesi, ısıl işlem öncesi ve sonrası alaşımların mikroyapısı ve mekanik özelliklerinin incelenmesini içerir. Üretilen alaşımlardan bir kısmı oda sıcaklığında diğer kısmı ise çökelme sertleşmesi işlemi uygulanarak çekme testine tabi tutulmuştur. Daha yüksek mukavemet ve deformasyon kabiliyetine sahip Mg alaşımları üreterek alaşımların mekanik özellikleri, mikroyapısı ve ısıl işlem çerçevesinde karakterizasyon işlemi yapılmıştır.

Due to their high specific strength and lightness properties, magnesium alloys are preferred in many sectors such as automotive, computer and aviation, especially in the defense industry and transportation sector. All these properties of magnesium such as good castability, high damping, good machinability and high thermal conductivity make magnesium a suitable metal for use. However, since magnesium has low strength and fracture toughness values as unalloyed, it is generally used by alloying. In this study, it was aimed to investigate the change in the microstructure and mechanical properties of the alloy by adding Titanium (Ti) and Rare Earth Element Neodymium (Nd) to the AT 33 Magnesium Aluminum and Tin alloy. In the production of alloys, Mg (99.85), Al (99.90%), Sn (99.90%), Al-6Ti and Mg-20Nd materials were used in the production by permanent casting method. CO2+%2SF6 was used as shielding gas during the casting process. Titanium and neodymium elements were added separately at the rate of 0.05-0.1-0.2% by weight to the AT 33 alloy, and microstructure and tensile tests were applied on all alloys produced. Tensile tests were applied for each alloy obtained. As a result of the experiments, the microstructural properties of the alloys were examined using scanning electron microscope (SEM) and optical microscope and the results were interpreted. As a result of the study, the microstructure of Mg3Sn3Al alloy; It was determined that the alloy consists of α-Mg dendrites, eutectic network and Mg2Sn intermetallic phases. The average grain size of α-Mg dendrites was calculated as 50-80 μm by grain size analysis of Mg3Sn3Al alloy. It was determined that the addition of titanium modified the microstructure of the AT 33 alloy, caused a decrease in the size of the α-Mg dendrites and increased the tensile strength values. However, with the addition of Neodymium, it was found that the AT 33 alloy caused an increase in tensile strength by forming intermetallics in the structure with the thinning of the microstructure. The tensile strength value of Mg3Sn3Al alloy was found to be 150 MPa. The addition of titanium and neodymium up to 0.1% by weight to the Mg3Sn3Al alloy increased the tensile strength values of the alloy. When the fracture surfaces were examined as a result of the tensile test, it was observed that the Mg3Sn3Al alloy of Ti and Nd additives underwent semi-ductile-brittle fracture behavior without brittle fracture. In addition, as a result of the tensile tests, it was determined that the % elongation values of Mg3Sn3Al alloy increased with the addition of Ti and Nd. For the heat treatment of 7 tensile samples taken from Mg alloys produced by the gravity casting method, the samples were first placed in a stainless-steel box. The stainless-steel box is filled with graphite powder to prevent the samples from oxidizing. After placing the tensile samples in the furnace in this way, the samples were kept in the single-phase zone at 430°C for approximately 24 hours. Then, all the phases (α and β) in the alloy were dissolved and instant cooling was applied to the samples one by one in order to prevent the formation of precipitates from the solid solution obtained as a single phase (α). Then, the samples were kept in an oven at 175°C for approximately 100 and 300 hours. Then, the samples were taken and aging was carried out by cooling them at room temperature (air). When we examine the microstructures of the alloys after heat treatment, it seems that they are quite different from the microstructures before the heat treatment. After heat treatment, the grains in the microstructures are separated from each other. Although the strength of the alloys whose microstructure was changed with the applied heat treatment did not change much, the amount of deformation increased. As a result, a more spherical microstructure was obtained after heat treatment. The lightness of structural components is very important to reduce costs, reduce environmental impact and energy consumption. To achieve this, one of the most popular methods is to lighten structural components by replacing materials such as steel with less dense materials such as aluminum or magnesium. However, such lighter materials cannot always meet the strength requirements in structural applications and therefore their mechanical properties need to be improved. Therefore, further material development is needed to improve mechanical properties. In addition, this thesis includes the precipitation hardening of magnesium alloys produced by the permanent mold casting method, the microstructure and mechanical properties of the alloys before and after heat treatment. Some of the alloys produced were subjected to tensile testing at room temperature, while the other part was subjected to precipitation hardening. By producing Mg alloys with higher strength and deformation ability, the characterization process was carried out within the framework of the mechanical properties, microstructure and heat treatment of the alloys. Non-ferrous alloys of titanium, aluminum and magnesium have wide applications in technological fields. Due to their unique properties, they are considered the most suitable alternatives to steel in structural applications. Among them, magnesium and its alloys constitute the lightest structural metals. With the development of technology, the use of Mg alloys in structural and biomedical applications is increasing day by day. In the formation of alloys, the compatibility of Mg with its alloys in other engineering materials was investigated. Classification of Mg alloys according to machining techniques can be categorized as cast magnesium alloys, wrought magnesium alloys for engineering applications. Magnesium alloys produced by mold and other casting techniques are casting alloys. In the recent past, cast alloys have been used for commercial and industrial applications. Although high pressure permanent mold casting is popular, sand or permanent mold casting methods are also used in the production of some Mg parts. With the sand-casting method, parts weighing up to 1.5 tons can be produced. Here, the cost of production is low, but since Mg has low density, its nutritious and runner design requires significant expertise. In permanent mold casting, it enables more solid parts to be produced because it provides fast solidification. In the past years, magnesium alloys have been used as a building material, especially in warplanes. In the following periods, Mg alloys were replaced by aluminum alloys due to reasons such as the amount of ore and production cost. However, recently, the use of Mg alloys has been brought to the agenda again due to increasing CO2 emissions and decreasing energy efficiency, lightness and special strength. Mg alloys, which are mainly used in automotive industry and aviation applications, are indispensable in our daily lives, even in the cases of devices such as mobile phones and laptops. The aim of this thesis is primarily to produce AT-33 Mg alloys containing Ti and Nd by casting method. The produced alloys were then heat treated to produce materials that can be easily formed, have higher strength and better corrosion resistance and wear behavior compared to the AT-33 Mg alloy. It aims to better understand the effect of alloying elements on the deformation behavior of Mg alloys. Four representative alloying elements were selected from among the various elements used in Mg alloys; The most commonly used alloying elements in Mg alloys are Al and Sn, Ti, which is added to Mg alloys and acts as a grain refiner, albeit slightly, and Nd, one of the rare earth elements used in many Mg alloys. Binary alloys containing these elements were cast in similar grain sizes and subjected to various thermomechanical processes. Deformation behaviors were analyzed by in situ SEM and EDS. The aim of this thesis is to produce magnesium alloys with higher strength and deformation ability by characterizing alloys within the framework of mechanical properties, microstructure and heat treatments.