İkiz Merdaneli Sürekli Döküm Yöntemiyle Eloksal Kalite Aa5005 Alüminyum Alaşımı Levha Üretimi Ve Karakterizasyonu
İkiz Merdaneli Sürekli Döküm Yöntemiyle Eloksal Kalite Aa5005 Alüminyum Alaşımı Levha Üretimi Ve Karakterizasyonu
Dosyalar
Tarih
2014-06-27
Yazarlar
Yeşilbağ, Beyza
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Institute of Science and Technology
Özet
Yeryüzünde en çok bulunan metallerden biri olan alüminyum, 19. Yüzyıl sonlarında ayrı ayrı Charles Hall ve Paul Herault tarafından alüminadan elde edilmesiyle uygulamalarda yer almaya başlamıştır. Hafifliği, kolay şekillendirilebilirliği gibi özellikleri sayesinde otomotiv, inşaat, havacılık ve uzay, denizcilik, medikal, gıda gibi pek çok sektörde giderek artan yaygınlıkla kullanılmaktadır. Korozyon dayanımının yüksek olması alüminyumun önde gelen tercih edilme nedenlerinden biridir. Alüminyumun havaya teması ile birlikte üzerinde birkaç nanometre kalınlığında doğal olarak bir oksit tabakası gelişmektedir. Normal koşullarda malzemede korozyonun ilerlemesini engelleyen bu koruyucu tabaka korozif ortamlardaki uygulamalarda yeterli gelmemektedir. Alüminyumun anodizasyonu (eloksal) doğal olarak oluşan oksit tabakasının kontrollü olarak elektrokimyasal yöntemler ile kalınlaştırılması işlemidir. Anodizasyon koruma amaçlı yapıldığı gibi, dekoratif amaçlarla da yapılabilmektedir. Bu amaçla direk olarak alüminyum anodizasyon sonrasında veya renklendirilerek görsel ve estetik uygulamalarda kullanılmaktadır. Anodizasyon işlemi çoğunlukla alüminyuma profil veya levha halindeyken uygulanmaktadır. Alüminyum levha üretimi için günümüzde yaygın olarak kullanılan iki yöntem mevcuttur. Bunların ilki, 1950'li yıllarda ortaya çıkan Direk Soğutmalı Döküm (DC) ve sıcak haddeleme ile levha elde edilmesidir. Direk soğutmalı döküm ile ingot olarak dökülen alüminyumun üst ve alt yüzeyleri frezelenerek katılaşma karakteristiği olan kolonsal yüzey taneleri ile döküm pislikleri giderilir ardından sıcak haddeleme için rekristalizasyon sıcaklığının üzerinde ısıl işlem yapılır. Elde edilen malzemenin yapısı tamamen homojendir. DC döküme alternatif olarak ortaya çıkan ikiz merdaneli döküm (TRC) yöntemi ilk yatırım ve işletme, işçilik, enerji maliyetlerinde yarattığı avantajlardan dolayı levha üretiminde tercih edilen yöntem haline gelmiştir. Kıyaslandığı üretim yöntemine göre daha az proses basamağı içeren ve daha mukavim olan ikiz merdaneli döküm, yöntemin getirdiği bazı karakteristik özelliklere sahiptir. Hızlı katılaşma ve aynı anda haddelemenin yarattığı plastik deformasyon nedeniyle oluşan bu karakteristik özellikler proses esnasında özel olarak kontrol edilmedikçe üretilen levhalar bazı uygulamalarda kullanılamamaktadır. Anodizasyon işlemi için genel olarak direk soğutmalı döküm ve sıcak haddeleme ile üretilen alüminyum levhalar kullanılmaktadır. İkiz merdaneli döküm ile üretilmiş levhalarda dökümden kaynaklı karakteristik bir yüzey izi olan ripple nedeni ile anodizasyon sonrasında beklenen sürekli görüntü elde edilememektedir. Bu çalışma ile ikiz merdaneli sürekli döküm ile anodizasyona uygun kalitede AA5005 alaşımı levha üretimi amaçlanmıştır. Bu doğrultuda döküm parametreleri düzenlenmiş ve farklı döküm sonrası proseslerin anodizasyona etkisi incelenmiştir. İkiz merdaneli sürekli döküm ile üretilen levhaların anodizasyon özellikleri ve özellikle 5005 alaşımının anodizasyon davranışı ile ilgili literatürde çalışma miktarının yok denecek kadar az olması bu çalışmanın özgün tarafını oluşturmaktadır. Çalışma sonucunda TRC ile üretilen alüminyum levhaların anodizasyon işlemi için engel teşkil eden ripple görüntüsünün döküm parametrelerinin kontrol edilmesi ile bitmeye yakın derecede azaltılmıştır. Magnezyum içeriğinden dolayı TRC ile dökümü dikkat gerektiren 5005 alaşımı ile yapılan bu çalışma döküm parametrelerinin iyileştirilmesi ile istenen görüntüye ulaşılabileceğinin mümkün olduğunu göstermiştir.
Aluminium is most widely used non-ferrous metal and one of the most abundant of all elements in the Earth's crust but due to its strong affinity to oxygen it is never found in elemental state. After the time that Charles Hall and Paul Herault obtain the aluminium from alumina in late of 19th century, it has started to be used in various applications. Due to its properties such like lightness, formability; it has a wide range of application area like automotive, construction, aerospace and aviation, marine, medical, food. Aluminium is almost used as alloyed, which improves its mechanical properties markedly. One of the reason that aluminium is favored for many applications is it corrosion resistivity. A few nanometer thick aluminium oxide film is immediately formed on aluminium substrate when exposed to air. The film that formed on substrate is a shield which prevents from further oxidation. At normal conditions, natural film is enough to prevent the material but at corrosive conditions it has to be developed. Anodizing of aluminium (or eloxal) is an electrochemical method that thicken natural oxide layer intentionally. The process gets its anodizing name from because the substrate is placed as anode of an electrical circuit. Anodizing increase corrosion and wear resistance and provides an esthetical appearance. Anodized parts can be dyed with various methods. Due to the formed anodic aluminum oxide layers structure, dyed parts preserves its color and exhibits good resistance to UV lights. Anodizing is limited to the substrates surface and does not increase the strength of the material and can be performed for various purposes like, increasing corrosion resistance at corrosive environments, improving surface hardness and abrasion resistance, improving adhesion for organic coatings, changing optical properties, manipulating dielectric properties. Aluminium profiles and sheets are mostly anodized aluminium parts. There are two common methods to produce aluminium sheets. The first and conventional one is Direct Chill Casting (DC) and hot rolling which has been used since 1950's. The production with this method begins with casting aluminium ingot by direct chill casting. After the ingot completely solidified, it is cut into smaller slabs. The columnar grains and impurities of casting at upper and lower surfaces are removed by scalping. Before hot rolling, slabs are heated over the recrystallization temperature. Finally, hot rolled plates are cold rolled to their final thickness. The sheets cast by DC have a homogenous structure due to scalping and heating operations. Twin roll casting (TRC) is an alternative method for producing aluminum sheets instead of DC. TRC has lower investment and operating costs, lower need for labor which results into being the favored method. To compare with DC, TRC has less process steps; it combines casting and hot rolling in one operation. The cast material are cold rolled after. Due to rolling and solidifying in a very limited time and space, it has higher strength. Scalping and heating processes are eliminated and the scrap rate is lower. Due to the rolling and solidifying at the same and limited time, products have a unique structure and properties. Rapid solidification keeps alloying elements and intermetallics in the solid solution which increases strength. Smaller and oriented grains exist at surface regions whereas equiaxed and bigger grains exist at the center. Because of its advantages TRC became the choice for producing aluminium sheets for many applications such as foils. Nevertheless, for some applications it has limited to use. Unless the production parameters are designed specifically and controlled strictly it takes effort to adapt it for that kind of applications. To be anodized sheets are generally chosen from DC products. Sheets produced with TRC have a surface mark named as "ripple". Ripples look like the lines at sand on the sea side and stand vertical to casting direction. To avoid the tidal moves during casting, parameters should be controlled. Ripple lines are the obtained reason that TRC sheets are not used for anodizing. Despite there are various work about anodizing and TRC separately, there is a lack of work which investigates the anodizing behavior of TRC sheets and 5005 aluminium alloy specifically. With its subject and aim this work distinguishes from other anodizing and TRC literature. This work aims to produce aluminum AA5005 alloy sheets by TRC which are convenient for anodizing treatment. For this purpose, the casting parameters are designed to produce sheets without ripples. To avoid tidal moves and ripples, liquid metal level in tundish and setback distance is decreased. 7 sets of castings are performed and the set with the best surface appearance are chosen for further processes. 1.5 mm thick H22 temper condition sheets are produced by 5 different processes and characterized by optical microscopy. At final conditions, sheets have almost none of ripples. Observing ripples on some of the sheets are thought to be resulted from the earlier castings traces on casting rolls. Sheets are cleaned, etched and anodized in laboratory type anodizing tanks. After anodizing, sheets are first inspected visually, and then imaged by scanning electron microscopy. According to investigations, sheets produced with the processes without homogenization annealing show better result after anodizing. Sheets produced with the 4th process which has recrystallization annealing shows the most homogenized pore structure at SEM images. Results of the experiments showed that changing the liquid level in tundish and setback distance has a critical effect for surface quality. When the tidal moves diminished, ripples are not seen at surface. It was also obtained that sheets produced with different process parameters show different anodizing behavior and homogenization annealing is not a preferred process for producing sheets to be anodized. It is concluded that when the production parameters are designed specially it is possible to produce 5005 alloy aluminium sheets for anodizing by twin roll casting.
Aluminium is most widely used non-ferrous metal and one of the most abundant of all elements in the Earth's crust but due to its strong affinity to oxygen it is never found in elemental state. After the time that Charles Hall and Paul Herault obtain the aluminium from alumina in late of 19th century, it has started to be used in various applications. Due to its properties such like lightness, formability; it has a wide range of application area like automotive, construction, aerospace and aviation, marine, medical, food. Aluminium is almost used as alloyed, which improves its mechanical properties markedly. One of the reason that aluminium is favored for many applications is it corrosion resistivity. A few nanometer thick aluminium oxide film is immediately formed on aluminium substrate when exposed to air. The film that formed on substrate is a shield which prevents from further oxidation. At normal conditions, natural film is enough to prevent the material but at corrosive conditions it has to be developed. Anodizing of aluminium (or eloxal) is an electrochemical method that thicken natural oxide layer intentionally. The process gets its anodizing name from because the substrate is placed as anode of an electrical circuit. Anodizing increase corrosion and wear resistance and provides an esthetical appearance. Anodized parts can be dyed with various methods. Due to the formed anodic aluminum oxide layers structure, dyed parts preserves its color and exhibits good resistance to UV lights. Anodizing is limited to the substrates surface and does not increase the strength of the material and can be performed for various purposes like, increasing corrosion resistance at corrosive environments, improving surface hardness and abrasion resistance, improving adhesion for organic coatings, changing optical properties, manipulating dielectric properties. Aluminium profiles and sheets are mostly anodized aluminium parts. There are two common methods to produce aluminium sheets. The first and conventional one is Direct Chill Casting (DC) and hot rolling which has been used since 1950's. The production with this method begins with casting aluminium ingot by direct chill casting. After the ingot completely solidified, it is cut into smaller slabs. The columnar grains and impurities of casting at upper and lower surfaces are removed by scalping. Before hot rolling, slabs are heated over the recrystallization temperature. Finally, hot rolled plates are cold rolled to their final thickness. The sheets cast by DC have a homogenous structure due to scalping and heating operations. Twin roll casting (TRC) is an alternative method for producing aluminum sheets instead of DC. TRC has lower investment and operating costs, lower need for labor which results into being the favored method. To compare with DC, TRC has less process steps; it combines casting and hot rolling in one operation. The cast material are cold rolled after. Due to rolling and solidifying in a very limited time and space, it has higher strength. Scalping and heating processes are eliminated and the scrap rate is lower. Due to the rolling and solidifying at the same and limited time, products have a unique structure and properties. Rapid solidification keeps alloying elements and intermetallics in the solid solution which increases strength. Smaller and oriented grains exist at surface regions whereas equiaxed and bigger grains exist at the center. Because of its advantages TRC became the choice for producing aluminium sheets for many applications such as foils. Nevertheless, for some applications it has limited to use. Unless the production parameters are designed specifically and controlled strictly it takes effort to adapt it for that kind of applications. To be anodized sheets are generally chosen from DC products. Sheets produced with TRC have a surface mark named as "ripple". Ripples look like the lines at sand on the sea side and stand vertical to casting direction. To avoid the tidal moves during casting, parameters should be controlled. Ripple lines are the obtained reason that TRC sheets are not used for anodizing. Despite there are various work about anodizing and TRC separately, there is a lack of work which investigates the anodizing behavior of TRC sheets and 5005 aluminium alloy specifically. With its subject and aim this work distinguishes from other anodizing and TRC literature. This work aims to produce aluminum AA5005 alloy sheets by TRC which are convenient for anodizing treatment. For this purpose, the casting parameters are designed to produce sheets without ripples. To avoid tidal moves and ripples, liquid metal level in tundish and setback distance is decreased. 7 sets of castings are performed and the set with the best surface appearance are chosen for further processes. 1.5 mm thick H22 temper condition sheets are produced by 5 different processes and characterized by optical microscopy. At final conditions, sheets have almost none of ripples. Observing ripples on some of the sheets are thought to be resulted from the earlier castings traces on casting rolls. Sheets are cleaned, etched and anodized in laboratory type anodizing tanks. After anodizing, sheets are first inspected visually, and then imaged by scanning electron microscopy. According to investigations, sheets produced with the processes without homogenization annealing show better result after anodizing. Sheets produced with the 4th process which has recrystallization annealing shows the most homogenized pore structure at SEM images. Results of the experiments showed that changing the liquid level in tundish and setback distance has a critical effect for surface quality. When the tidal moves diminished, ripples are not seen at surface. It was also obtained that sheets produced with different process parameters show different anodizing behavior and homogenization annealing is not a preferred process for producing sheets to be anodized. It is concluded that when the production parameters are designed specially it is possible to produce 5005 alloy aluminium sheets for anodizing by twin roll casting.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2014
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2014
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2014
Anahtar kelimeler
Alüminyum,
Anodizasyon,
İkiz Merdaneli Sürekli Döküm,
Aluminum,
Anodization,
Twin Roll Casting