Alüminyum alaşımlarına uygulanan  mikroark oksidasyon ve anodik oksidasyon işlemlerinin karşılaştırılması
Comparison between micro arc oxidation and  anodic oxidizing technichs on aluminium alloys
Yazar: MERİH ÖVÜNDÜR
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Aluminyum alaşımları hafif metal olusu ile tanınmaktadır ve başta otomotiv, inşaat, uzay, havacılık vb. pek çok sektörde uygulama alanı bulan ve oldukça ilgi çeken bir mühendislik malzemesidir. Düşük yoğunluğuna rağmen iyi bir korozyon direnci, yüksek termal ve elektriksel direnç gösterirler. Bu özelliklerine karşın; düşük aşınma direnci ve sertliğe sahip olması; alüminyum alaşımlarının endüstride daha yaygın bir şekilde kullanılmasının önüne geçmektedir.
Bu doğrultuda geleneksel anodik oksidasyon işlemi geliştirilmiş ve alüminyum alaşımları için alternatif bir yüzey modifikasyon tekniği olarak dikkatleri çekmiştir. Geleneksel anodik oksidasyon ile nispeten yüksek bir kaplama sertliği ve aşınma direnci elde edilmiş olsa da; halen endüstriyel uygulamalardaki beklentilerin çok altında bir aşınma direncine sahip olunmuş ve yöntem yetersiz kalmıştır.
Mikroark oksidasyon yöntemi ise hafif metallerin üzerinde daha kalın bir oksit kaplamasının elde edilebilmesi adına geliştirilmiş ve anodik oksidasyon mantığına dayanan elektrokimyasal bir proses olarak öne çıkmıştır. Buna karşın alüminyum alaşımlarının mikroark oksidasyonu; anodik oksidasyona kıyasla halen endüstriyel uygulamalarda sınırlı bir kullanım alanına sahiptir.  
Bu çalışmada AlSi9Cu3(Fe) döküm alaşımı ve AlMg3 (5754 kalite) dövme alaşımı üzerine ; ticari bir firmada endüstriyel ve özel parametreler ile yaptırılan anodik oksidasyon işlemi sonucu elde edilen oksit tabakası ile laboratuvar koşullarında üretilmiş mikroark oksidasyon yöntemine ait oksit tabakalarının morfolojileri ve tribolojik performansları karşılaştırılmıştır. Mikroark oksidasyon yönteminde voltaj kontrolü, akım kontrolü ve işlem süresinde değişiklikler yapılarak farklı kalınlık ve morfolojilerde oksit tabakaları elde edilmiştir. 
Tedarikçi firmada kaplanan malzemeler ile laboratuvar ortamında kaplanan mikroark oksidasyon numunelerinin yüzey incelemeleri, kesit incelemeleri, x-ışınları analizi, yüzey pürüzlülüğü tespiti, üç nokta eğme testi yapılmış, akabinde tribolojik performansların karşılaştırılması adına karsıt hareketli aşınma cihazı ile kuru ortamda aşınma testleri gerçekleştirilmiştir.Yapılan deneyler ve testler doğrultusunda; günümüz endüstrisinin en yaygın uygulamalarından biri olan alüminyum ve alaşımlarının anodik oksidasyon yöntemi ; gelişmekte olan mikroark okdisayonu yöntemi ile karşılaştırılmıştır.
Deney sonuçları değerlendirildiğinde ; alüminyum döküm ve dövme alaşımlarının ticari ve özel eloksal parametreleri sonucunda yüzeyde amorf bir oksit tabakası elde edildiği tespit edilirken mikroark oksidasyonu ile kalın bir kristalin oksit tabakası elde edildiği sonucuna varılmıştır.
Aşınma testlerinin sonuçları incelendiğinde ; hem alüminyum döküm hem de alüminyum dövme alaşımları için mikroark okdisayon yöntemi ile anodik oksidasyon yöntemine göre çok daha yüksek aşınma direncine sahip bir koruyucu oksit film tabakasının malzemelerin yüzeyinde oluşturulduğu görülmüştür.

Abstract:
Aluminium alloys are very attractive engineering materials for many applications and had been found many application due to their low density, good corrosion resistance, enhanced specific strength, high thermal and electrical conductivities. Hundreds of alloys and tempers make aluminium alloys useful for a very wide variety of applications from aircraft, aerospace, automotive, and shipbuilding to household products including pots, pans, and cooking utensils. Newer applications, such as drive shafts, radiators, cylinder heads, and suspension systems have proven to be most advantageous when dealing with weight vs strength vs cost consideration Despite these attractive properties ; the low wear resistance and relativelty low hardness limit alumunium alloy's more extensive use in the wear industry. 
In this respect, anodic oxidizing process which is an electro-chemical coating process appeared as an attractive surface modification technique in industry for over years. Although slight increase in hardness and wear resistance are achieved by anodic oxidizing process,  the enhancements in surface properties are still far from the requirements of the wear related applications.
On the other hand, the wear resistance of aluminium alloys had been studied to improve with different chemical, thermal and painting processes. Plasma spray coating which is a thermal barier coating , chrome coating, cataphoresis coating, electrostatic powder painting and etc. were several methods among those trials to develop properties of aluminium and also to raise of wear resistance which is normally not high-level in aluminium and its alloys. 
Recently, micro arc oxidation, which generates thick and hard oxide layers on light metals, appeared as an alternative electrochemical process against anodic oxidation process. Anodic oxidation emloys electrolytic cell which consists of anode, cathode, electrolite and power supply. 
Basic electrolites are used in the electrolytic cells of the anodic oxidation process. The anode material applied oxidation is attached to the positive electrode while the cathode material (generally stainless steel) is stuck to the negative electrode. As a result of voltage applied, sparks are appeared on the surface of the anode when the voltage reach the breakdown voltage. 
These sparks can instantaneous raise to high temperatures and are beginning of the micro arc oxidation process. In the next part of the process, oxidation occurs on the surface of the anode due to the electrochemical reactions between anode and cathode. During the existance of oxide layer process, sample molten material coming from discharge channels freezes on layer, so oxide layer gets thicker as time passes. Also the porous structure is also appeared with a better visuality. For power source of coating system, pulsed AC power unit has been used during processes. 
However, micro arc oxidation process has limited applications in aluminium industry as compared to anodic oxidation.There are various factors such electrolite, voltage, current density, impact rate, waiting and treatment time that changed surface properties of the material during the micro arc oxidation process.These factors give advantage to change surface properties easily and obtain product as required.
This study was initiated to compare the tribological and morphological properties of AlSi9Cu3(Fe) casting alloy and 5754 quality AlMg3 wrought alloy after anodic oxidation and micro arc oxidation processes. Anodic oxidation process was applied in a commercial workshop with the customized process parameters and general commercial parameters to have a comparison between micro arc oxidated samples which have been produced in laboratory conditions. Micro arc oxidation process parameters as voltage controlled, current controlled and process time have been changed to realize the changes in morphology and tribological performance of selected alloys.
After anodizing of samples at commercial supplier and micro arc oxidation in laboratory conditions; surface morphology and microstructure of the samples were examined by an energy dispersive spectrometer (EDS, Oxford Instruments) equipped scanning electron microscope (SEM, Hitachi TM-1000, JEOL JSM 6335F FEG and Philips XL 30 SFEG). Qualitative phase analysis of the coatings was carried out by an X-ray diffractometer (XRD). Mean surface roughness (Ra) of the samples was measured by using a surface profilometer (Veeco Dektak, 6M). Cross sectional microstructure of the samples was examined by a scanning electron microscope (SEM, Hitachi, TM-1000) and three point flexural tests have been performed on Shimadzu bending test device with until 900 bending. 
After surface characterization tests, the results showed that of the anodized alloys have an amorphous oxide layer where a crystalline thick oxide layer have been occured by micro arc oxidation process on selected aluminium alloys. 
On the other hand, with micro arc oxidation process; a more porous oxide layer have been occured on both aluminium casting alloys and aluminium wrought alloys compare to anodized surfaces.
In micro arc oxidation, the increasing voltage and increasing process time were providing a thicker oxide layer for aluminium casting alloys where also the roughness of surface were increasing from 1m to almost 5m values. On the other hand; the increasing time were more effective to thickness change of aluminium wrought alloys. For both samples; the dimension of micro pores were improved as determined by surface SEM photos under various of zoom in between 1000x to 10000x.
Following the surface characterization tests, tribological performances of anodizing and micro arc oxidized samples were compared on a reciprocating wear tester under dry sliding conditions. The dry sliding wear tests showed that the relative wear resistance of the oxide layers which generated by voltage-controlled micro arc oxidation is remarkably higher than that of the anodized alloys with commercial parameters. 
Tribological performance of current-controlled micro arc oxidation samples were also tested by comparing them with anodizing process within special parameters.. The only difference of specific parameters were process time compare to commercial parameters. The process times were increased for casting alloys (from 50 minutes to 60 minutes) and decreased for wrought alloys (from 30 minutes to 25 minutes) to have an equal coating thicknesses to compare the oxide layers obtained by micro arc oxidation and anodizing under objective circumstances. Even the process time increased for casting alloy's anodizing process, the maximum layer thickness were 10µm eventually. Therefore, each sample for wrought alloy and casting alloy which produced by anodizing and micro arc oxidation process were manuelly linished with 2500 mesh grinding paper under water solution. The results were also similar to "voltage-controlled MAO" and had higher relative wear resistance values than to anodizing samples with specific coating parameters. Micro arc oxidation of aluminium wrought alloy showed the greatest relative wear resistance among all other samples.
Three point flextural tests have been made on aluminium wrought alloy, 1 micro arc oxidation applied sample and 1 commercial anodizing applied sample have been tested. For micro arc oxidation process; Positive voltage has been chosen as 440V and negative voltage value was 88V while the process time 10 minutes.On the other hand, the commercial anodizing parameters were constant voltage as 15V and 30 minutes as process time.  Three point flexural test results showed that the coating bound obtained with micro arc oxidation process on selected wrought alloy were much more stronger than layer occured by anodizing process. Under stereo microscope; the scrathes and cracks were highly appeared on anodizing surface. The bending test were only made on wrought alloys because of the good elastical properties of aluminium wrought alloys compare to aluminium casting alloys.