The Effects Of Stearic Acid On Mechanical Alloying Process Of Iron Powder

Abstract

Toz metalurjisi, bilinen metal işleme yöntemleri içinde çok önemli bir yere sahiptir. Toz metalurjisinin üretim aşamaları; metal tozlarının üretimi, üretilen metal tozlarının karıştırılması, karıştırılan tozların preslenip şekillendirilmesi, sinterlenmesi ve gerekli son işlemlerin uygulanarak nihai ürünün elde edilmesi şeklindedir. Mekanik alaşımlama (MA), toz metalurjisinin karıştırma basamağında uygulanan, yüksek enerjili öğütme sistemidir. Kimyasal veya ısıl işlemlere başvurmadan gerçekleştirilen bir katı hal reaksiyonu olup, homojen dağılıma sahip, kontrollü ve ince tozları üretmek mümkündür. Mekanik alaşımlama prosesini etkileyen çeşitli parametreler vardır. Bu parametreler; öğütücü tipi, öğütme kabı ve bilye cinsi, öğütme hızı, öğütme süresi, bilye-toz oranı, öğütme kabı doluluk oranı ve öğütme atmosferi ve sıcaklığıdır. Mekanik alaşımlama işleminde, toz parçacıkları arasında soğuk kaynaklanma, kırılma ve tekrar kaynaklanma basamakları devamlı olarak tekrar eder. Sağlıklı bir mekanik alaşımlamanın gerçekleştirilebilmesi için, soğuk kaynaklanma ve kırılma basamakları arasında denge sağlanması gerekir. Tozların soğuk kaynaklan eğilimini azaltmak için, "proses kontrol ajanı (PKA)" adı verilen kimyasallar öğütülecek tozlar ile karıştırılır. En çok kullanılan proses kontrol ajanları; stearik asit, hekzan, metanol ve etanoldür. Bu çalışmada, atomize demir tozu ve proses kontrol ajanı olarak stearik asit kullanılarak, proses değişkenlerinden bazıları değiştirilerek veya proses kontrol ajanının miktarı değiştirilerek çeşitli deneyler yapılmış ve bu kapsamda seçilen stearik asitin Fe tozunun mekanik alaşımlanması sürecine olan etkileri incelenmiştir. Hazırlanan numunelerin yoğunlukları ve partikül boyutları ölçülmüştür. Taramalı elektron mikroskobu ve Fourier dönüşümlü kızılötesi spektroskopisi analizleri yapılmıştır. Numunelerin X-Işını faz analizleri yapılmış ve MA işlemi sonrasında faz dönüşümü olup olmadığı incelenmiştir. MA uygulanmış tozlara karbon analizi yapılıp, tozların MA öncesi ve sonrasında içerdikleri karbon miktarları karşılaştırılmıştır. Son olarak, MA yapılmış tozlara ısıl işlem uygulanmış ve sıcaklığın fonksiyonu olarak tozlarda meydana gelen ağırlık değişimleri incelenmiştir. Genel olarak, MA süresinin, PKA miktarının, başlangıç tozlarının boyutunun, bilye-toz oranının ve öğütme atmosferinin değiştirilmesi ile üretilen tozların yoğunluğunun ve partikül boyutunun değiştiği gözlenmiştir. Ayrıca FT-IR spektroskopisi analizi ile MA süreçlerinde kullanılan stearik asit gibi organik proses kontrol ajanlarının kontrolünün yapılabileceği görülmüş olup, bu analizle incelenen numunelerin içerdikleri stearik asitin durumu hakkında yorumlarda bulunulmuştur.

Powder metallurgy has a very important place in the metal processing methods. Powder metallurgy’s production stages are; the production of metal powders, mixing of the produced powders, pressing and forming of the powders, sintering and applying some final operations if they are necessary. Mechanical alloying (MA) is a high energy milling process which is performed at the mixing stage of the powder metallurgy. The MA process was invented at International Nickel Company (INCO) in 1966 by John Benjamin and his colleagues. They developed a material for gas turbine applications and other heat resistant alloys. They produced a nickel-based superalloy by combining oxide dispersion strengthening by MA process. There are a lot of advantages of the MA process. By this technique, homogeneous and fine powders can be obtained. New alloys, which can not be synthesized by other methods, can be produced by MA. Amorphous phases and amorphous materials can be produced obtained by MA. Some materials and alloys that contain nanoparticles can be produced by this technique. Furthermore, some composites or powders can be synthesized by MA through mechanochemical reaction. There are some process variables, which affect the MA process. Because MA is a complex procedure, process variables must be optimized to achieve the desired product phase, microstructure and properties. These process variables are; type of mill, type of grinding vial and balls, milling speed, milling time, ball-to-powder weight ratio, extent of filling the vial, milling atmosphere and milling temperature. Mechanial alloying process involves repeated cold welding, fracturing and rewelding of powder particles. A successful alloying among powder particles can occur only when a balance is maintained between cold welding and fracturing of powder particles. Some chemicals, which are called “process control agents (PCA)” are added to the powder mixture during milling to reduce the effect of excessive cold welding. The most important process control agents which are commonly used in practice are; stearic acid, hexane, methanol and ethanol. In MA process, iron (Fe) is a commonly used matrix material because of its ductile structure. When the matrix material isn’t ductile enough, MA can’t occur between the powders and the system works like a high energy milling. Therefore, a ductile matrix material is preferred in the MA process. In this study, effect of PCA was investigated on the MA performance of atomized Fe powder. For this purpose stearic acid was selected as PCA and effect of its presence and amount were studied against different MA process variables like milling time, ball-to-powder weight ratio and milling atmosphere. To this end, six different sets of experiments were carried out. In the first set, it was aimed to investigate the effect of milling time on the mechanically alloyed Fe powders and stearic acid. Fe powders and stearic acid were milled by Spex™ mill for 0, 15, 30, 60, 120, 240 and 480 minutes. Except the milling time, all other process variables were holded steady. Powders that milled for different times were characterized and their properties were examined. In the second set, it was aimed to investigate the effect of the amount of stearic acid on the mechanically alloyed Fe powders. Different amounts of stearic acid were mixed with Fe powders and they milled by Spex™ mill for the same time. Except the PCA amount, all process variables were holded steady. Powders that milled with different amounts of stearic acid were characterized and their properties were investigated. In the third set, it was aimed to investigate the effect of the particule sizes of the initial iron powders on the mechanically alloyed Fe powders and stearic acid. For this purpose, the initial iron powder was analyzed by sieve analysis and it is separated to five groups with different sizes (-150 µm, -150+106 µm, -106+63 µm, -63+45 µm and -45 µm). Initial Fe powders with different sizes and 3 wt.% stearic acid as PCA milled with Spex™ mill for 30 minutes. Milling conditions were the same in all cases with the exception of the initial iron powder size. Milled powders were characterized and their properties were examined. In the fourth set, to investigate the effect of stearic acid’s presence on the mechanically alloyed Fe powders and stearic acid was aimed. For this purpose, Fe powders were milled with and without the addition of stearic acid by Spex™ mill. All process variables were holded steady. Milled powders were characterized and their properties were investigated. In the fifth set, it was aimed to investigate the effect of the milling atmosphere on the mechanically alloyed Fe powders and stearic acid. Fe powder and 3 wt.% stearic acid were milled under air and argon atmospheres. Milling conditions were the same in all cases with the exception of the milling atmosphere. Milled powders were characterized and their properties were examined. In the last set, it was aimed to investigate the effect of ball-to-powder weight ratio on the mechanically alloyed Fe powders and stearic acid. For this purpose, Fe powders were milled with three different ball-to-powders weight ratios (5:1, 10:1 and 15:1). All other process variables were holded steady. 3 wt.% stearic acid as PCA and Spex™ mill were used. Milled powders were characterized and their properties were investigated. Characterization studies were performed on both powder state and bulk state after MA. For this purpose characterization of samples were done by helium gas pycnometer, laser particle size analyzer, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, X-ray diffraction analyzer and carbon analyzer. As the result of characterization studies, it was seen that stearic acid, which is added as a PCA, is decomposed by increasing milling times. Also it was noticed that stearic acid decomposed faster when the size of initial iron powder is smaller. Moreover, finer powders were obtained by the presence of stearic acid as a PCA, by MA. It was seen that milling atmosphere affects the speed of the decomposition of stearic acid during MA process. When the Fe powders were milled under air atmosphere, stearic acid decomposed after 60 minutes milling while it decomposed after about 80 minutes under argon atmosphere. Lastly, by increasing the ball-to-powder weight ratio, the degree of deformation and the speed of the decomposition of stearic acid increased. As a result of thesis study it was seen that during the MA process of Fe powder, stearic acid decomposes gradually. The milling time, the amount of PCA, the presence of PCA, the particle size of the pre-powders, the ball-to-powder weight ratio and the milling atmosphere affect the properties of the mechanically alloyed powders. The necessary PCA amount that must be added to MA process is a function of milling time, ball-to-powder weight ratio, milling atmosphere and the size of initial powders and they must be optimized up to the study.