Homojenizasyonun AA 6063 alüminyum alaşımnın içyapısı ve özelliklerine etkisi

Abstract

Mimari uygulamalarda yaygın olarak kullanılan AA 60 63 alüminyum alaşımının şekillendirilmesinde en fazla kullanılan üretim yöntemi ekstrüzyondur. Ekstrüzyonla üretim sırasında, düşük ekstüzyon yükünde yüksek ekstrüzyon hızlarına ulaşılması ve temiz bir ürün yüzeyi elde edilmesi amaçlanır. Malzemeye ekstrüzyon öncesi uygulanan homojenleştirme işlemi ile ürün yüzeyinin temiz olması, homojenleştirme sonrası uygulanan kontrollü soğutma ile de ekstrüzyon hızını belirleyen mikroyapının elde edilmesi sağlanmaktadır. Homojenleştirme sırasında döküm yapıda mevcut olan çökeltilerin bileşiminde bulunan Mg'un tamamı ve Si' un bir kısmı katı çözeltiye geçmektedir. Homojenleştirme süreci sonunda uygulanan hızlı soğutma Mg ve Si 'un katı çözeltide kalmasına neden olurken, kontrollü soğutma ile Mg2Si çökeltileri oluşturulmaktadır. Ekstrüzyon sürecinde yüksek hızlara ulaşılması, Mg2Si çökeltilerinin ekstrüzyon sürecinde yeniden katı çözeltiye geçmesine bağlıdır. Ürün yüzeyinin temizliğini ise malzemede bulunan Al-Fe-Si fazlarının kristal yapısı belirlemektedir. Bu çalışmada, beş ayrı sıcaklık ve bu sıcaklıklarda beş ayrı süre kullanılarak homojenleştirme deneyleri yapılmış, döküm yapıda bulunan fazların süre ve sıcaklığa bağlı olarak değişimi X-ışını ve elektron difraksiyon yöntemleri ile saptanmış, boyut dağılım analizleri Taramalı Elektron Mikroskop 'a bağlı görüntü analiz sistemi ile gerçekleştirilmiştir. Homojenleştirme deneyleri sonuçlarına göre seçilen bir homojenleştirme sıcaklık ve süresi için üç ayrı hızda soğutma deneyleri yapılmıştır. Bu deneyler sonucunda oluşan mikroyapılar Taramalı Elektron Mikroskop ve Transmisyon Elektron Mikroskop ile incelenmişlerdir. Homojenleştirme ve soğutma hızının ekstrüzyon parametrelerine etkisini incelemek amacıyla da iki ayrı redüksiyon oranı ve üç ayrı hız kullanılarak bir dizi ekstrüzyon deneyi gerçekleştirilmiştir. Elde edilen sonuçlarla, AA 6063 alaşımına ekonomik bir üretim yöntemi olan ekstrüzyonun uygulanmasında en yüksek verimin alınabilmesi için gerekli koşullar irdelenmiştir.

Aluminium and most of its alloys are extensively used in forming by extrusion processing. Heat- treatable AA 6000 series (Al-Mg-Si) alloys are the most popular alloys for producing shapes by extrusion and AA 6063 constitutes about seventy percent of all aluminium extrudates. Chemical composition, microstructural properties and thermal and mechanical treatments applied are the parameters that determine the formability of a material. Homogenisation is an elevated temperature treatment which changes the structure of an as cast ingot to make it more amenable to hot deformation so that higher strength and better surface finish is achieved upon deformation. As the main objective in extrusion processing is to obtain a product with desirable properties at maximum extrusion speed, careful control of all aspects of opertion is required. These include casting of the ingot, homogenisation treatment, reheating to extrusion temperature, extrusion and post-extrusion treatments. Besides its high extrudability, AA 6063 combines excellent corrosion resistance and good finishing characteristics with improved mechanical properties. Since AA 6063 posesses good surface finish it is suitable for anodic coating without any further surface processing. As a consequence of these properties AA 6063 is widely used for architectural purposes. Magnesium and silicon are the main alloying elements used in AA 6063 alloy and combine to form the stoichiometric Mg2Si precipitates. It is well known that, mechanical properties of AA 6063 are highly dependent on the Mg2Si content. While mechanical properties are improved by the increase in Mg2Si content, extrudability is decreased. The minimum amount of Mg2Si content required for solid solution hardenening of AA 6063 alloy is 0.3%. Silicon combines with iron in preference to magnesium and iron is always present as impurity in aluminium alloys. If silicon content of the alloy is below the VI minimum amount required to form the Mg2Si precipitates, mechanical properties will be adversely affected. For this reason the alloy should always contain excess silicon. Otherwise, the quantity of Mg2Si precipitates will be lower than the amount required for achieving the minimum strength values specified. Iron that is present in all aluminium alloys will combine with both aluminium and silicon to form a large variety of phases during solidification or subsequent thermomechanical processing and these phases determine the properties of the alloy such as recovery and recrysallisation behaviour, texture, electrical resistance, surface finish. Besides equilibrium 8-Al13Fe4, Al8Fe2Si and P~Al5FeSi intermetallic phases, various non- equilibrium phases may form depending upon the exact composition of the alloy and the cooling rate after the casting of the ingot. Extrudability of an ingot is increased by homogenisation treatment during which magnesium and some of the silicon present in the intermetallic phases are taken into solid solution. Through this process, Mg contained in the precipitates will be taken into solid solution and p-AlFeSi phase which is known to have detrimental effects on the surface finish of extrudates is transformed into more favourable o-AlFeSi phase and both the aspect ratio of the phases and the Fe/Si ratio are decreased. Magnesium and silicon in solid solution will combine to form Mg2Si depending upon the subsequent cooling rate which determines the size of the component. While the size of Mg2Si precipitates is determined by the cooling rate, extrusion rate is determined by the size of Mg2Si precipitates. Mg2Si precipitates should be small enough inorder to get into solid solution during extrusion processing. Etial 60 which is produced by Etibank Seydişehir Aluminium Plants and whose composition limits are stated to be within the composition limits of AA 6063 is selected as experimental material in present investigation. Since the microstructure of the as cast structure will determine the microstructure of homogenised structure, as cast ingots was examined with scanning electron microscope (SEM) for the beginning. Energy dispersive X-ray analyses (EDAX) carried out by the EDAX attachment of SEM revealed that compositionally two different precipitates were present in as cast structure. These were quaternary AlMgFeSi and ternary AlFeSi phases and of the other phases that may be present in dilute aluminium alloys, neither binary AlFe precipitates nor Mg2Si phase were found to exist. The exact composition of precipitates could not be determined by EDAX analyses as VI i the smallest dimension of the precipitates were smaller than the probe size used. For this reason matrix always contributed the spectrum acquired and X-ray diffraction and electron diffraction analyses were found to be essential for the determination of the phases present in both as cast and homogenised structures. The morphology of the precipitates at the mold casting interface were found to be different than the morphology of the precipitates at the grain interior. However, as the width of this region was 50um at most which is at thickness that will easily be removed during extrusion it will not have any effect on the extrusion properties. Thin foil specimens prepared from as cast structure were investigated with transmission electron microscope (TEM). SEM and TEM studies showed that second phase particles were mainly formed along the grain boundaries as continuous precipitates with high aspect ratio. There existed few Al-Fe-Si precipitates spherical in shape within the grains and the rate developed during the cooling of the ingot prevented the formation of Mg2Si precipitates. Inorder to determine the effect of time and temperature on homogenisation, heat treatments at five different temperature and five different time at each temperature were applied. The tempertures selected were 550, 560, 570, 580, 590°C and 0.5, 1, 2, 4, 8 hr intervals were applied for each temperature. The pieces were water quenched after heat treatment and the structure of the phases are examined using SEM and TEM and EDAX analyses of the precipitates were performed using SEM. It is concluded from the EDAX analyses that magnesium present in the precipitates could readily be taken into solid solution. Mg bearing particles were not detected to be present even in the specimen received from the piece that was heat treated at the lowest temperature for the shortest time. However, complete transformation of detrimental p-AlFeSi phase to more favourable a-AlFeSi phase and hence Si to be solutionised is a time consuming process and is increased with the increase in time and temperature of heat treatment. Meanwhile, aspect ratio of the precipitates changed as well and the change, in % area distribution of the particles with applied heat treatment was performed using the image analysis attachment of SEM. The analyses showed that major change in the areal distribution of the phases i.e. the aspect ratio took place during the heating of the ingot to the homogenisation temperature. TEM investigations of the homogenised samples showed that the formation of Mg2Si precipitates is prohibited by quenching applied following the homogenisation. Vlll As the exact composition of the precipitates could not be determined due to their size and X-ray diffraction and electron diffraction was found to be essential for the determination of the phases, selective dissolution of the matrix was applied to both as cast and homogenised structures. The second phase particles that were extracted from the matrix by selective dissolution were centrifugally precipitated. X-ray analysis of the remaining particles revealed that, from the ternary AlFeSi phases p-AlFeSi was present in as cast structure and AlMgFeSi phase could not be detected due to the reason that this phase is in minor amount in the alloy. Although the transformation of the p-AlFeSi phase to a-AlFeSi starts at the homogenisation temperature of 550°C which is the lowest homogenistation temperature applied, complete transformation did not take place at any of the times applied at this temperature. After a homogenisation treatment of 4 hours at 570°C complete transformation of the phase is obtained. As expected, within the temperature and time intervals applied complete transformation took place at higher homogenisation tempertures at shorter time intervals. As stated before, Mg2Si precipitates determine the mechanical properties of the extrudates. To achieve high extrusion speeds at constant load, Mg2Si precipitates should be present in the homogenised structure which will be taken into solid solution during extrusion and hardening of the extrudates will take place during the cooling of the product. For this reason homogenised ingots should either be quenched and a subsequent aging process applied or they should be cooled at a controlled rate after homogenisation so that an optimum structure can be formed. To determine a rate of cooling that will provide high extrusion rates, cooling at five different rates were performed. Among these, three different rates were selected according to the hardness values measured. The structures developed were examined using SEM and TEM techniques and it is observed that while quenching prohibits Mg2Si formation and leads to higher hardness, lower cooling rates leads to a fine distribution of the phase within the structure and the hardness is lower with respect to quenched sample. The size of the Mg2Si phases could not be measured since the precipitates were transformed into a hydroxide component during the electrolytic polishing applied for thin foil preparation of the specimens required for TEM investigations. Since Mg2Si distribution is important in determining the extrusion speed, extrusion experiments were performed using two different reduction ratios for each cooling rate. It is revealed that as the cooling rate after homogenisation decreases rate of extrusion increases and for the selected cooling rates extrusion is carried out ix successfully, resulting in good surface finish. It is concluded from the present study that of the various phases that may be present in dilute aluminium alloys, only p-AlFeSi is present in semicontinuously cast Etial 60 aluminium alloy whose composition fits the AA 6063 specifications. This phase can completely be transformed into oc-AlFeSi phase which is more favourable for extrusion processing by appropriate homogenisation treatment. While the transformation was determined to be complete at a heat treatment of 4 hr at 570°C it was completed at 590°C within 0.5 hr. However, at homogenisation temperatures lower than 570°C entire transformation of the phase did not occur even at the longest time of 8 hr applied. The aspect ratio of the AlFeSi phases is decreased mainly during the heating of the ingot to the homogenisation temperature, i.e, an important amount of decrease in aspect ratio is realised before the completion of the transformation of AlFeSi phases. For this reason it is concluded that there is not a relation between phase transformation and size change. For the formation Mg2Si phase which determines the extrudability and the strength of the material, both magnesium that exists in the AlMgFeSi phase and silicon that exists in AlMgFeSi and AlFeSi phases should be taken into solid solution during homogenisation treatment. Magnesium can readily be solutionized but time is required for silicon to get into solid solution. Due to this reason, homogenisation treatment to be performed at temperatures higher than 570°C and for relatively longer times is required not only for the completion of the phase transformation but for Si to get into solid solution as well. Formation of Mg2Si precipitates can be achieved either by application of an aging treatment to a quenched ingot or by controlled cooling after homogenisation. However, controlled cooling after homogenisation is prefered due to economical reasons. The microstructures thus achived will lead to the application of lower loads during extrusion and maximum strength of the extrudates will be obtained. Cooling rates applied during the experiments revealed that as the rate of cooling is decreased, extrudability is increased but the fact that Mg2Si precipitates may be far too coarsened by the application of very slow rates should be considered in industrial applications. For this reason in practice, cooling rates after homogenisation treatment should not be much lower than the cooling rate of 2°C/min which is the lowest cooling rate applied in this study.