Suni yaşlandırma işlem parametrelerinin AA6082 ve AA6056 alaşımlarının mikroyapı ve mekanik özelliklere etkisinin incelenmesi
Investigation of the effect of artificial aging process parameters on the microstructure and mechanical properties of AA6082 and AA6056 alloys
Yazar: DİLEK DENİZ
Danışman: PROF. DR. NİL TOPLAN
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Bu çalışmada ısıl işlem uygulaması yapılabilen AA6082 ve AA6056 serisi dövme alüminyum alaşımlarına uygulanan suni yaşlandırma işleminin mekanik özellikler ve mikroyapı üzerindeki etkilerinin araştırılması amaçlanmıştır. Sanayide biyet olarak tanımlanan hammaddeler, 7 metre uzunluğunda ve 178 mm çapında yarı sürekli döküm yöntemiyle üretilmektedir. Yarı sürekli döküm yöntemi ile imal edilen AA6056 ve AA6082 alüminyum alaşımı biyetler ekstrüzyon yöntemi ile şekillendirilerek çalışmada kullanılan lama profillerin numuneleri üretilmiştir. Çözeltiye alma işlemi üretilen biyetlere yapılan homojenizasyon ısıl işlemi ile başlamıştır. Biyetler; ekstrüzyon ile şekillendirilecek olan profilin geometrisine uygun olacak boyutlarda kesilmektedir. Ekstrüzyon işlemine geçilmeden önce kesilen biyetlere ön ısıtma yapılır ve çözeltiye alma işlemi tamamlanmış olur. Su verme işlemi ise sıcak iş takım çeliklerinden imal edilen kalıptan çıkan ve nihai şeklini alan alüminyum profilin 25°C sıcaklığa sahip olan su dolu havuzdan geçirilmesine dayanmaktadır. Su verme işlemi tamamlandıktan sonra profiller germe ve kesme uygulamalarından geçmiştir. Ekstrüzyon prosesinden 3000 mm olarak çıkan profiller yaşlandırma işlemi için 500 mm boya kesilmiştir. Suni yaşlandırma yapılmak üzere her bir alaşımdan 41 adet ve aynı sayıda yedek olmak üzere deney numuneleri hazırlanmıştır. Tüm numuneler biax kalem ile numaralandırılarak tanımlanmıştır. Her iki alaşıma da detayları deneysel çalışmalarda verilen sürelerde 150°C, 165°C, 180°C, 195°C ve 210°C sıcaklıklarda suni yaşlandırma işlemi yapılmıştır. Her iki alaşımdan ikişer numune doğal yaşlanmanın mekanik özelliklere ve mikroyapıya etkilerinin araştırılması için 3 ay oda sıcaklığında bekletilmiştir. Yaşlandırılan numuneler çekme testi numune hazırlığı için hassas kesim işlemi ile 310 mm uzunluğa kısaltılmıştır. Çekme deneyi ile AA6056 ve AA6082 alaşımlarının çekme ve akma mukavemeti değerleri tespit edilmiştir. Numunenin kalan kısmı ise sertlik ölçümleri ve mikroyapı analizleri için kullanılmıştır. Sertlik ölçümleri numunelerin ekstrüzyon yönüne paralel olan yüzeylerinden beşer adet ölçüm yapılarak ve yapılan ölçümlerin ortalamaları kaydedilerek tamamlanmıştır. Suni ve doğal yaşlandırılan numunelerin, kimyasal kompozisyonlarının belirlenmesi için spektral analiz yapılmıştır. Ekstrüzyon yönüne dik ve paralel olacak şekilde metalografik numune hazırlama süreçlerinden geçen her iki alaşıma ait numunelerdeki çökelti dağılımı optik mikroskop ile incelenmiştir. Alüminyum matris yapısındaki değişimleri, oluşan çökelti ve çökeltilerin yapıdaki dağılımları SEM-EDS analizleri ile detaylandırılmış ayrıca seçilen numunelere MAP analizi de yapılmıştır. Suni ve doğal yaşlandırılan numunelerin faz yapıları ise XRD cihazı ile tanımlanmıştır. Yapılan deneyler sonucunda, aynı sıcaklık ve sürede yaşlandırılan AA6056 alaşımı numunelerde AA6082 alaşımı numunelere göre daha yüksek mekanik özellikler elde edilmiştir. Suni yaşlandırma işleminde kullanılan süre ve sıcaklığın etkisi ile mikroyapıda meydana gelen çökeltiler ve malzemelerin mekanik özelliklerine etkileri tartışılmıştır.

Abstract:
This study was aimed to investigating the effects of artificial aging applied to 6XXX series wrought aluminum alloys, which can be heated and treated, on mechanical properties and microstructure.
Direct chill casting method and direct extrusion method were used to produce the samples to be used in the study. Raw materials, defined as billet in the industry, were produced by direct chill casting method with a length of 7 meters and a diameter of 178 mm. After the casting process was completed, the billet was homogenized and subsequently cooled. The main purpose of homogenization is to make the raw material extrudable by providing phase transformations in the microstructure of the billet.
AA6056 and AA6082 aluminum alloy billets manufactured by the direct chill casting method were shaped by the extrusion method and samples of the flat profiles used in the study were produced.
In extrusion production, billet front and back temperatures, extrusion beginning and end temperatures, press speed, bursting pressure , and billet length used was kept constant for both alloys. Since the aim of the study was to examine the effect of two different alloys on the mechanical properties and microstructure under different artificial aging conditions, the production parameters were kept constant.
The heat treatment processes used in the study consist of three stages. These are the processes of solution, quenching and aging, respectively.
The solution process started with the homogenization heat treatment of the produced billets. It continues until the aluminum starts to pass through the extrusion molds made of hot work tool steels, which is accepted as the beginning of the extrusion process. The quenching process is based on passing through a water-filled pool with a temperature of 25°C on profiles. After the quenching process was completed, the profiles went through tensioning and cutting applications. Profiles were cut to 3000 mm length. The profiles that came out of the extrusion process as 3000 mm were cut to 500 mm length for the aging process.
The aging process is basically divided into two artificial aging and natural aging. Heat treatment furnaces are used in artificial aging. It enables the production of materials in desired conditions by using different heat treatment recipes in heat treatment furnaces, which are called thermal furnaces in the industry. Natural aging, on the other hand,refers to the change in the mechanical properties of the material depending on time by natural means without any heat treatment after production by extrusion.
84 samples were prepared to be aged at different temperatures and times and their 84 spare samples were also prepared. All samples were numbered with a biax pen.
Both alloys were artificially aged at 150°C, 165°C, 180°C, 195°C and 210°C for the periods detailed in the experimental studies.
AA6082 and AA6056 samples were subjected to artificial aging in heat treatment furnaces at 150°C for 12, 16, 20, 24, 28, 32, 36 and 40 hours. In the aging process at 165°C, aging heat treatment was applied for 8, 10, 12, 14, 16, 18 and 20 hours. While periods of 4, 5, 6, 7, 8, 9, 10, 11 and 12 hours were used in the artificial aging process at 180°C; Periods of 2, 3, 4, 5, 6, 7, 8, 9 and 10 hours at 195°C were used. Finally, artificial aging heat treatment was applied at 210 °C for 1, 1.5, 2, 2.5, 3, 3.5, 4 and 4.5 hours.
Samples 83 and 84 were kept at room temperature for 3 months to investigate the effects of natural aging on mechanical properties and microstructure.
Mechanical properties change in samples that are subjected to aging heat treatment. The samples were expected to exhibit different mechanical properties for each temperature and time. In order to determine this, mechanical tests should be applied to the samples. Some of the mechanical properties that are expected to change are tensile strength and yield strength .
The tensile strength and yield strength of the material will be determined by applying the tensile test. The aged samples were shortened to 310 mm in length by fine cutting for tensile test sample preparation.The remaining part of the sample was used for hardness measurements and microstructure analysis. Hardness measurements were completed by making 3 measurements from the cross-section of the flat profile and from the same points in each sample and recording the averages of the measurements made.
Hardness measurements of the test samples were made with a Brinel device using 2.5mm diameter balls, applying 10 kg preload and 62.5 kg total load. According to the data obtained, the highest hardness value was obtained as 130 HB in 7 hours in the aging process at 180°C for the AA6056 alloy; The highest hardness for AA6082 alloy was obtained as 109.33 HB in 16 hours at 165°C. The highest hardness value in AA6056 alloy was obtained in the sample that was aged for 7 hours. This value is also the highest hardness value obtained at all temperatures and times used for both alloys.
The hardness values of the samples subjected to natural aging for 3 months at room temperature were recorded as 91.53 HB for the AA6056 alloy and 78.43 HB for the AA6082 alloy. Higher hardness values were obtained in the artificially aged samples compared to the naturally aged samples.
Tensile tests were performed on AA6082 and AA6056 alloy samples. Tensile test was applied to samples which were kept for natural aging process at room temperature for 3 months. When the tensile strength values of the AA6056 alloy samples were compared, it was found that the sample aged for 7 hours at 180 °C had the highest tensile strength; it was observed that, which underwent 4,5 hours at 210 °C, had the lowest tensile strength value. When the tensile strength values of the AA6082 alloy samples were compared, it was found that the sample aged for 36 hours at 150 °C hadthe highest tensile strength value; the lowest tensile strength value was observed, which underwent 4,5 hours at 210°C. In the AA6056 sample, where the tensile strength was recorded as the highest value, the yield strength also had the highest yield value of the study. The lowest yield strength value belongs to the sample, which was naturally aged for 3 months at room temperature. As a result of the tensile test applied to AA6082 alloy samples, the highest value of yield strength was seen in aged for 8 hours at 180°C. The lowest yield strength value was make firm in the sample, which was subjected to natural aging.
As a result of the artificial aging process applied to the AA6056 alloy samples; it was observed that the sample showing the highest mechanical properties was the sample aged at 180°C for 7 hours. It's tensile strength is 417.30 MPa, yield strength is 385.00 MPa and hardness value is 130 HB. The tensile strength of the sample, which was aged at 210 °C for 4.5 hours, was recorded as 356.81 MPa, yield strength of 322.20 MPa and hardness value of 109.33 HB. This shows the lowest strength result obtained by the artificial aging process applied to AA6056 alloy.
When two different alloys aged at the same temperature and time are compared, it is seen that the AA6056 alloy has a higher hardness value than the AA6082 alloy. AA6056 alloy has a higher hardness value than AA6082 in the samples applied natural aging process.
Tensile stress and yield stress values obtained by the tensile test were compared for AA6056 and AA6082 alloys. The chemical compositions of the samples, which were aged using different temperatures and times, were analyzed by a spectrometer, macro, and microstructural analyzes were made using an optical microscope and SEM microscope, and phase structures were made by XRD phase analysis. As a result of the experiments, when the AA6056 and AA6082 samples aged at the same temperature and time were compared, higher mechanical properties were obtained in the AA6056 alloy samples than in the AA6082 alloy samples.