T4 ve T6 ısıl işlemli 6061 alüminyum levhanın iki eksenli gerilmeler altında şekil değiştirmesinin incelenmesi
Investigation of deformation of T4 & T6 heat treated 6061 aluminum plate under biaxial stress
Yazar: EGEMEN UZ
Danışman: PROF. DR. ŞAFAK YILMAZ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Alüminyum levhalar = Aluminum sheets ; Isıl işlem = Heat treatment ; İki eksenli eğilme = Biaxial bending

Özet:
Sanayileşmede rekabetin giderek artması ve eş zamanlı olarak çevreyle ilgili yükselen duyarlılıkla birlikte ürünlerin hafifletilmesi tasarımda kaçınılmaz beklentilerden biri olmuştur. Geçmişten bu yana hafifliklerinden dolayı uçak-uzay endüstrisinde tercih edilmekte olan alüminyum alaşımları özellikle son yıllarda otomotiv endüstrisi tarafından da yüksek özgül dayanım, alışılmış imalat tekniklerine uygunluk ve korozyon direnci gibi özelliklerinden ötürü yoğun bir şekilde kullanılmaya başlanmıştır. Özellikle 5xxx ve 6xxx serisi alüminyum alaşımları korozyona dayanıklılığıyla dikkat çekmektedir. Tasarımcılar malzemeden yüksek dayanım bekleken imalat için öncelik şekillendirilebilirlik yani sünekliktir. Bu tez çalışmasında T6 (çözeltiye alınıp, suni yaşlandırılmış) ve T4 (doğal yaşlandırma) ısıl işlemi görmüş alüminyum alaşımlarının şekillendirme performansı incelenmiştir.
T6 ve T4 ısıl işlemli 6061 alüminyum alaşımının tek eksenli ve iki eksenli çekme deneylerindeki, düzenli ve düzensiz uzama bölgelerinin davranışı karşılaştırmalı olarak incelenmiştir. Metal malzemenin deformasyon davranışı, çekme yüküne maruz bırakıldığında, düzgün uzama alanı dahilinde Holloman pekleşme modeli denklemiyle uyumlu ama boyun verme sırasındaki genleme değerinin pekleşme üsteline sayı değeri olarak eşit olmadığı ve kopma sırasında gerçek gerilme değerinin Holloman modeliyle uyuşmadığı hatta daha az olduğu biliniyor. Bunun sebebi artan genleme değişiminden ileri gelen, malzeme dahilinde boşluk teşekkül etmesidir.
Sünek malzemelerin kırılma mekaniği çalışmalarında Gurson-Tveergaard-Needleman (GTN) hasar modeli çoğu kez kullanılmaktadır. Daha önce literatürde yayınlanmış dokuz adet GTN hasar modeli parametresinden üçü haricinde kalan altı adet parametre tek eksenli çekme metoduyla deneysel olarak elde edilmişti. Bu tez çalışmasındaysa, iki eksenli çekme durumuyla tek eksenli durumunun ne kadar yakınlaştığı incelenmektedir. Çalışma kapsamında şekil değişimiyle ortaya çıkan boşluk oranını tayin etmek maksadıyla boyun vermiş ve düzgün uzamış çekme testi numunesi kesitlerinde yoğunluk ölçümleri yapılmıştır. İlaveten elde edilen parametre değerlerinin doğruluğunu kontrol edebilmek için çekme testini simülasyonla tatbik edebilecek bir sonlu elemanlar modeli de ortaya konmuştur. Oluşturulan bu modelde malzemenin elastik bölgedeki özellikleri için literatürden faydalanılırken plastik bölgedeki özelliklerse uygulanan çekme testlerinden elde edilmiştir. Son adımda deneysel yolla elde edilen GTN hasar modeli parametre değerleri sonlu elemanlar uygulaması için veri olarak kullanılmıştır. Sonlu elemanlar simülasyonları sayesinde ampirik olarak elde edilen GTN model parametreleri doğrulanmıştır.

Abstract:
With the increasing competition in industrialization and environmental awareness, the lightening of products has been one of the inevitable expectations in design. Aluminum alloys which have been preferred in the aerospace industry for a long time due to their lightness from the past have been used intensively by the automotive industry in recent years. The auto industry uses aluminum for the vehicle frame and body, electrical wiring, wheels, lamps, paint, transmission, air conditioner condenser and pipes, engine parts (pistons, radiator, cylinder head) and magnets (for speedometers, tachometers and air bags).
Using aluminum for automobile manufacture instead of steel gives a number of benefits. On average, aluminum is 10% to 40% lighter than steel depending on the product. Vehicles made from aluminum have better acceleration, better braking and better handling. The rigidity of aluminum provides drivers with more immediate and precise control. The malleability of aluminum allows designers to engineer vehicle shapes optimized for maximum performance. Aluminum can absorb twice as much energy in a crash than the equivalent weight of steel. Aluminum can be used to increase the size and energy absorption capacity of a vehicle's front and back crumple zones, enhancing safety without increasing weight. Vehicles made from lighter aluminum require shorter stopping distances, helping to prevent collisions. Vehicles with aluminum components can be 24 percent lighter than those with steel components. This saves 0.7 gallons of fuel per 100 miles, a saving of 15 percent in fuel consumption over steel vehicles. Similar fuel savings are made when aluminum is used in hybrids, diesels and electric vehicles. Aluminum reacts with the oxygen in the air to form a microscopically thin layer of oxide. This layer is only 4 nanometers thick but provides excellent protection against corrosion. It even repairs itself if damaged. Especially the 5xxx and 6xxx series aluminum alloys attract attention due to its various characteristics. The 5xxx series profiles as one of the common use aluminum alloys which have Mg in 3-5%. It also call Al-Mg aluminum alloy. The characteristic is low density, high tensile strength and high elongation. Its weight is lower than other alloy in the same area. Therefore, it is widely use consumer electronics, aviation, and the normal industry. The widely use in 5xxx series alloy is 5052, 5005, 5083, 5A05. The other one of best materials of industry is 6xxx series aluminum alloy. The main chemical composition of it is the Mg and Si. Therefore, we call it Al-Mg-Si aluminum alloy. 6xxx series with its good plasticity and corrosion resistance characteristic become the most widely application alloy such as power lines, automotive, aircraft/spacecraft components and etc.. The widely use in 6xxx series alloy is 6005, 6063, 6061, 6060. While designers expect high strength from the material, formability is the priority for manufacturing. In this thesis, the forming performance of T6 (in solution and artificially aged) and T4 (natural aging) heat-treated aluminum alloys is investigated.
The behavior of regular and irregular elongation zones in uniaxial and multiaxial tensile tests of T6 and T4 heat-treated 6061 aluminum alloy has been investigated comparatively. It is known that the deformation behavior of the material is in accordance with the Holloman strain model equation in the region of uniform extension, but the unit strain value at the necking moment is not numerically equal to the hardening exponent and the actual stress value at fracture is lower than the Holloman model. The reason for this is the formation of voids in the material resulting from increased unit deformation.
The Gurson-Tveergaard-Needleman (GTN) damage model is frequently used in the fracture mechanics studies of ductile materials. Gurson suggested a model for void-related damage in porous domains including voids. After that, Tvergaard and Needleman made contributions to the original theory of porous metal plasticity. Therefore, the damage model of Gurson-Tvergaard-Needleman (GTN) has been broadly used for the estimation of deformation and fracture behavior of metals.
To estimate ductile damage in metals, researchers have conducted diverse studies via the GTN model. GTN damage model takes into account the void evolution in the course of the plastic deformation. Scholars have specified the phases of the void evolution through computed tomography (CT) or in-situ X-ray laminography. Cao and colleagues defined the ductile damage of high carbon steel by X-ray microtomography besides mechanical tests. They stated that the void density which characterizes void size increased exponentially with the effective plastic strain and the void nucleation process. Furthermore Yuenyong and coworkers applied a practical approach to establish the void nucleation related GTN model parameters: Direct current potential drop (DCPD). Void density has been correlated by them with the change in the resistance of the inspected zone.
Al-Mg-Si alloys have a broad range of utilization area including aerospace, aircraft and automotive industries as a result of their high strength/weight ratio, good corrosion resistance, formability, weldability, medium strength and low cost. For this reason, GTN model of Al alloys were investigated by the researchers, and they have identified GTN model parameters numerically for diverse manufacturing applications of aluminum alloys. In terms of tensile testing, microstructural analysis and simulations have revealed the GTN model parameters to compare the experimental outcomes with the finite element solver.
Moreover, Abbasi et al. utilized an artificial neural network and tensile testing results to identify the GTN model parameters. However, there are some researches that have experimentally obtained GTN model parameters of metals. Wcislik reported that quantitative image analysis and the final fracture scanning electron microscopy (SEM) photograph could be used to establish the final void volume fraction. He et al. also suggested to identify GTN model parameters of 5052-0 Al alloy by tensile tests and SEM micrographs. However, their experimental methodologies are not practical since they suggested digital image processing to get the final void volume fraction using fractographs. In addition, the standard deviation of the void nucleation distribution is based on values from literature.
Researchers obtained GTN model parameters for diverse metallic materials in the literature. Except for three of the nine GTN damage model parameters previously given from the literature; the remaining six coefficients were experimentally obtained by uniaxial tensile tests. In this thesis, it is examined how close the uniaxial case is in the multiaxial case. In order to determine the void ratio developed by deformation within the scope of the thesis study, density measurements were made in the sections of the tensile test specimen, which were properly elongated and necked. In addition, a finite element model was created to simulate the tensile test to check the accuracy of the parameters found. In this model, the properties of the material in the elastic region are taken from the literature. The properties in the plastic region were obtained from the tensile tests carried out. In the last stage, the experimentally obtained GTN damage model parameters were entered into the finite element software as data. The experimentally obtained GTN model parameters were verified by finite element simulations. For deformation applications, there are two kinds of finite element solver algorithms: explicit and implicit techniques. For the implicit one, the stress and strain at the integration points are revised by iteratively solving a set of nonlinear equations. However, for the explicit type, the stress and the strain at the integration points are updated by using known stress and strain states; therefore, iterative solving is reduced. A commercially available finite element solver, Abaqus, was used to develop the simulations of tests and a finite element model (FEM) model with the dimensions the test specimens that were consumed in the experiments were prepared. With this thesis, on the one hand, the findings that will fill the relevant gaps in the scientific and technical literature, and on the other hand, the outputs for the determination of the heat treatment parameters that will enable the efficient use of this method in the industry have been obtained. The results of the study are expected to contribute to the rapidly developing automotive and aerospace industries in our country and in the world, and to the entire manufacturing world where aluminum alloy plates (by cold forming) are used intensively.