ÖZET Anahtar Kelimeler: Sonlu elemanlar analizi, süperplastik deformasyon, deformasyon hızı, 7075 Al alaşımı, 2024 Al alaşımı Bu çalışmada, deformasyon hızına duyarlı malzemelerin süperplastik şekil verme işlemlerinin, sonlu elemanlar tekniği ile incelenmesinde kullanılabilecek pratik bir yöntem önerilmiştir. Yöntemin temelinde, önceden belirlenen zaman aralıklarında, önceden belirlenen bir kritere göre, elemanların o anki durumlarını tanımlamada kullanılan bir karşılaştırmanın yapılması ve gerekli elemanın özelliklerinin değiştirilerek analize devam edilmesi yatar. Deneysel çalışmada, 7075 Al ve 2024 Al alaşımları 1 mm başlangıç kalınlığından 0,55 mm, 0,40 mm ve 0,25 mm kalınlığa soğuk olarak haddelenmiştir. Bu kalınlıkta hazırlanan numuneler 475°C, 500°C, 525°C ve 550°C olmak üzere dört farklı sıcaklık ve 5x1 0"3, lxl 0'3, 5x1ü"4, 1x1 0-4 ve 5x1 0"5 sn"1 olmak üzere beş farklı başlangıç deformasyon hızında çekilmişlerdir. Sonlu elemanlar yöntemi analizlerinde kullanılan çekme modelinde, simetriden dolayı çeyrek numune kullanılmıştır. Süperplastik deformasyona yönelik gerçekleştirilen sonlu elemanlar yöntemi analizlerinde kullanılan geleneksel eşitliğin(akma gerilmesi değerini deformasyon hızının üstel biçimi olarak ifade eden eşitlik) yerine farklı bir yaklaşım önerilmiştir. Elemandaki deformasyon hızı üç değişik yaklaşımla tanıtılmıştır. Birincisinde, sonlu elemanlar yöntemi ile hesaplanan elemanın yer değiştirme hızı deformasyon hızı olarak alınmıştır. İkinci tip analizlerde sonlu elemanlar analizi ile o ana kadar elde edilen deformasyon miktarının değişim hızı elemanına ait deformasyon hızı kabul edilmiştir. Üçüncü de ise deformasyon hızı mühendislik zorlanması değerinden çıkarılmıştır. Ayrıca bu üç karşılaştırma kriterinden biri olan ikinci tip analiz, bir kahp içinde şekil verme analizinde kullanılmıştır. İlave olarak, kalıp içinde şekil verme analizinde karşılaştırma kademesinde ufak bir değişiklik yapılarak, karşılaştırmanın başlangıç durumuna göre değil, bir önceki karşılaştırma kademesindeki değerlere göre yapılması sağlanmıştır. Sonlu elemanlar analizlerinde kullanılan malzeme özellikleri, deneysel çalışma sonuçlarının basit bir düzenlemesi ile elde edilmiştir. Yapılan sonlu elemanlar analizleri neticesinde, önerilen her üç tip karşılaştırma ve yerine koyma yönteminin çekme numunesi analizlerinde kullanılabileceği gösterilmiştir. Kalıp içinde şekil verme analizlerinde ise karşılaştırmanın bir önceki karşılaştırma değerlerine göre yapıldığı analizin daha geniş bir uygulama alanına sahip olabileceği ortaya çıkarılmıştır. Deneysel çalışmalar sonucunda ise hemen hemen bütün numunelerin uygulanan şartlar altmda deformasyon yumuşaması davranışı gösterdikleri tespit edilmiştir. Ayrıca deneyde kullanılan 7075 ve 2024 alaşımları için maksimum uzamanın elde edildiği bir sıcaklık ve deformasyon hızı aralığının olduğu bulunmuştur. Bu değerler her iki alaşım için de 0,25 mm kalınlık, 525°C sıcaklık ve 5x10"* sn"1 başlangıç deformasyon hızı olarak tespit edilmiştir.
A COMPUTER AIDED NEW APPROACH FOR ANALYZING SUPERPLASTIC FORMING PROCESSES Keywords : Finite element analysis, superplastic deformation, strain rate, 7075 Al Alloy, 2024 Al Alloy Superplastic materials are polycrystalline solids which have the ability to show unusually high ductilities. Superplasticity is an important mode of deformation in metallic alloys with very small grain sizes and a deformation temperature of >0.5 Tm. If these conditions are fulfilled and maintained, then a material will show a high strain rate sensitivity of flow stress which is the most important mechanical characteristic of superplastic materials. For superplasticity, strain rate sensitivity(m) should be > 0.3. However, a high m does not necessarily result in a high tensile strain. A number of materials are prone to cavitation during superplastic flow, and this can lead to premature failure. In general, high elongations are observed over a rather limited range of intermediate strain rates and there is a decrease in the superplastic effect at both high and low strain rates. Although a fine grain size is an important requirement for superplasticity, it is not always possible to characterize the microstructure by this single parameter. It is important that the grain boundaries at high angle disordered. Other factors, including grain aspect ratio, grain size distribution, texture and the volume fractions of phases and their deformation characteristics can also influence superplastic behavior. Superplastic deformation is characterized by low flow stresses and this, combined with a high resistance to non-uniform thinning has lead to the superplastic forming of near net shape from sheet material. It is well established that grain boundary sliding is the primary deformation mode for superplastic flow. However, sliding between two grains cannot occur without the necessity for accommodation at the intersection with a third grain. Accommodation mechanisms are usually considered to determine the kinetics of superplastic flow, and may include diffusional and dislocational processes. Various theoretical models of superplasticity have been proposed, differing from each other primarily in the details of how the stress concentrations produced by grain boundary sliding. Superplasticity has been observed for a wide range of materials including metallic alloys, metal matrix composites, intermetallic phases, ceramics and minerals. A number of aluminum, titanium and nickel base alloys; aluminum based metal matrix composites have been used to produce aerospace parts. For titanium alloys the xviiisuccessive application of diffusion bonding and superplastic forming is used to produce relatively lightweight, complex cellular structures of high torsional rigidity. To take better advantage of the superplastic characteristics of the material, it is necessary to control the temperature and the strain rate during the manufacturing process. Since the material undergoes significant elongation, it also undergoes extreme thinning. The latter can be caused because of the particular geometry of the manufactured product, the characteristics of the material used, the lubrication, and the process parameters adopted. The design stage should take account of the real thickness distribution in order to avoid critical areas. The deformation process and variation of thickness should be controlled closely. At this stage, it is necessary, not only to design the product, but also to design the process in order to establish the optimum production parameters and to foresee the real geometry of the product. It is crucially important, in the process, to be able to control the pressure cycle in the order to obtain the optimum strain rate. To calculate optimum pressure cycle and for simulating the forming process, a finite element program can be used successfully. Numerical modeling can be used since analytical modeling would be limited only to some forms and to the use of largely approximated assumptions and experiments can be too expensive. The finite element method can be considered to be the most dependable both for analyzing complex geometry, and for taking into consideration all the phenomena involved in the manufacturing process. In this study, a different approach is proposed to use in the finite element analysis of superplastic deformation of strain rate sensitive material. The main aim is to model high deformation of strain rate sensitive materials by using basic flow rules that is not concerning the strain rate sensitivity. In this approach, a check and replacement schedule is used in the predefined time intervals. On the other hand an experimental study was also carried out. In the experimental study, two kind of aluminum alloys which are designated as 7075 and 2024 are rolled to the 0.55 mm, 0.40 mm and 0.25 mm thickness. Tensile test samples are machined from these rolled sheets. Uniaxial tensile tests were carried out at four different temperatures (475°C, 500°C, 525°C and 550°C) and five different initial strain rates (5xl0"3, lxlO"3, 5x10"*, 1x10^ and 5x1 0"5 sn1). In the finite element study tensile test sample is modeled, and three types of analysis is carried out. In the first analysis, displacement rates of each element are assumed as strain rates for each element. In the second analysis, strains of each element is calculated every predefined time intervals by the finite element analysis and strain rate of each element is calculated as strain change in the time period. In the third analysis, an equation which gives the strain rate of tensile test according to its gauge length and cross-head speed is used for the calculation of strain rates of each element. According to all these there definitions, results showed that each analysis type can be used for the finite element analysis of strain rate sensitive material's high deformation processes. A finite element computer code ANSYS was employed and used following a new approach. In this approach a step by step analysis was carried out and controlled to allocate a strain rate value with an element deformation. Having said using that the third definition face some restrictions. Furthermore, the finite element analyze is extended for the deformation process in a die. In these analysis two kind of time intervals are used. In the first attempt time interval is accepted as xixtotal analyze time. In the second attempt, the time interval is assumed as the difference between the last two check times. In both attempts, the results are in good agreement if the shape process takes place in a short time. Otherwise the second time interval method should be used. Finally, the experimental study results revealed that nearly all samples showed strain softening for the two types of aluminum alloys. It is shown that there is optimum temperature and strain rate range is present. The optimum process conditions can be given as 0.25 mm thickness, 525°C temperature and 5x1 0"4 s"1 strain rate. XX
