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Sonlu elemanlar metodu yardımıyla nokta direnç kaynağının soğuma analizi

The finite element modeling of the thermal distribution obtained resistance spot welding results

  1. Tez No: 66752
  2. Yazar: LÜTFİYE ÇEBİ
  3. Danışmanlar: DOÇ. DR. MURAT VURAL
  4. Tez Türü: Yüksek Lisans
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1997
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Konstrüksiyon Bilim Dalı
  13. Sayfa Sayısı: 72

Özet

ÖZET Sayısal analiz yazılımları, örnek olarak ANSYS yazılımı, mühendislere makina parçalarını modelleme, işletme durumundaki yükleri yükleme ve sıcaklık dağılımı, gerilme gibi fiziksel tepkileri inceleme olanağı verir. Bu işlem, firmalara birçok prototipi bilgisayar ortamında yapma, test etme, tekrar üretme ve optimum boyutlarda en ucuz üretim imkanı sağlar. Bazı konularda, örneğin biyomedikal uygulamalarda, yapay uzuvlarda ve göz lenslerinde, prototip testi imkansızdır veya istenmez. Sayısal analiz sonucunda, en iyi dizayn için maliyet, boyut, ısı kısıtları konarak en iyi çözüm bulunur. Üretimden veya kullanımdan önce en iyi tasarım yapılabilir. Bu çalışmada ki amaç; direnç nokta kaynağı yapıldıktan sonra oluşan ergiyen bölge ve ısının tesiri altında kalan bölgedeki sıcaklık dağılımının Ansys paket programı yardımıyla bulunmasıdır. Kaynak isteminin bu son periyodunda soğuma zamanına bağlı olarak, tanelerde kristalli ve iri taneli yapı oluşmaktadır. Bu arada, kaynak periyodu sırasında oluşan ısının büyük bir kısmı kaynak çekirdeğinin oluşumuna harcanırken, bir kısmı da çekirdeğin yakın çevresinde sıcaklık dağılımına bağlı olarak bazı metalurjik oluşumlara yol açmaktadır. Yani parçada gerilmeler ve çarpılmalar oluşmaktadır. Bu gerilmeleri ve çarpılmaları en aza indirmek veya parçanın kullanılacağı yere göre farklı bölgelere kaydırmak için, soğuma zamanı veya uygulanan kuvvetin miktarı değiştirilebilir. Bu model bilgisayarda hazırlanmasaydı, her farklı durum için yeni bir deney yapmak gerekirdi. Oysa bilgisayarda sayısal veriler değiştirilerek sonuçlar kısa zamanda görülebilir. iv

Özet (Çeviri)

SUMMARY The resistance spot welding which is used as a manifacturing process for joining sheet metal has a great importance in automotive industry, for decorative aims in construction industry, as well as in aircraft, ship, chemistry, food and space industries. Its welding process for joining two materials at their common interface is a complicated interaction of electrical, thermal, mechanical, metallurgical and surface phenomena. A brief description of this welding process is explaned, since it will help identify features that should be included in a theoretical model to provide a realistic simulation. In this welding, the electrode force is used to join the metals. The force produces a local deformation at the common interface to seat the workpiece properly and to establish good electrical contact before current flow. It should be noted that this initial load generates high contact stresses between the electrodes and workpiece. Repeated mechanical cycling can contribute to the mechanical fatigue and distortion of the electrode. Then melting will initiate at the common interface of the workpieces and spread to produce the weld nugget. This phase change from solid to liquid produces a change in material properties. During the weld cycle, the electrode load is maintained to offset the high internal thermal expansion and thereby, contain the molten metal within the workpiece. This prevents liquid metal expansion. The weld cycle is terminated by switching the current off while maintaining the electrode load. The final stage of the process that we will examine is the hold cycle, which establishes the metallurgical quality of the weld nugget. During this stage, the nugget cools and contracts. Mechanical load is essential to provide the necessary forging pressure to obtain a good metallurgical structure and to prevent the formation of shrinkage voids. In this study; the aim is only to analyze the temperature distribution using Finite Element Methods depends on the cooling time. It was VIaccepted that weld nugget geometry were obtained before and had 1400 C. Then the workpiece is left to cool down. The Finite Element Method of analysis is essentialy the numerical solution of the governing field or partial differential equations by the discretization of the continuum. According to this method, the original geometry is replaced by an array of elements consisting of nodes in the interior and on the surface of the material. The model response is sought for a single element such that the continuity of the function is preserved between the nodes while satisfying the gowerning field equations in the interior. For example, the finite element formulation for an elasticity problem can be derived by using the principle of virtual work for determining nodal displacements such that compatibility is satisfied exactly and equilibrium approximately in the interior. Once the nodal response of a single element has been derived, the global behavior of the solid is then obtained by summing up the contribution for each individual element. Accordingly, this formulation reduces the original problem to the solution of coupled simultaneous equations, which can be compactly written in matrix notation. As it is well known, this method has been developed to provide solutions to problems where an analytical or classical numerical approach is impossible. It may be applied to a wide variety of problems defined by quasiharmonic partial differantial equations, such as stress analysis, fluid, and thermal engineering etc. Its use is particularly interesting for problems involving complex geometries. One of the chief features of the finite element technique is the way curved boundaires can be realistically treated by using higher order isoparametric elements. Accurate solutions can be obtained in regions where the gradiens are steep by refining the mesh. It can be shown that as the nodal degrees of freedom are increased by refining the mesh, the solution will generally converge to the exact solution of the governing equation. The finite element method of analysis has been recognized as a powerpool tool and has been effectively used to obtain solutions in solid mechanics, heat transfer, and fluid dynamics. An axisymnetric finite element model was used to simulate the spot welding process during the hold cycles. For the case of equal thickness workpieces and equal electrode qeometry, only one quadrent of the model has to be constructed due to qeometric symmetry about the centerline of the electrodes and the common interface between the workpieces. This simplification of the model reduces the number of elements. Solid quadratic finite elements were used to represent the electrode and workpiece. vuThe boundary conditions are imposed on the finite element. The mechanical load was applied as a pressure distribution on the annuler end of the electrode. In addition, this element will allow relative slip between the electrode and woorkpiece due to the difference in their thermal expansion. Convective heat transfer to ambient air was specified on all the lateral surfaces of the workpiece except the surfaces in contact were specified as stress-free. At the beginning of the hold cycle, the initial condition of temperature of the weld nugget geometry is set equal to 1400 C while the other temperatures of the workpiece are specified at room temperature. During the phase, the transient temperature and thermal expansion chanqe rapidly. The Finite element model just described can be constructed from standart finite element computer codes having extensive element libraries which are on the market. In this investiqation, the ANSYS finite element program was used to construct the model The computer program essentially calculates the coefficients of the set of linear algebraic equations and then solves this system. The program can be divided into six different parts: l)Data Input: The data to be introduced are the number of constitutive materials and their respective thermal conductivity; and of the nodes located at the vertices of these elements and the co-ordinates of these nodes. Finally input data are introduced for boundary conditions. 2)Computation of stiffnes matrix and covection matrix. 3)Computation of the boundary conditions vectors for each node. 4)Matrices and boundary conditions vectors assembly. 5)Solution of the linear equations system in nodes temperature. 6)Results input. Isotropic temperature dependent material propertres were used. These properties were specified over a temperature ranqe from room temperature to above melting The mesh size at the end of the electrode was found to be sufficiently refined for the determination of the variables and their gradients in the region of interest. Since the temperature on the upper VUlsection of the electrode were small a coarse mesh could be used without sacrificing solution accuracy. The finite element model of the resistance spot welding process includes the fallowing features and capabilities. 1. Flat end electrode 2. Applied temperature 3. Cooling time 4. Convective heat transfer to air Finite element results 1. Temperature distribution in electrode and workpiece after cooling 2. Deformation of electrode and workpiece during welding eyele The analysis was conducted using class III copper and Type304 austenitic stainless steel material properties for the electrode and workprece respectively. Before the starting the analysis, it was accepted that weld nugget geometry had occured and the weld nugget had the temperature ratio which is taken with respect to the liquidus temperature equal to 2600 F (1427 C). Then the other temperatures were at room temperatures at 37 C. The weld nugget height and diameter were determined as 0.052 (1.32 mm) and 0.180 in (4.57 mm) respectively. Time at the beginnig was set tequal to 0.001 sn. In the analysis, the temperature distribution was investigated at the end of 0.04 sn. Of course, time can be changed until the correct joining was occured. At the end; it was investigated temperature distribution for a variaty of materials to be joined. As a conclusion, using a computer program realistic results can be obtained but in this study, at the beginning of the resistance spot welding was not observed. So, the results can not be correct. But if the model includes the electrothermal mechanical interaction and good temperature-dependent material properties, the finite element modeling of the resistance spot welding process can provide good simulation. It is common practice in industry to use welding conditions tabulated in handbooks as the starting point to develop a welding schedule. The results show that there is an another alternative : the use of a realistic analytic model. Finite element modeling and analysis is a well developed technique. Using this method, one can investigate weld nugget formation for a variety of materials to be joined and different electrode configurations. The use of an analytic tool by the process engineer provides him with another procedure to develop weld schedules before going into the shop floor to conduct tests. Many analytic experiements were conducted inexpensively using this model to determine the significant variables and the best place to measure them. In addition, the finite element model and the analysis IXconducted provided a better understanding of the welding physics which would be difficult to determine by experiment alone.

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