Nokta direnç kaynağında ısıl ve yapısal analizlerin sonlu elemanlar yöntemi ile incelenmesi
Thermal and structural analysis of resistance spot welding by finite elements method
Yazar: BAŞAR YAVUZ
Danışman: PROF. DR. MURAT VURAL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Direnç kaynağı = Resistance welding ; Kaynak = Welding ; Kaynak yöntemleri = Welding methods ; Sonlu elemanlar = Finite elements ; Sonlu elemanlar analizi = Finite element analysis ; Sonlu elemanlar yöntemi = Finite element method

Özet:
Elektrik nokta direnç kaynağı endüstride sacların birleştirilmesi sırasında yaygın olarak kullanılan bir birleştirme yöntemidir. Özellikle sac parçaların yoğun olarak kullanıldığı beyaz eşya, otomotiv gibi endüstri kollarında kendine geniş kullanım alanı bulmaktadır. Bu yöntemde saclara yüksek basınç altında elekrik akımı uygulanır. Bu akım sonucu oluşan ısı enerjisi de kaynak çekirdeğinin oluşturulmasında kullanılır. Elektrik direnç nokta kaynğında elektriksel, ısıl, mekanik ve metalürjik olaylar birbirleriyle bağdaşık ve etkileşim halinde etki ederler. Bu durum sistemin tüm etkileri tek bir seferde gösterecek bir matematik modelin oluşturulmasını zorlaştırmaktadır. Bu nedenle genellikle analiz tek bir etki üzerine yapılır ve bu şekilde sonuca ulaşılmaya çalışılır. Elektrik akımı sonucu oluşan ısı Joule kanununa göre oluşur. Bu ısının bir kısmı ortam dağılırken çoğunluğu çekirdeğin ergimesi ve katılaşarak birleştirilmesi sırasında kullanılır. Yöntemde uygulanan baskı kuvveti de ayrıca önem arz etmektedir. Elektrot kuvvetlerinin uyguladığı kuvvet ergimiş haldeki kaynak çekirdeğine baskı oluşturur ve kaynak banyosunun dağılmasını engeller. Ayrıca kaynak bölgesi için zararlı olan atmosferik gazların ve toz parçacıklarının kaynak bölgesine ulaşmasını da engellemiş olur. Nokta direnç kaynağında akımın düzgün bir şekilde akması öenmlidir. Elektrotların uyguladığı baskı kuvveti bu akımın da doğru bir şekilde akmasına yardımcı olmaktadır.
Bu parametrelerin doğru bir şekilde uygulanması ile kaliteli bir kaynak çekirdeği oluşumu sağlanır. Kaliteli bir kaynak birleştirmenin sahip olduğu dayanımı belirlemektedir. Bu nedenle kaynak kalitesi feragat edilebilecek bir özellik değildir. 
Sıcaklık dağılımı da metalürjik yapıyı dolayısıyla kaynak kalitesini ve dayanımı etkilemektedir. 
Bu çalışmada kaynak işlemi sonucunda ki ısı dağılımı, uygulanan baskı kuvvetleri nedeniyle oluşan deformasyon ve en yüksek gerilme durumlarının tespiti amacıyla sonlu elemanlar yöntemi kullanılarak bir model oluşturulmuştur. Daha sonra oluşturulan bu model yardımıyla bir sümülasyon çalışması yapılmıştır. Bu model yardımıyla deneysel olarak tespit edilmesi zor ve maliyetli olan nokta direnç kaynağına ait parametrelerin incelenmesi ekonomik ve hızlı bir şekilde sağlanmış olur.
Ayrıca bu çalışma değişik malzemler için gerekli olan optimun ısıl ve kuvvet değerlerinin belirlenmesi sırasında veri kaynağı olarak kullanılabilir.

Abstract:
Resistance spot welding is widely used in the industrial applications. It is used to join the sheet materials. RSW can be applied into the automation easily. Especially in automotive industry, it is the main welding method. It has thermal, structural and electrical phenomena together. This makes the experimental analysis very hard. In this method first we put sheet metals together, and then RSW electrodes applied pressure and give electrical current. After that, system stops the current flow while pressure is still applied. The last, system release the pressure and we can take the sheet metals. Current flow creates heat. Heat is created with Joule law. This heat melts the material. Sheet materials join and solidified in each other. In sheet metals heat should applied in small amount and in short time. Because, high heat flow in sheet metals create distortions. 
RSW has some parameters. Some parameters change the welding quality. Welding time and welding current have more effect than the other parameters. If we increase the welding time metals can burn. If we decrease the time then we take low quality weld area. Current is also important. According to Joule law current generates the heat. When the high current is applied, cracks can be created. When the low current is used, then we need more time for joining. This increases the weld area and decreases the welding quality. 
Heat affected zone is the most important topic in the all welding methods. This area has its own microstructure. Heating and cooling time determine the microstructure in this area. This area also identifies the quality of the welding. Because big heat affected zone changes the materials microstructure.
In this paper, the thermal and structural analysis in resistance spot welding is examined with using finite element analysis. For this research S235, 304 and Al-7075 are used as sheet metal's material. 
Finite element analysis is widely used in engineering analysis. Engineers and researchers want to examine the system in order to make better systems. Generally, experimental analyses are expensive and hard to set up. With these difficulties, people make this analysis by using computers. Every machine or application can be modeled by using mathematical models. Some of them have many equations which are hard to solve. Computers are used to solve these equations. Finite element method is used for this approach. Every system has infinite points. Finite element method describes the system with finite points. Systems represented with these new model which gives the original system response. FEM gives the result with some errors. These all points are named as node. Nodes link with each other. These nodes create the elements. Elements can be 1, 2 or 3 dimensional. FEM is a numerical method. It is used in fluid mechanics, structural-thermal analysis, vibrations etc. 
FEM try to find changes in nodes by using some equations. In order to reach the solution, system should be fixed in some points.
Finite element method has some steps in order to reach the results. First, system is divided into the small parts which are called as element. For the elements physical behaviors are defined. Equations are created for these elements. Unknown data are solved by using equation system. The last, desired element data are found.
For the complex systems, thousands of equations are exist. When the system become more complex, then the equations are also become complex and hard to solve. These equations cannot solve analytically. Computers are used for this kind of situation. In some models computers cannot solve the system, as well.
In this method, 1-D and 2-D elements are used. 1-D element carries the force parallel to its axes. These kinds of elements are used to model the bridge or truss system. The other option is using 2-D element. It is used generally pressure vessel system analysis. If the thickness of the material is much lower than the other dimensions, then 2-D elements are good for it. These elements are located differently in order to get 3-D system. On the other hand some computer programs use their own element types. 
Resistance spot welding is a very fast method and has different phenomena together. That's why researchers cannot easily test it experimentally. In order to examine the process, researchers use some FEM. However, researchers cannot analysis all phenomena in one step. In some cases, some special programs are used to examine. 
In thermal analysis, Fourier hear equation is used. Some reference points and initial conditions are added. After that thermal analysis can be done.
In structural analysis, there are two basic equations. One of them is balance equations. The other one is structure equations. Von mises yield criterion is used for the structural system analysis.
For this paper, two sheet metals are used. These metals are joint by using resistance spot welding. And also finite element model is used to examine the thermal and structural behavior while the welding operation has been done.
Finite model has nearly 150000 nodes and 28000 elements. The model has 100x1x10 (mm) dimensions. Ambient temperature is added as 22 C. Three types of materials are used as sheet metals which are S235, 304 and Al-7075. All thermal and structural properties are added to the system. And 5000 N force applied to the system as an electrode pressure. 
In analysis process, we accepted that weld nugget has occurred in 0.3 s. After that system starts to cool while 5000 N force is applied in 1 s. As a result we try to understand thermal and structural behavior of system. 
Temperature distribution, deformation and max equivalent stress are found for three materials.
For S235, we find the temperature distribution. Deformation is found 0.003 mm and max equivalent stress is measured as 1153 MPA.
For 304, temperature distribution is found in different times. Deformation is found 0.003 mm and max equivalent stress is measured as 1147 MPa.
For Al-7075, temperature distribution is found, as well. Deformation is measured as 0.03 mm and max equivalent stress is measured as 634 MPA during the welding operation.
With this research, we get some outcomes. Temperature distribution has low heat flow until 1 second. After this, it cools in high rate.
Temperature distribution has circular shape. It has not elliptical shape like the weld nugget.
There are symmetrical deformation occurs around the weld nugget. This deformations occur due to electrode applied force.
Max equivalent stress has max value instantly. However, this value decreases in some point after the welding operations.
Max equivalent stress occurred around the weld nugget. And also some stress occurrence exists at the edge of the electrode applying area in the sheet metal.
By using this research we can do some new research in the future. Optimum force can be found for different type of materials.
The prediction of the deformation at the electrode can be examined.
After the long using of electrode, some cracks may occur. We can predict the crack occurrence speed.
Max equivalent stress should be decreased. We can use this research in order to decrease the max equivalent stress.