Using the conformal mapping method, the current density distribution at the contact interface and the constriction resistance of a single and multiple line contact were calculated for various space angles. The distribution of heat generation rate in neighborhood of a single line contact was also determined for various space angles. The results of the calculation revealed that the influence of space angle on the current density distribution, contact resistance and heat generation rate distribution is considerable and can not always negligible.
The influence of the axisymmetric contact pressure distribution on the current density in the circular contact surface between two conducting semi-infinite bodies was also theoretically analyzed by using the newly proposed mechanical asperity contact model and the electrical current flow model. The shape of the nominal current density distribution approximates to the distribution shape of the contacting pressure but the current density on a micro contact approximates to the inverted shape on the contacting pressure, and that the effect of the pressure distribution shape on the overall contact resistance is not negligible and must be considered, when the mean pressure level is lower than several percentage of the hardness of the contacting materials. On the other hand, increasing mean pressure level causes both the current densities to increase as approaching the contact surface margin and the effect of the pressure distribution shape on the overall contact resistance to weaken.
The effect of contact force, micro asperity and contamination on the potential, joule heat generation rate distribution and the temperature response was analyzed and discussed by using a hybrid finite element model. In this model, the analytical solution of constriction resistance for single and multiple line contacts and asperity deformation approximation were adopted to consider the effect of micro contacts for calculating the potential and temperature distribution using finite element method. The results of calculations revealed that the effect of those on the temperature, potential and joule heat rate distribution in the resistance spot welding system (as one of the conducting bodies system) is considerable when many deformable asperities exist in the contact surfaces, and therefore not always negligible.
In this thesis, a new method was also investigated to measure the high current in resistance welding process. A measuring unit was developed by using a strain gage attached on the outer surface of a steel ring. The steel ring was placed around a section of the secondary loop of the welding machine, and was deformed by electro-magnetic forces induced by the high welding current. The strain gage signal of ring deformation is enough to determine the welding current in resistance spot welding, especially when direct current is used for the welding.
저항점 용접 공정은 용접물 내부의 전기저항 및 용접물과 용접물, 용접물과 전극간의 접촉부에서 발생하는 접촉저항에 의해 발생하는 저항열을 이용하여 용접부를 얻는 공정이다. 이중 특히 접촉저항은 용접공정의 초기단계에서 가장 중요한 발열원이며 전기전도도가 큰 금속을 용접할수록 그 발열원으로서의 중요도는 커진다. 그러나 이러한 중요성을 가짐에도 불구하고 지금까지의 저항점 용접공정 해석에서는 접촉저항에 관한 이론적인 해석 및 고려가 비교적 찾기 힘든 실정인 바, 이는 전기접촉 현상이 가압력 및 표면상태 등에 큰 영향을 받는 복잡한 현상으로써 저항점 용접공정의 해석에 적합하도록 이론적인 모델을 만들기 힘들었기 때문이다. 본 논문의 경우는 전기 저항 점 용접시 집중적인 열의 발생이 예상되는 접촉부에 대한 해석적인 전류흐름 모델을 세워 가압력을 고려한 접촉저항을 예측하고 이에의 한 결과를 용접부 전체의 온도해석을 위한 수치적 모델에 응용함으로서 보다 정확한 너겟 형성 과정의 예측 및 최적 용접조건 선정에 도움이 되는 이론적인 저항점 용접공정 모델을 제시함을 그 목적으로 한다.
