연속파형 Nd:YAG 레이저를 이용한 이차전지 극주 SS41의 용접특성에 관한 연구

Abstract

It is difficult to process the cutting-edge industrial technologies with the conventional methods due to new materials and miniaturization. A laser manufacturing system requiring a high precision processing step based on the laser has come into the spotlight with the development of advanced machining process technologies. The laser process is widely used in welding, cutting, heat treatment and marking. Although the price of laser devices is expensive, they work fast, and their materials barely transform. It is advantageous to use them in thin plate welding because of their fast speed with a contactless method. Especially, laser welding and cutting technologies are actively used in motor industry. However, the level of environmental pollution has become serious as the related industry has developed. Regarding this point of view, the motor industry focuses on developing electric or hybrid vehicles and researching key components in those cars. As a representative research about a secondary battery, an Argon TIG welding method was applied; however, since the performance of its weldability was not enough, a laser welding method has been applied in order to complement it. In this study, we employ the T-joint welding technique between a pole plate and tap to increase the output of a secondary battery. As a basic test, we tried to find the optimal condition for the lap joint welding between a pole plate in a secondary battery and a low carbon steel SS41, which is tap material. From this test, the weldability is measured in comparison with welding characteristics by using a Ni-coated specimen of SS41 and according to nozzle types. As a result, the laser power is 1800W, and the laser beam welding speed is 1.8m/min for the optimal welding condition in case of a cross nozzle when lap joint welding was applied to every speed. The above welding condition is good for a coaxial nozzle, and a laser beam welding speed is fine for 2.0m/min. However, since it is lap joint welding, porosity is made at welding areas. Lap joint welding is applied to Ni-coated SS41. Overall, porosity defects were not found, but crack issues appeared on joint areas. It is considered that nickel has influence on the crack issues. For T-joint between a pole plate and tap as a research objective, we tested SS41 and a Ni-coated SS41 specimen with a coaxial nozzle and cross nozzle, respectively. For the test condition, the laser power was 1800W, and the laser beam welding speed was 1.8m/min. As a result, the cross nozzle shows a uniform welding bead and welding depth while the quality of welding at front bead is low in the coaxial nozzle due to the high pressure of shielding gas, and the molten pool is deeply formed by the shielding gas. It might be advantageous to increase pressure than to weld at the coaxial nozzle in cutting.