리튬이온전지의 펄스 Nd:YAG레이저 용접시 용접결함의 발생원인과 방지대책

Abstract

Laser material processing is a very fast growing technology for various industrial applications, because of many advantages. Its major advantage of less and controlled heat input has been exploited successfully for the very critical application of battery case and electrolyte injection hole hermetic welding.

This thesis is about the occurrence source of weld-defects and its solution methods in pulsed Nd:YAG laser welding of lithium-ion battery. Lithium-ion battery is composed of aluminium which is not easily welded, because of high reflexibility and fast thermal transfer velocity. Therefore, aluminium is subject to weld-defects like a crack. Because of this, we need to find out the source making weld-defects and its preventive methods.

In this study, we used Nd:YAG laser welding machine, which has a short wave length, is better with a CO2 laser, since the short wave length gives smaller spot size and results in higher laser beam absorbance on an aluminium surface.

The welding method of battery case has changed over joint geometry from welding of side position to flat one. The welding result of flat position showed defect free and welding reliability. However, the welding of electrolyte injection hole is much more complicated than that of battery case.

We accomplished leak test in water by air compressor in order to check the points of weld-defect and observed cross section. As a consequence, we found major occurrence position and source of crack. This solidification crack was occurred at the crossing of 1st and 2nd bead. The formation of solidification crack caused weld instability, which affected by solidification shrinkage and hardening.

Therefore, welding method of electrolyte injection hole has got to be improved to reduce and prevent crack. Such solidification crack was reduced and prevented when crossing of 1st and 2nd bead was removed. In order to remove crossing of 1st and 2nd bead, method of ramp down was used. Furthermore, weld trajectory and defocused distance have been changed to reduce shrinkage stress and heat input.

The results give a good agreement with the experimental, where a sound weld shape and crack-free weld bead were obtained. In conclusion, this show that the welding stability is greatly affected by modulation of laser pulse shape for the same laser energy and welding parameters.