LPG특수선 재액화장치용 순티타늄판의 Nd:YAG 레이저 용접성에 관한 연구

Abstract

Titanium and its alloys have excellent corrosion and erosion resistance, high specific strength and creep properties in high temperature, and these properties make them using various fields of application. For example, the chemical plant, sports and leisure equipments, aero spaces, and so on. Titanium heals itself even though the oxidation layer(TiO2) is broken in the sea water. Therefore, it has outstanding resistance for the stress corrosion cracking, crevice corrosion, pitting and microbiologically influenced corrosion. Additionally, pure titanium has good formability which bring out to the best material for the heat exchanger plate.

Conventional plate type heat exchanger has prevented fluid leakage with gaskets between the plates by mechanical joining method, but gaskets are hardened in high temperature and pressure. Thus, LPG re-condenser which is always in high temperature and pressure, is jointed its plates by welding. EBW and diffusion bonding make high quality welds, but it should be done in vacuum conditions. And arc weldment has large HAZ and possibility of distortion because of large heat input. Hence, laser welding is applied to get sound beads with small heat input and high aspect ratio in the air.

CP titanium(ASTM grade 1) was used by experimental materials. It was welded by pulsed and CW Nd:YAG laser. The best welding conditions for production were drew out after investigation of welding parameters and heat sources.

The principal laser welding factors for titanium are as followed

First, the pulse width which can control the peak power is the most important parameters of pulsed laser. And the laser power and welding speed that can control the heat input are the most important parameters in CW laser. Secondly, the weld bead changes its color to gold, brown, blue and gray with oxidation. And nitrogen is more effective to the embrittlement of welds than oxygen. Finally, if weldments are exposed in the air, microstructure of weld metal is changed to martensitic structure. Conclusionally, the most important thing for titanium welding is the selection of shield conditions.