하이브리드 용접에 의한 세립강 용접부의 미세조직변화에 관한 연구

Abstract

A laser beam welding and an electric arc welding were conjugated in order to apply the positive points of each welding method such as high speed, low thermal load, deep penetration, and high productivity. It has been widely studied for the automation industry of a pipeline welding more about the fiber laser-MIG conjugated welding namely the hybrid welding. This work is comprised of two research section. In the first section, the MIG welding was combined with a fiber laser welding to make up the hybrid welding. The weld shapes, microstructure and mechanical properties for weld zones was investigated after the hybrid welding or only fiber laser welding with 700 MPa grade Ultra Fine Grained(UFG) high strength steel. The amount of acicular ferrite and HAZ(heat affected zone) were observed larger after hybrid welding compared with after only fiber laser welding using by optical microscope and SEM. Vickers hardness was used to compare the mechanical properties of transformed microstructure at the top and bottom area of fusion zone after hybrid or only fiber laser welding. The quantitative metallography on the weld metals was carried out after hybrid welding or only fiber laser welding. Vickers hardness on weld metal after fiber laser welding was higher than after hybrid welding. In the second section, in order to understand the effect of filler wire, the microstructure and mechanical properties in weld zones after the hybrid welding with 700 MPa grade Ultra Fine Grained(UFG) high strength steel were investigated on the various filler wires containing different amount of Ti, Mn. The distribution of alloy composition of filler wire in weld metal was analysed by EPMA. The amount of Ti, Mn elements on weld metal at the top zone(arc zone) was higher than bottom zone(laser zone). The microstructure evolved on weld metal is different from each filler wires. The proportions of the grain boundary ferrite and second phase in weld metal were increased with manganese content, which caused to the slight increase of Charpy impact toughness. The proportions of acicular ferrite was not increased.