CO₂ Welding of Galvanized Steel

Abstract

It is well-known that CO₂ welding of galvanized steel often makes a porosity, such as a blow hole or a worm hole, in weld metal.
In this study, in order to investigate the mechanism of porosity formation process in weld metal of galvanized steel, zinc analysis on inner surface of blow hole, gas chromatography analysis for gas in porosity and gas evolution test of weld metal of galvanized steel were carried out.
Moreover, several types of welded joint were fabricated by the best welding conditions expected for giving less porosity, and their weld results and static strength was evaluated.
As the results, the following matters were concluded.
(1) The formation process of porosity was considered as follows: Hydrogen which was expected to exist in Fe-Zn compound layer formed on mild steel surface as zinc was coated, was came out into molten pool during welding. As the thermal instability, such as large spatter formation, was occurred in molten pool, continuous gas evolution from melted metal was intercepted, and the porosity was formed in weld metal at that time.
(2) It was clarified that the welding condition which gave less porosity was low welding voltage, low welding current and low welding speed for 1.6-2.3 mm thichness galvanized plate. And it was noticed that arc point must be hold the best position to make arc stable.
(3) Appearance of obtained weld bead and X-ray results of welded joints were estimated good, and joint efficiency was about 1.0 in all types of welded joint except for specimens in which the large weld distortion or melt out hole were occurred.