A serious problem in the spot welding of hot dip galvannealed high strength steel sheets is the occurence of violent spattering of molten metal in the nugget and formation of explosion.
A study was carried out to develop hot dip galvannealed high strength steel sheets having excellent resistance to explosion and also to develop welding methods for supressing the formation of cavities by explosion of the nugget.
Results proved that the occurence of explosion depends upon the sort of iron-zinc compounds in the coating, and that when the coating has Γ phase of thickness more than 0.3 micrometers, explosion occurs, often exceeding a rate of 10%. As the Γ phase increases the electric resistance of the coating, it accelerates the formation of nugget in the initial period of welding and melts rapidly into the nugget. Melting of the Γ phase increases the zinc concentration in the nugget and causes the formation of cavities, because the vapour pressure of zinc in the molten nugget is extremely high.
Based upon the results of study, countermeasures were taken with regard to galvannealing and welding conditions.
