The Effect of welding condition on defects, microstructure, mechanical properties and formability of CO_2 laser welded 800MPa grade TRIP steel was investigated. Bead-on-plate welding was carried out under various power, welding speed and shield gas. The number of porosity reduces with increasing the welding speed. The number of porosity in joints used Ar-50%He mixing gas is lower than that of joints used Ar gas. The maximum hardness was obtained at the weld metal and the HAZ near the weld metal. The value was the same regardless of the welding speed used and was nearly equal to that of water quenched raw metal. In the perpendicular tensile test to the weld axis, the joints produced at optimum condition were fractured at the base metal and the tensile property was nearly equal to the raw metal. In a parallel tensile test, the strength of the joints was higher than that of the base metal, but elongation was found to be lower than that of the raw metal because a crack was initiated in the bead at the strength levels corresponding to the tensile strength of the base metal and was propagated perpendicular to tensile direction. Elongation and formability were further improved using low power or Ar+He mixing gas compared to high power or Ar gas.
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