Abstract
Fibre laser welding of CP780 and TRIP690 steel sheets with a thickness of 1.5 mm was evaluated. Sound welds were obtained at the heat input from 43.3 to 95.0 J mm⁻¹. The highest 488 and 476 HV0.1 microhardness values were measured in the coarse-grained HAZ of CP780 and TRIP690 steel, respectively. The microstructure in the fusion zone consisted mainly of martensite and lower bainite. The microstructure in the coarse-grained HAZs contained martensite, lower bainite, upper bainite, and retained austenite. The microstructure of the inter-critical HAZs consisted of martensite and ferrite with an increased portion of ferrite with the distance from the fusion zone. The softening regions were indicated in the sub-critical HAZs of both steels. Joint tensile strength exceeded the tensile strength of TRIP690 steel. The apparent elongation was concentrated mainly in TRIP690 steel because of the transformation strengthening of the weld region during the welding process.

Key words
CP780 steel, TRIP690 steel, fibre laser welding, microhardness, microstructure

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