Abstract
The fibre laser welding of dual phase DP600 steel and bake hardening BH220 steel were studied with a concentration on the microstructure of butt joints, but microhardness and tensile properties were also measured. The temperature peaks influenced the microstructure of the fusion zone and heat affected zone. The microstructure in the fusion zone was predominantly built of acicular ferrite with small areas of upper and lower bainite, and martensite. The similar microstructure was observed in the heat affected zone near DP600 steel. The microstructure in the heat affected zone near BH220 steel consisted mostly of coarse ferrite and acicular ferrite. The microhardness increased in the fusion zone and in the
heat affected zone near DP600 steel, which was the consequence of martensite and bainite formation in these areas. The tensile strength of the fibre laser welded joints exceeded the strength of the BH220 steel base material with failure occurring in this steel.

Key words
dual phase steel, bake hardening steel, fibre laser welding, microstructure, microhardness

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