Abstract
Microstructures in the sub-zones of butt joint consisting of dual phase DP980 steel and complex phase CP780 steel with a thickness of 1.5 mm welded using fibre laser process were evaluated. The microstructure in the fusion zone was built of an approximately equal portion of martensite and lower bainite. In both coarse-grained heat affected zones near DP980 and CP780 steel dominant presence of lath martensite and a minor portion of lower bainite were observed. Similar microstructures were created in both fine-grained heat affected zone near DP980 and near CP780 steel but with the presence of ferrite and tempered martensite in region heated to lower peak temperatures. Mainly M₃C carbides precipitated in lower bainite, and mostly MX carbonitrides precipitated in martensite. Created microstructures corresponded with microhardness profile across the joint. The highest microhardness of 476 HV_0.1 was measured in the coarse-grained region of the heat affected zone near DP980 steel with a high portion of martensite.

Key words
dual phase steel, complex phase steel, fibre laser welding, microstructure, microhardness

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