Microstructures and microhardness profiles of butt joints welded by fibre laser were evaluated. Dual phase steel HCT980X and bake hardening steel HX220BD sheets with thickness of 1.2 mm were welded at five different welding parameters. The joints showed microstructural changes from ferrite and martensite in dual phase steels or ferrite in bake hardening steels to acicular ferrite, bainite and martensite in the fusion zones and heat affected zones. The welding resulted in a significant microhardness increases in the fusion zones and even more in the heat affected zones near dual phase stell HCT980X. The microstructures observed in particular weld zones corresponded well with microhardness profiles measured across the joints.
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