The fibre laser welding of two different dual phase steels HCT980X and HCT600X was evaluated. The sheets with thickness of 1.2 mm were welded using the solid-state fibre laser IPG type YLR 4500 with maximum output 4.5 kW and wave length 1060 nm at graduated beam powers and welding speeds. The microstructure of welded samples was analysed, microhardness and tensile strength were measured. The heat input influenced the microstructure of the fusion zone and heat affected zone. The microhardness increased in the fusion zone and even more in the heat affected zone of HCT980X steel, which was the consequence of martensite and bainite formation in these areas. The tensile strength of joints achieved the strength of the HCT600X steel and all joints fractured in the HCT600X base metal.Keywords: dual phase steels, fibre laser welding, microstructure, microhardness, tensile strength IntroductionDual phase (DP) steels, which are the important part of advanced high strength steels, have been used in manufacture of lightweight automobiles to reduce fuel consumption without compromising other attributes such as safety, performance, recyclability and cost. Some of the major advantages of the dual phase steels include their superior mechanical properties in comparison with standard steels, moderate price thanks to small amount of alloying additions, as well as excellent technological properties, together with good weldability and machinability [1][2][3][4]. The excellent mechanical properties are the consequence of their multiphase structure. Microstructure of DP steel consists of 30-70 % martensite in the fine-grained, spherical ferrite matrix and 1-10 % of metastable retained austenite. As a result, the steel is characterised by high tensile strength up to 1180 MPa with unit elongation up to 27 %. Most often the microstructure of DP steel is developed as a result of accelerated cooling of thin sheets after cold rolling in the range between Ac1 and Ac3 and controlled air-water mist cooling to ambient temperature. The process is flexible one and allows for different combinations of relative volume ratio of ferrite and martensite. The dual phase steels are used in automotive industry for frames and crossbeams, vertical beams, side impact beams, and safety elements [4][5][6]. Dual phase steels have been used for producing of tailor welded blanks. Tailor welded blanks are semi-finished parts that consist of at least two single sheets that are welded together prior the forming process. The sheets can exhibit different mechanical properties, thickness or coatings.
Microstructural changes and microhardness profiles of laser-welded dual-phase DP600 and bakehardened 220BH steels were evaluated. The joints of these steels were butted with different welding parameters. They exhibited the microstructural changes, from ferrite and martensite in dual-phase steels or ferrite in bake-hardened steels to acicular ferrite, bainite and martensite in the fusion and heat-affected zones of the weld joints. As a result of welding, microhardness increases significantly in the fusion zones and even more in the heat-affected zones near DP600 steel.
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