SPECIAL FEATURES OF WELD FORMATION FOR LASER WELDING OF SHEET STEEL 09MN2SI

Studies of special features for the formation of the structure and permanent joints properties during laser welding of 09mn2si sheet steel obtained after narrow-stream plasma cutting with a new PMVR-5.3 type plasma torch without additional machining of the gouging welds are presented. More efficient operation of the gas dynamic stabilization system for PTA of this plasma torch improves the cutting quality and, as a result, increases the absorption coefficients of radiation, penetration and laser welding performance. It is shown that a carbon dioxide laser welding contributes to a weld joint formation having a «dagger» shape and structure with a weld zone and an adjacent heat-affected zone (HAZ) on both sides of the CLW. At the same time, HAZ is heterogeneous in structure and consists of two subzones – overheating and normalization. In the weld zone, an increase in the hardness of the base metal is noted, and the distribution itself is symmetrical with respect to the CLW and fully reflects the location and extent of the crystallization and heat-affected zones in accordance with the data of metallographic analysis. In this case, the maximum value of microhardness is observed near the CLW, where a finely dispersed troostite-type structure is formed during crystallization. It is shown that the developed complex cutting and welding technology does not change the chemical composition of the steel in the weld vicinity. Based on the results of mechanical tests, conclusions were drawn about an increase in strength by about 30...35 % and impact strength with a slight decrease in the ductility of weld-in connections, as well as the absence of microcracks, unwelded spots and stretching of metal when stripping along fusion lines, which proves high quality of laser welding using the proposed technology.