Comparison of feasibility, microstructure and performance of hybrid laser arc, activated flux tungsten inert gas and friction stir welding for thick plate of innovative ultra-pure ferritic stainless steel

An innovative grade of ferritic stainless steel, ultra-pure 18Cr–2Mo thick plate, was designed and produced for special industrial application. In order to maintain its mechanical properties after joining, three advanced joining methods, hybrid laser arc welding, activated flux tungsten inert gas welding and friction stir welding, were selected and conducted to connect the thick plates. The feasibility of three joining methods, the microstructure and mechanical properties were compared, and the results have demonstrated that the sound joint was successfully produced using the selected parameters through friction stir welding. The obtained hardness and impact toughness of the weld zone were satisfying. In terms of activated flux tungsten inert gas welding, the crack will be created due to microstructural brittleness. And as for hybrid laser arc welding, the weld zone is narrow, and the addition of wire during welding for the top weld metal area leads to higher formation ratio of low-angle grain boundaries, which is beneficial to performance of the joint. However, there is still a weak area in the fusion line of the welded joint. The result has illustrated that the welding of innovative ultra-pure ferritic stainless steel thick plate by friction stir welding is feasible.