The paper deals with the application of semi-Friction Stir Processing (s-FSP) to the improvement of surface contact within the flange joints used in casings of high-voltage switchgears which are filled with electroinsulating SF6 gas. The goal of the research was to design the appropriate tool (shape and size) as well as to select parameters for this s-FSP process which results in the removal of casting defects from the surface layer of the final product. The characteristics of the proposed process is the redesign of a tool shape, which is not equipped with an element penetrating and stirring the material (pin). Moreover, the process does not require the usage of sophisticated devices or tilting the tool relative to the treated surface during the process. The elaborated process is useful for repairing the defective castings, and in the case of high-voltage switchgears, it allows to achieve the required tightness of the joints.