“…Since FSW is performed at a temperature lower than the melting point of the materials to be welded, FSW can produce joints with fewer defects or porosity, low residual stresses, etc., compared with other fusion welding methods [3,4]. The FSW technique has developed very rapidly since its emergence and it has now been expanded to many high melting point metallic materials, including Cu, Ti, Fe, stainless steels, and even high carbon steels, which were considered to be unweldable materials by fusion welding methods because of the formation of the brittle martensite phase [5][6][7][8]. Therefore, with the increasing effort to improve fuel efficiency in industry, the use of the FSW technique must be in strong demand in the near future for the welding of light materials, especially aluminum and magnesium alloys.…”