SPARK PLUG FOR INTERNAL COMBUSTION ENGINE

摘要

In a spark plug having a precious metal tip joined to the front end portion of a ground electrode, the stable junction state of the precious metal tip is guaranteed for a long time. A precious metal tip 32 on the side of a ground electrode 27 is not directly joined to the front end portion of the ground electrode 27 but indirectly joined with a mounting part 51 interposed therebetween. The mounting part 51 includes a base part 53 which has a disc shape and is provided with a flange part 52 on its outer periphery and a protruding part 54 which protrudes from the base part 53 and has a columnar shape. First, in the state where the precious metal tip 32 is in contact with the protruding part 54 of the mounting part 51, laser welding or the like is performed thereon to form a fused part 42 and obtain a complex, and the base part 53 of the mounting part 51 is joined to a flat surface of the ground electrode 27 by resistance welding. The grain size of the grains of the mounting part 51 in the vicinity of the fused part 42 is smaller than the grain size of the grains thereof in the vicinity of the ground electrode 27, and the grain size of the grains of the flange part 52 of the mounting part 51 is smaller than the grain size of the grains of the protruding part 54.