SPARK PLUG FOR INTERNAL COMBUSTION ENGINE

摘要

This aims to enable a spark plug having a precious metal tip joined to the leading end of an earth electrode, to retain the joined state of the precious metal tip stably for a long term. The precious metal tip (32) on the side of the earth electrode (27) is joined not directly but indirectly through a pedestal portion (51) to the leading end of the earth electrode (27). The pedestal portion (51) includes a disc-shaped base portion (53) having a flange portion (52) at its outer circumference, and a cylindrical protrusion (54) protruding from the base portion (53). At first, the precious metal tip (32) can be subjected, while being made to abut on the protrusion (54) of the pedestal portion (51), to a laser welding treatment, thereby to form a molten portion (42) as a complex structure, and the base portion (53) of the pedestal portion (51) is joined to the flat face of the earth electrode (27) by a resistance welding treatment. In the pedestal portion (51), the particle diameter of the crystal grains near the molten portion (42) is made larger than that of the crystal grains of the earth electrode (27). In the pedestal portion (51), moreover, the particle diameter of the crystal grains of the flange portion (52) is made smaller than that of the crystal grains of the protrusion (54).