Spark plug and process for producing same

A production process of a spark plug, the spark plug comprising:

• a rod-shaped center electrode extending in an axis direction of the spark plug;
• a cylindrical insulator having an axial hole formed therein in the axis direction and retaining the center electrode in the axial hole;
• a cylindrical metal shell disposed on an outer circumference of the insulator;
• a ground electrode made of a nickel-based alloy, extending from a front end portion of the metal shell and bent at a substantially middle portion thereof in such a manner as to define a spark gap between a front end portion of the ground electrode and a front end portion of the center electrode;
• and a nickel-based plating layer formed on parts of surfaces of the ground electrode and the metal shell, the production process comprising:
• a plating film application step for performing a nickel plating treatment on the metal shell to which the ground electrode has been joined, thereby applying a plating film to substantially the whole of the surfaces of the metal shell and the ground electrode;
• a molten layer formation step for forming a molten layer in which metal materials of the plating film and the ground electrode are molten together by irradiation of a laser beam or electron beam onto at least a center-electrode-side part of the portion of the ground electrode to be bent;
• and
• a spark gap defining step for bending the portion of the ground electrode to be bent to define the spark gap between the front end portion of the ground electrode and the front end portion of the center electrode,
• wherein, in the molten layer formation step, the laser beam or electron beam is irradiated in such a manner that a maximum length between a point of a surface of the plating layer closest to the molten layer and a point of the molten layer opposite from the surface of the plating layer in a thickness direction of the ground electrode is larger than or equal to a thickness of the plating layer, and
• wherein, in the molten layer formation step, the laser beam or electron beam is irradiated in such a manner that the maximum length between the point of the surface of the plating layer closest to the molten layer and the point of the molten layer opposite from the surface of the plating layer in the thickness direction of the ground electrode is 200 μm or smaller.