Patent ID: 8624472

Claim:
A spark plug for an internal combustion engine, comprising: an insulator having an axial hole; a center electrode fixedly inserted into the axial hole so as to project from a front end of the insulator; a metallic shell externally holding the insulator; and a ground electrode joined to the metallic shell and having a distal end portion which faces a front end portion of the center electrode so as to define a spark discharge gap between a front end portion of the center electrode and the distal end portion of the ground electrode, wherein: a noble-metal chip formed from a platinum (Pt) alloy which contains platinum as a main component is joined to at least one of the front end portion of the center electrode and the distal end portion of the ground electrode, and the electrode to which the noble-metal chip is joined is formed from a nickel (Ni) alloy which contains nickel as a main component; the noble-metal chip is joined via a weld portion formed by fusing and mixing the nickel alloy of the electrode and the platinum alloy of the noble metal chip by laser welding; and the weld portion has a plurality of acicular or rhizoid microcracks, wherein, as viewed on a section of the weld portion, the microcracks have an average length within a range of 50 μm to 500 μm inclusive, wherein at least one of the nickel alloy forming the electrode to which the noble-metal chip is joined and the platinum alloy forming the noble-metal chip contains one or more additives selected from the group consisting of at least one of elements belonging to Groups 3A and 4A of the Periodic Table and oxides of these elements, wherein the weld portion has a fusion zone along the circumference of a joint interface between the noble-metal chip and the at least one electrode, wherein the total content of the one or more additives in the nickel alloy forming the electrode to which the noble-metal chip is joined and in the platinum alloy forming the noble-metal chip is within a range of 0.005% by mass to 0.3% by mass inclusive, and wherein the nickel alloy forming the electrode to which the noble-metal chip is joined contains chromium (Cr) in an amount of 10% by mass to 30% by mass inclusive and aluminum (Al) in an amount of 0.5% by mass to 3.0% by mass inclusive.