Spark plug and method of manufacturing the same

1. A spark plug, comprising:

a rod-shaped center electrode that extends in an axial direction;

an insulator with a shaft hole that extends in the axial direction, the insulator holding the center electrode inside of the shaft hole;

a metal shell that surrounds and holds a part of the insulator in a circumferential direction; and

a ground electrode that includes a base end welded to the metal shell, wherein

the metal shell and the ground electrode are joined via a fusion portion, the fusion portion being formed by melting both the ground electrode and the metal shell together by welding,

an end face of the base end of the ground electrode is melted entirely, and

when a smallest thickness of the fusion portion in the axial direction is A, said smallest thickness being provided at a side of the fusion portion facing the center electrode;

when, in a cross section that includes a center line of the ground electrode and the axis, and is parallel to the axis, a length of a ground-electrode-side melted boundary is B, said boundary being formed between the fusion portion and the ground electrode; and

when a thickness of the ground electrode is C,

conditions of A≧0.2 mm and B>C are satisfied.

2. The spark plug according to claim 1 , wherein

in the cross section, a shape of the ground-electrode-side melted boundary includes any of a curved line, a plurality of straight lines, and a combination of a curved line and a straight line.

3. The spark plug according to claim 1 , wherein in the cross section,

when a length between a first end point and a second end point in an axial direction is D, the first end point being an end point on an opposite side of the center electrode at a metal-shell-side melted boundary formed between the fusion portion and the metal shell, the second end point being an end point on an opposite side of the center electrode at the ground-electrode-side melted boundary; and

when a length between a third end point and a fourth end point in the axial direction is E, the third end point being an end point on a side of the center electrode side at the metal-shell-side melted boundary, the fourth end point being an end point on a side of the center electrode side at the ground-electrode-side melted boundary,

a condition of D>E is satisfied.

4. The spark plug according to claim 1 wherein

in the cross section, a shape of the metal-shell-side melted boundary includes any of a curved line, a plurality of straight lines, and a combination of a curved line and a straight line.

5. The spark plug according to claim 4 , wherein

in the cross section, the shape of the ground-electrode-side melted boundary is a convex shape to the metal shell side.

6. The spark plug according to claim 5 , wherein

a noble metal tip is joined to a distal end portion of the ground electrode.

7. The spark plug according to claim 4 , wherein

the shape of the metal-shell-side melted boundary in the cross section includes at least two or more portions, said portions including both or either of a convex portion that is convex to the metal shell side and/or a concave portion that is concave to the ground electrode side.

8. The spark plug according to claim 1 , wherein

the ground electrode is formed of noble metal or alloy containing noble metal.

9. A method for manufacturing the spark plug according to claim 1 , comprising the steps of:

preparing a metal shell workpiece that becomes a metal shell after welding;

preparing a ground electrode workpiece that becomes a ground electrode after welding; and

welding the metal shell workpiece and the ground electrode workpiece, wherein

the ground electrode workpiece has a higher melting point than a melting point of the metal shell workpiece, and

in the cross section, when a thickness of the ground electrode workpiece is F; and when a thickness of an end face of the metal shell workpiece at a side welded to the ground electrode workpiece is G,

a condition of F>G is satisfied.