International Patent Application Publication WO2012/067199, corresponding to U.S. Pat. No. 9,257,817 B2, discloses a spark plug, which includes a spark-generating part employing an electrode tip made of a noble metal durable against spark generation. This document discloses a process for joining the electrode tip to an electrode body by laser welding, while forming a weld part between their contact surfaces, wherein the weld part has a variation of shape. For enhancement of strength of joint between the electrode tip and the electrode body, this joint is implemented by joining the entire contact surfaces of these components.
However, joining the entire contact surfaces between the electrode tip and the electrode body by laser welding tends to cause an increase in amount of a melted portion of material and thereby cause the electrode tip to tilt or sink during the laser welding. This may cause so-called welding drop under some conditions. The welding drop can cause a problem such as failure of ignition and decay of durability, when adhered to a surface of the electrode.
In view of the foregoing, for welding the electrode tip and the electrode body of the spark plug, it is desirable to enhance the joint strength while suppressing the occurrence of welding drop.