New Design of Copper–Inconel 601 Ground Electrode Spark Plug Based on a Thermo-Mechanical Model

Inconel 601 is one material of choice for intermediate- to high-temperature protective coatings for spark plugs’ ground electrodes. Production of ground electrodes of spark plugs implies the following operations: the tamping of the copper core in an Inconel 601 cup, cold-forming of the assembly, annealing, welding, and bending of the final spark plug. On the production line, the use of Inconel 601 as a protective coating for ground electrodes leads to possible cracking in the welded area after bending. In the present paper, possible causes of cracking are analyzed. It is clearly shown that a combination of Copper –Inconel interface oxidation, Inconel yielding during the heat treatment, and micro-movements during bending lead to cracks in the welded area of the ground electrode. First, the detrimental effect of gaps, between Copper and Inconel 601, is shown experimentally. Second, a thermo-mechanical analysis combined with SEM (Scanning Electron Microscopy) observations identified the annealing treatment and interface oxidation as the main cause of gaps. Third, bending simulations show the relation between these gaps and cracking. Finally, a new ground electrode design, preventing cracks, is suggested.