Spark Plug Melt Sag Suppresses Oxide Scale Growth
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Solution Overview
Problem
The reliability of spark plugs with precious metal tips is compromised due to the growth of oxide scales at the interface between the precious metal tip and the electrode base material, caused by thermal expansion differences, leading to cracking and potential detachment of the tip.
Innovation Solution
A spark plug design featuring a melt sag formed over the entire circumference of the precious metal tip, which suppresses air entry and oxide scale formation, thereby reducing stress and improving joint reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the precious metal tip is joined to the electrode base material by laser welding, then the joint strength is improved, but oxide scale forms at the interface due to thermal expansion differences during heating and cooling cycles
Solution Approach 1:
The patent introduces a melt portion as an intermediary layer between the precious metal tip and the electrode base material. This melt portion has intermediate composition and thermal expansion properties, acting as a buffer that reduces stress concentration at the interface during thermal cycling, thereby preventing oxide scale formation and improving joint reliability
Solution Approach 2:
The patent modifies the physical and chemical parameters of the interface region by creating a melt portion with controlled composition gradient. By adjusting the melting depth and composition of the melt portion, the thermal expansion coefficient mismatch is reduced, suppressing oxide scale growth at the interface
2Reliability
If the melt portion is made thicker to suppress stress, then crack formation is reduced, but the device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent optimizes the thickness and composition parameters of the melt portion to achieve the minimum effective thickness needed for stress suppression. By controlling the laser welding parameters (energy density, pulse duration, scanning speed), the melt portion thickness is precisely controlled to be sufficient for reducing thermal stress without excessive complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The melt sag effectively prevents oxide scale growth and crack extension, enhancing the reliability and durability of the joint between the precious metal tip and the electrode base material, even under repeated heating and cooling cycles.
Implementation Method 1
the melt sag having a predetermined shape is formed over the entire circumference on the side surface of the precious metal tip, entry of the air into the interface between the precious metal tip and the melt portion can be suppressed, thereby suppressing formation of an oxide scale at the interface
Implementation Method 2
an oxide film (hereinafter also referred to as an oxide scale) may be formed on the surface of the melt portion. The oxide scale is formed, at the melt portion interface, so as to gradually grow from an outer peripheral portion near the outside air toward the inside of the melt portion interface
Implementation Method 3
heating and cooling cycles are repeated, whereby a stress is caused by a difference in thermal expansion coefficient between the precious metal tip and the electrode base material
Data Source
Figure 1
Figure 2(A)~2(B)
Figure 3
AI summary
Growth of an oxide scale at an interface between a precious metal tip and an electrode base material is suppressed. A melt portion (26) includes a melt sag (28) over an entire circumference of a side surface of the precious metal tip (27). In a cross section, including a center axis, of an electrode: a length of a line S corresponding to an end surface of the precious metal tip (27) is D; virtual straight lines apart from the center axis by a distance of "9D/20" are L1, L2; intersection points of the virtual straight lines L1, L2 and the interface between the precious metal tip and the melt portion are P1, P2; a virtual straight line connecting the intersection points P1 and P2 is L3; of both end points of the line S, the end point on the same side as the virtual straight line L1 is an end point P3, and the end point on the same side as the virtual straight line L2 is an end point P4; a straight line passing each end point P3, P4 and parallel to the center axis is a virtual straight line L4, L5; of end points of the melt sag (28), the end point on the virtual straight line L4 is P5, and the end point on the virtual straight line L5 is P6; and intersection points of the virtual straight lines L4, L5 and the virtual straight line L3 are P7, P8. In this case, each of a distance X1 between the intersection point P7 and the end point P5 and a distance X2 between the intersection point P8 and the end point P6 is 0.092 mm or more.