Zinc-Coated Spot Weld Structure for Crack-Resistant Joints
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Solution Overview
Problem
Existing spot welding techniques for zinc-based coated steel sheets often result in cracking due to liquid metal embrittlement, and methods to prevent cracking either decrease productivity or compromise corrosion resistance.
Innovation Solution
The solution involves controlling the coating structure near the spot welds by forming a high-melting-point FeAl alloy layer with an average thickness of 0.3 µm or more and a zinc-based coated layer with an average thickness of 2.0 µm or more, which suppresses cracking and maintains high corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If spot welding is performed on zinc-based coated steel sheets, then productivity is maintained, but cracking occurs due to liquid metal embrittlement
Solution Approach 1:
The patent applies preliminary action by forming an Al-containing zinc-based coated layer on the steel sheet before spot welding. This pre-formed coating layer with specific composition (Al content of 3-10 mass%) creates a protective barrier that prevents liquid metal embrittlement during subsequent welding operations, allowing high-speed welding without cracking
Solution Approach 2:
The patent applies parameter changes by controlling the aluminum content in the zinc-based coated layer within a specific range (3-10 mass%). This parameter optimization ensures the coating has sufficient melting point to prevent LME while maintaining weldability, resolving the contradiction between productivity and weld quality
2Reliability
If hold time is increased during welding, then cracking is suppressed, but productivity decreases
Solution Approach 1:
The patent changes the chemical composition parameters of the coated layer by incorporating aluminum at 3-10 mass%, which fundamentally alters the welding behavior. This composition modification enables crack suppression through normal welding cycles without requiring extended hold times, thus maintaining high productivity while achieving reliable crack-free welds
3Reliability
If the coated layer is removed before welding, then cracking is suppressed, but corrosion resistance deteriorates
Solution Approach 1:
The patent applies local quality by creating a coated layer with specific local characteristics - an Al-containing zinc-based coating with 3-10 mass% Al that provides crack suppression locally at the weld zone while maintaining the protective corrosion-resistant properties of the zinc-based coating throughout the entire steel sheet surface
Solution Approach 2:
The patent uses composite materials by creating a multi-component zinc-based coated layer containing aluminum (3-10 mass%). This composite coating structure combines the crack-suppression benefits of aluminum with the corrosion-resistant properties of zinc, achieving both objectives simultaneously without removing the coating
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
This approach effectively suppresses cracking in spot welds and enhances the corrosion resistance of the welds, even when multiple steel sheets including zinc-based coated steel sheets are spot-welded.
Implementation Method 1
cracking may disadvantageously occur in a weld. This is considered to be due to what is called liquid metal embrittlement in which a low-melting-point metal coated layer on surfaces of a steel sheet changes into a liquid phase during the welding process and enters grain boundaries of the steel sheet to decrease grain boundary strength and cause cracking
Implementation Method 2
a spot weld produced by resistance spot welding (hereinafter, also referred to as 'spot welding') of multiple steel sheets
Data Source
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Figure 3
AI summary
An object is to provide a spot welded member having a spot weld formed by spot-welding multiple steel sheets including a zinc-based coated steel sheet. According to the present invention, a spot welded member includes a spot weld formed by holding a sheet stack of multiple steel sheets between a pair of electrodes and spot-welding the sheet stack, in which at least one of the multiple steel sheets is a high-strength zinc-based coated steel sheet having a tensile strength of 780 MPa or more, the high-strength zinc-based coated steel sheet having a coating with an Al content of 0.5% or more by mass, and in which the heat shock region of the spot weld outwardly extending from an edge of a corona bond area includes a coated layer including an FeAl alloy layer having an average thickness of 0.3 µm or more and a zinc-based coated layer having an average thickness of 2.0 µm or more on the FeAl alloy layer at the interface between the base steel sheet of the high-strength zinc-based coated steel sheet and the coating.