Welded Joint Plating Structure for LME Crack and Corrosion Resistance
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Solution Overview
Problem
Existing spot welding techniques fail to adequately inhibit liquid metal embrittlement (LME) cracking and ensure corrosion resistance in welded joints, particularly in high-strength steel sheets, and there is a lack of study on designing plated steel sheets to address these issues.
Innovation Solution
A welded joint design featuring a first steel sheet with a plating layer facing a second steel sheet without a plating layer, a boundary plating layer between them, and a specific Mg/Zn composition ratio in the plating layers to inhibit LME cracking and enhance corrosion resistance, with a tensile strength of 780 MPa or more and an area ratio of the MgZn2 phase at the cross-section of the boundary plating layer of 10% or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If spot welding is performed on plated steel sheets to join high strength steel sheets, then the tensile strength of the welded joint is improved, but liquid metal embrittlement (LME) cracking occurs due to metal component penetration into grain boundaries
Solution Approach 1:
The invention applies different plating layer configurations to different steel sheets in the welded joint. Specifically, one steel sheet has a plating layer while the other has no plating layer or a different plating layer, creating local quality differences that prevent LME cracking at the welded joint while maintaining high tensile strength
Solution Approach 2:
The invention changes the compositional parameters of the plating layers, specifically controlling the Mg/Zn ratio to satisfy a specific relationship. This parameter change prevents the formation of harmful intermetallic compounds that cause LME cracking, while maintaining the desired mechanical properties of the welded joint
2Strength
If high strength steel sheets with tensile strength of 780 MPa or more are used to improve structural strength, then the strength of the welded joint is improved, but the susceptibility to LME cracking increases
Solution Approach 1:
The invention changes the chemical composition parameters of the plating layers, specifically controlling the Mg and Zn content to satisfy a specific Mg/Zn ratio relationship. This parameter change prevents the formation of liquid metal embrittlement-prone intermetallic compounds, allowing high strength steel sheets to be welded without LME cracking
Solution Approach 2:
The invention creates a composite structure consisting of high strength steel sheets with specially designed plating layers. The plating layers, with their controlled MgZn2 phase content, form a composite material system that combines the high strength of the steel sheets with the crack-resistant properties of the engineered plating structure
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 solution effectively inhibits LME cracking and secures excellent corrosion resistance, even in high-strength steel sheets, by stabilizing Zn at steel grain boundaries and promoting a protective oxide layer, as demonstrated by the examples provided.
Implementation Method 1
stabilizing Zn at steel grain boundaries
Implementation Method 2
promoting a protective oxide layer
Data Source
AI summary
The present disclosure inhibits liquid metal embrittlement (LME) cracking and improves corrosion resistance in a welded joint obtained by spot welding a first steel sheet and a second steel sheet. In the welded joint of the present disclosure, a first plating layer is provided on a surface of the first steel sheet facing the second steel sheet, no plating layer is present on or a second plating layer is provided on a surface of the second steel sheet facing the first steel sheet, and a boundary plating layer is provided between the first steel sheet and the second steel sheet in a range of 0.5 mm from an end part of the corona bond toward an outside of the spot welded part. A higher tensile strength of a tensile strength of the first steel sheet and a tensile strength of the second steel sheet is 780 MPa or more, an area ratio of a MgZn2 phase at the cross-section of the boundary plating layer is 10% or more, and the first plating layer and the second plating layer satisfy a predetermined Relation I.