Asymmetric Plated Welded Joint for LME-Resistant Auto Members
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Solution Overview
Problem
Liquid metal embrittlement (LME) cracking occurs during spot welding of plated steel sheets, particularly in high-strength steel, due to metal components penetrating grain boundaries, and existing solutions focus on welding processes rather than the design of the steel sheets themselves.
Innovation Solution
A welded joint design featuring a first steel sheet with a plating layer facing a second steel sheet without a plating layer, where a boundary plating layer with specific Al and Mg compositions and a high tensile strength is applied between the sheets, satisfying Relations I and II, and having an internal oxide layer to inhibit LME cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If plating layers are provided on both steel sheets for corrosion protection, then corrosion resistance is improved, but LME cracking occurs due to metal component penetration into grain boundaries
Solution Approach 1:
The invention applies different plating configurations to different steel sheets: one steel sheet has a plating layer while the other has no plating layer or a different plating layer. This local differentiation prevents LME cracking by avoiding the presence of plating layers on both sheets, while still maintaining adequate corrosion protection through the single plating layer or alternative plating design.
2Strength
If high strength steel sheets with tensile strength of 780 MPa or more are used, then structural strength is improved, but susceptibility to LME cracking increases
Solution Approach 1:
The invention uses high strength steel sheets (tensile strength ≥780 MPa) for one of the steel sheets to ensure structural strength, but combines this with an asymmetric plating configuration where only one sheet has a plating layer. This local differentiation allows the high strength steel to provide necessary structural integrity while the plating asymmetry prevents LME cracking.
3Reliability
If conventional plating configurations are used on both steel sheets, then corrosion protection is maintained, but manufacturing complexity increases due to need for special plating layer designs
Solution Approach 1:
The invention simplifies the plating configuration by applying plating layers to only one steel sheet or using different plating layers on each sheet, rather than applying identical plating layers to both sheets. This asymmetric approach maintains adequate corrosion protection while reducing the complexity of plating layer design and manufacturing.
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 proposed design effectively inhibits LME cracking by controlling the Al/Zn and Mg/Zn ratios in the plating layers and incorporating an internal oxide layer, ensuring the stability of the liquid phase at the steel grain boundaries and reducing the penetration of Zn, thus preventing brittle fracture.
Implementation Method 1
the metal components in the plating layers penetrate the grain boundaries of the steel sheets and sometimes causes liquid metal embrittlement (LME) cracking
Implementation Method 2
incorporating an internal oxide layer, ensuring the stability of the liquid phase at the steel grain boundaries and reducing the penetration of Zn, thus preventing brittle fracture
Data Source
AI summary
The present disclosure inhibits liquid metal embrittlement (LME) cracking 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 η-Zn phase at the cross-section of the boundary plating layer is 5% or more, and the first plating layer and the second plating layer satisfy predetermined Relations I and II.