Plated Steel Spot-Welded Joint Structure for LME Crack Suppression
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Solution Overview
Problem
Existing technologies do not sufficiently address the suppression of Liquid Metal Embrittlement (LME) cracking in welded joints, particularly in spot welding of galvanized steel sheets, where the plating structure before welding is not adequately controlled.
Innovation Solution
A welded joint is created using a plated steel sheet with a plating layer containing a controlled amount of Al, ranging from 0.10 to 1.50 mass %, and a specific chemical composition, which improves the plating layer structure and enhances LME resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional spot welding method is used on galvanized steel sheets, then welding process is simple, but LME cracking occurs due to zinc penetration along crystal grain boundaries
Solution Approach 1:
The plating layer is pre-formed with controlled Al content (0.03-1.50 mass%) and specific phase composition (η phase ratio of 10-100%) before welding. This preliminary structural preparation ensures that during spot welding, the plating layer resists zinc penetration and prevents LME cracking, eliminating the need for complex post-welding treatments or process modifications.
Solution Approach 2:
The invention changes the chemical composition parameters of the plating layer by controlling Al content within 0.03-1.50 mass% and Fe content within 0.01-2.00 mass%, and controls the phase structure parameter by maintaining η phase ratio between 10-100%. These parameter optimizations improve LME resistance without complicating the welding process.
2Reliability
If Al content in plating layer is increased to form FeAl alloy layer, then LME resistance improves, but plating layer composition control becomes more difficult
Solution Approach 1:
The invention optimizes Al content within a specific range of 0.03-1.50 mass%, which is sufficient to form the necessary FeAl alloy layer and control η phase formation, but not excessive to cause manufacturing difficulties. This balanced parameter selection achieves LME resistance while maintaining manufacturability.
Solution Approach 2:
The plating layer is designed as a composite structure containing multiple phases (η phase and Γ phase) with controlled proportions. The η phase (Zn-Fe-Al alloy phase) provides LME resistance while the Γ phase (Zn-Fe alloy phase) maintains plating integrity. This composite approach achieves superior performance with manageable composition control.
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 solution effectively suppresses or reduces LME cracking in spot welding, thereby improving the strength and corrosion resistance of the welded joint.
Implementation Method 1
at the plating layer of the separation part of a region up to 500 μm from an end of the pressure weld, a ratio of an area ratio of an η phase with respect to a total of area ratios of the η phase and Γ phase is 10 to 100%
Implementation Method 2
zinc converted to a liquid phase by the weld heat input penetrates to the inside of the steel sheet along the crystal grain boundaries to cause embrittlement
Data Source
AI summary
Provided is a welded joint comprising a plurality of steel sheets stacked together, a spot weld having a nugget joining the plurality of steel sheets, and a pressure weld and heat affected zone formed around the nugget, and a separation part positioned around the pressure weld, wherein one or more of the plurality of steel sheets is a plated steel sheet comprising a base steel sheet and a plating layer formed on at least a surface corresponding to a stacking surface of the plurality of steel sheets of surfaces of the base steel sheet, the plating layer at the separation part at an outside of the heat affected zone has a predetermined chemical composition, and at the plating layer of the separation part of a region up to 500 m from an end of the pressure weld, a ratio of an area ratio of an η phase with respect to a total of area ratios of the η phase and Γ phase is 10 to 100%.