Galvanized Steel Spot Welds With Fe-Rich Zn Layer Against LME Cracking
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Solution Overview
Problem
Existing resistance spot welding methods for high strength steel sheets with surface treatments, such as galvanized coatings, face issues like liquid metal embrittlement cracking due to the low melting point of the coated layers, which limits the application of these sheets and increases production costs, and are not effective in suppressing cracking across various chemical compositions and operational conditions.
Innovation Solution
The method involves increasing the concentration of Fe in the Zn alloy layer between steel sheets to a certain value, along with specific welding conditions and post-heat treatment, to prevent liquid Zn from being present during tensile stress, thereby suppressing liquid metal embrittlement cracking without removing the coated layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high strength steel sheets with galvanized coatings are used for automotive parts, then corrosion resistance is improved, but liquid metal embrittlement cracking occurs during resistance spot welding
Solution Approach 1:
The patent changes the chemical composition parameters of the steel sheet, specifically controlling the Si content to 0.01-3.0 mass% and P content to 0.01-0.50 mass%, to prevent LME cracking while maintaining corrosion resistance through galvanized coatings
Solution Approach 2:
The patent creates a composite structure by combining high strength steel base material with controlled alloying elements (Si, P) and galvanized coatings, forming a multi-layer composite that resists both corrosion and LME cracking
2Productivity
If conventional resistance spot welding is applied to high strength steel sheets with surface treatments, then production efficiency is maintained, but cracking occurs in the weld
Solution Approach 1:
The patent modifies the chemical composition parameters of the steel sheet (Si: 0.01-3.0 mass%, P: 0.01-0.50 mass%) to enable conventional resistance spot welding processes to produce crack-free welds, thereby maintaining production efficiency while improving weld quality
3Strength
If the galvanized coating is removed before welding, then LME cracking is prevented, but corrosion resistance is reduced and production cost increases
Solution Approach 1:
The patent changes the base steel composition parameters (Si: 0.01-3.0 mass%, P: 0.01-0.50 mass%) to make the galvanized coating compatible with resistance spot welding, eliminating the need for coating removal and associated costs while preventing LME cracking
Solution Approach 2:
The patent converts the potentially harmful effect of the galvanized coating (causing LME cracking) into a beneficial feature by controlling the base steel composition to prevent cracking, allowing the coating to remain for corrosion protection without causing welding defects
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 liquid metal embrittlement cracking in resistance spot welds, allowing for the use of high strength steel sheets with galvanized coatings without removing the coatings, thereby improving weld strength and reducing production costs while maintaining corrosion resistance.
Implementation Method 1
a high welding current is applied between the upper and lower welding electrodes for a short time... a spot-shaped weld 4 is obtained by utilizing resistance heat generated by the application of the high welding current
Implementation Method 2
the low-melting point metal coated layer on the surface of the steel sheet melts during welding... the molten low-melting point metal penetrates into grain boundaries of the base material
Data Source
AI summary
A resistance spot welded member includes overlapping steel sheets resistance-spot-welded together at a spot-shaped weld called a nugget, and a Zn alloy layer formed between two of the overlapping steel sheets. At least one of the steel sheets is a galvanized steel sheet having a Zn-based coated layer on its steel sheet surface. The resistance spot welded member satisfies the following formulas (1) and (2):CFe≥20(1)0<L≤500(2)where CFe denotes the concentration (mass %) of Fe in the Zn alloy layer, and L denotes the distance (μm) from the edge of the nugget to the position at which CFe is measured.