Brazed Lap Joint Structure to Suppress Zinc-Induced LME Cracking
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Solution Overview
Problem
Liquid metal embrittlement (LME) cracking occurs during the welding of zinc-based-plated steel sheets due to the diffusion of molten zinc into grain boundaries, leading to weakened joint integrity, which is not adequately addressed by existing technologies.
Innovation Solution
A brazed lap joint design with specific grain size and boron concentration controls, combined with controlled brazing conditions, including a copper-based filler metal and limited heat input, to suppress LME cracking and enhance joining strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If zinc-based-plated layer is applied to improve corrosion resistance, then corrosion resistance is improved, but LME cracking occurs during welding due to zinc diffusion into grain boundaries
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by adding boron (0.0005-0.005 mass%) and controlling grain boundary characteristics. This parameter change creates a grain boundary structure that resists zinc diffusion, thereby preventing LME cracking while maintaining the zinc-based-plated layer for corrosion protection.
Solution Approach 2:
Boron acts as an intermediary element that modifies the grain boundary structure. The boron-enriched grain boundaries serve as a barrier that mediates between the zinc-based-plated layer and the steel matrix, preventing harmful zinc diffusion while allowing the plated layer to function for corrosion resistance.
2Strength
If welding heat is applied to join steel sheets, then joining strength is achieved, but LME cracking occurs due to thermal shrinkage and zinc diffusion
Solution Approach 1:
The invention applies preliminary anti-action by pre-modifying the grain boundaries with boron before welding. This preliminary treatment creates a resistant structure that counteracts the harmful effects of subsequent welding heat and zinc diffusion, preventing LME cracking while maintaining joint strength.
Solution Approach 2:
The invention changes the microstructural parameters of the steel sheet by controlling grain size (average effective grain size of 7.0 μm or less) and boron concentration at grain boundaries. These parameter changes make the material more resistant to LME cracking during welding while maintaining adequate joining strength.
3Reliability
If grain size is reduced to suppress LME cracking, then LME resistance is improved, but manufacturing complexity increases
Solution Approach 1:
The invention changes the chemical composition parameters (adding boron in controlled amounts) to achieve the desired grain boundary characteristics. This chemical parameter change is simpler to implement than mechanical grain size control methods, as it can be achieved through standard steelmaking processes without additional equipment or complex manufacturing steps.
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 LME cracking while maintaining high joining strength and corrosion resistance, ensuring reliable joint performance.
Implementation Method 1
a concentration of B atoms at a prior austenite grain boundary of the second steel sheet excluding the brazed zone is 2.0 atm % or more
Implementation Method 2
a concentration of B atoms at a prior austenite grain boundary of the second steel sheet excluding the brazed zone is 2.0 atm % or more
Implementation Method 3
a brazed zone including a brazing metal joining an end surface of the first steel sheet and a surface of the second steel sheet
Implementation Method 4
an average effective grain size of the second steel sheet in a root portion, a toe portion, and a middle portion between the root portion and the toe portion is 15.0 μm or less
Data Source
AI summary
A brazed lap joint according to an aspect of the present invention includes: a first steel sheet and a second steel sheet, the first steel sheet and the second steel sheet being stacked; a zinc-based-plated layer, the zinc-based-plated layer being an overlapping surface of the first steel sheet and the second steel sheet; and a brazed zone including a brazing metal joining an end surface of the first steel sheet and a surface of the second steel sheet, and a heat-affected zone around the brazing metal, in which a concentration of B atoms at a prior austenite grain boundary of the second steel sheet excluding the brazed zone is 2.0 atm % or more, an average effective grain size of the second steel sheet excluding the brazed zone is 7.0 μm or less, an average effective grain size of the second steel sheet in a root portion, a toe portion, and a middle portion between the root portion and the toe portion is 15.0 μm or less, a Vickers hardness of the brazing metal is 250 or less, and a leg length of the brazed lap joint on the second steel sheet side is 2.0 mm or more.