Flux-Free Brazing Sheet Layer Design for Stable Open-Part Joining
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Solution Overview
Problem
The existing flux-free brazing methods using Al—Si—Mg alloy filler metals face challenges with the rapid generation of a liquid phase in the intermediate layer, leading to insufficient joining, especially in complex joint shapes like corrugated fins, due to the lower solidus temperature and excessive liquid phase formation, which results in the brazing filler metal flowing out and failing to function effectively.
Innovation Solution
The use of a brazing sheet with an Al—Si-based outermost surface layer and an Al—Si—Mg-based intermediate layer, optimized with specific Si particle distribution and composition, where the outermost layer contains 2-13% Si and the intermediate layer contains 4-13% Si and 0.1-5.0% Mg, with controlled liquidus temperature difference and cladding ratios, to prevent premature melting and ensure effective brazing filler metal flow in flux-free brazing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an Al-Si-Mg alloy brazing filler metal is used for flux-free brazing, then the Mg decomposes the Al oxide film to enable joining, but the MgO film grows easily on the surface and inhibits joining in open-part joints
Solution Approach 1:
The brazing filler metal is segmented into a multi-layer structure with an outermost layer (Al-Si-based, Mg-free) and an intermediate layer (Al-Si-Mg-based). This segmentation prevents MgO film formation on the surface while maintaining Mg's oxide-decomposing function in the intermediate layer, resolving the contradiction between enabling joining and preventing harmful MgO growth.
Solution Approach 2:
Different layers are assigned different chemical compositions and functions: the outermost layer is designed to be Mg-free to suppress MgO film growth on the surface, while the intermediate layer contains Mg to decompose Al oxide films. This local quality differentiation allows each layer to perform its specific function optimally.
2Reliability
If the intermediate layer has lower solidus temperature due to Mg addition, then Mg can decompose oxide film effectively, but the liquid phase generates more rapidly and flows out from material end
Solution Approach 1:
The Si content in the intermediate layer is precisely controlled within 4-13 mass% to adjust the liquidus temperature. By optimizing this parameter, the liquid phase generation is controlled to occur at appropriate timing, preventing premature flow out while maintaining effective oxide film decomposition through Mg.
Solution Approach 2:
The intermediate layer uses a composite Al-Si-Mg alloy composition where Si and Mg work synergistically: Si controls the liquidus temperature and liquid phase generation timing, while Mg provides oxide film decomposition capability. This composite material approach balances the contradictory requirements.
3Quantity of substance
If the Si content in the intermediate layer increases to control liquid phase, then liquid phase ratio increases, but the brazing filler metal flows out and stops functioning as effective fluid brazing filler metal
Solution Approach 1:
The Si content in the intermediate layer is optimized within 4-13 mass% to control the liquidus temperature and liquid phase generation timing. This parameter optimization ensures that sufficient liquid phase is generated at the appropriate stage to maintain brazing filler metal function, while preventing excessive flow out that would cause loss of effectiveness.
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 achieves stable and efficient joining in open-part joints by controlling the Si particle distribution and oxide film decomposition, ensuring strong and durable bonds comparable to conventional brazing methods, even in complex geometries like automobile heat exchangers.
Implementation Method 1
the Mg in the melt-activated brazing filler metal deoxidizes and decomposes the Al oxide film (Al2O3) on the joining part surface
Implementation Method 2
the Mg in the melt-activated brazing filler metal deoxidizes and decomposes the Al oxide film (Al2O3)
Implementation Method 3
the MgO film grows more easily on the surface of the Mg-added brazing filler metal
Implementation Method 4
a liquid phase brazing filler metal is generated more rapidly than the outermost surface layer during the brazing heating process
Data Source
AI summary
A brazing sheet for flux-free brazing has an outermost surface brazing filler metal layer, consisting of an Al—Si-based alloy containing 2 to 13% Si in mass %, and an intermediate brazing filler metal layer, consisting of an Al—Si—Mg-based alloy containing 4 to 13% Si and 0.1 to 5.0% Mg in mass %, which are cladded on one or both sides of a core material. In the outermost surface brazing filler metal layer, the number of Si particles having a circle equivalent diameter of 1.75 μm or more is 10% or more of the number having a circle equivalent diameter of 0.8 μm or more, as observed in the direction of the surface layer. The intermediate brazing filler metal layer contains less than 3000 per 10000 μm2 of Si particles having a circle equivalent diameter of 0.25 μm or more, as observed in a cross section of the brazing filler metal layer.