Al-Si-Mg-Bi Brazing Sheet for Flux-Free Joints Without MgO Films
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Solution Overview
Problem
The existing flux-based brazing methods for aluminum heat exchangers, particularly those with Mg-added alloys, face challenges in achieving stable joints due to the growth of stable MgO films, especially in open joint configurations, which impedes joining and requires a flux-free brazing solution for improved reliability and wider application.
Innovation Solution
A flux-free aluminum brazing sheet with a multilayer structure, featuring an Al-Si-Mg-Bi-based brazing material finely and densely dispersed with Mg-Bi compounds, where the Mg-Bi-based compounds are uniformly distributed to suppress oxide film growth and enhance brazability, ensuring good joinability without the use of flux in a non-oxidizing atmosphere.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fluoride-based flux is used for brazing Mg-added aluminum alloys, then joining can be achieved, but MgO film growth occurs on the surface impeding stable joint formation
Solution Approach 1:
The invention extracts and removes the flux component from the brazing process, developing a flux-free brazing method that eliminates the harmful MgO film growth caused by fluoride-based flux while maintaining effective joining through controlled Bi particle distribution in the brazing material
Solution Approach 2:
The invention introduces Bi particles as an intermediary substance in the brazing material that prevents oxide film formation and promotes stable joining without requiring flux, thereby mediating between the Mg-added aluminum alloy and the brazing process to achieve reliable joints
2Reliability
If Bi particles are added to suppress oxide film formation, then brazability improves, but coarse Bi particles (5.0 μm or more) cause low-temperature melting and oxide deposition before braze melting
Solution Approach 1:
The invention changes the size parameter of Bi particles from coarse (5.0 μm or more) to fine (0.01 μm or more and less than 5.0 μm), which prevents low-temperature melting and oxide deposition before brazing while maintaining effective oxide film suppression during the brazing process
Solution Approach 2:
The invention creates local quality differences by distributing Bi particles of specific size ranges throughout the brazing material, ensuring that fine Bi particles are present to suppress oxide film formation at critical locations without the harmful effects of coarse Bi particles
3Object-generated harmful factors
If coarse Mg-Bi compounds (5 μm or more) are used to suppress oxide film formation during material manufacturing, then oxide film suppression is effective, but the compounds do not dissolve during braze heating
Solution Approach 1:
The invention changes the size parameter of Mg-Bi compounds from coarse (5 μm or more) to fine (0.01 μm or more and less than 5.0 μm), enabling the compounds to dissolve effectively during braze heating and release Bi particles that suppress oxide film formation, thereby achieving both oxide film suppression and brazing 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 enables stable and reliable flux-free brazing by preventing oxide film growth, improving brazability, and maintaining joint integrity, even in open joint configurations, thus overcoming the limitations of traditional flux-based methods.
Implementation Method 1
Mg in the brazing material that has been melted and activated reduces and decomposes an Al oxide film (Al 2 O 3) on the surface of a joint
Implementation Method 2
it is most important to uniformly concentrate Bi on the surface during braze melting in order to further improve brazability in a Bi-added Al-Si-Mg-based brazing material
Implementation Method 3
the Mg-Bi compound is reliably dissolved during the braze heating to generate metal Bi
Data Source
Figure 1~2
Figure 3
AI summary
An aluminum brazing sheet has a multilayer structure of two or more layers of at least a core material and a brazing material, wherein an Al-Si-Mg-Bi-based brazing material containing, by mass%, 0.01% to 2.0% of Mg, 1.5% to 14.0% of Si, and 0.005% to 1.5% of Bi is clad on one surface or both surfaces of the core material to be located at an outermost surface of the aluminum brazing sheet, in the Al-Si-Mg-Bi based brazing material, there are more than 10 Mg-Bi-based compounds having a diameter of 0.01 µm or more and less than 5.0 µm when observed in a surface layer plane direction and there are less than 2 Mg-Bi-based compounds having a diameter of 5.0 µm or more, and in the brazing material, there are less than 5 Bi particles having a diameter of 5.0 µm or more when observed in the surface layer plane direction.