Mg-Bi Aluminum Brazing Sheet for Stable Flux-Free Joining
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Solution Overview
Problem
The existing flux-based brazing methods for aluminum heat exchangers often result in brazing defects due to the reaction of fluoride flux with Mg, limiting the use of Mg-added high-strength alloys, and flux-free methods face challenges with unstable joinability, especially in open joint shapes where a stable oxide film grows, impeding joining.
Innovation Solution
Incorporating Mg and Bi into the aluminum alloy to form a Mg—Bi compound with specific size and distribution, which dissolves and diffuses during heating, reducing surface tension and oxide film growth, and using an Al—Si-based brazing material with controlled Si and Bi content to enhance brazability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Mg is added to the brazing material to decompose Al oxide film, then joinability is improved, but MgO film grows on the material surface impeding joining
Solution Approach 1:
The patent changes the chemical composition parameters by adding Bi element (0.003-2 mass%) to the Al-Si-Mg brazing material. This compositional modification alters the oxidation behavior during brazing, suppressing MgO film formation while maintaining the ability to decompose Al oxide film, thus resolving the contradiction between improving joinability and preventing harmful MgO growth
Solution Approach 2:
Bi acts as an intermediary element that mediates between Mg's reducing action and oxygen's oxidizing action. The Bi element interferes with the direct reaction between Mg and oxygen, preventing excessive MgO formation while allowing controlled decomposition of Al oxide film, thereby enabling stable joining in open joint configurations
2Stability of the object's composition
If Mg content in brazing material is reduced to suppress MgO film growth, then oxide film stability is improved, but insufficient Mg reduces Al oxide film decomposition capability
Solution Approach 1:
The patent modifies the compositional parameters by introducing Bi (0.003-2 mass%) and optimizing Mg content (0.003-3 mass%), creating a new chemical system where Bi compensates for reduced Mg activity. This parameter change allows lower Mg content while maintaining sufficient oxide film decomposition capability through Bi's influence on oxidation kinetics
Solution Approach 2:
The patent creates a composite brazing material system (Al-Si-Mg-Bi) that combines multiple elements with complementary functions. Mg provides oxide film decomposition capability while Bi suppresses MgO formation and enhances Bi's own reducing action, creating a synergistic composite material that resolves the contradiction between oxide film stability and decomposition capability
3Reliability
If Bi is added to reduce surface tension and improve wetting, then brazability is improved, but coarse Bi particles oxidize and deposit on surface impeding joining
Solution Approach 1:
The patent严格控制 Bi content to 0.003-2 mass% and controls particle size distribution, preventing coarse particle formation. This parameter control ensures Bi remains sufficiently fine to dissolve and diffuse uniformly during brazing, reducing surface tension without forming oxidizable coarse particles that would deposit on the surface
Solution Approach 2:
The patent ensures uniform local distribution of Bi throughout the brazing material matrix, creating consistent local conditions for controlled oxidation and dissolution. This uniform distribution prevents localized coarse particle formation and ensures steady-state Bi release during brazing, improving wetting while minimizing harmful oxide deposition
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 without flux, maintaining joinability even in open joint configurations by controlling the Mg and Bi content and distribution, suppressing oxide film growth, and improving the strength and corrosion resistance of the alloy.
Implementation Method 1
Mg in the brazing material that has been melted and activated reduces and decomposes an Al oxide film (Al2O3) on the surface of a joint
Implementation Method 2
the generated Bi diffuses into the surface of a material and concentrates uniformly
Implementation Method 3
mixed in a molten braze to reduce a surface tension
Data Source
AI summary
An aluminum alloy for flux-free brazing provided for brazing performed via an Al—Si-based brazing material without a flux in a non-oxidizing atmosphere without depressurization, includes: by mass %, 0.01% to 2.0% of Mg; and 0.005% to 1.5% of Bi, wherein in the aluminum alloy, there are more than 10 Mg—Bi-based compounds having a diameter of 0.01 μm or more and less than 5.0 μm in terms of equivalent circle diameter per 10,000-μm2 visual field and there are less than 2 Mg—Bi-based compounds having a diameter of 5.0 μm or more per 10,000-μm2 visual field in a cross section parallel to a rolling direction, and in the aluminum alloy, there are less than 5 Bi particles having a diameter of 5.0 μm or more in terms of equivalent circle diameter per 10,000-μm2 visual field in the cross section parallel to the rolling direction.