Flux-Free Brazing Sheet Layering for Open Aluminum Joints
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Solution Overview
Problem
The existing flux-free brazing method using Al-Si-Mg alloy filler metals faces challenges with open joints, where the MgO film growth inhibits joining due to its stability, and the intermediate layer's lower solidus temperature causes premature melting and flow issues, leading to insufficient joining, especially in complex shapes like corrugated fins.
Innovation Solution
Optimizing the Si particle distribution in both the outermost and intermediate brazing filler metal layers, with specific Si and Bi content ranges, and controlling the liquidus temperature difference between the layers to prevent premature melting and ensure effective Mg diffusion for oxide film decomposition, while using a non-oxidizing gas atmosphere to suppress oxide film growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Mg is added to the brazing filler metal to deoxidize and decompose the Al oxide film, then joining capability is improved, but MgO film grows more easily on the surface and inhibits joining
Solution Approach 1:
The brazing filler metal is divided into multiple layers: an outermost surface layer with Mg-free or low-Mg composition and intermediate layers with Mg-added composition. This segmentation allows the Mg to be contained away from the surface, preventing MgO film growth while still enabling deoxidation and joining capability through controlled Mg diffusion during brazing.
Solution Approach 2:
Different regions of the brazing filler metal are given different compositions tailored to their specific functions. The outermost layer has composition optimized for oxide film resistance and surface stability, while intermediate layers have composition optimized for deoxidation and joining. This local quality differentiation resolves the contradiction between needing Mg for joining and avoiding MgO formation.
2Productivity
If the intermediate layer has lower solidus temperature due to Mg addition to promote melting and joining, then brazing efficiency is improved, but the intermediate layer melts before the outermost layer and flows out, stopping effective brazing
Solution Approach 1:
The composition parameters of each layer are precisely controlled to achieve different solidus temperatures. The outermost layer is designed with higher solidus temperature than the intermediate layers, creating a controlled melting sequence. This parameter optimization ensures intermediate layers melt first for efficient joining while preventing premature flow and maintaining structural stability.
3Strength
If Si content in the outermost surface layer is increased to form molten brazing filler metal and fillet, then wetting and joining quality are improved, but the material becomes hard and brittle, making production difficult
Solution Approach 1:
The Si content in the outermost surface layer is optimized within a specific range (2-13 mass%) to achieve the desired balance. This parameter optimization ensures sufficient Si is present to form molten brazing filler metal and fillet for good wetting and joining quality, while preventing excessive Si that would cause the material to become hard and brittle, thus maintaining ease of production.
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 effective flux-free brazing with improved wetting and joining quality on open joints, ensuring strength and durability comparable to conventional methods in heat exchanger applications.
Implementation Method 1
On the outermost surface brazing filler metal layer, a molten brazing filler metal is formed by Si at the time of brazing and a fillet of the joining part is formed
Implementation Method 2
the diffusion of Mg from the intermediate layer is delayed
Implementation Method 3
the Mg in the melt-activated brazing filler metal deoxidizes and decomposes the Al oxide film (Al 2 O 3 ) on the joining part surface
Implementation Method 4
brazing can be performed flux-free in a non-oxidizing gas atmosphere with an oxygen concentration of 100 ppm or less
Data Source
Figure 1
Figure 2
Figure 3(a)~3(b)
AI summary
Brazing sheet for flux-free brazing, wherein 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%, are cladded on one side or both sides of a core material, and furthermore, in the outermost surface brazing filler metal layer, the number of Si particles having a circle equivalent diameter of 1.75 µm or more among 10% or more of the number of those having a circle equivalent diameter of 0.8 µm or more, as observed in the direction of the surface layer, and 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, and wherein aluminum members are joined to each other without using flux in a non-oxidizing gas atmosphere under normal pressure with an oxygen concentration of 100 ppm or less, using the brazing sheets.