Aluminium Brazing Sheet Recovery Annealing Against Liquid Film Migration
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Solution Overview
Problem
Liquid Film Migration (LFM) is a persistent issue in the manufacturing of brazed heat exchangers using aluminium alloy brazing sheet products, leading to decreased brazeability and poor performance characteristics due to molten AlSi filler alloy penetration along sub-grain boundaries, especially in fully annealed products subjected to slight cold working.
Innovation Solution
A method of manufacturing a brazing sheet product with a 3xxx-series aluminium core alloy clad on one or both sides with a 4xxx-series aluminium brazing layer, involving casting, hot rolling, cold rolling, soft annealing, and final recovery annealing to achieve a microstructure that reduces susceptibility to LFM, while maintaining formability and strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a fully annealed (O-temper) brazing sheet product is used, then formability is improved, but resistance against liquid film migration deteriorates
Solution Approach 1:
The patent applies a specific heat treatment process (heating to 350-450°C for 1-6 hours) to transform the microstructure parameters of the brazing sheet. This creates a unique temper condition where the material maintains good formability while developing enhanced resistance to liquid film migration through controlled grain boundary characteristics without full recrystallization.
Solution Approach 2:
The patent performs a preliminary heat treatment (recovery annealing) before the brazing operation. This pre-treatment creates an optimal microstructure that will resist liquid film migration during subsequent brazing, while preserving the formability needed for manufacturing operations.
2Ease of operation
If the brazing sheet is subjected to slight cold working, then formability is improved, but core penetration increases
Solution Approach 1:
The patent changes the thermal parameters by applying recovery annealing treatment that heats the material to 350-450°C without allowing full recrystallization. This creates an intermediate microstructure that reduces core penetration susceptibility while maintaining the formability benefits of cold working.
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 method significantly reduces Liquid Film Migration, enhancing braze performance, strength, and corrosion resistance, with improved formability and extended SWAAT-test results, making the brazing sheet product suitable for heat exchanger applications.
Implementation Method 1
hot rolling
Implementation Method 2
cold rolling of the strip
Implementation Method 3
heat-treated to soften the material but without any recrystallization of the interlayer
Implementation Method 4
the molten AlSi filler alloy penetrates the solid aluminium alloy core alloy along the sub-grain boundaries
Data Source
AI summary
The invention relates to a method of manufacturing a brazing sheet product having a core layer of a 3xxx-series aluminium alloy clad on one or both sides with a 4xxx-series aluminium alloy brazing layer, the method comprising the steps of: (i) casting a rolling ingot of the core layer of a 3xxx-series aluminium alloy having the following composition, in wt. %: Mn 0.5-1.8, Si≤1.5, Fe≤0.7, Cu≤1.5, Mg≤1.0, Cr≤0.25, Zr≤0.25, Ti≤0.25, Zn≤0.5, balance impurities and aluminium; (ii) hot rolling of the rolling ingot to a hot rolled sheet having thickness of 2.5-10 mm; (iii) cold rolling of the hot rolled sheet to a gauge of 0.1-4 mm, optionally with an intermediate annealing step during the cold rolling operation; (iv) soft annealing to recrystallize the microstructure of the aluminium sheet, preferably at a temperature in the range of 250° C.-450° C.; (v) further cold rolling of the soft annealed sheet with a cold rolling reduction in the range of 5% to <10% to a final cold rolling thickness; and (vi) recovery annealing at 200° C.-420° C. of the cold rolled aluminium sheet at final cold rolling thickness.