The defects of aluminum brazing primarily stem from the material's high oxidizability and the close melting ranges of base and filler metals. Here's a detailed breakdown:
1. Oxidation and Surface Preparation: Aluminum forms a stable oxide layer (aluminum oxide, Al2O3) that prevents filler metals from wetting the surface. This necessitates the suppression of this oxide layer before brazing. This can be achieved chemically through corrosive fluxes, base or acid attacks, or the use of magnesium, or mechanically by sanding. Failure to properly suppress or remove this layer can lead to poor wetting and weak joints.
2. Melting Range Issues: The melting ranges of base and filler metals in aluminum brazing are very close, which complicates the process. Some aluminum alloys cannot be brazed due to these close melting points. The brazing method must ensure precise temperature control and uniform thermal distribution to prevent overheating of the base material, which could lead to melting or distortion.
3. Pre-Braze Cleaning and Furnace Conditions: Proper cleaning is crucial to remove all oxides, contaminants, and oils. The furnace cycle must be controlled to avoid issues like distortion and uneven temperature distribution. Rapid ramping or quenching can lead to problems such as distortion, quench cracking, and splatter of the braze alloy.
4. Loss of Mechanical Properties: Brazing can lead to the irreversible loss of mechanical properties in materials that rely on cold working and annealing for their strength. This is particularly relevant for hardenable alloys like those in the 6xxx series (Al-Si-Mg), where certain precautions must be taken due to their low solidus temperatures.
5. Limited Alloy Compatibility: Not all aluminum alloys are suitable for brazing. Precipitation-hardened alloys, especially from the 2xxx (Al-Cu) and 7xxx (Al-Zn-Mg) series, are generally not brazable due to their low melting points. Even within brazable alloys, specific conditions may be required, as seen with alloys 7004, 7005, and 7072.
6. Brazing Techniques and Equipment: The choice of brazing technique (e.g., flame brazing, furnace brazing) and the setup of the brazing equipment are critical. Incorrect techniques or poorly calibrated equipment can lead to common problems such as splatter, quench cracking, and distortion.
In summary, the defects in aluminum brazing are largely due to the challenges posed by aluminum's high oxidizability, the close melting points of base and filler metals, the need for meticulous pre-braze preparation and furnace control, and the limitations in alloy compatibility. Addressing these issues requires careful selection of materials, precise process control, and appropriate brazing techniques.
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