Fluxes in Aluminium Brazing
Fluxes play a critical role in the aluminium brazing process by facilitating the formation of sound and reliable joints. They aid in the removal of oxides, promote wetting of the base metal and filler material, and facilitate heat transfer during the brazing operation. In this article, we will explore the functions of fluxes in aluminium brazing, discuss different types of fluxes commonly used, and provide guidelines for proper flux selection and application techniques to ensure successful brazing operations.
Functions of Fluxes in Aluminium Brazing:
-
- Oxide Removal:
- Fluxes act as a cleaning agent, removing oxides, dirt, and other contaminants from the surface of the aluminium.
- Oxides on the aluminium surface hinder the wetting and flow of the filler material, leading to weak or incomplete joints.
- Fluxes react with the oxides, forming a molten slag that can be easily removed, exposing a clean metal surface for effective brazing.
- Wetting and Spreading:
- Fluxes promote wetting, which is the ability of the molten filler material to spread and form a continuous and uniform bond with the base metal.
- They reduce the surface tension between the filler material and the base metal, enabling better capillary action and flow into the joint.
- Proper wetting ensures complete filling of the joint and the formation of strong metallurgical bonds.
- Heat Transfer:
- Fluxes enhance heat transfer by improving the thermal conductivity between the base metal and the filler material.
- They aid in the dissipation of heat, ensuring uniform heating and cooling throughout the brazing process.
- Efficient heat transfer helps prevent overheating or underheating of the joint, contributing to the formation of high-quality brazed joints.
- Oxide Removal:
Types of Fluxes Used in Aluminium Brazing:
- Chloride-Based Fluxes:
- Chloride-based fluxes, such as zinc chloride or ammonium chloride, are commonly used for aluminium brazing.
- They are effective in removing oxides and promoting wetting, resulting in strong and reliable joints.
- However, chloride-based fluxes can be corrosive and require thorough post-brazing cleaning to remove any residue.
- Fluoride-Based Fluxes:
- Fluoride-based fluxes, including potassium fluoroaluminates or potassium aluminium fluoride, are widely used in aluminium brazing.
- They offer excellent oxide removal and wetting properties while being less corrosive than chloride-based fluxes.
- Fluoride-based fluxes are compatible with a wide range of aluminium alloys and exhibit good flow characteristics.
- Non-Corrosive Fluxes:
- Non-corrosive fluxes, such as organic-based or water-soluble fluxes, are gaining popularity in aluminium brazing applications.
- These fluxes offer effective oxide removal and wetting capabilities without causing corrosion on the base metal.
- Non-corrosive fluxes simplify the post-brazing cleaning process and are environmentally friendly.
Guidelines for Flux Selection and Application Techniques:
- Consider Base Metal and Filler Material Compatibility:
- Select a flux that is compatible with both the base metal and the filler material to ensure proper oxide removal and wetting.
- Assess Corrosion Resistance Requirements:
- Determine the level of corrosion resistance needed for the specific application and select a flux accordingly.
- If corrosion resistance is critical, opt for non-corrosive or low-corrosive fluxes.
- Follow Manufacturer’s Recommendations:
- Consult flux manufacturers for their recommendations on flux selection, application techniques, and cleaning procedures.
- Manufacturers provide specific guidelines based on flux composition, alloy compatibility, and brazing conditions.
- Apply Flux Properly:
- Apply the flux evenly and uniformly to the joint surfaces using a brush, spray, or dip method.
- Ensure that the flux adequately covers the entire joint area for effective oxide removal and wetting.
- Remove Flux Residue:
- After brazing, thoroughly clean the brazed assembly to remove any flux residue.
- Residual flux can be removed through appropriate cleaning methods, such as rinsing with water, using neutralizing agents, or employing specialized flux removers.
