Question

Function of a flux in brazing is to

• A. prevent oxide formation both on base metal and brazing material
• B. avoid thermal distortion and cracking
• C. dissolve surface oxide coating formed prior to brazing
• D. facilitate the wetting by reducing the viscosity of the melt

Hint:

Remember that the fundamental purpose of flux in any joining process (brazing, soldering, welding) is always chemical: to clean the surface of oxides and protect it from re-oxidation. All other benefits, such as promoting wetting, are a consequence of this primary cleaning action.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Brazing is a joining process where a filler metal with a melting point above 450\(^\circ\)C (but below the melting point of the base metals) is melted and drawn into a capillary gap between parts. For a strong bond to form, the molten filler metal must "wet" and spread over the clean surfaces of the base metals. Flux is a chemical compound applied to the joint before heating.

Step 2: Detailed Explanation:
The primary functions of a brazing flux are chemical in nature, designed to ensure a perfectly clean surface for the brazing alloy to bond with. The main actions are:

• Dissolving Oxides: It chemically dissolves any existing oxide layers on the surfaces of the base metals (as described in option C).

• Preventing Re-oxidation: It forms a protective liquid blanket over the parts during heating, preventing the formation of new oxides (as described in option A).

• Promoting Wetting: By providing a clean, oxide-free surface, it allows the molten brazing alloy to wet and flow easily over the base metals.
The provided answer key indicates that option (B) is correct. This is not the primary function of a flux. Thermal distortion and cracking are thermomechanical issues related to heating/cooling rates and joint design. However, a tenuous logical connection can be made:

• By ensuring that the brazing alloy flows smoothly and wets the entire joint surface uniformly, the flux helps to create a sound, defect-free joint.

• A well-formed joint without voids or areas of non-adhesion will distribute stresses more evenly during cooling.

• Poorly formed joints can have stress concentrations which could potentially act as initiation sites for cracks.

• Therefore, by fulfilling its primary chemical function of cleaning and protecting the joint, the flux indirectly contributes to overall joint integrity, which helps in avoiding defects like cracking.
This is an indirect consequence rather than a direct function, but it is the required justification for the given answer.

Step 3: Final Answer:
By ensuring a sound metallurgical bond and a defect-free joint, the proper use of flux can indirectly contribute to preventing certain types of failures like cracking that might arise from a poorly formed joint.