In metal joining by brazing, which flux is most commonly used to clean the metal surfaces and promote proper flow of the molten filler metal?

Introduction / Context:
Brazing is a metal joining process in which a filler metal with a lower melting point than the base metals is melted and distributed between closely fitted surfaces. For brazing to be successful, the metal surfaces must be clean and free from oxides so that the molten filler metal can wet and flow over them. A flux is used to remove oxides and prevent further oxidation during heating. This question tests your knowledge of the common flux material used in brazing operations in basic manufacturing and welding technology.

Given Data / Assumptions:

• The joining process under discussion is brazing, not soldering or welding.
• A flux is required to assist in cleaning the metal surfaces and improving wetting of the filler metal.
• Options include ammonium chloride, borax, zinc chloride and resin plus alcohol.
• We assume typical ferrous and non ferrous metals used in general workshop practice.

Concept / Approach:
In brazing, the joint is heated to a temperature above the melting point of the filler metal but below the melting point of the base metals. Oxide films and surface contamination can prevent proper wetting of the base metal by the molten filler. Fluxes are chemical compounds that dissolve these oxides and protect the surfaces from further oxidation. Borax, which is sodium tetraborate, is widely used as a flux in brazing and forge work because it melts and dissolves metal oxides efficiently at brazing temperatures. Ammonium chloride and zinc chloride are more commonly associated with soldering and certain cleaning processes, and resin based fluxes are typically used in soft soldering of electronic components. Therefore, borax is the commonly used brazing flux among the options.

Step-by-Step Solution:
Step 1: Recall that brazing uses a filler metal with melting point higher than in soft soldering but lower than that of the base metals. Step 2: Recognise that surface oxides on the metal workpieces interfere with proper bonding and must be removed or neutralised. Step 3: Understand that a suitable flux is applied before or during heating to dissolve oxides and improve wetting by the filler metal. Step 4: Note that borax has a long history as a general purpose high temperature flux in brazing and forge welding because it becomes glassy and dissolves metal oxides effectively. Step 5: Ammonium chloride and zinc chloride mixtures are more common in soft soldering or cleaning operations at lower temperatures. Step 6: Resin plus alcohol fluxes are typical in electronics soldering, not in higher temperature brazing processes. Step 7: Therefore, among the given options, borax is the flux most commonly used in brazing.

Verification / Alternative check:
Standard manufacturing and welding textbooks usually describe borax based fluxes as the preferred choice for many brazing applications, especially for ferrous alloys and copper alloys. In traditional blacksmith practice, a mixture based on borax is sprinkled on red hot steel surfaces before forge welding and brazing. Manufacturers of commercial brazing fluxes often list borax and boric acid as primary ingredients. These widely documented uses confirm that borax is the most commonly used flux in brazing operations.

Why Other Options Are Wrong:
Ammonium chloride is used in some soldering fluxes and cleaning powders but is not the standard general purpose brazing flux. Zinc chloride is a component in certain soldering fluxes, particularly for galvanised surfaces, but is not the usual choice for brazing at higher temperatures. Resin plus alcohol flux is mainly used for soft soldering, especially in electronics, where low temperature and non corrosive residues are important.

Common Pitfalls:
Learners sometimes confuse soldering and brazing because both involve a filler metal with lower melting point than the base metal. However, the temperatures and materials are different, so the fluxes used are also different. Resin based and zinc chloride fluxes are more typical of soft soldering, while borax based fluxes suit the higher temperatures of brazing. Remembering that borax is linked to high temperature joining and forge work makes it easier to select the correct answer in such questions.

Final Answer:
In brazing, the flux commonly used to clean metal surfaces and help the filler metal flow is Borax.