Historians and archaeologists have confirmed that brazing is one of the oldest methods of joining materials. Dating back to 1500-2000 BC, ancient brazing was primarily used for making jewelry and crafts, but welding was found to exist in natural forms in metals like gold and silver, leading to the derivation of new metals and alloys.

Now, from household tools to high-tech aerospace equipment, silver brazing has been widely applied as a science in various fields. Initially, silver brazing was used for welding metals and metal alloys, but it has now expanded to connect other materials beyond metals, such as ceramics and metal-ceramics. When a new material or a new use for an existing material arises, silver brazing often plays a major role in welding; in this regard, operators or those preparing to implement brazing should understand the basic principles of welding to assist in selecting the correct filler alloy for successful welding. To help readers gain more information on this topic, we have listed the following common questions and provided answers:

How to distinguish brazing from other welding methods?

By definition, the American Welding Society stipulates that filler metal (filler alloy) melts below the melting temperature of the workpiece being welded but above 450 °C, and that the alloy fills the workpiece through capillary action to form a joint is considered brazing.

What are silver filler materials and hard filler materials?

The above two terms are used to describe silver-containing fillers but are colloquial or non-standard expressions. Welding conducted above 450 °C is brazing; conversely, below 450 °C it is soldering.

Are there other metals besides silver that can be used as filler materials (filler)?

Commonly used alloys include copper, copper-phosphorus, nickel, aluminum, and gold in addition to silver.

Why is silver filler so popular?

Because silver filler can connect many different metals, alloys, and other materials, and it has a lower melting temperature which makes it easier to use.

What is capillary action?

The American Welding Society defines capillary action as the phenomenon where molten solder flows against gravity between closely spaced surfaces of workpieces and ultimately forms a joint.

What factors can affect capillary action?

Many factors can affect capillary action; the size of the gap in the welding apparatus is a major factor, as well as the contact area of the workpieces being welded.

What is the appropriate welding gap?

For most standard types of joints, the filler metal is an alloy of silver and copper-phosphorus; a recommended welding gap is between 0.002-0.005 inches. Of course, if you have any questions, feel free to contact our technical experts.

What are common types of brazed joints?

There are mainly three types of welded joints: butt joints, lap joints, and plug joints, as well as different mixed types of these three joints.

What are the main factors affecting the surface energy of base materials?

Contaminants and oxides.

What contaminants are included?

The most common contaminants include lubricants for drawing, cutting fluids, polishing agents, machine oil, and lubricating oil.

How to remove oxides?

Using flux during the welding process is the most effective method for removing oxides.

Are there different types of flux?

Yes. Flux can be mainly classified into three categories: chemical fluxes, reducing air fluxes, and vacuum fluxes. When using copper-phosphorus alloy to weld copper workpieces, phosphorus can act as a flux to remove oxides.

Can flux be used to remove other contaminants?

No. Flux is not a degreaser or solvent; it cannot be used to remove other contaminants.

Should paste flux be removed after welding?

Yes, paste flux should be removed because all paste fluxes contain fluorides which can weaken joint strength if left on the workpiece.

What is the importance of heating in the welding process?

Improper heating is a major issue in silver brazing that can affect joint integrity.

What is the most difficult aspect of heating?

Knowing when the inner surface reaches a uniform appropriate temperature is the most difficult part of heating because operators cannot see the inner surface and it is also difficult to measure this surface in large-scale production.

Why is uniform temperature important?

Capillary action is the mechanism by which molten filler metal melts and forms joints. When the temperature across the entire capillary action area reaches uniformity, only then can the filler alloy (filler) be drawn to the hottest surface.

Can uniform heating ensure that filler alloys fill and form joints?

No. Uniform heating alone is not sufficient to ensure that fillers flow smoothly and ultimately form joints; brazing requires multiple techniques working together. The operator's professional skills are extremely important for achieving a good joint. Additionally, there may be areas under capillary action that do not receive uniform heating; in such cases, fillers may be drawn to the nearest mating surfaces due to capillary forces.

What is the difference between silver filler material and copper-phosphorus alloy containing silver?

Copper-phosphorus alloys (with or without silver) are not as versatile as silver fillers; they are mainly used for welding metal materials with copper as the base material. The phosphorus element in copper-phosphorus fillers also acts as a flux. In contrast, silver fillers can be used not only for metals welded with copper-phosphorus alloys but also for workpieces with other metals as base materials.

Besides copper, are there other base materials suitable for copper-phosphorus alloy?

Copper-phosphorus alloy can also be used for connections with brass, silver, tungsten, and molybdenum. However, these materials require the use of a filler to complete the welding.

Why is copper-phosphorus alloy not suitable for welding with iron and nickel as base materials?

Iron and nickel elements can react with phosphorus to form brittle phosphides, weakening the joint strength.

What are solidus temperature and liquidus temperature?

During brazing, the solidus temperature is when the filler reaches a certain temperature and begins to melt; the liquidus temperature is when the alloy reaches a certain temperature and completely turns into a liquid.

What is low eutectic/eutectic?

Low eutectic/eutectic is a term used to describe when an alloy or temperature reaches a certain point where the alloy transforms into a liquid. A common silver eutectic alloy contains 72% silver and 28% copper, with an eutectic temperature of 1435F.

What factors should be considered when selecting silver filler?

Workpiece material, heating method, automation, strength, internal pressure, working temperature, corrosive environment/media, fluidity, ductility, conductivity, and cost are all important considerations. For more information, feel free to contact our technical experts.