The Most-Often-Asked Questions About Silver Brazing
– And the Answers.

Historians and archeologists have been able to establish that brazing is one of the oldest methods of joining materials. This process dates back to a time period of 1500-2000 B.C.  Although brazing started in ancient times primarily as a tool in the making of jewelry and art items, the joining of metals found in natural form such as gold and silver grew as new metals were discovered and alloys developed. 

Today, silver brazing is a science used in a variety of applications ranging from joining metals for garden tools to high tech aerospace assemblies. Silver brazing originally was used with metals and metal alloys. It has expanded to include the joining of materials other than metals, such as cermets and ceramics. As new materials are developed and new applications for existing material are created, silver brazing assumes a much larger role as a method of joining. For this reason, users or potential users of silver brazing alloys should have an understanding of the basic principles to assist in the selection and/or its application. To help in gaining this information, the more commonly asked questions (and answers) are listed.

1) What separates brazing from other methods of joining with heat such as soldering and welding?

Ans. By definition, the American Welding Society has established brazing as the use of a filler metal which melts at a temperature greater than 840F (450C) and below the melting temperature of the material being joined. Also, the filler metal is distributed in the joint by capillary action.

2) What is silver solder or hard solder?

Ans. Both terms are slang or non-standard terms used to describe brazing with a silver containing alloy. By the definition noted above, soldering is performed at a temperature below 840F, and brazing above 840F.

3) Are there filler metals other than silver brazing alloys?

Ans. Yes. The other more common alloys are:  copper, copper-phosphorus, nickel, aluminum, and gold.

4) Why is silver brazing so popular?

Ans.  Its ability to join a wide variety of dissimilar metals, alloys, and other materials has made it very popular. Also, silver brazing alloys have a relatively low melting temperature making them easy to use.

5) What is capillary action?

Ans. The American Welding Society states that capillary action is the force by which liquid in contact with a solid is distributed between closely fitted faying surfaces of the joint to be brazed or soldered.

6) What affects capillary action?

Ans. There are many factors affecting capillary action.  The major influences are those that affect the capillary spacing or gap and those that affect the surface energy of both the liquid alloy and the surface to which the braze alloy is to be in contact.

7) What is the proper gap?

Ans. For the more common standard type of joints, a gap of .002-.005 inches is suggested for both silver and copper phosphorus filler metals.  We recommend that you contact Wolverine's technical experts if you have a question regarding a specific application.

8) What are the common types of braze joints?

Ans. There are three (3) types of braze joints: butt, lap, and scarf.  Other joints would be combinations of these.

9) What are the major factors that affect the surface energy on the base material to be joined?

Ans. Contaminants and oxides are the major factors that affect surface energy.

10) What are some of the contaminants?

Ans. The most common contaminants are; drawing compounds, cutting fluids, polishing compounds, oils, and greases.

11) How are oxides removed?

Ans. The most effective ways to remove oxides are with a reducing or fluxing material.

12) Are there different types of fluxing material?

Ans.  Yes. There are three classifications of fluxing material: chemical, reducing atmosphere, and vacuum. Phosphorus is also a reducing element in the joining of copper to copper with a copper phosphorus braze alloy.

13) Will the flux remove other contaminants?

Ans. No. The flux will not remove other contaminants. It is not a detergent, a solvent, or a degreaser.

14) Should the chemical or paste flux be removed after brazing?

Ans. Yes. Practically all paste fluxes contain fluoride. Fluoride left on the braze assembly will support corrosion.

15) Of the various factors for proper brazing, how important is the heating process?

Ans. Improper heat procedure is a major problem in silver brazing and can impact the integrity of the joint.

16) What is so difficult about heating?

Ans. The difficulty in heating is knowing when and if the inside surfaces have reached a uniform proper temperature. The operator cannot see the inner surfaces, and it is not easy to measure this surface during mass production.

17) Why is uniform temperature important?

Ans. Capillary action is the mechanism through which the filler metal flows into the joint. When the inside capillary spacing is uniform throughout the joint, the alloy will be drawn to the hottest surface.

18) Will uniform heat guarantee proper filling of the braze alloy into the joint?

Ans. No. Unfortunately, uniform heat alone does not guarantee that the braze alloy will flow smoothly and evenly into the joint. Brazing requires a certain level of craftsmanship. Operator expertise is a critical component in achieving a good braze joint. In addition, there may be a situation in which the capillary spacing is not uniform. In this example, the braze alloy will be drawn by capillary force to the closest fitting surface. In other words, a .002 inch gap area within a joint will pull alloy from a .012 inch area.

19) What is the difference between silver brazing alloys and copper phosphorus alloys containing silver.

Ans. The copper phosphorus alloys (with or without silver) are not as versatile as the silver brazing alloys. They are used primarily in the joining of copper to copper. The phosphorus in the copper phosphorus acts as the flux. The silver brazing alloys can not only be used on the same base materials as the copper phosphorus alloys, but can also be used on many other base materials.

20) Besides copper, what other base materials can use copper phosphorus alloys?

Ans. Copper phosphorus alloys can be used on brass, silver, tungsten and molybdenum. They are not self fluxing on these materials.

21) Why can’t copper phosphorus be used on ferrous or nickel base alloys?

Ans. The iron and nickel will react with the phosphorous forming intermetallic brittle phosphide compound, resulting in a weak joint.

22) What are solidus and liquidus temperatures?

Ans. Upon heating a braze alloy, the temperature at which the alloy starts to melt is the solidus temperature. The temperature at which the alloy has become completely liquid is the liquidus temperature.

23) What is eutectic?

Ans. "Eutectic" describes an alloy or temperature where the alloy turns liquid at a single temperature. The most common silver eutectic alloy is 72% Ag-28% Cu which has a eutectic temperature of 1435F.

24) What factors should be considered in the selection of a silver brazing alloy and its form?

Ans. Some of the more important factors are: base material, method of heating, automation, strength, internal pressure, service temperature, corrosive environment, self fluxing, ductility, conductivity, and cost. For more detailed information, please contact Wolverine's technical experts.

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