Guidelines for Selecting the Right Brazing Alloy

There is a wide range of brazing alloys on the market, and for many applications a number of them will do the job. The more specific the technical and economic requirements, the more obvious the correct choice becomes. There are many choices that affect the integrity of the assembly. Some of these are: joint design and proper fitup; cleaning procedure to prepare surface; fluxing practice; proper cooling and flux removal; method of heating; and choice of brazing alloy. Most often, the design, methods and materials are specified. Therefore, the remaining choices are limited. How do you choose a brazing alloy? Are there any specific precautions to follow? It is these questions that are the topic of this document.

There are many types of brazing alloys that fall within several alloy groups. Within these groups are various alloy combinations that alter the melting temperatures and other properties of brazing alloy. Brazing alloys also come in a variety of forms. Such a wide variety of brazing alloys and forms exist to fulfill a wide variety of requirements.

The brazing alloy is defined generally as the metal which is added during the brazing process to complete a joint. It's melting temperature, by definition, is above 840°F, but below the melting point of the base metals. A brazing alloy should also fulfill the following criteria;

• It must "wet" or flow evenly across the surfaces of the base metal, and through capillaries as small as 0.001-inch;
• It must resist alloy separation into solid and liquid phase during brazing (liquidation);
• It must provide a strong bond by partially alloying with the base metals;
• And it must meet conditions of corrosion resistance, ductility, thermal and electrical conductivity, or other properties dictated by the organization having the quality responsibilities.

Brazing alloys comprise a variety of metals and therefore, have a variety of melting temperatures and melting ranges. Thus far, we have used the term "melting temperature" to describe the temperature at which a metal turns from solid to liquid.

Low vs. High Temperature

Low-temperature brazing alloys, as the name implies, begin melting at lower temperatures. The advantages are higher productivity as heating times are reduced, and greater economy from lower energy consumption. On occasion, however, brazing alloys with relatively high melting points are desirable.

For example, an engine component that will be routinely subjected to operating temperatures up to 1200°F must be brazed with a high temperature brazing alloy. Any other braze would fail, rendering the component useless. Step brazing is a second, and more common, reason for using high-temperature brazing alloys. Step brazing occurs when two or more joints are formed successively and in close proximity. The first joint is brazed using a high temperature brazing alloy, and the second using one of the lower temperature. In this procedure, the heat required for the second braze is below the melt temperature of the first braze, allowing assembly in stages.

The heat treatment process and properties of some base metals dictate the temperature range of the brazing alloys. Base metals that are to be heat treated may require either high- or low-temperature brazing alloys. If the braze is completed first, a high-temperature brazing alloy is called for to withstand the temperatures used during the heat treating process. An example would be joining of low alloy carbon steels with copper or copper zinc brazing alloys (2000-2100°F), followed by normalize and hardening procedures (1500-1600°F).

Silver - Copper

Silver is sometimes used alone as a brazing alloy; however, it is most often used in combination with other metals such as copper. An alloy of silver and copper will begin to melt at a temperature of almost 600°F below that of pure copper. In addition to reducing the melting point, silver also enhances capillary flow. The eutectic combination of silver and copper (72% silver, 28% copper) as discussed earlier, is often selected for furnace brazing applications where a very narrow melting point is required.

Copper - Zinc

Zinc is also alloyed with copper to reduce the melting temperature. These alloys can be used for the same base materials as copper. However, the corrosion resistance is generally inadequate for joining copper, silicon bronze, copper-nickel or stainless steel. These alloys are used for general purpose work, most often in braze welding applications such as auto-body repair work.

Silver - Copper Zinc

Silver, copper and zinc were first used to make industrial silver brazing alloys. These alloys combine the attractive features of the silver-copper and the copper-zinc alloy groups. Compared with the silver-copper alloy group, these alloys have a lower melting temperature at the same silver content. Zinc also helps improve wetting on ferrous metals. Compared with the copper-zinc alloy group, the added silver improves the flow properties of the alloy and the corrosion resistance.

Active Brazing Alloys

Active metals, such as titanium, are also added to some silver brazing alloys to enable direct brazing to non-metallic surfaces such as graphite or ceramics. These alloys are commonly referred to as active brazing alloys.

Copper - Phosphorous

Copper-phosphorous base alloys are widely used for joining copper and copper alloys. The phosphorous not only lowers the melting temperature of the brazing alloy, but also reacts with and reduces copper oxides or tarnish present on the surface of the metals to be brazed. This is a function usually performed by the brazing flux, and, therefore, when copper-phosphorous alloys are used in brazing copper the usual flux may be omitted. Other oxides (such as zinc in brass) are not reduced by phosphorous; therefore, flux is needed to produce satisfactory joints. While especially suitable for joining copper and its alloys, this alloy group may also be used on silver, tungsten and molybdenum. Copper phosphorous alloys are not used on ferrous or nickel-containing alloys because iron and nickel form glass-like compounds with phosphorous, and the joints produced may be brittle.

Silver - Copper Phosphorous

A group of alloys has evolved from the original 93% copper, 7% phosphorous alloy. Silver has been added in amounts of up to 18% to alter the melting characteristics and properties of the final braze joint. Increasing the amount of silver contained in these alloys improves the flow properties, making the brazing alloy easier to work with and better suited to filling larger gaps joints. Other properties that are improved with the addition of silver are ductility, electrical conductivity, and the tendency to form larger fillets.

Braze Alloy Forms

Brazing alloys are available in many forms: rod, wire, powder, sheet and strip. A broad selection of preformed shapes such as rings, washers, discs and slugs is also available. Typically, rod and wire are used for simple face feeding. Preforms, however, are increasingly used for longer production runs or for applications in which face feeding is less economical or less technically feasible, as in furnace brazing. Brazing alloys are often pre-placed in the form of paste. Brazing paste consists of brazing alloy powder, a binder, and quite often a flux. Although brazing pastes are generally more expensive than brazing preforms they are preferable for some applications due to the ease of pre-application.

Economic Criteria

The real cost of braze goes beyond the cost of the raw materials. Costs encompass the entire brazing operation preparation time, the cost of the brazing alloy itself, heating costs, and the cost of the required brazing procedures. Every application has many trade-offs, which should be carefully examined in any cost analysis.