There are various types of brazing alloys available in the market. The more specific the technical and economic requirements for the alloy, the easier it is to find a suitable brazing material. Several factors affect the integrity of the assembly, including joint design, cleaning procedures for welding surfaces, flux, proper cooling and removal of flux residues, heating methods, and the choice of brazing alloy. Most of the time, the design, methods, and material choices are clear, so the remaining influencing factors are limited. How to choose a brazing alloy? What needs to be considered in advance? This is what this article will discuss.

The diverse brazing alloys can be divided into several groups, each containing mixtures with different melting temperatures and other properties; the forms of brazing alloys are also varied; many brazing alloys are used to meet different brazing requirements.

Brazing alloys are typically defined as materials used as filler metals to complete joint connections in brazing, with a melting temperature higher than 840 °F but lower than that of the workpiece. A brazing alloy should possess the following characteristics:

It must maintain wettability and fluidity on the surface of the welded workpiece and can operate through capillary action as small as 0.001 inches;It can prevent the alloy from separating into solid and liquid states during the welding process;It must provide a strong joint through interaction with parts of the welded workpiece;It must meet quality performance requirements for corrosion resistance, ductility, thermal & electrical conductivity, and other user demands.

Brazing alloys consist of various metals, resulting in multiple melting temperatures and ranges. To date, we use the term 'melting temperature' to describe the temperature at which a metal transitions from solid to liquid.

Comparison of high and low temperatures

Low-temperature brazing alloys begin to melt at lower temperatures as their name suggests. Their advantage is that productivity increases with shorter heating times, leading to greater economic efficiency due to lower energy consumption. However, sometimes it is necessary to have brazing alloys with higher melting temperatures.

For example, if engine components need to operate at a temperature of 1200 °F, then a brazing alloy with a higher melting temperature must be used for welding. Other brazing materials would not suffice; otherwise, it could lead to component failure. For layered hard brazing, high-temperature brazing materials are often required. Layered hard brazing is typically used when welding two or more adjacent joints. The first joint uses high-temperature brazing material while the second uses a relatively lower temperature alloy; during this process, the second joint is heated below the melting temperature of the first joint.

The heating method and some properties of the workpiece determine the temperature range for brazing alloys. If brazing is completed first, high-temperature alloys need to be selected; for instance, using copper or copper-zinc alloys (2000-2100 °F) to weld low-carbon steel alloys and then normalizing and hardening the workpiece at temperatures of 1500-1600 °F.

Silver-Copper Alloy

Silver is sometimes used alone as a brazing alloy but is more often found in mixtures with other metals like copper. Silver-Copper alloys begin to melt at temperatures around 600 °F (below pure copper's melting temperature). In addition to lowering melting points, silver can enhance capillary flow. Easily melted silver-copper alloys (containing 72% silver and 28% copper) are commonly used in torch welding with narrower melting ranges.

Copper-Zinc Alloy

Zinc is often combined with copper to lower melting temperatures. These alloys can be used for welding similar base material workpieces like copper; however, their corrosion resistance is usually insufficient for connecting copper with silicon bronze, copper-nickel or stainless steel. Copper-zinc alloys are commonly used in various brazing tasks, particularly in automotive parts repair.

Silver-Copper-Zinc Alloy

The first use of silver, copper, and zinc together was for manufacturing industrial silver brazing alloys. These alloys combine significant features of silver-copper and copper-zinc alloys; compared to silver-copper alloys with equivalent silver content, they have lower melting points. Zinc can also improve wettability on iron metals. The presence of silver enhances fluidity and corrosion resistance compared to copper-zinc alloys.

Active Alloys

Reactive metals like titanium can also be added to silver alloys so they can directly braze non-metal surfaces such as graphite or ceramics; these are typically referred to as active alloys.

Copper-Phosphorus Alloy

Copper-phosphorus based alloys are widely used for connections between copper and its alloys. Phosphorus not only lowers the melting temperature of brazing alloys but also reacts with workpieces to remove surface copper oxides or rusted areas—this is a function that flux should perform; therefore, when using copper-phosphorus alloy for welding copper, flux may be omitted. Other oxides (like zinc in brass) cannot be removed by phosphorus; thus having an adequate flux is essential for satisfactory joints. This type of alloy is particularly suitable for connecting copper and its alloys but can also be used for welding silver, tungsten, and molybdenum. Copper-phosphorus alloys should not be used on iron or nickel-containing alloys because iron and nickel react with phosphorus to produce glassy mixtures that make joints prone to brittleness.

Silver-Copper-Phosphorus Alloy

Adding 18% silver to an alloy containing 93% copper and 7% phosphorus alters its melting performance and joint characteristics; as silver content increases, fluidity improves making it easier to use and suitable for filling larger gaps in joints. Other enhanced properties due to silver inclusion include ductility and conductivity.

Forms of Alloys

Brazing alloys exist in various forms: rods, wires, sheets, and strips; other pre-formed types include rings, washers, discs, and blocks. Typically, welding rods or wires are used in simple face-to-face welding, while pre-formed solder products are used in longer production processes or in situations where face-to-face welding is not feasible (due to lack of economic viability and technicality), such as in furnace torch welding. Alloys also often exist in paste form, with solder paste containing alloy powder, adhesives, and flux. Although solder paste is usually much more expensive than brazing products, it becomes the preferred choice in some cases due to ease of operation.

Economic viability

The cost of welding is higher than the cost of raw materials; it includes the preparation time for brazing, the cost of the filler material itself, heating costs, and other necessary welding costs. Each welding process has many aspects that need to be weighed, and strict consideration must be given when calculating costs.