Hard Solder Materials: Key Properties & Applications

Due to its high strength, hard solder can be used for welding stressed components and in high-temperature working environments.

Aluminum-based Solder

Aluminum-based solder is primarily used for brazing aluminum and aluminum alloys. By adding 1% to 1.5% mass fraction of Mg to aluminum alloys, it can be used for vacuum brazing of aluminum alloys.

Aluminum-based solder can be made into bimetal composite plates, i.e., solder with 5% to 10% of the base metal thickness on both sides (referred to as aluminum brazing plate). This is used for brazing large areas or densely joined components, such as diesel engine coolers.

Silver-based Solder

Silver-based solder is the most widely used type of hard solder. It has a moderate melting point, good workability, wets many metals, and possesses good strength, ductility, thermal conductivity, electrical conductivity, and resistance to various corrosive media. Therefore, it is used for brazing low carbon steel, structural steel, stainless steel, copper and its alloys, brass alloys, and refractory metals.

Silver solders used for vacuum brazing should not contain volatile elements such as phosphorus, cadmium, zinc, magnesium, or lithium, as they would affect the brazing process and quality. Table 5-62 lists the grades, compositions, properties, and applications of vacuum-grade silver-based solder.

Copper-based Solder

Pure copper can also be used as solder, with a melting point of 1083℃. When used as solder, the brazing temperature is approximately 1100-1150℃. To prevent oxidation of the workpiece, pure copper solder is mostly used for brazing steel and copper and their alloys under reducing, inert, or vacuum conditions.

When using copper-zinc solder, zinc easily evaporates, resulting in an increased melting point of the solder and the formation of porosity in the joint. In addition, zinc vapor is toxic and detrimental to workers’ health. To reduce zinc evaporation, a small amount of silicon can be added to copper-zinc solder.

There is also a type of copper-phosphorus solder, which is based on Cu-P and Cu-P-Ag alloys and is mainly used for brazing copper and copper alloys. Because copper-phosphorus solder can fill joint clearances and act as a flux, it is called self-fluxing solder.

It is widely used in the manufacturing of electric motors and refrigeration equipment. However, copper-phosphorus solder cannot be used for brazing steel, nickel alloys, and copper-nickel alloys with more than 10% omega (Ni).

Adding nickel and cobalt to copper can improve its heat resistance, making it a high-temperature copper-based solder. Table 5-65 shows the compositions, properties, and applications of high-temperature copper-based solder.

Manganese-based Solder

Manganese-based solder is used for working at relatively high temperatures.

It has good ductility and can be made into various shapes. It wets stainless steel and high-temperature alloys well, without causing strong corrosion or intergranular penetration in stainless steel. Manganese-based solder is suitable for brazing in a protective gas atmosphere, requiring high gas purity.

It is not suitable for flame brazing and high-vacuum brazing. Table 5-66 shows the compositions, characteristics, and applications of manganese-based solder.

Nickel-based Solder

Nickel-based solder has excellent corrosion resistance and heat resistance, and is used for brazing high-temperature working parts. Nickel has a very high melting point (1452℃) and insufficient thermal strength, so alloying elements must be added to reduce its melting point and improve its thermal strength for use as solder.

Gold-based Solder

Gold-based solder can reduce the transition of the gold plating layer to the solder, which is beneficial for the stability of the gold plating layer. Refer to table 5-68 for its chemical composition, characteristics, and applications.

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