Brazing Material Selection: A Guide for Engineers

The Selection Principles of Brazing Materials

There are various types of brazing materials, and there are many factors to consider during their use. In principle, the selection of brazing materials should be considered from the following aspects.

Matching of Brazing Materials with Base Metals

For a given base metal, the selected brazing material should have an appropriate melting point, good wettability, and filling capability for the base metal. The interaction between the brazing material and the base metal should yield beneficial results and avoid the formation of brittle intermetallic compounds.

For example, copper-phosphorus brazing materials cannot be used to braze steel and nickel, as they would form extremely brittle phosphide phases at the interface. Similarly, cadmium-based brazing materials, when used to braze copper, can easily form brittle copper-cadmium compounds at the interface, leading to embrittlement of the joint. Ideally, there should be mutual solubility in both liquid and solid states.

Matching of Brazing Materials with Brazing Methods

Different brazing methods have different requirements for the performance of brazing materials. For instance, when using flame brazing, the brazing material’s melting point should differ from the base metal’s melting point as much as possible to avoid potential local overheating, burning, or melting of the base metal.

When using resistance brazing, it is preferable for the brazing material’s electrical resistivity to be slightly higher than that of the base metal to improve heating efficiency. For furnace brazing, the brazing material should contain fewer volatile elements to ensure that its performance is not affected by the evaporation of alloying elements over a relatively long brazing time.

Ensuring Compliance with Usage Requirements

Different products have varying requirements for the performance of brazed joints under different working environments and conditions. These requirements may involve electrical conductivity, thermal conductivity, operating temperature, mechanical properties, sealing, oxidation resistance, and corrosion resistance. When selecting brazing materials, the most important usage requirements should be carefully considered.

For joints with low strength and operating temperatures, soft brazing materials are suitable. Joints intended for low-temperature operation should use brazing materials with low tin content. For joints requiring high-temperature strength and good oxidation resistance, nickel-based brazing materials are suitable. Boron absorbs neutrons, so brazing materials containing boron cannot be used in that field.

For electrical components requiring good conductivity, brazing materials with high tin content, such as tin-lead brazing materials, or those with high silver content, such as silver-based brazing materials, should be chosen. Vacuum-sealed joints should use vacuum-grade brazing materials, while aluminum brazed joints requiring good corrosion resistance should use aluminum-silicon brazing materials, and so on.

The most common usage requirements for brazed joints are strength, oxidation resistance, and corrosion resistance. When in doubt, samples can be taken for testing to determine whether the joint meets the necessary working time, temperature, and strength requirements.

Requirements for Brazing Structure

Due to the complexity of brazing structures, it is sometimes necessary to pre-process the brazing material into forms such as rings, washers, shims, foils, and powders, and place them in or near the brazing gap. Therefore, when selecting brazing materials, careful consideration should be given to whether their processing performance allows them to be shaped as needed.

Meeting the Needs of Brazing

When brazing quenched and tempered 2Cr13 steel components, BAg40CuZnCd brazing material, with a brazing temperature below 700°C, can be selected to prevent annealing of the workpiece. When brazing cold-worked hardened copper, brazing materials with a brazing temperature not exceeding 300°C should be selected to avoid softening of the base metal after brazing.

Production Costs

Production costs include the cost of brazing materials, forming and processing costs, and costs related to brazing methods and equipment investment. When the production batch size is small, priority should be given to the performance and quality of the product.

When the performance is similar, brazing materials that are inexpensive and readily available should be chosen. In large-scale production, reducing the cost of brazing materials is of significant economic importance.

Correctly selecting brazing materials is crucial to ensuring high-quality brazed joints. The comprehensive considerations for determining the selection should include the mutual compatibility of brazing materials and base metals, the usage requirements of brazed parts, existing equipment conditions, and economic factors.

Table 5-78 lists the priority order for selecting brazing materials based on the matching of brazing materials with base metals, as summarized from production practices.

Table 5-78: Matching and Selection Order of Soldering Materials and Base Metals

Base MetalAluminum-based brazing filler metalCopper-based brazing filler metalSilver brazing filler metalNickel-based brazing filler metalCobalt-based brazing filler metalGold-based brazing filler metalPalladium-based brazing filler metalManganese-based brazing filler metalTitanium-based brazing filler metal
Copper and Copper Alloys3126457
Aluminum and Aluminum Alloys1
Titanium and Titanium Alloys2435671
Carbon Steel and Alloys12684537
Martensitic Stainless Steel6715243
Austenitic Stainless Steel3716542
Precipitation Hardening High-Temperature Alloys28134567
Non-precipitation Hardening High-Temperature Alloys67451238
Hard Alloys and Tungsten Carbide15674328
Precision Alloys and Magnetic Materials21673548
Ceramics, Graphite, and Oxides32784651
Refractory Metals78654231
Polycrystalline Diamond, Gemstones86451273
Metal Matrix Composites143895672
Note: In the table, 1-9 represents the sequence of matching and selection from first to last.

When selecting a brazing material, the following points should be considered:

1.Whenever possible, choose a brazing material whose main components are the same as those of the base material, ensuring good wettability between the two.

2.The liquidus temperature of the brazing material should be at least 40-50°C lower than the solidus temperature of the base material.

3.The melting range of the brazing material, i.e., the temperature difference between the solidus and liquidus lines, should be minimized to avoid process difficulties and melting segregation.

4.The main elements of the brazing material and the base material on the periodic table should be close to each other, reducing the risk of electrochemical corrosion and enhancing joint corrosion resistance.

5.The brazing material should exhibit high chemical stability at brazing temperatures, with low vapor pressure and low oxidizability, preventing compositional changes during the brazing process.

6.The brazing material should ideally have good formability, allowing it to be shaped into wire, rod, sheet, foil, powder, etc., as per process requirements.

Considering the working conditions, the following general principles apply:

1.For joints operating below 300°C under low loads, copper-based brazing materials are preferred. For applications requiring improved gap filling in long joints, a small amount of boron can be added to copper to form Cu-Ni-B brazing material.

2.For joints operating at 300-400°C under low loads, copper-based and silver brazing materials are suitable, with copper-based materials being the more economical choice.

3.For joints operating at 400-600°C under oxidizing, corrosive conditions and high stress, manganese-based, palladium-based, gold-based, or nickel-based brazing materials are recommended. For critical components, Au-22Ni-6Cr or Au-18Ni brazing materials are preferred due to their excellent brazing process performance and moderate brazing temperatures (980-1050°C), resulting in joints with outstanding comprehensive mechanical properties.

4.For joints operating at 600-800°C, palladium-based, nickel-based, or cobalt-based brazing materials are suitable. Ni-Cr-B-Si series brazing material with good fluidity is preferred, but it is not suitable for parts with a thickness less than 0.5mm due to its relatively high boron content.

5.For joints operating above 800°C, nickel-based and cobalt-based brazing materials can be used, but brazing materials containing phosphorus and Ni-Cr-B-Si series brazing materials are not recommended due to difficulties in meeting strength and oxidation resistance requirements.

Additionally, based on the specific usage requirements of the joints, the following selections can be made:

1.For corrosion-resistant and oxidation-resistant joints, gold-based, silver-based, cobalt-based, palladium-based, nickel-based, or titanium-based brazing materials are usually chosen.

2.Considering strength, the general order from high to low is: cobalt-based, nickel-based, palladium-based, titanium-based, gold-based, manganese-based, copper-based, silver-based, aluminum-based.

3.Considering electrical performance, gold-based, silver-based, copper-based, and aluminum-based brazing materials are usually chosen. When all requirements are met, priority is given to the cheaper copper-based brazing materials.

4.For special requirements in welded parts, brazing materials should be selected according to specific demands. For example, in the nuclear industry, brazing materials that contain boron are not allowed, as boron absorbs neutrons.

Table 5-79 shows the brazing materials suitable for various metal material combinations for reference.

Selection of Brazing Materials and Fluxes for Brazing Carbon Steel and Low Alloy Steel

Refer to Table 5-80 for the selection of brazing materials and fluxes for brazing carbon steel and low alloy steel.

Selection of Brazing Materials for Aluminum and Aluminum Alloys

The selection of brazing materials for typical aluminum and aluminum alloys, as well as the brazing characteristics for soft and hard brazing, can be found in Table 5-81 to Table 5-83.

Selection of Brazing Materials for Magnesium and Magnesium Alloys

Due to the absence of standard grade designations domestically, for the convenience of users in selecting brazing materials and fluxes, Table 5-84 lists the selection of brazing materials for several magnesium alloys in the United States, along with their chemical compositions and properties for user reference.

Selection of Brazing Materials for Zirconium and Zirconium Alloys

The physical and chemical properties of zirconium and zirconium alloys are extremely similar to titanium and titanium alloys. Zirconium and its alloys are primarily used in nuclear reactors, requiring joints with excellent corrosion resistance.

Brazing materials with a beryllium mass fraction of 4% to 5% are generally selected for zirconium alloys, as they offer excellent corrosion resistance. In special circumstances, pure Ag, BAg60CuSn, BAg92CuLi, and Al-Si brazing materials can also be used.

Tables 5-58 to 5-68 in this chapter can be referenced for the selection of brazing materials.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top