Introduction to Brazing Knowledge

I. What is Brazing?

• Welding: Welding refers to a joining method that connects two separate solid objects into one integral piece by inducing atomic (molecular) bonding force through appropriate physical and chemical processes.
• Brazing: Brazing belongs to solid-phase joining. Unlike fusion welding methods, during brazing the base metal does not melt. A filler metal with a melting temperature lower than that of the base metal is used. The heating temperature is set below the solidus line of the base metal but above the liquidus line of the filler metal. The filler metal melts while the workpieces do not. This process utilizes the molten filler metal to wet the base metal, fill the joint gap, and mutually diffuse with the base metal, thereby firmly joining the workpieces together.

Based on the melting point of the filler metal, brazing is categorized into soft brazing and hard brazing:

(1) Soft Brazing: The filler metal melts below 450°C, resulting in joint strength typically less than 70 MPa. It is commonly used for welding conductive, airtight, and watertight components in the electronics and food industries. Tin-lead alloy soldering is the most prevalent type.
(2) Hard Brazing: The filler metal melts above 450°C, producing joint strength generally greater than 200 MPa. The joints exhibit high strength, and some can operate at high temperatures. Hard brazing employs a wide variety of filler metals, with aluminum-, silver-, copper-, and nickel-based alloys being the most commonly used.

II. Characteristics of Brazing

1. Compared to fusion welding methods, brazing offers the following advantages:

(1) The brazing heating temperature is relatively low, resulting in minimal impact on the microstructure and properties of the base metal.

(2) Brazed joints are smooth and even, offering an aesthetically pleasing appearance.

(3) Welding distortion is relatively small. Particularly with brazing methods employing uniform heating (e.g., furnace brazing), workpiece distortion can be minimized, making it easier to ensure dimensional accuracy.

(4) Certain brazing methods can produce dozens or even hundreds of joints simultaneously, leading to high productivity.

(5) It enables the joining of dissimilar metals or alloys, and metals to non-metals.

1. However, brazing also has its own limitations:

Brazed joints generally have lower strength and poorer heat resistance compared to fusion welds. The significant difference in composition between the base metal and the filler metal can lead to galvanic corrosion, resulting in relatively poor corrosion resistance. Additionally, brazing often requires higher assembly precision.

III. Applications of Brazing

Brazing is not suitable for welding general steel structures or heavily loaded, dynamically loaded components.

It is primarily used in manufacturing precision instruments, electrical components, dissimilar metal structures, and complex sheet metal structures such as sandwich panels and honeycomb cores. It is also commonly employed for brazing various types of wires and carbide cutting tools.

Brazing finds extensive applications in machinery, electric motors, instruments, radio equipment, thermal management/heat sinks, new energy sectors, etc. Examples include carbide tools, drill bits, bicycle frames, heat exchangers, piping, and various containers. In the manufacturing of microwave waveguides, electron tubes, and electronic vacuum devices, brazing is sometimes the only feasible joining method.

IV. Common Materials Used in Brazing

1. Filler Metal: Refers to the material used to form the joint during brazing.

(1) The filler metal should have an appropriate melting temperature range, typically at least several tens of degrees Celsius lower than the melting temperature of the base metal.

(2) At the brazing temperature, it should exhibit good wettability to ensure complete filling of the joint gap.

(3) The filler metal and base metal should interact through diffusion to form a strong bond.

(4) The filler metal should have a stable and uniform composition, minimizing the loss of alloying elements during the brazing process.

(5) The resulting brazed joint should meet the product's technical requirements, satisfying demands for mechanical properties, physical-chemical properties, and service performance.

(6) The filler metal should be economical. It should contain minimal or no rare or precious metals. It should also meet productivity requirements for the brazing operation.

(7) The filler metal should possess good workability/formability to facilitate fabrication into various shapes.

1. Flux: Refers to the solvent used during brazing. The function of the flux is to remove oxides from the surfaces of the base metal and filler metal, protecting them. It reduces oxidation during heating, improves the wettability of the filler metal on the base metal, and effectively dissolves or disrupts the oxide films on the workpiece and filler metal surfaces.

2. Temperature: The main process parameters in brazing are the brazing temperature and holding time. The brazing temperature is typically selected to be 25-60°C above the liquidus temperature of the filler metal to ensure the filler metal can fully fill the joint gap.