Brazing is a metal joining process that uses a filler metal (braze alloy) with a melting point lower than that of the base metals but higher than 450℃. The process heats the assembly to a temperature where the braze alloy melts completely while the base metals remain in a solid state. Driven by capillary action, the molten braze alloy flows into the narrow gap between the faying surfaces of the base metals, then undergoes wetting and spreading to form a metallurgical bond with the base metals after cooling and solidification.

 

    - Surface Cleaning: Oxide films, grease, and contaminants on the base metal surfaces severely impair the wetting ability of molten braze alloy and capillary penetration. Mechanical cleaning methods include grinding, sandblasting, and wire brushing to remove surface oxides and roughness; chemical cleaning uses acidic or alkaline solutions to dissolve oxides and degrease, followed by thorough rinsing and drying to avoid residual corrosives.

    - Assembly and Gap Control: Joint clearance is a critical factor affecting brazing quality, typically controlled within the range of 0.02–0.2 mm. Too narrow a gap restricts capillary flow of the braze alloy, while too wide a gap fails to maintain sufficient capillary force, leading to insufficient filling and reduced joint strength. Fixtures are often used to fix the assembly position and ensure gap uniformity during heating.

    - Braze Alloy and Flux Placement: Braze alloy can be pre-placed at the joint interface in forms of foil, wire, powder, or paste. Flux is used to remove oxide films on the base metal and braze alloy surfaces during heating, improve wetting performance, and prevent re-oxidation. It is applied by brushing, spraying, or mixing with the braze alloy, with the dosage adjusted according to joint size and base metal type.

 

    - Torch Brazing: Uses an oxy-fuel flame as the heat source, featuring simple equipment, high flexibility, and suitability for on-site repair or small-batch production of components such as copper tube fittings and small steel assemblies. Precise temperature control is required to avoid overheating local areas.

    - Furnace Brazing: Places the assembled workpiece in a sealed furnace for uniform heating, enabling simultaneous brazing of multiple joints. It is ideal for mass production of complex components like heat exchanger cores and aerospace precision parts. Atmosphere-controlled furnace brazing (using inert gas or reducing atmosphere) or vacuum furnace brazing can eliminate the need for flux and prevent oxidation.

    - Induction Brazing: Relies on high-frequency alternating current to generate eddy currents in the base metal, realizing rapid and localized heating. The process offers short cycle times, minimal thermal deformation, and is widely used for brazing high-volume components such as automotive air conditioner tubes and electrical connector pins.

    - Vacuum Brazing: Conducts the brazing process in a high-vacuum environment (10⁻³–10⁻⁵ Pa), eliminating the need for flux and avoiding oxidation, pollution, and residual flux corrosion. It is the preferred method for joining high-performance materials like stainless steel, titanium alloys, nickel-based superalloys, and metal-ceramic composites in aerospace and nuclear engineering fields.

 

    - Brazing Temperature: Must be set 20–50℃ higher than the melting point of the braze alloy to ensure full melting and fluidity of the braze alloy, while strictly below the solidus temperature of the base metals to prevent grain coarsening or melting of the base material.

    - Holding Time: Determines the extent of metallurgical reaction between the braze alloy and base metals. Excessively long holding time causes over-erosion of the base metal and grain growth, reducing joint performance; insufficient holding time leads to incomplete wetting and poor bonding. Typical holding time ranges from 5 to 60 minutes, depending on the workpiece size and brazing method.

    - Atmosphere Control: For oxidation-sensitive materials (e.g., aluminum, titanium alloys), vacuum or inert gas protection is mandatory during heating to avoid surface oxidation and ensure braze alloy wetting.

 

    - Residue Removal: For brazing processes using corrosive flux, post-treatment steps such as water washing, alkaline washing, or pickling are required to remove residual flux, preventing long-term corrosion of the joint.

    - Stress Relief Annealing: Heating the brazed workpiece to 200–300℃ and holding for a certain period can eliminate residual stresses in the joint area, improving the fatigue resistance of the brazed component.

    - Quality Inspection: Non-destructive testing methods including X-ray inspection, ultrasonic testing, and dye penetrant testing are used to detect internal defects such as unfilled gaps, porosity, and cracks, ensuring joint reliability.

 

    - Enables joining of dissimilar materials (e.g., metal-ceramic, copper-steel, aluminum-stainless steel) that are difficult to weld by fusion welding methods.

    - Causes minimal thermal deformation of the workpiece, maintaining the precision of complex components.

    - Allows simultaneous brazing of hundreds of joints in a single heating cycle, significantly improving production efficiency.

    - Produces smooth and uniform joints without the need for subsequent machining in most cases.