Flux Coated Brazing Alloys

A1: Flux coated brazing alloy is a brazing material with a brazing filler metal (such as silver-based, copper-based alloys) as the core and a flux coating on the surface. The flux plays a role in removing oxides and preventing re-oxidation during brazing, and it is widely used in brazing scenarios with high joint quality requirements.

A2: In terms of composition, flux coated brazing alloys have a surface flux coating, while ordinary brazing filler metals are single brazing materials without flux; in terms of operation, flux coated brazing alloys can be used directly without additional flux, while ordinary brazing filler metals require separate flux application; in terms of effect, flux coated brazing alloys ensure uniform flux distribution, reducing the risk of insufficient or excessive flux, and the brazing quality is more stable.

A3: In high-temperature environments, select nickel-based flux coated brazing alloys with high-temperature resistance; in humid or corrosive environments (such as coastal areas), prioritize copper-based or silver-based alloys with good corrosion resistance; for electrical connection scenarios, choose alloys with stable electrical conductivity, such as silver-based flux coated brazing alloys.

A4: Common methods include torch brazing, furnace brazing, induction brazing, and resistance brazing. Torch brazing is suitable for small workpieces and on-site operations; furnace brazing is applicable to batch brazing of complex components; induction brazing is suitable for local heating of parts with high precision requirements.

A5: Thoroughly remove oil, rust, dust, and oxide films on the surface of the base metal. Mechanical methods (such as sandblasting, grinding) or chemical methods (such as pickling) can be used; for parts with high precision, alcohol or acetone can be used for degreasing to avoid affecting the wettability of the brazing filler metal.

A6: Common defects include incomplete wetting (brazing filler metal does not spread evenly), porosity (small holes in the joint), slag inclusions (residual flux or oxides), and cracks (caused by excessive stress). Incomplete wetting is mainly due to insufficient base metal cleaning or inappropriate brazing temperature; porosity is often caused by gas entrapment during brazing.

A7: Ensure the base metal surface is thoroughly cleaned to remove all oxides and contaminants; control the brazing temperature within the recommended range (too low reduces fluidity, too high causes flux failure); design a reasonable brazing gap (generally 0.05-0.2mm) to facilitate the spread of the brazing filler metal.

A8: The brazing temperature should be 30-50℃ higher than the melting point of the brazing filler metal core but lower than the melting point of the base metal. For example, BAg-3 (melting point 600-650℃) is usually brazed at 630-700℃; BNi-1 (melting point 1050-1100℃) is brazed at 1080-1150℃.

A9: They should be stored in a dry and ventilated environment with a temperature of 15-30℃ and relative humidity not exceeding 60%; avoid contact with water, corrosive gases, and direct sunlight; keep the packaging sealed, and return unused alloys to a moisture-proof container in time.

A10: In most cases, yes. Residual flux on the joint surface may cause corrosion, so it should be cleaned with hot water, ultrasonic cleaning, or special cleaning agents; for high-precision parts, polishing can be performed to remove burrs; for high-temperature service parts, stress relief annealing may be required.

A11: In principle, no. Different types have different melting points, flux compositions, and performance. Mixing may cause uneven melting, flux failure, and unqualified joints. Special cases require brazing process assessment to verify feasibility.

A12: The gap should be determined according to the alloy type and base metal. For high-fluidity alloys (such as silver-based), a small gap (0.05-0.2mm) is suitable; for copper-based alloys with lower fluidity, the gap can be 0.1-0.3mm; use fixtures to fix the workpiece during brazing to prevent gap changes due to thermal deformation.

A13: Possible reasons include excessive storage humidity causing flux to absorb moisture and deteriorate; excessively high brazing temperature leading to flux volatilization or decomposition; or expired alloys, as flux loses activity over time.

A14: Tensile and shear tests detect mechanical strength; airtightness tests (such as pressure tests, helium leak detection) check sealing performance; metallographic analysis observes internal structures (e.g., slag inclusions, porosity); for electrical brazing, conductivity tests are required.

A15: Consider the melting points of both base metals to ensure the brazing temperature is lower than both; select alloys whose brazing filler metal can wet both base metals; ensure flux compatibility with both to avoid harmful reactions. For example, brazing copper and stainless steel uses silver-based flux coated brazing alloys.

A16: Slightly damp alloys can be used after drying at 80-100℃ for 1-2 hours, but flux activity may decrease; severely damp alloys (with caked or discolored flux) may cause pores or cracks and are recommended to be scrapped.

A17: Use low-temperature brazing alloys to reduce thermal stress; adopt symmetrical brazing sequences for even heating; fix workpieces with fixtures during brazing; for thin-walled parts, use induction brazing with fast heating/cooling.

A18: First, remove slag inclusions by mechanical methods (e.g., grinding); re-clean the brazing area; select new alloys and re-braze with appropriate temperature and heating speed; perform performance tests after re-brazing.

A19: Too long a brazing time may cause excessive flux volatilization, reduced activity, and increased oxidation risk; too short a time may result in insufficient wetting and incomplete spreading of the brazing filler metal. The time should match the temperature and be controlled according to the alloy specification.

A20: Select alloys with good fluidity to ensure full filling of the brazing gap; control brazing temperature and time to avoid pores; thoroughly clean the base metal to prevent contaminants affecting sealing; perform post-brazing airtightness tests and rework if necessary.

A21: Preheat workpieces and alloys to remove moisture; strengthen ventilation to reduce environmental humidity; use moisture-resistant flux coated alloys; shorten alloy exposure time in air to avoid moisture absorption.

A22: Flux coated brazing alloys have surface flux, suitable for small-diameter and precision brazing; flux-cored wires have internal flux, with higher flux content, suitable for large-area brazing. The former is convenient for manual operation; the latter is often used in automatic equipment.

A23: Check and adjust the heating method to ensure uniform heating of the brazing area; adjust the brazing angle to make the alloy flow evenly; replace worn fixtures that cause workpiece position deviation; select alloys with appropriate fluidity according to the joint shape.

A24: Excessively rough surfaces may trap contaminants, affecting wetting; too smooth a surface may reduce the brazing filler metal's adhesion. The roughness should be moderate (generally Ra 1.6-6.3μm) to balance cleaning and adhesion.

A25: Prioritize alloys with high mechanical strength (such as nickel-based or copper-phosphorus alloys); select those with good fluidity to ensure full gap filling; ensure compatibility with the pipeline material to resist medium corrosion; test joint pressure resistance after brazing.

A26: It may be due to too large a brazing gap, causing uncontrolled flow; too high a temperature increasing fluidity; or excessive heating in local areas, making the alloy flow to low-temperature zones.

A27: Conduct salt spray tests to simulate atmospheric corrosion; perform immersion tests in corrosive media (e.g., acids, alkalis) to observe corrosion conditions; analyze surface corrosion morphology through metallography; compare with standard corrosion rates for evaluation.

A28: Too thick a coating may cause excessive residual flux and slag inclusions; too thin a coating may result in insufficient oxide removal and poor wetting. The thickness should be uniform and meet the alloy's factory specifications.

A29: Choose alloys with high electrical conductivity (such as silver-based); ensure low contact resistance of joints; select low-temperature brazing alloys to avoid damaging component insulation; ensure joints have sufficient mechanical strength.

A30: It may be due to over-high brazing temperature causing grain coarsening; excessive diffusion between the brazing filler metal and base metal; or flux residue reacting with the base metal. Post-brazing annealing can reduce brittleness.

A31: The rate should be moderate: too fast may cause uneven temperature distribution and local overheating; too slow may lead to flux premature volatilization. Adjust according to the workpiece thickness—faster for thin workpieces, slower for thick ones.

A32: The environment should be free of dust, oil mist, and corrosive gases to avoid contamination of the brazing area; workbenches and fixtures should be kept clean; operators should wear clean gloves to prevent hand oil from soiling workpieces.

A33: Check if the alloy type matches the base metal; re-braze with appropriate temperature/time to ensure sufficient wetting; if caused by pores or slag inclusions, remove defects and re-braze; perform strength tests after rework.

A34: Long-term storage may cause flux to absorb moisture or lose activity, reducing oxide removal ability; the brazing filler metal may oxidize slightly, affecting fluidity. Alloys should be used within the shelf life, and stored in sealed packaging if unused for a long time.

A35: Select alloys with low melting points to reduce thermal impact on components; choose those with good fluidity for precise gap filling; prioritize alloys with uniform flux coating to avoid residue affecting precision; match the alloy color with the component for appearance if needed.