Brazing is a metal joining process where a filler metal is heated above melting point and distributed between two or more close-fitting parts by capillary action. The filler metal flows into the gap between the parts and forms a strong joint. Braze alloys, sometimes called filler metals, are used to join metal components. The brazing process uses capillary action to distribute the molten braze filler metal evenly between two or more metal pieces. When the braze metal cools and solidifies it forms a strong metallic bond between the metal components. Brazing differs from soldering in that it involves higher temperatures and the use of a filler metal with a melting point above 450°C (840°F), while soldering uses filler metals with lower melting points below 450°C (840°F). Brazing is often used to join metals like steel, copper, brass, and aluminum. It has applications in automotive, construction, electronics, machinery, and household appliances.
Advantages of Brazing
Brazing has several advantages that make it useful for many industrial and manufacturing applications:
Joins Dissimilar Metals
One key advantage of brazing is its ability to join dissimilar metals. Many braze alloys and filler metals are compatible with different base metals like steel, copper, aluminum, and more. This allows brazing to join dissimilar metal components like copper and steel. The different metals don’t actually fuse or mix together. Rather, the molten braze filler metal flows between the gaps and adheres to the metal surfaces through metallurgical bonding. This gives brazing an edge over other joining processes that may struggle with dissimilar metals.
Does Not Melt the Base Metals
In brazing, the base metals do not reach their melting points. Only the braze filler metal melts and flows into the joint. This localized heating puts less stress on the base metals. And because the base metals do not melt, brazed joints retain more of the base metal’s original strength characteristics. This allows for strong joints while minimizing distortion of the original parts. It also makes brazing suitable for delicate parts and heat-sensitive materials.
Excellent Gap Filling Ability
Brazing can bridge gaps between parts and completely fill joints up to 0.03 – 0.05 inches (0.75 – 1.25 mm). The molten filler metal spreads deeply into joints by capillary action. This gives brazed joints inherent strength even with imperfect fit-up between parts. It also reduces the need for expensive precision machining of joint surfaces.
Provides Stress Relief
The high heat of the brazing process relieves residual stresses in metal parts. This reduces the risk of distortion or cracking during subsequent machining, forming, or service life. Brazing’s stress relief ability makes it well-suited for heat treatment or parts subject to thermal fatigue.
Can Join Complex Geometries
The free-flowing nature of molten braze filler allows it to form bonds even in complex joint geometries. Brazing can access hard-to-reach internal areas and create continuous joints between complex assemblies or unusual designs. This gives designers more flexibility compared to welding or mechanical fasteners.
Does Not Require Force
Brazing does not use mechanical force or compression at the joint interface. The molten filler metal penetrates joint gaps through capillary attraction only. This reduces risks of part movement, distortion, or damage during the brazing process. It also eliminates the need for very precise fit-up or fixturing of assemblies.
Disadvantages of Brazing
While brazing has many advantages, it also comes with some limitations to consider:
High Cost of Equipment and Materials
Brazing can require significant upfront investments in equipment like furnaces, gases, and automation. The filler metals and fluxes used are also costly compared to other joining methods. This makes brazing best suited for higher volume production where the costs can be distributed over many parts. It may be excessive for small batches or prototypes.
Possible Distortion or Damage from Heat
Although less than welding, brazing does involve concentrated heat input to the joint area. Sensitive parts or materials may still be at risk for overheating, distortion, metallurgical damage, or corrosion if proper techniques are not followed. Precautions are necessary with delicate or thin-walled assemblies.
Difficulty With Dissimilar Metal Corrosion
Joining dissimilar metals carries a risk of galvanic corrosion over time. The differences in electrochemical potential between metals sets up local corrosion cells at the joint. Careful braze filler selection, joint design, and protective coatings are required to combat dissimilar metal corrosion.
Limited Strength
Brazed joints have lower strength than welded or bolted joints in certain situations. The filler metals have lower strength than most base metals. And only a thin layer of filler is present at the joint interface. Brazing may not work for highly stressed joints or dynamic loads.
Braze Filler Metal Residue
Excess braze filler metal left on part surfaces requires removal through grinding, sanding, or other methods. Complete cleaning can be challenging with complex geometries. Remaining braze residue can negatively impact joint appearance or function.
High Operator Skill Required
Achieving consistent, high-quality brazed joints requires training and experience. Operators need proper technique when positioning parts, applying heat, adding filler, fluxing, and cleaning. Automation and strict process controls can overcome variability in manual brazing.
Brazing Equipment
Several important pieces of equipment are used in brazing processes:
Furnaces
Furnaces heat parts to brazing temperature and maintain proper thermal conditions. Common types include batch furnaces, continuous furnaces, vacuum furnaces, and induction brazing systems. Furnace design is based on production volume, metal types, and joint complexity.
Brazing Torches
Small handheld torches provide localized heating for manual brazing. Mapp gas, propane, and oxy-fuel torches are common. Automated torches on controlled robots or machines give more precision and consistency.
Fillers and Fluxes
Braze fillers are metal alloys with melting points above 450°C (840°F). Fluxes remove oxides and promote filler flow. Correct fillers and fluxes are required to wet and join each metal type. Preforms, pastes, powders, and rods provide flexibility.
Fixtures and Tooling
Fixtures properly locate and align parts for joint access. Tooling like resistance heated platens and induction coils speed heating in automated brazing. Quenching systems cool brazed parts. Containment tools capture excess fillers.
Metrology Equipment
Temperature monitoring and recording ensures proper thermal processing. Joint inspection verifies filler penetration and surface condition. Mechanical testing equipment checks ultimate strength.
Common Braze Filler Metals
Some frequently used braze filler metals include:
| Filler Metal | Key Characteristics |
|---|---|
| Silver Alloys (Ag) | – Join copper, brass, steel – Excellent wetting and flow – Ductile filler |
| Gold Alloys (Au) | – Join copper, nickel, steel – Resists corrosion – Biocompatible |
| Copper Alloys (Cu) | – Join copper, steel, nickel – Economical filler – High strength |
| Aluminum Alloys (Al) | – Join aluminum, magnesium – Withstand vibration – Anodic to most metals |
| Nickel Alloys (Ni) | – Join stainless steel, nickel – Withstand high temps – Resists corrosion |
The percentages of key alloying elements in these filler metals provide optimal melting ranges, wetting, and metallurgical compatibility with different base metals. Selection depends on joint design, service environment, temperature, electrical/thermal requirements, and cost.
Common Brazing Processes
Some of the most common modern brazing techniques include:
Torch Brazing
Manual brazing using handheld torches. Operator heats parts locally while adding braze metal and flux. Commonly used on small repairs and custom fabrication.
Furnace Brazing
Parts and filler metal placed in batch or continuous furnaces with protective atmospheres. Automated processes used in high-volume manufacturing.
Induction Brazing
High-frequency alternating current generates heat in part edges through electromagnetic induction. Localized heating focuses energy and minimizes distortion.
Resistance Brazing
Electric current is passed through resistive heating elements or the parts themselves to generate heat. Allows precise control and automation for mass production.
Vacuum Brazing
Brazing in low-pressure vacuum environments. Protects against oxidation while removing trapped gases or contaminants from joint.
Dip Brazing
Parts are dipped into molten braze bath allowing capillary action to draw metal into joint. Excellent gap filling for complex geometries.
Aluminum Brazing
Specialized aluminum brazing alloys and fluxes join aluminum parts exposed to nitrogen gas atmospheres. Forms corrosion resistant aluminum joints.
Common Brazed Components
Brazing produces strong, sealed joints suitable for many components, including:
Heat Exchangers
Compact plate-fin and tube-and-fin heat exchangers used for HVAC, refrigeration, aerospace, and automotive applications. Brazing seals leaky joints.
Electrical Contacts
Silver/copper alloy brazes permanently join copper or brass electrical connectors and contacts. Ensures conduction through vibration or thermal cycles.
Metal Matrix Composites
Reinforcing fibers or particles are brazed into lightweight metal matrices to create high-strength composites for aviation and defense.
Magnet Wire
Fine copper wire coated with insulation and brazed into coils used in electric motors and transformers. Brazing gives reliable connections.
Carbide Tools
Tungsten carbide cutting tips brazed onto steel tool bodies for strength, shock resistance, and tool life in machining applications.
Medical Implants
Complex titanium, stainless steel, and cobalt chrome implants are vacuum brazed for strength and biocompatibility inside the human body.
Brazing Safety
Brazing requires adherence to important safety practices:
Respiratory Protection
Fumes from molten metals, flux, and organic binders require extraction or respirators to avoid breathing hazards.
Protective Clothing
Gloves, aprons, and other PPE protect operators from hot metals and UV radiation.
Eye/Face Protection
Shielded masks safeguard against flying bits of molten filler, flux spatter, and ultraviolet/infrared rays.
Fire Prevention
Sparks and open flame demand special preventive measures. Quenching brazed parts in water avoids potential fires.
Fume Extraction
Built-in ventilation and fume extraction removes toxic gases and smoke released from the heating process.
Safe Material Handling
Proper procedures for filler metals prevent contact with flux residues. Mechanical aids lift heavy assemblies.
Following documented protocols and safety training for each process ensures brazing does not put workers or facilities at risk.
Brazing Codes and Standards
Key organizations providing codes, specifications, training, and certifications for brazing include:
– American Welding Society (AWS) – Develops brazing codes and training/certification programs. Administers brazing competency tests.
– ASME Boiler & Pressure Vessel Code – Provides rules for brazed joints in pressurized tanks, piping, and heat exchangers.
– SAE International – Issues aerospace and automobile manufacturing standards covering brazing procedures, acceptance criteria, inspection, and more.
– American Bureau of Shipping – Marine standards and rules referencing proper brazing techniques, materials, qualifications, etc. for ship components.
– International Organization for Standardization – ISO quality standards with requirements for brazing procedures, welders, materials, and quality systems management.
Using qualified personnel and following the proper codes ensures brazed components meet the highest quality and safety levels for their service application.
Conclusion
Brazing provides a versatile metal joining process capable of producing strong, sealed joints across a range of materials and geometries. Heating a filler metal to flow across close-fitting parts allows brazing to bridge gaps, damp vibrations, and create leak-tight seals in challenging assemblies. While brazing requires upfront investments in training and equipment, the benefits of joining dissimilar metals, minimizing distortion, and automating complex assemblies make it an essential industrial manufacturing technique. Following safety protocols and industry codes allows companies to implement high-quality brazing tailored to their specific component needs.
