Aluminum Brazing Explained: Advantages, Disadvantages, Quality and Performance

What is Aluminum Brazing?

Metallurgical mount at 30x of aluminum braze showing voids

Aluminum brazing is a specialized process that uses filler metal to bond two parts without melting the substrate.  It is unique in its ability to create a strong, reliable joint without compromising the integrity of the aluminum. This is done by using a filler metal such as copper or silver that has a higher melting point than the aluminum being brazed.  A metallurgical bond is formed between the filler metal and the aluminum through a combination of wetting and capillary action.

Key Differences Between Brazing and Welding

While both brazing and welding create bonds between metal pieces, the mechanisms behind the bond is entirely different.  Welding fuses the base metals directly, often with the addition of filler material. Aluminum welding typically requires temperatures exceeding 1,200 °F, and produces bonds as strong as the base metal itself, ideal for high-stress applications.

Brazing on the other hand, joins metals by melting a filler into the joint without melting the base metals. Aluminum brazing typically occurs between 600 °F to 800 °F. Capillary action draws the molten filler into the joint, forming a strong, uniform bond as it cools. This method is especially valuable for joining delicate or complex assemblies where high heat could warp or damage components.

Flux plays a critical role in brazing.  It is a chemical substance applied to  aluminum pieces before heating.  Its main functions are to prevent oxidation during brazing, clean the metal surfaces by breaking down contaminates such as oil or grease, and help the molten filler flow evenly into the join, ensuring a strong uniform bond.

Braze welding is a joining method that combines elements of both welding and brazing.  In braze welding, the filler metal is deposited onto the heated surface of the base metals, similar to welding.  The resulting bond, formed by a combination of adhesion and metallurgical interaction, is stronger than joints achieved by traditional brazing.  Braze welding is particularly useful for repairs where full fusion welding is impractical.

Feature	Welding	Brazing	Braze Welding
Temperature Range	1,200°F+	600 – 800°F	1,000 – 1,200°F (varies with filler)
Heat Impact	Base metals melt	Base metals do not melt	Base metals do not melt, surface is heated
Bond Strength	As strong as base metal	Strong, but less than welding	Stronger than brazing, but less than full fusion weld

Advantages of Aluminum Brazing

• Low thermal impact: Base metals do not melt, and are minimally heated, making this method ideal for delicate, or heat sensitive components.

• Reduced residual stress: Lower heat input from the brazing process produces less distortion and residual stress in the base metal compared to welded joints 

• Joins dissimilar metals: Because base metals do not melt, brazing can reliably join dissimilar base metals, expanding design flexibility.  On the other hand, because the melting points of the base metals are critical, welding is best suited to similar metals.

• Aesthetic: Brazed joints have a tidy appearance, require no post processing or cleaning.

• Simple and cost-effective equipment: Unlike welding, which requires expensive machines and high electrical power, brazing aluminum can be performed with a handheld torch.  This makes brazing highly portable, and economical compared to welding.

Disadvantages of Aluminum Brazing

• Limited strength for high-stress applications: While brazed joints are strong, they may not be suitable for applications that experience high mechanical loads or fatigue. Alternative methods such as fastening or welds provide higher strength.

• Requires skilled technicians: Achieving reliable and consistent brazed joints depends on proper technique, temperature control, and flux/filler material selection.

• Joint design constraints:  Joints must have tight gaps and specific geometry to facilitate capillary action and good filler flow.

• Service temperature limitation: The strength of a brazed joint is limited by the melting point of the filler metal.  Joints can fail if exposed to temperatures near or above the filler’s melting range during service.

• Potential for galvanic corrosion:  When joining dissimilar metals, galvanic corrosion can occur if not properly protected. 

Corrosion and Quality Assessment of Aluminum Brazed Components

Evaporators in cyclic corrosion chamber, pressurized with nitrogen gas and setup for continuous pressure monitoring during exposure to detect leaks

While aluminum brazing can create strong and reliable joints, the long-term performance of depends on proper material selection and skilled workmanship.  Brazed joints are also vulnerable to environmental stressors such as moisture, heat and galvanic corrosion.  Laboratory corrosion testing is essential for manufacturers, helping to optimize material choices and process design to ensure quality of brazed components over time.

To assess the durability of brazed joints, a variety of corrosion and inspection methods are commonly used. Continuous salt spray (fog) and cyclic corrosion testing using standard methods such as  ASTM B117, ISO 9227 ASTM G85, and SAE J2334 simulates long-term exposure to moisture, temperature changes, and contaminated environments.  After exposure, inspection techniques such as leak detection or examination of braze cross section mounts under optical microscopy are used to assess braze quality.  Optical microscopy can reveal voids, incomplete wetting, or other microstructural issues that can contribute to joint degradation over time. Components are commonly inspected for leaks by pressurization with helium, nitrogen, or air.  Leaks can indicate areas where the joint may be compromised.  Microscopic analysis of cross sections can be used to examine the microstructure, evaluate porosity, and identify any effects from temperature exposure. 

Cross section mount at 100x showing silicon diffusion, an effect of elevated temperature during brazing

Elevated temperature during brazing can cause various microstructural issues.  These include grain growth in the base metal, excessive filler penetration, or formation of brittle intermetallic compounds.  This can weaken the braze  and act as crack initiation sites.  High temperatures can cause excessive flow or cause vaporization of volatile components in the flux, which can lead to porosity or voids within the braze.