Brazing is a metal-joining process that bonds pieces together using a filler metal heated above 840°F (450°C), without melting the base materials themselves. That distinction is what separates it from welding, where the workpieces themselves are melted and fused together. Brazing sits in a middle ground: hotter than soldering, but gentler than welding, producing joints that can reach tensile strengths above 200 MPa depending on the filler metal used.
How Brazing Differs From Welding
The American Welding Society classifies brazing as a “liquid-solid phase” process. “Liquid” refers to the filler metal, which is melted. “Solid” refers to the base metals, which stay intact. In welding, both the filler and the base metals melt together into a shared pool that solidifies into one piece. This is a fundamental difference in how the bond forms, and it changes everything about joint design, heat management, and which materials you can work with.
Because brazing doesn’t melt the parent metal, it introduces far less thermal distortion. Thin-walled parts, precision assemblies, and heat-sensitive components can be joined without warping. Welding’s intense heat can also change the grain structure of the metal around the joint, creating a heat-affected zone that may be weaker or more brittle than the original material. Brazing largely avoids this problem.
How Capillary Action Creates the Bond
Brazing relies on a physical phenomenon called capillary action to pull molten filler metal into the joint. When two metal surfaces are placed very close together, the tiny imperfections and irregularities on their surfaces create microscopic channels. Molten filler metal is drawn through these channels the same way water climbs up a narrow glass tube: the liquid molecules are attracted to the solid surface (adhesion), and that attraction pulls the rest of the liquid along behind it.
This is why joint gap matters so much. If the gap between parts is too wide, the capillary force weakens and the filler can’t fully bridge the space. If the gap is too tight, there may not be enough room for the filler to flow. For most materials brazed with flux, the ideal clearance falls between 0.002 and 0.005 inches (roughly 0.05 to 0.13 mm). At these distances, capillary forces are strong enough to distribute filler evenly throughout the entire joint.
Testing has shown that joint strength is remarkably sensitive to this gap. The strongest joints, reaching 135,000 psi, occurred at a clearance of just 0.0015 inches (0.038 mm). This precision is one reason brazing is favored in manufacturing environments where parts can be machined to tight tolerances before assembly.
Capillary action can’t occur during welding because the base metal melts. There’s no solid surface for the liquid to travel along. This is the core mechanical reason brazing and welding produce such different types of joints.
Common Filler Metals
The filler metal you choose depends on the materials being joined, the operating temperature of the finished assembly, and the strength required. Most brazing fillers fall into a few broad families:
- Silver-based alloys are among the most versatile. They flow well, work across a wide range of base metals, and produce joints with good oxidation resistance at temperatures up to about 800°F (427°C). Combined with other metals, silver-based fillers cover a melting range up to roughly 1,800°F (1,000°C). Sub-categories include pure silver, silver-copper, and silver-copper-zinc blends.
- Copper-phosphorus alloys are commonly used for joining copper to copper, such as in refrigeration and plumbing. The phosphorus acts as a built-in flux on copper surfaces, so no separate flux is needed in many applications. Ideal joint clearances for these fillers range from 0.001 to 0.005 inches for shorter joints.
- Aluminum-silicon alloys are designed for joining aluminum parts. They require tighter clearances, often 0.000 to 0.002 inches for furnace brazing, because aluminum’s oxide layer and thermal properties demand precise control.
- Gold-based alloys are used in aerospace, electronics, and medical devices where corrosion resistance and reliability are critical. They’re expensive but produce extremely clean, strong joints in controlled atmospheres.
Joining Dissimilar Metals
One of brazing’s biggest advantages over welding is its ability to join metals that are difficult or impossible to weld together. Welding requires compatible melting points and metallurgy between the two base metals. If you try to weld copper to steel, for example, the vastly different melting temperatures and thermal properties create a weak, unreliable fusion. Brazing sidesteps this entirely because neither base metal needs to melt.
Silver-based fillers can join steel to copper, brass to stainless steel, and even tungsten or molybdenum to other metals. Specialized alloys exist for brazing titanium and aluminum to dissimilar metals, though galvanic corrosion between unlike metals needs to be considered in the design. Nickel-based fillers handle high-performance combinations like stainless steel to superalloys, or carbide tips to steel tool bodies, which is how many cutting tools are manufactured.
How Strong Are Brazed Joints?
A well-designed brazed joint can be surprisingly strong. In tensile testing of steel-to-stainless-steel joints, silver filler metal produced average strengths around 160 MPa, with peak values reaching 246 MPa. Copper-nickel fillers joining titanium alloys to steel have achieved up to 260 MPa when brazed at the right temperature. These numbers depend heavily on joint design, clearance, and filler selection, but they show that brazing produces structurally meaningful bonds, not just seals.
Filler choice makes a dramatic difference. In the same study comparing silver and brass fillers on identical joints, silver filler averaged 160 MPa while brass filler managed only about 11 MPa. This is why filler selection isn’t an afterthought: it’s one of the most important decisions in the process.
Brazed joints also distribute stress differently than welds. Because the filler flows across the entire mating surface, the load is spread over a large area rather than concentrated along a bead. Lap joints, where one piece overlaps another, take particular advantage of this by maximizing the bonded surface area.
The Brazing Process Step by Step
Brazing follows a consistent sequence regardless of the heat source. First, the joint surfaces are cleaned to remove oils, oxides, and contaminants that would prevent the filler from wetting the metal. Then the parts are fitted together with the correct clearance, often held in place with fixtures, clamps, or gravity.
A flux is typically applied to the joint area. Flux is a chemical compound that dissolves metal oxides as the joint heats up, keeping the surfaces clean so the molten filler can adhere. Some brazing environments, like vacuum furnaces or controlled-atmosphere ovens, eliminate the need for flux by removing oxygen from the equation entirely.
Heat is then applied. This can come from a torch (the most common method for repair and small-batch work), an induction coil, a furnace, or even a dip in molten salt. The goal is to bring the entire joint area to a uniform temperature above the filler’s melting point but well below the base metal’s melting point. Once the filler melts, capillary action pulls it into the joint. After cooling, any remaining flux residue is cleaned off.
Safety Considerations
Brazing produces fumes that can be hazardous, particularly when certain metals are involved. OSHA requires mechanical or local exhaust ventilation when brazing with zinc-bearing or cadmium-bearing filler metals in enclosed spaces. Cadmium fumes are especially dangerous and require local exhaust ventilation or supplied-air respirators.
Even with common silver-based fillers, adequate ventilation matters. Flux fumes can irritate the respiratory system, and prolonged exposure to metal fumes of any kind carries health risks. Brazing at lower temperatures than welding doesn’t mean lower risk from fume exposure, since many brazing fillers contain metals that produce toxic vapors at relatively modest temperatures.
Where Brazing Is Used
Brazing is widespread in HVAC and refrigeration, where copper tubing joints need to be leak-tight and withstand vibration. The automotive and aerospace industries use it for heat exchangers, turbine components, and any assembly where tight tolerances and dissimilar metals are involved. Carbide cutting tools are almost universally brazed: a hard carbide tip is joined to a tougher steel body, combining wear resistance with structural strength in a way neither material could achieve alone.
Electronics and medical devices rely on brazing for clean, precise joints that won’t introduce contamination. Jewelry making uses brazing (often called “silver soldering” in that trade) to join precious metals without destroying fine details. In plumbing, the joints in copper water lines are typically brazed or soldered depending on the pressure and temperature requirements of the system.