Brazing filler metals divide into four main families: silver (BAg), copper-phosphorus (BCuP), brass (RBCuZn), and nickel (BNi). Each family has multiple alloys optimized for specific base metals, temperatures, and applications. Silver alloys give the strongest, most versatile joints. Copper-phosphorus alloys are self-fluxing on copper. Brass is cheap and adequate for general steel work. Nickel handles high-temperature and corrosion-resistant applications.
Choosing the right filler metal depends on three factors: what base metals you’re joining, what temperature the joint will see in service, and how much you can spend on filler. The AWS classification system tells you exactly what each alloy does.
How to Read AWS Filler Metal Designations
The AWS A5.8 standard classifies brazing filler metals. The designation encodes the alloy family:
- BAg = Silver brazing alloy (B = brazing, Ag = silver)
- BCuP = Copper-phosphorus brazing alloy
- RBCuZn = Brass brazing rod (R = rod, B = brazing, CuZn = copper-zinc)
- BNi = Nickel brazing alloy
- BAlSi = Aluminum-silicon brazing alloy
- BAu = Gold brazing alloy
The number following the family designation identifies the specific alloy composition. Higher numbers don’t mean better; they’re simply catalog identifiers.
Silver Brazing Alloys (BAg Series)
Silver brazing alloys are the most versatile brazing filler metals. They wet and flow on virtually all metals except aluminum and magnesium, produce strong joints with good ductility, and operate at relatively low temperatures.
Common BAg Alloys
| AWS Class | Silver % | Solidus/Liquidus (F) | Best For | Notes |
|---|---|---|---|---|
| BAg-1 | 45% | 1,145/1,145 | General purpose, narrow clearance | Eutectic (melts/flows at same temp). Excellent capillary flow. Contains cadmium. |
| BAg-1a | 50% | 1,160/1,175 | Similar to BAg-1, cadmium-free | Narrow melting range. Good flow. Safer than BAg-1. |
| BAg-2 | 35% | 1,125/1,295 | Wider gaps, general purpose | Wide melting range helps fill larger clearances. Contains cadmium. |
| BAg-5 | 45% | 1,225/1,370 | Carbide brazing, general | Cadmium-free. Higher flow temp than BAg-1. Good for carbide tool tips. |
| BAg-7 | 56% | 1,145/1,205 | Stainless steel, food grade | Cadmium-free. Tin content helps wet stainless. Good corrosion resistance. |
| BAg-24 | 50% | 1,220/1,305 | General purpose, cadmium-free | Economical cadmium-free option. Wide melting range. |
| BAg-34 | 38% | 1,200/1,310 | HVAC, general | Good balance of cost and performance. Cadmium-free. |
Cadmium Warning
Some older BAg alloys (BAg-1, BAg-2, BAg-3) contain cadmium. Cadmium fume is extremely toxic and can cause fatal lung damage from a single heavy exposure. Modern practice favors cadmium-free alloys (BAg-1a, BAg-5, BAg-7, BAg-24, BAg-34). If you must use cadmium-bearing alloys, a supplied-air respirator is mandatory, not just a fume extractor.
When to Use Silver Alloys
- Joining dissimilar metals (steel to copper, stainless to brass)
- Food-grade and potable water applications (cadmium-free required)
- Joints requiring high strength and ductility
- Thin-wall assemblies where minimal heat is preferred
- Stainless steel joints
- Electrical connections requiring conductivity
Copper-Phosphorus Alloys (BCuP Series)
BCuP alloys are the workhorses of copper brazing. They’re self-fluxing on copper-to-copper joints (the phosphorus acts as a deoxidizer), which makes them fast and convenient for plumbing and HVAC work.
Common BCuP Alloys
| AWS Class | Silver % | Phosphorus % | Solidus/Liquidus (F) | Best For |
|---|---|---|---|---|
| BCuP-2 | 0% | 7.25% | 1,310/1,460 | Copper to copper, tight clearance. Economical. |
| BCuP-3 | 5% | 6.3% | 1,190/1,480 | Copper to copper, general. Most common HVAC alloy. |
| BCuP-5 | 15% | 5% | 1,190/1,475 | Copper to copper and copper to brass. Best flow characteristics. |
| BCuP-6 | 2% | 7.1% | 1,190/1,460 | Copper to copper. Low silver content reduces cost. |
BCuP Rules
- Self-fluxing on copper-to-copper only. The phosphorus provides the fluxing action. On copper-to-brass or copper-to-steel, you need external flux even with BCuP alloys.
- Never use BCuP alloys on ferrous metals (steel, stainless, cast iron). The phosphorus forms brittle iron phosphide at the interface, creating a weak, crack-prone joint.
- Never use BCuP on nickel alloys for the same reason; phosphorus creates brittle nickel phosphide.
BCuP-5 (15% silver, often called “15% silver solder” in the trade) is the premium copper brazing alloy. It flows beautifully, fills gaps better than the lower-silver alloys, and produces the strongest joints. For copper pipe brazing specifics, see brazing copper pipe.
Brass Brazing Alloys (RBCuZn Series)
Brass brazing rod is the old-school, general-purpose brazing filler for steel. It’s a copper-zinc alloy that flows at higher temperatures than silver alloys but costs a fraction of the price.
Common Brass Alloys
| AWS Class | Composition | Solidus/Liquidus (F) | Best For |
|---|---|---|---|
| RBCuZn-A | 59.5% Cu, 40% Zn, 0.5% Sn | 1,630/1,650 | Steel, cast iron, copper alloys |
| RBCuZn-C | 58% Cu, 32% Zn, 10% Ni | 1,690/1,715 | Steel, stainless, nickel alloys. Higher strength. |
| RBCuZn-D | 48% Cu, 42% Zn, 10% Ni | 1,690/1,715 | Surfacing/buildup. Higher zinc for wear resistance. |
When to Use Brass Alloys
- Joining steel to steel where silver brazing is too expensive
- Cast iron repair (braze welding with brass rod is the standard repair method for cast iron)
- Buildup and surfacing on worn steel parts
- Large joints where the volume of filler makes silver cost-prohibitive
Brass brazing requires flux (typically AWS Type FB3-A borax-based flux). The higher brazing temperature means more heat input and more distortion compared to silver brazing.
Nickel Brazing Alloys (BNi Series)
Nickel brazing alloys operate at high temperatures (1,700-2,150F) and produce joints that resist high-temperature service, corrosion, and oxidation. They’re used primarily in furnace brazing (vacuum or controlled atmosphere) for aerospace, turbine, and chemical processing applications.
Common BNi Alloys
| AWS Class | Brazing Range (F) | Best For |
|---|---|---|
| BNi-1 | 1,950-2,150 | Stainless steel, high-temp alloys. General purpose. |
| BNi-2 | 1,780-1,900 | Lower flow temp. Good gap filling. Most widely used BNi. |
| BNi-3 | 1,850-1,900 | Highest corrosion resistance. No chromium or boron. |
| BNi-7 | 1,630-1,700 | Lowest flow temp BNi. Good for heat-sensitive assemblies. |
Nickel alloys require vacuum or inert/reducing atmosphere brazing. They’re not used with flux or torch brazing in typical shop environments.
Joint Clearance by Filler Type
Capillary action is the driving force in brazing. The gap between the parts must be tight enough for surface tension to draw the filler in, but not so tight that the filler can’t enter.
| Filler Family | Optimal Clearance at Brazing Temp | Maximum Clearance |
|---|---|---|
| BAg (silver) | 0.001-0.005" | 0.005" |
| BCuP (copper-phosphorus) | 0.001-0.005" | 0.005" |
| RBCuZn (brass) | 0.002-0.005" | 0.010" |
| BNi (nickel) | 0.001-0.005" | 0.005" |
At brazing temperature is the key phrase. Metals expand when heated. A joint with 0.003" clearance at room temperature may have 0.005" at brazing temperature if the outer part expands more than the inner part (tube-in-tube joint). Design the clearance based on the differential expansion at brazing temperature.
Flux Requirements
| Filler | Base Metals | Flux Required? | Flux Type |
|---|---|---|---|
| BAg (silver) | Steel, stainless, copper | Yes | AWS FB3-A (white paste) |
| BCuP | Copper to copper | No (self-fluxing) | None needed |
| BCuP | Copper to brass or steel | Yes | AWS FB3-A |
| RBCuZn (brass) | All | Yes | AWS FB3-A or FB3-C |
| BNi (nickel) | All | No (furnace atmosphere provides) | Vacuum or inert/reducing gas |
Flux Application
Apply flux as a paste to the joint area before heating. Cover all surfaces that will reach brazing temperature. The flux melts and becomes active at about 1,050-1,100F, protecting the surfaces from oxidation as the temperature rises to the brazing range.
After brazing, remove flux residue. Most brazing fluxes are water-soluble when hot but become glassy and hard when they cool. Quenching the joint in hot water immediately after brazing dissolves the flux. Cooled flux requires soaking or chemical cleaning.
Leaving flux residue on the joint is not acceptable. Flux residue is corrosive. It absorbs moisture and attacks the base metal over time. Clean all brazed joints.
Selecting the Right Filler: Decision Guide
- Copper-to-copper pipe or tubing? BCuP-5 (15% silver, self-fluxing). BCuP-2 if budget is tight.
- Copper-to-brass or copper-to-steel? BAg-5 or BAg-34 with flux.
- Steel-to-steel, general purpose? RBCuZn-A (brass) with flux if cost matters. BAg-5 or BAg-24 with flux for higher quality.
- Stainless steel? BAg-7 with flux for food/corrosion applications. BAg-5 for general stainless.
- Dissimilar metals? BAg-1a or BAg-5. Silver alloys wet nearly all metals.
- High-temperature service (above 500F)? BNi alloys in furnace brazing. Silver and brass joints lose strength above 400-500F.
- Carbide to steel (tool tips)? BAg-5 or BAg-24.
Filler Metal Forms and Pre-Placement
Brazing filler metals come in several physical forms. The right form depends on the brazing method: hand-fed torch work or production furnace brazing.
Rod/Wire: The most common form for torch brazing. The operator hand-feeds the rod into the heated joint. Available in straight lengths (rod) or on spools (wire) for automated feeding. Diameters range from 1/32" to 3/16".
Strip and preforms: Stamped or cut shapes (rings, washers, discs, slugs) pre-placed in the joint before heating. Used in furnace brazing, induction brazing, and any process where the operator can’t hand-feed filler during heating. Pre-placed filler produces the most consistent joint fill because the exact amount of filler is controlled by the preform dimensions.
Paste: Filler metal powder mixed with flux and binder into a paste that’s applied by syringe, brush, or automated dispenser. Used for small, precise joints in electronics, medical devices, and production brazing. The paste stays where you put it during assembly, then melts and flows during heating.
Cladding (brazing sheet): Aluminum sheet with a thin layer of brazing filler alloy bonded to one or both surfaces. Used for fabricating aluminum heat exchangers and multi-joint assemblies. During furnace brazing, the clad layer melts and brazes all joints simultaneously. This is how automotive radiators are manufactured.
For information on torch brazing technique with these fillers, see brazing copper pipe. For production brazing with induction heating, see induction brazing explained.
Storage and Handling
Brazing filler metals should be stored in their original packaging in a clean, dry environment. Contaminated filler produces poor wetting, voids, and weak joints.
Keep filler dry. Moisture causes porosity in the brazed joint. BCuP and BAg rod stored in humid conditions may develop surface oxidation that impairs flow. Wipe rod clean before use if it shows discoloration.
Avoid skin oils. Handle filler with clean gloves or only at the end you won’t be using. Skin oils on the filler surface can cause voids and poor wetting, particularly on precision joints.
Don’t mix alloys. Contamination from one filler type on another can produce unpredictable melting behavior. Keep different alloys in separate containers.
Flux shelf life. Brazing flux paste has a shelf life (typically 1-2 years). Old flux loses activity and may not adequately protect the joint from oxidation. If your flux has dried out, hardened, or separated, replace it.
For joint strength comparisons between brazing and welding, see brazing vs welding strength.