# Copper-phosphorus brazing alloys

Cu-P and Ag-Cu-P brazing fillers: classifications, melting behavior, substrate and service limits, joint fit, and process control.

Reviewed: 2026-11-01

Tags: consumables, welding, brazing, copper-phosphorus

Lucas Milhaupt's **Sil-Fos 15** sheet gives **643°C solidus**, **705°C flow point** and **802°C true liquidus**. Its brazing range is **705–815°C**. Phosphorus provides self-fluxing action on clean copper-to-copper joints. Select compatible flux for brass or bronze, and use a separately approved filler for steel or nickel-base joints. [Sil-Fos 15 TDS, p. 1](https://www.lucasmilhaupt.com/Lucas/Technical-Documents1/Sil-Fos15TDSEnglish.pdf)

## Family and process role

Copper–phosphorus and silver–copper–phosphorus fillers wet copper and compatible alloys by capillary flow. Keep phosphorus-bearing and phosphorus-free silver fillers distinct when specifying steel joints. [Johnson Matthey brazing definitions](https://matthey.com/products-and-markets/pgms-and-circularity/pgm-industrial-products/metal-brazing-tips-and-advice), [Aufhauser PhosCopper selection guidance](https://www.brazing.com/products/phosCopperBrazingAlloys.aspx)

Useful procurement identifiers include the exact trade name, classification, supplied form, nominal composition and tolerances, and batch documentation. Manufacturer examples illustrate the family:

| Named filler | Manufacturer's composition information | Listed classification |
| --- | --- | --- |
| Aufhauser PhosCopper 0 | Ag 0%, Cu 92.75%, P 7.25% nominal | AWS A5.8 BCuP-2 |
| Aufhauser PhosCopper 15 | Ag 15%, Cu 80%, P 5% nominal | AWS A5.8 BCuP-5 |
| Harris 2P | Ag 1.8–2.2%, P 6.8–7.2%, Cu remainder | ISO 17672 CuP 280 |

These are manufacturer declarations, not a reproduction of the full standards or proof that every similarly named product has identical tolerances. Harris distinguishes its composition ranges from actual batch values available through an inspection certificate or material test report. [Aufhauser alloy table](https://www.brazing.com/products/phosCopperBrazingAlloys.aspx); [Harris 2P TDS, pp. 1–2](https://ch-delivery.lincolnelectric.com/api/public/content/dbc9664cd4aa459c9b74d177618a09d4?v=0ddc1786)

Silver content alone does not rank fluidity or gap tolerance. Aufhauser describes PhosCopper 0 for tight fit and PhosCopper 15 for wider gaps. Compare the formulation and product guidance. [Aufhauser PhosCopper products](https://www.brazing.com/products/phosCopperBrazingAlloys.aspx)

## Solidus, liquidus, and working cycle

**Solidus** is the highest temperature at which the filler is fully solid; **liquidus** is the lowest at which it is fully liquid. An alloy with a melting range passes through an intermediate solid-plus-liquid condition. These terms describe the filler, not an allowable service temperature or a complete heating schedule. [Johnson Matthey terminology](https://matthey.com/products-and-markets/pgms-and-circularity/pgm-industrial-products/metal-brazing-tips-and-advice)

The Sil-Fos 15 TDS is dated **November 2016**. Its footnote distinguishes useful flow from the **802°C true liquidus**. [Sil-Fos 15 TDS, p. 1](https://www.lucasmilhaupt.com/Lucas/Technical-Documents1/Sil-Fos15TDSEnglish.pdf)

The same TDS warns of **liquation** during slow heating: low- and high-melting constituents can separate, leaving residue. Prolonged heating and small clearances can also cause base-metal erosion. Develop heating, placement, dwell, and geometry together. [Sil-Fos 15 TDS, pp. 1–2](https://www.lucasmilhaupt.com/Lucas/Technical-Documents1/Sil-Fos15TDSEnglish.pdf)

As a separate product example, Harris 2P lists **645°C solidus and 788°C liquidus**. These values belong to that formulation; they should not be combined with another supplier's clearance or brazing-cycle recommendation to create an undocumented recipe. [Harris 2P TDS, p. 1](https://ch-delivery.lincolnelectric.com/api/public/content/dbc9664cd4aa459c9b74d177618a09d4?v=0ddc1786)

## Self-fluxing has a substrate boundary

Phosphorus provides self-fluxing action when these fillers join **copper to copper**. That property does not eliminate preparation: grease, dirt, and oxide still obstruct wetting and capillary distribution. Lucas Milhaupt's brazing fundamentals distinguish cleaning from fluxing and describe phosphorus-bearing fillers as the copper-to-copper exception to the usual need for separate flux. [Lucas Milhaupt brazing fundamentals, cleaning and fluxing](https://www.lucasmilhaupt.com/Brazing-Academy/Brazing-Fundamentals)

For **brass, bronze, or copper-to-copper-alloy joints**, select the flux recommended for the actual filler and substrate. Italbras explicitly distinguishes flux-free copper-to-copper use from copper-alloy joints requiring flux. Do not extend self-fluxing to a plated surface merely because copper lies underneath it. Confirm the surface actually being wetted and the manufacturer's approval for that combination. [Italbras copper-phosphorus alloy guidance](https://www.italbras.it/brazing/lcopper-phosphorus-alloys-and-copper-alloys/?lang=en)

**Ferrous and nickel-base materials require particular caution.** Italbras warns against steel and nickel alloys because brittle intermediate phases can form. An ordinary copper-tube filler therefore cannot be assumed suitable for a copper-to-steel component. Select a separately approved dissimilar-joint filler and process. [Italbras substrate restrictions](https://www.italbras.it/brazing/lcopper-phosphorus-alloys-and-copper-alloys/?lang=en)

Service chemistry is a separate restriction from base-metal compatibility. Harris states that phosphorus-containing fillers, including 2P, must not be used where the braze encounters **sulfur or sulfur compounds in service**. Italbras also cautions against sulfur-contact service. A successful initial braze or leak test does not establish compatibility with an aggressive operating environment. [Harris 2P corrosion guidance](https://ch-delivery.lincolnelectric.com/api/public/content/dbc9664cd4aa459c9b74d177618a09d4?v=0ddc1786); [Italbras service restrictions](https://www.italbras.it/brazing/lcopper-phosphorus-alloys-and-copper-alloys/?lang=en)

## Joint fit and a bounded selection example

Capillary clearance is a process variable, not a defect to conceal with a large external fillet. Evaluate fit **at brazing temperature**: expansion of the parent parts can change the cold gap. Johnson Matthey directs users to the optimum gap in the chosen product's data and identifies joint overlap as another factor affecting strength. [Johnson Matthey joint-design guidance](https://matthey.com/products-and-markets/pgms-and-circularity/pgm-industrial-products/metal-brazing-tips-and-advice)

Aufhauser lists **0.002–0.006 in** clearance for **PhosCopper 15**—approximately **0.051–0.152 mm** by conversion. Confirm the clearance convention, temperature basis, tolerances, and supplier interpretation before using this as a tube diametral difference. The range does not automatically apply to a silver-free filler. [Aufhauser PhosCopper 15 recommendation](https://www.brazing.com/products/phosCopperBrazingAlloys.aspx)

For a copper refrigeration assembly, establish base-metal and surface identity, service chemistry, joint gap/overlap, and the approved filler before choosing a rod or preform. Then qualify heating and filler delivery on representative joints. Assess joint penetration, erosion, leakage, and the application's required mechanical performance; an attractive fillet is only external evidence. Lucas Milhaupt identifies base metals, clearance, and filler choice as factors in joint strength. [Lucas Milhaupt brazing FAQ](https://www.lucasmilhaupt.com/Resources-Library/FAQ)

## Flux cleanup and process documentation

When flux is required, match its active temperature range and time stability to the actual cycle. Remove residues using that flux manufacturer's procedure. Lucas Milhaupt describes hot-water cleaning for water-soluble residues **after the filler has fully solidified**; that is not permission to quench every assembly regardless of thermal-shock sensitivity. Keep flux removal distinct from oxide-scale removal and avoid applying one cleaning chemistry indiscriminately to all parts. [Lucas Milhaupt brazing fundamentals, joint cleaning](https://www.lucasmilhaupt.com/Brazing-Academy/Brazing-Fundamentals)

Retain the filler identity, batch/certificate requirements, flux, fit specification, heating method, and acceptance criteria with the procedure. Product classifications and typical property tables support selection; they do not replace assembly qualification or establish a universal pressure rating. Review the actual filler and flux safety data sheets and the heating equipment's instructions for the process being used. [Harris 2P TDS, safety and batch-documentation notes](https://ch-delivery.lincolnelectric.com/api/public/content/dbc9664cd4aa459c9b74d177618a09d4?v=0ddc1786)

## Related reference

[Welding methods](../../processes/welding-methods/index.md)