What is the scope of AWS A5.31 / A5.31M?
AWS A5.31/A5.31M is the American Welding Society specification that prescribes classification requirements for fluxes used in brazing and braze welding.
The standard covers fluxes intended for use with brazing and braze welding filler metals, such as those classified under AWS A5.8/A5.8M.
These fluxes are classified according to three primary criteria: the filler metal type with which they are intended to be used, the physical form in which they are supplied, and their activity temperature range.
The classification system employs the designator “FB” to indicate a flux for brazing and braze welding applications.
The current edition is AWS A5.31M/A5.31:2022, which is the third edition and revises the 2012 edition.
The 2022 edition prescribes requirements for the classification of eighteen fluxes.
The specification also addresses general requirements, testing procedures, packaging, and marking, and includes an appended guide concerning the classification system and the intended use of the fluxes.
The standard makes use of both U.S. Customary Units and the International System of Units (SI), and each system must be used independently of the other.
Verify the current edition number and any superseded designations with the latest edition.
What is the classification system used in AWS A5.31 / A5.31M?
The AWS A5.31 classification system is built around the “FB” prefix, which stands for Flux for Brazing.
The designation that follows the prefix is structured to convey three essential pieces of information: the filler metal family, the physical form, and the activity temperature range.
The first digit following “FB” indicates the filler metal group with which the flux is intended to be used.
The letter suffix that follows the digit indicates the specific sub-group within that filler metal family, which correlates with the physical form of the flux and its activity temperature range.
This alphanumeric system allows welding engineers and brazing operators to select a flux that is chemically compatible with the filler metal and metallurgically appropriate for the base metals being joined.
The classification is not based on the chemical composition of the flux alone; rather, it is performance-based, with the activity temperature range serving as a critical selection parameter.
Verify the complete designator rules and any additional classification criteria with the latest edition.
What is the complete list of AWS A5.31 / A5.31M classifications?
The classifications below are compiled from publicly available AWS reference tables and standard summaries.
The 2022 edition prescribes requirements for eighteen fluxes for brazing and braze welding.
Because AWS standards are periodically revised, verify the complete and current list with the latest edition.
FB1-A
FB1-B
FB1-C
These classifications designate fluxes intended for use with aluminum-silicon brazing filler metals, such as BAlSi types classified under AWS A5.8/A5.8M.
All three are supplied in powder form.
FB1-A has an activity temperature range of 580–615 °C (1080–1140 °F).
FB1-B has an activity temperature range of 560–615 °C (1040–1140 °F).
FB1-C has an activity temperature range of 540–615 °C (1000–1140 °F).
These fluxes are used for brazing aluminum and aluminum alloys.
Verify the complete FB1 series and any additional sub-groups with the latest edition.
FB2-A
This classification designates a flux intended for use with magnesium brazing filler metals, such as BMg types.
It is supplied in powder form and has an activity temperature range of 480–620 °C (900–1150 °F).
This flux is used for brazing magnesium alloys.
Verify the complete FB2 series with the latest edition.
FB3-A
FB3-C
FB3-D
FB3-E
FB3-F
FB3-G
FB3-H
FB3-I
FB3-J
FB3-K
FB3-L
FB3-M
The FB3 series is the largest and most widely used family within AWS A5.31.
These fluxes are intended for use with silver-based brazing filler metals (BAg types), copper-phosphorus brazing filler metals (BCuP types), and in some cases copper, nickel, gold, and copper-zinc brazing filler metals.
The physical forms within this series include paste, powder, liquid, and dispensable paste.
FB3-A is a paste flux for BAg and BCuP filler metals with an activity range of 565–870 °C (1050–1600 °F).
FB3-C is a paste flux for BAg and BCuP filler metals with an activity range of 565–925 °C (1050–1700 °F).
FB3-D is a paste flux for BAg, BCu, BNi, BAu, and RBCuZn filler metals with an activity range of 760–1205 °C (1400–2200 °F).
FB3-E is a liquid flux for BAg and BCuP filler metals with an activity range of 565–870 °C.
FB3-F is a powder flux for BAg and BCuP filler metals with an activity range of 560–870 °C (1200–1600 °F).
FB3-G is a suspension flux for BAg and BCuP filler metals with an activity range of 565–870 °C.
FB3-H is a suspension flux for BAg filler metals with an activity range of 565–925 °C.
FB3-I is a suspension flux for BAg, BCu, BNi, BAu, and RBCuZn filler metals with an activity range of 760–1205 °C.
FB3-J is a powder flux for BAg, BCu, BNi, BAu, and RBCuZn filler metals with an activity range of 760–1205 °C.
FB3-K is a liquid flux for BAg and RBCuZn filler metals with an activity range of 760–1205 °C.
FB3-L is a dispensable paste flux for BAg and BCuP filler metals with an activity range of 565–870 °C.
FB3-M is a dispensable paste flux within the same composition family.
Verify the complete FB3 series and the precise activity temperature ranges with the latest edition.
FB4 series — fluxes for aluminum bronze brazing
FB4-A
This classification designates a flux specifically formulated for brazing base metals that contain aluminum, such as aluminum bronze.
It is supplied in paste form and has an activity temperature range of approximately 595–870 °C (1100–1600 °F).
The classification nomenclature for FB4-A includes specific base metal types as part of its designation.
This flux is suitable for base metals containing up to approximately 9% aluminum.
Verify the complete FB4 series and the specific base metal designations with the latest edition.
Higher-numbered and specialty classifications
Additional flux classifications beyond the FB1 through FB4 series may exist within the A5.31 system or in related AWS documents such as B8.19.
Some references to FB5, FB6, and FB7 designations appear in technical literature, but these may not be independently classified within the current edition of AWS A5.31.
Fluxes for specialized applications such as aluminum brazing in controlled atmospheres and nickel brazing are addressed in other AWS publications.
For a definitive enumeration of every classification, consult the latest edition of AWS A5.31/A5.31M and related AWS brazing documents.
The activity temperature range is one of the most critical selection parameters for a brazing flux.
It defines the temperature window within which the flux becomes chemically active and capable of dissolving or reducing the oxide layers on the base metal and filler metal surfaces.
Below this range, the flux remains inert and does not promote wetting.
Above this range, the flux may break down, evaporate, or lose its protective capability, leading to oxidation and poor joint quality.
The activity temperature range must be matched to the melting and flow temperature of the brazing filler metal being used.
The physical form of the flux — powder, paste, liquid, or suspension — determines how the flux is applied to the joint and its suitability for different brazing methods.
Paste fluxes are convenient for manual application and are widely used in torch brazing.
Powder fluxes are often mixed with water or a binder to form a paste at the point of use and are commonly used in furnace brazing.
Liquid fluxes are used in automated or immersion applications, while suspension fluxes combine fine powder particles in a liquid carrier for spray or dip application.
The selection of form and activity range should be based on the brazing process, the base metals, the filler metal, and the joint configuration.
Verify the specific activity temperature ranges and application requirements with the latest edition.
What are the key selection criteria for brazing fluxes?
Selecting the correct brazing flux involves matching the flux classification to the filler metal, the base metal, and the brazing process parameters.
The filler metal type is the primary determinant: FB1 fluxes are designed for aluminum-silicon filler metals, FB2 for magnesium filler metals, FB3 for silver and copper-phosphorus filler metals, and FB4 for aluminum bronze applications.
The base metal composition must also be considered, as some fluxes are formulated to handle refractory oxides that form on certain alloys, such as aluminum-containing bronzes.
The brazing temperature must fall within the activity temperature range of the flux, and the flux must remain active throughout the brazing cycle.
The physical form should be compatible with the application method and the production environment.
For copper-to-copper joints using copper-phosphorus filler metals, flux may not be required at all, as the phosphorus content in the filler metal acts as a deoxidizer.
In controlled-atmosphere furnaces using hydrogen, nitrogen, argon, or vacuum, flux is generally unnecessary because the atmosphere itself prevents oxidation.
Flux is not a substitute for proper surface preparation; all joint surfaces must be mechanically or chemically cleaned before brazing.
Verify specific selection guidance with the latest edition and the flux manufacturer’s recommendations.
AWS A5.31/A5.31M does not exist in isolation; several companion specifications provide the filler metal classifications and related product requirements that complement the flux specification.
AWS A5.8/A5.8M is the primary specification for brazing and braze welding filler metals and provides the filler metal classifications that are referenced in the A5.31 flux classification system.
AWS A5.7/A5.7M specifies copper and copper-alloy bare welding rods and electrodes, which are relevant for certain brazing and braze welding applications.
AWS A5.14/A5.14M specifies nickel and nickel alloy bare welding electrodes and rods, which are used with FB3-D, FB3-I, and FB3-J fluxes for high-temperature brazing.
AWS B8.19 provides information about fluxes that are not currently classified within A5.31M/A5.31.
The ASME Boiler and Pressure Vessel Code, Section II, Part C, adopts AWS filler metal and flux specifications as SFA specifications; users should confirm the corresponding SFA edition for their application.
Consulting these specifications together provides a comprehensive framework for brazing flux and filler metal selection, qualification, and quality assurance.
The AWS filler metal and consumables classification system is organized by product type and intended process, with each specification addressing a distinct category of materials.
AWS A5.31 is unique within this system in that it addresses fluxes, which are consumable chemicals rather than metallic filler metals.
The specification complements the filler metal classifications in AWS A5.8/A5.8M by providing the chemical agents necessary to promote wetting and oxide removal during the brazing cycle.
The “A5” series covers filler metals and related consumables for arc welding, brazing, and braze welding, and the numbering sequence roughly corresponds to the chronological development of the specifications.
The “/A5.31M” suffix indicates that the specification includes both U.S. Customary Units and SI units, a convention applied to AWS standards that have been harmonized with ISO practices.
The 2022 edition represents the third edition of the specification and revises the 2012 edition, reflecting ongoing updates to testing procedures and classification criteria.
Users should always confirm that they are working from the most current edition of any AWS specification, as classifications and requirements may be added, revised, or withdrawn between editions.
Verify the current edition number and any superseded designations with the latest edition of AWS A5.31/A5.31M.
