Brazing-processes

techniques and materials:

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Brazing-processes are indeed very old technology already employed in Prehistoric times to join gold jewelry.

This new, edited and enriched page, summarizes the essential features of this group of processes.

It provides definition and explanations and assists with the understanding of critical aspects and requirements.

Finally it refers to specific additional pages where essential information is elaborated further on.

Visit the NEW Page on Welding Overview, for a thorough Introduction to Welding.
Visit also the NEW Page on Process-selection, for Understanding the Selection procedure and
the NEW Page on Process-optimization for improved productivity.

Definition of Brazing-processes (AWS):
A group of joining processes that produces coalescence of materials by heating them to the brazing temperature in the presence of a filler metal having a liquidus above 450 °C (840 °F) and below the solidus of the base metal.

The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action.

Let us remind that liquidus (Latin for liquid state) is the minimum temperature at which all components of a mixture (such as an alloy) are in a liquid state.

And that solidus (the Latin word for solid) is the temperature below which a mixture (such as an alloy) is completely solid.

Faying surfaces are those of materials in contact with each other, either joined or about to be joined.

The above definition clarifies that the Brazing-processes:

produce coalescence or intimate joining by metallurgical union between the elements being brazed together.
is by no means limited to metals: although metal brazing is quite common, other base materials like ceramics, graphite and cermets can be brazed.
is performed at well defined brazing temperatures, above the accepted arbitrary limit and below the solidus (the highest temperature at which a material is still completely solid) of the base metals.
causes the molten filler metal wet the surfaces and be drawn and distributed into the joint by capillary force (surface tension).

One well known example of Brazing-processes involving non metals is that of ceramic cutting points (sintered carbide tips) brazed to metallic supports, for machine tools like lathes and milling machines. This joining process lends itself to automatic operation, if the conditions are correct.

How to braze metal is an art and a science.

Brazing-processes must be mastered for exploiting successfully the technology.

What are these metal Brazing-processes?

They are those meeting the description above.
Note that nothing limits the heating methods employed, the kind of materials, or the purposes.

Similar joining processes using temperatures below the said limit are called soldering.

Alternatively Adhesive bonding is generally carried out at room temperature, except that low temperature curing may be used for completing the joint.

When are metal Brazing-processes preferred to welding?

They should be preferred if:

joining cannot be performed satisfactorily by welding
joining requirements and quality can be assured at lower total cost

As examples we may consider:

Joining dissimilar materials
Avoiding excessive warping
Avoiding damage to base materials

Advantages

Ability to join dissimilar (welding incompatible) materials,
use of lower heat input than for welding, with less distortion and less detrimental metallurgical influence on heat treated materials,
availability of a large range of filler alloys, each one adapted to particular base materials and a certain temperature range,
ability to join wide different thicknesses,
ability to join vast areas,
ability to perform many joints at the same time,
ability to preserve coating or cladding without damage
ability to save on finishing operations,
development of adequate strength, dependent on the extent of the brazed surfaces,
possibility to use localized or general heat sources as required,
heating can be simultaneously used for additional treatments (aging, stress relieving),
step fabrication of different joints in sequence from higher to lower temperatures,
capability of economic manufacturing,
relatively easier than welding application process.

Limitations

joint must conform to requirements of form, overlap, length and clearance,
filler-alloys are generally more expensive,
flux is needed (one more variable) to obtain wetting of base metal surfaces, and elimination of its residuals is required to prevent corrosion,
color of filler-alloy may be different from that of base metal,
permitted finishing processes may be limited,
resistance to corrosion of joint may be less than that of base material.
unfavorable interaction of base materials with liquid filler metal may occur.

Brazing-processes are available for manual or automated brazing application.

The most common manual brazing process is performed by using an oxy-acetylene flame from a torch under complete control of the brazing operator.

Suitable kits are available in the market.

For a selection of modern OXY-Gas Welding and Brazing Equipment, Torches and Kits, you may wish to have a look at My Store.

Special automatic equipment reproducing the manual techniques with gas torches is used to provide economic solutions to repetitive production.

In typical mass production high frequency induction heating is used within the space of a coil, where well controlled high temperature is reached very fast.

Complex constructions are realized by Brazing-processes with different joints individually prepared (preplacing filler-alloy) beforehand.

This permits the realization of brazing of all joints at one time in a furnace, under protective atmosphere (from oxidation), or in a heated bath of salts (Dip processing is used for aluminum parts) as soon as brazing temperature is reached.

Other applications combine the heating for Brazing-processes with that needed for metallurgical purposes like heat treatments as aging or stress relieving.

The use of filler alloys with progressively decreasing melting temperatures permits to perform sequential Brazing-processes sub-operations (in separate activities), called step operations, in a furnace, at different times.

That is advantageous if the need arises because of assembling requirements, so that following joints are made at a lower temperature that does not compromise the earlier ones.

See our new page on Brazing Joint Design

Filler alloys

For the largest part of base metals, three are the types of filler-alloys most used in Brazing-processes: Copper base, Nickel base and Silver base.

For other alloys the base metal is either Aluminum, Magnesium or Gold. Within each type many alloys of various composition are available differing essentially in melting range and ease of flow at high temperature.

An Article on the Selection of Silver Alloy based Filler Metal was included (4) in our Practical Welding Letter No. 03 of November 2003. To see the article click on PWL#03.

An Article on Furnace-Brazing was published (7) in the May 2004, Issue No. 09 of Practical Welding Letter. Click on PWL#09 to read it.

An Article on the Selection of High Temperature Filler Metal Alloys for Brazing-processes was published (4) in the September 2004, Issue No. 13 of Practical Welding Letter. To read the article, click on PWL#013.

An Article on Filler Precious Metals in Brazing-processes was published (4) in the issue No.14 of Practical Welding Letter of October 2004.
To see the Article click on PWL#014.

An Article on Induction_Brazing was published (7) in the issue No.14 of Practical Welding Letter of October 2004. To see the Article click on PWL#014.

An Article on joining of Ceramic Materials was published (4) in the Issue 23 of Practical Welding Letter for July 2005.
Click on PWL#023 to read it.

An Article on Brazing-processes for joining Heat Resisting Alloys was published (4) in the Issue No.24 of Practical Welding Letter for August 2005.
To read it click on PWL#024.

An Article on Dew Point Measurement for Furnace Brazing Atmospheres was published (11) in the Issue No.24 of Practical Welding Letter for August 2005. To read it click on PWL#024.

An Article reporting on recent Brazing-processes development work by NASA on Brazing in Space was published (4) in Issue 28 of Practical Welding Letter for December 2005. To read the article (in Section 4) click on PWL#028.

An Article on Electrolysers (that provide Hydrogen for Furnace processing) was published (11) in the Issue 33 of Practical Welding Letter for May 2006. To read it click on PWL#033.

An Article on Filler Metal for Brazing Titanium Alloys was published (4) in Issue 46 of Practical Welding Letter for June 2007.
Click on PWL#046 to read it.

An Article on Cleaning for joining was published (2) in Issue 52 of Practical Welding Letter for December 2007.
Click on PWL#052 to read it.

An Article on Filler Metals for Diffusion Joining was published (4) in Issue No. 53 of Practical Welding Letter for January 2008.
Click on PWL#053 to read it.

An Article on Filler Metals for honeycomb Brazing-processes was published (4) in Issue 55 of Practical Welding Letter for March 2008.
Click on PWL#055 to read it.

An Article on Gaseous Filler Metals for novel Brazing-processes was published (4) in Issue 56 of Practical Welding Letter for April 2008.
Click on PWL#056 to read it.

An Article on Corrosion Resistant Aluminum Brazing Filler Metals was published (4) in issue 60 of Practical Welding Letter for August 2008.
Click on PWL#060 to read it.

An Article on Brazing Defects was published (7) in Issue No. 62 of Practical Welding Letter for October 2008.
Click on PWL#062 to read it.

An Article on Brazing Carbon Steel to Stainless was published (7) in issue 69 of Practical Welding Letter for May 2009.
Click on PWL#069 to read it.

An Article on Filler Metals for Brazing Graphite and Carbon/Carbon Composites was published (4) in Issue 73 of Practical Welding Letter for September 2009.
Click on PWL#073 to read it.

An Article on Brazing Flux Removal was published (3) in Issue No. 75 of Practical Welding Letter for November 2009.
Click on PWL#075 to read it.

An Article on Filler Metal for Spot Brazing Titanium to Nickel was published (4) in Issue No. 81 of Practical Welding Letter for May 2010.
Click on PWL#081 to read it.

An Article on Selecting Active Brazing Filler Metal was published (4) in Issue No. 97 of Practical Welding Letter for September 2011.
Click on PWL#097 to read it.

An Article on How to do it well: Furnace Brazing Copper was published (3) in Issue No.99 of Practical Welding Letter for November 2011.
Click on PWL#099 to read it.

An Article on Low Temperature Brazing Filler Metals for Joining Titanium was published (4) in Issue No. 103 of Practical Welding Letter for March 2012.
Click on PWL#103 to read it.

An Article on Filler Metals for Active Brazing was published (4) in Issue No. 109 of of Practical Welding Letter for September 2012.
Click on PWL#109 to see it.

An Article on Amorphous Filler Metal for Vacuum Brazing Ceramics to Titanium was published (4) in Issue 136 of Practical Welding Letter for December 2014.
Click on PWL#136 to see it.

An Article on Filler Metal for brazing a copper cable to a copper rivet was published (4) in Issue 137 of Practical Welding Letter for January 2015.
Click on PWL#137 to see it.

An Article on Filler Metal for Aluminum Brazing was published (4) in Issue 139 of Practical Welding Letter for March 2015.
Click on PWL#139 to see it.

An Article on New Silver-Free Brazing Filler Metals was published (4) in Issue 140 of Practical Welding Letter for April 2015.
Click on PWL#140 to see it.

An Article on How to get dry Gas at the Brazing Furnace was published (3) in Issue 141 of Practical Welding Letter for May 2015.
Click on PWL#141 to see it.

An Article on Aim for Small, Concave Braze Fillets was published (3) in Issue 148 of Practical Welding Letter for December 2015.
Click on PWL#148.

An Article on A new method of brazing stainless steel parts was published (7) in Issue 151 of Practical Welding Letter for March 2016.
Click on PWL#151.

An Article on Advanced Microwave Brazing was published (2) in Issue 161 of Practical Welding Letter for January 2017.
Click on PWL#161.

An Article on Brazing Filler Metal Alloys was published (4) in Issue 166 of Practical Welding Letter for June 2017.
Click on PWL#166.

An Article on Resistance Brazing Filler Metal was published (4) in Issue 167 of Practical Welding Letter for July 2017.
Click on PWL#167.

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Filler metals for Brazing-processes are available in the form of foil, wire or paste (made up by a metal powder and a binder).

Paste can be obtained in precharged dispensers for manual or machine application, or as a thick suspension to be applied by brush.

Tip!: Pastes have an expiry date, because of the binder (usually six months from manufacturing date), and must be stored as recommended by manufacturers, possibly under refrigeration.

For mass production, preforms of designed shape can be purchased, either bent from wire or blanked from strip, eliminating scrap and preparation work, and easing preplacement in the joint.

See our new page on Brazing Heating.

See our page on Brazing Inspection and remember, when designing brazed joints, that inspectability (the ability to perform suitable inspection) must be included in design.

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References and Specifications

The following documents are important reference material for anyone involved in brazing operations. We recommend to learn and use the most significant of them for your activities.

ANSI/AWS B2.2/B2.2M:2010
Specification for Brazing Procedure and Performance Qualification
Edition: 3rd
American Welding Society / 07-Jul-2009 / 86 pages

ANSI/AWS C3.2M/C3.2:2008
Standard Methods for Evaluating the Strength of Brazed Joints
Edition: 4th
American Welding Society / 04-Jan-2008 / 44 pages

ANSI/AWS C3.3:2008
Recommended Practices for Design, Manufacture, and Inspection of Critical Brazed Components
Edition: 3rd
American Welding Society / 21-Apr-2008 / 44 pages

ANSI/AWS C3.4M/C3.4:2007
Specification for Torch_Brazing
Edition: 2nd
American Welding Society, 24-Aug-2007, 26 pages

ANSI/AWS C3.5M/C3.5:2007
Specification for Induction_brazing
Edition: 2nd
American Welding Society, 24-Aug-2007, 26 pages

ANSI/AWS C3.6M/C3.6:2008
Specification for Furnace_brazing
Edition: 3rd
American Welding Society, 12-Sep-2007, 28 pages

ANSI/AWS C3.7:2005
Specification for Aluminum_brazing
American Welding Society, 29-Jun-2005, 28 pages

ANSI/AWS C3.8M/C3.8:2011
Specification for the Ultrasonic Examination of Brazed Joints
Edition: 3rd
American Welding Society / 13-Jul-2011 / 30 pages

ANSI/AWS D10.13/D10.13M:2001
Recommended Practices for the brazing Copper Pipe and Tubing for Medical Gas Systems
American Welding Society, 01-Jan-2001, 21 pages

ANSI/AWS A5.8M/A5.8:2011
Specification for Filler Metals for Brazing and Braze Welding
Edition: 10th
American Welding Society / 17-Jun-2011 / 62 pages

ANSI/AWS A5.31M/A5.31:2012
Specification for Fluxes for Brazing and Braze Welding
Edition: 2nd
American Welding Society / 17-Feb-2012 / 38 pages

AWS BRH
Brazing-Handbook, Fifth Edition
American Welding Society, 01-Jan-2006, 700 pages

AWS WHB-1.9
WELDING HANDBOOK, 9th Edition, Volume 1
Welding Science and Technology,
American Welding Society, 01-Jan-2001, 650 pages

AWS WHB-2.9
WELDING HANDBOOK, 9th Edition, Volume 2
Part 1: Welding Processes (AWS WHB V2)
American Welding Society, 28-Apr-2004, 720 pages

AWS WHB-3.9
WELDING HANDBOOK, 9th Edition, Volume 3
Part 2: Welding Processes
American Welding Society , 01-Jan-2007, 669 pages

AWS WHB-4.9
WELDING HANDBOOK, 9th Edition, Volume 4
Materials and Applications Part 1
American Welding Society / 2010 / 760 pages

ISO 3677
Filler metal for soft soldering, braz. and braze welding - Designation
International Organization for Standardization, 01-Jun-1992, 2 pages

BS EN 13347:2002
Copper and copper alloys. Rod and wire for welding and braze welding
British-Adopted European Standard, 21-Dec-2002, 20 pages

Brazing Articles published in the Welding Journal are available online:
Braze Q & A

Brazing Footprints
Case Studies in High-Temperature Brazing
by Robert L. Peaslee

is available for downloading from
http://www.asminternational.org/pdf/spotlights/BrazingFootprints5.pdf

Interested readers are urged to download and save them for learning and future reference.

Watch the following Video on

How-To Braze Copper Tube

https://www.youtube.com/watch?v=07ODV2Bdqow

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