Brazing-copper:Hard Challenge for Correct Outcome.SOLUTIONS with Effective, Practical AdviceBrazing-copper and most of the copper alloys is described as easy and practical in that these materials, except possibly for a few exceptions, are readily brazeable. Attention has to be paid to select suitable process and filler metal to prevent possible problems. The influence of brazing heat on base metal microstructure should be assessed to examine if any effects like annealing are desirable or not. In previous pages of this Website we dealt with Brazing in general, with Brazing Joint Design, and with Brazing Heating that describes the different methods of heating for brazing. Base Metals Problems are found when trying to braze tough pitch copper containing oxygen, in hydrogen based furnace atmospheres, because high pressure steam is formed in the solid metal with destructive results. Also torch brazing of tough pitch copper should be avoided as it may produce embrittlement. Deoxidized coppers are readily brazed, even without flux in protective atmospheres if Brazing-copper phosphorous filler metal is used, because phosphorus is a powerful deoxidizer. However the use of flux is generally recommended because it improves the process. Copper alloys containing chromium or aluminum must be thoroughly cleaned to remove oxides that interfere with brazing. Furthermore the use of special AWS flux type FB4-A is recommended. Fluxes are used to braze brasses which are alloys of copper and from 5 to 40% zinc. Overheating or heating brasses for too long may leave voids because zinc tends to vaporize. Bronzes were originally alloys of copper and tin. Modifications were introduced that conserved the name even if tin content was reduced or eliminated. Most important categories now include phosphor-bronze, silicon-bronze and aluminum bronze. Phosphor-bronze alloys should be brazed only after stress relieving at 290 to 345 0C (550 to 650 0F), because in cold worked condition they are prone to hot cracking. These are important considerations to remember when approaching Brazing-copper of particular copper alloys. Filler Metals ANSI/AWS A5.8/A5.8M:2004 ISO 3677 Brazing-copper with copper base filler metals is commonly done, one has just to take care of a few precautions depending on the type of base metal. A few recommendations can be found in Issue 34 of Practical Welding Letter for June 2006. Click on PWL#034 to see it. A Table of some Copper Base brazing filler metals by their AWS classification is shown hereafter:
A Table listing copper base brazing alloys can be found also in a commercial publication. See Filler Copper Alloys . Brazing-copper with silver base filler metals is also very popular. A few indications and a Table of some wires and rods by their AWS classification can be found in Issue 3 of Practical Welding Letter for November 2003. Click PWL#03 Brazing-copper can also be performed with precious metal base filler metals, in particular with a gold alloy named BAu-4 containing 82% Au and 18% Ni, but its use is limited to high cost applications for vacuum and electronic devices where low vapor pressure is important.
Fluxes Apart from the special flux mentioned above (type FB4-A) to be used for removing tough oxides generated by the presence in the base metal of chromium or aluminum, other fluxes are available commercially. Their ingredients, generally borates, fluorides and chlorides, are selected for their activity at definite temperatures corresponding to the brazing temperature range of the different filler metals. Suppliers recommendations may refer also to the base metals. Fluxes for Brazing-copper are supplied as powders or slurries, but also as liquids for special applications. Torch brazing is sometimes performed by having the fuel gas pass through a container containing liquid flux. Brazing-copper All the methods described in our page on Brazing Heating can be used, the selection being generally determined by expediency and economy, provided the requirements of the final product are met satisfactorily. Furnace brazing is the process whose main advantage is its adaptability to mass production of repetitive brazed units. Furnaces must be adequate to manage the required mass with suitable atmosphere and temperature controls. Brass is not furnace brazed, generally. Gases containing sulfur should not come in contact with copper or copper alloys.
AWS A5.31-92(R2003)
ANSI/AWS D10.13/D10.13M:2001
The Copper Development Association Inc. makes available to anybody upon request a hardcopy leaflet presenting examples of Qualification documents: Brazing Procedure Specifications. Resources 1. - Articles: An article from the AWS Welding Journal of August 2002 on Tips for Soldering and Brazing Copper Tubing can be found at An article from The Fabricator of May 15, 2003 on Brazing copper tubing can be found at An article from Practical Welding Today of March 13, 2007 on Brazing-copper and copper alloys can be found at An Article from International Copper Association, Ltd. 2. - Information from Commercial Sources: From Handy & Harman on Fluxes on Copper & Copper Alloys:
From Aufhauser
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Brazing
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