Resistance-brazing

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Resistance-brazing was briefly mentioned in this website as one of the processes using localized heating for brazing.

See my page on Brazing Heating.

Process

In Resistance Brazing, the joint to be brazed is made part of an electric circuit in which the local heat developed by the resistance to the passage of electric current melts the brazing filler metal and joins the elements.

Resistance-brazing is therefore suitable for applications in which rapid and localized heating is required and where electrodes can apply the pressure required to establish electrical contact.

The heat is generated in the workpieces, in the electrodes or in both, depending on the dimensions and on the resistivity of the respective materials.

Advantages of Resistance-brazing

  • Rapid localized heating in well defined areas
  • Clean operation without flames, surroundings unaffected
  • High quality joints with non specialized workforce
  • Can be made portable for applications on heavy equipment
  • Basic equipment relatively inexpensive
  • Automatic operations possible with additional accessories

Limitations of Resistance-brazing

  • Components must be electrically conductive
  • Large joint area are unpractical
  • Limited to low temperature brazing filler metals
  • Local heating can cause distortion

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Equipment

Modified (if necessary) Spot Welding Machines can be used for Resistance-brazing, although the controlling parameters must be especially adapted to the brazing process.

In particular the rate of heating, strongly dependent upon the rate of pressure application, should equalize rapidly, providing uniform heat across the brazed area.

Essential elements are the transformer, the electrodes and the pressure source for squeezing the workpieces between them.

Electrodes & Tongs

In Resistance-brazing, generally water cooled high conductivity electrodes work well with low conductivity (high resistivity) base metals, in that resistive heat will be concentrated at the joint.

For brazing high conductivity workpieces, the heat, generated in low conductivity (high resistivity) electrodes, will be transferred by thermal conduction to the joint through the base metal.

Depending on the metals of the workpieces (see above), a few materials are suitable for electrodes, as determined by trials. Among them also a few types of graphite can be selected to exploit the fact that molten brazing metal does not wet graphite.

The surface condition of Resistance-brazing electrodes may deteriorate with use. The state of their surface must be checked regularly and dressing operations must be performed as needed.

For local applications on complex structures, small hand held specially made pliers called tongs are used, providing electric current (through cables running inside water cooled hoses) and squeezing pressure between the electrodes.

Resistance-brazing Parameters

Assuming that electrode material is selected as explained above, right conditions should include proper attention to the following factors:

  • Cleanliness of all surfaces in the joint including electrodes
  • Correct fit between elements
  • Uniform heating up to and at brazing temperature
  • Avoidance of overheating
  • Holding at brazing temperature no longer than necessary
  • Maintaining required pressure until the end of brazing cycle
The contact area between workpiece and electrodes should preferably equal the braze area, but never be less than half of it.

Preparation for Resistance-brazing

First one has to clean all the surfaces, including electrodes, with the most effective means, depending on the types of possible surface contaminants.

Cleaning is critical to obtaining acceptable brazed joints. If brazing cannot be performed within a few hours after cleaning, the assemblies must be dismounted and all cleaning steps repeated anew.

Brazing flux, intended only to remove light oxidation, cannot make up for inadequate cleaning.

A suitable brazing flux in easily dispensed form should be distributed on the surfaces of the joint to be brazed. A close fit is required for acceptable results.

Alignment and fit-up can be enabled by suitable fixtures, if necessary.

Resistance-brazing Filler Metals

Filler metal as shims or foil should be carefully preplaced between the abutting surfaces of the joint.

The type of filler metal should be thoughtfully selected depending on the base materials and on the functions the brazed joint must accomplish.

In general the materials with the lowest brazing temperature should be preferred, typically based on silver.

Also copper-phosphorus alloys can be used for Resistance-brazing of Copper and copper alloys. For aluminum alloys, filler metals of the aluminum-silicon type but their melting range risks to overlap that of certain base metals.

Operation

Resistance-brazing can be performed in manual mode or in various degrees of automated modes, depending on configuration and production requirements.

In manual operation the operator is responsible for assembly and alignment, and controls the process variables, current, pressure and time.

In automated modes the assembly preparation can be manual or automated. Once the Resistance-brazing cycle is started, the process variables are rigidly fixed, possibly with feedback control.

At the end of the brazing cycle, pressure must be maintained until the joint cools down, either naturally or by some quenching means (air or water).

Then the brazed assembly must be taken out of the fixture, if any, and all flux residues must be removed in hot water or by other means.

Visual inspection will follow to ascertain Resistance-brazing correct performance. Additional nondestructive testing operations may be needed depending on how critical are service conditions.

See also the following document:

ANSI/AWS C3.9/C3.9M:2009
Specification for Resistance Brazing
American Welding Society / 30-Jul-2008 / 26 pages

A special Issue of Practical Welding Letter, the Mid June Bulletin No.74, full of Resources on Resistance Welding was published on June 14, 2012.
Click on PWL#106B to see it.

An Article on Cooling a Group of Resistance Spot Welders was published (3) in Issue 131 of Practical Welding Letter for July 2014.
Click on PWL#131 to see it.

An Article on Follow-up on VFAW was published (2) in Issue 151 of Practical Welding Letter for March 2016.
Click on PWL#151.

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

To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.

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Watch the following Video on

Manual Resistance Brazing Equipment

http://www.youtube.com/watch?v=_qe2MRUEEmw


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