Good Silver Brazing Practice
Brazed joints should be designed with an appropriate capillary gap into which the filler metal can flow when it is molten. When joining different materials consideration should be made for their different rates of expansion which could cause the joint to open up too wide or become too narrow at brazing temperature. This factor is important because the strength of a joint will vary with both the joint gap and the degree of overlap between parent metals. The optimum joint gap at brazing temperature varies according to the brazing alloy being used and so the calculated joint gap should be matched with this figure.
- For lap joints use an overlap length of 3-4 x the thickness of the thinnest parent metal part in the joint.
- For tubular parts of sizes up to 25mm in diameter the joint overlap should be equal to one tube diameter.
Brazing alloys should be chosen based on their suitability for a particular application. Consideration should be given to a number of factors:
- The design of the component to be brazed
- The parent metals being joined
- The environment the joint needs to function in
- The brazing process:– heating method; degree of automation
- The economics of the job.
In general the brazing alloys that are easiest to use are the high silver, free flowing filler metals with low melting ranges. Filler metal alloys with higher brazing temperatures and longer melting ranges are more difficult to use and result in bigger fillets, longer heating cycles and more difficult to remove flux residues and heat stain.
- Components that are to be brazed or soldered should be free from grease and any surface oxide prior to brazing.
- Grease and oil such as engineering lubricants need to be removed using a degreasing solvent.
- In some cases hot, soapy water may also be effective.
- Surface oxide can be removed with a medium grade abrasive cloth. This will promote good bonding.
Note: Brazing fluxes do not clean components and cannot remove grease, oil or other contaminants.
For a silver brazing process in air a flux is necessary to prevent oxidation of the components during heating to brazing temperature. The choice of flux is as important as the choice of the brazing alloy and care should be taken to ensure that the working range of the flux is compatible with the brazing temperature.
Brazing flux must have suitable life at temperature and should be suitable for both the heating method and the parent metals. Ideally flux should be applied to both components before assembly and prior to heating. The flux must melt and become active below the melting point of the brazing alloy and it must stay active right through the brazing cycle removing the oxides formed on the parent metals.
If the flux residues appear blackened and glassy the flux has probably been exhausted during heating. This could be because either insufficient flux has been used or the components have been overheated. Alternatively a flux with greater time and temperature stability should be used.
There are several options for a heat source for silver brazing in air which include: hand torch; fixed burner; induction and resistance heating. Torch or fixed burners can use different fuel gas combinations are available from oxy-acetylene, oxy-propane, through to air / natural gas.
Follow these steps when heating a component for silver brazing:
- For brazing it is essential that the parent metals be heated evenly to the correct temperature.
- At the correct temperature for brazing the flux should be clear and flow over the joint area.
- When brazing copper with an alloy from the Meta-Braze™ silver-copper-phos range it should be glowing a dull cherry red indicating the correct temperature has been reached.
- Brazing rod, strip or wire should then be applied to the joint opening. If brazing the correct temperature has been reached the brazing alloy will melt and flow.
- If the brazing alloy does not flow more heat should be applied
- Free flowing filler metal alloys will flow into and around the joint by capillary attraction.
- A sluggish flowing filler metal alloy should be applied along the entire joint, building up a fillet.
- Molten brazing filler metal always flows to the hottest part of the joint
- Heating should continue as the filler metal flows into the joint. For torch brazing indirect heating should be used.
- Once brazing is complete heating should be discontinued.
It is important to select an appropriate cooling regime. In some cases when the silver brazing alloy has solidified the joint can be water quenched and this will help remove flux residues. However this should not be done on materials where there is a mis-match of contraction rates or where thermal shock might cause cracking, for example on tungsten carbide to steel joints. During cooling care must be taken not to damage the component or cause cracking due to thermal shock.
It is important to remove flux residues after brazing. Specific details about how to remove flux residues can be found on the datasheet for each flux.