Why do voids appear in brazed joints?

Voids are not uncommon in brazing but 4 common sources of issues are:

• Surface Contamination

• Zinc, cadmium and lead

• Braze Cycle and Temperature Issues

• Poor Joint Design

Surface Contamination

It's easy to be flippant about this and just assume that the furnace will burn these out. However any oils, dirt etc left on the surface of the parts often contain constituents that can volatize and gas out during the process.

These gasses will try to find a route out, the contaminants on the outside of the braze parts will therefore often burn out and the gas will escape harmlessly into the furnace atmosphere. However the gas formed from the contaminants on the inside of the parts can get trapped and will present themselves as bubbles inside the braze joint as it solidifies.

The other issue of course is that not all contaminants will turn into a gas but will remain on the surface. This can prevent your BFM from flowing over this part of the join and creating another void.

Its not very exciting and possibly insulting to remind experienced braze operators that all parts must be thoroughly cleaned prior to brazing and then parts must be handled with gloved hands to prevent contamination.

Zinc, Cadmium and Lead

There are some elements that should never be used in a vacuum furnace.

Zinc and cadmium are used in some BFMs to lower brazing temperatures and increase flow rates. These two metals are also found in some platings of braze parts. Lead can be found in some steels or brasses, its added to enhance machinability of the metals.

However all three of these metals will quickly outgas. These can not only leave voids in your joint but can also leave holes in the base metal creating potential for leaks.

None of these metals should be used in a vacuum furnace, the fumes will contaminate the furnace and the pumps. If you are torch or induction brazing these parts ensure you allow for adequate ventilation as the fumes will be noxious!

Braze Cycle and Temperature Issues

Clearly temperature and cycle control is easier on a braze furnace where it can be programmed. It is less easy to control on torch brazing and the temptation for an operator to turn up the heat to "speed up" the process a little.

The problem about over-heating (in either a furnace or flame process) is that in overheating the joint can result in an excess of outgassing taking place and the issues mentioned above happening.

It can be useful when working with a new process to use thermo-monitor devices in the furnace or on the belt with the parts and then examine your join afterwards. Some parts may require an inert gas to suppress outgassing (this can also carry some risks - see the post on boron reacting with nitrogen!)

Poor Joint Design

In the ideal world all gaps will be consistent and the part will be perfectly centred and not allowed to move. However we appreciate that in the real world tolerances can be less than desireable and gaps can often be wider than wanted.

If the design is poor then when the metals heat they will expand and contract at different rates - there are likely to be temperature differentials at various parts of the cycle - this can allow for movement in the parts.

Thin brazed joints will obviously also solidify more quickly than wider joints. As a brazed part cools you can find that the thinner joint "locks" before the wider joint, as the wider joint cools the liquid can not draw the joint together as it shrinks - this has the capacity to create voids in the wider joint.

In summary the issues highlighted above can cause voids and other instances where you may think "why is the filler not flowing properly?".

Gas bubbles are relatively easy to recognise as the tend to have round edges and appear clear and shiny on the inside.

Voids from contaminants are more likely to be varied in shape, size and edge. The inside of the voids are likely to be discoloured and show presence of residues etc.

Voids from poor gap clearance will show differences in the distances between the edges of the metal surfaces compared to proper joints on the same assembly.