1. Surface contamination
2. Base metal and brazing filler metal (BFM) constituents
3. Brazing methods/temperatures used
4. Poor joint fit up
Let's now look a bit more closely at the fourth source of these voids in brazed joints – poor joint fit ups – and then add a brief review and summary to this topic.
Avoiding/suppressing voids in brazed joints:
1. Be sure all surfaces of parts to be brazed are very clean and free from any lubricants, oils, greases or oxides that might outgas during heating.
2. When flame brazing or induction brazing, do not overheat the brazed joint or the BFM. Practice will enable the brazer to uniformly melt and flow the BFM throughout the brazed joint with a minimum of gas bubbles.
3. When furnace brazing:
a) Atmosphere continuous belt furnace – Carefully control furnace temperatures and belt speeds so that the assemblies do not overheat during their passage through the furnace. This can be determined by placing thermocouples on a set of parts going through the furnace and then examining cross-section photomicrographs of brazed joints. Modify belt speed and furnace set temperatures until cross sections show complete BFM flow with a minimum of gas bubbles in the joint.
b) Vacuum furnaces – Monitor as shown above using thermocouples, but it is also very important that the level of vacuum is such that it will not cause metallic elements to vaporize. As pressure levels get less and less in a vacuum furnace, the temperatures at which metals vaporize gets lower and lower. Standard "vapor-pressure charts" show this. It may be necessary to braze in a partial pressure of inert gas in the vacuum furnace, achieved by backfilling the vacuum furnace during the brazing process. This process, performed by many brazing companies, involves backfilling a vacuum furnace to a pressure of approximately 100 microns or more, using argon or nitrogen in order to "suppress" the outgassing of any metallic elements in the base metals or BFMs involved.
Concluding ThoughtsWhen examining voids in a braze joint via cross-sectional micrographs:
1. Gas-bubble voids usually tend to have rounded edges, and the inside of the voids typically appear clear and shiny.
2. Voids resulting from surface contamination can have very differing shapes and edges. The inside surfaces of the voids can be discolored and may show presence of residues or surface irregularities as compared to properly brazed surfaces. Microprobe analysis of the inside surfaces of a void can sometime pinpoint the elements present in the void to assist in determining its cause.
3. Voids resulting from poor gap clearances usually show measurable differences in the distance between the faying surfaces as compared to that of properly brazed joints in the same assembly.
4. To prevent gas-bubble voids, be sure parts are kept very clean prior to and during brazing, and be sure temperatures used for brazing are not excessive. In vacuum-furnace brazing, this may also necessitate the use of a backfilling gas to build up a partial pressure in the vacuum furnace.