1. Surface contamination
2. Base metal and brazing filler-metal (BFM) constituents
3. Brazing methods/temperatures used
4. Poor joint fitup
Let's briefly look a little more closely at the third source of these voids in brazed joints, brazing methods/temperatures used. In the next posting, we'll look at the last item.
3. Brazing Methods/Temperatures UsedIt is very important to control the brazing temperature and time as much as possible to minimize outgassing of metal constituents that can form bubbles in a brazed joint. It is not difficult to control the temperatures of furnace brazing since it can be programmed to within a few degrees of desired temperature. It is usually more difficult to control actual temperatures involved in flame brazing than for furnace brazing.
When brazing temperatures are allowed to go too high, there is a strong thermodynamic driving force to move the metal from solid to liquid to gas. I've frequently witnessed flame-brazing operations where the flame setting used by the brazer is too intense, resulting in overheating of the joint and outgassing of BFM constituents as he/she tries to "speed up the process" to get more production done. Although that person may be brazing more parts per hour, etc., the quality of such joints is open to question! Gases formed from such overheating can result in excess fumes in the breathing zone as well as lots of gas bubbles (voids) in a brazed joint.
Proper training and practice is essential to be able to bring the brazing temperature to a point where the BFM will melt and flow throughout the brazed joint but not be so high as to cause the liquid BFM to turn to a gas, resulting in imperfect joints.