Well, the parts I was involved with were for aircraft use. I was part of the quality team that assessed discrepant material for disposition. It might be used, reworked or scrapped.
Parts were typically bead blasted with glass beads then chemically cleaned with mild acid. No dirt of any kind, no dust, no oil, no finger prints allowed on the parts. I would suspect that chainsaw parts might not be cleaned as carefully.
The parts were then assembled using gravity to hold them in place, they often had pilots that fit in holes to assure alignment. Braze wire preforms or chips were inserted between the parts or right at the outside of the joint, touching both parts. A few to hundreds of parts were put on a rack and loaded into a vacuum chamber, in this case it was about 3 feet in diameter and 5 feet long. One end opened for admittance. The air was pumped out and electric heat applied to melt the braze alloy. No flux was used. From the color of the damaged part, it would appear that a bronze alloy was used in this case, the sprockets I have seen looked very similar. Sometimes silver alloys are used, there are hundreds of different alloys used depending on the base metal, temperature desired and other factors. The parts were cooled in the vacuum, perhaps over night, they came out of the chamber all shiny and clean.
Open furnaces can also be used, normally with a reducing atmosphere. The prevents oxides forming at brazing temperature. The parts are cleaned and loaded similarly but a metal conveyor carries them through the hot zone of the furnace. This process is continuous and it often used for heat treating metal parts as well as brazing.
The photo above looks like the parts were perhaps oily, the braze was applied under the points of the spur, perhaps a wire formed in a spur like shape. The dirt on the parts prevented the alloy from flowing out. As can be often seen on these assemblies, braze can flow out near the rim of the drum.
It is possible that the parts were hand brazed, that method is still used for relatively large production runs. Clutch drums are sold in large enough quantities however that hand brazing would not seem economical. Even so, it looks like the parts were not cleaned properly. Close inspection might reveal what the exact nature of the problem was. It is pretty clear that the part was defective however.
Nice call 67L36Driver.
Bad joints are probably way more common than we know, a good joint is very nearly as strong as the base metal. A poor joint might last 10 years or more, eh?
