What design feature supports capillary flow in larger-diameter brazed joints?

• A. Thick outer shells without internal features.
• B. External coatings.
• C. Eliminating any shoulders to create a straight bore.
• D. Properly prepared internal shoulders, grooves, or features that create consistent capillary paths.

Answer: (D)

Capillary flow relies on small, well-defined gaps that draw molten filler metal into the joint. In larger-diameter brazed joints, you need internal features that create narrow, continuous capillary paths. Properly prepared internal shoulders, grooves, or similar features establish consistent gaps throughout the joint, guiding the filler metal and promoting an even, complete fill. External coatings or thick outer shells don’t provide those internal capillary channels, and removing internal shoulders to make a straight bore eliminates the very geometry that drives capillary action.