Question

The primary mechanism that draws the molten filler metal into the narrow gap between the workpieces during brazing is

• A. gravity flow
• B. surface tension
• C. capillary action
• D. viscosity gradient

Hint:

Capillary action requires:
- Good wetting (Contact angle \( \theta \rightarrow 0 \)).
- Tight, uniform joint clearances (typically \( 0.025\text{ to }0.1\text{ mm} \)).
- Clean, oxide-free surface interfaces (achieved by using flux).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question asks for the principal physical mechanism that facilitates the flow and distribution of liquid filler metal into the tight clearance joint of a brazed assembly.

Step 2: Key Formula or Approach:
The driving pressure \( \Delta P \) that pulls a wetting liquid into a narrow gap of width \( d \) is governed by the capillary pressure equation:
\[ \Delta P = \frac{2 \gamma \cos\theta}{d} \]
where:
\( \gamma \) is the surface tension of the liquid filler metal.
\( \theta \) is the contact angle (wetting angle) between the liquid filler and the solid base metal.
\( d \) is the joint clearance.

Step 3: Detailed Explanation:

• Role of Capillary Action:
- In brazing, the base metals are not melted. Instead, they are heated, and a filler metal with a melting point above \( 450^{\circ}\text{C} \) is introduced.
- If the liquid filler metal wets the solid base metal (i.e., the contact angle \( \theta \lt 90^{\circ} \)), the surface energy balance drives the liquid to spontaneously climb and spread into the narrow joint clearance.
- This fluid flow mechanism is known as capillary action.

• Optimizing Joint Clearance:
- If the joint clearance \( d \) is too wide, the capillary pressure becomes too low, and the filler will not fill the gap fully.
- If the clearance is too tight, the flow is restricted by viscous resistance. Typical optimal clearances range from \( 0.025-0.1\text{ mm} \).
- Gravity flow and viscosity gradients do not provide the primary driving forces for this flow.


Step 4: Final Answer:
Capillary action is the fundamental physical mechanism that pulls the molten filler metal into the narrow joint gap during brazing.
Therefore, the correct choice is option (C).