Question:medium
Which ONE of the following is FALSE with respect to filler material used in brazing?
Which ONE of the following is FALSE with respect to filler material used in brazing?
Show Hint
Brazing depends on capillary action, which needs HIGH wettability and low viscosity to pull filler into the joint gap.
Updated On: Aug 5, 2026
- Melting temperature of the filler material must be less than the base metal.
- Wettability of the base metal by the filler material must be low.
- Viscosity of the filler material must be low for penetration into the interface.
- Chemical reactivity between the filler material and the base metal must be low.
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Set up the capillary flow condition.
Filler metal rises into the gap between two plates mainly through capillary action, and the height it can reach follows a simple physical rule.
$h = \frac{2 \gamma \cos\theta}{\rho g r}$
Here $\gamma$ is the surface tension of the molten filler, $\theta$ is the contact angle (wetting angle), $\rho$ is density, and $r$ is the gap width.
Step 2: Link this formula to wettability.
Good wetting means a small contact angle $\theta$, which makes $\cos\theta$ large and pushes the capillary rise $h$ higher.
So low wettability (large $\theta$, small $\cos\theta$) would give poor or no capillary flow into the joint, which defeats the whole point of brazing.
Step 3: Match each option against known brazing needs.
$\text{Melting point of filler} < \text{melting point of base metal (needed, matches A)}$
$\text{Wettability should be HIGH, not low (contradicts B)}$
$\text{Low viscosity helps flow into thin gaps (matches C)}$
$\text{Low reactivity avoids brittle joints (matches D)}$
Step 4: Pick the false statement.
Since brazing needs high wettability for capillary flow to work, the statement claiming it must be low goes against the actual requirement.
That makes option (B) the false one, while (A), (C) and (D) all describe genuine filler metal needs.
Final Answer:
Wettability must be high, not low, so option (B) is the false statement. \[ \boxed{\text{Option (B)}} \]
Filler metal rises into the gap between two plates mainly through capillary action, and the height it can reach follows a simple physical rule.
$h = \frac{2 \gamma \cos\theta}{\rho g r}$
Here $\gamma$ is the surface tension of the molten filler, $\theta$ is the contact angle (wetting angle), $\rho$ is density, and $r$ is the gap width.
Step 2: Link this formula to wettability.
Good wetting means a small contact angle $\theta$, which makes $\cos\theta$ large and pushes the capillary rise $h$ higher.
So low wettability (large $\theta$, small $\cos\theta$) would give poor or no capillary flow into the joint, which defeats the whole point of brazing.
Step 3: Match each option against known brazing needs.
$\text{Melting point of filler} < \text{melting point of base metal (needed, matches A)}$
$\text{Wettability should be HIGH, not low (contradicts B)}$
$\text{Low viscosity helps flow into thin gaps (matches C)}$
$\text{Low reactivity avoids brittle joints (matches D)}$
Step 4: Pick the false statement.
Since brazing needs high wettability for capillary flow to work, the statement claiming it must be low goes against the actual requirement.
That makes option (B) the false one, while (A), (C) and (D) all describe genuine filler metal needs.
Final Answer:
Wettability must be high, not low, so option (B) is the false statement. \[ \boxed{\text{Option (B)}} \]
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