Question

Alloy A has a narrow freezing range as compared to Alloy B. Considering both alloys solidify under identical conditions, which statement is correct regarding feeding?

• A. A requires more riser volume than B
• B. B is more prone to micro shrinkage
• C. A shows dendritic feeding difficulty
• D. B shows directional solidification

Hint:

Freezing Range Rules:
- Narrow Freezing Range \(\rightarrow\) Planar front solidification \(\rightarrow\) Macro-shrinkage (piping) \(\rightarrow\) Easy to feed.
- Wide Freezing Range \(\rightarrow\) Mushy zone solidification \(\rightarrow\) Micro-shrinkage (interdendritic porosity) \(\rightarrow\) Difficult to feed.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question compares the solidification and feeding behavior of two alloys with different freezing (solidification) temperature ranges.
Alloy A has a narrow freezing range (e.g., pure metals or eutectic alloys), while Alloy B has a wide freezing range (e.g., solid-solution alloys).

Step 2: Key Formula or Approach:
The freezing range of an alloy is the difference between its liquidus temperature \( T_L \) and solidus temperature \( T_S \):
\[ \Delta T_{\text{freeze}} = T_L - T_S \]
Alloys with a wide freezing range (\( \Delta T_{\text{freeze}} \) is large) solidify over a broad temperature span, creating a large, sluggish solid-liquid "mushy zone" during casting.

Step 3: Detailed Explanation:

• Solidification in Wide Freezing Range Alloys (Alloy B):
- During cooling, dendritic crystals grow extensively throughout the liquid volume.
- Because \( \Delta T_{\text{freeze}} \) is wide, this mushy zone containing solid dendrites and liquid exists for a long time.
- The growing dendrite arms interlock, creating narrow, tortuous channels that restrict the flow of liquid metal.
- This makes it highly difficult for molten metal from the riser to feed the shrinkage occurring at the roots of these dendrites.
- Consequently, this feeding difficulty leads to widespread, microscopic shrinkage cavities (interdendritic porosity or micro-shrinkage) dispersed throughout the casting.

• Solidification in Narrow Freezing Range Alloys (Alloy A):
- Alloy A solidifies with a sharp, planar front (skin-forming behavior).
- Feeding is straightforward because there is a distinct boundary between solid and liquid, allowing the riser to easily feed the shrinkage, concentrating any shrinkage cavity in the riser or as a single macro-cavity.
- Hence, Alloy B is much more prone to micro-shrinkage.


Step 4: Final Answer:
Alloy B (with the wide freezing range) is more prone to micro-shrinkage due to dendritic feeding difficulties in its large mushy zone.
Therefore, the correct choice is option (B).