Lesson 8.1: Metal Casting (Moulding, Solidification, Casting Defects)

Metal casting is a fundamental manufacturing process. GATE and PSU exams often test casting methods, solidification principles, and defect analysis.

🔹 1. Introduction

• Definition: Metal casting is the process of pouring molten metal into a mould to obtain desired shape.

• Applications: Engine components, turbine blades, machine parts, pumps, valves

• Key Concepts: Moulding, solidification, cooling rate, defects, casting metals

🔹 2. Moulding Process

• Steps:

  1. Pattern making (wood, metal, plastic)

  2. Mould preparation (sand, investment, permanent moulds)

  3. Core making (for internal cavities)

  4. Pouring molten metal

  5. Cooling and solidification

  6. Shakeout and finishing

• Types of Moulding:

  • Sand casting

  • Die casting

  • Investment casting

  • Shell moulding

  • Permanent mould casting

🔹 3. Solidification

• Process: Molten metal cools and forms a solid

• Nucleation & Growth: Determines grain structure

• Cooling Rate:

  • Rapid cooling → Fine grains → High strength

  • Slow cooling → Coarse grains → Lower strength

• Directional Solidification: Helps prevent shrinkage defects

🔹 4. Casting Defects

• Common Defects:

  1. Porosity: Gas entrapment or shrinkage cavities

  2. Cold Shut: Incomplete fusion of metal streams

  3. Misrun: Molten metal solidifies before filling mould

  4. Hot Tears / Cracks: Thermal stresses during cooling

  5. Sand Inclusion: Foreign particles embedded in casting

• Prevention: Proper mould design, controlled pouring, adequate venting, correct cooling

🔹 5. Solved Examples (PYQ Style)

1. Identify suitable casting method for a turbine blade

2. Calculate solidification time using Chvorinov’s rule

3. Analyze causes of porosity in sand casting

4. Suggest solutions to prevent cold shuts and misruns

🔹 6. Practice Exercises

1. Draw steps of sand casting process

2. Calculate solidification time for given casting dimensions

3. List defects and remedies in die casting

4. Explain effect of cooling rate on microstructure

5. Compare different casting methods with applications

🔹 7. Summary

• Metal Casting: Molten metal poured into mould to form shape

• Moulding Types: Sand, die, investment, shell, permanent

• Solidification: Grain formation, cooling rate, directional solidification

• Defects: Porosity, cold shut, misrun, cracks; prevention methods

• Exam Tip: Focus on casting methods, defects, and solidification calculations