Casting of Super Alloys

Question 1

What are the 3 types of casting for super alloys?

Answer 1

Conventional Casting

Directionally Solidified (DC) Casting

Single Crystal (SC) Casting

Question 2

Describe conventional casting.

Answer 2

Uses a ceramic mould, solidification begins at the mould walls and the grain structure is formed by many nucleation sites starting grain growth. The growth continues until growing grains impinge on each other.

Question 3

Describe directionally solidified (DC) casting.

Answer 3

The mould is heated and supported on a water-cooled chill plate. The mould is then slowly lowered out of the furnace. Grains nucleate at the chilled end and grow upward, eliminating transverse boundaries. The resulting casting is then stronger in the direction of the centrifugal forces.

Question 4

Describe single crystal (SC) casting.

Answer 4

The mould is similar to DS casting but has a constriction (corkscrew or pigs tail) which is designed to let only the most favourably oriented grain grow through. All others are intercepted at the walls of the constriction. Again the mould is slowly lowered to prevent other grains nucleating at the mould walls. The lack of grain boundaries in these castings makes them more resistant to CREEP and THERMAL SHOCK.

Question 5

What is the casting process used for?

Answer 5

Fabrication of turbine blades.