Shaping Processes Flashcards
What are the reasons for choosing casting
- It is more practical to fix the alloy composition in the liquid state before solidification
- Complex Shapes
- Use of specialist materials
What is a grain
A small crystal comprising of a number of atoms.
What are the three zones in a casting grain
- Chill Zone
- Columnar Zone
- Equiaxial Zone
Explain the grain structure at each of the casting zones
Chill zone is where the liquid cools rapidly. Nucleation starts here due to the undercooling resulting in small grains. In the columnar zone the preferentially orientated grains grow outwards. Nucleation no longer occurs. At the equiaxial zone heterogeneous nucleation occurs with random orientations. However nucleation rate is still small so grain size is larger than the chill zone.
What is the benefit of a smaller grain size
A smaller grain size achieves a more uniform distribution of alloying elements so the yield stress is consistent across the part
What are inoculants and how are they used
Inoculants provides sites for heterogeneous nucleation. They take the form of a dispersion of fine solid particles added to the melt.
How does segregation occur
Following the phase diagram implies that on solidification the equilibrium composition is equal to that of the alloy, however this is not true. In practice the diffusion of the solute into the solid is too slow an a concentration gradient is locked in.
What are the two types of segregation
Micro-segregation is across grain boundaries
Macro-segregation is across the whole casting
What are the consequences of segregation
Non uniform distribution of alloying elements so a variation is yield stress
Non uniform distribution of impurities that could form solid phases which reduces toughness
How can we avoid segregation
Reduce grain size (inoculants)
Add alloying additions that react with the impurities forming a fine dispersion
Homogenise the casting to redistribute the solute
How can we achieve fine grain castings?
- Grain boundary hardening
- Inoculants
