Principles of Solidification and Casting Flashcards
Advantages and Disadvantages of Sandcasting
+ves
Low-cost moulds
High Tm alloys can be cast
Economical for small and large production
Fast production, automation if large scale
-ves
poor dimensional accuracy due to phase change
poor surface finish, post processing required
sand mould can deform
labour intensive/skilled
Advantages and Disadvantages of shell moulding
+ves
Highley repeatable
less post-processing compared to SC
less sand used (saving cost)
thin section
-ves
higher production cost than SC
pattern cost is higher
limited mass and size
Advantages and Disadvantages of gravity die casting
+ves
close dimensional tolerances
superior surface finish
fast cooling rates = fine microstructure
fast cycle time
-ves
only low Tm alloys
high tooling costs of die
coatings required to protect die
only simple shapes
porosity
Advantages and Disadvantages of pressure die casting
+ves
high precision
very smooth surface finish
very thin sections
high production rates, fast production
complex shapes
-ves
cannot post-process, ruins surface finish
high forces to keep die closed
high tooling cost of dies
long lead times, to make dies
low Tm alloys
porosity
Advantages and Disadvantages of investment casting
+ves
very complex shapes, re-entrant angles/cavities
high dimensional tolerance
highly reliable
wax is easy to shape
-ves
long lead times
long production times
mould is not reusable
subject to turbulent flows
What are the features of a good nucleating agent
have one crystal dimension similar to the solid being nucleated
must be wetted by the liquid
small spherical cap and hence small contact angle
why is a small spherical cap beneficial in nucleation
a small spherical cap results in less atoms being present, hence less undercooling is required for nucleation.
what are the 2 types of porosity found in castings
microporosity - from air that is trapped in the mould before casting, liquid metal can also contain dissolved gasses
macroporosity - caused by the underfilling of a mould, inadequate supply of liquid to make up for volumetric contraction
how can we reduce microporosity
bubble a scavenger gas through the liquid metal to remove dissolved gases
flush moulds so that no air or inclusions are is trapped within them before pouring cast
make sure everything is dry, water vapour leads to hydrogen absorption
how do gas bubbles nucleate
poorly wet particles (inclusions) are likely to nucleate gas bubbles
Describe the shape of the growth front and its affect on the metal
the growth front is always planar due, it dictates the microstructure and thus the properties of the cast material
what are the 3 types of growth and under what conditions do they take place
PLANAR - no undercooling, slow cooling
CELL - some undercooling
DENDRITIC - lots of undercooling, fast cooling
What affect does dendritic growth have on the material
dendritic channels prevent the mould from being filled sufficiently, leading to shrinkage, macroporosity and cast defects
What determines the rate of solidification
heat transfer - more heat transferred out of a mould the quicker it can solidify.
what are the key features of permanent moulds
high tooling costs
takes long time to manufacture mould (lead time)
economical as can be reused
higher quality surface finish compared to sand
good for high production rate
