Casting Flashcards
(30 cards)
Freezing range has an inversely proportional relationship with what?
Fluidity
What assumptions can be made with a shiny surface finish
A really shiny surface finish is likely not done from casting alone, meaning post processing took place
Investment vs shell casting
• Investment has a better surface finish
• Investment has a finer slurry (finer slurry means a thin mixture that can fit smaller crevices but also can make a more compact and smoother surface finish)
Lost Wax
• faster cooldown gives a finer grain structure
• finger grain structure usually leads to higher strength and ductility
What are the two routes you can go with regarding due casting?
Hot chamber or cold chamber
Define cold chamber and give some elements around it
• when the furnace is not directly integrated into the system.
• best suited for metals with higher melting points
- higher melting points has a direct correlation to higher strength and higher viscosity
- due to higher viscosity, higher pressure will be required of the press to push molten metal through
•faster cooling rate ~> smaller grains ~> higher strength
Define and describe some elements surrounding hot chamber
• furnace is directly integrated into the system
• best suited for metals with lower melting points
• metals will be quite fluid due to lower melting points
- less pressure will be required for the press
- molten metal will move easier
• slower cooling rate than cold chamber
- slow cooling rate ~> bigger grains ~> lower strength
• slow cooling rate due to the integrated furnace, thus the whole system is at a higher temperature
Expendable mold techniques
• Sand
• Ceramic
• Investment
• Plaster
• Evaporative pattern
• Shell
(S~C~I~P~E~S)
Permanent mold techniques
• Slush
• Squeeze
• Semisolid
• Pressure
• Die
• Centrifugal
(S~S~S~P~D~C)
What is the purpose of a riser in normal gravity casting
They are basically resovoirs for hot metal that helps to supply more metal into the casting during the cooling process to avoid air bubbles or shrinkage in the mold
Important considerations in casting
• solidification and cooling of metal in mold
• flow of the molten metal into the mold cavity, and design of gating systems or pathways for molten metal to fill the cavity
• influence of the mold material
Is a high or low freezing range desirable?
Low freezing range
Low melting temperature alloys are easier to work with
Less input energy to melt metal. Less pressure required, like in the case of hot chamber casting (check those reasons)
What are nucleating agents?
Chemical compounds added into hot metal in order to promote nucleation in cooling
Aspects of fluidity:
• viscosity
• surface tension
• inclusions (insoluble solid particles entrapped in the fluid)
• solidification pattern of the alloy
• mold design
• mold material and surface characteristic
• degree of superheat (difference between solidification temperature and pouring temperature
• rate of pouring
• heat transfer
What kind of objects are used in slush casting?
Hollow, ornamental and decorative objects
What is chronov’s rule?
Solidification time is a function of the volume of a casting and its surface area
How can dendrites contribute to cooling and be used to strengthen metals for after the cooling process is complete?
Dendrite arms are not particularly strong and can be broken up by agitation or mechanical vibration in the early stages of solidification (also known as semisolid metal forming and rheocasting). This process results in finer grain size, with equipped nondendritic grains distributed more uniformly throughout the casting.
What is thixotropic behavior of alloys?
Viscosity decreases when the Liquid Metal is agitated
Why are sprue’s tapered?
In free falling liquid, the cross sectional area or the stream decreases as the liquid gains velocity downward. If sprues had a constant cross section, there will be regions where the liquid loses contact with the sprue walls, resulting in ASPIRATION (a process where air is drawn in or entrapped in the liquid).
Why are low shrinkage alloys most helpful during the casting process?
Why does shrinkage occur?
During cooling, metallic bonds shorten in length when the energy in the structure is lost due to heat losses and cooling
Categories of casting defects
A - metallic projections (flash, swell, roughness)
B - cavities (porosity, shrinkage, etc)
C - discontinuities (cracks, hot/cold tearing)
D - defective surface (folds, scars, oxide scale)
E - incomplete casting (misrun)
F - incorrect dimensions and shape
G inclusions
Why do hot tears occur?
Casting cannot shrink freely during cooling because of constraints in various parts of the molds and cores. Exothermic compounds may be used (like exothermic padding) to control cooling at critical sections to avoid hot tearing
