10-08-2012, 12:06 PM
Casting Defects and Design Issues
castdefect.pdf (Size: 1.4 MB / Downloads: 285)
Porosity
Types
–due to gases – smooth bubbles
–due to shrinkage – rough voids
•Not a problem for ingots
–parts that will be deformation processed
–as long as it is not exposed to air (corrosion)
–can be healed
Porosity due to Gases
Smooth bubbles
–result from entrapped gases
–solubility in liquid is high, in solid is low, so gas is rejected during cooling
•Sievert’s law
S = kpg0.5
–S = solubility
–k = constant
–pg = partial pressure of gas over melt
Remedies for Gas Bubbles
•Control atmosphere
–vacuum
–gases with less solubility
•Proper venting to let gases out
•Proper design of runners and gates to avoid turbulence
Porosity due to Shrinkage
•Rough bubbles - voids
•Stages
–cooling liquid
–rejects latent heat at melting point
•alloys become slushy - liquid and solid co-exist
–cooling solid
Porosity / Shrinkage Solutions
•Risers allow molten metal to flow into mold to make up for shrinkage
•Design flow so no part freezes early
–large channels
•“Flexible” molds
–allow metal to shrink, not hold metal
Design Rules Summary
•Uniform wall thickness
•Flat parting lines
•Gradual thickness transitions
•Draft for removal
–tapers: 0.5 to 2 degrees
•Surface of mold gives surface of part