Solidification defects Flashcards
What is the definition of a defect?
A discontinuity found in the volume or surface of a part, that (negatively) affects its strength due to local stress consentration generated close to the defect itself.
What rules the solidification structure in large ingots and similar products?
Thermal exchange conditions and by segregation phenomena. It depends on distance from surface.
What are the three regions in an ingot cast?
- Skin zone (or chill zone), made up of small equiaxed grains
- columnar dendritic region
- equiaxed central region
What is an inclusion?
Any material that is trapped inside a mineral during its formation.
What is the difference between endogenous incusions and exogenuos inclusions?
- Endogenous: results from chemical reactions between the molten steel and reactants contained in the slag ot injected in the melt
- Exogenous: particles og external substances accidentally trapped into the melt through erosion of refractory walls, casting gates, etc
How does cold shots form?
When a small portion of liquid metal comes into contact with the surface of the die and rapidly solidifies.
What is a blister?
A blister consists of a small surface area that was blown up owing to pressure
developed in sub-surface regions. When gas-related internal pressure is high enough and the constraint given by the die
is removed, then the surface area is free to expand and produces blisters
What are sinks?
Sinks are similar to blisters, but they refer to situations where a negative pressure is acting in sub-surface regions of a casting.
A sink is related to the presence of a local sub-surface shrinkage cavity. It occurs when a solidification hot spot localizes close to the metal/die interface
What are joints?
Joints are formed when a metal flow portion, at least partially solidified (covered with an oxide film), meets another hot metal vein that can flow around it. The presence of different microstructures corresponding to the different flows becomes visible.
What is a vortex?
When a partially solidified metal region is subjected to a spiral flow drawn by mould filling, it results in a particular spiral distribution of oxide film and/or microstructures
What are ejection marks?
Defects caused by the presence of an undercut in the mould, as a result of damage or of material soldering phenomena. During ejection phase of the casting, interference generate scratches, material losses or buildup of accumulated material at edges.
What are thermal fatigue (solidification defect)?
Small cracks formex on edges or on die surfaces due to the repetition of the stress-strain cycles induced by the rapid heating and cooling stages. The liquid metal infiltrates into cracks during casting and produces surface relieves on the part
How do gas porosity happen?
Gases can be dissolved into molten metals. On solidification the solubility of the gas abruptly decreases, gas atoms are rejected into the liquid originating bubbles that may be trapped by the solidification front before floating toward the surface, leading to gas porosity.
What is shrinkage porosity?
Due to volume contraction, the pressure in the liquid melt may fall and become negative with respect to external pressure, providing the driving force to nucleate voids (or surface sinks).
What are hot tears?
Tension stresses generated into the cast component due to cooling can cause cracking or hot tears when material ductility is not sufficient
often due to geometrical features
