Identify some of the important advantages of shape-casting processes
- Can create very complex geometries, including both internal and external shapes
- Some casting processes are net shape, others are near net shape
- Can produce very large parts
- Some casting methods well suited for mass production
- Can be performed on any metal heated to a liquid stat
What are some limitations / disadvantages of casting
- Poor dimensional accuracy
- Poor surface finish
- limitations on mechanical properties
- shrinkage due to cooling
- safety hazards and environmental hazards present
What is a factory that performs casting operations usually called
What is the difference between an open mold and a closed mold
Open Mold: liquid metal is simply poured into the cavity of the mold
Closed Mold: Passageway, called gating system, provided to permit molten metal to flow from outside the mold into the cavity
Name the two basic mold types that distinguish casting processes
Expendable Mold Casting
Permanent Mold Casting
Which casting process is the most important commercially
What is the difference between a pattern and a core in sand molding/
Pattern: mold cavity is formed using a pattern. Typically made of wood, metal, plastic, or other materials and has the shape of the part to be cast. Cavity formed by packing sand around the pattern in both the cope and drag sections of the mold
Core: A form placed inside the mold cavity to define interior gemoetry
What is meant by superheated?
Amount of heat that must be removed from molten metal between pouring and when solidification commences
Superheat refers to temeprature difference between pouring temperature of the molten metal and the temperature of the molten metal when solidification commences.
Why should turbulent flow of molten metal be avoided?
Turbulence in fluid flow is characterized by erratic variations in the magnitude and direction of the velocity throughout the fluid
Tends to accelerate the formation of metal oxides that can become entrapped during solidification, thus degrading quality of casting. Can also aggravate mold erosion. gradual wearing away of mold surfaces due to impact of flowing mold cavity
What is the continuity law as it applies to the flow of molten metal in the casting?
The volume rate of flow remains constant throughout the liquid
Volume flow rate is equal to the velocity multiplied by the cross-sectional area of the flowing liquid
What does heat fusion mean in casting
The amount of heat required to solidify metal from liquid state
How does solidification of alloys differ from solidification of pure metals
Solidify at a constant temperature equal to its freezing point, which is the same as its melting point
Freeze over a temperature range rather than a single temperature. Exact range depends on the alloy system and the particular composition.
What is eutectic alloy?
Constitute exception to general process by which alloys solidify
Particlular composition in an alloy system by which the solidus and liquidus are at the same temp
solidification occurs at a constant temperature called eutectic temp
What is Chvorinov's rule in casting?
Total solidification time required for casting to solidify after pouring.
Total solidification time = mold constant ( volume / area ) ^ (n =2)
Identify the three sources of contraction in a metal after pouring
(1) liquid contraction during pouring
(2) contraction during the phase change from solid to liquid, solidification shrinkage
(3) thermal contraction of the solidified casting during cooling to room temperature
what is a chill in casting?
Internal or external heat sinks that cause rapid freezing in certain regions of the casting