Ch 7 Fundamentals of Metal Casting

What are some of the limitations and disadvantages of casting?

Disadvantages include: (1) limitations on mechanical strength properties; (2) porosity (3) poor dimensional accuracy (4) safety hazards due to handling of hot metals (5) environmental problems.

Identify some of the important advantages of shape-casting processes

(1) complex part geometries are possible (2) some casting operations are net shape processes, meaning that no further manufacturing operations are needed to accomplish the final part shape (3) very large parts are possible to produce (4) they are applicable to any metal that can be melted (5) some casting processes are suited to mass production.

Identify the three sources of contraction in a metal casting after pouring.

(1) contraction of molten metal after pouring (2) solidification shrinkage during transformation of state from liquid to solid (3) thermal contraction in the solid state

Name the two basic mold types that distinguish casting processes.

(1) expendable molds (2) permanent molds

What is the local solidification time?

the time required for a particular location in a casting to solidify once nucleation has begun

What is the total solidification time?

the time required for the casting to solidify completely after the casting has been poured

In foundry work, a runner is which one of the following: (a) channel in the mold leading from the downsprue to the main mold cavity, (b) foundryman who moves the molten metal to the mold, or (c) vertical channel into which molten metal is poured into the mold?

(a) channel in the mold leading from the downsprue to the main mold cavity

The upper half of a sand-casting mold is called which of the following: (a) cope or (b) drag?

(a) cope

Sand casting is which of the following types: (a) expendable mold or (b) permanent mold?

(a) expendable mold

Total solidification time is defined as which one of the following: (a) time between pouring and complete solidification, (b) time between pouring and cooling to room temperature, (c) time between solidification and cooling to room temperature, or (d) time to give up the heat of fusion?

(a) time between pouring and complete solidification

In casting, a flask is which one of the following: (a) beverage bottle for foundrymen, (b) box which holds the cope and drag, (c) container for holding liquid metal, or (d) metal which extrudes between the mold halves?

(b) box which holds the cope and drag

In a sand-casting mold, the V/A ratio of the riser should be (a) equal to, (b) greater than, or (c) smaller than the V/A ratio of the casting itself?

(b) greater than

Turbulence during pouring of the molten metal is undesirable for which of the following reasons (two best answers): (a) it causes discoloration of the mold surfaces, (b) it dissolves the binder used to hold together the sand mold, (c) it increases erosion of the mold surfaces, (d) it increases the formation of metallic oxides that can become entrapped during solidification, (e) it increases the mold filling time, and (f) it increases total solidification time?

(c) it increases erosion of the mold surfaces (d) it increases the formation of metallic oxides that can become entrapped during solidification

A riser in casting is described by which of the following (three correct answers): (a) an insert in the casting that inhibits buoyancy of the core, (b) gating system in which the sprue feeds directly into the cavity, (c) metal that is not part of the casting, (d) source of molten metal to feed the casting and compensate for shrinkage during solidification, and (e) waste metal that is usually recycled?

(c) metal that is not part of the casting (d) source of molten metal to feed the casting and compensate for shrinkage during solidification (e) waste metal that is usually recycled?

During solidification of an alloy when a mixture of solid and liquid metals is present, the solid-liquid mixture is referred to as which one of the following: (a) eutectic composition, (b) ingot segregation, (c) liquidus, (d) mushy zone, or (e) solidus?

(d) mushy zone

Chvorinov's rule states that total solidification time is proportional to which one of the following quantities: (a) (A/V)n, (b) Hf, (c) Tm, (d) V, (e) V/A, or (f) (V/A)2; where A = surface area of casting, Hf = heat of fusion, Tm = melting temperature, and V = volume of casting?

(f) (V/A)2

What are the different cooling rates?

- Rapid cooling produces equiaxed (roughly round) grains - Slow cooling towards the interior forms long columnar grains that grow towards the center

What issues are associated with solidification?

- The time for a metal to freeze - Shrinkage - Directional solidification - Riser design

What are dendrites?

- Tree-like structures that form during the solidification - Slow cooling rates produce dendrites with larger branch spacing - Faster cooling rates produce finer spacing - Very fast cooling rates produce no dendrites

Name the characteristics of a riser

- réservoir in the cold which is a source of liquid metal to compensate for shrinkage during solidification - the riser must be designed to freeze after the main casting in order to satisfy its function - thickest part of casting-last to freeze, riser should feed directly yo these regions

Different types of risers

- top riser - side riser - blind riser - open riser

What are the casting shrinkage the sum of?

1. Liquid shrinkage the volume of the liquid decreases during cooling 2. Solidification shrinkage the volume of the material decreases during the transition from liquid to solid state 3. Solid shrinkage the volume of the solid decreases during cooling

What are the factors affecting the pouring operation?

1. Pouring temperature is the temperature of the molten metal as it is introduces into the mold. 2. Pouring rate refers to the volumetric rate at which the molten metal is poured into the mold. 3. Turbulence in fluid flow is characterised by erratic variations in the magnitude and direction of the velocity throughout the fluid.

When are casting techniques used?

1. When the finished shape is so large or complicated that other methods would be impractical. 2. When a particular alloy is so low in ductility that forming by either hot or cold working would be difficult. 3. In comparison to other fabrication processes, casting is the most economical.

The heat energy required to heat the metal to a molten temperature sufficient for casting is the sum of?

1. the heat to raise the temperature to the melting point. 2. the heat of fusion to convert it from solid to liquid. 3. the heat to raise the molten metal to the desired temperature for pouring

What is a chill in casting?

A chill is a heat sink placed to encourage rapid freezing in certain regions of the casting

What is a eutectic alloy?

A eutectic alloy is a particular composition in an alloy system for which the solidus and liquids are at the same temperature

What is the difference between an open mold and a closed mold?

An open mold is open to the atmosphere at the top; it is an open container in the shape of the desired part. A closed mold has a cavity that is entirely closed by the mold, with a passageway (called the gating system) leading from the outside to the cavity. Molten metal is poured into this gating system to fill the mold.

What are the old classifications?

Expendable mold - sand - mixed with binders or bonding agents - after solidification of casting: the gold is broken and cannot be reused Permanent mold - made of metals that retain their strength at high temperatures - casting can be removed without destroying the mold - can be reused

What is fluidity in the context of metal casting?

Fluidity is a measure of the capability of a metal to flow into and fill the mold before freezing. It defines to the great extend the quality of casting.

How does solidification of alloys differ from solidification of pure metals?

Pure metals solidify at a single temperature equal to the melting point. Most alloys (exceptions are eutectic alloys) start to solidify at the liquidus and complete solidification occurs at the solidus, where the liquidus is a higher temperature than the solidus.

Which casting process is the most important commercially?

Sand casting is the most important casting process.

Name different types of casting shrinkage

The bulk of the shrinkage occurs as - CAVITIES, if solidification begins at all surfaces of the casting - PIPES, if one surface solidifies more slowly than the others.

What is the continuity law as it applies to the flow of molten metal in casting?

The continuity law, or continuity equation, indicates that the volumetric flow rate is constant throughout the liquid flow

What are some of the factors that affect the fluidity of a molten metal during pouring into a mold cavity?

The factors include: (1) pouring temperature above the melting point, (2) metal alloy composition, (3) viscosity of the liquid metal, and (4) heat transfer to the surroundings

What is the difference between a pattern and a core in sand molding?

The pattern determines the external shape of the cast part, while a core determines its internal geometry if the casting includes a cavity

Why should turbulent flow of molten metal into the mold be avoided?

Turbulence causes: (1) it accelerates formation of oxides in the solidified metal and (2) it causes mold erosion or gradual wearing away of the mold due to impact of molten metal