ME 286 Midterm

(b), (e).

Ferrous metals include which of the following (two correct answers): (a) aluminum, (b) cast iron, (c) copper, (d) gold, and (e) steel?

(c), (d), and (e).

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?

(f).

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?

The recrystallization temperature is the temperature at which a metal recrystallizes (forms new grains) rather than work hardens when deformed.

Define the recrystallization temperature for a metal.

Viscosity is the resistance to flow of a fluid material; the thicker the fluid, the greater the viscosity.

Define viscosity of a fluid.

(c) and (d).

Deformation processes include which of the following (two correct answers): (a) casting, (b) drilling, (c) extrusion, (d) forging, (e) milling, (f) painting, and (g) sintering?

(d).

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?

Generally production quantity is inversely related to product variety. A factory that produces a large variety of products will produce a smaller quantity of each. A company that produces a single product will produce a large quantity.

How are product variety and production quantity related when comparing typical factories?

A shaping process changes the geometry of the work material (machining or forging). A surface processing operation does not alter the geometry, but instead alters surface of the work (painting or plating).

How does a shaping process differ from a surface processing operation?

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.

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

In a compression test, the specimen cross sectional area increases as the test progresses; while in a tensile test, the cross sectional area decreases.

How does the change in cross sectional area of a test specimen in a compression test differ from its counterpart in a tensile test specimen?

S = 0.7 TS, on average.

How is shear strength S related to tensile strength TS, on average?

G = 0.4 E, on average.

How is the shear modulus of elasticity G related to the tensile modulus of elasticity E, on average?

Advantages include (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; (4) they are applicable to any metal that can be melted; and (5) some casting processes are suited to mass production.

Identify some of the important advantages of shape-casting processes.

The three contractions occur due to (1) contraction of the molten metal after pouring, (2) solidification shrinkage during transformation of state from liquid to solid, and (3) thermal contraction in the solid state.

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

(b).

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?

Expendable molds are produced sequentially with the other steps because each casting requires its own mold. The mold cost is a component in the total cost to produce each casting. With permanent mold processes, the mold is an expensive piece of specialized tooling that is used for many castings; its cost is apportioned over the total quantity of parts made throughout the life of the mold.

In analyzing the costs in a casting operation, there is a basic difference in the way mold fabrication is treated between expendable mold processes and permanent mold processes. What is the difference?

(b).

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?

(a).

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?

When the material is perfectly plastic and does not strain harden.

In what case does the strength coefficient have the same value as the yield strength?

(d).

Inventions of the Industrial Revolution include which one of the following: (a) automobile, (b) cannon, (c) printing press, (d) steam engine, or (e) sword?

(c).

Mining is classified in which one of the following industry categories: (a) agricultural industry, (b) manufacturing industry, (c) primary industry, (d) secondary industry, (e) service industry, or (f) tertiary industry?

The three basic categories of engineering materials are (1) metals, (2) ceramics, and (3) polymers. A fourth category, composites, is a non-homogeneous mixture of the other types and therefore is not a basic category.

Name the three basic categories of materials.

The two categories are (1) expendable mold processes, and (2) permanent mold processes.

Name the two basic categories of casting processes.

The two mold types are (1) expendable molds and (2) permanent molds.

Name the two basic mold types that distinguish casting processes.

(f) and (g).

Particulate processing of metals and ceramics involves which of the following steps (two best answers): (a) adhesive bonding, (b) deformation, (c) forging, (d) material removal, (e) melting, (f) pressing, and (g) sintering?

(a).

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

(b). It is the elastic region that is characterized by a proportional relationship between stress and strain. The plastic region is characterized by a power function the flow curve.

The plastic region of the stress strain curve for a metal is characterized by a proportional relationship between stress and strain: (a) true or (b) false?

(b).

The shear strength of a metal is usually (a) greater than or (b) less than its tensile strength?

(a).

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

A split pattern is a pattern that consists of two pieces; a match plate pattern consists of the two split patterns attached to opposite sides of a plate.

There are various types of patterns used in sand casting. What is the difference between a split pattern and a match plate pattern?

(a).

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?

(c) and (d).

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?

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.(4)

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

General defects include: (1) misruns, in which the casting solidifies before filling the mold cavity; (2) cold shuts, in which two portions of metal flow together but there is lack of fusion at the joint; (3) cold shots, where solid globules of cast metal become entrapped in the casting; (4) shrinkage cavity, which is a depression on the casting surface or an internal void in the casting caused by solidification shrinkage; (5) microporosity, which is a network of small voids throughout the casting caused by localized solidification shrinkage; and (6) hot tearing, which is a crack in the casting caused by a mold that does not yield to the metal during the early stages of solidification shrinkage.

What are some of the general defects encountered in casting processes? Name and briefly describe three.

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

What are some of the limitations and disadvantages of casting?

The operations include (1) trimming, in which the sprues, runners, risers, and flash are removed, (2) core removal, (3) surface cleaning, (4) inspection, (5) repair if needed, (6) heat treatment, and (7) machining.

What are some of the operations required in sand casting after the casting is removed from the mold? (7)

Common die-casting metals include zinc, tin, aluminum, brass, and magnesium.

What are the common metals used in die casting?

A primary industry is one that cultivates and exploits natural resources, such as agriculture or mining. A secondary industry takes the outputs of primary industries and converts them to consumer and capital goods. Examples of secondary industries are textiles and electronics. A tertiary industry is in the service sector of the economy. Examples of tertiary industries are banking and education.

What are the differences between primary, secondary, and tertiary industries? Give an example of each category.

The two subclasses of assembly processes are (1) permanent joining and (2) mechanical fastening. Examples of permanent joining include welding or adhesive bonding. Examples of mechanical fastening include threaded fasteners, such as nuts and bolts, and rivets.

What are two subclasses of assembly processes? Provide an example process for each subclass.

Heat of fusion is the amount of heat energy required to transform the metal from solid state to liquid state.

What does heat of fusion mean in casting?

Capital goods are those purchased by companies to produce goods or provide services. Examples of capital goods are aircraft and construction equipment.

What is a capital good? Provide an example.

Chaplets are metal supports of various designs used to hold the core in place in the sand mold.

What is a chaplet?

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

What is a chill in casting?

A eutectic alloy is a particular composition in an alloy system for which the solidus and liquidus temperatures are equal. The temperature is called the eutectic temperature. Hence, solidification occurs at a single temperature, rather than over a temperature range.

What is a eutectic alloy?

A foundry.

What is a factory that performs casting operations usually called?

A pattern is a full-sized model of the part, enlarged to account for shrinkage and machining allowances in the final casting.

What is a pattern in sand casting?

Shell molding is a casting process in which the mold is a thin shell, typically 9 mm (3/8 in) made of sand held together by a thermosetting resin binder. The surface of the shell-mold cavity is smoother than a conventional green-sand mold, and this smoothness permits easier flow of molten metal during pouring and better surface finish on the final casting.

What is a shell molding?

Flash is a thin portion of metal at the exterior of a casting that results from molten metal being squeezed into the spaces between the die halves of the mold at the parting line, or into the clearances around the cores and ejector pins.

What is flash in die casting?

Hardness is defined as the resistance to indentation of a material. It is tested by pressing a hard object (sphere, diamond point) into the test material and measuring the size (depth, area) of the indentation.

What is hardness, and how is it generally tested?

In investment casting, a pattern made of wax is coated with a refractory material to make the mold, after which the wax is melted away prior to pouring the molten metal. The term investment comes from one of the less familiar definitions of the word invest, which means "to cover completely," this referring to the coating of the refractory material around the wax pattern.

What is investment casting?

The term castability refers to the ease with which a cast part can be produced. It depends on (1) the part design, (2) the metal used for the casting, and (3) the proper selection of casting process to economically satisfy production requirements. (3)

What is meant by the term castability and upon what factors does it depend?

Superheat is the temperature difference above the melting point at which the molten metal is poured. The term also refers to the amount of heat that is removed from the molten metal between pouring and solidification.

What is meant by the term superheat?

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

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

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

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

A core is a full-scale model of the interior surfaces of the part, inserted into the mold cavity prior to pouring, so that the molten metal will flow and solidify between the mold cavity and the core to form the casting's external and internal surfaces.

What is the difference between a pattern and a core?

An open mold is open to the atmosphere at the top; it is an open container in the desired shape which must be flat at the top. A closed mold has a cavity that is entirely enclosed 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 is the difference between an open mold and a closed mold?

In true centrifugal casting, a tubular mold is used and a tubular part is produced. In semicentrifugal casting, the shape is solid; an example is a railway wheel. The mold is rotated so that centrifugal force is used to distribute the molten metal to the exterior of the mold so that the density of the final metal is greater at the outer sections.

What is the difference between true centrifugal casting and semicentrifugal casting?

Chvorinov's rule is summarized: TTS = Cm(V/A)2, where TTS = total solidification time, Cm = mold constant, V = volume of casting, and A = surface area of casting.

What is the relationship known as Chvorinov's rule in casting?

Work hardening, also called strain hardening, is the increase in strength that occurs in metals when they are strained.

What is work hardening?

The usual properties are (1) strength ability to maintain shape in the face of the flowing metal, (2) permeability ability to allow hot air and gases to escape from the cavity, (3) thermal stability ability to resist cracking and buckling when in contact with the molten metal, (4) collapsibility ability to give way during shrinkage of the solidified casting, and (5) reusability can the sand be reused to make other molds?

What properties determine the quality of a sand mold for sand casting?

Sand casting is the most important casting process.

Which casting process is the most important commercially?

Hot chamber machines are faster because cold chamber die casting machines require molten metal to be ladled into the chamber each cycle from an external source. Ladling takes more time than injecting the molten metal into the die as in the hot-chamber operation.

Which die-casting machines usually have a higher production rate, cold chamber or hot chamber, and why?

(a), (d), and (e).

Which of the following are the three basic types of static stresses to which a material can be subjected (three correct answers): (a) compression, (b) hardness, (c) reduction in area, (d) shear, (e) tensile, (f) true stress, and (f) yield?

(a), (e), and (f).

Which of the following industries are classified as secondary industries (three correct answers): (a) beverages (b) financial services, (c) fishing, (d) mining, (e) power utilities, (f) publishing, and (g) transportation?

(a) and (d).

Which of the following processes start with a material that is in a fluid or semifluid state and solidifies the material in a cavity (two best answers): (a) casting, (b) forging, (c) machining, (d) molding, (e) pressing, and (f) turning?

(a) and (c).

Which of the following riser types are completely enclosed within the sand mold and connected to the main cavity by a channel to feed the molten metal (two correct answers): (a) blind riser, (b) open riser, (c) side riser, and (d) top riser?

(d).

Which one of the following is a machine used to perform extrusion: (a) forge hammer, (b) milling machine, (c) rolling mill, (d) press, (e) torch?

(c).

Which one of the following is the correct definition of ultimate tensile strength, as derived from the results of a tensile test on a metal specimen: (a) the stress encountered when the stress strain curve transforms from elastic to plastic behavior, (b) the maximum load divided by the final area of the specimen, (c) the maximum load divided by the original area of the specimen, or (d) the stress observed when the specimen finally fails?

(a).

Which one of the following materials has the highest hardness: (a) alumina ceramic, (b) gray cast iron, (c) hardened tool steel, (d) high carbon steel, or (e) polystyrene?

Different hardness tests and scales are required because different materials possess widely differing hardnesses. A test whose measuring range is suited to very hard materials is not sensitive for testing very soft materials.

Why are different hardness tests and scales required?

Because of necking that occurs in the test specimen.

Why cannot a direct conversion be made between the ductility measures of elongation and reduction in area using the assumption of constant volume?

Turbulence causes the following problems: (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.

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