Metal Casting

Question 1

Types of manufacturing:

Answer 1

Metal casting
Metal forming Machining
Welding
Powder metallurgy

Question 2

Technical process:

Answer 2

apply physical and/or chemical processes
to alter the geometry, properties, and/or appearance of a starting material to make parts or products.

Question 3

Economic process:

Answer 3

transform materials into items of greater
value by one or more processing and/or assembly
operations.

Question 4

Engineering Materials:

Answer 4

Metals
Ceramics
Polymers
Composites

Question 5

Solidification processes:

Answer 5

Casting and moulding processes start with a work
material heated to a fluid or semifluid state.

(1) Pour the fluid into a mould cavity.

(2) Allow the
fluid to solidify, after which the solid part is removed from
the mould.

Question 6

Deformation processes:

Answer 6

The starting workpart is a ductile solid that is shaped by the application of forces exceeding the yield strength.

Examples:
(a) forging
(b) extrusion

Question 7

Material removal processes:

Answer 7

The starting material is a solid (ductile or brittle), from
which material is removed so that the resulting part has
the desired geometry.

Examples:
machining such as: (a) turning
(b) drilling
(c) milling; grinding and non-traditional machining

Question 8

Particulate processes:

Answer 8

The starting material is powder.

The common process consists of pressing and sintering.

Question 9

Welding processes:

Answer 9

Two or more parts are coalesced (form one mass) at their contact surfaces
by the application of heat and/or pressure.

Examples: arc welding, resistance welding

Question 9

What is casting:

Answer 9

Casting is a process in which molten metal flows by
gravity or other force into a mould where it solidifies in the shape of the mould cavity.

All variety of metals can be cast, ferrous or nonferrous.

Question 10

Principle of casting:

Answer 10

Melt the metal,
Pour it into a mould,
Let it cool and solidify

Question 11

Advantages of casting:

Answer 11

Can create complex part geometries.

Can create both external and
internal shapes.

Net shape or near net shape.

Can produce very large parts.

Some casting methods are suited to mass production.

Question 12

Disadvantages of casting:

Answer 12

Limitation on mechanical properties.

Poor dimensional accuracy and
surface finish for some processes.

Safety hazards to humans when
processing hot molten metals.

Environment problems.

Question 13

The mould in casting:

Answer 13

The mould contains a cavity whose geometry determines
the shape of the cast part.

The actual size and shape of the cavity must be slightly
oversized to allow for shrinkage that occurs in the metal during
solidification and cooling and for machining allowances.

Moulds are made of a variety of materials, including sand,
plaster, ceramic and metal.

Question 14

Types of mould casting:

Answer 14

Expendable-mould casting.

Permanent-mould casting.

Question 15

Expendable-mould casting:

Answer 15

The mould must be destroyed to remove the casting.

Mould materials: sand, plaster, wax etc.

Examples: sand casting, investment casting.

More complex shapes are possible, but production rates
are limited.

Question 16

Permanent-mould casting:

Answer 16

The mould can be used over and over to produce many
castings.

Mould materials: metals (or less commonly, a ceramic
refractory material).

Examples: die casting, low-pressure casting.

Shapes are limited by the need to open the mould, but production rates are high.

Question 17

Parts of sand casting moulds:

Answer 17

Mould

Cavity

Core

Gating System

Riser

Question 18

Mould:

Answer 18

Mould consists of cope (upper half) and drag (bottom half).

The two halves are contained in a box (flask), and separate at the parting line.

Question 19

Cavity:

Answer 19

The cavity defines the external surfaces of a cast part.

It is formed by packing sand around a pattern. The pattern is usually made oversized (why?).

Sand for the mould is moist and contains a binder to maintain its shape.

Question 20

Core:

Answer 20

The core defines the internal surfaces of a cast part.

It is placed inside the mould to define the interior geometry of the part.

Question 21

Gating System:

Answer 21

The gating system is the channel (or network of channels),
by which the molten metal flows into the cavity from outside the mould.

It typically consists of a downsprue, through which the
metal enters a runner that leads to the main cavity.

At the top of the downsprue, a pouring cup is often used to minimise splash and turbulence.

Question 22

Riser:

Answer 22

The riser connected to the main cavity is a reservoir in the
mould that serves as a source of liquid metal to
compensate for shrinkage during solidification.

It must be designed to freeze after the main casting.

Question 23

Heating the Metal:

Answer 23

Heating furnaces are used to heat the metal to a molten
temperature sufficient for casting.

The heat energy required is the sum of:

• Heat to raise the temperature to the melting point
• Heat of fusion to convert the metal from solid to liquid
• Heat to raise the molten metal to the pouring temperature

Question 24

Pouring the molten metal:

Answer 24

The metal must flow into all regions of the mould before solidifying. Factors affecting pouring: pouring temperature, pouring rate, and turbulence.

Question 25

Solidification of metals:

Answer 25

Solidification differs depending on whether the metal is a pure element or an alloy. A pure metal solidifies at a constant temperature. Most alloys freeze over a temperature range, which depends on the alloy system and composition.

Question 26

Solidification time:

Answer 26

Total solidification time TTS is the time taken between pouring and complete solidification. It is dependent on the size and shape of the casting by an empirical relationship (Chvorinov’s rule). SEE EQUATION IN NOTES

Question 27

What does Chvorinov’s rule tell us?

Answer 27

A casting with a higher V/A ratio cools and solidifies more slowly than one with a lower ratio. Its use in riser design – To feed molten metal to main cavity, TTS (total solidification time) for riser must be greater than TTS for main casting. Since mould constants of riser and casting are equal, design the riser to have a larger V/A ratio so that the main casting solidifies first and the effects of shrinkage are minimised. Its use in directional solidification control

Question 28

Shrinkage during solidification and cooling:

Answer 28

* Shrinkage of a cylindrical casting: (0) Starting level of molten metal immediately after pouring; (1) Reduction in level caused by liquid contraction during cooling. (2) Reduction in height and formation of shrinkage cavity caused by solidification shrinkage; (3) Further reduction in height and diameter due to thermal contraction during cooling of the solid metal Note: solidification shrinkage occurs in nearly all metals because solid phase has a higher density than liquid phase

Question 29

Directional solidification:

Answer 29

To minimise the damaging effects of shrinkage, it is desirable for regions of the casting most distant from the liquid metal supply to freeze first and for solidification to progress from these remote regions towards the riser(s). – This is achieved by observing Chvorinov’s rule. E.g., locate sections of the casting with lower V/A ratios away from the riser.

Question 30

Riser design:

Answer 30

The geometry of a riser is normally selected to maximise the V/A ratio. (details in Chvorinov’s rule) It is also desirable to reduce the riser volume V to minimise waste in production. Note, the connection between the riser and the main cavity must be designed such that it does not freeze before the casting.

Question 31

Sand casting:

Answer 31

Nearly all alloys can be sand cast. * Versality – wide range in part size and production quantities. * Sand casting includes not only the casting operation itself, but also pattern making and mould making.

Question 32

Patterns:

Answer 32

* A full-sized model of the part, slightly enlarged to account for shrinkage and machining allowances (if any). * Pattern materials: wood, plastics and metals. * Types of patterns used in sand casting: Solid pattern, Split pattern, Match-plate pattern, Cope-and-drag pattern

Question 33

Cores:

Answer 33

Full-scale model of the internal surface of the part. It is inserted into the mould prior to pouring. The molten metal flows and solidifies between the mould cavity and the core to form the casting’s external and internal surfaces. It may require supports to hold it in position in the mould cavity during pouring, called chaplets.

Question 34

Sands:

Answer 34

Silica (SiO2) or silica mixed with other minerals. Good refractory properties for high temperatures. Other important features of sands include grain size, size distribution and grain shape. Small grain size provides a better surface finish on the cast part. Large grain size is more permeable to allow escape of gases during pouring. Irregular grain shape strengthens the mould due to interlocking. Sands are held together by a mixture of water and clay. (typically 90% sand, 3% water and 7% clay by volume)

Question 35

Desired sand mould properties:

Answer 35

Strength: to maintain shape and resist erosion. Permeability: to allow hot air and gases to pass through voids in the sand. Thermal stability: to resist cracking and buckling on contact with the molten metal. Collapsibility: ability to give way and allow casting to shrink without cracking the casting; ability to remove the sand from the casting during cleaning. Reusability: to reuse the sand to make other moulds.

Question 36

Sand moulds are classified as:

Answer 36

Green-sand mould – contains moisture at time of pouring. Dry-sand mould – uses organic binders and is baked. Skin-dried moulds – cavity surfaces is dried to a depth

Question 37

Investment casting:

Answer 37

* A pattern made of wax is coated with a refractory material to make the mould, after which the wax is melted away prior to pouring the molten metal. * Also known as the lost wax casting. * It is a precision casting process – capable of making castings of high accuracy and intricate detail.

Question 38

Investment casting steps:

Answer 38

1) Wax patterns are produced. 2) Several patterns are attached to a sprue to form a tree. 3) The pattern tree is coated with a thin layer of refractory material – a slurry of very fine-grained silica or other refractory mixed with plaster. 4) The full model is formed by covering the coated tree with sufficient refractory material to make it rigid. 5) The mould is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity. 6) The mould is preheated to a high temperature, the molten melt is poured, and it solidifies. 7) The mould is broken away from the finished casting, and parts are separated from the sprue.

Question 39

Advantages of Investment casting:

Answer 39

Parts of great complexity and intricacy can be cast. Close dimensional control and good surface finish. Wax can be recovered for reuse. A net shape process – additional machining is not normally required. All types of metals, including high temperature alloys, can be investment cast.

Question 40

Disadvantages of Investment casting:

Answer 40

A relatively expensive process as many steps are involved. Investment castings are normally small in size.

Question 41

Permanent-mould casting process:

Answer 41

Basic permanent-mould (gravity) casting process. Low-pressure casting. Die casting. Squeeze casting and semisolid metal casting. Centrifugal casting.

Question 42

Basic permanent-mould casting:

Answer 42

The flow of molten melt into cavity is caused by gravity. Permanent-mould casting uses a metal mould constructed of two sections designed for easy, precise opening and closing. – Moulds used for casting lower melting-point alloys (e.g., Al, Mg, Cu alloys) are commonly made of steel or cast iron. – Moulds used for casting steels must be made of refractory material, due to the very high pouring temperatures. Metal cores can be used. If withdrawal of a metal core would be difficult, sand cores can be used (also called semipermanent-mould casting)

Question 43

Basic permanent-mould casting steps:

Answer 43

1) The mould is preheated and coated. Preheating facilitates metal flow. Coatings aid heat dissipation and lubricate the mould surfaces for easier separation of the cast part. 2) Cores (if used) are inserted, and the mould is closed. 3) Molten metal is poured into the mould where it solidifies. 4) As soon as the metal solidifies, the mould is opened and the casting is removed. This is to prevent cracks from developing in the casting due to cooling contraction.

Question 44

Advantages of basic permanent-mould casting steps:

Answer 44

Close dimensional control and good surface finish. More rapid solidification caused by metal mould results in a finer grain structure, so stronger castings are produced.

Question 45

Disadvantages of basic permanent-mould casting steps:

Answer 45

Generally limited to lower melting-point metals. Simple part geometries compared to sand casting because of the need to open the mould. High cost of mould (thus suited to high volume production)

Question 46

Low-pressure casting:

Answer 46

The liquid metals is forced into the mould cavity under low pressure (approx. 0.1 MPa) from beneath. Advantages: clean molten metals from the centre of the ladle is introduced, thus gas porosity and oxidation defects are minimised (improved mechanical properties)

Question 47

Die casting:

Answer 47

The molten metal is injected into the mould cavity under high pressure (typically 7 to 350 MPa). The pressure is maintained during solidification, after which the mould is opened and the part is removed (why?). Moulds in this casting operation are called dies. Die casting machines are designed to hold and accurately close the two halves of the mould and keep them close while the liquid metal is forced into the cavity.

Question 48

Hot-chamber die casting:

Answer 48

Metal is melted in an internal container attached to the machine, and a piston injects the liquid metal under high pressure (typically 7 to 35 MPa) into the die cavity. High production rate – 500 parts per hours are not uncommon. Applications are limited to lower melting-point metals that do not chemically attack the plunger and other mechanical components. Casting metals: zinc, tin, lead and magnesium.

Question 49

Hot-chamber die casting steps:

Answer 49

1) With the die closed and the plunger withdrawn, molten metal flows into the chamber. 2) The plunger forces metal in the chamber to flow into the die, maintaining the pressure during cooling and solidification. 3) The plunger is withdrawn, the die is open and the solidified part is ejected.

Question 50

Cold-chamber die casting:

Answer 50

Molten metal from an external melting container is poured into an unheated chamber, and a piston injects the metal under high pressure (typically 14 to 140 MPa) into the die cavity. Still high production process (although cycle rate lower than hot-chamber die casting). Casting metals: aluminium, brass, magnesium alloys.

Question 51

Cold-chamber die casting steps:

Answer 51

1) With the die closed and the ram withdrawn, molten metal is poured into the chamber. 2) The ram forces metal to flow into the die, maintaining the pressure during cooling and solidification. 3) The ram is withdrawn, the die is opened and the part is ejected.

Question 52

Moulds for die casting:

Answer 52

* Materials for moulds (dies) -Usually made of tool steel, mould steel or maraging steel. – Tungsten and molybdenum (good refractory qualities) are used when steel and cast iron are die cast. * Ejector pins are required to remove the part from the die when it opens. * Lubricants must be sprayed onto the cavity to prevent sticking. * Venting holes and passageways must be built into the dies at the parting line to evacuate the air and gases in the cavity.

Question 53

Advantages of die casting:

Answer 53

Economical for large production quantities. Close dimensional control and good surface finish. Thin sections are possible, down to 0.5 mm. Rapid cooling provides small grain size and good strength to the casting.

Question 54

Disadvantages of die casting:

Answer 54

Generally limited to lower melting-point metals. Part geometry must allow for removal from the die.

Question 55

Casting quality 1:

Answer 55

Misrun: a casting solidifies before completely filling the mould cavity. Cold shut: two portions of the metal flow together but there is a lack of fusion due to premature freezing. Cold shot: metal splatters during pouring; consequently solid globules form and become entrapped in the casting.

Question 56

Casting quality 2:

Answer 56

Shrinkage cavity: a depression in the surface or internal voids in the casting, caused by solidification shrinkage. Microporosity: a network of small voids distributed within the casting caused by localised solidification shrinkage or gases (entrapped or released due to solubility limit). Hot tearing: the casting is restrained from contraction by an unyielding mould.

Question 57

Product design considerations:

Answer 57

Geometric simplicity. Dimensional tolerance and surface finish. Machining allowances

Question 58

Geometric simplicity:

Answer 58

Although casting can be used to produce complex part geometries, simplifying the part design usually improves castability. Avoid unnecessary complexities.– Sharp corners and angles should be avoided, because they are sources of stress concentrations and may cause hot tearing and cracks. Section thickness should be uniform to avoid shrinkage cavities. Drafts facilitate the removal of pattern from mould in expendable-mould casting, as well as the removal of part from mould in permanent-mould casting. Design change can reduce the need for coring.

Question 59

Dimensional tolerance and surface finish:

Answer 59

Dimensional accuracy and finish vary significantly, depending on which casting process is used.

Question 60

Machining allowances:

Answer 60

Almost all sand castings must be machined to achieve the required dimensions and part features. Additional material, called the machining allowance, is left on the casting in those surfaces where machining is necessary. Typical machining allowances for sand castings range between 1.5 and 3 mm.