Advanced Metal Casting(MAE611)

Dr K.Srinivasulu Reddy

Department of Mechanical EngineeringAdama Science & Technology University

Course OutlineUnit I Properties of Materials for CastingFerrous and non – ferrous materials, fluidity and the variables affecting fluidity, mold factors, measurement of fluidity, metal-matrix composites and their properties and suitability as casting materials, selection of materials for castings, melting of metals, specific consideration to cast iron, steel, Al, Mg and Titanium .

Unit II Solidification of Metals Crystallization and the development of cast structure, nucleation and growth, constitutional super cooling, columnar and dentritic growth, independent nucleation, critical radius of nucleus, eutectic and peritectic freezing, the structure of castings and its control, grain shape and orientation, directional solidification, grain size control, feeding of castings, feeding characteristics of alloys, geometric influence on solidification, agitation feeding methods, insulation and exothermic feeding, pressure feeding, design of feeder head, modification and padding, differential cooling and solidification modeling.

Unit III Properties of Moulding Materials and TestingFunctional requirements of mould materials such as green sand, bonding materials, non-siliceous refractories, loam molding mixtures, mould surface coatings, sand reclamation and various sand testing techniques and the international standards for testing foundry sand.

Course Outline Contd..Unit IV Casting Design Casting design considerations, mould design for casting, gating system design, riser design, and CAD/CAM rapid prototyping, initial considerations in design, technical characteristics, dimensional and surface characteristics, metallurgical characteristics, process and alloy selection, inserts, bimetal casting techniques and surface treatments of castings, moulding factors in casting design, design considerations at the casting stage, machining factors in casting design, mould design for sand and die casting, gating system design and riser design.

Unit V Casting Defects and Quality ControlFinishing operations of castings, causes and remedies of various defects, shaping faults, inclusions, gas defects, contraction, cracking, distortion, factors affecting dimensional accuracy, quality assessment and control, product testing procedures, destructive, non-destructive testing of casting, quality and process control in the foundry, pollution control in foundries.

Unit VI Automation and Special Casting Techniques Shell, die, centrifugal casting, unorthodox shaping techniques, squeeze castings, metal matrix composites, gravity die castings- investment- CL process, V-process, lost foam process-foundry mechanization.

Lecture Objectives

Advantages of casting

Limitations of casting

Applications of casting

Metal Casting Terminology

Steps involved in making casting

Metal Casting- History

• Earliest metal shaping method known to human being.

• Casting is a three step process

- Pouring molten metal into a mould patterned after the part to be

made.

- Allowing it to solidify.

- After solidification removing metal from the mold either by breaking it

or taking the mold apart.

• Discovery of casting process dates back to 3500 BC in Mesopotamia.

• In early stages of casting only single pieces like copper axes, and other

flat objects were made.

• In later periods, when round objects were required to be manufactured,

the mould was split into two or more parts to facilitate the withdrawal of

the round objects.

Copper Age (~4000 B.C.)

The oldest known casting in existence, a pure copper frog, was cast in Mesopotamia.

The first metal to be used was copper. (3200B.C)

Bronze Age (~3000 B.C.)

Bronze was the first alloy used.

Bronze = Copper & 5-10% Tin

Iron Age (~2000 B.C.)

Iron smelting began in Egypt.

Metal Casting- History cont…• Great improvements were done by Chinese around 1500 B.C.

Specialized in multi-piece moulds for making highly intricate jobs.

• Indus valley civilization is known for their extensive use of casting of copper and bronze for tools, ornaments, weapons and utensils. But not much improvements in technology.

• India is credited with the invention of crucible steel.

• Casting technology in India since 300 BC, during the period when Alexander the Great invaded India.

• The example of the metallurgical skills of ancient India can be seen from the famous iron pillar located near Qutab Minar in Delhi. Made by pure malleable iron during Chandragupta II era (375-413 AD). Rusting is practically zero and very negligible in the buried portion.

History

3200 B.C.E(Before Common Era) A copper frog, the oldest known casting in existence, is cast in Mesopotamia.

233 B.C. Cast iron plowshares are poured in China.

500 A.D. or C.E(Common Era) Cast crucible steel is first produced in India, but the process is lost until 1750, when Benjamin Huntsman reinvents it in England.

1455 Dillenburg Castle in Germany is the first to use cast iron pipe to transport water.

1480 Birth of Vannoccio Biringuccio (1480-1539), the "father of the foundry industry," in Italy. He is the first man to document the foundry process in writing.

1709 Englishman Abraham Darby creates the first true foundry flask for sand and loam molding.

1750 Benjamin Huntsman reinvents the process of cast crucible steel in England.

1809 Centrifugal casting is developed by A. G. Eckhardt of Soho, England.

1896 American Foundrymen's Association (renamed American Foundrymen's Society in 1948 and now called the American Foundry Society) is formed.

1897 Investment casting is rediscovered by B.F. Philbrook of Iowa. He uses it to cast dental inlays.

1947 The Shell process, invented by J. Croning of Germany during WWII, is discovered by U.S. officials and made public.

1953 The Hotbox system of making and curing cores in one operation is developed.

1958 H.F. Shroyer is granted a patent for the full mold process, the forerunner of the expendable pattern (lost foam) casting process.

1968 The Coldbox process is introduced by L. Toriello and J. Robins for high production core making.

1971 The Japanese develop V-Process molding. This method uses unbonded sand and a vacuum.

1971 Rheocasting is developed at Massachusetts Institute of Technology.

Weight:6 tonsHeight:7 mMP: Casting & ForgingRemark: Corrosion resistance is negligible

This iron pillar dating to 400 A.D., remains standing today in Delhi, India. Corrosion to the pillar has been minimal a skill lost to current ironworkers.

Iron Pillar ~ 400A.D.

Cast Iron (~800)

Chinese were the first in the production of cast iron.

1750

Steel becomes the world’s most used material.

Sectorwise Consumption of Castings

Product Mix

Various Types of Castings Produced

Various Types of Castings Produced

Metal Casting- Advantages- Practically any material (ferrous/non-ferrous) can be cast .

- Very simple and inexpensive tooling required for making casting molds Ideal method for making trial production or production in small lots.

- Complex shapes with internal cavities or hollow sections can be produced very easily.

- Castings are cooled uniformly from all sides Elimination of direction properties.

- Large parts of any size and weight can be produced.

- Ideally suited for certain metals and alloys which can only be processed by casting due to metallurgical considerations.

- Very competitive with other processes.

Metal Casting- Limitations & Applications

Limitations

- Parts made using normal sand casting has problems like dimensional

accuracy and surface finish .

Special casting methods like die casting has to be used.

- Sand casting process is labor intensive

Machine moulding and foundry mechanization should be done.

- Lots of manufacturing defects due to moisture present in sand castings.

Applications

- Cylindrical blocks, liners, machine tool beds, pistons, piston rings, mill rolls,

wheel housings, water supply pipes and specials, and bells.

AdvantagesThe metal casting process is extensively used in manufacturing because of its many advantages.

1.Molten material can flow into very small sections so that intricate shapes can be made by this process. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.2.It is possible to cast practically any material that is ferrous or non-ferrous.3.The necessary tools required for casting molds are very simple and inexpensive. As a result, for production of a small lot, it is the ideal process.4.There are certain parts made from metals and alloys that can only be processed this way.5.Size and weight of the product is not a limitation for the casting process.

Limitations

1.Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique. Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish. Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process.

2.The metal casting process is a labor intensive process

What is metal casting

Expendable Mold & Expendable Pattern

Melt Spinning Casting

Lost Foam Casting

Single Crystal Casting

Investment Casting

Expendable mold & Permanent Pattern

Ceramic Mold Casting

Die casting - Hot chamber machine

Permanent mould

Die casting - Cold chamber machine

Permanent Mold Contd…

Centrifugal Casting Squeeze Casting

Metals processed by casting •Sand casting – 60% •Investment casting – 7% •Die casting – 9% •Permanent mold casting – 11% •Centrifugal casting – 7% •Shell mold casting – 6%

Metals processed by casting •Sand casting – 60% •Investment casting – 7% •Die casting – 9% •Permanent mold casting – 11% •Centrifugal casting – 7% •Shell mold casting – 6%

Permanent mold - Die casting –hot chamber –cold chamber-Centrifugal casting-Squeeze casting

Expendable molds –sand –shell –slurry –investment •lost wax •lost foam

Share of Casting Processes

Capabilities

•Dimensions –sand casting - as large as you like –small - 1 mm or so •Tolerances –0.12 mm to 2.54 mm •Surface finish –die casting (1-3 mm) –sand casting (10-25 mm)

Metal Casting Terminology

Metal Casting Terminology cont..

• Flask: A moulding flask is one which holds the sand mould intact. Depending

upon the position of the flask in the mould structure it is referred to by various

names such as drag-lower moulding flask cope-upper moulding flask and

cheek-intermediate moulding flask used in three-piece moulding. It is made

up of wood for temporary applications and more generally of metal for long-

term use.

• Pattern: Pattern is a replica of the final object to be made with some

modifications. The mould cavity is made with the help of the pattern.

• Parting line: This is the dividing line between the two moulding flasks that

makes up the sand mould. In split pattern it is also the dividing line between

the two halves of the pattern.

• Bottom board: This is a board normally made of wood which is used at the

start of the mould making. The pattern is first kept on the bottom board, sand

is sprinkled on it and then the ramming is done in the drag.

Metal Casting Terminology cont..

• Moulding sand: It is the freshly prepared refractory material used for making the mould cavity. It is a mixture of silica, clay and moisture in appropriate proportions to get the desired results and it surrounds the pattern while making the mould.

• Backing sand: It is what constitutes most of the refractory material found in the mould. This is made up of used and burnt sand.

• Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the moulding cavity to give a better surface finish to the castings.

• Core: It is used for making hollow cavities in castings.

• Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten metal is poured.

• Sprue: The passage through which the molten metal from the pouring basin reaches the mould cavity. In many cases it controls the flow of metal into the mould.

Metal Casting Terminology cont..

• Runner: The passage ways in the parting plane through which

molten metal flow is regulated before they reach the mould cavity.

• Gate: The actual entry point through which molten metal enters

mould cavity.

• Chaplet: Chaplets are used to support cores inside the mould cavity

• Chill: Chills are metallic objects which are placed in the mould to

increase the cooling rate of castings to provide uniform or desired

cooling rate.

• Riser: It is a reservoir of molten metal provided in the casting so that

hot metal can flow back into the mould cavity when there is a

reduction in volume of metal due to solidification.

Terminology involved

Flask: for holding the mold intact

Cope: Upper half of mold flask

Drag: Bottom half of mold flask

Parting line: Separation line of the two mold halves

Pattern: replica of the part of be made, mold cavity is formed with pattern

Core: to create internal geometrical surfaces, hollow cavities

Pouring basin: funnel shaped cavity at the top of the mold into which molten metal is poured

Sprue: passage for the molten metal to reach mould cavity from pouring cup

Gate: actual entry point thru which molten metal enters mold cavity

Runner: passage way in the parting plane thru which molten metal reaches the mold cavity

Riser: reservoir of molten metal for the casting to compensate for shrinkage during solidification

Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.

Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.

Vent: Small opening in the mold to facilitate escape of air and gases.

Sand Mould Making Procedure

1

2

3

4

Sand Mould Making Procedure Contd..

• First a bottom board is placed either on the moulding platform or on the floor, making the surface even.

• The drag moulding flask is kept upside down on the bottom board along with the drag part of the pattern at the centre of the flask on the board.

• There should be enough clearance between the pattern and the walls of the flask which should be of the order of 50 to 100 mm.

• Dry facing sand is sprinkled over the board and pattern to provide a non-sticky layer.

• Freshly prepared moulding sand of requisite quality is now poured into the drag and on the pattern to a thickness of 30 to 50 mm.

Sand Mould Making Procedure cont..

• Rest of the drag flask is completely filled with the backup sand and uniformly rammed to compact the sand.

• The ramming of sand should be done properly so as not to compact it too hard, which makes the escape of gases difficult, nor too loose so that mould would not have enough strength.

• After the ramming is over, the excess sand in the flask is completely scraped using a flat bar to the level of the flask edges.

• Now, with a vent wire which is-a wire of 1 to 2 mm diameter with a pointed end, vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during casting solidification.

• This completes the preparation of the drag.

Sand Mould Making Procedure cont..

• The finished drag flask is now rolled over to the bottom board exposing the pattern.

• Using a slick, the edges of sand around the pattern is repaired and cope half of the

pattern is placed over the drag pattern, aligning it with the help of dowel pins.

• The cope flask on top of the drag is located aligning again with the help of the pins.

• The dry parting sand is sprinkled all over the drag and on the pattern.

• A sprue pin for making the sprue passage is located at a small distance of about 50

mm from the pattern.

• Also a riser pin if required, is kept at an appropriate place and freshly prepared

moulding sand similar to that of the drag along with the backing sand is sprinkled.

• The sand is thoroughly rammed, excess scraped and vent holes are made all over

in the cope as in the drag.

Sand Mould Making Procedure cont..

• The sprue pin and the riser pin are carefully withdrawn from the flask.

• Then, the pouring basin is cut near the top of the sprue.

• The cope is separated from the drag and any loose sand on the cope and drag removed.

• Interface of the drag is blown off with the help of bellows.

• Now, the cope and the drag pattern halves are withdrawn by using the draw spikes and rapping the pattern all around to slightly enlarge the mould cavity so that the mould walls are not spoiled by the withdrawing pattern.

Sand Mould Making Procedure cont..

• The runners and the gates are cut in the mould carefully without spoiling the mould. Any excess or loose sand found in the runners and mould cavity is blown away using the bellows.

• Now, the facing sand in the form of a paste is applied all over the mould cavity and the runners which would give the finished casting a good surface finish.

• A dry sand core is prepared using a core box. After suitable baking, it is placed in the mould cavity.

• The cope is replaced on the drag taking care of the alignment of the two by means of the pins.

• The mould now is ready for pouring.

Sand mold - openedSand mold - opened

Sand mold - closedSand mold - closed

67

Mixing moulding sand with binders & adhesivesMixing moulding sand with binders & adhesives

Filling sand in moulding flasksFilling sand in moulding flasks

Melting furnaceMelting furnace

Pouring molten liquidPouring molten liquid

Knock outKnock out

Heat treatmentHeat treatment

Machining Machining

Final products of castingFinal products of casting

Casting Flow Chart

Sand Casting Process Flow

Patterns Materials used for pattern Types of patterns

Pattern

- Pattern is a replica of the object to be made by the casting process

- Patterns are used to mold the sand mixture into the shape of the casting.

- Made of wood, plastic, metal, POP, rubber, wax and polystyrene

80

81

FUNCTIONS OF THE PATTERN

1. A pattern prepares a mold cavity for the purpose of making a casting.

2. A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.

3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern.

4. Patterns properly made and having finished and smooth surfaces reduce casting defects.

5. A properly constructed pattern minimizes the overall cost of the castings.

82

PATTERN MATERIALS

Some materials used for making patterns are:

Wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins.

The pattern material should be:

1. Easily worked, shaped and joined

2. Light in weight

3. Strong, hard and durable

4. Resistant to wear and abrasion

5. Resistant to corrosion, and to chemical reactions

6. Dimensionally stable and unaffected by variations in temperature and humidity

7. Available at low cost

Pattern Materials

Wood• Most commonly used pattern material. • Varieties of woods: Pine, mahogany, teak, sal and deodar

Advantages

- Easily available

- Low weight

- Cheap

- Easy to shape

Disadvantages

- Absorbs moisture from moulding sand distortions/warpages and dimensional changes occur.

- Proper seasoning and upkeep of wood is necessary for large scale use.

83

Usual materials: wood, metal and plastics.

84

Metals & Alloys: For large quantities of molds, better accuracy, finish and higher pressures in ramming as in machine molding, metal patterns are recommended even though costlier than wood.

Ex: cast iron, aluminum and copper alloys

Plastics: Combine the advantage of wood-like lightness, easy machinability and economy with the wear-resistance and dimensional stability of metals.

Sometimes used as wear resistant coating on wooden patternEx: Epoxy resins

Polystyrene: Polystyrene or expanded Thermocole, which has special property that gasifies on heating.

For single quantity castings like prototypes, it is very easy to make a pattern which is not taken out but either burnt off (or) metal is poured through gating directly into the mold without removing the pattern.

As the polystyrene gasifies leaving negligible residue , the casting formed is sound.

Comparative Characteristics of Metallic Pattern Materials

Pattern metal Advantages Disadvantages

Aluminium alloys

Good machinability

High corrosion resistance

Low density

Good surface finish

Low strength

High cost

Grey cast iron

Good machinability

High strength

Low cost

Corrosion prone

SteelGood surface finish

High strengthCorrosion prone

Brass & bronze

Good surface finish

High strength

High corrosion resistance

High cost

Lead alloys Good machinabilityHigh cost

Low strength85

Pattern Materials based on Expected Life

Number of castings produced before pattern equipment repair

Pattern material

Small castings(under 600 mm)

2,000 Hard wood

6,000 Aluminum, plastic

100,000 Cast iron

Medium castings(600 – 1,800 mm)

1,000 Hard wood

3,000 Aluminum, plastic

Large castings(above 1,800 mm)

200 Soft wood

500Hard wood metal reinforced

86

Types of Patterns

1. Single piece pattern

2. Split/two piece pattern

3. Gated pattern

4. Cope and drag pattern

5. Loose piece pattern

6. Follow board Pattern

7. Sweep pattern

8. Skeleton pattern

87

1.Single Piece Pattern• Inexpensive and simplest type of pattern.

• Made of single piece.

• Used in cases where the job is simple and doesn’t create withdrawal

problems.

• Applications in very small scale production

88

2.Split/Two Piece Pattern• Mostly used for intricate castings.

• Pattern is split into 2 halves: 1 part in drag and other in cope.

Split surface of the pattern is the parting plane of the mould.

• Two halves of the pattern are aligned using the dowel pins fitted in

cope half which match precisely with holes made drag half.

Dowel Pins

89

3. Gated Pattern

• Improved version of simple or single piece pattern.

• Gating and runner system integrated in this pattern.

• Hand cutting of runners and gates are eliminated

Productivity improvement of a moulder.

These are made of wood or metal and specially used for mass productions of small castings

In mass production, a number of castings are prepared in a single multicavity mould by joining a group of patterns.

In such type of multicavity mould, gates or runners for the molten metal are formed by connecting parts between the individual patterns

90

• Similar to split or two piece pattern.

• In addition to splitting the pattern, the cope and drag halves of the pattern along with the gating and risering systems are attached separately to the metal or wooden plates along with the alignment pins.

• The cope and drag moulds may be produced using these patterns separately by two moulders but they can be assembled to form a complete mould.

• These types of patterns are used for castings which are heavy and inconvenient for handling as also for continuous production.

91

4. Cope and Drag Pattern

92

5.Loose Piece Pattern• Used when the contour of the part is such that withdrawing the pattern from

the mould is not possible.

During moulding the obstructing part of the contour is held as a loose

piece.• After moulding is over, first the main pattern is removed and then the loose

pieces are recovered through the gap generated by the main pattern.• Moulding with loose pieces is a highly skilled job and is generally expensive

Should be avoided where possible.

93

6.Follow Board Pattern• Adopted for those castings where there are some portions which are

structurally weak and if not supported properly are likely to break under the

force of ramming.

• Bottom board is modified as a follow board to closely fit the contour of

the weak pattern

support it during the ramming of the drag.

• During the preparation of the cope, no follow board is necessary because

the sand which is compacted in the drag will support the fragile pattern.

94

7.Sweep Pattern• Used to sweep complete pattern by means of a plane sweep.

• Used for generating large shapes which are axis-symmetrical

• Ex. Bell shaped or cylindrical.

• Greatly reduces the cost of a 3-dimensional pattern.

• Suitable for large castings ex. bells for ornamental purposes which

are cast in pit molds.

95

96

8.Skeleton Pattern• Used for production of only few castings of large weight and size

• Skeleton pattern made of wood is used for building final pattern which

is made by packing sand around the skeleton.

• The desired form is obtained by removing the excess sand using a

strickle (foundry tool used to shape a mold in sand).

• Skeleton depends upon the geometry of the workpiece.

• Skeleton pattern used for very large castings required in small

quantities where large expense on complete wooden pattern is not

economical.

97

Skelton Pattern Contd..

98