Casting Manufacturing Processes

Chapter 10Fundamentals of Metal Casting

Chapter 11Metals for Casting

Traditional Manufacturing Processes

Casting

Forming

Sheet metal processing

Cutting

Joining

Powder- and Ceramics Processing

Plastics processing

Surface treatment

FUNDAMENTALS OF METAL CASTING

Overview of Casting TechnologyHeating and PouringSolidification and Cooling

Classification of solidification processes

Casting

Refractory mold pour liquid metal solidify, remove finish

Process in which molten metal flows by gravity or other force into

a mold where it solidifies in the shape of the mold cavity Melt the metal Pour it into a mold Let it

freeze

Can create complex part geometries Can create both external and internal shapes complex geometry, internal cavities, hollow sections

• VERSATILE: small (~10 grams) very large parts (~1000 Kg)• ECONOMICAL: little wastage (extra metal is re-used)

• ISOTROPIC: cast parts have same properties along all directions

Casting Advantages:

Some casting processes are net shape; others are near net shapeCan produce very large parts Some casting methods are suited to mass production

Disadvantages of Casting

Different disadvantages for different casting processes:

Limitations on mechanical propertiesPoor dimensional accuracy and surface finish for some processes; e.g., sand castingSafety hazards to workers due to hot molten metalsEnvironmental problems

Big parts: engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, big bells, big statues, and pump housings Small parts: dental crowns, jewelry, small statues, and frying pans All varieties of metals can be cast, ferrous and nonferrous

Parts Made by Casting

Different Casting Processes

Process Advantages Disadvantages Examples

Sand many metals, sizes, shapes, cheap poor finish & tolerance engine blocks, cylinder heads

Shell mold better accuracy, finish, higher production rate

limited part size connecting rods, gear housings

Expendablepattern

Wide range of metals, sizes, shapes

patterns have low strength

cylinder heads, brake components

Plaster mold complex shapes, good surface finish

non-ferrous metals, low production rate

prototypes of mechanical parts

Ceramic mold complex shapes, high accuracy, good finish

small sizes impellers, injection mold tooling

Investment complex shapes, excellent finish small parts, expensive jewellery

Permanent mold

good finish, low porosity, high production rate

Costly mold, simpler shapes only

gears, gear housings

Die Excellent dimensional accuracy, high production rate

costly dies, small parts,non-ferrous metals

gears, camera bodies, car wheels

Centrifugal Large cylindrical parts, good quality

Expensive, few shapes pipes, boilers, flywheels

Sand Casting

Open Molds and Closed Molds Sand Casting

Sand Casting

cope: top half

drag: bottom half

core: for internal cavities

pattern: positive

funnel sprue runners gate cavity {risers, vents}

Metal is poured into Cavity through a Channel called sprue, which transmits the molten metal via runner into the mold cavity At top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into down sprue. The runner should not be big because it will increase the amount of the waste metal. It should not be small because this enhances rapid solidification in the runner causing a blockage

Gating System

At the bottom of the sprue there is a gap called well for the collection of the unwanted sand, which comes with the flowing metal

Reservoir in the mold 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

Riser

To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimumSince the geometry of the riser is normally selected to maximize the V/A ratio, this allows reduction of riser volume as much as possible

Sand Casting Considerations..

(d) taper

- do we need it ?

Mold cavity

chaplet

Mold cavity

chaplet

(e) core prints, chaplets

- hold the core in position - chaplet is metal (why?)

(f) cut-off, finishing

Forming the Mold Cavity

(a) How do we make the pattern?

[cut, carve, machine]

(b) Why is the pattern not exactly identical to the part shape?The pattern is usually oversized to allow for shrinkage of metal as it solidifies and cools , post-processing

- pattern outer surfaces; (inner surfaces : core)

(c) parting lineThe reason for this is to remove the tasted part easier from the mold

- how to determine?

Mold cavity is formed by packing sand around a pattern, which has the shape of the part

Pouring the Molten Metal• For this step to be successful, metal must flow into all regions of the

mold, most importantly the main cavity, before solidifying

• Factors that determine success:

– Pouring temperature

– Pouring rate

– Turbulence

Pouring the Molten Metal

Solidification of Metals

Transformation of molten metal back into solid stateSolidification differs depending on whether the metal is a pure element or an alloy

Directional Solidification

To minimize 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 toward the riser(s)

Thus, molten metal is continually available from risers to prevent shrinkage voids The term directional solidification describes this aspect of freezing and methods by which it is controlled

Achieving Directional SolidificationDesired directional solidification is achieved using Chvorinov's Rule to design the casting itself, its orientation in the mold, and the riser system that feeds itLocate sections of the casting with lower V/A ratios away from riser, so freezing occurs first in these regions, and the liquid metal supply for the rest of the casting remains open

Chills ‑ internal or external heat sinks that cause rapid freezing in certain regions of the casting

A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point)

Cooling curve for a pure metal during casting

Solidification of Pure Metals

Solidification of Pure Metals

Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface immediately after pouringSkin thickness increases to form a shell around the molten metal as solidification progressesRate of freezing depends on heat transfer into mold, as well as thermal properties of the metal

Characteristic grain structure in a casting of a pure metal, showing randomly oriented grains of small size near the mold wall, and large columnar grains oriented toward the center of the casting

Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of casting

Solidification of some Compounds

Solidification Time

Solidification takes timeTotal solidification time TTS = time required for casting to solidify after pouringTTS depends on size and shape of casting by relationship known as Chvorinov's Rule

where TST = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent with typical value = 2; and Cm is mold constant

n

m AV

CTST

• Cm depends on mold material, thermal properties of casting metal, and pouring temperature relative to melting point

• Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using same mold material, metal, and pouring temperature, even though the shape of the part may be quite different

Mold Constant in Chvorinov's Rule

What Chvorinov's Rule Tells Us

A casting with a higher volume‑to‑surface area ratio cools and solidifies more slowly than one with a lower ratio

To feed molten metal to main cavity, TST for riser must be greater than TST for main casting

Since riser and casting mold constants will be equal, design the riser to have a larger volume‑to‑area ratio so that the main casting solidifies first

This minimizes the effects of shrinkage

There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the cast product.

Casting Defects: 1) Misruns (due to rapid solidification in the runner) 2) Cold shuts (due to rapid solidification before complete filling of the mold) 3) Cold shots (due to splattered globules of metal during pouring) 4) Shrinkage cavity (due to lack of riser system) 5) Microporosity (due to localized solidification shrinkage) 6) Hot tearing (due to the die's prevention of contraction)

Casting Quality

Shrinkage of a cylindrical casting during solidification and cooling: (0) starting level of molten metal immediately after pouring; (1) reduction in level caused by liquid contraction during cooling (dimensional reductions are exaggerated for clarity in sketches)

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

(dimensional reductions are exaggerated for clarity in our sketches) Thus, solidification causes a reduction in volume per unit weight of metalException: cast iron with high C content ; Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change

Defects related with sand molds : 1) Sand blow 2) Pinholes 3) Sand wash 4) Scabs 5) Penetration 6) Mold shift 7) Core shift 8) Mold crack

Defects related with sand molds

Inspection Methods: 1) Visual Inspection to detect obvious defects such as Misruns, surface flaws. 2) Dimensional measurements to ensure that tolerances have been met. 3) Metallurgical, chemical, physical, and other tests related with the quality

a) Pressure testing to locate teaks in the casting b) Radiographic methods magnetic particle tests, use of fluorescent penetrants, and supersonic testing to detect either surface or internal defects in casting. c) Mechanical testing to determine properties such as tensile strength and hardness.

Inspection Methods

Shell mold casting - metal, 2-piece pattern, 175C-370C- coated with a lubricant (silicone)- mixture of sand, thermoset resin/epoxy- cure (baking) - remove patterns, join half-shells mold- pour metal- solidify (cooling)- break shell part

Expendable Mold Casting

- Styrofoam pattern- dipped in refractory slurry dried- sand (support)- pour liquid metal- foam evaporates, metal fills the shell- cool, solidify- break shell part

polystyrenepattern

patternsupport

sand

moltenmetal

polystyreneburns;gas escapespolystyrene

pattern

patternsupport

sand

moltenmetal

polystyreneburns;gas escapes

Plaster-mold, Ceramic-mold casting

Plaster-mold slurry: plaster of paris (CaSO4), talc, silica flour

Ceramic-mold slurry: silica, powdered Zircon (ZrSiO4)

- The slurry forms a shell over the pattern- Dried in a low temperature oven- Remove pattern- Backed by clay (strength), baked (burn-off volatiles)- cast the metal- break mold part

Plaster-mold: good finish (Why ?) plaster: low conductivity => low warpage, residual stresslow mp metal (Zn, Al, Cu, Mg)

Ceramic-mold: good finishhigh mp metals (steel, …) => impeller blades, turbines, …

Investment casting (lost wax casting)

(a) Wax pattern (injection molding)

(b) Multiple patterns assembled to wax sprue

(c) Shell built immerse into ceramic slurry immerse into fine sand (few layers)

(d) dry ceramic melt out the wax fire ceramic (burn wax)

(e) Pour molten metal (gravity) cool, solidify [Hollow casting: pouring excess metal before solidification

(f) Break ceramic shell (vibration or water blasting)

(g) Cut off parts (high-speed friction saw) finishing (polish)

Vacuum casting

Similar to investment casting, except: fill mold by reverse gravity

Easier to make hollow casting: early pour out

Permanent mold casting

MOLD: made of metal (cast iron, steel, refractory alloys)

CORE: (hollow parts)- metal: core can be extracted from the part- sand-bonded: core must be destroyed to remove

Mold-surface: coated with refractory material

- Spray with lubricant (graphite, silica)- improve flow, increase life

- good tolerance, good surface finish

- low mp metals (Cu, Bronze, Al, Mg)

Die casting

- a type of permanent mold casting- common uses: components for rice cookers, stoves, fans, washing-, drying machines, fridges, motors, toys, hand-tools, car wheels, …

HOT CHAMBER: (low mp e.g. Zn, Pb; non-alloying)(i) die is closed, gooseneck cylinder is filled with molten metal(ii) plunger pushes molten metal through gooseneck into cavity(iii) metal is held under pressure until it solidifies(iv) die opens, cores retracted; plunger returns(v) ejector pins push casting out of ejector die

COLD CHAMBER: (high mp e.g. Cu, Al)(i) die closed, molten metal is ladled into cylinder(ii) plunger pushes molten metal into die cavity(iii) metal is held under high pressure until it solidifies(iv) die opens, plunger pushes solidified slug from the cylinder(v) cores retracted(iv) ejector pins push casting off ejector die

Centrifugal casting

- permanent mold- rotated about its axis at 300 ~ 3000 rpm- molten metal is poured

- Surface finish: better along outer diameter than inner,- Impurities, inclusions, closer to the inner diameter (why ?)

Casting Design: Typical casting defects

Casting Design: Defects and Associated Problems

- Surface defects: finish, stress concentration

- Interior holes, inclusions: stress concentrations

2a

2b

0

0

max

max = 0(1 + 2b/a)

2a

2b

0

0

max

max = 0(1 + 2b/a)

Casting Design: guidelines

(a) avoid sharp corners(b) use fillets to blend section changes smoothly(c1) avoid rapid changes in cross-section areas

Casting Design: guidelines

(c1) avoid rapid changes in cross-section areas (c2) if unavoidable, design mold to ensure

- easy metal flow- uniform, rapid cooling (use chills, fluid-cooled tubes)

Casting Design: guidelines

(d) avoid large, flat areas- warpage due to residual stresses (why?)

Casting Design: guidelines

(e) provide drafts and tapers- easy removal, avoid damage- along what direction should we taper ?

Casting Design: guidelines

(f) account for shrinkage- geometry- shrinkage cavities

Casting Design: guidelines

(g) proper design of parting line

- “flattest” parting line is best

Further reading: Chapters 10-16, Kalpakjian & Schmid

Summary