Metal Casting

Lecture 3

Summary of Casting

Processes

TABLE 11.1Process Advantages LimitationsSand Almost any metal cast; no limit

to size, shape or weight; low tooling cost.

Some finishing required; somewhat coarse finish; wide tolerances.

Shell mold Good dimensional accuracy and surface finish; high production rate.

Part size limited; expensive patterns and equipment required.

Expendable pattern Most metals cast with no limit to size; complex shapes

Patterns have low strength and can be costly for low quantities

Plaster mold Intricate shapes; good dimensional accu- racy and finish; low porosity.

Limited to nonferrous metals; limited size and volume of production; mold making time relatively long.

Ceramic mold Intricate shapes; close tolerance parts; good surface finish.

Limited size.

Investment Intricate shapes; excellent surface finish and accuracy; almost any metal cast.

Part size limited; expensive patterns, molds, and labor.

Permanent mold Good surface finish and dimensional accuracy; low porosity; high production rate.

High mold cost; limited shape and intricacy; not suitable for high-melting-point metals.

Die Excellent dimensional accuracy and surface finish; high production rate.

Die cost is high; part size limited; usually limited to nonferrous metals; long lead time.

Centrifugal Large cylindrical parts with good quality; high production rate.

Equipment is expensive; part shape limited.

Basic FeaturesPattern and Mould

◦ A pattern is made of wood or metal, is a replica of the final product and is used for preparing mould cavity

◦ Mould cavity which contains molten metal is essentially a negative of the final product

◦ Mould material should posses refractory characteristics and with stand the pouring temperature

◦ When the mold is used for single casting, it made of sand and known as expendable mold

◦ When the mold is used repeatedly for number of castings and is made of metal or graphite are called permanent mould

◦ For making holes or hollow cavities inside a casting, cores made of either sand or metal are used.

Melting and Pouring◦Several types of furnaces are available for melting metals and their selection depends on the type of metal, the maximum temperature required and the rate and the mode of molten metal delivery.

◦Before pouring provisions are made for the escape of dissolved gases. The gating system should be designed to minimize the turbulent flow and erosion of mould cavity.The other important factors are the pouring temperature and the pouring rate.

Solidification and Cooling

◦ The properties of the casting significantly depends on the solidification time cooing rate.

◦ Shrinkage of casting, during cooling of solidified metal should not be restrained by the mould material, otherwise internal stresses may develop and form cracks in casting.

◦ Proper care should be taken at the design stage of casting so that shrinkage can occur without casting defects.

Removal, Cleaning, Finishing and Inspection

◦ After the casting is removed from the mould it is thoroughly cleaned and the excess material usually along the parting line and the place where the molten metal was poured, is removed using a potable grinder.

◦ White light inspection, pressure test, magnetic particle inspection, radiographic test, ultrasonic inspection etc. are used

Classification of casting processes

Open and Closed Mould

Sand Casting (Expandable-mould, Permanent-pattern Casting)

Pattern geometry

Use of chaplets to avoid shifting of cores

Possible chaplet design and casting with core

Production steps in sand casting including pattern making and mold making

PatternsVariety of patters are used in casting

and the choice depends on the configuration of casting and number of casting required◦ Single-piece pattern◦ Split pattern◦ Follow board pattern◦ Cope and drag pattern◦ Match plate pattern◦ Loose-piece pattern◦ Sweep pattern◦ Skeleton pattern

(a)Split pattern

(b) Follow-board

(c) Match Plate

(d) Loose-piece

(e) Sweep

(f) Skeleton pattern

Pattern allowancesShrinkage allowanceDraft allowanceMachining allowanceDistortion allowance

Moulding Materials Major part of Moulding material in sand

casting are1. 70-85% silica sand (SiO2)2. 10-12% bonding material e.g., clay cereal etc.3. 3-6% water

Requirements of molding sand are:(a) Refractoriness(b) Cohesiveness(c) Permeability(d) Collapsibility

The performance of mould depends on following factors:

(e) Permeability(f) Green strength(g) Dry strength

Effect of moisture, grain size and shape on mould quality

Melting and Pouring The quality of casting depends on the method of

melting. The melting technique should provide molten metal at required temperature, but should also provide the material of good quality and in the required quantity.

Pouring vessels

Molten metal is prevented from oxidation by covering the molten metal with fluxes or by carrying out melting and pouring in vacuum

Ladles which pour the molten metal from beneath the surface are used

The two main consideration during pouring are the temperature and pouring rate

Fluidity of molten metal is more at higher temperature but it results into more amount of dissolved gases and high temperature also damage the mould walls and results into poor surface quality of the casting

To control the amount of dissolved gases low, the temperature should not be in superheated range

In ferrous metals, the dissolved hydrogen and nitrogen are removed by passing CO. In non-ferrous metals, Cl, He, or Ar gases are used.

Therefore, fluidity and gas solubility are two conflicting requirements. The optimum pouring temp. is therefore decided on the basis of fluidity requirements. The temp. should be able to fill the whole cavity at the same time it should enter inside the voids between the sand particles.

Cooling rate depends on casting material and configuration. It also depends on volume and surface area of the casting also.

The pouring rate should be such that solidification does not start and the cavity is completely filled without eroding mould surface and undue turbulence.

The Gating System1. Minimize turbulent flow so that

absorption of gases, oxidation of metal and erosion of mould surfaces are less

2. Regulate the entry of molten metal into the mould cavity

3. Ensure complete filling of mould cavity, and

4. Promote a temperature gradient within the casting so that all sections irrespective of size and shape could solidify properly

The Gating System

A: pouring basin

B: WeirC: SprueD: Sprue wellE: RunnerF: IngatesG: Runner

break upH: Blind J: Riser

Use of chills

Cooling and Solidification

Pure metal

Alloy

Mechanism of SolidificationPure metals solidifies at a constant temp.

equal to its freezing point, which same as its melting point.

The change form liquid to solid does not occur all at once. The process of solidification starts with nucleation, the formation of stable solid particles within the liquid metal. Nuclei of solid phase, generally a few hundred atom in size, start appearing at a temperature below the freezing temperature. The temp. around this goes down and is called supercooling or undercooling. In pure metals supercooling is around 20% of the freezing temp.

A nuclease, more than a certain critical size grows, and causes solidification.

By adding, certain foreign materials (nucleating agents) the undercooling temp. is reduced which causes enhanced nucleation.

In case of pure metals fine equi-axed grains are formed near the wall of the mold and columnar grain growth takes place upto the centre of the ingot.

In typical solid-solution alloy, the columnar grains do not extend upto the center of casting but are interrupted by an inner zone of equiaxed graines.

My adding typical nucleating agents like sodium, magnesium or bismuth the inner zone of equiaxed grained can be extended in whole casting.

Solidification TimeOnce the material cools down to

freezing temperature, the solidification process for the pure metals does not require a decrease in temperature and a plateau is obtained in the cooling curves, called thermal arrest. The solidification time is total time required for the liquid metal to solidify.

Solidification time has been found to be directly proportional to volume and inversely proportional to surface area.

Location of Risers and Open and Closed Risers

• Top riser has the advantage of additional pressure head and smaller feeding distance over the side riser.

• Blind risers are generally bigger in size because of additional area of heat conduction.

Why Riser?The shrinkage occurs in three stages,

1.When temperature of liquid metal drops from pouring to zero temperature

2.When the metal changes from liquid to solid state, and

3.When the temperature of solid phase drops from freezing to room temperature

The shrinkage for stage 3 is compensated by providing shrinkage allowance on pattern, while the shrinkage during stages 1 and 2 are compensated by providing risers.

The riser should solidify in the last otherwise liquid metal will start flowing from casting to riser. It should promote directional solidification. The shape, size and location of the risers are important considerations in casting design

Cleaning and Finishing1. Casting is taken out of the mould by

shaking and the Moulding sand is recycled often with suitable additions.

2. The remaining sand, some of which may be embedded in the casting, is removed by means of Shot blasting.

3. The excess material in the form of sprue, runners, gates etc., along with the flashes formed due to flow of molten metal into the gaps is broken manuaaly in case of brittle casting or removed by sawing and grinding in case of ductile grinding.

4. The entire casting is then cleaned by either shot blasting or chemical pickling.

5. Sometimes castings are heat treated to achieve better mechanical properties.

Casting DefectsDefects may occur due to one or more of the following reasons:◦ Fault in design of casting pattern◦ Fault in design on mold and core◦ Fault in design of gating system and

riser◦ Improper choice of moulding sand◦ Improper metal composition◦ Inadequate melting temperature and

rate of pouring

Classification of casting defectsCasting defects

SurfaceDefect

Internal Defect

Visible defects

BlowScar

BlisterDropScab

PenetrationBuckle

Blow holesPorosityPin holesInclusions

Dross

WashRat tailSwell

MisrunCold shutHot tear

Shrinkage/Shift

Surface Defects

These are due to poor design and quality of sand molds and general cause is poor ramming

Blow is relatively large cavity produced by gases which displace molten metal from convex surface. Scar is shallow blow generally occurring on a flat surface. A scar covered with a thin layer of metal is called blister. These are due to improper permeability or venting. Sometimes excessive gas forming constituents in moulding sand

Drop is an irregularly-shaped projection on the cope surface caused by dropping of sand.

A scab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold.

Penetration occurs when the molten metal flows between the sand particles in the mould. These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it.

Buckle is a vee-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face. A proper amount of volatile additives in moulding material could eliminate this defect by providing room for expansion.

Internal Defects

The internal defects found in the castings are mainly due to trapped gases and dirty metal. Gases get trapped due to hard ramming or improper venting. These defects also occur when excessive moisture or excessive gas forming materials are used for mould making.

Blow holes are large spherical shaped gas bubbles, while porosity indicates a large number of uniformly distributed tiny holes. Pin holes are tiny blow holes appearing just below the casting surface.

Inclusions are the non-metallic particles in the metal matrix, Lighter impurities appearing the casting surface are dross.

Visible Defects

Insufficient mould strength, insufficient metal, low pouring temperature, and bad design of casting are some of the common causes.

Wash is a low projection near the gate caused by erosion of sand by the flowing metal. Rat tail is a long, shallow, angular depression caused by expansion of the sand. Swell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand.

Misrun and cold shut are caused by insufficient superheat provided to the liquid metal.

Hot tear is the crack in the casting caused by high residual stresses.

Shrinkage is essentially solidification contraction and occurs due to improper use of Riser.

Shift is due to misalignment of two parts of the mould or incorrect core location.

Casting with expendable mould: Investment Casting

Advantages and Limitations

Parts of greater complexity and intricacy can be cast

Close dimensional control 0.075mmGood surface finishThe lost wax can be reusedAdditional machining is not required in normal

coursePreferred for casting weight less than 5 kg,

maximum dimension less than 300 mm, Thickness is usually restricted to 15mm

Al, Cu, Ni, Carbon and alloy steels, tool steels etc. are the common materials

Permanent mould casting: Die casting

Graphite+oil

General Configuration of a Die Casting Machine

In Die casting the molten metal is forced to flow into a permanent metallic mold under moderate to high pressures, and held under pressure during solidification

This high pressure forces the metal into intricate details, produces smooth surface and excellent dimensional accuracy

High pressure causes turbulence and air entrapment. In order to minimize this larger ingates are used and in the beginning pressure is kept low and is increased gradually

Cycle in Hot Chamber Casting

Cycle in Cold Chamber Casting

Centrifugal Casting

• A permanent mold made of metal or ceramic is rotated at high speed (300 to 3000 rpm). The molten metal is then poured into the mold cavity and due to centrifugal action the molten metal conform to the cavity provided in the mould.

• Castings are known for their higher densities in the outer most regions.

• The process gives good surface finish

• Applications: pipes, bushings, gears, flywheels etc.

Comparison of Casting Processes