Manufacturing Methods MAE 4500

Chapter 7 - Metal Casting

FIG. 6-5

FIG. 7-1

FIG. 7-3

FIG. 7-4

Casting PropertiesFundamental Properties:

ViscosityThe flow of the liquid metal inside the mold is controlled by its viscosity.

a. T>Tm

Viscosity is a function of superheat.

Superheat = (T - Tm) / Tm

b. TS < T < TL

Viscosity is a function of the structure and hence a function of the freezing

range.

Freezing range = (TL - TS) / TL

Viscosity low Viscosity high

Variation of viscosity in an eutectic system

FIG. 7-5

Gases and Solidification

Solid

Alloy

LiquidSolubility

TTm

S Pg1/2

Pg is the partialpressure of the gas over the melt

Similar to FIG. 6-17

To eliminate:1. Reduce pg ----> vacuum2. Melting with insoluble gas

(inert gas such as argon)3. Adding elements that react with the gas and form oxides

Al, Si, Ca with steelsPhosphorus with Cu and

bronze

Technological PropertiesFluidity:

This term indicates the capability of the molten metal to flow into the cavities of the system before freezing.

Fluidity is a function of:a. Fluid flow (viscosity)

• Heat transfer• Superheat• Mold temperature• Mold type

b. Solidification mechanism

Pure metalsand eutectic

alloys

Solid solutionalloys

Improvedfluidity

Fluidity Evaluation

Fluidity indexIt is measured by the length of the solidifiedspiral or length of the solidified thin plate.

FIG. 7-6

Heat Extraction and SolidificationProgressive vs. Directional Solidification

Directional solidification is important for the quality of the casting (solidification proceeds from the most remote section to the points of feeding).1. Feeder heads (risers)

They serve as wells of liquid metal to compensate for shrinkage during

solidification.2. Chills

They are heat-absorbing devices for fast freezing.

Risers (feeder heads) should have a high volume-to-area ratio compared to the product.

Directionalsolidification

Progressivesolidification

Aids to directionalsolidification

Improper positioningof riser

Chill

Effect of Process Parameters on Product Properties

1. Mold temperature2. Nucleating agents3. Breaking dendrites

a. Vibration during solidificationb. Centrifugingc. Forced flow (pressure die casting)d. Shearing (rheocasting)e. Pressure application during solidification

(squeeze casting)

(Grain size, porosity, and homogeneity control)(Grain size, porosity, and homogeneity control)

Fl

uidi

ty

Effect of mold temperature on properties and production rate

FIG. 7-7

Classification of casting processes

FIG. 7-12

Sequence of Operations of Sand Casting

FIG. 7-19

Similar to FIG. 7-15

Pattern Design

Match Plate Cope and Drag Plates

(a)

Characteristics of a Sand Mold

Similar to FIG. 7-17

FIG. 7-14

Investment Casting (Lost-wax Process)

FIG. 7-22

Centrifugal Casting

FIG. 7-26

Permanent molds

SIMILAR TO FIG. 7-24 and FIG. 7-25

Hot Chamber Die Casting

Cold Chamber Die Casting

Illustration of a Die Casting Machine