Week 2

Metal –Casting Processes

Source : Manufacturing Processes for Engineering Materials,5Source : Manufacturing Processes for Engineering Materials,5rdrd Edition., S. Kalpakjian and S. Schmid, 2008 , Edition., S. Kalpakjian and S. Schmid, 2008 , Additional references in the last slide

Metals processed by casting

Sand casting – 60% Permanent mold casting – 11% Die casting – 9% Investment casting – 7% Centrifugal casting – 7% Shell mold casting – 6%


Types of Parts Made

Engine blocks Pipes Jewelry


Mold Features

The following is a gravity casting system.2 principles of fluid flow are relevant to gating design:

Bernoulli’s theorem and the law of mass continuity.


Ferrous casting alloysCast irons represent the largest amount of all metals cast and can cast into complex shapes.

Types of irons:

Gray cast iron Ductile iron (nodular iron) White cast iron Malleable iron Compacted-graphite iron Cast steels Cast stainless steels


Nonferrous casting alloys

Types of alloys:

Aluminum-based alloys Magnesium-based alloys Copper-based alloys Zinc-based alloys High-temperature alloys


Components of Casting

Path: Sprue -> Well -> Runner -> Mold cavity Riser: Compensate volume loss due to shrinkage Location of riser? Core: Make holes Core print Draft: Prevent collapse

of sand


Expendable-Mold Permanent-Pattern Casting Processes


Sand Casting

Types of sand molds 3 types: green-sand, cold-box, and no-bake

molds. Green molding sand is mixture of sand, clay, and

water and is inexpensive. In skin-dried method, castings has high strength,

better accuracy and surface finish. In no-bake mold process, a synthetic liquid resin

is mixed with the sand and hardened in room temperature.


Sand Casting


Shell-mold casting Can produce castings with close

dimensional tolerances Good surface finish Low cost.


Ceramic Mold Manufacture

FIGURE 5.18 Sequence of operations in making a ceramic mold .


Description: Green Sand Casting Capabilities

Horacio Elizondo Author

April 2003Date

Alting,Leo. “Manufacturing Processes Reference Guide.” 1994Reference

Shape Material Conserve Material Consolidation Function – Sub function


Expendable-Mold ,Expendable-Pattern Casting Processes


Expendable-pattern casting (lost foam) Evaporative Pattern Casting

FIGURE 5.20 Schematic illustration of the expendable-pattern casting process, also known as lost-foam or evaporative-pattern casting .


Investment Casting (lost-wax process)

Source: Schematic illustration of investment casting (lost wax process). Castings by this method can be made with very fine detail and from a variety of metals. Source: Steel

Founders' Society of America .Source : Manufacturing Processes for Engineering Materials,5Source : Manufacturing Processes for Engineering Materials,5rdrd Edition., S. Kalpakjian and S. Schmid, 2008 , Edition., S. Kalpakjian and S. Schmid, 2008 , Additional references in the last slide

Description: Investment Casting Capabilities


April 2003Date




Investment Casting


Permanent-Mold Casting Processes


Continuous-Casting

(a) The continuous-casting process for steel. Note that the platform is about 20 m (65 ft) above ground level. Source: American Foundrymen's Society.

(b) (b) Continuous strip casting of nonferrous metal strip. Source: Courtesy of Hazelett Strip-Casting Corp.


Description: Permanent Mold Casting Capabilities


April 2003Date




Pressure & Hot-Chamber Die Casting

FIGURE 5.23 The pressure casting process, utilizing graphite molds for the production of steel railroad wheels. Source: Griffin Wheel Division of

Amsted Industries Incorporated .

FIGURE 5.24 Schematic illustration of the hot-chamber die-casting process .


Description: Hot Chamber Die Casting Capabilities


April 2003Date




Cold-Chamber Die Casting

FIGURE 5.25 Schematic illustration of the cold-chamber die-casting process. These machines are large compared to the size of the casting, because high forces are required to keep the two halves of the die closed

under pressure .Source : Manufacturing Processes for Engineering Materials,5Source : Manufacturing Processes for Engineering Materials,5rdrd Edition., S. Kalpakjian and S. Schmid, 2008 , Edition., S. Kalpakjian and S. Schmid, 2008 , Additional references in the last slide

Die Casting


Centrifugal Casting

FIGURE 5.26 Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped hollow parts can be cast by this process .


Semicentrifugal Casting

FIGURE 5.27 (a) Schematic illustration of the semicentrifugal casting process. Wheels with spokes can be cast by this process. (b) Schematic illustration of casting by centrifuging. The molds are placed at the

periphery of the machine, and the molten metal is forced into the molds by centrifugal forces .


Properties of Die-Casting Alloys

TABLE 5.6 Properties and typical applications of common die-casting alloys.


Rotor Microstructure

FIGURE 5.22 Microstructure of a rotor that has been investment cast (top) and conventionally cast (bottom). Source: Advanced Materials and Processes, October

1990, p. 25. ASM International .Source : Manufacturing Processes for Engineering Materials,5Source : Manufacturing Processes for Engineering Materials,5rdrd Edition., S. Kalpakjian and S. Schmid, 2008 , Edition., S. Kalpakjian and S. Schmid, 2008 , Additional references in the last slide

Fluid Flow and Heat Transfer

Bernoulli’s theorem Based on

- principle of conservation of energy - frictional losses in a fluid system

Conservation of energy requires that,

Constant 2

2

g

v

g

ph

h = elevationp = pressure at elevation

v = velocity of the liquidρ = density of the fluid

fg

v

g

ph

g

v

g

ph

2

2

222

2

211

1


Fluid Flow and Heat Transfer- Fluid flow

Mass continuity States that for an incompressible liquid the rate

of flow is constant.

Subscripts 1 and 2 pertain to two different locations in the system.

2211 vAvAQ Q = volumetric rate of flowA = cross-sectional area of the liquid streamv = velocity of the liquid



Sprue profile Relationship between height and cross-sectional area

at any point in the sprue is given by

Velocity of the molten metal leaving the gate is

When liquid level reached height x, gate velocity is

1

2

2

1

h

h

A

A

ghcv 2

xhgcv 2Source : Manufacturing Processes for Engineering Materials,5Source : Manufacturing Processes for Engineering Materials,5rdrd Edition., S. Kalpakjian and S. Schmid, 2008 , Edition., S. Kalpakjian and S. Schmid, 2008 , Additional references in the last slide


Flow characteristics Reynold’s Number Ratio of momentum (inertia) to viscosity Fluid flow in gating systems is turbulence, as opposed to

laminar flow. (Which flow is preferred?) Reynolds number, Re, is used to characterize this aspect

of fluid flow.

Higher the Re, greater the tendency for turbulent flow.

vD

Re

v = velocity of the liquidD = diameter of the channelρ = density n = viscosity of the liquid.


Critical Reynold’s Number

• Re ~ 2,000– Laminar to turbulent transition

– Eddies begin to form

• Re > 20,000– very turbulent


How fast would a stream of honey 1 in. in diameter need to be turbulent?

Density (ρ) = 1.43 g/cm3 (at 20oC)Viscosity () = 189 poise (at 20.6oC)

vD

Re

9.18

1430254.0Re

v

turbulent flow transition Re ~ 2,000

Re = 2,000 = 1,430 * V * 0.0254/18.9

V = 1,040 m/s )This ignores shear thinning(.

Example 1


The desired volume flow rate of the molten metal into a mold is 0.01 m3/min .

The top of the sprue has a diameter of 20 mm and its length is 200 mm.

What diameter should be specified at the bottom of the sprue in order to prevent aspiration?

What is the resultant velocity and Reynolds number at the bottom of the sprue if the metal being cast is aluminium and has a viscosity of 0.004 N-s/m2

Solution

Since d1 = 0.02 m

The metal volume flow rate is Q= 0.01 m3/min = 1.667×10-4 m3/s 1Top, 2 bottom

Therefore

Example 5.2 Design and analysis of a sprue for casting

m/s 531.01014.3

10667.14

4

11

A

Qv

24221 m 1014.3002.0

44

dA


m/s 45.12 v

Assuming no frictional losses, and recognizing that the pressure at the top and bottom of the sprue is atmospheric

Thus ,

fg

v

g

ph

g

v

g

ph

2

2

222

2

211

1 0

81.92

81.92

531.02.0

22

2

v

g

p

g

p atmatm

242 m 1015.1 A22vAQ 45.1101.667 2

4 A

22 4

dA

24

41015.1 d

mmd 12


745,11

004.0

2700012.045.1Re

vD

In calculating the Reynolds number

3/2700 mkg

As stated above, this magnitude is typical for casting molds, representing a mixture of laminar and turbulent flow

An Re value of up to 2000 represents laminar flow An Re Between 2000 and 20,000 it is a mixture of laminar and turbulent flow and is generally regarded as harmless in gating systems for casting Re values in excess of 20,000 represent severe turbulence.

v = velocity of the liquidD = diameter of the channelρ = density n = viscosity of the liquid.


Physical Properties of Materials

TABLE 3.3 Physical Properties of Various Materials at Room Temperature.


Heat transfer

Heat flow depends on casting material and the mold and process parameters.

Temperature distribution in the mold-liquid metal interface is shown below.


22

,int_

1

4

casting

casting

moldmoldmoldinitilmoldpomelting

CastingCasting

A

v

ckTTt

2

,int_

casting

casting

initilmoldpomelting

CastingCasting

A

v

TTht

Solidification time (t) for an insulating mold (α mold << α casting; k mold << k casting)

Solidification time (t) for a conducting mold (Biot # =hl/k < 0.17)

ΔH = latent heat for the process = Hf + ΣCiΔTHf = latent heat of solidification (fusion)V = volumeA = areah = heat transfer coefficientC = specific heatρ = density


Chvorinov’s rule for solidification time for a conducting mold

t = K (V/A)

Chvorinov’s rule for solidification time for an insulating mold

t = cooling timeK = a constantV = volumeA = area

2

A

VKt

Cooling time (t) for a solid object for a small Biot number (Biot # =hl/k < 0.17)

finalcastingpomelting

initilcastingmoldcastingcasting

casting

casting

TT

TT

h

C

A

vt

,int_

,ln


moldtotaltotal

gate

moldf hhh

gA

At

2

2

total

bapouring

sprueofbottom

sprueoftop

sprueoftop

spreueofbottom

h

h

v

v

A

A sin,

..

..

..

..

Filling time for a bottom-gated mold

For no aspiration

Mold filling time estimate

gategatevA

volumeMoldt

.


Material properties:Data for solid materials at room temperature

MaterialSpecific heat (C)(kJ/kg-oC)

Density (ρ)(kg/m3)

Thermal conductivity(k) (W/m-oC)

Sand1.1615000.60

Aluminum0.902700202

Nickel0.44891092

Magnesium1.071700156

Copper0.398970385

Gray cast iron0.441712542.7


MaterialMelting point(oC)

Latent heat ofsolidification (fusion)

)Hf) (kJ/kg(

Specific heat (C))kJ/kg-oC(

Viscosity)(

)mPa-s(

Aluminum6603961.051.3

Nickel14532970.73---

Magnesium6503841.381.04

Copper10832200.522.1

Gray cast iron

12512110.345.25

Data for liquid materials


Solidification time

Solidification time is a function of the volume of a casting and surface area (Chvorinov’s rule).

Effects of mold geometry and elapsed time on skin thickness and its shape are show.

n

C

Area Surface

Volumetion timeSolidifica C = constant

n = 2


Three pieces being cast have the same volume but different shapes. One is a sphere, one a cube, and the other a cylinder with a height equal to its diameter. Which piece will solidify the fastest and which one the slowest? Use n = 2.

SolutionThe volume is unity

Respective surface areas are

Respective solidification times t are

Example 5.3Solidification times for various solid shapes

54.522 :Cylinder

66 : Cube

84.44

34 :Sphere

2

2

3/2

rhrA

aA

A

2area Surface

1tion timeSolidifica

CtCCt cylindersphere 033.0 028.0 043.0 cubet


Shrinkage

Shrinkage in casting causes dimensional changes. Cracking is a result of:

1. Contraction of the molten metal

2. Contraction of the metal during phase change

3. Contraction of the solidified metal

For L->S, always think of

Solidification Shrinkage!!


Melting Practice and Furnaces

Melting has a direct bearing on the quality of castings. Fluxes are inorganic compounds that refine the molten

metal by removing dissolved gases and various impurities.

The metal charge may be composed of commercially pure primary metals, which can include remelted or recycled scrap.


Casting Alloys

Source : Manufacturing Processes for Engineering Materials,5rd Edition., S. Kalpakjian and S. Schmid, 2008 , Additional references in the last slideSource : Manufacturing Processes for Engineering Materials,5rd Edition., S. Kalpakjian and S. Schmid, 2008 , Additional references in the last slideP.P.P File Prepared by the author and publisher and other sourcesP.P.P File Prepared by the author and publisher and other sources

Bibliography (References). Manufacturing Processes for Engineering Materials,5rd Edition., S. Kalpakjian and S. Schmid, Prentice

Hill 2008 Materials and Processes in Manufacturing, 10th Edition, E. Paul DeGarmo, J. T. Black, Ronald A. Kohser

,2007.Manufacturing Engineering & Technology , Serope Kalpakjian , 6th Edition, Prentice Hall,2009.TECNOLOGI CO DE MONTERREY Mechanical Manufacturing, Professor Arturo Molina , October 2004.E. Paul DeGarmo et al, “Materials And Processes in Manufacturing”, Wiley Publishing Company 2003.

John E. Schey, “Introduction To Manufacturing Processes” McGraw-Hill Book Company,1988.Courtney, T. H., “ Mechanical Behavior of Materials”, N. Y., McGraw-Hill, 1990.E., George E. and George F., “ The Testing of Engineering Materials”, McGrraw-Hill Book Company.

1982.“Hardness Tests”, Metals Park, Ohio: ASM International, 1987.Harmer E., George E. and George F., “ The Testing of Engineering Materials”, McGrraw-Hill Book

Company. 1982.Halmshaw R. “ Non-Destructive Testing”, Edward Arnold, 1991.Courtney, T. H., “ Mechanical Behavior of Materials”, N. Y., McGraw-Hill, 1990.Jonathan S. Colton , Manufacturing Processes and Engineering, Georgia Institute of Technology, 2009.Pohlandt K., “ Material Testing for Metal forming Industry”, N.Y Springer 1989.Lawrence E. Doyle, “Manufacturing Process And Materials For Engineers”, Prentice-Hall, Third

Edition.T.T. EL-Midany & M.A. Mansour , Manufacturing Technology, King Abdulaziz University.S.F. Kvav et al., Machine Tool Operations, McGraw-Hill Book Company.Geoffrey Boothroyd, Fundamentals of Metal Machining and Machine Tools, McGraw-Hill Book

Company.Cold and Hot Forging Fundamentals and Applications, Taylan Altan ,al, ASM,2007.Fundamentals of Metal Forming ,Robert H. Wagoner,Jean-Loup,Wiley,1997.Metal Forming ,Willam F. Hosford Robert M. ,second Edition, PTR, 1993.

Week 2

Documents

Transcript of Week 2