Lecture 5 6-Casting

8/10/2019 Lecture 5 6-Casting

http://slidepdf.com/reader/full/lecture-5-6-casting 1/44

Manufacturing Processes-I (TA 201A)

6 credit course

Course Instructor

K. Balani

Department of Materials Science and Engineering

6194 (office); [email protected]

http://lattice.mme.iitk.ac.in/~kbalani/TA201/TA201.htm

1



2

Casting



3

http://grassroutesjourneys.blogspot.com/2011/05/dhokra-tribal-art-of-odisha.html

http://elitehandicraft.co

m/About_us.htm

Manifold Turbine Blade

Dhokra art form

Manifold

Turbine blade



www.jepsculpture.com/bronze.shtml

http://www.jepsculpture.com/bronze.shtml

http://www.jepsculpture.com/bronze.shtml



Complex, 3-D shapes

• Near net shape• Low scrap• Relatively quick process

• Intricate shapes• Large hollow shapes• No limit to size• Reasonable to good surface finish



Metals processed by casting

• Sand casting – 60%• Investment casting – 7%

• Die casting – 9%• Permanent mold casting – 11%• Centrifugal casting – 7%• Shell mold casting – 6%



Capabilities

• Dimensions– sand casting - as large as you like– small - 1 mm or so

• Tolerances– 0.005 in to 0.1 in

• Surface finish

– die casting 8-16 micro-inches (1-3 μm)– sand casting - 500 micro-inches (10-25μm)



Casting fundamentals

Interestingly, every steps boils down to solidification….and everything is intimately related

Sound casting

Steps for sand casting

1. Pattern making2. Sand making3. Mold making4. Core making

5. Metal making6. Pouring7. Solidification8. Machining9. Quality investigation



Taper in patternsfor ease of removalfrom the sand mold

Typical metal match-plate pattern used insand casting



Cores – made of sand aggregates

› Possess:

Strength

Permeability

Ability to withstand heat

Collapsibility

› Anchored by core points

› Chaplets are used to keep the core from moving



Sa nd m old ing m a c hines:

› Vertical flaskless molding

› Sandslinger

› Impact molding

› Vacuum molding

The sand-casting operation

F



Furnaces

Blast furnace

Basic Oxygen Furnace

Electric Arc Furnace Induction Furnace



Sand Casting



16

Pouring



Nucleation

The probability of nucleation occurring at point in the parent phase is

same throughout the parent phase

In heterogeneous nucleation there are some preferred sites in the

parent phase where nucleation can occur

Homogenous

Heterogenous

Nucleation

NucleationSolidification + Growth=

Liquid → solid walls of container, inclusions

Solid → solid

inclusions, grain boundaries,

dislocations, stacking faults

C li f t l



19

Cooling curve for pure metal

http://practicalmaintenance.net/?p=1085

C li f All



20

Cooling curve for Alloy

http://practicalmaintenance.net/?p=1176



Microstructure - Dendrites

P i A l i



Pouring AnalysisHeating to a desired temperature

Heat energy requirement

1. The heat to raise the temperature to the melting point.

2. The heat of the fusion to convert it from solid to liquid.

3. The heat to raise the molten metal to the desired temperature for

pouring- Super heat .∆H = ρV{Cs(Tm- To) + ∆Hf + C

l (T p-Tm)}

ρ= density of metal = gm/cm3

V= volume of metal = cm3

Cs= Specific heat of solid metal (J/gm/0c)C

l = Specific heat of liquid metal (J/gm/0c)

∆Hf = Heat of fusion on unit mass (J/gm)

Tp= Pouring Temperature (oC)

Tm= Melting temperature (oC)

To= Starting temperature (oC)

Pouring the molten metal



Pouring the molten metal

Care:1. Pouring Temperature

2. Pouring rate

3. Turbulence

Engineering analysis of pouring

Bernoulli’s TheoremSum of the energies (head pressure, kinetics and friction) at any

two points in a flowing liquid are equal.

H= head(cm)

P1= pressure on the liquid (N/cm2

)ρ= density, gm/cm3

F = head loss due to friction (cm)

1 and 2 are at two locations

v1

= velocity at point 1(cm/sec)

g = gravitational acceleration (cm/s2 )

2 2

1 1 2 2

1 1 2 2

2 2

P v P vh F h F

g g ρ ρ + + + = + + +



Simplified Bernoulli’s equation:2 2

1 2

1 2

2 2

v vh h

g g+ = + (Ignoring friction force or drag)

and P= 1atm.h2= 0 ; v1= 0

2 2

1 21 2

2 2

v vh h

g g+ = +

21

2

vh

g⇒ =

Hence,2 2v gh=

v2= Flow velocity at point (2)

Another important relationship during pouring is the continuity

law: Volume rate flow = Constant through out the liquid

1 1 2 2Q v A v A= × = × A1 & A2 area =cm2

Hence, Area increases, velocity decreases

Q: Why the sprue is tapered downward to reduce the area?

h1

h2

h



27

2 2v gh=

=(2x9.81x0.3)1/2 = 2.43m/s=243 cm/s

1 1 2 2Q v A v A= × = ×

Velocity

Volume rate flow:

=24.3 x 3 = 729 cm3/s

Time to fill mold cavity of 1000 cm3

= 1000/ 729 = 1.37 s

S lidifi ti Ti



Solidification Time

TST(Total solidification time) = Cm(V/A)n

V= Volume of the casting (cm3)

A= Surface area of the casting (cm2)

n= Exponent (≈2)Cm = Mold constant (min/cm2)

Cm depends on

1. Casting operation

2. Mold material

3. Thermal properties ofcast metal

4. Pouring temperature

Total solidification time= time required for the casting to solidify after

pouring.

Solidification time:

Empirical relationship (Chvorinov’s Rule)

Size of the casting

Shape of the casting



1 2

V V

A A

⟩

TST1 ⟩ TST2

Cooling will be slow for a

casting with higher volume to

weight ratio.

Casting Design Thumb rule (Riser size):

TST of Riser > TST of actual casting

Hence, Riser should solidify last. Since Riser provides

liquid metal to cast cavity.

Direction solidification



30

In order to avoid shrinkage problem,

Furthest section should solidify first and riser solidifies last.

Directional solidification aspect of freezing and methods by

which it is controlled……..(chills are used to incorporate directional

solidification).

Chills – internal or external heat sinks that cause rapid freezing

in certain regions of the casting (thin section where V/A ratio is highest)

Direction solidification

Casting Defects : S d f t t d ll ti



Casting Defects : Some defects are common to any and all casting

processes.

Phase Change &Shrinkage



Phase Change & Shrinkage

Shrinkage cavity- This defect is a depression in the surface or an internal void in the

casting, caused by solidification shrinkage that restricts the amount of the molten

metal available in the last region to freeze. It often occurs near the top of the casting, in

which case it is referred to as “pipe”. The problem can often be solved by proper riserdesign.



Shrinkage



Due to shrinkage giving rise toa funnel-like cavity

• Solutions– insulate top (glass wool)– heat top (exothermic mixture - thermit)

Pipe Defect



35

Misruns- A misrun is a casting

that has solidified before

completely filling the mold cavity.

Typical causes include (1) fluidity

of the molten metal is in sufficient,(2) pouring temperature is too low,

(3) pouring is done too slowly, and

or (4) cross section of the mold

cavity is too thin.

Cold shut- occurs when two

portions of the metal flow together

but there is lack of fusion betweenthem due to premature freezing. Its

causes are similar to those of a

misrun.



Some defects are related to the use of sand molds, and therefore



Sand blow- This defect consist of a balloon shaped gas

cavity caused by release of mold gases during pouring. It

occurs at or below the casting surface near the top of thecasting. Low permeability, poor venting , and high

moisture content of the sand mold are the usual causes.

Some defects are related to the use of sand molds, and therefore

they occur only in sand castings.

Pinhole- A defect similar to sand blow involves the formation of many

small gas cavities at or slightly below the surface of the casting.

Sand wash- A wash is an irregularity in the surface of the casting that

results from erosion of the sand mold during pouring. The contour ofthe erosion is imprinted into the surface of the final cast part.

Scab- This is a rough area on the surface of the casting due to encrustations

of sand and metal. It is caused by portions of the mold surface flaking off

during solidification and becoming imbedded in the casting surface.



Penetration- When the fluidity of the liquid metal is high, it may

penetrate into the sand mold or sand core. After freezing, the surface of

the casting consists of a mixture of sand grains and metal. Harder

packing of the sand mold helps to alleviate this condition.

Mold shift- This is manifested as a step in the cast product

at the parting line caused by sidewise displacement of the

cope with respect to the drag.

Core shift- A similar movement can happen with the core, but the

displacement is usually vertical. Core shift and mold shift are caused by

buoyancy of the molten metal

Molds crack- If mold strength is insufficient, a crack may develop,

into which liquid metal can seep to form a “fin” on the final casting.

Other Casting Processes



39

Other Casting Processes

Shell Molding



40

Investment casting



Die casting



42

Die casting

Slip casting



43

p g

Product design



g

Geometric simplicity – cores to avoid Corners- no sharp corner and angles – hot tearing Section thickness – uniform in order to avoid shrinkage cavities – hot spots Draft Machining allowance Surface finish

Lecture 5 6-Casting

Documents

Transcript of Lecture 5 6-Casting