ie 337: materials & manufacturing processes lectures 7&8: introduction to metal casting...

IE 337: Materials & Manufacturing Processes

Lectures 7&8: Introduction to Metal Casting

Chapters 10 & 6

Assignment

HW 2 due today CH 21,22 and 24 Problems In Assignments folder

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This Time

Basic Process Casting Terminology Heating Pouring Phase diagrams Solidification

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Flow of Molten Liquid Requires Heating

Heat Transfer of Liquid in Mold Cavity During and After Pouring

Solidification into Component

Casting

Common process attributes:

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Metal Casting Processes Categories

1. Expendable mold processes - mold is sacrificed to remove part Advantage: more complex shapes possible Disadvantage: production rates often limited by

time to make mold rather than casting itself

2. Permanent mold processes - mold is made of metal and can be used to make many castings Advantage: higher production rates Disadvantage: geometries limited by need to open

mold

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Overview of Sand Casting

Most widely used casting process, accounting for a significant majority of total tonnage cast

Nearly all alloys can be sand cast, including metals with high melting temperatures, such as steel, nickel, and titanium

Parts ranging in size from small to very large Production quantities from one to millions

7Sand casting mold

Sand Casting

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Gating System

Channel through which molten metal flows into cavity from outside of mold

Consists of a downsprue, through which metal enters a runner leading to the main cavity

At top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue

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Riser

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

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Heating the Metal

Heating furnaces are used to heat the metal to molten temperature sufficient for casting

The heat required is the sum of: 1. Heat to raise temperature to melting point

2. Heat of fusion to convert from solid to liquid

3. Heat to raise molten metal to desired temperature for pouring

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Heating the Metal

1. Heat to melting point: VCs(Tm-To)

2. Heat of fusion to convert from solid to liquid: VHf

3. Heat molten metal to pouring temp.: VCl(Tp-Tm)

density, V = volume, Cs = specific heat,

Hf = latent heat of fusion, T = temperature

• Properties vary with temperature and phase• Melting may occur over a temperature range• Heat loss to the ambient

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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

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Pouring Calculations 1

Minimum mold filling time, MFT

MFT =V/Q

Q: volumetric flow rate, cm3/s

V: mold cavity volume, cm3

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Pouring Calculations 2

Volumetric flow rate remains constant

Q =v1A1 = v2A2

Q: volume rate of flow cm3/s

v: velocity of the liquid metal

A: cross-sectional area, cm2

CONTINUITY EQUATION

Increase in area results in decrease in velocity

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Pouring Calculations 3

Flow of liquid metal through gating system & mold

v = (2gh)1/2

v: velocity of the liquid metal at base of sprue

g = gravitational acceleration, 981 cm/s2

h = height of sprue, cm

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Example 1

The downsprue leading into the runner of a certain mold has a length = 175 mm.

The cross-sectional area at the base of the sprue is 400 mm2.

The mold cavity has a volume = 0.001 m3. Determine: (a) the velocity of the molten metal

flowing through the base of the downsprue, (b) the volumetric flow rate, and (c) the time required to fill the mold cavity.

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Example 1: Solution

(a) Velocity v = (2gh)0.5

= (2 x 9810 x 175)0.5 = 1853 mm/s

(b) Volume flow rate Q = vA = 1853 x 400 = 741,200 mm3/s

(c) Time to fill cavity MFT = V/Q

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Solidification of Pure Metals

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

Solidification and Cooling

Figure 10.8 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).

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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

Microstructure: I

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Most alloys freeze over a temperature range rather than at a single temperature

Solidification of Alloys

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

Microstructure: II

Micro-segregation

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

Microstructure: II

Macro-segregation

Phase Diagrams

A graphical means of representing the phases of a metal alloy system as a function of composition and temperature

A phase diagram for an alloy system consisting of two elements at atmospheric pressure is called a binary phase diagram Other forms of phase diagrams are

discussed in texts on metallurgy and materials science

Phase Diagrams

Composition is plotted on the horizontal axis and temperature on the vertical axis

Any point in the diagram indicates the overall composition and the phase or phases present at the given temperature under equilibrium conditions

Copper-Nickel Phase Diagram

Figure 6.2 Phase diagram for the copper‑nickel alloy system.

Copper‑Nickel (Cu-Ni) Alloy System

Solid solution alloy throughout entire range of compositions below the solidus

No intermediate solid phases in this alloy system

However, there is a mixture of phases (solid + liquid) in the region bounded by the solidus and liquidus

Chemical Compositions of Phases

The overall composition of the alloy is given by its position along the horizontal axis

However, the compositions of liquid and solid phases are not the same These compositions can be found by

drawing a horizontal line at the temperature of interest

Where the line intersects the solidus and liquidus indicates the compositions of solid and liquid phases, respectively

Example

Determine compositions of liquid and solid phases in the Cu-Ni system at an aggregate composition of 50% nickel and a temperature of 1316oC (2400oF)

Inverse Lever Rule – Step 1

The phase diagram can be used to determine the amounts of each phase present at a given temperature Using the horizontal line that indicates

overall composition at a given temperature, measure the distances between the aggregate composition and the intersection points with the liquidus and solidus, identifying the distances as CL and CS, respectively

Inverse Lever Rule – Step 2

The proportion of liquid phase present is given by

L phase proportion =

And the proportion of solid phase present is given by

S phase proportion =

)( CLCSCS

CLCSCL

Applications of the Inverse Lever Rule

Methods for determining chemical composition of phases and amounts of each phase are applicable to the solid region of the phase diagram as well as the liquidus‑solidus region

Wherever there are regions in which two phases are present, these methods can be utilized

When only one phase is present, the composition of the phase is its aggregate composition under equilibrium conditions, and the inverse lever rule does not apply

Tin-Lead Phase Diagram

Figure 6.3 Phase diagram for the tin‑lead alloy system.

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Mechanical properties of 2 identical composition samples with different cooling rates

Effect of Solidification Rate

Solidification Time

Solidification takes time Total solidification time TST = time required for

casting to solidify after pouring TST depends on size and shape of casting by

relationship known as Chvorinov's Rule

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Chvorinov's Rule

where TST = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent usually taken to have a value = 2; and Cm is mold constant

n

m A

VCTST

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Mold Constant in Chvorinov's Rule

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

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Example 1

In casting experiments performed using a titanium alloy and a zircon sand mold, it took 155 s for a cube-shaped casting to solidify. The cube was 50 mm on a side.

If the same alloy and mold type were used, find the total solidification time for a cylindrical casting in which the diameter = 30 mm and length = 50 mm.

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Example 1: Solution

Cube Volume V = (50)3 = 125,000 mm3

Cube Area A = 6 x (50)2 = 15,000 mm2

Cube (V/A) = 125,000/15,000 = 8.33 mm

Cm = TST/(V/A)2

= 155/(8.33)2

= 2.23 s/mm2

Cylinder Volume V = D2L/4 = (30)2(50)/4

= 35,343 mm3

Cylinder Area A = 2D2/4 + DL = (30)2/2 + (30)(50)= 6126 mm2

Cylinder (V/A) = 35,343/6126 = 5.77 mm

TST = Cm(V/A)2

= 2.23 (5.77)2 = 74.3 s39

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

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

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Shrinkage in Solidification and Cooling

Figure 10.8 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).

Shrinkage in Solidification and Cooling

Figure 10.8 (2) 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 solid metal (dimensional reductions are exaggerated for clarity).

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Solidification Shrinkage

Occurs in nearly all metals because the solid phase has a higher density than the liquid phase

Thus, solidification causes a reduction in volume per unit weight of metal

Exception: cast iron with high C content Graphitization during final stages of freezing causes

expansion that counteracts volumetric decrease associated with phase change

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Shrinkage Allowance

Patternmakers account for solidification shrinkage and thermal contraction by making mold cavity oversized (See Table 10.1)

Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance

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Example 2

A mold cavity has the shape of a cube, 100 mm on a side. Determine the volume and dimensions of the final cube after cooling to room temperature if the cast metal is copper.

Assume that the mold is full at the start of solidification and that shrinkage occurs uniformly in all directions.

For copper, solidification shrinkage is 4.9%, solid contraction during cooling is 7.5%.

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Example 2: Solution

Volume of cavity V = (100)3 = 106 mm3

Volume of casting V = 106(1-0.049)(1-0.075) = 879,675 mm3

Dimension on each side of cube = (879,675)0.333 = 95.82 mm

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You should have learned

Basic Process Sand Casting Terminology Heating Pouring Phase diagrams Solidification

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Next Class

Casting Continued Chapter 11