L12 Casting

Question 1

What is casting?

Answer 1

Casting processes involve the introduction of molten metal into a cavity where upon solidification the metal takes the shape of a cavity

Question 2

What are some advantages of casting as compared to machining?

Answer 2

• Able to produce intricate shapes (including internal cavities) in a single piece
• Scalable from very small to very large
• A near net shape process with minimal waste and finshing

Question 3

What are 4 Important Factors of Casting?

Answer 3

• Solidification
• Heat transfer
• Fluid flow
• Mold material

Question 4

In what way does a pure metal solidify? ie. When it reaches a freezing point, does the temperature stay constant? What physical properties change as metal transforms from liquid to solid?

Answer 4

Pure metals have a clearly defined melting/freezing point.Solidification takes place at a constant temperature until all the metal has frozen.Volume decreases –> density changes

Question 5

What is the difference between solidification of alloys compared with pure metals?

Answer 5

Unlike pure metals, alloyssolidify over a range of temperatures.

Question 6

Solidification of alloys, where does freezing begin?

Answer 6

Freezing begins at theliquidus line and completes at the solidus line (TempL - TempS : Freezing range)

Question 7

Within the 2 phase region, the alloy during solidifcation has a”blank” texture. How can its composition be approximated?

Answer 7

• “Mushy”

* The composition may be approximated by its phase diagram

Question 8

As the alloymetal solidfies what forms?

Answer 8

• Dendrites are formed that interlock as the material cools
• Dendrites are crystals that develop with a multi branch tree like form
• Form between the liquid and solid phase (ie two phase region)

Question 9

Name the following dendrites

Answer 9

• Columnar Dendrite
• EquiaxedDendrite
• EquiaxedNon-Dendrite

Question 10

“____” is a strong fuction of cooling rate.What are the 3 types of cooling rates and their purposes?

Answer 10

Grain Size

• Slow Cooling (10^2 K/s) creates coarse dendrite structures
• Medium Cooling (10^4 K/s) creates a finer structure
• Fast Cooling (10^8 K/s) creates amorphic strucutres

Question 11

What addition allowsequiaxed grains throughout the solid?

Answer 11

Nucleating agents allow equiaxed grains throughout the solid (Favourable mechanical properties)

Question 12

What are the 3 compositions ofEquiaxed grains? Which is the most desired?

Answer 12

• We can have pure metal, solid solution, or heterogeneous nucleation
• Heterogeneous nucleation is the desired composition

Question 13

Theflow of heat and molten metals has an affect on cast parts”___” and performance?

Answer 13

Cast part “properties”

Question 14

3basic principles (math related) applied to begin analyzing fluid flow and mold geometry are…

Answer 14

• Bernoulli’s Theorem
• Mass Continuity
• Reynolds Theorem

Question 15

What are thelaminar and turbulent conditions for Reynolds Number?

Answer 15

• Re

* Re > 20000 (Turbulent)

Question 16

Fill in the blanks of this single gravity casting system (The pic shows up when you actually do the question, dont delete anything. If its too small, go to the lecture slides)

Answer 16

I am so proud of you for doing this

Question 17

What is the purpose of the runner and gate system in thesingle gravity casting system?

Answer 17

To deliver metal to the mold and trap contaminants

Question 18

What is a blind riser in thesingle gravity casting system?

Answer 18

Allows for extra metal to be added and replace material lost during shrinkage and cooling

Question 19

What is the mold cavity?

Answer 19

Represents the final mold part

Question 20

When is a core usedin the single gravity casting system?

Answer 20

If the part hasinternal cavities

Question 21

What is fluidity?

Answer 21

Ability of a molten metal to fill a cavity

Question 22

What are the materialproperties thataffectfluidity with regards to casting?

Answer 22

• Viscosity
• Surface Tension (ST)
• Inclusions
• Solidification Pattern

Question 23

Explain how viscosityaffects fluidity

Answer 23

Viscosity
* Increase in vis. decreases fluidity

Question 24

Explain how surface tensionaffects fluidity

Answer 24

Surface Tension (ST)

• Increase in ST decreases fluidity
• Metal oxides usually have higher ST

Question 25

Explain how Inclusionsinfluence fluidity

Answer 25

Inclusions

* Decrease fluidity

Question 26

Explain how Solidifcation patterns influence fluidity

Answer 26

Solidification Pattern

* The freezing range of an alloy is inversely proportional to its fluidity

Question 27

5 Casting parameters that affect fluidity?

Answer 27

Mold Designs

• Space, runner, riser Mold Materials
• Thermal properties and surface roughness Degree of Superheat
• Temp, above melting point Rate of Pouring
• Slow -solidifies, fast -turbulence Heat Transfer
• Affects the metals viscosity

Question 28

Heat Flow through casting can vary depending on 4 factors…

Answer 28

• Casting Material
• Mold Material
• Geometry
• Process Parameters

Question 29

In thin sections of a cast, do flow rates need to be high or low? Why?

Answer 29

• Flow needs to be high to avoid premature chilling and solidfication, but not too high such as to cause turbulence
• Excessive turbulence causes air entrapment

Question 30

What does Chvorinov’s Rule Desribe?

Answer 30

It is a formula that indicates how long it will take a metal to solifiy in a casting process.

Question 31

Metals typically shrink during what three phases?

Answer 31

1. contraction as a liquid2. contraction through phase change3. contraction as a solid

Question 32

When does the most shrinkage occur?

Answer 32

Contraction as a solid

Question 33

What metal is an exception to shrinkage when cooling?

Answer 33

Cast Iron(expands when cools, graphite precipitates which has a high specific volume)

Question 34

What is bad about shrinkage?

Answer 34

Causes dimensional changes that cause hot tearing, which is essentially cracking