Materials- Metals: Solidification and Joining Processes

Question 1

How does planar growth work?

Answer 1

In well-inoculated liquid solidification occurs by planar growth. Material solidifies layer by layer in front of a solidification front which removes heat from the liquid-solid interface by conduction.

Question 2

What is a well-inoculated liquid?

Answer 2

Liquid containing nucleating agents so at equilibrium heterogeneous nucleation can occur producing fine-grained equiaxed structure.

Question 3

Graph of temperature against distance from solid-liquid interface for planar growth

Answer 3

Solid region left, liquid region right of vertical dotted line. Horizontal dotted line goes out right of vertical one representing freezing temperature. Curved line up (not origin) from bottom left to intersection of dotted lines. Then curved line up with less steep gradient so horizontal dotted line is a tangent. The curved lines represent actual temperature of that point in the solid/liquid.

Question 4

Describe dendritic growth

Answer 4

Liquid not well-inoculated so has to be undercooked before solid forms. A small solid protuberance at interface is encouraged to grow since liquid ahead of solidification front is undercooled. Dendritic growth proceeds proceeds until liquid is not undercooled anymore, after which planar growth takes over. Dendritic normally a small fraction of total growth in pure metals.

Question 5

Graph of temperature against distance from solid-liquid interface for dendritic growth

Answer 5

Same position of dotted lines as for planar growth and same curve in solid region. At intersection of dotted lines, curve drops below horizontal one for the the undercooled liquid region, then comes back up and above freezing temperature.

Question 6

What does rate of growth of solid (solidification) depend on?

Answer 6

The cooling rate, or the rate of heat extraction. Higher cooling rate means shorter solidification time.

Question 7

Chvorinov’s rule

Answer 7

ts=B(V/A)^n
Where ts is time required for a simple casting to completely solidify and s is subscript
B is constant
n is constant (normally 2)
V is volume of material
A is surface area in contact with mold

Question 8

What does rate of solidification to properties of material and why?

Answer 8

Faster time reduces secondary dendrite arm spacing (SDAS). Reduced SDAS can result in improved properties such as tensile strength and ductility.

Question 9

What is the chill zone of a cast structure and what happens here?

Answer 9

Region next to mold wall. First solid forms here and consists of fine grains which heterogeneously nucleate on the mold surface (random orientations). Many nuclei form due due large initial undercooling on the cold mold wall.

Question 10

What is the columnar zone of a cast structure and what happens here?

Answer 10

Region just inside of chill zone and is thicker than chill zone. Contains elongated grains oriented in a particular crystallographic direction because grains grow fastest in certain directions. Grains often grow perpendicularly from casting wall resulting in anisotropic properties in columnar region. Formation if this zone influenced more by growth than nucleation phenomena.

Question 11

What is the equiaxed zone of a cast structure and what happens here?

Answer 11

Zone in centre of casting containing rounded, randomly orientated grains, resulting in isotropic properties. Formation of equiaxed zone is nucleation-controlled process. May not form if solid continues to grow in columnar fashion until solidification complete.

Question 12

What is directional solidification and what does it result in?

Answer 12

Mold is heated from one end and cooled from the other, resulting in a columnar microstructure with all grain boundaries aligned along an axis. Better creep and fracture resistance obtained.

Question 13

How is single crystal growth achieved and what is it use for?

Answer 13

Uses a small seed crystal with preferred crystallographic direction and heat transfer is carefully controlled in the furnace. Example is for silicon for semiconductors as polycrystalline materials are not effective for electronic use due to presence of grain boundaries.

Question 14

What is epitaxy?

Answer 14

The process by which one material is made to grow in an oriented fashion using a substrate with matching crystallographic direction

Question 15

What is epitaxial growth

Answer 15

The growth of highly oriented or single crystal thin films on a substrate.

Question 16

Describe powder metallurgy

Answer 16

Involves the compaction of powdered metal and then melting it in a 3D shape one layer at a time to produce a more dense piece.

Question 17

What is powder metallurgy good for?

Answer 17

Metals with low ductilities because only small plastic deformation of the powder particles need occur. Parts that require very close dimensional tolerances may be economically produced using this technique.

Question 18

What is a plate?

Answer 18

Finished shape generally with a thickness over 6mm

Question 19

What is a sheet?

Answer 19

Material of less than 6mm thickness

Question 20

What are rails and sections?

Answer 20

Finished complex shape in cross section

Question 21

What are wires and bars?

Answer 21

Finished shape with width and length roughly equal

Question 22

What is welding?

Answer 22

When two or more metal parts are joined to form a single piece. It is a metallurgical process involving some diffusion. Actual melting of the pieces occurs in the vicinity of the bond. A filler metal may be used to facilitate the process.

Question 23

Examples of welding methods

Answer 23

Arc welding, gas welding, brazing, soldering

Question 24

Describe the various zones in a welded metal

Answer 24

The area of metal all as one above the joint on the surface is the weld metal comprised of the base metal and the filler metal. Area just below that still as one is fused base metal. Larger area below that is not as one and is heat-affected zone (HAZ) which may experience microstructural and property alterations. All zones under weld metal go back to surface in curve.

Question 25

What happens when a plain carbon steel is welded together?

Answer 25

There is recrystallisation. The residual stresses weaken the joint. There is transformation to austenite and then to other microstructures depending on the cooling rate (e.g pearlite or martensite).

Question 26

How can the diminishing of the properties of the HAZ from welding be reduced?

Answer 26

Rapid heating and cooling in the welding process to minimise recrystallisation and grain growth. This helps to retain its strength.

Question 27

What is weldability?

Answer 27

The ease of welding a material

Question 28

Considerations when welding different C% steels

Answer 28

Low C% steels are easy to weld and the strength of the welded regions is stronger than the base steel.
Medium and high C% steels are more difficult to weld because martensite forms in the welded region, reducing toughness.
Steels that have been quenched and tempered previously shouldn’t be welded since martensite May form in the weld.

Question 29

What is inversely proportional to weldability?

Answer 29

Hardenability

Question 30

What is neutral hardening?

Answer 30

Aka martensitic or quench hardening. Consists of austenitising, quenching and tempering in order to retain a tempered martensite or bainite structure. Used to achieve high hardness/strength in steel. Chemical composition of steel surface of the parts not intended to be changed during the process.