Metals- Solid State Processes

Question 1

What do solid state deformation processes do?

Answer 1

Induce a shape change in a solid metal workpiece by plastic deformation under forces applied by tools and dies. Workpiece generally produced initially by casting.

Question 2

Primary and secondary solid state processes

Answer 2

Primary: rolling, extrusion, forging
Secondary: wire drawing, machining

Question 3

Features of forging

Answer 3

Produces discrete parts with a set of dies
Finishing operations usually required
Elevated temperatures usually needed
Die and equipment costs relatively high

Question 4

Features of rolling

Answer 4

Produces flat plates or structural shapes (e.g I-beams)
The piece gets longer and thinner but not wider
High speed of production
High capital investment required

Question 5

Features of extrusion

Answer 5

Production of long lengths of constant cross section
Round billet placed into chamber and forced through a die opening by a ram (with dummy block in between)
Die dictates final shape of product
Normally carried out at elevated temperatures
Pieces then cut to length
Batch process

Question 6

What is rolling?

Answer 6

The process of reducing the thickness (gauge) or cross section of a workpiece by compressive forces applied through a set of rolls.
Flat rolling produces a plate or sheet. Can be done at room temperature or at elevated temperatures

Question 7

Why is hot rolling used?

Answer 7

The metal is softer at higher temperatures so it deforms with a smaller load than at cooler temperatures

Question 8

Cluster mill

Answer 8

Used for rolling. The work roll is connected to two intermediate rolls on either side which are connected to driven rolls. These are next to bearing shafts and the bearing backing in the housing. Very rigid and used for rolling thin sheet of high strength materials like stainless steel

Question 9

Steckel mill

Answer 9

Used for rolling. Coiled metal is passed back and forth through a roll until it is the right thickness. Can apply a tension this way to reduce the load requirements in the roller

Question 10

Rolling force equation

Answer 10

F=χwrt(R(h0-hf))σflow
χ is friction factor
w is width of material deforming
R is radius of rolls
h0 and hf are initial and final thicknesses of material
σflow is flow strength of material

Question 11

How does increasing temperature affect the rolling load?

Answer 11

Decreases flow stress so decreases rolling load

Question 12

Surface defects in rolling

Answer 12

May be present on plate or sheet resulting from inclusions in the material, scale, rust or roll markings.
Can reduce defects caused by scale by prior treatment of the ingot

Question 13

Structural defects from rolling

Answer 13

Affect integrity of the product such as wavy edges, cracking, centreline porosity or alligatoring (where material explodes while rolling).
Wavy edges solved by pulling metal apart by edges.
Other reduced by better control of homogenisation prior to rolling

Question 14

How does punch extrusion work?

Answer 14

A punch is pressed into the billet which extrudes around it

Question 15

Condition for extrusion for steel and aluminium

Answer 15

Aluminium 350-550°C

Steel 1000-1200°C

Question 16

Direct and indirect extrusion

Answer 16

Direct: billet slides relative to the container wall
Indirect: die moves towards the billet so no relative billet/container motion
Friction conditions are different
Energy requirements and surface quality better with indirect

Question 17

What is impact extrusion used for?

Answer 17

Similar to indirect and used to making hollow shapes

Question 18

Hydrostatic extrusion

Answer 18

Chamber is filled with a fluid that transmits the pressure to the billet. No friction at the container walls

Question 19

How to see flow pattern of the billet experimentally

Answer 19

Billet is halved and a grid machined onto one side. Halves then joined back together and extruded. Subsequent splitting of the two halves reveals the flow pattern. Quite expensive and limited information gained

Question 20

Extrusion process variables

Answer 20

Container temperature, pad temperature, die temperature, friction conditions at billet/tooling interfaces

Question 21

How else can the extrusion process be modelled?

Answer 21

Using finite element modelling. Can validate experimentally. Can use for data that is otherwise difficult to obtain. Input data like hot working data and friction characteristics. Can find stress, strain and temperature distributions

Question 22

Formulae for extrusion

Answer 22

Extrusion ratio: R=A0/Af (Original and final CSA)
Extrusion pressure: P/σ=a+bln(R) (a and b constants)
Extrusion strain rate=6vln(R)/D (v extrusion velocity, D billet diameter)

Question 23

Problem with extrusion

Answer 23

Only works with certain alloys that are more extrudable than others

Question 24

Defects with extrusion

Answer 24

Surface cracking if temperatures too high or low or if extrusion speed is too high or low. Need to analyse closely to find which cause. Piping occurs when impurities in billet moves towards centre. Can solve by changing flow pattern or machining the billet surface prior to extrusion. Or could leave a discard by not extruding 10% of the billet.

Question 25

Components produced by forging

Answer 25

Crankshafts, gears, wheels, turbine blades and many other structural components

Question 26

Forgeability

Answer 26

Capability of a metal to undergo deformation without severe surface cracking. Good forgeability means able to be shaped by low forces without cracking. Can be quantified using upsetting tests or hot twist tests

Question 27

Upsetting force/ forging load formula

Answer 27

P=σ(1+2μr/3h)
μ is friction factor
r is effective radius of metal
h is effective height of metal

Question 28

Open die forging

Answer 28

Deformation of a cylindrical specimen between two platens. Sometimes used as initial step in more complex forging processes. Barrelling takes place due to friction at the platen/metal interface and when cold metal deformed between hot dies. Material at interface cools rapidly so shows greater resistance to deformation than material at centre

Question 29

Impression die forging

Answer 29

Material acquired the shape of the die cavities whilst being deformed. Some material flows sideways to form a flash which cools quickly and helps the rest of the workpiece fully fill the die. Because high friction encourages the filling of the entire die cavity and metal assumes correct shape. Flash is recycled.

Question 30

Steps of impression die forging

Answer 30

Start with blank bar stock. Edging, blocking, finishing, trimming.

Question 31

Closed die forging

Answer 31

No flash formed and workpiece completely surrounded by dies. Proper control of volume of material important to achieve final dimensions. Can do near net shape forging where part formed is very close to final required dimensions and dies are machined to greater accuracy than in other operations. Means less machining needed after forging. Aluminium and magnesium suited to this as forging loads are low and there is little die wear.

Question 32

Isothermal forging

Answer 32

Dies heated to same temperature as workpiece. Cooling of specimen eliminated and low flow stress maintained. Can be carried out over very narrow temperature range at longer times. Much greater control of microstructural and mechanical properties. Very good dimensional accuracy but is expensive compared to other routes.

Question 33

Defects with forging

Answer 33

Surface cracking leading to problems with fatigue failure or corrosion. Excess material can cause buckling or lapping. Complet filling of a die can be hard so dies design must try to eliminate small radii where the material can fold over itself.

Question 34

What is a problem with all 3 deformation processes?

Answer 34

Anisotropy