Plastic- Extrusion of Aluminium Alloys Part 2

Question 1

Choice of died for case study 1

Answer 1

Flat faced dies with minimal added lubricant are perfectly acceptable and there is no need to use conical dies

Question 2

Formula for extrusion speed

Answer 2

Vext=R.Vram
Where Vext is the speed at which the extrusion exits the die.
Vram is the ram speed.
R is the extrusion ratio

Question 3

What does the extrusion ratio determine?

Answer 3

The degree of deformation imparted to the billet

Question 4

Define the extrusion ratio and five formulae

Answer 4

The ratio of the CSA of the container to the CSA of the extrusion.
R=A0/Ae
Assumes container diameter is the billet diameter. Not true when inserting the billet but true for most of extrusion cycle.
If cylindrical extrusion then R=D0^2/De^2

Question 5

Describe the geometry of the deformation zone and formula for its volume

Answer 5

A come with an angle of 45°.
Volume is 1/3 x base x height.
πD0^3/24

Question 6

Volume flow rate of material through deformation zone and how this is used to find time taken for material to pass through the deformation zone

Answer 6

Volume flow rate=Vram.πD0^2/4 (ram velocity times container CSA).
Divide volume of deformation zone by volume flow rate to get time.
t=D0/6Vram

Question 7

Formula for extrusion strain rate

Answer 7

Take strain (ln(R)) and divide by time.
ε•=ε/t=6.Vram.ln(R)/D0

Question 8

Define extrusion pressure

Answer 8

Normally defined as the maximum load applied by the press for extrusion to occur divided by the CSA of the billet.
Pe=F/A0

Question 9

What factors determine the extrusion pressure?

Answer 9

Degree of deformation imparted to the billet.
Friction between billet and container.
Flow stress of material being extruded.
Increasing any of these factors increases the required extrusion pressure.

Question 10

Formula for extrusion pressure for indirect extrusion (need to know)

Answer 10

P0=σ0(A+B.ln(R))
σ0 is flow stress
A is generally 0.8
B is generally 1.8
R is extrusion ratio

Question 11

Formula for extrusion pressure for direct extrusion (need to know)

Answer 11

Needs amending to take into account container wall friction.
Pe=P0(1+4μL/D0)
Where P0 is extrusion pressure formula for indirect.
Pe is actual extrusion pressure.
μ is the coefficient of friction between the billet and container.
D0 is container diameter.
L is corrected length of billet after upsetting (changes over extrusion cycle)

Question 12

Value of extrusion ratio for case study 1

Answer 12

150^2/25^2=36

Question 13

What is the press capacity for case study 1?

Answer 13

566MPa

Question 14

Ideal choice for extrusion speed and temperature

Answer 14

Fastest speed at lowest temperature

Question 15

Choosing between fast and slow cooled material in terms of extrusion pressure

Answer 15

Both σ0 and P0 increase with increasing speed and decreasing temperature. Slow cooled has lower P0 for any combination of speed and temperature. The fastest speed available (35mm/s) with lowest T available (350C) for fast cooled gives P0 greater than press capacity (566MPa) so cant be used. This is not a problem for slow cooled so choose that one

Question 16

Heat balance for temperature rise during extrusion

Answer 16

Heat generated by deformation - heat transferred to tooling = heat accumulated by material.
0.9Fdx/dt-hA(TB-TT)=mCdTB/dt
F is extrusion load, x is ram displacement, t is time.
h is heat transfer coefficient between tooling and billet.
A is interface area between tooling and billet.
T sub B is billet temp, T sub T is tooling temp, m is mass of billet.
C is heat capacity of billet.
0.9 factor takes into account not all of work done is converted to heat.

Question 17

What factors affect the temperature rise during deformation?

Answer 17

Anything that increases the load (F) increases the T rise (dTB).
Decreasing tooling temperature (TT) decreases the T rise.

Question 18

What is the lowest melting point for the fast and slow cooled material?

Answer 18

Fast: 575C
Slow: 550C
Means more combinations of high T and high speed that slow cooled can’t do without localised melting

Question 19

What do extrusion limit diagrams look like?

Answer 19

Ram speed vs initial billet temperature (constant extrusion ratio) or extrusion ratio vs initial billet temperature (constant ram speed). There are two diagonal lines sloped towards each other (going up). In the region between these lines extrusion is possible. Left of this region extrusion is impossible due to insufficient pressure (press capacity). Right of the region extrusion impossible due to extrude melting.

Question 20

What factors affect extrusion limit diagrams plotting extrusion ratio against initial billet T?

Answer 20

The operating window is significantly larger for indirect extrusion (higher peak of triangle). The operating window decreases with increasing ram speed (lower peak of triangle).

Question 21

What are the two energy requirements for extrusion?

Answer 21

Heating the material from room temperature to the extrusion temperature.
Extrude the material.

Question 22

Total energy required to extrude the material

Answer 22

P+ρC(TB-T(RT))
P is extrusion pressure
ρ is density
T sub B is billet temperature
T sub RT is room temperature

Question 23

How does billet temperature effect energy costs?

Answer 23

Increasing TB means the energy required to heat increases but the energy required to extrude (P) decreases. However extra energy required for heating far outweighs the reduction in extrusion energy so lower TB leads to lower overall energy costs

Question 24

Choice of homogenisation treatment, ram speed and temperature

Answer 24

Slow cool
Maximum speed of 35mm/s
350°C

Question 25

What type of press is normally used for Al alloys?

Answer 25

Horizontal press with a run out table for the product. Normally also have stretchers to keep the extrusion straight

Question 26

What sort of speeds are generally used for extrusion?

Answer 26

For Al low speeds are used to avoid surface defects (10-50mm/s). For high strength materials higher speeds are used to prevent cooling (up to 500mm/s). This is because billet needs to be at very high temperature for extrusion to be possible which is often much higher than the max operating T of the container material. Therefore the container is much cooler than the billet which causes cooling of the billet.

Question 27

What extrusion ratios, temperatures, dies and lubrications can be used generally?

Answer 27

For Al up to 400:1 can be used but generally 10:1 to 100:1. For steels use less than 40:1.
For Al T in range 250-500C used with container 50-100C cooler.
For Al unlubricated flat faced dies are used. For high strength materials lubricated conical dies are used.

Question 28

How does tube extrusion work?

Answer 28

Have a hollow billet so that a fixed mandrel can extend through it to the die (connected to dummy block?). The billet is then pressed out of the die as a cylinder. Problems with oxidation of the internal surface.

Question 29

How is practical modelling of extrusion done?

Answer 29

Billet is halved and a grid is machined onto one side. The halves are then joined back together and extruded. Subsequent splitting of the two halves reveals the flow pattern. Can individually vary the pad T, container T, die T and friction conditions at the billet/tooling interfaces to obtain a picture of how the billet flow is affected by changes in extrusion conditions.

Question 30

Advantage of finite element modelling

Answer 30

Can do experimental validation with gridded billets. Need for expensive and difficult plant trials reduced. Information and data difficult to obtain experimentally can be obtained. Output data includes stress, strain and T distributions

Question 31

How can changing the process parameters affect product quality and integrity?

Answer 31

The shape of the flow path of the surface layers (oxide layers) is changed. Higher due temperature keeps them closer to surface and not flowing inside.