Rolling Extrusion and Drawing

Question 1

Rolling

Answer 1

• Metal rolling plastically deforms the work material by compressive forces between two constantly spinning rolls
• reduces the thickness of the metal and affects the grain structure

Question 2

Spreading

Answer 2

• The plastic deformation causing reduction in thickness will also cause an increase in the width of the part

Question 3

Why hot roll?

Answer 3

• converts the case grain structure into a wrought grain structure
• rolling above the re-crystallization temp breaks apart the old grain structure and reforms a new one

Question 4

Draft: (Rolling)

Answer 4

• The reduction in thickness is measured by the difference in thickness before and after the reduction
• The volume before and after is constant

Question 5

No slip point:

Answer 5

• Surface velocity of the rolls and that of the material are exactly the same
• Rolls rotate at a constant speed faster than the movement of the material, after the no slip point the material moves faster than the rollers

Question 6

Rolls are subject to these 4 extreme operating conditions

Answer 6

• Large Forces
• Bending Moments
• Thermal stresses
• Wear

Question 7

Roll material is selected for

Answer 7

• strength
• rigidity
• wear resistance

Question 8

Common material chosen for rollers

Answer 8

• Cast iron
• Cast steel
• Forged steel

Question 9

Ways to reduce the roll force

Answer 9

• Lower Friction
• Rolls with smaller radii
• taking smaller reductions per pass
• raising the workpiece temperature

Question 10

Defects in metal rolling

Answer 10

• Wavy edges (caused by bending of the rolls)
• Edge and internal cracks (caused by low material duality and barreling of the edges)
• Alligatoring (homogeneous deformation caused from a defect in the original cast ingot)

Question 11

What is extrusion

Answer 11

• A workpiece of certain length and cross section is forced to flow through a die of smaller cross section forming it to the new cross section

Question 12

Uniform flow pattern (mostly homogeneous) happens when

Answer 12

there is no friction at the interfaces

Question 13

Dead metal zone develops when

Answer 13

Friction along all interfaces is high

Question 14

Total force of extrusion is the combination of 3 main contributors:

Answer 14

• “Ideal force”: Just result from homogeneous deformation(no friction or shear)
• Frictional Forces: increase with length of contact path between billet and die
• Shear deformation: increases with die angle

Question 15

What are the 4 types of extrusion methods

Answer 15

• Direct (or forward) extrusion
• Indirect (or backwards) extrusion
• Hydrostatic extrusion
• Impact extrusion

Question 16

Direct Extrusion

Answer 16

• a cylindrical billet is placed in a chamber and forced through a stationary die by a ram

Question 17

Indirect Extrusion

Answer 17

• a die moves towards the billet, and there is no relative motion between the billet and the container

Question 18

Hydrostatic Extrusion

Answer 18

• The container is filled with fluid that transmits pressure to the billet, which is then extruded through the die

Question 19

Impact Extrusion

Answer 19

• A punch descends at a high speed and strokes the billet, extruding the piece in the opposite direction

Question 20

Direct Extrusion Advantages and Disadvantages

Answer 20

Advantages:
- the extrusion process is the most simple

Disadvantages:
- Ram must overcome frictional forces between billet and chamber

Question 21

Indirect extrusion Advantages and Disadvantages

Answer 21

Advantages:
- Negligible friction between the billet and the container, reduces the pressure required in operation

Disadvantages:

• more complex
• product must be extracted through the hollow ram

Question 22

Hydrostatic extrusion Advantages and Disadvantages

Answer 22

Advantages:

• negligible friction between the billet and the container walls
• hydrostatic pressure increases material ductility

Disadvantages
- the material needs to be at a lower temperature

Question 23

Impact extrusion Advantages and Disadvantages

Answer 23

Advantages:
- a rapid process for mass producing thin-walled hollow shapes

Disadvantages:
- limited to small parts

Question 24

Defects from Extrusion

Answer 24

Surface Cracking

Pipe, tailpipe or fishtail

Internal Cracking

Question 25

Surface Cracking

Answer 25

• At high temps:
  if extrusion speed, temp, and friction are too high the surface temp rises leading to surface cracking
• At low temps:
  if the billet sticks to the die land, it will stay stuck until enough pressure builds up up to move it

Question 26

Pipe, tailpipe or fishtail

and how to reduce it

Answer 26

• oxide impurities collected from the dead zone are drawn toward the center of the billet

Reduce by:

• reducing friction and minimize temperature gradient
• machine billet prior to extrusion

Question 27

Internal Cracking

Answer 27

develops at the center of an extruded product

Variables that lead to this:

• die angle
• Extrusion ration
• friction

Question 28

Drawing

Answer 28

Drawing is similar to extrusion, except

• the extrusion material is being bushed through die opening (bar under compression)
• Drawing material is being pulled through the die opening (bar under tension)

Question 29

Fundamental differences between extrusion and drawing

Answer 29

• extrusion can provide large reductions in cross section

- In drawing, the amount of cross sectional reduction is much more limited because metal is being pulled through

Question 30

defect specific to drawing

Answer 30

Formation of seams (longitudinal scratches)