Rolling Extrusion and Drawing Flashcards Preview

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Flashcards in Rolling Extrusion and Drawing Deck (30):
1

Rolling

- 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

2

Spreading

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

3

Why hot roll?

- 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

4

Draft: (Rolling)

- The reduction in thickness is measured by the difference in thickness before and after the reduction

- The volume before and after is constant

5

No slip point:

- 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

6

Rolls are subject to these 4 extreme operating conditions

- Large Forces
- Bending Moments
- Thermal stresses
- Wear

7

Roll material is selected for

- strength
- rigidity
- wear resistance

8

Common material chosen for rollers

- Cast iron
- Cast steel
- Forged steel

9

Ways to reduce the roll force

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

10

Defects in metal rolling

- 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)

11

What is extrusion

- 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

12

Uniform flow pattern (mostly homogeneous) happens when

there is no friction at the interfaces

13

Dead metal zone develops when

Friction along all interfaces is high

14

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

- "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

15

What are the 4 types of extrusion methods

- Direct (or forward) extrusion

- Indirect (or backwards) extrusion

- Hydrostatic extrusion

- Impact extrusion

16

Direct Extrusion

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

17

Indirect Extrusion

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

18

Hydrostatic Extrusion

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

19

Impact Extrusion

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

20

Direct Extrusion Advantages and Disadvantages

Advantages:
- the extrusion process is the most simple

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

21

Indirect extrusion Advantages and Disadvantages

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

22

Hydrostatic extrusion Advantages and Disadvantages

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

23

Impact extrusion Advantages and Disadvantages

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

Disadvantages:
- limited to small parts

24

Defects from Extrusion

Surface Cracking

Pipe, tailpipe or fishtail

Internal Cracking

25

Surface Cracking

- 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

26

Pipe, tailpipe or fishtail

and how to reduce it

- 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

27

Internal Cracking

develops at the center of an extruded product

Variables that lead to this:
- die angle
- Extrusion ration
- friction

28

Drawing

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)

29

Fundamental differences between extrusion and drawing

- 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

30

defect specific to drawing

Formation of seams (longitudinal scratches)