Plasric- Case Study 3: Wire Drawing

Question 1

What is wire drawing?

Answer 1

An operation where the cross sectional area of a bar is reduced by pulling it though a converging die

Question 2

What is most of the plastic deformation in wire drawing caused by?

Answer 2

Compressive forced due to the reaction of the die on the metal

Question 3

What temperature is wire drawing usually performed at and what is the starting stock?

Answer 3

Usually performed at room temperature and the starting stock typically extruded or rolled bar 5-10mm in diameter

Question 4

Describe the two types of drawing and what they are used for

Answer 4

Wet drawing: used for copper and copper alloys and where the whole drawing apparatus is submerged in a lubricating bath.
Dry drawing: used for more other metals. Soap powder used to lubricate wire. Ferrous alloys often precoated with Cu, Sn, lime or zinc phosphate. Starting rod cleaned by pickling is acid bath to remove surface scale so that die wear is reduced.

Question 5

How are large diameter rods and wires produced?

Answer 5

On drawbenches. The rod is placed in a swaging machine, inserted through the die and clamped in the jaws of the drawhead. The drawhead is moved by a chain drive or hydraulic mechanism.

Question 6

Drawbench stats

Answer 6

Available for 1.5MN force and 30m run out. Draw speed vary from 10-100m/min

Question 7

How are small diameter wires produced?

Answer 7

Rod is placed in a swaging machine and drawn through the first die onto a capstan or bull block. The wire is passed round the capstan with sufficient turns to provide grip. Then through the second die and onto the second capstan etc.

Question 8

Stats for making small diameter wires

Answer 8

For coarse wire the reduction will typically be 20-50% per pass whilst for fine wires it will be 15-25%. Drawing speed may exceed 1500m/min.

Question 9

What else may be required when making small diameter wires?

Answer 9

Depending on the material used and reduction required, intermediate anneals may be needed. E.g non-ferrous and low carbon steel wires were produced in a number of tempers

Question 10

Features of wire drawing dies

Answer 10

Typically made of tungsten carbide (very wear resistant). For drawing left to right, the die has larger opening on the left with two different slopes. Small exit gap is the bearing surface which guides the wire or rod as it exits the die. First slope is the entrance angle (less steep) and is large enough to allow for the addition of lubricant which adheres to the die. Next slope is the approach angle (steeper) which defines the section where the reduction occurs.

Question 11

Die semi-angle

Answer 11

Symbol α. Think is basically the approach angle and is angle between the die surface, before the bearing surface, and the horizontal. Most important parameter and varies between 4 and 12°

Question 12

Task for case study 3

Answer 12

Producing copper wires for power cables. Supplier used to provide 5mm diameter rod which we reduce to under 1mm diameter through a series of wire drawing machines. New supplier only provides 8mm diameter rod. Using a spare wire drawing machine need to devise a drawing schedule for this machine to reduce the 8mm rod to 5mm

Question 13

Data for the case study

Answer 13

Stress strain data from tensile tests on the 8mm wire. Maximum drawing load for the drawing machine is 4kN. The drawing machine can handle a maximum of 4 reductions. The dies are submerged in an oil/water lubricant which gives a friction coefficient between the die and copper wire of 0.1

Question 14

Decisions to be made for case study 3

Answer 14

Number of drawing passes required.
Die design (angles)

Question 15

What are the 3 contributions to the the work done during wire drawing?

Answer 15

Useful work: directly contributes to the required shape change.
Work done against friction: the friction between the material and the die in the approach angle.
Redundant work: done deforming (shearing) the material that does not contribute to overall shale change

Question 16

Formula for drawing stress only accounting for useful work

Answer 16

σD=σbar.ln(A0/A1)
Where σbar is mean flow stress of material
A0 is initial CSA of wire
A1 is final CSA of wire
ln(A0/A1) is strain imparted to the material during drawing.
Mean flow stress is average of the yield stress of the material before and after the drawing operation (so /2)

Question 17

What does the fact that wire drawing is normally conducted cold mean?

Answer 17

The material will work harden during the operation

Question 18

When considering friction, what will affect the drawing load?

Answer 18

If the coefficient of friction increases then σD increases.
If the approach angle α increases then the contact area between the material will decrease and so the drawing load will decrease.

Question 19

Drawing stress equation accounting for friction

Answer 19

σD=σbar.ln(A0/A1)+(μ/tan(α)).σbarln(A0/A1)

Question 20

How is redundant work done?

Answer 20

During wire drawing the material is sheared in one direction as it enters the deformation zone and then sheared again when it exits the deformation zone. This shearing requires work to be done on the material but does not contribute to the overall shape change of the material.

Question 21

Formula for drawing stress taking friction and redundant work into account

Answer 21

σD=σbar.ln(A0/A1)+(μ/tan(α)).σbarln(A0/A1)+2/3.σbar.α
Where α is in radians

Question 22

When considering redundant work how does approach angle affect drawing stress?

Answer 22

As α increases the amount of shearing that the material undergoes as it enters and exits the die will increase so the drawing stress will increase. (Contrast to effect α has on friction)

Question 23

What will the optimum solution for case study 3 ensure?

Answer 23

The drawing stress for each pass is less than the flow stress of the wire (so the wire doesn’t break).
The drawing load for each pass is less than 4kN (the capacity of the machine)

Question 24

What does the optimum approach angle vary with?

Answer 24

Depends on ln(A0/A1)

Question 25

Formula for optimum approach angle and drawing force

Answer 25

Sin(αopt)=rt((3μ/2).ln(A0/A1))
F=σD.A1

Question 26

Order of steps to see if a solution is possible (number of passes)

Answer 26

1. Find strain from ln(A0/A1)
2. Find optimum approach angle in rad
3. Total strain is ln(A0/A1)+redundant strain (2/3 x α)
4. Find product flow stress using total strain and stress strain data given.
5. Find mean flow stress using average between total strain and 0 strain.
6. Calculate drawing stress using full formula.
7. Calculate drawing load from F=σD.A1
8. Check drawing stress doesn’t exceed product flow stress
9. Check drawing load isn’t greater than the max for the machine.

Question 27

How to check if a multiple pass solution works

Answer 27

Find the strain ln(A0/A1) required (not total) to get from overall start to end diameters. Divide this by number of passes. Calculate the first die diameter from this strain and the A0 which is 8mm. Do for the next die where the output from previous is input (A1 goes to A0). Perform all other steps and calculations and checks the same as before. Entry flow stress will be product flow stress from previous pass.

Question 28

How many passes are needed for case study 3 and why?

Answer 28

Need the full 4 passes available. Main reason is to keep the drawing load below 4kN which is only possible is all passes when using 4. Issue of wire breaking only for 1 pass solution

Question 29

Why might the 5mm wire need annealing?

Answer 29

By drawing from 8mm to 5mm we have considerably hardened the wire (150 to 350 MPa). If the material is not annealed it may cause problems downstream as the rest of the production line is geared up to accept 5mm copper wire in the soft condition

Question 30

What defects can the wrong approach angle lead to?

Answer 30

Too shallow approach angle means there is only surface deformation and you get chevron or central bursting.
Too steep approach angle means you get shaving (outer parts of rod don’t go through the die)

Question 31

What does bad lubrication lead to?

Answer 31

Snakeskin. Surface defects

Question 32

What can case wire breaks and why are they a problem?

Answer 32

Any defects in the material being drawn (oxides, inclusions, porosity) can lead to wire breaking at stresses lower than the yield stress of the material. In continuous wire drawing operation a wire break will cause serious disruption to the production line. Is essential that the quality/cleanliness of the starting material is high as this will determine the maximum reduction that can be performed.

Question 33

Types of tube drawing

Answer 33

Sinking.
Plug drawing (plug in die and material flows around it).
Mandrel drawing