ME5603 Metal Forming Technology

ME5603 Metal Forming Technology

By: A/P Lee Kim Seng

Earing occurs when a cup is deep-drawn from a circular blank.

This is due to the material of the blank which is not isotropic.

4 ears normally appear on the cup symmetrically situated.

Earing give rise to scrap material in which the ears had to be cut off which is undesirable.


When a cup is drawn in, the flange circumference is constantly being reduced and this tends to cause the material to wrinkle.

Wrinkling can be prevented by using blank holder with suitable loading.


There are large residual stresses left in cups after drawing.

In the axial direction there is residual tension on the outside and compression on the inside, resulting from bending and unbending over the die lip.

These stresses are largest near the top of the wallbecause of the bending that occurred near the end of the draw, when there was little net tension.

These residual stresses induce a bending moment in the wall which is balanced by hoop tension near the top of the cup.


In some materials this hoop tension can lead to splitting of the walls by stress-corrosion cracking unless the cups are stress relieved.

Other metals, especially ones with aligned inclusions, crack immediately as the cup leaves the die.

Ironing, which causes the entire section to yield, greatly diminishes the magnitude of the residual stresses.

Con’t..


Calculated blank diameters for symmetrical shells (American Society of Tool Engineers 1959, by permission of McGraw-Hill Book Co.)


Lubrication improved drawability and reduces the friction between metal contact surfaces, thus reducesthe maximum punch load.

Solid lubricant – e.g. Teflon sheet, Titanium nitrite


The objective of the blank holder is to prevent wrinklingbut at the same time allows the flange to thicken during the drawing operation.

Minimum Hold-down Pressure of Flange Area

2.76400Phosphor Bronze1.72/2.07250/300Brass1.38200Copper1.38/3.45200/500Aluminum Alloy0.69100Aluminum6.91000Austenitic Stainless Steel3.45500Low Carbon SteelMPa (N/mm2)lbf/in2Annealed Material


The minimum tool radii which will yield a part are actually very small and are about the same as the minimum bend radii. However, with such tooling, the maximum blank size which can be cupped without rupturing is rather small.

Larger tool radii permits parts of equal diameter to be drawn to a considerable greater depth or with a wider flange.


Tool radii are usually keep between 5 to 10 times the blank thickness. Die radius should be kept below 20 times blank thickness.

If the punch radius exceeds 30% of the punch cross-section diameter the finished part will be less strong.


Radial Clearance between Punch & Die

Clearance of 10/20% over blank thickness are commonly used.


Consider hoop element at periphery and assume constant volume of the element

ππππdoto∆∆∆∆ro= ππππd t ∆∆∆∆r

If the material is isotropic, then =to

t∇∇∇∇

ro∇∇∇∇

r

to

t∴∴∴∴ ππππdoto = ππππd t

doto2 = d t2

ddot = to√√√√By: A/P Lee Kim Seng

If punch diameter is dp,

the thickness of edge as it enters the gap between punch & die is

dp

dot = to√√√√dp

do = draw ratiodraw ratiowhere

The maximum draw ratio is known as drawabilitydrawabilityWhich 2.3


Reduction is expressed as

r = do

do - dp

do

dp1 -= ordo

dp1 -100 %( )

where do = Diameter of blankdp = Diameter of punch

ifdo

d11 -r1 = , ,d1

d21 -r2 =d2

d31 -r3 =


The total reduction do

dn1 -r1/n =

do

d1= 1 -d1

d2. . …….d2

d3

dn-1

dn

= 1 - (1 - r1)(1 - r2)(1 - r3)……..(1 - rn)

total reduction = 1 -total draw ratio

1

If r1 = r2 = r3 = …….. rn

Then r1/n = 1 - (1 - r1)nBy: A/P Lee Kim Seng

E.g. For 2 operations at 50% reduction per operation

r1/2 = 1 - (1 - 0.5)2

= 0.75= 75%

r1/3 = 1 - (1 - 0.5)3

= 0.875= 87.5%

1

2

E.g. For 3 operations at 50% reduction per operation


Theoretical Aspect

Ref: An approximate analysis for determining the maximum punch load in axisymmetrical deep drawing

J. L. Duncan & W. Johnson

9 MTDR Conf 16th /20th Sept 1968 By: A/P Lee Kim Seng

Assumptions:

1. The material is isotropic.

2. There is no change in thickness, t.

3. There is no bending stresses.

4. The surface area remains constant.

5. Friction is neglected.

6. Punch load-displacement diagram is assumed to be of sinusoidal shape.

The following assumptions were made in the approximate mean work analysis.

Cont..


Cont..Assuming constant volume, the final cup wall height can be obtained by equating the volume of material in the initial flange b ≤≤≤≤ a ≤≤≤≤ ao and the final cup wall.

ππππ (ao2 - b2) t = 2ππππbht

ao2 - b2

2b∴∴∴∴ h =

b2

h = (R2 - 1) ----- (1)

ao

bR = = Draw Ratiowhere


A mean radius a in the flange is chosen such that the areas of flange on either side of a are equal. I.e

ππππ (ao2 - a2) = ππππ (a2 - b2)

2ao

2 + b2a = √√√√ 2R2 + 1= b √√√√ ----- (2)

During drawing, the element at radius a is deformed under plane strain condition (constant thickness) into a cylindrical cup of mid-wall radius b.

Cont..


Thus for an annular element initially at radius r, the total hoop strain

ba

εεεεθθθθ = ln = - lnab

= - lnb √√√√ 2

R2 + 1

b

----- (3)∴∴∴∴ ε εεεθθθθ = - ½ ln2

R2 + 1

Cont..


Since thickness is constant, then the operation is conducted under plane strain

εεεεz = 0 & εεεεθθθθ + εεεεr + εεεεz = 0 ∴∴∴∴

∴∴∴∴ ε εεεθθθθ = - εεεεr

Effective strain

εεεε [ (εεεεθθθθ - εεεεr)2 + (εεεεr - εεεεz)2 + (εεεεz - εεεεθθθθ)2 ] ½=3

√√√√2

[ 4εεεεθθθθ2 + εεεεθθθθ

2 + εεεεθθθθ2 ] ½=

3√√√√2

εεεεθθθθ=√√√√32

-

Cont..


∴∴∴∴ εεεεθθθθ = - ½ ln2

R2 + 1

∴∴∴∴ ε εεε =√√√√31

ln2

R2 + 1----- (4)

Cont..

εεεεθθθθεεεε = √√√√32

-


dw = σσσσ1A1dl1 + σσσσ2A2dl2 + σσσσ3A3dl3

= V (σσσσ1dεεεε1 + σσσσ2dεεεε2 + σσσσ3dεεεε3)

= V { (σσσσ1’ + σσσσm) dεεεε1 + (σσσσ2’ + σσσσm) dεεεε2 + (σσσσ3’ + σσσσm) dεεεε3}

Cont..


σσσσm (dεεεε1 + dεεεε2 + dεεεε3) = 0 ∴∴∴∴

dw = V {σσσσ1’dεεεε1 + σσσσ2’dεεεε2 + σσσσ3’dεεεε3} ∴∴∴∴ ----- (a)

σσσσ1’dεεεε1 =

σσσσ2’dεεεε2 =

σσσσ3’dεεεε3 = dλλλλ ----- (b)

From Levy & Mises

dw = V (σσσσ1’2 + σσσσ2’2 + σσσσ3’2) dλλλλ∴∴∴∴ ----- (c)

also σσσσ = √√√√ 32 (σσσσ1’2 + σσσσ2’2 + σσσσ3’2) ----- (d)

Cont..


& √√√√ 23 (dεεεε1

2 + dεεεε22 + dεεεε3

2)εεεε =d ----- (e)

d εεεε 2 dλλλλ2 (σσσσ1’2 + σσσσ2’2 + σσσσ3’2)23

=

dλλλλ223

=σσσσ 2

32

dλλλλ249

= σσσσ 2

d εεεε dλλλλ23

= σσσσ∴∴∴∴ ----- (f)

Cont..


From Equation (c)dw

= (σσσσ1’2 + σσσσ2’2 + σσσσ3’2) dλλλλ∴∴∴∴V

Substitute Equation (d) into (c) dλλλλ23

= σσσσ 2

d εεεε32

= σσσσ23

----- (g)d εεεε= σσσσdwV

∴∴∴∴ ��

σσσσ d εεεεεεεε

εεεεo

W = V = Volume ×××× Area under σσσσ / εεεε curve

Cont..


The plastic work done per unit volume on the material initially at a is

��


0

If the material is assumed to be of rigid linear strain hardening type, then the integral

��


0

=

2σσσσo + σσσσ

εεεε

----- (5)

Cont..


It is assumed that the mean plastic work done per unit volume for all element in the blank in the regionb ≤≤≤≤ a ≤≤≤≤ ao is equal to the plastic work done on a unit volume initially at a, radius a.

Since the volume of material in this region of the blank is

V = ππππ (ao2 - b2) t

= ππππb2 t (R2 - 1) ----- (6)

W = V ��


0

∴∴∴∴ The total work done in forming the cup will be

Cont..


----- (7a)��


0W = ππππb2 t (R2 - 1)∴∴∴∴

= ππππb2 t (R2 - 1)2

σσσσo + σσσσ√√√√31

ln2

R2 + 1

∴∴∴∴ V = ππππb2 t (R2 - 1)

----- (7)(R2 - 1)(σσσσo + σσσσ) ln2

R2 + 1=

2√√√√3ππππb2 t

W

Cont..


Assuming as an approximation that punch load versus punch stroke characteristic is a sine curve. i.e.

P = Pmax sinπχπχπχπχh

----- (8)

The work done by the punch will be the area under the curve

Pmax sinπχπχπχπχh

dχχχχ��

h

0W =


(Con’t)

Pmax cosπχπχπχπχh

hππππ

-h

0=

Pmax2hππππ

= ----- (9)W

Equating Equation (9) = Equation (7)

Pmax2hππππ

(R2 - 1)(σσσσo + σσσσ) ln2

R2 + 1=


Pmax sinπχπχπχπχh

dχχχχ��

h

0W =


b2

h = (R2 - 1)Since

(R2 - 1)(σσσσo + σσσσ) ln2

R2 + 1=


Pmaxb (R2 - 1)

ππππ∴∴∴∴

(σσσσo + σσσσ) ln2

R2 + 1=

2√√√√3ππππ2 b t

Pmax ----- (10)

----- (10a)��


0Pmax = ππππ2 b tor

(Con’t)


σσσσo ln2

R2 + 1=

√√√√3ππππ2 b t

Pmax

εεεε =√√√√31

ln2

R2 + 1

NB: σσσσ is obtained from σσσσ vs. εεεε curve when

For zero strain hardening

In practice, due to thinning t1 < t∴∴∴∴ Pmax (expt) is slightly less than that predicted.

Initial Thickness

(Con’t)


Ironing is an operation that sizes and thins the walls of drawn cup (hollow components).

Advantages:• Deformation forces and the stress are smaller

when compared with cold extrusion process.• Better wall-thickness tolerance and surface finish.

The preform for the ironing process is a drawn or extruded cup, which is pushed through one or more ironing rings.


(Con’t)

Ironing in process


The competitive process of ironing are flow formingand hollow forward extrusion.

If ironing (wall thinning) is employed during cupping and redrawing, a more uniform wall thickness and increased cup height result.

Cupping Ironing

(Con’t)


Often, only the top of the wall is ironed, this being achieved by controlling the clearance between the punch and the die lip, so no ironing occurs until the thickened material on the outer part of the flange flows over the die lip.

Ironing Rings & Can Tooling

(Con’t)


Ironing adds to the total punch force, but if it occurs only late in the draw after the maximum punch force is reached, it does not affect drawabilitydrawability.

(Con’t)


Ironing decreases the degree of earing, besides producing a thinner and more uniform wall thickness.

Since the wall thickness in the valleys between ears is greater than at the eared portions, the increase in height due to ironing is greater at the valleys.

A more uniform height (I.e., less produced earing) results.

(Con’t)


Two ways of deep drawing:i. Draw & Redrawii. Draw & Iron

• Carbide Punch & Die• Draw into shallow cup• Redraw into shallow cup


After re-drawing, ironing is done in a few stages.

Con’t..


Subsequently the cans are trimmed, necked and flanged.

Con’t..


Studies with coated cutting tools indicate that a a contributory factor to the success of coating is the low coefficient of friction between the tool and the work piece.

The major advantages of applying coatings include:• Longer tool life,• Increased productivity,• Improved surface finishes and closer tolerances,• Reduce forming forces.which result in cost savings.

It was reported that coating increased tool life of some forming tools by as much as 75 times.


3.09,0003,000M2Tap6.

35.070,0002,000M2Extrusion punch

5.4.623,0004,950M2Form tool4.

10.01,000,000100,000D2Thread rolling die

3.9.1100,00011,000M2Trim die2.75.01,500,00025,000D2Draw die1.

CoatedUncoatedImprovement X

Ave life (ops)Tool Material

Tool type

Average tool life of various tools (Sheet Metal Industries, V60, P303)


2223HfN4112Al2O3

2444Ti(C,N)1333TiN3551TiC

Friction Resistance

Heat Resistance

Chemical Resistance

Resistance to Abrasive Wear

Coating Material

Rating: 1 = Best, 5 = Worst

Since the flow of material in the deep drawing process is highly influenced by the frictional force present, TiN coating is an appropriate choice, due to the fact that TiN has the highest level of lubricity among the various coatings. By: A/P Lee Kim Seng

Thickness of coating: Usually about 2 to 5 micronsWick has shown that coating thickness > 8 microns tends to reduce its wear resistance.Further increase to 12 microns or more, the brittle properties can dominate and the coating may be chipped off, or even cracked from the substract.

Coating ProcessesTwo most common coating deposition methods are chemical vapor deposition (CVD) and physical vapor deposition (PVD), where thin solid films are grown from the vapor phase.

CVD 850-1250 °°°°CPVD 150-500 °°°°C By: A/P Lee Kim Seng

ME5603 Metal Forming Technology

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