Deep Drawing, Formability. Deep drawing

Transcription

1 Deep Drawing, Formability Deep drawing 1

2 Principle punch, blankholder ring, blank, die ring Deep drawing of metal sheet is used to form containers A flat blank is formed into a cup by forcing a punch against the center portion of a blank that rests on the die ring Tooling: punch, blankholder ring, blank, die ring 2

3 Redrawing blankholder punch for redrawing blank workpiece die ring Final shape of a container can be reached by more drawing steps, these operations are called as redrawings 3

4 Combined drawings First drawing Second drawing More drawing operations can be combined into one tool using multiple ram system in a hydraulic press 4

5 Other characteristics of deep drawing Easiest way is to draw cylindrical parts from circle disc, but The process is capable of forming box (rectangular) shapes or shell-like containers Special variants are liquid pressure forming and rubber die forming These processes result in near net shapes for many purposes. 5

6 Deep drawing ratio Limiting Drawing Ratio (LDR) β= D o /d n where D o is the diameter of the first (largest) blank and d n is the smallest cup diameter that can be successfully drawn Drawing Ratio in general: β i = d i /d i+1 First drawing β 1 =2,2 1,8 redrawing β i =1,4 1,1 (copper, aluminium, mild steel) 6

7 Calculation of blank diameter In case of cup-like components the blank is circular Area of blank equal to theareaofcup: D π 2 d π = 1 + d πh 1 4 h d 1 D 0 = d d 1 h D o 7

8 Calculation of drawing force Drawing force: F d,max = n*π*d*t*uts Example: Cup diameter: d= 45,7 mm Sheet thickness: t = 0,5 mm Ultimate Tensile Strength: UTS = 320 MPa Drawing coefficient: n = 0,7 0,95 F d,max = 0,9*π*45,7*0,5*320 = N 8

9 Drawing tool Bélyeg punch Ráncgátló blankholder Munkadarab alakítás közbeni állapotban workpiece Húzógyűrű Die ring 9

10 Drawing errors (1) Earing Other errors: Buckling and wrinkling Fracturing Stress corrosion cracks 10

11 Drawing errors (2) Buckling and wrinkling, causing fracturing 11

12 Drawing errors (3) Fracturing 12

13 Deep Drawing Example 13

14 Analysis of a cup drawing The cup to be drawn: Diameter: 60,3 mm Height: 104,8 mm Blank diameter (earing is eliminated): D 0 = d 2 n + 4d n h n D 0 = 60, ,3 104,8 D 0 = 170,1 mm 14

15 Calculation of drawing ratio Suggested drawing ratio First drawing ~2 Second drawing 1,2 1,25 Third drawing 1,15 1,18 Further drawings 1,1 1,12 etc. Calculations by approximative (iterative) way 15

16 Results Nr. of operations Height in mm Diameter in mm Drawing ratio 0. blank - 170,1-1. Cup drawing 65,1 85,7 1,98 2. Redrawing 85,7 69,9 1,23 3. Redrawing 104,8 60,3 1,16 16

17 Drawing force Cup drawing (first drawing) Diameter: 85,7 mm Thickness: 2 mm UTS: 320 MPa F d max = n π d t UTS F d max = 0,9 π 85, F d max = N 155kN 17

18 Fluid forming hidromec process Punch, blankholder Seal, fluid, fluid holder Fluid forming uses only one solid die half. Forming pressure is applied by the action of hydraulic fluid, which forces the blank to assume the shape of the rigid tool Sometimes a flexibile membran is used to separate blank and the fluid 18

19 Strech drawing clamping die Used for drawing car body panels Blank is clamped, rigid die gives the shape of sheet metal Change in thickness: s 1 <s 0 workpiece 19

20 Formability Testing of Sheet Metals 20

21 Representation of Strain True or logarithmic strain: ϕ The integral of the incremental change in length dl, divided by the actual length L: d ϕ = dl L ϕ = L 1 L 0 dl L = ln L 1 L 0 21

22 22 Equivalent strain Strain in the direction of 3 axes: φ 1 = φ L φ 2 = φ w φ 3 = φ th Equivalent strain: 2 ) 3 1 ( 2 ) 3 2 ( 2 ) 2 1 ( 3 2 ϕ ϕ ϕ ϕ ϕ ϕ ϕ + + = e

23 True stress-strain (flow stress) curve Flow stress curve: σ f = σ f (φ e ) if temperature (T) and strain rate, is constant. In general (n: strain hardening exponent; m: strain rate sensitivity) σ σ σ f f f = σ = = f n c ϕ n c ϕ ( ϕ ; & ϕ ; T ) e e e ( if & ϕ e m e ( if & ϕ = e & ϕ ; T e T dϕ dt = = const.) const.) 23

24 Tensile test: Plastic strain ratio (r) Measurement L 0 w 0 φ L = ln(l 1 /L 0 ); φ w = ln(w 1 /w 0 ); φ th = - (φ L + φ w ) ( as φ L + φ w + φ th = 0 ) 24

25 b = w; s = th 25

26 Plastic strain ratio (r) Evaluation Calculation: r = φ w / φ th Definition: ratio of the true width strain devided to the true thickness strain The r value frequently changes with direction in the sheet Test specimens should be machined parallel (0 o ), perpendicular (90 o ), and (45 o ) related to the rolling direction 26

27 Measure of anisotropy Test 27

28 Measure of anisotropy Average normal anisotropy: r + r45 + r m = r90 Planar anisotropy: r = r 45 r r 90 The value r m determines the limiting drawing ratio, and r is in correlation with the extent of earing. A combination of high r m and low r provides optimal drawability. 28

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30 Average normal anisotropy and hardening exponent of metals Mild steel Drawing steel Austenitic steel Copper Aluminium Titanium n 0,2-0,5 0,22-0,26 0,4-0,55 0,35-0,5 0,2-0,3 0,05 r 1,0-1,4 1,4-1,8 0,9-1,2 0,6-0,9 0,6-0,8 3,0-5,0 30

31 Forming Limit Diagram (1) Major engineering strain, % Failure Safe Safe Minor engineering strain, % 31

32 Forming Limit Diagram (2) 32

33 Forming Limit Diagram (3) Sheet metal can be deformed only to a certain level before local thinning (necking) and failure occur FLD shows the limit of necking (or failure) as function of minor and major strain Strains can be evaluated from the deformation of circle grids plotted on the surface of sheet metal 33

34 Other formability tests Ball punch test (Erichsen test) Hydraulic bulge test Hemispherical dome test Cup drawing test (drawing test) 34

35 Ball punch test Well known as Olsen or Erichsen test Die The cup height at fracture is used as the measure of strechability Specimen Punch Blankholder ring 35

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37 Ball punch test Examples 37

38 Hydraulic bulge test Material characterisation in biaxial streching Testing to much higher strain levels than those achievable in tensile testing Research in plasticity theory Lock bead Hydraulic fluid Die ring Base plate 38

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40 Hemispherical dome test Lubricated punch is used for deformation of sheet metal The dome height at fracture is measured The test yields reproducible results Die ring Blankholder ring Punch Lock bead 40

41 Cup drawing test Circular blanks of various diameters are used Tooling is standardised Limiting drawing ratio (LDR) is the ratio of the diameter of the largest blank that can be successfully drawntothe diameter of the punch 41

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43 Cup drawing test Examples 43