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w w w . a u t o s t e e l . o r g

Great Designs in Steel is Sponsored by: AK Steel Corporation, ArcelorMittal, Nucor Corporation, Severstal North America Steel Corporation w w w . Inc. a u and t o sUnited t e e States l.org

ANALYSIS OF TRIMMING PROCESSES FOR STAMPED BODY PANELS Sergey Golovashchenko Ford Motor Company Andrey Ilinich Oakland University

www.autosteel.org

OUTLINE 1. Introduction – trimming process 2. Trimming issues: burrs, slivers and splits from the trimmed surface 3. Influence of trimming conditions on burrs and slivers generation 4. Robust trimming method 5. Stretching of trimmed surface 5.1. Cases when stretching of the sheared surface occur 5.2. Methods of testing 5.3. Testing technique employed 5.4. Modes of fracture during stretching of trimmed blanks 5.5. Influence of trimming conditions on ability of trimmed samples to stretch 6. Conclusions

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CONVENTIONAL TRIMMING PROCESS Schematic of the trimming process Clamping pad

Sharp upper trim steel

Sharp lower trim steel

Blank

c

• Quality of trimmed surface is very sensitive to the accuracy of alignment of upper and lower dies • Burrs, slivers and splits are typical issues in conventional trimming • Tooling alignment is especially difficult in curved areas of the trimming line

www.autosteel.org

EXPERIMENTAL TOOLING EMPLOYED IN TRIMMING EXPERIMENTS Upper plate Upper block Sliding plates Steel block Nitrogen cylinder

Lower plate Lower block Clamping pad Die insert Sample Punch insert

www.autosteel.org

TRIMMED EDGE OF AHSS DP500 AFTER CONVENTIONAL TRIMMING PROCESS Clamping pad

Sharp upper trim steel

Sharp lower trim steel

Blank

t

5%

C 10%

• Clearance C ~ 10% t • C can be increased during trimming process • C may not be uniform around the perimeter of the blank

15%

20%

30%

www.autosteel.org

40%

MECHANISM OF BURR AND SLIVER FORMATION IN CONVENTIONAL TRIMMING part

offal

steel DP600

part

Sliver crosssection

Lower trim steel

Hair-like sliver

Accumulated damage DQSK steel

sliver on the offal side

www.autosteel.org

offal

ROBUST TRIMMING PROCESS Clamping pad

Dull upper trim steel

Blank

Sharp lower trim steel

US Patent 7197970

Elastic pad

Cross-sections of trimmed edges from advanced high-strength steel DP500 5%

10%

15%

20%

30%

www.autosteel.org

40%

MECHANISM OF ROBUST TRIMMING DP500, t=0.65 mm

DP600, t=1.48mm

Theoretical approach discussed in: S.Golovashchenko, Int.J.Mech.Sci. V.48, N12, 2006

A crack starts from the free surface near the lower trim edge and propagates to the stretched surface on top of the blank.

www.autosteel.org

CONVENTIONAL AND ROBUST TRIMMING PROCESS (DQSK mild steel, t=0.74mm) Conventional trimming Z=2%

Z=10%

Z=20%

Z=30%

Z=40%

Robust trimming (with scrap support and dull upper shearing edge) Z=2%

Z=10%

Z=20%

Z=30%

www.autosteel.org

Z=40%

CONVENTIONAL AND ROBUST TRIMMING PROCESS (DP600 steel, t=0.74mm) Conventional trimming 5%

10%

20%

30%

40%

50%

Robust trimming (with scrap support and dull upper shearing edge) 5%

10%

20%

30%

www.autosteel.org

40%

50%

WHEN SPLITS FROM SHEARED SURFACE ARE EXPECTED? S.Sadagopan GDIS’2004

1. Stretch flanging and hemming

2. Drawing of blanks with with windows M. Shi & X. Chen - SAE Paper 2007-01-1693

SAE Paper 2007-01-1693

3. Stretching of the sheared edge

www.autosteel.org

TESTING OF SHEARED EDGE FORMABILITY Hole expansion test

Half-a-dog bone tensile test

EDM

Sheared surface

•Reproduces strain gradient •Varying the clearance requires multiple tools •Typical hole diameter D= 10mm •Results may vary as f (t/D) •Burrs are smaller compared to trimming

Strip tensile test

Smoothed surface

Sheared surface

•No strain gradient

•No strain gradient

•Requires surface preparation from offal side

•Requires min surface preparation

•May reproduce variety of trimming conditions in one tool

•Appropriate only for materials with substantial n-value •May reproduce variety of trimming conditions in one tool

www.autosteel.org

TESTING PROCEDURE EMPLOYED TO DEFINE STRETCHING PERFORMANCE OF TRIMMED SAMPLES Testing equipment

75

Scrap Part Scrap Part Scrap Part Trimmed surface

Tensile test Part 12.7

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OBSERVED FAILURE MODES Schematic of the test

Failure modes

Load - displacement curves Load

Mode 1

Mode 1 Mode 2+1 Mode 2 Displacement

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Mode 2 Mode 2+1

OBSERVED FAILURE MODES Mode 1

Mode 2+1

Mode 2 Mode 2+1

A Mode 2

View A (Mode 2)

Mode 1

Major crack

Multiple starting cracks Mode 2+1

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FRACTURE INITIATION IN TENSILE TEST Samples of trimmed with conventional process (C=40% DP500, t=0.65 mm)

fracture initiation Samples trimmed with robust process (C=40%, DP500, t=0.65 mm)

fracture initiation www.autosteel.org

FREQUENCY OF FAILURE MODES Conventional Trimming Process, DP500, t= 0.65mm Mode 1

Mode 2 10 9 8 7 6 5 4 3 2 1 Mode 2 Mode 2+1 Mode 1

0 8.5%

16.6%

24.7%

32.9%

41.0%

49.2%

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FREQUENCY OF FAILURE MODES Robust Trimming Process, DP500, t= 0.65mm Mode 1

Mode 2 8 7 6 5 4 3 2 1 0

Mode 1 44.4%

36.3%

28.1%

20.0%

15.1%

11.0%

6.9%

Mode 2

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STRETCHING OF TRIMMED SAMPLES 40

Robust process

Total Elongation, %

35 30 25 20

Conventional process

15 10 5 0 0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

clearance between the shearing edges/t·100%

Material DP500, t=0.65 mm www.autosteel.org

60.0%

CONCLUSIONS 1.

Conventional trimming technology with sharp shearing edges provided good quality of sheared surface and ability to stretch along trimmed surface for DP 500 and DP 600 if the clearance between sharp shearing edges did not exceed 20%.

2.

Increasing the clearance over 20% in conventional trimming process resulted in quick deterioration of quality and significant reduction of formability.

3.

The robust trimming process provided significantly smaller burrs in a wide variety of clearances between the shearing edges, stable total elongation along trimmed surface, and elimination of slivers.

www.autosteel.org

w w w . a u t o s t e e l . o r g

Great Designs in Steel is Sponsored by: AK Steel Corporation, ArcelorMittal, Nucor Corporation, Severstal North America Steel Corporation w w w . Inc. a u and t o sUnited t e e States l.org