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Great Designs in Steel is Sponsored by:AK Steel Corporation, ArcelorMittal, Nucor Corporation,

Severstal North America Inc. and United States Steel Corporation


ANALYSIS OF TRIMMING PROCESSES FOR STAMPED BODY PANELS

Sergey GolovashchenkoFord Motor Company

Andrey IlinichOakland University


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


CONVENTIONAL TRIMMING PROCESS

Sharp lowertrim steel

Clamping padSharp upper

trim steel

Blank

• 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

Schematic of the trimming process

c


Upper plate

Upper block

Sliding plates

Steel block

Nitrogen cylinder

Lower plate Lower block Clamping pad Die insert Sample Punch insert

EXPERIMENTAL TOOLING EMPLOYED IN TRIMMING EXPERIMENTS


5% 10% 15% 20% 30% 40%

TRIMMED EDGE OF AHSS DP500 AFTER CONVENTIONAL TRIMMING PROCESS


Clamping padSharp upper

trim steel

BlankC

• Clearance C ~ 10% t

• C can be increased during trimming process

• C may not be uniform around the perimeter of the blank

t


Lower trim steel

Sliver cross-section

Hair-like sliver

sliver on the offal side

DQSK steel

steel DP600part offal

MECHANISM OF BURR AND SLIVER FORMATION IN CONVENTIONAL TRIMMING

part offal

Accumulated damage



Clamping pad

Elastic pad

Dull upper trim steel

Blank

5% 10% 15% 20% 30% 40%

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

US Patent 7197970

ROBUST TRIMMING PROCESS


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

MECHANISM OF ROBUST TRIMMING DP600, t=1.48mm DP500, t=0.65 mm

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


Z=2% Z=10% Z=20% Z=30% Z=40%

Z=2% Z=10% Z=20% Z=30% Z=40%

Conventional trimming

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

CONVENTIONAL AND ROBUST TRIMMING PROCESS (DQSK mild steel, t=0.74mm)


5% 10% 20% 30% 40% 50%Conventional trimming

5% 10% 20% 30% 40% 50%Robust trimming (with scrap support and dull upper shearing edge)

CONVENTIONAL AND ROBUST TRIMMING PROCESS (DP600 steel, t=0.74mm)


WHEN SPLITS FROM SHEARED SURFACE ARE EXPECTED?

S.Sadagopan GDIS’2004 1. Stretch flanging and hemming

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

3. Stretching of the sheared edgeSAE Paper 2007-01-1693


Hole expansion test Half-a-dog bone tensile test Strip tensile test

•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

EDM Sheared surface

Smoothed surface

Sheared surface

•No strain gradient

•Requires surface preparation from offal side

•May reproduce variety of trimming conditions in one tool

•No strain gradient

•Requires min surface preparation

•Appropriate only for materials with substantial n-value

•May reproduce variety of trimming conditions in one tool

TESTING OF SHEARED EDGE FORMABILITY


TESTING PROCEDURE EMPLOYED TO DEFINE STRETCHING PERFORMANCE OF

TRIMMED SAMPLES

Part

Trimmed surface

12.7

75

Scrap

Scrap

Scrap

Part

Part

Part

Tensile test

Testing equipment


Load

Displacement

Mode 1Mode 2+1Mode 2

Mode 2Mode 1

Schematic of the test

Load - displacement curves

Failure modes

Mode 2+1

OBSERVED FAILURE MODES


Mode 1 Mode 2Mode 2+1

A

View A (Mode 2) Major crack

Multiple starting cracks

OBSERVED FAILURE MODES

Mode 2

Mode 2+1

Mode 2+1

Mode 1


FRACTURE INITIATION IN TENSILE TESTSamples 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


8.5% 16.6% 24.7% 32.9% 41.0% 49.2%

Mode 1Mode 2+1

Mode 20123456789

10

Mode 1 Mode 2

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


6.9%

11.0

%

15.1

%

20.0

%

28.1

%

36.3

%

44.4

%

Mode 1

Mode 201234

56

7

8

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

Mode 1 Mode 2


0

5

10

15

20

25

30

35

40

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0%

clearance between the shearing edges/t·100%

Tota

l Elo

ngat

ion,

%

STRETCHING OF TRIMMED SAMPLES

Conventional process

Robust process

Material DP500, t=0.65 mm


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.

CONCLUSIONS



Great Designs in Steel is Sponsored by:AK Steel Corporation, ArcelorMittal, Nucor Corporation,

Severstal North America Inc. and United States Steel Corporation

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