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Great Designs in Steel is Sponsored by:AK Steel Corporation, ArcelorMittal, Nucor Corporation,
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ANALYSIS OF TRIMMING PROCESSES FOR STAMPED BODY PANELS
Sergey GolovashchenkoFord Motor Company
Andrey IlinichOakland University
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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
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
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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
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5% 10% 15% 20% 30% 40%
TRIMMED EDGE OF AHSS DP500 AFTER CONVENTIONAL TRIMMING PROCESS
Sharp lowertrim steel
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
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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
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Sharp lowertrim steel
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
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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.
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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)
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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