Use of Servo Press with a CNC Hydraulic Cushion in Forming AHSS

Servo presses and CNC cushions

Introduction
• DP 980 – DP 1200 steels are used by various OEM’s and
suppliers
• UHSS – DP 1200 (by Nippon Steel/Sumitomo Metals/Kobe
Steel) is used for B-Pillar in selected Nissan models
• R&D is being conducted for forming 1400 MPa steels to
produce structural parts
• Main challenges are: flow stress data, formability, flanging and
edge fracture, springback, temperature and lubrication, and press and tool deflection

Material properties
• E-modulus (tensile test, loading unloading test, tension
compression test)
• Strain rate dependency at room and elevated temperature
Bulge test (biaxial):
 Effective stress vs. strain (flow
stress data)
• Biaxial
• Large strains
• Not accurate for small strains
Tensile test (uniaxial):
 Engineering stress vs. strain
• Uniaxial
• Relatively small strains
• Yield stress
• Tensile stress
• Uniform elongation
• Total elongation
• E-modulus

Material properties: Tensile Test + Bulge Test

Formability:
Uniform Elongation, Total Elongation, Bulge Height, and FLD

• Complex microstructure of AHSS materials can lead to significantly different hole expansion ratio (HER)
• In DP steels the difference in hardness between the hard and soft phases (martensite and ferrite) is large and this facilitates the formation of micro-cracks
• Tests with the current ISO standard (ISO 16630) for hole flanging (10 mm hole diameter) show large scatter. Larger hole diameter (20 mm to 75 mm) are used in recent studies

E-modulus changes with strains. Variation of E-modulus as a function of strain can be obtained through loading-unloading tensile test. However, determination of E- modulus through a tensile test is not accurate and can cause error in springback prediction.

Lubrication tests:
1) Draw Bead Test
2) Strip Drawing Test
3) Twist Compression Test (TCT)
4) Cup Drawing Test (CDT)

The test conditions in cup drawing test (CDT) are closer to the real stamping conditions than TCT. In stamping there is heat generation due to friction and also due to plastic deformation of the material.

Materials used in the study:
• DP980 1.2mm
• DP590 1.4mm
• CP800 1.4mm
• TRIP1180 1.2mm
• TWIP980 1.3mm
• TWIP900 1.1mm

AIDA Servo Press Tests and PAM-STAMP Simulations

The thinning value around the corner of the panel is measured and compared with simulation results.

• Objective:
― Determine how the servo ram motion control can influence the drawability

• Forming conditions:
― 2-in drawing depth
― 2000-3000 (psi) BH pressure
(nitrogen cylinder)
― No spacer used
― Three blanks tested for each ram
motion condition
― Using attach / detach mode reduces
the cycle time

• Objective:
― Determine how the servo cushion can influence drawability
The optimum variable blank holder force which
avoid both the wrinkle and failure at the part will
be determined through FE simulations.

• Objective:
―Determine the servo press motion to obtain the “Best”
possible blanked / sheared edge quality

• Blanking with three operation modes and with two different speeds is
investigated
• For the evaluation of the edge quality Hole Expansion Test will be run
(in cooperation with EWI and KTH)
• The sheared edge is evaluated (under the microscope) and will be
correlated with the HER test