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

          2018-11-12 11:04:00
          LUQIMENG
          Original
          2426
          Introduction
          Challenges 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

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