Jay Sartkulvanich and Taylan Altan represent the Center for Precision Forming at Ohio State University (www.cpforming.org and www.ercnsm.org).
Forming Advanced High-Strength Steels (AHSS) sheet metal presents new challenges over forming traditional steels. Ohio State University’s Center for Precision Forming (CPF) leads research efforts on precision forming and fabrication of lightweight materials. CPF’s research activities include: warm forming and warm hydroforming of aluminum and magnesium alloy; hot stamping of boron steels; and advanced applications of servo presses. The latest AHSS formability research initiatives from CPF on springback, shearing, edge stretching, deep drawing, hot stamping, servo presses, die materials, die-coatings and lubrication are summarized in this article.
The Center for Precision Forming (CPF), located at the Ohio State University in Columbus, OH, conducts research relevant to sheetmetal forming. The National Science Foundation (NSF) partially funds this activity together with participating companies. The mission of CPF is to serve as a center for the creation and dissemination of knowledge in precision forming and fabrication of lightweight materials, which ultimately impact next-generation products. CPF uses extensively the knowledge base developed at the Engineering Research Center for Net Shape Manufacturing (ERC/NSM) at The Ohio State University (Director Dr. Taylan Altan), established in 1985.
Considering the interests of the stamping industry, research at the CPF currently focuses on stamping of advanced high-strength steels (AHSS), warm forming and warm hydroforming of aluminum and magnesium alloys, hot stamping of boron steels, and advanced applications of servo presses.
Advanced High-Strength Steel (AHSS) use has been increasing in the automotive industry, due to advantages related to weight reduction as well as crashworthiness and safety. However, there are several challenges in forming AHSS. For example, common knowledge of material behavior of mild steels cannot be applied to AHSS. In addition, fracture in bending and edge flanging is more critical in AHSS. A number of studies indicate that fracture in forming of AHSS occurs much earlier (at lower strains) than when forming mild steels. Due to their high strength properties, AHSS also yield more springback and wear tooling faster than when forming mild steels. Furthermore, variations in material properties from batch to batch affect the process and the design procedure.
CPF has conducted various research activities related to these challenges. Major research topics related to forming AHSS include
- determination of material properties under biaxial deformation,
- prediction of springback considering E-modulus variation,
- evaluation of various stamping lubricants,
- investigation of galling behavior of various die materials and coatings,
- fracture prediction in stretch bending.
New test developed for flow-stress properties
Flow-stress properties are required for computer simulation and analysis of the forming process. Usually, these data are determined using tensile tests. However, the data obtained from tensile tests are only for relatively small strains (to 0.2 for AHSS). At CPF, the Viscous Pressure Bulge (VPB) test was developed to determine flow-stress data under biaxial deformation, allowing data to be obtained at larger strains (to 0.4 for AHSS). VPB tests of various AHSS (e.g. DP600, TRIP 780 and DP980) and other sheetmetals were conducted for several industrial sponsors. VPB tests were also used to evaluate variation of the formability of the same material across different batches/coils.
Figure 1. Schematic of a VPB test (a) before the test and (b) after the test.
Research leads to improved springback conditions
Many research studies demonstrate that the elastic modulus of AHSS decreases with increasing plastic strain, and it is necessary to consider the unloading modulus variation for accurate springback predictions. At CPF, tensile tests and bending tests with V-dies and S-dies were conducted to determine the variation of unloading modulus as well as to validate springback predictions obtained from finite element (FE) simulations. Figure 2 illustrates bending experiments for various curvature geometries and bending conditions, using an S-die.
Figure 2. Bending experiments using an S-die to determine springback of various deformation modes. Press and tooling is pictured on the left, with various shapes formed using the S-die on the right.
Extensive lubrication, die-material and die-coating tests
New materials, such as galvanized AHSS, aluminum alloys and stainless steels, require advanced lubrication systems to enhance formability and reduce scrap rates. CPF has been evaluating lubricants as well as die materials/coatings for various automotive companies and suppliers by emulating the conditions that exist in stamping production. For this purpose, the cup drawing test and the strip drawing test are used.
Figure 3. (a) Cup-drawing test, (b) strip-drawing test and (c) cup-ironing test, all performed for evaluation of lubricants and die materials/coatings.
Testing the shearing process and its influence on edge stretching
Blanking and shearing of AHSS presents some difficulties due to a very high shearing load that could significantly reduce tool life and yield poor quality of the sheared edge. In addition, the formability of the edges subjected to tension, such as during stretch flanging, is highly influenced by the edge quality. CPF is conducting studies on the fundamental understanding of the shearing process and its influence upon edge stretching.
Predicting fracture in sheet bending
Fracture in bending of AHSS is another critical subject. At CPF, preliminary bending experiments and FE simulations were conducted to establish a procedure to predict fracture (shear vs. tensile) in sheet bending, with and without a stretching condition. Currently, deep-draw tooling with stretching capability is being developed to study the influence of punch radius and stretching level on bending failure.
Forming aluminum and magnesium alloys at higher temperatures allows deeper draws
Aluminum and magnesium alloys are being increasingly used in automotive applications, also due to their lightweight and high strength-to-weight ratios. However, their applications are limited to shallow parts because of their low formability. Recent research demonstrated that these materials have higher formability when formed at elevated temperatures (250 to 300 C). CPF is conducting studies with industrial partners on hydroforming of these materials. This study shows that the sheet hydroforming at elevated temperatures can form a cup with larger cup height and better reverse bulge profile than when forming at room temperature.
Research targets hot stamping
Hot stamping (or hot press forming, or press hardening), developed in 1980s, has been increasingly used to manufacture crash-resistant components (e.g. side impact beams, pillars). In this process, boron-steel blanks are first heated to austenite temperatures (900-950 C) inside a furnace and subsequently transferred to an internally cooled dieset. The formed part is cooled down very rapidly inside the closed die, which is internally cooled by water circulation, completing the quenching (martensitic-transformation) process. As a result, the tensile strength of hot-stamped steels can achieve levels to 1,600 MPa (about 230 ksi). CPF is developing advanced process-simulation techniques, using FEA, for optimizing die design and reducing cycle times in hot stamping.
Optimizing servo presses
During the last few years, a number of automakers and suppliers have purchased and installed servo presses. Compared to mechanical presses, servo presses offer several advantages, such as adjustable stroke length, short cycle times, adjustable ram position/velocity synchronized with the feeder, considerable savings in energy, and ability to dwell at bottom-dead-center. The flexibility to adjust slide motion allows for decreasing the tool impact speed while reducing the cycle time. Thus, formability and productivity are improved. CPF is collaborating with research institutes and press builders to conduct research on
- development of a methodology to optimize the ram velocity control path (e.g. variable speed, dwelling, restriking) during deep drawing of sheetmetal parts,
- feasibility of hybrid modeling for press health assessment, diagnosis, prognosis and applications in production. Experiments on servo presses are planned and will be conducted during the first half of 2011.
Workshops and conferences
Every year, CPF disseminates research findings to industry through workshops and conferences. CPF hosted one workshop on Lubrication in Stamping (September 2008) and two workshops on Forming of AHSS (April 2009 and November 2010). The aim was to bring together interested individuals to discuss issues related to the practice of advanced stamping technology. Personnel from various automotive OEMs, steel suppliers, die makers, and die material and lubricant suppliers attended the workshops and participated in panel discussions. For more information and an update of activities at the CPF, please visit our website, www.cpforming.org or www.ercnsm.org, or contact Dr. Taylan Altan (director of CPF), at 614-292-5063, firstname.lastname@example.org.
- How can the research described above be incorporated to improve your metal-stamping operations?