A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions
Summary
This protocol proposes a novel biaxial testing system used on a resistance heating uniaxial tensile test machine in order to determine the forming limit diagram (FLD) of sheet metals under hot stamping conditions.
Abstract
The hot stamping and cold die quenching process is increasingly used to form complex shaped structural components of sheet metals. Conventional experimental approaches, such as out-of-plane and in-plane tests, are not applicable to the determination of forming limits when heating and rapid cooling processes are introduced prior to forming for tests conducted under hot stamping conditions. A novel in-plane biaxial testing system was designed and used for the determination of forming limits of sheet metals at various strain paths, temperatures, and strain rates after heating and cooling processes in a resistance heating uniaxial testing machine. The core part of the biaxial testing system is a biaxial apparatus, which transfers a uniaxial force provided by the uniaxial testing machine to a biaxial force. One type of cruciform specimen was designed and verified for the formability test of aluminum alloy 6082 using the proposed biaxial testing system. The digital image correlation (DIC) system with a high-speed camera was used for taking strain measurements of a specimen during a deformation. The aim of proposing this biaxial testing system is to enable the forming limits of an alloy to be determined at various temperatures and strain rates under hot stamping conditions.
Introduction
The automotive industry is facing a huge global challenge of reducing fuel consumption and minimizing environmental pollution from vehicle emissions. Weight reduction is beneficial to improving the performance of automobiles and can directly reduce energy consumption1. Due to the low formability of sheet metals at room temperature, hot stamping and cold die quenching processes (referred to as hot stamping)2 are used to improve the formability of alloys and thus to obtain complex shaped components in automotive applications.
A forming limit diagram (FLD) is a useful tool to evaluate the formability of an alloy3. Out-of-plane tests, such as the Nakazima test4,5, and in-plane tests, such as the Marciniak test6,7,8, are conventional experimental methods to obtain the FLDs of sheet metals under various conditions9,10,11. A servo-hydraulic biaxial testing machine has also been used to investigate the formability of alloys at room temperature12,13.
However, none of the methods above are applicable to formability tests under hot stamping conditions, since a cooling process prior to forming is required along with control of the heating and cooling rates. The deformation temperature and strain rate are difficult to obtain accurately. Therefore, a novel formability testing system is proposed in this study to experimentally determine the forming limits of sheet metals under hot stamping conditions.
Protocol
Representative Results
Discussion
Conventional formability test methods used to determine forming limits are usually applicable only at room temperature. The presented technique can be used to evaluate the formability of metals for hot sheet stamping applications by introducing a novel biaxial testing apparatus to a resistance heating uniaxial testing machine. This cannot be performed using conventional methods for hot stamping applications. The setup of heating and cooling systems and the DIC system is critical to controlling the uniformity of temperatu…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This research was supported by the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 604240, project title “An industrial system enabling the use of a patented, lab-proven materials processing technology for Low Cost forming of Lightweight structures for transportation industries (LoCoLite).”
Materials
| Aluminium Alloy | Smiths Metal | 6082 | Specimens machining |
| Laser cutter | LVD Ltd | HELIUS 25/13 | Laser cutting specimens |
| CNC machine | HAAS Automation | TM-2CE | Machine specimens by milling |
| Vernier caliper | Mitutoyo | 575-481 | Thickness measurement |
| Resistance heating uniaxial testing machine | Dynamic System Inc | Gleeble 3800 | Thermo-mechanical materials simulator |
| High flow quench system | Dynamic System Inc | 38510 | For air cooling |
| Thermocouples | Dynamic System Inc | K type | |
| Nozzles | Indexa | Nozzle flared 1/4 inch bore | |
| Welding cables | LAPP Group | H01N2-D | |
| High-speed camera | Photron | UX50 | For DIC testing |
| Camera lens | Nikon | Micro 200mm | |
| Lamp | Liliput | 150ce | 300W |
| Laptop | HP | Campaq 2530p | For images recording |
| Biaxial testing apparatus | Manufactured independently | All parts were designed and machinced by authors for biaxial testing | |
| Steel | West Yorkshire Steel | H13 | Mateials of the biaxial testing apparatus |
| Image correlation processing software | GOM | ARAMIS | Non-contact measuring system and data post-pocessing |
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