Page 12 - Metallforming Magazine December 2020

Cutting Edge By Eren Billur, Ph.D.
Heat-Assisted Adiabatic Cutting
of High-Strength Steel
In the last two Cutting Edge columns, I explained how heating can improve the formability of boron-alloyed steels and some aluminum alloys. In both cases, we heat the sheet metal in a furnace to their optimum forming temperature. Here we’ll discuss a heat-assisted cutting technology where the sheet metal blank itself generates the heat required for cutting, without the need for a furnace or other heating device.
How Does a Blank Heat Itself?
Most metal formers should have experi- enced that a part exiting the press may be quite hot. This occurs because the sheet metal generates heat as it deforms. The temperature of the part may depend on several factors, including the amount of work done (press force and working stroke length), and how quickly the part is produced (directly pro- portional to stroke).
Fig. 1 illustrates tensile-test results, record-
ed with a thermal camera, for two different
steel grades—DC04 (a mild steel similar to a deep drawing grade), and HCT980X (similar to DP 1000). The high-speed thermal camera captured the temperature increase in the fracture zone.
When pulling the mild steel at 50 mm/sec., the temper- ature at the fracture zone peaks at 69.5 C. When quadrupling the test speed, heat cannot dissipate as quickly as during the first condition. Also, due to strain-rate effects, about 5 percent more work is performed, and the temperature rises to 107.2 C. Note: After only 0.5 sec., the maximum temper- ature drops to 94.0 C. The heating and cooling cycle occurs
Eren Billur is the founder of Billur Metal Form, a con- sulting, engineering and training company in Ankara, Turkey. He studied at Baskent University and Virginia Commonwealth University, received a PhD in mechan- ical engineering from The Ohio State University, and worked as a researcher at the Center for Precision Forming. His areas of expertise include material char- acterization, sheet metal forming processes, and finite element simulations. He has authored/co- authored more than 20 scientific papers (including proceedings) and contributed to four books, including “Hot Stamping of Ultra High Strength Steels,” pub- lished in 2018.
Eren Billur
Billur Metal Form, Founder eren@billur.com.tr
Fig. 1—Thermal-camera images of tensile specimens, just before fracture.
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10 MetalForming/December 2020
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Fig. 2—A comparison of the heating zones during (a) conven- tional blanking, and (b) adiabatic cutting.
in very short time intervals and is localized in a very confined area of deformation.
When we pull the higher-strength HCT980X alloy, the peak temperatures are 123.9 C and 135.4 C, respectively.
How is This an Advantage?
Heating the blank typically is a disadvantage for metal