Sergey Fedorovich Golovashchenko

The Effect of Tool–Sheet Interaction on Damage Evolution in Electromagnetic Forming of Aluminum Alloy Sheet

A study of the effect of tool-sheet interaction on damage evolution in electromagnetic forming is presented. Free form and conical die experiments were carried out on 1 mm AA5754 sheet. Safe strains beyond the conventional forming limit diagram (FLD) were observed in a narrow, region in the free form experiments, and over a significant region of the part in the conical die experiments. A parametric numerical study was undertaken, that showed that tool-sheet interaction had a significant effect on damage evolution. Metallographic analysis was carried out to quantify damage in the parts and to confirm the numerical results.

Analysis of the Increased Formability of Aluminum Alloy Sheet Formed Using Electromagnetic Forming

One of the main challenges associated with the use of aluminum alloys in the automotive industry is increasing their limited formability. Electromagnetic forming has been considered recently as a way of addressing this issue. Increases in formability for several commercial aluminum alloys have been reported in electromagnetic (EM) and other high speed forming processes. These increases are typically attributed to high strain rate and inertial effects; however, these effects alone cannot account for the increases in formability observed. The present authors have previously reported that the increased formability is likely due to damage suppression caused by the tool/sheet interaction. This paper presents an analysis of this interaction and how it affects the formability of the sheet. Experimental and numerical work was carried out to determine the details of the forming process and its effects on formability, damage evolution and failure. It has been determined that when the sheet makes contact with the tool, it is subject to forces generated due to the impact, and very rapid bending and straightening. These combine to produce complex non-linear stress and strain histories. The predictions indicate that relatively little damage is generated in the process except in specific areas of the parts. Damage measurements agree with the predicted trends and fractographic analysis shows that parts formed with the EM process do not fail in pure ductile failure, but rather in a combination of plastic collapse, shear fracture and ductile failure. The majority of the plastic deformation occurs at impact, leading to strain rates in the order of 10,000 s -1 . It is concluded that the rapid impact, bending and straightening that results from the tool/sheet interaction is the main cause of the increased formability observed in EM forming. The tool/sheet interaction produces a non-plane stress condition, very high strain rates and highly non-linear strain paths.

Springback Calibration Using Pulsed Electromagnetic Field

In stamping practice, springback of the blank is often compensated by developing the shape of the blank, which provides the targeted shape after its springback. In this paper we attempt to address the root cause of springback phenomenon dictated by residual stresses in the blank after completion of the stamping process. New process of springback calibration has been developed based upon the idea of elimination of elastic residual stresses in stamped blank. This technology attempts to address the issues of sheet material variability and stamping process variations, which make geometrical compensation more difficult. Simple set of experiments demonstrated that formed blank can be clamped to the targeted shape and then subjected to pulsed electromagnetic field.

Formability and Damage in Electromagnetically Formed AA5754 and AA6111

This paper presents the results of experiments carried out to determine the formability of AA5754 and AA6111 using electromagnetic forming (EMF), and the effect of the tool/sheet interaction on damage evolution and failure. The experiments consisted of forming 1mm sheets into conical dies of 40° and 45° side angle, using a spiral coil. The experiments showed that both alloys could successfully be formed into the 40° die, with strains above the conventional forming limit diagram (FLD) of both alloys. Forming into the higher 45° cone resulted in failure for both materials. Metallographic analysis indicated that damage is suppressed during the forming process. Micrographs of the necked and fractured areas of the part show evidence that the materials do not fail in pure ductile fracture, but rather in what could be a combination of plastic collapse, ductile fracture and shear band fracture. The failure modes are different for each material; with the AA5754 parts failing by necking and fracture, with significant thinning at the fracture tip. The AA6111 exhibited a saw tooth pattern fractures, a crosshatch pattern of shear bands in the lower half of the part, and tears in the area close to the tip. Both areas showed evidence of shear fracture. This experimental study indicates that there is increased formability for AA5754 and AA6111 when these alloys are formed using EMF. A major factor in this increase in formability is the reduction in damage caused by the tool/sheet interaction.

Trimming of Advanced High Strength Steels

Modern product design and manufacturing often utilizes a wide variety of materials. Where once low carbon steel predominated, a variety of different materials such as aluminum alloys and advanced high-strength steels (AHSS) are now being utilized. Although such alternative materials may provide a variety of benefits in manufacturing and design, these same materials may present difficulties when subjected to manufacturing processes originally designed for low carbon steel. One such manufacturing area where difficulties may arise is in trimming operations. A defect that may arise directly in the trimming operation are burrs. Burrs decrease the quality and accuracy of stamped parts and cause splits in stretch flanging and hemming. Current standards limit the production of burrs through accurate alignment of the upper and lower edges of the trim knives. The clearance between the shearing edges should be less than 10% of the material thickness. For automotive exterior sheet, this requires a gap less than 0.06mm. Unfortunately, tolerances often exceed the capabilities of many trim dies resulting in the production of burrs. To satisfy the current standards of quality and to meet customer satisfaction, stamped parts frequently need an additional deburring operation, which is often accomplished as a metal-finish operation and conducted manually. The objective of the research described in this paper was to study the mechanisms of burr generation and the impact on AHSS formability in stretch flanging. Results on both the conventional trimming process and a recently developed robust trimming process, which has the potential to expand tolerances of trim die alignment, will be discussed.Copyright

Sharp flanging and flat hemming of aluminum exterior body panels

Limitations of conventional flanging and hemming technologies require increased radii of flanges and roped hems when aluminum alloys are used for production of closure panels. A new process of flanging has been developed based upon the idea of redistributing plastic strains through the larger area, delivering additional metal into the bending zone, and creating an additional axial compression. Comparison of the newly developed and conventional flanging process indicated that the new process expands the bending ability of aluminum alloy 6111-T4 by allowing an additional 10% of prestrain to the panel compared with previous forming operations. The advantages of the new flanging operation can be transferred to the hemming operation by allowing an additional 10% of prestrain through the whole sequence of forming and assembly operations. Employment of suggested flanging technology makes possible the flat hemming operation of panels stamped from aluminum alloy 6111-T4 if the thickness of the interior panel is 1 mm or more.

Quantitative Microstructural Analysis of Formability Enhancement in Dual Phase Steels Subject to Electrohydraulic Forming

Under certain conditions, strain rate sensitive materials such as dual phase steels, show formability improvement under high strain rate forming which is known as hyperplasticity. In this research, two commercial dual phase steel sheets, DP500 and DP780, were formed under quasi-static conditions using the Nakazima test and under high strain rate conditions by electrohydraulic forming (EHF) into a conical die. Macro-strains, measured from electro-etched circle grids with an FMTI analyzer, showed remarkable formability improvement in EHF specimens. Micro-strains, i.e., the strains in the ferrite and the martensite, were calculated by quantitative metallography of more than 7000 ferrite grains and 10,500 martensite islands. The goal was to investigate the deformation improvement of the constituents under EHF. Around 20 and 100% deformation improvements were observed in ferrite and martensite, respectively. Furthermore, as a micro-mechanical modeling technique, correlation of the micro-strains with the macro-strains was investigated by applying the mixture rule. Results showed a reasonable correlation between the macro and micro-scale strains; however in banded microstructures, the strain in the martensite should be determined precisely for more accuracy.

An Improved Numerical Integration Method for Springback Predictions

In this investigation, the focus is on the springback of steel sheets in V‐die air bending. A full replication to a numerical integration algorithm presented rigorously in [1] to predict the springback in air bending was performed and confirmed successfully. Algorithm alteration and extensions were proposed here. The altered approach used in solving the moment equation numerically resulted in springback values much closer to the trend presented by the experimental data, Although investigation here extended to use a more realistic work‐hardening model, the differences in the springback values obtained by both hardening models were almost negligible. The algorithm was extended to be applied on thin sheets down to 0.8 mm. Results show that this extension is possible as verified by FEA and other published experiments on TRIP steel sheets.

Electromagnetic Forming and Joining for Automotive Applications

In this paper some options of how electromagnetic forming (EMF) can assist to expand the capabilities of conventional forming and joining technologies are discussed. Three different areas where EMF has the potential for a significant expansion of capabilities of traditional technologies are reviewed: 1) restrike operation to fill sharp corners of automotive panels; 2) low energy method of springback calibration; 3) joining of closed frames with an openable coil. Each of these applications was demonstrated in laboratory conditions and the description of the tooling is provided in the paper. Suggested design of a flat concentrator collecting induced electric currents from a flat coil was demonstrated for a corner filling operation and a springback calibration. An efficient technique of fabricating the flat coil from a flat plate by using water jetting technology enables a cost effective coil design, which can be reinforced by a system of non-conductive bars. The insulation of the coil is produced from the flat sheet of insulation material. Suggested design allows the coil to be repaired if a shortcut or fracture of insulation strips happens. A technology of low-energy calibration of stamped parts provides an option of working with a wider variety of materials including aluminum alloys, mild steels, and advanced high-strength steels. This technology is demonstrated for calibration of U-channels.

Improvement of formability of 6xxx aluminum alloys using incremental forming technology

Aluminum sheet is becoming increasingly common as an automotive body panel material. The heat-treatable aluminum alloys of the 6xxx series are widely used as an outer panel material, due to their ability to precipitation harden during the paint-bake cycle, resulting in improved dent resistance. Increasing the formability of these alloys would allow for multiple parts of less complex geometry to be combined into a single more complex part, thereby avoiding the costs associated with any subsequent joining operations. Incremental forming is a process that can improve material formability through the use of short, recovery heat treatments applied between increments of deformation. The objective of this study was to investigate the incremental forming behavior of 6111-T4 an alloy, which is often used for exterior body panel applications. Interrupted tensile testing was used to simulate the incremental forming process. The effect of different heat-treatment parameters on mechanical properties was analyzed. The heat treat regimen developed for uniaxial testing was then applied to a series of plane strain tests using a hemispherical punch, to simulate the more complex states of stress found in forming operations.