High-velocity sheet metal forming offers many opportunities. It can solve many problems that are difficult to overcome with traditional metal-forming techniques. Furthermore, practical high-velocity forming methods are well developed, although not commonly employed.
By A.A. Tamhane, M. Padmanabhan, G. Fenton, M. Altynova
and G.S. Daehn
Department of Materials Science and Engineering ,
The Ohio State University, Columbus OH
V.J. Vohnout, Department of Industrial and Systems
Engineering,
The Ohio State University, Columbus OH
V.S. Balanethiram, Delphi Chassis Systems, Livonia, MI
Often, the question is asked, "If high-velocity forming techniques are so useful and powerful, why aren’t they used more?" We offer the following analogy to address this question.
Suppose that a classically educated, but sheltered, engineer is asked to devise a procedure to drive nails into wood. If he or she is unaware of the concept of the hammer the engineer is likely to develop something that looks like a modern press. The device might be built so that it precisely aligns a nail normal to the piece of wood and has an actuator that moves at a controlled displacement rate (possibly with high force) and drives the nail slowly into the board.
Other engineers might applaud this approach as it offers much control and precision. By way of added improvements, the engineering community would work on issues such as the stability and buckling of the nail as well as the challenge of making a truly portable nail driver. Over time, others would improve on this approach. Standards would be developed and the viability of many companies might become dependent on its continuation.
Now imagine another engineer suggesting that this common but somewhat elegant process could be replaced by simply banging on the head of the nail to drive it into the wood. While this has many advantages in terms of simplicity, cost, stability of the nail and portability, it might encounter some resistance as it appears some control over the process is lost and there might be a substantial learning curve in developing good hammers and the skills needed to wield them properly. Fortunately the hammer was developed long before conventional engineering practices.
In some sense this analogy parallels the state of sheet metal forming technology today. Forming typically is accomplished with the motion of massive matched tools with precise control of static forces and slow displacement rates. In effect, we now are suggesting that much might be accomplished by hurling chunks of metal into dies.
While at first glance (and with the current state of the art) it appears much control is lost with this kind of procedure, in fact, many current problems might be solved and many operations made simpler through its use. However, a fresh look at sheet metal forming is required to appreciate such techniques.
Fig.1 -- Schematic illustration of the electro-hydraulic forming process
is shown in Fig.1
This article will focus on four primary attributes that make high-velocity forming a viable production alternative.
High-velocity metalforming was studied fairly extensively between about 1955 and 1970 [1-3]. Several methods were considered including explosive forming with both high explosives and mixtures of combustible gases. Also, methods based on the discharge of capacitor banks were explored. The latter have intrinsic advantages in that there is no need to store explosives and the discharge can be highly reproducible. We believe that such methods have the best potential for commercial use in mass-production. Two capacitor-discharge-based forming methods are electrohydraulic and electromagnetic forming.
In electrohydraulic forming, an electric arc discharge is used to convert electrical energy to mechanical energy. A capacitor bank delivers a pulse of high current across two electrodes, which are positioned a short distance apart while submerged in a fluid (water or oil). The electric arc discharge rapidly vaporizes the surrounding fluid creating a shock wave. The workpiece, which is kept in contact with the fluid, is deformed into an evacuated die. A schematic illustration of the electrohydraulic forming process is shown in Fig. 1.
The potential forming capabilities of submerged arc discharge processes were recognized as early as the mid 1940s. During the 1950s and early 1960s, the basic process was developed into production systems. This work principally was by and for the aerospace industries. By 1970, forming machines based on submerged arc discharge, were available from machine tool builders. A few of the larger aerospace fabricators built machines of their own design to meet specific part fabrication requirements.
Electrohydraulic forming is a variation of the older, more general, explosive forming method. The only fundamental difference between these two techniques is the energy source, and subsequently, the practical size of the forming event.
Very large capacitor banks are needed to produce the same amount of energy as a modest mass of high explosives. This makes electrohydraulic forming very capital intensive for large parts. On the other hand, the electrohydraulic method was seen as better suited to automation because of the fine control of multiple, sequential energy discharges and the relative compactness of the electrode-media containment system.
Electromagnetic forming is the only high velocity forming technique to gain significant acceptance in commercial metal working. The electromagnetic forming technique has been in use commercially for the last 30 years. Mostly, it has been used for joining and assembly of concentric parts. The minimal springback inherent in all high velocity forming processes provides high-quality joints.
Fig.2 -- Schematic illustration of electromagnetic forming showing a)
solenoidal and b) flat forming coils.
One of the most common applications of electromagnetic forming is the compression crimp sealing and assembly of axi-symmetric components such as automotive oil filter canisters. As the name implies, in this technique, electromagnetic forces are used to form the material. A current pulse from a capacitor bank is passed through a coil that is placed in close proximity to a workpiece. The current pulse causes a high-magnetic field around the coil. This field induces an eddy current in the workpiece and an associated secondary magnetic field. The two fields are repulsive and the force of magnetic repulsion causes deformation of the workpiece.
The designed shape and electrical characteristics of the coil depend on the workpiece. Coils can be developed for most practical forming geometries including forming of flat sheets. Fig. 2 is a schematic illustration of electromagnetic forming showing a) solenoidal and b) flat forming coils.
The nature of the electromagnetic forming process makes it highly suitable for automation. Results obtained are very repeatable because energy discharge characteristics are controlled essentially by the non-changing electrical parameters of the system and precise control of capacitor bank charge voltage. The fundamental physical characteristic of this technique is that the deformation forces initially are only magnetic body forces generated within the material by eddy currents induced by the drive coils. Surface pressures only occur upon contact with the form tool. This can provide deformation capabilities that are difficult to obtain with other forming methods.
Currently, formability limitations preclude the application of hard-to-form materials for many components. The use of aluminum in automobile bodies is an example. While replacement of steel with aluminum can cut body-in-white weight by more than 30 percent [4], difficulties in forming aluminum require that only the weakest, most formable alloys be considered. In addition, the cost of fabricating these aluminum components is much greater than when using steel. But, improvements in formability seen in high-velocity forming might permit the application of high-strength aluminum in vehicles with reduced manufacturing costs.
While we believe that improvements in formability are probably the most compelling reasons to consider high-velocity forming, the purpose of this article is to provide a review of the field, so only a brief overview of this issue is provided. Several papers present more comprehensive coverage of this issue [5-11]. A complete understanding of how formability is affected by high-deformation velocity is lacking. However, some things are clear. The main fundamental factor controlling tearing of the sheet, which is different in high-velocity versus quasi-static forming, is that inertial effects can be large. That is, acceleration of material in the vicinity of a potential tear trying to initiate or where the material impacts the die can be significant. This offers resistance to propagation of the tear. These effects almost always seem to make forming easier.
Comparisons of numerical models and experimental observations on the effect of velocity expose the current limits on our understanding of ductility. Ductility has been studied with simple axi-symmetric expansion of Al 6061 T4 (aluminum) rings. Experiments with electromagnetic expansion of rings are preferred because they simplify the analysis of material response when subjected to a high strain rate [12]. Stress on the rings nominally is uni-axial. For thin rings, strains to failure are generally about twice that seen in quasi-static tensile tests. We also can see that total elongation to failure increases with increasing discharge energy; i.e., with increasing metalforming velocity. Ring expansion at velocities ranging from 90 to 170 meters/sec. have been studied.
The height of the specimen rings also is found to have an effect on ductility. Fig. 3a shows the maximum deformation that can be obtained without failure in free expansion of rings of varying heights. The maximum deformation is seen to increase with increasing ring height, an effect that has not been discussed adequately in literature to date. With the use of suitable dies, much larger deformations can be obtained, without failure, by constraining the ring prior to failure.
Fig.3 -- Ring experiments with a solenoidal electromagnetic
coil. a) Maximum deformation without failure in free expansion: a. Undeformed 1 mm ring,
deformed, b. 1 mm - 35 percent circumferential elongation, c. 2 mm - 33 percent, d. 4 mm -
41 percent, e. 8 mm - 67 percent and f. 16 mm - 70 percent rings. All rings had diameters
equal to ring "a" prior to deformation. b) Increase in deformation without
failure by using a die. By increasing deformation velocity and using dies to stop rings
before failure, large deformations can be obtained.
An example is shown in Fig. 3b. A 1 mm ring sample, which exhibits a maximum of 34 percent elongation without failure in free expansion, is expanded using a die that corresponds to a 50 percent elongation. The same energy discharge level results in more than 60 percent uniform elongation (and failure) in free expansion. The ring exhibits 50 percent elongation with no failure when contained by a die. Tensile testing of 6061 T4 aluminum samples yielded a value of 26 percent elongation (averaged over three tests). Another interesting effect of increasing deformation velocity is that the value of the anisotropy ratio (r), (the ratio of width strain to thickness strain) varies. Fig. 4 shows variations in r as a function of total true circumferential strain, which depends on metalforming velocity. Here, r is seen to increase monotonically with total circumferential strain and with decreasing wall thickness.
Fig.4 -- Variation in observed anisotropy ratio (r) with strain
for 1 mm rings at various strains and varying wall thickness. The inner diameter of all
rings was identical.
Usually r value is a material parameter that is fairly constant in uni-axial tension. The variation in r value shown in Fig. 4 is rather unusual, and not completely understood. However, it still can be useful. The r value is a measure of resistance to thinning of a sheet. Large r values therefore are desirable in sheet forming. It appears that r values can be varied by manipulation of the forming procedure.
An increase in ductility can be explained, at least partially, on the basis of inertial stabilization of neck growth during high-rate forming [5,7]. Note that since there is a significant increase in ductility with velocity for many materials, forming limits of those materials become a function of the metalforming velocity. Therefore, efforts to predict forming capabilities of methods operating in the range of velocities studied here must include this effect.
The nominal bi-axial formability of aluminum, copper and interstitial-free iron sheets also have been studied in our group by Balanethiram et. al. [5,6], using the electrohydraulic technique (high strain rate) and fluid pressure forming (low strain rate). A comparison of dome height specimen sheets formed into a 90 deg. conical die using high- and low-strain rates is shown in Fig. 5. Samples formed at the high strain rates showed significantly larger dome heights and thickness strain.
Fig.5 -- Samples formed at the high strain rates showed
significantly larger dome heights and thickness strain.
The surface of sample sheets also was circle gridded and the strain states after deformation were plotted on a forming limit diagram. The high strain rate data showed forming limits that far exceed those predicted by the conventional forming limit diagram. A possible high-rate forming limit curve also was drawn on the plot. We consistently saw that forming in dies resulted in higher forming limits than ring expansion. Beneficial effects of die wall impact (giving an ironing-like effect) may be responsible.
Fig.6 -- Schematic of experiemental assembly for sheet wrinkling
experiments.
The practical "forming window" in a metalforming operation is usually limited by necking or tearing problems on one hand and wrinkling on the other. In conventional metalforming processes, this "forming window" for aluminum is very narrow as tearing is severe and wrinkling and springback are pronounced. High-rate forming techniques make aluminum more formable by widening the forming window on both sides.
Fig.7 -- Samples exhibiting reduction in wrinkling with
increasing capacitor discharge energy (increasing metal velocity).
A schematic of the experimental setup used to test the effects of sheet velocity on wrinkling is shown in Fig. 6. A vacuum chamber houses a flat electromagnetic coil. A sheet metal blank is placed over the coil. The die into which the metal is to be formed is positioned directly above the sheet metal. Discharge of the capacitor bank propels the aluminum sheet at the die.
For the wrinkling studies, a male die in the shape of a cone section of semi-apex angle 45 deg. was used. Sheets of 1100-O Al were formed over the die. At low capacitor discharge energies (approximately 1.69 kJ), significant wrinkling occurred. When the energy level was increased to 2.25 kJ, the extent of wrinkling declined and was completely absent at 3.38 kJ and 4.69 kJ.
Fig.8 -- CALE simulation of a sheet forming problem with a
clamped sheet and a flat forming coil.
A photograph of samples formed at different energy levels is shown in Fig. 7. Similar trends in wrinkling were observed in 6061 T-6 Al. Note that no blank holders were used in this experiment. Tooling was kept very simple.
Inertial effects are thought to be responsible for reduced wrinkling. A brief, one-dimensional example can demonstrate how this might occur. Consider a slender bar (like an arrow), thrown at a wall. When pressed against the wall statically or when projected at low speeds, the bar buckles. However, when projected at high speeds, buckling does not occur.
Fig.9 -- Profile of a clamped sheet deformed using a flat
forming coil, simulated by CALE.
This fact is important to military long rod penetrators. For buckling to occur, particles should move in the lateral direction. At high velocities, when inertial forces dominate, lateral movement is not possible. A similar two-dimensional effect can be visualized for the sheet samples. For wrinkling to take place, particles should be able to change their trajectories. But, when inertial effects are large, this is not possible and wrinkling is suppressed.
Researchers in this field 30 years ago were unable to simulate these complex, highly dynamic processes. Thus the great majority of their work was purely empirical in nature. Electromagnetic and electrohydraulic forming now are amenable to high-resolution modeling using generalized hydrocodes running on work stations.
For example, in electromagnetic forming, stresses and velocity-versus-time profiles for expanding ring experiments using solenoid coils have been predicted accurately [9,13]. Computer codes that can model more complex two-dimensional problems also are available. CALE, a "C" language-based code, originally developed as an astrophysics code at Lawrence Livermore National Laboratory, now is being used to model these forming processes and the subsequent material response [8,13].
Fig. 8 shows an example of a CALE simulation of a sheet forming problem. A flat spiral coil is used to form a clamped metal sheet. Numbered lines indicate lines of magnetic flux around the current-carrying elements in the simulation.
Fig. 9 shows a profile of the sheet through the deformation process. Deformation begins at the edges of the sheet and progresses towards the center. A high-speed camera was used to capture the action. The predicted time-profile of the deformation agrees with the profile obtained in an experiment by Takatsu et al under similar conditions [14]. CALE accurately simulates the trajectory and profile of the deforming sheet metal workpiece. Such numerical tools are essential if this technique is to be developed to its potential.
The Boeing company swages end fittings on torque tubes for its new model 777 aircraft using the electromagnetic forming technique. Tests show that these components are far superior to ones made earlier using riveted fittings.
Recently, The AWS Group of Wheeling, WV, which makes components for speakers, designed Ti (titanium) diaphragms for its speakers based on a new concept. However, forming the 0.002 in.-thick diaphragms proved to be impossible using either matched tool forming or spinning.
To overcome forming limitations, AWS, in collaboration with The Ohio State University, developed an electromagnetic forming process using a flat spiral electromagnetic coil. The workpiece, a 0.002 in. thick Ti sheet, is laid on top of an Al driver sheet, which is kept in contact with the spiral coil. A current pulse is passed through the coil, causing the driver and the workpiece to be launched at an evacuated female die. A schematic view of the forming process and a photograph of the manufactured part are shown in Fig. 10. Diaphragms now are being made at AWS using the electromagnetic forming technique.
A potentially major application of high-velocity forming technology exists in the fabrication of aluminum alloy automotive body panels. Because of poor formability at low (quasi-static) forming velocities, aluminum workpieces tend to fail at sharp corners and bends when processed with conventional matched tooling. High-rate forming and hybrid-forming (combined high rate and low rate) concepts, are being examined to solve this problem.
One possible solution is to use electromagnetic coils and one-sided (either male of female) dies and form the complete part using only these coils. This is the technique used to form the AWS speaker previously discussed. Automotive body panels, however, are much larger in size than existing parts successfully made by this method.
Although there are no fundamental limitations to the size of parts that can be made by electromagnetic forming, larger parts require more energy, which translates into larger capacitor banks and higher initial capital expenditure. As a result, hybrid forming processes are being considered, where electromagnetic methods would be used only to form areas of the workpiece that can not be formed conventionally.
In principle, both electromagnetic and electrohydraulic forming can be used in such a hybrid process. A matched tool set with electromagnetic coils built into sharp corners and other difficult-to-form contours is one way of forming these parts. Matched tools would be used to form sections of the workpiece that can be formed easily at low velocities using mechanical energy from the press.
This semi-formed workpiece then would be subjected to high-velocity forming with electromagnetic coils used to complete the forming operation. Similarly, a quasi-static, fluid-pressure process, with an electrical discharge in the fluid at the end of the pressure cycle to form sharp corners and bends, could represent another hybrid method for making difficult parts.
Fig.10 -- Speaker component manufactured at AWS: a) schematic of
the manufacturing setup and b) photograph of the component.
High-rate forming technology has, for years, demonstrated significant capability in forming difficult materials for the aerospace industry. The technology now is being studied for application to automotive aluminum sheet forming. This involves development of electromagnetic or electrohydraulic processes for forming the large sheets required for auto body panels, and decreasing cycle times for these and the new hybrid processes. For an electromagnetic hybrid, there is a significant engineering opportunity for the design and development of compact, long-lived, forming coils capable of being integrated into the tool assembly. However, for many simpler parts, electromagnetic forming can be used right away.
A basic understanding of material response to high-velocity forming, beyond empirical relationships, will be required for efficient, effective design of a hybrid process. Researchers in Materials Science at The Ohio State University and other institutions have been pursuing an understanding of the fundamental phenomena occurring in materials under very high rates of strain. The promise of hyperplastic forming technology has attracted widespread interest from several industries.
A "Hyperplastic Forming Consortium" has been formed at The Ohio State University to pool resources and further develop this technology. Members in this consortium range from primary metals producers to automobile and aerospace companies. Under the auspices of this consortium, a research program is being conducted to understand the fundamentals of material behavior during high-rate forming while concurrently developing practical forming technology. Even though more aspects of material behavior are beginning to be understood and workable processes are being developed, further research is essential.
The authors would like to acknowledge the financial support for this research provided by the National Science Foundation through a National Young Investigator award to Glenn Daehn (DMR 9258172) and by the Hyperplastic Forming Consortium (HFC).
More information about participation in the HFC can be obtained by contacting G.S. Daehn, 477 Watts Hall, 2041 College Rd., Columbus, OH 43210; Ph: 614/292-6779.
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