Electromagnetic forming is a method of forming sheet metal or thin walled tubes that is based on placing a work-coil in close proximity to the metal to be formed and running a brief, high intensity current pulse through the coil. If the metal to be formed is sufficiently conductive the change in magnetic field produced by the coil will develop eddy currents in the work piece. These currents also have associated with them a magnetic field that is repulsive to that of the coil. This natural electromagnetic repulsion is capable of producing very large pressures that can accelerate the work piece at high velocities (typically 1-200 meters/second). This acceleration is produced without making physical contact to the work piece. The electrical current pulse is usually generated by the discharge of a capacitor bank. This field has been developed by many individuals and companies and is widely used for the forming and assembly of tubular and sheet work pieces. Several excellent reviews of the field are available, including Moon, F. C., Magneto-Solid Mechanics, ASTME, High Velocity Forming of Metals, revised edition (1968); Plum, M. M., Electromagnetic Forming, Metals Handbook, Maxwell Laboratories, Inc., pp. 644-653; and Belyy, I. V., Fertik, S. M. and Khimenko, L. T., Electromagnetic Metal Forming Handbook, Khar'kov State University, Khar'kov, USSR (1977) (Translation from Russian by M. M. Altoynova 1996), all of which are hereby incorporated herein by reference. Examples of prior art patents involving electromagnetic forming include U.S. Pat. No. 4,947,667 to Gunkel et al., U.S. Pat. No. 4,531,393 to Weir et al., U.S. Pat. No. 5,353,617 to Cherian et al., U.S. Pat. No. 3,998,081 to Hansen et al., U.S. Pat. No. 5,331,832 to Cherian et al., U.S. Pat. No. 5,457,977 to Wilson, U.S. Pat. No. 4,619,127 to Sano et al., U.S. Pat. No. 4,473,862 to Hill, U.S. Pat. No. 4,151,640 to McDermott et al. and U.S. Pat. No. 5,016,457 to Richardson et al., all of which are hereby incorporated herein by reference.
Electromagnetic forming can be carried out on a wide range of materials and geometries within some fundamental constraints. First, the material must be sufficiently electrically conductive to exclude the electromagnetic field of the work-coil. The physics of this interaction have been well characterized.
It is an object of the present invention to provide apparatus and methods that take advantage of such actuators and to use them in conjunction with, mold and tool bodies.
Although not limited in their application to the automobile industry, many of the problems solved and advantages achieved with the apparatus and methods of the present invention can be appreciated by reference to the problems faced in the forming of sheet metals in that industry.
The automotive industry is currently interested in producing automobile body parts from aluminum alloys. The weight saving of up to 50% of the body-in white and its attendant gains in fuel efficiency are largely responsible for this interest. Additionally, the superior recycle characteristic of aluminum is recognized as becoming of increasing importance as the total life cycle cost of automobiles becomes an issue . [Du Bois 1996, Henry 1995]
The press forming of aluminum alloys have problems in comparison to steel principally due to very low strain rate hardening, low r (strain ratio) value and high galling tendency. In particular the lack of strain rate hardening behavior in aluminum alloys at room temperature is troublesome since this is the characteristic that allows post uniform plastic strain in a sheet metal. All good draw quality sheet steels have enhanced strain rate sensitivity which is identifiable by a long arching stress-strain curve. The press forming handicap of aluminum alloys, measured by the lack of strain rate sensitivity, is shown by the direct comparison of the stress-strain curves for typical auto body steel and aluminum sheet FIG. 10 which was adapted from an Aluminum Association report [Al Assoc.,1996]
Despite the press working "fussiness" of aluminum, car builders are currently using aluminum for selected body panels such as hoods outer door skins and trunk lids. These are parts that are geometrically simple and can be stretch-draw formed with conventional matched tools. However, the propensity of aluminum alloys to neck and tear at relatively low strain levels, makes many of the more geometrically complex body parts extremely difficult or impossible to produce in aluminum with conventional matched tools. A side-by-side comparison of two automobile door-inner panels from the same stamping die was conducted to manifest the material characteristics shown in FIG. 10. A fully formed panel of specified production steel sheet that was produced after set-up trials indicated satisfactory tool performance. A second panel of 6111-T4 aluminum of the same gauge as the steel was processed directly after the steel panel. The aluminum panel showed wrinkling and large splits that occurred within the first 25% of the tool stroke, which was not unexpected.
Fluid pressure forming methods such as Verson-Wheelon, ABB or Hydroform can extend the formable geometry for aluminum sheet somewhat but at the cost of long cycle time leading to unacceptably low production rates. Fluid pressure methods have high capital equipment costs compared to conventional press machines due principally to the high static operating pressures.
Several aluminum alloy exhibit superplastic creep behavior which can be utilized to produce very complex sheet part geometries. Current superplastic forming methods also suffer from inherently long cycle times in addition to requiring high temperatures and specialized alloys. Control of superplastic forming is inherently more complex in that it requires the explicit control of worksheet temperature and forming gas pressure during the forming cycle. The capital costs equipment costs are also significantly greater than the conventional [Laycock,1982].
A compromise solution might be to change the part designs to shapes which can be produced in aluminum using current production methods. Another solution would be a new sheet forming method which could overcome the formability shortcomings of aluminum alloys while maintaining acceptable production rates (150-300 parts/hr. for large body panels). Such a processes would be less restrictive for the automobile designers and thus more appealing to the industry. In addition, this improved forming performance must be attainable with capital equipment and tooling expenditures which will maintain competitive production part costs. To this end, it would be an added advantage if this new method could actually provide a reduction in tooling costs compared to current practice. Such a cost reduction may be attainable if, for instance, the new method required only a single part-surface tool instead of a precisely matched pair. Single-sided form tools, currently used in the fluid forming processes need fewer trials and subsequent geometry alterations before producing good parts. Another highly beneficial attribute of the new process would be implementation using the installed press machines that are currently used by the industry for conventional sheet metal stamping.
Hypothetically, a method that would completely fulfill the performance criteria listed above might be designed using a "clean sheet" approach. However it is quite likely that many of the attributes of current processes would be re-invented. Most complex technologies emerge in a evolutionary manner, incrementally with occasional forward leaps. Therefore, an examination of existing methods for evidence of partial solutions to the total problem is appropriate.
It is therefore an object of the present invention to produce hybrid apparatus and methods that go further toward meeting the ideal performance goals than the prior art devices and methods.
The existing processes of interest as components of a combined hybrid method are; conventional matched tools, fluid pressure processes and the high velocity, impulse power processes. The common characteristic that these methods share is a general insensitivity to alloy type or inherent restriction of forming rate. Superplastic forming has been omitted under this same rational, although near term developments in superplastic forming may indeed increase its viability as a production method for aluminum auto body panels. Each of the included methods have a significant track record in some production niche and have attributes which are partial solutions to the overall problem of production stamping of aluminum alloy sheet. In the interest of clarity, the characteristics of these methods are briefly described below. If more detailed information on these constituent methods is desired, the reader is referred to any good text or handbook of industrial metal forming practice [e.g. Lange, 1985, Lascoe, 1988].
Matched Tools
The use of matched tools is the most common method of producing sheet metal parts in the auto industry. If aluminum parts for the body-in-white could be produced in matched tooling, with the same level of development effort as steel parts, the auto industry would look no further. Any other potential benefits of a new method would, unfortunately, be ignored in favor of the more familiar method.
In matched tool forming a flat sheet blank is pressed into the desired shape between a male and female set of form tools. The female tool, usually referred to as the die, carries, in essence, the outside shape of the part. Similarly, the male tool, referred to as the punch, carries the inside shape of the part. In addition to the punch and die, virtually all matched tool sets have a third component called the blank holder which holds the blank in position against the die face and assist forming by controlling sheet draw-in.
The matched tool forming method is essentially a position control process. When the tool halves are closed on the sheet blank to a predetermined shut height, the part is fully formed. Since forces need not be directly controlled, the press machines and controls required for this process can be very simple in their fundamental design. The most commonly used press machines are mechanical, based on some variation of the simple slider-crank mechanism. Hydraulic presses, which can provide independent control of speed and position of the tool halves during the forming stroke which can benefit forming. However, the tool set must still be brought to the same closed position for the part to be fully formed.
Sheet forming with matched tooling is the process that the industry has a great deal of accumulated knowledge about. Essentially, the entire installed press machine population of the industry is optimally designed for the matched tool method.
The cost of producing matched tools is highest of the tool costs of the conventional processes of interest here. Tooling for other sheet forming methods such as fluid pressure forming, can be significantly less expensive and produced in less time since only one form surface is required. However fluid pressure methods has not displaced conventional matched tool forming to any significant extent. The reason is simply that tooling cost are not the principle driving force in auto body part production.
Fluid Pressure Forming
The fluid pressure processes used past and present have demonstrated certain of the desired traits of the process of the present invention. Principle among these traits is an extended forming capability as measured by Limit Draw Ratio (LDR). Further, the extended LDR is applicable to many of the hard-to-form alloys. [Yossifon and Tirosh, 1990, Nakamura and Nakagawa, 1987]
Fluid pressure sheet forming is a force control process as opposed to position control required for matched tool method. In fluid pressure forming, the blank sheet is forced over a male punch tool or into a female die by the pressure action of a fluid (usually oil or water). Since the pressurized fluid replaces the action of one of the tool halves of the matched tool method, fluid pressure forming has also been called "universal die" forming. Fluid pressure forming has been most successfully applied to smaller parts using large, expensive, slow, specialized press machines. Fluid pressure sheet forming machines are structurally heavier than matched tool (conventional) press machines for a given size of part. The larger machine structure is a direct consequence of the very high static pressure required to forming small inside (free) corner radii. The high pressure is applied over the entire plan area of the part, generating very large structural loads in the machine frame. These high loads are quite disproportional to the level of plastic work done to the part. In order to reduce the high peak pressures, it is common to employ auxiliary forming tool sections. The auxiliary tool sections are placed in partially formed part to act as pressure concentrators at the sharper part features. Since the machine must go through another cycle, this use of auxiliary tool sections approaches the cost of a full secondary operation.
High Velocity Forming
High velocity sheet forming, also referred to as "high energy rate" forming is not well known outside of the aerospace industry. However, this forming technology has been in commercial use, in some form, for close to a century [Ezra, 1973]. The first applications were the forming of large domes from plate using chemical explosives. Later, electromagnetic pulses and submerged electric arc (electro-discharge, electro-hydraulic) discharges were employed to generate very high power events which resulted in producing the very high deformation rates characteristic of these processes. The deformation velocities generated in the electromagnetic and electrohydraulic processes are lower than the velocities achievable with explosives but are still 100 to 1000 times greater than the deformation rates of the quasi static processes like matched tool or fluid pressure forming (.about.0.1 vs. 100 m/s). Such high deformation rates are known to significantly extend the deformation capacity of many metals [Wood 1963, Orava 1967]. FIG. 11 summarizes the results of some early experiments in high velocity forming of sheet metals. Note that FIG. 11 reports average strain rather than maximum strain at failure which has become the more accepted figure of merit since the introduction of Forming Limit Diagrams (FLD). FIG. 12 shows the results of more recent experiments in high velocity forming of aluminum alloys presented in FLD data format. It should be noted that the data of FIG. 11 is for unconstrained "free" dome tests while certain high velocity data in FIG. 12 could be confounded by an ironing effect due to impact with a covering conical die cap. The ironing effect compliments the primary hyper-plastic effect of inertial stabilization of necking.
Hyper-plasticity under free flow conditions has been chiefly attributed to suppression of local necking due to material inertia rather that changes in the constitutive behavior of the material. Although, much higher than conventional sheet forming rates, the velocities of these "high rate" processes generate strain rates that are generally lower than rates associated with changes in constitutive behavior (10.sup.2 -10.sup.3 Vs 10.sup.4 sec.sup.-1) [Follansbee and Kocks 1988.] Results of analytic and numerical simulations indicates that the inertia of material mass itself resists the high velocity changes inherent in the formation of local necking regions at high deformation rates [Fyfe and Rajendran 1980, Banerjee 1984, Fressengeas and Molinari 1985, Han and Tvergaard 1994, Hu and Daehn 1995]. Many of the commercial metals including aluminum alloys have demonstrated increases in ductility of 100% or more in comparison to the elongation obtained at low, quasi-static rates [Wood 1963, Balanethiram and Daehn 1992]. The extended ductility is available over a broad range of work piece velocities which are specifically material dependent but generally lie between 50 and 300 m/sec. The upper deformation velocity limit for a material is dependent on specimen geometry, and boundary conditions which determine whether or not plastic deformation front "wave" propagation effects can become significant [von Karman and Duwez, 1950]. Except for cases of essentially simultaneous, uniform deformation such as in the electromagnetic expansion of thin rings, "wave" fronts will be present.
The high velocity processes were extensively investigated during the twenty year period from approximately 1955 to 1975. By 1962, a bibliography containing hundreds of abstracts was published by the USAF [Strohecher, 1962]. In 1968, a textbook summarizing all the then current methods was publish by the American Society of Tool and Manufacturing Engineers [Bruno, 1968]. Texts covering specific methods were published by other authors [Rienhart, 1963, Ezra, 1973]. Interest in high velocity metal forming was principally centered in the aerospace industry and directed by military and space craft applications. Explosive forming of large radar domes and missile nose caps proved to be superior in part quality and cost when compared to welded fabrications [Arcojet General 1961]. This success led to application to smaller parts and eventually to the development of several machine based systems. These systems attempted to capitalize on the hyperplasticity and complex shape forming characteristics of the various processes for higher volume applications. Machine systems based on chemical explosives, electro-hydraulic and electromagnetic pulse were developed. The most widely used during the late sixties and early seventies was the electro-hydraulic method. However to date, only the electromagnetic pulse method has gained significant acceptance outside the aerospace industry.
Since the electromagnetic pulse and to a lesser extent, electro-hydraulic methods have the greatest potential of meeting the requirements, such as cycle time, of automotive type of manufacturing, only these two high velocity forming methods will be discussed further.
Electromagnetic
Electromagnetic sheet forming, also known as magnetic pulse forming, is based on the repulsive force generated by the opposing magnetic fields in adjacent conductors. The primary field is developed by the rapid discharge of a capacitor bank through the "driver coil" conductor and the opposing field results from the eddy current induced in the "work piece" conductor. Therefore, a fundamental requirement for this type of electric pulse energy is that the work piece must be an electrical conductor. The efficiency of electromagnetic forming is directly related to the resistance of the work piece material. Materials which are poor conductors can only be effectively formed with electromagnetic energy if a auxiliary driver plate of high conductivity is used to push the work piece.
Electromagnetic forming of axisymmetric parts, using either compression or expansion solenoid type forming coil is, to date, the most widely used of the electric pulse energy methods. The common application is for the swaging of tubular components onto coaxial mating parts for assembly. Not as common is the forming of shallow shells from flat sheets using flat spiral coils. FIG. 13 shows schematics of the general classes of electromagnetic forming coils and work pieces. Note that axisymmetric or tube compression forming onto a male form tool is also possible.
Electromagnetic pulse forming is currently used in the automotive industry most commonly for crimping and swaging operations on tubular type parts. One high production example of the industrial application of electromagnetic pulse forming is the pressure tight crimping of canister type oil filter assemblies.
Electromagnetic forming can be performed under low efficiency conditions without coils. In this case the work piece itself forms part of the direct current path closing the circuit on the charge source. For this reason it could also be called "direct" electromagnetic forming. If the part pre-form is such that the current flow is parallel to itself, the driving form pressure can be contained completely within the part. If the initial part geometry does not permit a parallel current flow, then an insulated "reaction" blocks of highly conductive material must be placed close to the part area to be formed, opposite to the direction of desired deformation. An opposing eddy current will be induced in the reaction block which can generate the desired repulsive magnetic forming pressure on the part. This condition is the inverse of more conventional electromagnetic forming where the induced eddy current is in the work piece. In general, part geometries will allow only a single current loop path. Therefore, such "direct" forming will tend to have rather low electromagnetic force efficiency compared to separate multi-turn coils which can generate greater force per ampere on the work piece.
Electro-Hydraulic
Submerged electric arc discharge has been commonly referred to in the literature as electro-hydraulic forming. The essential characteristics of this class of electric pulse power forming is the rapid discharge of kilo-joule levels of electric energy across a pair of electrodes submerged in a suitable fluid. The resulting arc vaporizes the nearby fluid, generating a small zone of plasma with of temperature in the thousands of degrees Kelvin and correspondingly high pressure. The rapid expansion of the plasma kernel transfers energy through the fluid to the work piece by a pressure shock wave followed by the momentum of the fluid displaced by the expanding gas bubble. The gas bubble actually expands and contracts several times before it dissipates in a manner analogous to the ring-down of the current through the coil in electromagnetic forming. The majority of the deformation work is done by the first expansion just as it is mostly accomplished by the first half pulse of current in the electromagnetic case.
The initiation of the arc can be assisted by the use of a small diameter "bridge"0 wire placed between the electrodes. It has been demonstrated that the use of a bridge wire provides for more consistent results by producing a more repeatable arc event in position and strength. However, the use of a bridge wire also makes the process more difficult to automate. Both variations have been used in commercial electro-hydraulic forming machines. FIG. 14 is a design schematic of a electro-hydraulic forming system. The pressure shock wave carries about half the energy from the discharge. The other half of the discharge energy is carried by the kinetic energy of the moving fluid surrounding the plasma bubble. However, the fluid kinetic energy is shown to provide the majority of the usable deformation energy [Caggiano et al 1963, Ezra,1973]. Although, the pressure shock can be directed by reflectors to focus on the work piece, the energy of the fluid momentum can not be easily directed and much is dissipated against the containment structure. One disadvantage of EH forming is that its energy efficiency is much lower than EM, due in part to the basic spherical nature of the pressure wave front, which is less efficient than a plane wave in most applications. The efficiency of electro-hydraulic forming is dependent on several system parameters and is generally given as 5-10% for most applications with a maximum of 15%.[Bruno, 1968].
An allied method, similar to electro-hydraulic should be briefly described here for completeness. This method, termed Shock Tube Hydraulic, the deformation energy is transferred to the work piece by the action of pressure shock and fluid momentum as in electro-hydraulic. The difference lies in the manner in which the pressure shock wave is generated and the proportion of the total energy contained in fluid momentum. In Shock Tube Hydraulic, the shock wave is generated by the rapid repulsion of a conducting driver plate with one side in contact with the working fluid, from a fixed coil conductor carrying the discharge current. A tube surrounding the driver plate and coaxial with its velocity serves to direct the fluid energy to a specific area. A schematic of one possible design of a shock tube assembly is shown in FIG. 15. FIG. 15 shows coil 160, driver plate 161, vellows 162, vacuum chamber 163, guide tube 164, die surface 165 and metal sheet 166. The basic effectiveness of this method has been demonstrated by the hydrodynamic equivalent method of a drop hammer on a water column. The use of a shock tube generated pressure pulse was also shown to be more than twice as energy efficient as compared to electro-hydraulic forming methods [Vafiadakis et al 1965]. It is not known whether the electro-magnetic version of the shock tube hydraulic presented here has been reduced to practice to date.
Electro-hydraulic systems were investigated by several of the U.S. auto makers, but considered to be too slow for even limited production on the smaller parts that the machines of that time could handle. Further, there were process control problems with these machines which further reduced the attractiveness to highly cost competitive, high volume industries.
During the 1960's, a decade before the Oil Crisis, there was not a strong interest in fuel savings from the weight reduction available with aluminum auto bodies. Without a serious need for the improved forming of aluminum alloy sheet or the general extended plasticity provided by the high velocity methods, the auto industry of the sixties had no inclination to seek solutions to the short comings of the high velocity forming processes in wide spread use by aircraft manufacturers.
The aerospace industry continues to utilize all of the high velocity forming methods to some extent, including electro-hydraulic. However, in recent years the electro-hydraulic process has been largely supplanted by improved fluid pressure forming systems. This is due, in part, to the fact that the size capacity of most electro-hydraulic machines were similar to the new fluid pressure forming systems. Further, the tooling for a quasi-static pressure process is lighter and often less expensive since it does not need to withstand the shock loading inherent in the electro-hydraulic process. The newer fluid pressure forming systems have increased peak pressure and reduced cycle time while improving the process repeatability by computerized pressure profile control. In contrast, there has not been any further improvements to the electro-hydraulic machines since the early 1970's. Consequently, electro-hydraulic forming is used in new applications by aerospace fabricators principally for parts which require higher peak forming pressures than the quasi-static fluid forming systems can generate. [Rorh Corp.]
The high velocity methods of sheet forming are the least common of the methods described herein. Table 1.1 is therefore provided as a summary of the past applications of these methods to forming of sheet metal stampings.
TABLE 1.1 ______________________________________ Matrix of electrically driven, high velocity forming processes and sheet metal part type Part Type* Shallow Deep Drape Tube Process Pan Draw Form Form ______________________________________ EM commonly not done uncommon very electro-mag- done muti-shots to-date common netic coils male or difficult due male tools male or good conductor female tools to rapid conductors female tools work pieces non-conduct- decrease in OK low conduct- ing best energy repeatability ing best repeatability transfer with OK repeatability good sheet medium good medium- deform. production assembly high operations production high production CEM new, new, not new, not new, patents coil-less promising practical practical awarded electro-mag- male or muti-shots muti-shots male or netic female tools difficult due difficult due female tools good conductor non-conduct- to rapid to rapid assembly work pieces ing best decrease in decrease in operations medium- energy energy high high transfer with transfer with production production sheet sheet deform. deform. EH commonly less common not practical most electro- done female tools, common hydraulic male or conducting female tools no conductivity female tools OK only restrictions conducting repeatability conducting on work OK problem OK repeatability low repeatability problem production OK medium multi-shots low to production medium production to-date EHS possible possible not practical possible electro-mag- male or female tools, female tools netic hydraulic female tools conducting conducting shock tube conducting OK OK no conductivity OK low repeatability restrictions repeatability production OK on work OK multi-shots medium medium production production ______________________________________ * Part type descriptions: (informal)
Shallow Pan: Parts principally stretch-formed with mostly bosses and narrow beads having depths up to approximately 15.times. sheet thickness
Deep Draw: Parts whose depth to breath ratio and geometry require sheet to be pulled in to limit plastic strains.
Drape Form: Similar to Shallow Pan type parts but can be deeper if sides have sufficiently open angle. Completely ballistic, no blank restraint
Tube Form: Parts formed by expansion or compression of simple tube section pre-forms, usually axisymmetric. Includes clinching assembly of multiple components.
Accordingly, it is an object of the present invention to provide improved apparatus and methods for the forming of metal work pieces, such as auto body size parts of aluminum alloy sheet. It is another object of the present invention to provide improvement in metal forming as measured, for instance, by the extent to which the new method increases the geometric forming limits of aluminum alloys in comparison to those obtainable using the prevalent commercial method of matched tool forming.
The potential advantages and disadvantages of each variation of the methods of the present invention is briefly discussed herein, along with the rational for proceeding with the MT-EM methods of the present invention.
In view of the following disclosure, other advantages of the invention, and the solution to other problems using the invention, may become apparent to one of ordinary skill in the art.