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. [25, 41]

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. Steels have significant 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 aluminum sheet Figure 1.1 which was adapted from an Aluminum Association report [2]. Story [66] and Hayashi [38] provide good overviews of all the problems common in press working aluminum alloys.

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. Figure 1.2 graphically illustrates this point and the manifestation of the material characteristics shown in Figure 1.1 with a side by side comparison of two automobile door-inner panels from the same stamping die. Panel A is a fully formed panel of specified production steel sheet that was produced after set-up trials indicated satisfactory tool performance. Panel B is a 6111-T4 aluminum panel of the same thickness as the steel and was processed directly after the steel panel. The wrinkling and large splits in the aluminum panel occurred within the first 25% of the tool stroke and were not unexpected. This simple demo/experiment was conducted only to produce a baseline for the application trials (see chapter 5) of the new forming method, the subject of this thesis.

Fluid pressure forming methods such as Verson-Wheelon, ABB or Hydroform , described in references [1, 69, 70], 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.

Some, specially processed aluminum alloys exhibit superplastic creep behavior which can be utilized to produce very complex sheet part geometries. Current superplastic forming, like hydraulic methods, 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 equipment costs are also significantly greater than the matched tool method [49].

A compromise solution might be to change the part designs to shapes that can be produced in aluminum using current production methods. Another solution, would be a new sheet forming method which could overcome the formability short-comings of aluminum alloys while maintaining acceptable production rates (150 - 300 parts/hr. for large body panels). Such a process would be less restrictive for the automobile designers and thus more appealing to the industry. This improved forming performance must be attainable with capital equipment and tooling expenditures that 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 an evolutionary manner, incrementally with occasional forward leaps. Therefore, an examination of existing methods for evidence of partial solutions to the total problem is appropriate. The obvious next step is to consider how aspects of existing

methods may be combined to produce a new, hybrid method that goes further toward meeting the ideal performance goals than the constituent methods used alone.

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. (It should be noted here, without further explanation, that near term developments in superplastic forming might 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 over all 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 [50, 51].
 

1.1 Matched Tools

Matched tools are 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 that are required for this process can be very simple. 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 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 principal driving force in auto body part production.
 

1.2 Fluid Pressure Forming

The fluid pressure processes used, past and present, have demonstrated certain of the desired traits of the described hypothetical process. 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. [72, 57]

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, 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 form 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. 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 second operation.
 

1.3 High Velocity Forming

High velocity sheet forming, also referred to as "high energy rate" forming is not well know outside of the aerospace industry. However, this forming technology has been in commercial use, in some form, for close to a century [28]. The first applications were the forming of large domes from plate using chemical explosives. Later, electromagnetic pulses and submerged electric arc discharges (electro-hydraulic) were employed to generate very high power events which resulted in producing the high deformation rates characteristic of these processes. The deformation velocities generated in the electromagnetic and electro-hydraulic processes are 100 to 1000 times greater than the deformation rates of the quasi static processes like matched tool or fluid pressure forming (~0.1 m/s vs. ~100 m/s). Such high deformation rates are known to significantly extend the deformation capacity of many metals [74, 58]. Figure 1.3 summarizes the results of some early experiments in high velocity forming of sheet metals. Note that Figure 1.3 reports average strain rather than maximum strain at failure that has become the more accepted figure of merit since the introduction of Forming Limit Diagrams (FLD). Figure 1.4 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 Figure 1.3 is for unconstrained "free" dome tests while certain high velocity data in Figure 1.4 could be confounded by an ironing effect due to impact with the covering conical die cap. The ironing effect compliments the primary hyper-plastic effect of inertial stabilization of necking.

FLD for 6061 T4 (solutionized and quenched) aluminum and added data from low and high rate forming experiments. Engineering strains are used throughout.

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 [64]. In 1968 a textbook summarizing all the then current methods was published by the American Society of Tool and Manufacturing Engineers [17]. Texts covering specific methods were published by other authors [62, 28]. 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. 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 was developed. The most widely used during the late sixties and early seventies were 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.
 

1.3.1) 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 "workpiece" conductor. Therefore, a fundamental requirement for this forming method is that the workpiece must be an electrical conductor. The efficiency of electromagnetic forming is directly related to the resistance of the workpiece material. Materials, which are poor conductors, can only be effectively formed with electromagnetic energy if an auxiliary driver sheet of high conductivity is used to push the workpiece.

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. Figure 1.5 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 forming can be performed, with lower efficiency, 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" block 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 workpiece. 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.
 

1.3.2) Electro-hydraulic

Submerged electric arc discharge is 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 surrounding 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. 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" 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. Figure 1.5 is a design schematic of an 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 [17, 25]. 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 lower than EM for axisymmetric parts, due in part to the basic spherical nature of the pressure wave front. 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% [17].

Electro-hydraulic systems where investigated by several of the US. automakers 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 that further reduced the attractiveness to highly cost competitive, high volume industries.

During the 1960ís, a decade before the Oil Crisis, there was no interest in fuel savings from the weight reduction available with aluminum auto bodies. Without a serious need for forming of aluminum alloy sheet the auto industry of the sixties had no inclination to seek solutions to the production shortcomings of the high velocity forming processes. During this same period or the general extended plasticity provided by the high velocity methods gained 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 capacities 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 no further improvements to the electro-hydraulic machines have been made since the early 1970ís. 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. [32]

The high velocity methods of sheet forming are the least common of the methods described in this chapter. Table 1.1 is therefore provided as a summary of the past applications of these methods to forming of sheet metal stampings.
 

* Part type descriptions: (informal)

Shallow Pan : Parts principally stretch-formed with mostly bosses and narrow beads having depths up to approximately 15x sheet thickness

Deep Draw: Parts whose depth to breadth ratio and geometry requires 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
 

1.4 Overview

The object of this research is the design and verification of an improved method for the forming of auto body size parts of aluminum alloy sheet . The improvement is measured by the extent by 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.

In Chapter 2 the proposed new method is described in general along with two possible manifestations. The potential advantages and disadvantages of each variation is briefly discussed along with the rational for proceeding with the MT-EM method in the research effort described by this thesis. Chapter 2 closes with a discussion of the critical issues to be resolved before the proposed MT-EM method can be considered verified at the proof-of-concept level. The technical issues related to the full commercial application of the MT-EM method are also reviewed.

In Chapter 3, the experiments required for the investigation of the fundamental viability of MT-EM aluminum sheet forming are described. The experimental hardware, procedures and methods of measurement of effects are explained in detail before the presentation of the experimental results.

The requirements and necessity for numerical modeling of the MT-EM forming method are discussed in Chapter 4. The fact that no single code, currently available, is capable of modeling the entire process is presented. Several simulation programs that can be used to model different regimes of the process are described. The use of a smooth particle hydro- code, GEM, for the modeling of the dynamic electromagnetic pulse forming regime of the process is discussed and the coupon testing model results from GEM are presented.

An overview of a design methodology useful for the application of the MT-EM forming process to commercial part manufacture is the primary subject of Chapter 5. The basic coil and tool design considerations are discussed in some detail. Trials involving two full scale automotive body panel parts are described and presented as real examples of preliminary, low production application of the MT-EH process for the manufacture of large auto body aluminum stampings.