IML Research

Impulse Forming

Traditionally the methods we use to form sheet metal involve the careful control of displacements and static forces using large presses.  Here we present an alternate mode of metal forming where impulse and high velocity are used to form sheet metal. 

Impulse-based metal forming is actually quite easy to carry out.  In particular, electromagnetic forming is carried out by driving a large pulsed current through a conductive coil in close proximity to a metal workpiece.  This can easily drive the workpiece at velocities over 100 m/s. Formability can be dramatically improved in electromagnetic forming.  This requires high velocity and is the improvements are due to both inertial and constitutive effects.

There are many opportunities in developing new approaches using electromagnetic forming for sheet metal forming. Example: springback can be easily controlled by imposing modest local plastic deformation in a metallic-formed sheet. The impact that takes place in electromagnetic forming can be used for decorative embossing. This produces features like those form coining and even micro-features like diffraction gratings.  This is amiable to much larger areas, at much lower equipment costs. 

Lastly, tooling and systems can be very lightweight and configured quickly.

Methods of Impulse Forming:

-Vaporizing Foil Actuator

-Electromagnetic Pulse

-Pulsed Laser

What is Agile Metal Forming

The term agile sheet metal forming here indicates the ability to rapidly respond to customer needs by being able to quickly move from a specified design to producing a number of high-quality, dimensionally-acceptable items.   The most challenging issue related to making the first run of parts is related to the design, tryout, and finishing of production tooling.  In later runs of the production line, the changeover between one toolset to another often becomes the most time-consuming and expensive issue.  All of this is thematically similar to lean manufacturing, however, there the emphasis is on management procedures and the choosing new systems that are more re-configurable, that are often based on automation and vision systems, etc. 

Here, the focus remains on sheet metal that still requires some hard tools to reproducibly provide acceptable dimensional tolerance and reproducibility.  However, the focus will be on shifting manufacturing methods to reduce lead time, by reducing the need for matched toolsets and precision alignment between tools, and reducing the need for tuning the system by modifying tool shapes

Agile Sheet Metal Forming, or: using a minimum set of easily or modified hard tools to manufacture high-value sheet metal articles that meet conventional dimensional and property specifications.

The Market Pull for Agile Metal Forming

Affluent consumers are fueling a growing demand in custom items in businesses such as designer clothing boutiques, shops for custom golf clubs, and providers of custom motorcycles are becoming more numerous.  Elements within the department of defense also procures items for special missions and forces in this way; military vehicles are commonly built in fleets of less than 500.  

Mass customization is growing very very quickly. Consumers are demanding high quality, custom products and fast.  There are some very interesting new business models appearing for satisfying consumers with rapidly-produced premium products, Zazzle and Cafe ress have shown rapid growth with on-demand graphic clothing and paper goods.  This concept is now being taken to 3-D manufactured goods in clothing by companies such as Style Shake and into hard goods by Ponoko.  

Fast mass customization does not yet apply to sheet metal, because the supporting technology is not there yet.  Impulse methods can change this.

The Technical Elements of Agile Sheet Metal Forming

1) One sided dies eliminates half of what needs to be manufactured and reduces many problems with dimensional tolerance.  One-sided dies used in hydroforming and superplastic forming.   

2) Minimal Static Forces In order to react large static forces, large presses are required, as are heavy tooling sets. Dynamic or impulse forces do not require large press frames.  The blacksmith can produce large intricate components using just a hammer.  His impulses are relatively short in time and can be absorbed by the mass of the hammer and anvil. This allows him to use a very light and agile forming tool (the hammer).

3) Rapidly Produced Tools Often it can take months to produce a toolset for sheet metal forming.  This is unacceptable in the agile metal forming paradigm.  Polymer, cast ceramic or rapid sintered tools are essential even if they cannot make long production runs.  The goal here is to change designs quickly. 

4) Reduced Process Steps Sheet metal manufacturing is often progressive in nature. In deeply formed components it is common to use multiple operations where one stretches the metal rather uniformly to thin it and create surface area and the next step produces the required shape.  Or, one step may provide a shape, the next a cut, and another flange.  Agile manufacture seeks to combine these operations into a smaller number of steps. The ability to embed electromagnetic actuators in semi-traditional tooling makes this possible.

5) Increased Degrees of Freedom Traditional stamping has very few degrees of freedom.  Once punch and die geometries are set, about the only adjustments one can make are to change binder pressure (possibly as a function of press stroke) or change lubrication conditions.  This causes problems when there are significant variations in the properties of incoming materials and, for example, springback characteristics change.  To the other extreme, the blacksmith has almost infinite discretion to modify his process to accommodate new materials or modified customer needs.

6) Late Stage Product Differentiation Processes that can be used to add a feature to a product (embossment, cutout, feature line, etc.), after it has been nominally finished can allow quick adaptation to customer demands. Impulse-based methods have the ability to produce local features without gross product distortion.

Conformal Interference Joining

Conformal interference joints are mechanical interlocks between two components, which can be composed of similar or dissimilar materials. Electromagnetic forming and vaporizing actuators can accelerate closed shapes inward or outward with velocities greater than 100 m/s. Upon impact under these conditions, one material will conform closely to the other and a state of residual stress will remain, holding the two components together. Joint strength exceeds the strength of the weaker of the two materials in a properly designed joint. Thus, this method represents a complementary method to welding. Unlike welding, however, there are no heat-affected zones and arbitrary dissimilar materials can be joined. This method is practiced commercially; we are working to extend the methods and the design science.

Conformal interference joint made by electromagnetic forming

Aluminum and steel joint with failure in the parent aluminum post tensile testing

Related Links:

-Conformal Joining

A perfect sheared edge is characterized by low burr height, small rupture zone, low amount of roll over, low penetration depth, and a large shear zone. Increased punch velocity leads to improved quality of sheared edges and a reduction in penetration depth and rollover (Breiting, 1998).
Golowin (2008) implemented punchless electromagnetic shearing by using path actuators to drive metal sheets into single-sided shearing dies. That work showed that at high speeds during direct electromagnetic shearing, even thermally conductive materials show improved sheared edge quality as compared to quasistatic shearing.
Another advantage that high-speed shearing offers is the reduction in force on the shearing die. If the shear stress wave velocity is less than the shearing velocity, all the deformation is concentrated on the cutting edge, and the energy required for shearing is lowered. 

Example of Electromagnetic Shearing

Example of the reduced burr height due to impulse forming