Workshop Description
MOTIVATION
NIST's work on the fundamental mechanisms of work hardening is driven
by industrial needs. Metal forming is an enormous industry in the
United States, with value added amounts in the tens of billions of dollars
annually. It is one of the nation's largest consumers of mild and
high strength steel, as well as aluminum alloys. The automobile industry
is the largest single user of sheet metal: sheet metal products comprise
approximately 33% of the weight of an automobile. An improvement
in the efficiency of metal forming operations would have a large impact
on the U.S. economy.
The cost of forming complex metal parts such as automobile panels is
dominated by two factors: the large initial cost of designing and producing
the die, and the cost of the metal sheet. NIST work in this area
is aimed at reducing the time and expense of designing dies. Most
dies are still designed using rules-of-thumb that are based upon long experience
with specific metal alloys. Increasingly, however, dies are being
designed using Finite Element Analysis (FEA) which predicts the behavior
of the metal sheet during stamping. In practice, the die never produces
an acceptable part on the first attempt, and the die must be extensively
redesigned and reworked. The redesign process is done by hand
using intuition by experienced craftsmen. Reworking involves resurfacing
the die contour, reprogramming the cutter path, and re-machining.
This expensive and time consuming process is repeated until a properly
shaped panel is produced with an acceptable failure rate. For mild
steel parts, a total of 8-10 tryout steps are often required.
Improvements in this process are extremely important to the automotive
industry, which is attempting to switch to high strength steels and aluminum
alloys to save weight and improve fuel economy. The behavior of these
materials during stamping is vastly different from the currently used mild
steel, and poorly understood. The time and cost of die production
for use with these materials is projected to be unacceptably high without
improved FEA modeling tools. Such improvements would eliminate much
of the trial-and-error that currently exists, thereby decreasing the time
and cost of new product development, and allowing production of lower-tolerance
parts. Since improved FEA would primarily reduce the high initial
fixed costs for a stamping run, small-run manufacturing costs would be
reduced by an even greater margin.
The fundamental reason why current FEA codes are inadequate is that
the models used to predict the behavior of the metal sheet are strictly
empirical and do not reflect the microstructural changes that are responsible
for the observed changes in mechanical behavior during deformation.
Understanding this process is a necessary prerequisite to developing more
accurate deformation models based on changes in the dislocation microstructure.
HISTORY
Research on the microstructural basis of work hardening (dislocations)
was a tremendously active field in the 1940's through roughly the mid seventies.
Work in the field diminished significantly at this time for several reasons,
in large part due to a perception that the problem was too complex to solve
adequately with the tools (theoretical, computational, and experimental)
then available. Funding sources dried up and work in this field came
mostly to a halt (although a few researchers continued to make slow but
steady progress).
The first NIST workshop on work hardening and dislocations was held
at NIST in the summer of 1995. This meeting was informal and exploratory
in nature, and was held to determine whether the past twenty years of progress
in science and technology provided the necessary tools for further advancements
in the basic understanding of work hardening. The unanimous conclusion
was that the time was ripe for a renewed effort, and that a renaissance
in the field was likely in the near future. In large part, this conclusion
was driven by new large-scale computer simulations of dislocations in 3D
being conducted by Kubin and Canova in France. However, it became
clear that significant progress could only be achieved through collaborative
efforts. Owen Richmond from ALCOA challenged all of the participants
to work on developing a quantitative model for the deformation of single-crystal
Al.
Robb Thomson and Lyle Levine at NIST began working full time in this
area, with most of the early research aimed at developing new experimental
methods for studying the development of dislocation structures in situ
and in bulk-thickness samples. Work was aimed primarily at aluminum,
due in large part to an active collaboration with researchers at ALCOA.
Over the next few years, NIST personnel organized American Physical
Society focus sessions on the fundamental mechanisms of mechanical properties
(primarily work hardening). The organizers of these focus sessions
were:
Lyle Levine (NIST) & Robb Thomson (NIST), Kansas City, MO,
March 1997
Lyle Levine (NIST) & Hussein Zbib (WSU), Los Angeles, CA, March
1998
From the large response to these meetings, it was clear that activity in
the field was growing rapidly. A second (much larger) workshop was
planned to be held in conjunction with a Short Course on the same topic
being held by LLNL. The workshop was held in Pleasanton, CA in June
1998, and it was organized by Lyle Levine & Robb Thomson (NIST) and
Hussein Zbib & John Hirth (WSU).
DESCRIPTION OF WORKSHOP
The purpose of the workshop was to explore the current status of this
multi-disciplinary field and to promote collaborative efforts. The
workshop was open only to invited participants. The invitation list
included all of the most prominent U.S. researchers in the field along
with a few very prominent researchers from outside the U.S.
RESULTS
All of the participants agreed that the workshop was a great success.
During the presentations and the discussion sessions, several important
points were made. The most important point was that there exists
a severe lack of communication between researchers working on different
length and time scales. For example, researchers conducting atomistic
simulations want their work to be relevant to work going on at larger length
scales, but they were mostly in the dark concerning what simulations would
be relevant. On the opposite end of the spectrum, researchers who
are developing three dimensional dislocation dynamics codes require atomistic-level
information on stacking fault energies, kink pair nucleation processes,
cross-slip nucleation, and junction formation and breaking. This
workshop was therefore instrumental in allowing important connections to
made between the diverse research groups.
The groundwork was also laid for several collaborative research endeavors
that directly affect NIST's research efforts in this field. Drs.
Levine and Thomson from NIST are currently involved in two large multi-institution
collaborations with people they met at this workshop. In addition
to participants from several national laboratories and Universities, these
two collaborations include participants from more than 20 companies including
steel and aluminum producers, FEA software companies, automobile manufactures,
and aerospace companies.
Finally, it was decided at the workshop that international communication
in the field was lacking, and that a full international conference on the
fundamentals of work hardening should be held. Work on this
conference began immediately. The conference, called Dislocations
2000, will be held at NIST on June 19-22, 2000. Full details may
be found on the NIST Dislocations
2000 web site.