melting and fusing together of two pieces of material to make onehold
together much of the industrial world. Your safety while driving
in a car depends in part on the reliability of more than 3,000 welds.
If a weld were to fail, the results could be catastrophic. Welds
make possible airplanes, metal bridges, office buildings, and high-pressure
tanks as well as all sorts of high-technology devices. Welding is
the most widely used method for joining metals and is typically
stronger, lighter, and cheaper than other joining methods such as
riveting and bolting.
Forge welding has been around
almost since people began to work with metals. Then, in the late
19th century, Sir Humphrey Davy discovered the electric arc, and
modern welding was born. The materials that welders use have changed
over the years and today include not just metals but also polymers,
ceramics, and composite and engineered materials. Lasers, electron
beams, and plasma arcs supplement traditional electric and torch
welding methods. Yet for all this history, basic knowledge about
the welding process is surprisingly sparse. Conventional inspection
techniques are not adequate to indicate how a weld evolves in time.
Welding may be old, but the science of welding is in its infancy.
Livermore has a vital interest
in knowing all it can about welding. Dependable welds are important
for maintaining the performance and safety of nuclear weapons. Welds
will also play a key role in the success of the Department of Energys
planned repository for long-term storage of nuclear wastes, which
will potentially be located at Yucca Mountain in the Nevada desert.
Waste canisters will have three layers of containment: a convenience
can inside an inner can inside an outer can. The lids of the inner
and outer cans will be welded shut. The canistered waste must remain
impervious to attack by air, moisture, and the surrounding environment
for thousands of years. All told, more than 100 miles of welds will
be required at the repository.
Metallurgist John Elmer,
Livermores expert on welding, has been researching details
of the welding process since the early 1990s with physical chemist
Joe Wong, whose specialty is synchrotron x-radiation experiments.
They are currently working with another metallurgist, Todd Palmer.
The three are also collaborating with colleagues at Pennsylvania
State University and Oak Ridge National Laboratory.
Over the last several years,
the team has succeeded in producing maps of the microstructural
changes that occur in and around the weld area as a metal melts
and resolidifies. More recently, their experiments have revealed
second-by-second changes in a metals microstructure during
welding. In contrast, conventional diagnostic techniques can examine
the material only before and after welding or can derive information
about changes only indirectly and after the fact. These experiments
at Livermore have given us the first real-time look at the welding
process, says Lou Terminello, division leader in the Chemistry
and Materials Science Directorate.
of a fusion weld. A liquid weld pool is created through the
interaction of an intense heat source and the substrates being
joined. Melting on the front side of the weld pool eliminates
the interface between the materials, while solidification on
the back side of the weld pool fuses the substrates together
to create a solid joined part. Surrounding the fusion zone is
a heat-affected zone, where the substrate is heated to temperatures
up to the melting point of the metal being joined. Solidification
in the fusion zone and solid-state phase transformations in
the heat-affected zone are responsible for dramatic changes
in the microstructure and properties of the welded joint.
Take the Heat
When two pieces of material
are being welded together, high heat rapidly melts the solid material,
which quickly cools and solidifies again as the heat source moves
away. Adjacent to the immediate weld area, or fusion zone, is the
heat-affected zone (HAZ). As the name HAZ implies, the material
there is affected by the high heat of the welding process but does
Heat causes changes in the
material. The three well-known basic phases of a material are gas,
liquid, and solid. But for many materials, multiple solid phases
exist at various temperatures or at various combinations of temperature
and pressure. At sea level1 atmosphereplain old H2O
may form several kinds of ice, each of which is a different solid
phase. Iron undergoes three solid-state phase transformations as
its temperature increases from room temperature to 1,535°C,
where it melts. Carbon also has several solid phases, including
graphite and diamond. No one would confuse graphite and diamond.
Each one is still carbon, but their crystal structures are very
When a material is welded,
its crystalline structure changes. It is these microstructural changes
that interest Elmer. They can affect the strength of the material
as well as its corrosion resistance, ductility, and mechanical properties.
Any or all of the changes could either enhance the quality of the
weld or reduce the welds integrity. We want to be able
to understand the welding process by modeling it and then predict
the changes that will occur, says Elmer. But first,
we need to gather real experimental data during welding to understand
the fundamental properties of the process.
top view of the microstructure of titanium from the fusion zone,
through the heat-affected zone, and into the base metal (30
times magnification): (a) the base metal, (b) the small-grained
portion of the heat-affected zone where the gamma phase has
partially transformed to the beta phase, (c) the large-grained
portion of the heat-affected zone where gamma-phase titanium
has fully transformed into the beta phase, and (d) the fusion
zone. Note the dramatic changes in grain structure.
Joe Wong has been
performing experiments with synchrotron radiation to examine materials
for the past two decades. He and others helped to develop the experimental
facility at the Stanford Synchrotron Radiation Laboratory back in
Synchrotron radiation is
a particularly intense form of electromagnetic radiation. Highly
energetic charged particles traveling at almost the speed of light
and deflected in a magnetic field emit synchrotron radiation. This
intense, highly collimated radiationmillions of times more
powerful than that from a conventional x-ray tubecan probe
the atomic structure and electronic states of matter. Experiments
that would have taken hours with an x-ray tube source take milliseconds
radiation spans the electromagnetic spectrum from infrared to hard
x rays. X rays are ideal for probing matter because the wavelength
of x-radiation is about the same size as an atom. Thus, with synchrotron
x rays, the team can make direct observations of phase transformations
in welds, watching microstructural changes as they evolve.
Synchrotron radiation sources
at Stanford and elsewhere around the world are used by scientists
working in many fieldsby materials scientists like Elmer and
Wong to study the dynamic properties of solid and amorphous materials,
by biomedical researchers to study proteins and other large biomolecules,
by medical workers for coronary angiography and other forms of imaging,
and by geologists for structure characterizations and trace-element
analyses of minerals.
The Livermore team is using
x rays from the 31-pole x-ray wiggler at Stanford Synchrotron
Radiation Laboratory for their experiments. In this device, an x-ray
beam wiggles between an array of 31 magnetic poles, gathering intensity
along the way. By carefully directing this small, intense synchrotron
beam at a given location in a weld, they can obtain an x-ray diffraction
pattern to identify the phases present in the material at that location
during the welding process. The x-ray diffraction pattern depends
on the atomic structure of the material. The diffraction pattern
is the fingerprint of a materials crystal structure,
says Wong. Liquid is chaotic with no long-range order,
he continues, so there is no diffraction.
A rendering of (b) the experimental setup for real-time investigations
of welds using synchrotron radiation. The x-ray beam enters
from the lower left through a pinhole to provide spatial resolution
of 180 micrometers. During welding, this spatially resolved
beam is aimed at a specific location of the weld where diffraction
takes place. The diffracted beams are captured in real time
using a silicon photodiode linear array detector. The weld is
produced by a gas tungsten arc on a revolving solid bar of the
material being studied.
Simple to Complex
The teams first experiments
examined titanium welds. Titanium is popular in manufacturing because
of its corrosion resistance and light weight. Also, titanium has
two well-characterized solid-phase transitions at ambient air pressure
before it melts. In pure titanium, the alpha phase exists from room
temperature to 882°C. At these temperatures, titanium has a
hexagonal-close-packed crystalline structure. At 882°C, pure
titaniums crystalline structure changes to the beta (body-centered-cubic)
phase, which it maintains until it reaches the liquid phase at 1,670°C.
As the liquid titanium cools, the phase transformations are reversed.
Because these phase transformations occur over such a wide temperature
range, titanium is a relatively easy material to study.
Using the experimental setup
shown in the figure below, a metal bar rotates under a gas tungsten
arc, taking 6 minutes for a full revolution. An intense x-ray beam
from the synchrotron source passes through a pinhole to allow researchers
to resolve features as small as 180 micrometers. During welding,
the x-ray beam is aimed at specific points around the heat source.
A silicon photodiode linear array detector records the diffraction
patterns during the experiment.
team maps phase transformations by performing a series of sequential
linear scans from the centerline of the weld and out into the HAZ.
In every row, 30 to 40 x-ray diffraction patterns are collected,
spaced 0.25 millimeters apart. Each row requires one revolution
of the cylinder. After completion of the first row, the welding
heat source is moved 1 millimeter from its previous position to
collect data in the next row, and so on.
This spatially resolved x-ray
diffraction (SRXRD) technique is unique to Livermore for the study
of welding. Spatial resolution is the key to collecting useful
in situ phase transformation data during welding, says Elmer.
The grain structure of commercially
pure titaniumor any solid material for that matterchanges
during welding. It is subjected to peak temperatures hundreds of
degrees higher than the melting point, followed by rapid cooling.
These temperature fluctuations alter the microstructure of the material
nonuniformly to create the HAZ adjacent to the weld fusion zone.
Solid-state phase transformations that occur in the HAZ create gradients
of both microstructure and properties between the liquid metal in
the fusion zone and the unaffected base metal farthest from the
weld. Within the HAZ, the most severe microstructural changes occur
close to the fusion zone, where the peak temperatures are the highest.
Researchers had suspected
for some time that annealing and recrystallization occur in the
colder portions of the HAZ in titanium. They also knew that both
partial and complete alpha-to-beta transformations take place in
the hotter portions of the HAZ. But what they had not been able
to determine was the exact size and location of these regions.
Using SRXRD, the Livermore
team found six regions in the HAZ around the liquid titanium weld
pool, each with an identifiable diffraction pattern. From their
diffraction data, they could follow the evolution of the phase transformations,
at various locations and at various temperatures. This research
resulted in a diffraction map of the HAZ [part (c) of the figure
below] that shows the location of all the phases with respect to
the transition temperatures.
Titanium was a good
place to start with our experiments, says Elmer. But
steels are welded much more frequently. So their next sets
of experiments dealt with carbonmanganese steel and stainless
steels. While these alloys have more complex phase changes than
pure metals, their phase transformations can be studied with the
Duplex stainless-steel alloys
consist of austenite and ferrite solid phases, each of which has
different crystal structures and magnetic properties. Here, they
found five principal phase regions between the liquid weld pool
and the unaffected base metal that contribute to the final microstructure
observed in the HAZ.
These typical diffraction patterns from titanium welds represent
the six different regions (on the right axis) that exist in
the heat-affected zone (HAZ) during welding. (b) Evolution of
spatially resolved x-ray diffraction patterns measured across
the alpha-to-beta (α to β) phase transformation isotherm
(temperature = 915ÉC) in titanium (Ti). Three zones are shown
on the right axis: alpha-phase titanium (αAR-TI),
alphabeta (α+β-Ti) coexistence zone, and beta-phase
titanium (β-Ti). (c) A spatially resolved x-ray diffraction
phase map showing the locations of all the titanium diffraction
patterns with respect to the transition temperatures for a titanium
weld. The welding heat source is located at 0 on the horizontal
Phase mapping experiments
performed using the SRXRD method are useful for observing phase
changes under quasi-steady-state heating and cooling conditions.
The next step was to examine the changes that occur at a single
spot as a function of time. Wong developed a time-resolved x-ray
diffraction (TRXRD) technique that takes a set of x-ray diffraction
patterns at a single location adjacent to or within a stationary
spot weld. When the detector is clocked for durations of tens to
hundreds of milliseconds, phase transformation may be observed on
a much shorter time scale than is possible with moving welds. Changes
in the diffraction pattern show directly how phase changes are taking
place as a function of time and temperature. As the temperature
goes up and then down, the metal at the weld becomes liquid and
then solidifies. With TRXRD, the Livermore team has been able to
examine the solidification and subsequent solid-state phase transformations
in a number of different materials for the first time.
For example, TRXRD has proved
useful for examining the solidification behavior of austenitic stainless
steels. In these stainless steels, the presence of residual ferrite
in the austenitic microstructure affects the integrity of welds.
Researchers have long been interested in understanding how residual
ferrite in the microstructure evolves. For more than 50 years, those
who work with welds have known that the composition of the weld
is important and have developed methods for assuring that the austeniteferrite
ratio was appropriate for each specific need. Numerous studies have
examined the rate of solidification, which affects the microstructure
and relative percentage of austenite and ferrite in the final weld.
Livermore was the first to make direct observations of the ferrite
and austenite phases and the dynamics of this transformation. The
Livermore team found directly, for the first time, that ferrite
is the first phase to solidify from the liquid weld pool in a 304
stainless-steel alloy. The ferrite phase existed as the only solid
phase for 500 milliseconds before beginning to transform into the
austenite phase. The ferrite-to-austenite transformation took an
additional 200 milliseconds of cooling, during which both phases
coexist. The combined results showed that the majority of the ferrite
phase transformed to the austenite phase by the time the weld had
cooled to a temperature of 1,100°C.
Monte Carlo simulation of grain-size evolution in welds.
image of a duplex stainless-steel weld obtained in real time
during a synchrotron experiment.
Elmer and Palmer have also worked with modeling experts at Pennsylvania
State University, where a research group has spent many years developing
models to predict the temperatures present throughout a weld. By
combining the results of the SRXRD experiments with the modeling
results, the evolution of observed phase transformations can be
more fully understood. As part of their collaboration, they performed
three-dimensional Monte Carlo simulations of the growth of grains
during gas tungsten arc welding of titanium, shown in the figure
The LivermorePenn State
collaboration has continued to study phase transformations in duplex
stainless steels. SRXRD observations of the phases present around
the weld pool of an arc-welded 2205 duplex stainless steel have
been combined with the results of a Penn State heat-transfer model
to produce a thorough map of the phase transformations occurring
in the heat-affected zone. An infrared image of a duplex stainless-steel
weld, taken during the synchrotron experiments, is shown in the
figure directly above.
Further analysis of the data
available in the diffraction patterns allowed the team to determine
the amount of ferrite and austenite present at each location. The
top figure below shows the variation in the ferrite volume fraction
as a function of location around the weld pool. This is the first
time the phenomenon was observed and quantified.
Once again demonstrating
its unique capabilities, the SRXRD technique allowed the team to
observe a decrease in the ferrite volume fraction at rather large
distances from the weld pool (on the order of 9 millimeters). This
change in the ferrite volume fraction was unexpected and had not
been previously observed. Because evidence for this reaction disappears
as the welding process continues, SRXRD provides the sole means
available for monitoring these phase transformations.
of a spatially resolved x-ray diffraction experiment portray
the dominant phase transformations and the regions over which
they occur in the heat-affected zone during welding of duplex
x-ray diffraction results show phase transformations during
weld solidification and cooling of a flux-cored arc-welding
Leads to Smarter Welds
This pioneering work is not
going unnoticed by the welding research community. Elmer was named
a Fellow of the American Welding Society in 2000. And in May 2001,
the society honored a paper by Elmer, Wong, and colleagues at Penn
State with the prestigious William Spraragen Memorial Award. Their
article on modeling of titanium welding was selected the best paper
of 2000 in the Welding Journal Research Supplement.
The ultimate purpose of all
research on welding is to move useful information out to the welders
of the world, to help them make better welds. In fact, Livermore
synchrotron investigations of welds, combined with computer modeling
and postweld characterization of microstructure, are beginning to
do just that.
Powdered filler metal additions,
which include aluminum, in flux-cored arc-welding electrodes alter
the microstructure and properties of the resulting welds in unpredictable
and undesirable ways. In the figure above, TRXRD results show phase
transformations during the solidification and cooling of a weld
in a mild steel consumable welding electrode. This figure comprises
over 500 diffraction patterns, taken at the rate of 20 patterns
per second, and indicates an unexpected nonequilibrium solidification
of the weld.
translates into a possible safety hazard for welded structures.
To mitigate the hazard, this research, which is being done in collaboration
with Oak Ridge National Laboratory, is now being used to help design
new self-shielded welding electrodes with improved weld properties
for safer building and bridge construction. You cant get much
more useful than that.
fusion welding, phase transformation, solidification kinetics, spatially
resolved x-ray diffraction (SRXRD), stainless steel, synchrotron
radiation, time-resolved x-ray diffraction (TRXRD), titanium, x-ray
information contact John Elmer (925) 422-6543 (firstname.lastname@example.org)
Joe Wong (925) 423-6385 (email@example.com).
received his B.S. and M.S. in metallurgical engineering from
the Colorado School of Mines in 1979 and 1981, respectively,
and his Sc.D. in metallurgy from the Massachusetts Institute
of Technology in 1988. After working briefly at Lawrence Livermore
in the early 1980s, he rejoined the Laboratory as a postdoctoral
scientist in 1988 and was named group leader for Materials
Joining in 1989, a position he continues to hold. The group
is responsible for electron- and laser-beam welding, vacuum
brazing, and diffusion bonding.
Elmer has written or
cowritten over 60 technical papers on materials joining, metallurgy,
rapid solidification, the interactions of high-energy-density
beams and materials, and the kinetics of phase transformations
under nonisothermal conditions. He is a member of the American
Welding Society (AWS) and the American Society of Metals International.
In 2000, he was made a fellow of AWS; in 1991 and 2000, he
received the William Spraragen Award from AWS; and in 1995,
he received the Professor Masubuchi-Shinsho Corporation Award
WONG received a B.Sc. in pure and applied chemistry in 1965
and a B.Sc. in physical chemistry in 1966 from the University
of Tasmania, Australia. In 1970, he received his Ph.D. in physical
chemistry from Purdue University, and in 1986, he received a
D.Sc. from the University of Tasmania. In 1986, he joined Lawrence
Livermore as a senior chemist.
Wongs primary research
interests include glass science and materials science. He has
also examined the chemical dynamics and phase transformation
of various materials and processes using high-resolution electron
microscopy, various kinds of spectroscopy, and novel synchrotron
instrumentation. He has written or cowritten over 175 journal
articles, holds 7 U.S. patents, and has received numerous prizes
and awards, most recently (with John Elmer) the William Spraragen
Memorial Award from the American Welding Society for the best
paper published in Welding Journal Research Supplement in 2000.