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Developments in Aluminum Heat Treating
11-10-02
The discovery of the precipitation hardening capability of aluminum alloys at the beginning of the 20th century opened up high performance applications in automobiles to aerospace. A hundred years later there is still much to learn and exploit from both the fundamental understanding and practical implementation aspects of aluminum heat treating. This article discusses some areas of recent progress.
First let's begin with a review of the basic process. Heat treatable aluminum alloys gain strength from subjecting the material to a sequence of processing steps called solution heat treatment, quenching, and aging. The primary goal is to create sub-micron sized particles in the aluminum matrix, called precipitates, which in turn influence the material properties. The resulting heat treated products carry a "T" temper designation, for example, 6061-T6. While simple in concept, the process variations required depending on alloy, product form, desired final property combinations, etc. make it sufficiently complex that heat treating has become a professional specialty unto itself.
The first step in the heat treatment process is solution heat treatment. The objective
of this process step is to place the elements into solution (hence the name) that
will eventually be called upon for precipitation hardening. Developing solution
heat treatment times and temperatures has typically involved extensive trial and
error experimentation, partially due to the lack of accurate process models. Work
now underway through the Center for Heat Treating Excellence (CHTE) at Worcester
Polytechnic Institute's Metal Processing Institute seeks to improve on this situation.
Under the direction of Professors Morral and Brody from the University of Connecticut,
a project entitled "Solution Heat Treatment of Aluminum Alloys: Effect on Microstructure
and Service Properties" builds on the knowledge base that exists in industry and
in the metallurgical literature to develop quantitative models for predicting
the response to solution heat treatment of cast and wrought aluminum alloys. This
program is just in the early phases and is one of four efforts in this Center.
Further information is available at http://www.wpi.edu/Academics/Research/CHTE
or www.eere.energy.gov/industry/supporting_industries/pdfs/integrated_heat_treatment.pdf.
Another development is the use of fluidized bed technology to achieve rapid heat
treatment. A project sponsored by the Department of Energy's Office of Industrial
Technologies NICE3 program has focused on an in-line fluidized bed aluminum heat
treatment system for cast components. The process heats components individually
in a continuous process mode instead of using the more conventional method of
batching a number of parts together. This approach is not only said to save energy
but also significantly reduce solution heat treatment time.
Quenching is the second step in the process. Metallurgically, its purpose is to
retain the dissolved alloying elements in solution for subsequent precipitation
hardening. Generally the more rapid the quench the better from a properties standpoint,
but this must be balanced against the twin concerns of part distortion and residual
stress if the quench is non-uniform. Balancing these competing demands has been
the focus of much of the development in this area, and work continues. Research
at Purdue University is focused on improving the uniformity of spray quenching
of complex shaped aluminum alloy extrusions and castings by better understanding
the heat transfer characteristics of water sprays and utilizing finite element
analysis models to predict thermal history and resultant properties of parts quenched
with multiple, overlapping sprays. Additional information on this project can
be found at http://widget.ecn.purdue.edu/BTPFL/Fac_Staff/Projects/spray_quench_complex.html.
Also aimed at reducing distortion during quenching is an extension of the fluidized
bed heat treating project described above. Funded with a NIST Advanced Technology
Program award, Technomics, LLC will build and demonstrate a fluidized bed quenching
system that uses a dry fluidized media instead of water or air to achieve the
desired cooling. The hypothesis to be tested initially on aluminum casting alloys
is that the use of the dry fluidized media eliminates the formation of vapor barriers
during quenching, which is a primary source of distortion in water quenching,
while still producing suitably rapid cooling to achieve desired properties. After
bench scale demonstration with the assistance of the WPI Materials Processing
Institute group, a full-scale system is to be built and tested at AMCAST on cast
aluminum automotive components including wheels, suspension arms, and suspension
knuckles. Please see atp.nist.gov/awards/00-00-4680
for more information on this effort.
Aging either at room or moderately elevated temperature after the quenching process
is used to produce the desired final product property combinations. The underlying
metallurgical phenomenon in the aging process, namely precipitation hardening,
has been of continuous interest to scientists in the aluminum field since its
inadvertent discovery in the early 1900's by Alfred Wilm. Due to the small size
of the precipitate particles, early understanding was hampered by the lack of
sufficiently powerful microscopes to actually see them. With the availability
of the transmission electron microscope (TEM) with nanometer-scale resolution,
researchers were able to actually image many precipitate phases and build on this
knowledge to develop improved aluminum alloy products. Recent developments in
analytical capabilities now offer the metallurgist atomic resolution and microchemical
analysis, which has lead to new insights on the structure of precipitates in aluminum
alloys. The 8th International Conference on Aluminum Alloys (ICAA8) provided a
snapshot of some of the more current results, and the Proceedings are recommended
for those seeking more depth.
With improved understanding of the precipitation hardening process has come commercial implementation of new aging practices. One that has been very useful in the automotive arena has dealt with alloy design and processing for auto body sheet alloys for optimal strengthening response during the elevated temperature processes used in conventional paint baking cycles. Preaging of aluminum sheet to what is being called the "T4P" temper has been applied prior to sheet forming to enhance the response during paint baking while maintaining the desired sheet formability. Also, the Manufacturing Science and Technology Group at CSIRO in Australia (http://www.csiro.au) has developed a novel aging treatment that increases both strength and fracture toughness. Their process involves interrupting the conventional high temperature artificial aging cycle with a dwell period at a lower temperature, which induces secondary precipitation and improves the precipitate distribution. The process is said to be effective for wrought and cast aluminum alloys.
To complete this discussion on progress in the area of heat treatment, it is worthwhile to note a new online resource developed through the Center for Heat Treating Excellence at WPI called the CHTE Global Database. While still in the early stages of development, the goal is to provide a compendium of ongoing heat treat research in one location. You can access the database at http://www.wpi.edu/Academics/Research/CHTE/search.html.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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