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Advances in Microstructure Quantification and Control in Extruded Products
9-27-02
Metallurgy 101 teaches us that processing affects microstructure, which in turn determines properties. One active area of study that promises to have an important impact is how extrusion processing parameters influence the metal grain structure and subsequently surface characteristics in aluminum extruded products.
Extrusion of aluminum is a highly versatile process used by companies large and small, producing products for markets ranging from transportation to building and construction to a wide variety of consumer products. According to the Aluminum Statistical Review for 2001, published by the Aluminum Association, extrusion and tube products represented 14.2% of the U.S. aluminum industry shipments in 2001. Previously in this column I have described an Aluminum Association-supported program underway at Cornell University to provide improved tools for structural designers to use in maximizing the benefits of aluminum extrusions. In addition to this work, there are also efforts directed towards improving the extruded aluminum product itself.
In the extrusion process, a cylindrical aluminum alloy billet at elevated temperature is forced through a die having the desired final product shape. While conceptually simple, the specifics of producing an extruded product with the desired shape is a much more complex problem. In addition to the requirement of meeting increasingly demanding tolerances and achieving the desired microstructure necessary for final properties, the process must also be economically viable. There are a number of important aspects to be considered in the extrusion process, one of which is consistently producing an extruded product surface with the desired mechanical and appearance characteristics. The broad category of surface quality for extrusions includes surface defects such as speed cracking, streaking, and roughening as well as the underlying surface grain structure. It is this latter area that will be the focus of this article. Some areas where surface grain structure can be important include finishing response, anisotropy of mechanical properties, and increased propensity for surface fracturing.
The process of extrusion die design primarily focuses on producing an extruded product with the required dimensional tolerances and macroscopic appearance that can be extruded as rapidly as possible to enhance productivity. The complex metal flow in the extrusion process often creates different deformation conditions in different regions of an extruded product. Features in the die such as variations in extrusion ratio within the part as well as choke and bearing modifications to control metal flow rates can affect the thermomechanical state of the extruded aluminum alloy. These factors, coupled with effects from the incoming extrusion billet structure as well as the extrusion temperature, can lead to variations in the grain structure, which are especially apparent at the extruded product surface.
The increased availability of advanced microstructural analysis techniques has helped to increase our understanding of the relationship of deformation and metallurgical conditions and the resulting surface grain structure. In particular, the electron backscattered diffraction (EBSD) technique has been used to reveal crystallographic texture of individual grains and substructure in deformed products, giving the metallurgist a clearer picture of microstructural evolution. Work in the extrusion area has been a focus of the Institute of Metal Forming under the direction of Prof. Wojciech Misiolek at Lehigh University. The article entitled "Effect of Die Design on Microstructure of Extruded Aluminum" by Claves, Misiolek, and Kelly, published in ET 2000: Proceedings of the Seventh International Aluminum Extrusion Technology Seminar provides an example of this approach in action. In this work, the effects of variations in extrusion die design were related to the resulting grain structure, as revealed by the EBSD technique, providing insight into the flow patterns and shear forces occurring within the die.
This work has now been extended through a DOE-OIT Aluminum Industries of the Future
project underway at Lehigh entitled "Surface Behavior of Aluminum Alloys Deformed
Under Various Processing Conditions". Being carried out with support from Alcoa,
Inc., the project seeks to develop the microstructural response models for surface
behavior and then integrate them with process models as a design package. Further
information on this project is available at http://www.eere.energy.gov/industry/aluminum/pdfs/alloysurfacebehavior.pdf
. With this type of information incorporated in appropriate models, the ability
to modify die designs for improved microstructural control may be possible.
A second initiative in the extrusion area is another DOE-OIT Aluminum Industries
of the Future project entitled "Superior Aluminum Extrusions". This is a collaborative
effort lead by Pacific Northwest National Laboratory and including two other National
Laboratories (Oak Ridge National Laboratory and Lawrence Livermore National Labroartory)
as well as commercial participants Alcoa and Boeing with the goal of developing
extrusion process and property prediction models for aluminum alloy extrusions
to increase material yield. A very ambitious program, it will address not only
the surface-related issues discussed previously but also mechanical properties
and residual stresses, seeking to produce a grand set of combined process models
for basically the entire extrusion processing operation. While still too early
in the project to know the success or failure of this approach, it does serve
to illustrate the interest and level of effort being directed to this area. Information
on this project is found at http://www.eere.energy.gov/industry/aluminum/pdfs/alextrusions.pdf.
No discussion of grain structure control in extruded products would be complete
without some reference to a very different approach that is under investigation
called equal channel angular extrusion. While rightly the subject of an article
all its own, briefly the purpose of this process is to produce ultrafine grain
structures through redundant deformation processes. While interesting academically
but not practically due to the limited size capability of the process, some recent
effort in modifying the process to be continuous bears watching. A brief description
of this approach is found at http://www.eere.energy.gov/industry/aluminum/pdfs/plasticdeformation.pdf.
Hopefully it is apparent from this brief review that there is a lot of interest in the area of grain structure quantification and control in extruded products. Certainly that is not to suggest that other factors are unimportant, and subsequent articles will address some of the key issues in extrusion as well as other wrought processing methods.
Article courtesy of Secat, Inc. - Research Resource for the Aluminum Industry
www.secat.net
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