Hot Extrusion of Nanostructured Al-Powder Alloys: Grain Growth Control and the Effect of Process Parameters on Their Mic
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UCTION
NANOSTRUCTURED materials have been the subject of extensive research in recent years thanks to their remarkable potential in terms of physical and mechanical properties.[1–4] A variety of techniques, such as mechanical alloying and rapid solidification, have been applied successfully to prepare nanostructured materials.[5] However, when these materials are prepared by the aforementioned methods, they usually result in powders or foils that cannot be used directly in structural applications. Therefore, the development of consolidation processes suitable for nanostructured materials, particularly of nanostructured powder materials, is of great interest. Processing powder into a bulk nanostructured material offers significant challenges. The low temperature required to prevent grain growth limits the processing window (maximum temperature and time at maximum temperature) to which the powder can be exposed with modest or no grain growth. The research published to date has shown that extrusion is one of the most effective processes for the consolidation of powder materials.[6–9] In the present work, we investigated the influence of the two main technical parameters—strain rate and temperature—of A.M. JORGE, Jr., M.M. PERES, C.S. KIMINAMI, C. BOLFARINI, and W.J. BOTTA, Professors, are with the Department of Materials Engineering, Federal University of Sa˜o Carlos, Rod. Washington Luiz, km 235, 13565-905 Sa˜o Carlos, SP, Brazil. Contact e-mail: [email protected] J.B. FOGAGNOLO, Professor, is with the Materials Engineering and Science Post Graduation Program, Sa˜o Francisco University, 13251-900 Itatiba, SP, Brazil. Manuscript submitted May 19, 2008. Article published online September 17, 2009 3314—VOLUME 40A, DECEMBER 2009
hot extrusion on the microstructure and mechanical properties of bulk material consolidated from nanostructured aluminum alloy powder. We have chosen to study two Al-based compositions due to their importance as structural materials in several applications, where low density, high mechanical strength, and corrosion resistance are desirable.
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MATERIALS AND METHODS
Rapidly solidified aluminum alloy powder with a nominal composition of Al-3.0Fe-0.42Cu-0.37Mn (wt pct) (to be referred to as Al(Fe)), rich in precipitates, was prepared using argon gas atomization. This procedure resulted in nanostructured aluminum alloy powder whose microstructure showed an average grain size of 25 nm. The average particle size was 75 lm. Cold pressing was used to transform the nanostructured powder into cylindrical preforms with a relative density of about 0.96 and with an initial diameter of 26.2 mm, in preparation for consolidation processing. The preforms were then consolidated into 7.9-mm-diameter bars of bulk material by hot extrusion at three different temperatures, 375 C, 400 C, and 425 C; an extrusion ratio (ER) of 10:1; and ram speeds (RSs) of 1, 15, and 30 mm/min. For purposes of comparison, the effect of precipitation on the microstructure and mechanical properties of the Al(Fe) alloy was compared to t
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