The Effect of Die Bearing Geometry on Surface Recrystallization During Extrusion of an Al-Mg-Si-Mn Alloy
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LOCALIZED recrystallization on the surface of an extrudate, known as a Peripheral Coarse Grain (PCG) layer, is a common occurrence during extrusion of medium- to high-strength extruded Al-Mg-Si-Mn alloys such as AA6082. The PCG layer is known to degrade
Y. MAHMOODKHANI and M.A. WELLS are with the Mechanical and Mechatronics Engineering Department, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L3G1, Canada. Contact e-mail: [email protected] J. CHEN and W.J. POOLE are with the Department of Materials Engineering, The University of British Columbia, 309-6350 Stores Road, Vancouver, BC V6T1Z4, Canada. N.C. PARSON is with Rio Tinto Aluminum, Arvida Research and Development Center, Saguenay, QC G7S4L2, Canada. Manuscript submitted February 6, 2019. Article published online August 30, 2019 5324—VOLUME 50A, NOVEMBER 2019
properties such as surface quality, strength, fracture toughness, stress corrosion susceptibility, fatigue resistance, and machinability.[1–3] It has been shown that the formation and severity of the PCG layer depend on alloy composition, extrusion temperature, extrusion ratio, and the homogenization practice.[1,4,5] Typically, the formation and thickness of the PCG layer are exacerbated by higher extrusion temperatures, faster velocities, higher extrusion ratios, slower cooling rates after extrusion, and the use of alloys which do not form dispersoids (i.e., Mn/Cr bearing precipitates 20 to 100 nm in size formed during homogenization).[6] The mechanism responsible for the formation of the PCG layer is still not fully developed but theories point to the characteristics of the surface layer where a combination of very large shear deformation (equivalent strains > 10) and high deformation temperatures (near the melting point of the alloy) develop due to the friction between the extrudate and the die. For example, van METALLURGICAL AND MATERIALS TRANSACTIONS A
Geertruyden et al.,[4] de Peri and Misiolek[2] proposed that a very fine equiaxed grain structure is produced at the surface by Continuous Dynamic Recrystallization (CDRX) or Geometric Dynamic Recrystallization (GDRX). It was hypothesized that this microstructure was highly unstable, with a high local stored energy and a high degree of heterogeneity which under the right conditions could undergo abnormal grain growth to produce large surface grains. On the other hand, Eivani[3] proposed that the key processing parameter responsible for the formation of the PCG layer in Al-Mg-Zn alloys (AA7020) is temperature. For example, in this work, no evidence of a PCG layer was found with an extrusion temperature of 460 C and a ram velocity of 5 mm/s, but PCG was observed for extrusion temperatures above 500 C using the same ram velocity. Further, Charit and Mishra[7] studied the formation of abnormal grains in friction stir welding of Al-Mg-Zn (7xxx) alloys and suggested that it was the dissolution of Mg-Zn containing precipitates which would allow recrystallization/grain growth to occur by reducing the local Smith–Zener drag from the
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