On the Influence of Strain Rate and Number of Passes on Grain Refinement in Al-Mg-Si Alloy Processed by Cyclic Expansion
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JMEPEG https://doi.org/10.1007/s11665-020-05264-0
On the Influence of Strain Rate and Number of Passes on Grain Refinement in Al-Mg-Si Alloy Processed by Cyclic Expansion Extrusion V. Babu, Balasivanandha Prabu Shanmugavel, and K.A. Padmanabhan Submitted: 23 April 2020 / Revised: 8 October 2020 / Accepted: 11 October 2020 The effects of the strain rate (3.1 3 1022 s21, 1.5 3 1022 s21 and 7.7 3 1023 s21) and the number of passes on grain refinement in an Al-Mg-Si alloy (AA 6063) processed by cyclic expansion extrusion (CEE) at 150 °C are discussed. Specimens processed up to 8 passes at a strain rate of 7.7 3 1023 s21 displayed an increased micro-hardness value of 114 HV and tensile strength of 195 MPa, which represent 200 and 65% increase, respectively, on the properties of the parent material. The electron backscatter diffraction images revealed that the slowest strain rate grain refinement was maximal on reaching an average grain size of 3 lm after 8 passes. The grain size distribution in the material after the CEE process was uniform, and a majority of grain boundaries were of the low-angle type. The TEM analysis also confirmed that after 8 passes, the grain refinement was maximum at a strain rate of 7.7 3 1023 s21. A reduction in the degree of dynamic recrystallization with an increasing strain rate (3.1 3 1022 s21 and 1.5 3 1022 s21) decreases the extent of fine grain formation. As a result, the strength properties were inferior at the higher strain rates due to the well-known Hall–Petch (H–P) effect, i.e., grain refinement decreases at the higher strain rates. In fact, the H–P relation is obeyed in the material processed at all the three different strain rates. Another feature is that in the CEE processed material, the H–P slope (ky) was less steep and the intercept value (ro) greater as the strain rate decreased. Keywords
cyclic expansion extrusion, grain refinement, mechanical properties, severe plastic deformation
1. Introduction Aluminum alloys are extensively used in aircraft, automobile and other structural applications (Ref 1, 2). In recent years, due to the demands from industries, an extensive research on aluminum alloys has been carried out to improve their mechanical properties. The 2xxx and 7xxx series alloys have been replaced by 6xxx alloys in many applications due to their good weldability, better formability and superior corrosion resistance (Ref 3). AA 6063 alloy in T6 condition contains phases of Mg2Si, FeAl3 and Fe3SiAl12. Silicon and magnesium combine to form Mg2Si (magnesium silicide). Magnesium silicide (Mg2Si) precipitates formed by the aging of AA 6063 alloy enable the alloy to attain full strength (Ref 4). However, the precipitation/age hardening improves the strength of the alloy only to some extent. Therefore, the focus has been on the development of ultrafine grained (UFG)/nanostructured (NS) aluminum alloys in order to improve the metallurgical and mechanical properties (Ref 5). Ultra- (100 nm-1000 nm) and nano-(< 100 nm) grain formation in aluminum alloys enhances the mechanical proper
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