On the Influence of Strain Rate and Number of Passes on Grain Refinement in Al-Mg-Si Alloy Processed by Cyclic Expansion Extrusion

The effects of the strain rate (3.1 × 10−2 s−1, 1.5 × 10−2 s−1 and 7.7 × 10−3 s−1) 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 × 10−3 s−1 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 µm 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 × 10−3 s−1. A reduction in the degree of dynamic recrystallization with an increasing strain rate (3.1 × 10−2 s−1 and 1.5 × 10−2 s−1) 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 (σo) greater as the strain rate decreased.