The Effect of Four Types of Heat Treatment on the Mechanical Properties of Al 6351 Alloy

Abstract

The present research focuses on examining the alterations in the microstructural characteristics and mechanical properties of Al 6351 alloy as a result of various thermal treatments. Four distinct processes were analyzed: solution treatment followed by quenching and aging (T6), retrogression and re-aging (RRA), high-temperature pre precipitation (HTPP), and a modified version of RRA involving interrupted aging. The T6 treatment produced the finest dispersion of precipitates, achieving maximum ultimate tensile strength (UTS) of 320 MPa and yield strength (YS) of 270 MPa; however, it decreased ductility to 12% due to limited dislocation movement caused by the precipitates, which were shown to become immobilized at high concentrations. The RRA treatment exhibited a slight improvement in yield strength, resulting in UTS and YS of 290 MPa and 10% elongation, respectively, as a consequence of a finer and more uniform precipitate distribution after re-aging. HTPP, on the other hand, demonstrated a reduction in strength compared to T6, with UTS and YS at 310 MPa and 260 MPa, respectively, attributed to precipitate coarsening occurring during pre-aging at elevated temperatures. The increasing ductility trend of 13% was associated with initial GP zone size. The modified RRA process yielded the highest strength levels, reaching UTS of 350 MPa, YS of 300 MPa, and an enhanced toughness reflected in improved impact energy of 20 J, likely due to the denser alignment of precipitates; however, it showed slightly lower elongation of 11% than T6 or RRA. The tensile fracture behavior was elucidated through FESEM-based fractography analysis, which revealed cracks propagating uniformly along the length. The results of this investigation highlight the necessity of precisely controlling heat treatment parameters to achieve specific mechanical properties in Al 6351. This research provides valuable insights into managing strength, ductility, and toughness via solid solution grain size distribution and the density of Mg2Si precipitates, which is beneficial for selecting appropriate heat treatments across various industries.