Analysis of abnormal grain growth in 6061 aluminum alloy during high-temperature solution treatment following forging

Abstract

The conventional forging process for 6061 aluminum alloy typically involves hot forging followed by solution heat treatment (SHT). However, during this process, the grain size can rapidly grow to the millimeter scale, resulting in uncontrollable and undesired coarse grains. In this study, the misorientation at the growth front of abnormal grains in forged 6061 aluminum alloy during solution heat treatment was measured, estimating the grain boundary energy to satisfy the energy state conditions for triple junction line wetting. This indicates that some particles, distant from the abnormally grown grains, are actually identical to the abnormally grown grains and are three-dimensionally connected. An improved combined process was proposed to suppress abnormal grain growth: replacing air cooling with water quenching after hot forging and adding room-temperature deformation prior to heat treatment. The lower limit of single-pass deformation is set between 10 % and 20 %, with 20 % deformation in a single pass showing a noticeable suppressive effect on AGG without causing cracking, this approach ensures that even large free forgings, approximately 150 mm in cross-section, maintain normal grain structures in both the core and edge regions, the average grain size is approximately 80 μm.