Effect of hot forging condition on the microstructure formation process
in a 6061 aluminum alloy

Manabu Nakai*,** and Goroh Itoh***
*Casting & Forging Research Section, Aluminum & Copper Business,
Daian Plant, KOBE STEEL, LTD.
**Major in Materials Science, Graduate School of Science and Engineering, Ibaraki University
***Department of Mechanical Engineering, College of Engineering, Ibaraki University

Medium-strength 6061 aluminum alloy is a suitable material for general-purpose structural use because of a good corrosion resistance and a good toughness., Table 2, Table 3 The effect of hot forging conditions on the microstructure in a 6061 aluminum alloy has been closely investigated, and the mechanism for the microstructure formation has been discussed. Specimens of 6061 aluminum alloy ingots shown in Table 1 were hot-forged at various temperatures and strain rates as shown in Table 2, and then T6-tempered. Microstructures of as-forged as well as T6-tempered were characterized mainly with EBSD and TEM.

Table 3 shows the mechanical properties of the T6-tempered samples. Non-recrystallized structures were formed by hot forging at all the Zener Hollomon parameters (Z parameters) from 1.1106s-1 to 1.41012s-1, which consisted of grains surrounded by high and low angle grain boundaries (Fig. 1). The microstructure became refined with increasing Z (Figs. 2 and 3). This refinement was attributable to grain subdivision mechanism that occurred during hot forging. Because of finer microstructure prior of solution treatment and non-occurrence of recrystallization, the microstructure of middle Z was finer than that of low Z. In the low Z and the middle Z conditions, these microstructures were thermally stable, did not change even after high temperature solution heat treatment. In contrast, in the high Z condition, a small number of nuclei were considered to form in the deformation microstructure because of high density of dislocations, resulting in coarse recrystallized microstructure.

[Published in Journal JILM, Vol. 65, No. 4 (2015), 123-130]

Table 1 Chemical composition of the 6061alloy specimen in mass %.
Composition range standardized by Aluminum Association is also indicated.
Table 2 Hot forging conditions. Initial ingots with a diameter of 60 mm and height of 90 mm
were forged at constant temperatures by a reduction 71.1%.
T : temperature, 0 : initial strain rate, Z : Zener-Hollomon parameter.
Table 3 Mechanical properties of each specimen T6-tempered. σu : ultimate tensile strength,
σ0.2 : 0.2% proof stress, δ : elongation to failure.
Fig. 1 Orientation maps of the three T6-tempered specimens at t /2. Grain boundary map is superimposed. Boundaries with misorientation angle θ are shown in blue and red when θ15°and 15°>θ2°, respectively.
Fig. 2 Orientation maps of the three specimens after hot forging at t /2. Grain boundary map is superimposed. Boundaries with misorientation angle θ are shown in blue and red when θ15°and 15°>θ2°, respectively.
Fig. 3 TEM micrographs of the three specimens after forging at t /2.