Annealing of Aluminum and Aluminum Alloys: Part One

요약:

The main advantages of aluminum are well known including but not restricted to a high strength-to weight ratio, high wear resistance, high reflectivity, and excellent heat and electrical conductivity.
Annealing of aluminum alloys provides the opportunity to soften work hardened materials to relieve stresses and overall stabilize the properties.

Aluminum (Al) alloys are used in a wide variety of applications in the automobile, marine and aerospace industries, thanks to their low specific gravity, high strength-to weight ratio, high wear resistance, high reflectivity, excellent heat and electrical conductivity, low melting point, negligible gas solubility (with the exception of hydrogen), excellent castability and good corrosion resistance.

These material characteristics allow for aluminum to replace more traditional materials such as steel and cast iron without sacrificing performance or reliability.

The general types of heat treatments applied to aluminum and its alloys are:

  • Preheating or homogenizing, to reduce chemical segregation of cast structures and to improve their workability
  • Annealing, to soften strain-hardened (work-hardened) and heat treated alloy structures, to relieve stresses, and to stabilize properties and dimensions
  • Solution heat treatments, to effect solid solution of alloying constituents and improve mechanical properties
  • Precipitation heat treatments, to provide hardening by precipitation of constituents from solid solution.

The distorted, dislocated structure resulting from cold working of aluminum is less stable than the strainfree, annealed state, to which it tends to revert. Lower-purity aluminum and commercial aluminum alloys undergo these structural changes only with annealing at elevated temperatures. Accompanying the structural reversion are changes in the various properties affected by cold working. These changes occur in several stages, according to temperature or time, and have led to the concept of different annealing mechanisms or processes.

Recovery. The reduction in the number of dislocations is greatest at the center of the grain fragments, producing a subgrain structure with networks or groups of dislocations at the subgrain boundaries. With increasing time and temperature of heating, polygonization becomes more nearly perfect and the subgrain size gradually increases. In this stage, many of the subgrains appear to have boundaries that are free of dislocation tangles and concentrations. Recovery annealing is also accompanied by changes in other properties of cold worked aluminum. Complete recovery from the effects of cold working is obtained only with recrystallization.

Recrystallization is characterized by the gradual formation and appearance of a microscopically resolvable grain structure. The new structure is largely strain-free-there are few if any dislocations within the grains and no concentrations at the grain boundaries.

Grain Growth After Recrystallization. Heating after recrystallization may produce grain coarsening. This can take one of several forms.

The aim of the study of Young Gun Ko and Kotiba Hamad was to investigated the effects of heat treatment (annealing) on the microstructure of ultrafine grained 6061 Al alloy samples fabricated by a differential speed rolling (DSR) process. The samples were fabricated using two passes DSR with 75% thickness reduction and a speed ratio of 1:4. The DSR-deformed 6061 Al alloy sample exhibited a lamellar boundary structure composed mostly of subgrains surrounded by low-angle grain boundaries. After annealing, the DSR-deformed 6061 Al alloy samples exhibited coarse grained structure and transformed from lamellar to equiaxed, where both the grain size and grain shape aspect ratio increased with increasing annealing temperature. The fraction of grain boundaries with high misorientation angles increased progressively during annealing, to ~77% at annealing temperature of 350°C.

The chemical composition of the 6061 Al alloy sheets used in the present work was (in wt %): 0.9 Mg, 0.71 Si, 0.5 Fe, 0.24 Cu, 0.19 Cr, 0.12 Mn, 0.05 Zn, 0.05 Ti, with Al comprising the balance. The as-received sheets were cut into small plates, 0.4 cm (thickness), 4 cm (width) and 10 cm (length). The plates were then homogenized for 3 h at 550°C and air cooled to obtain a fully annealed structure (Figure 1). The homogenized plates were then subjected to the room-temperature DSR deformation using two identical rolls (220 mm in diameter). To impose the shear deformation by DSR, a speed ratio of 1:4 was used, where the lower roll speed was fixed to ~3 m/min. The plates were subjected to two passes with 50% thickness reduction per pass, so that the total thickness reduction was 75% after the DSR deformation (two passes). During the DSR deformation, the plates were rotated by 180° around their rolling direction between the adjacent passes. This route was found to be beneficial for controlling the material with a fine grain structure. The heat treatment was carried out on the DSR-deformed plates for 1 h at various temperatures of 150–400°C with an interval of 25°C.



Figure 1: a) Homogenization conditions of the as-received 6061 Al alloy sheets and b) the obtained microstructure (DSR: Differential Speed Rolling: A.C. Air Cooled)

Figure 2a shows microhardness of the DSR-deformed 6061 Al alloy samples as a function of the annealing temperature. The curve obtained in Figure 2a exhibited the following three different regions:
• the microhardness decreases slightly until ~225°C. This range corresponds to the recovery in the materials
• The microhardness decreases sharply, i.e., at a steeper slope on the curve, from 225 to 350°C and
• the curve levels off at temperatures higher than 350°C. The sharp decrease of the microhardness in the second region of the curve suggests the occurrence of material softening induced by recrystallization.

Figure 2b shows the softened fraction at the various annealing temperatures used in this work. After annealing at 225°C for 1 h, the estimated softened fraction of the annealed sample showed a ~2.99% reduction compared to the DSR-deformed sample. The softened fraction of the sample increased significantly after annealing at temperatures between 225 and 350°C, where the samples annealed at 275, 325 and 350°C showed ~45%, ~79.8% and ~99.8% softened fraction compared to the as-deformed sample.



Figure 2: Effect of the annealing temperature on (a) microhardness and (b) softened fraction of the DSR-deformed 6061 Al alloy. The different color remarks along the dashed line in this figure indicate the annealed samples used to investigate the microstructural evolution during annealing.

The microstructural evolution of the samples annealed at 225°C, 275°C, 325°C and 350°C, which belong to the second region (ii) as indicated by the colored symbols in Figure 2a, were investigated to understand the softening behavior of the DSR-deformed 6061 Al alloy plates during the heat treatment. The EBSD maps (grain boundaries maps) in Figure 3 showed that the lamellar structure was gradually broken-down to form the equiaxed grains. On the other hand, at 275°C, some areas showed the lamellar structure obtained by the initial DSR deformation, as indicated by the dashed rectangles in Figure 3b. This was shown further by partitioning of the recrystallized grains of the samples treated at 225, 275, 325 and 350°C using the criteria of grain orientation spread GOS (Figure 3e–h). The most obvious and expected observation from Figure 3e-h is that the fraction of the recrystallized grains increased with increasing the temperature, where recrystallizations corresponding to ~18%, ~31%, 52% and ~84% were measured after the heat treatment at 225, 275, 325 and 350°C, respectively. In addition, the larger fraction of recrystallized microstructures of the sample annealed at 350°C (Figure 3h) resulted in the low microhardness values, as shown in Figure 2a.



Figure 3: Grain boundaries maps and recrystallized microstructures of the DSR-deformed 6061 Al alloy annealed for 1 h at 225°C (a, e), 275°C (b, f), 325°C (c, g), 350°C (d, h).


References

1. M. Fawzy Ibrahim: Effects of Be, Sr, Fe and Mg interactions on the microstructure and mechanical properties of aluminum based aeronautical alloys, PhD thesis, University of Quebec at Chicoutimi, February 2015, Accessed July 2018;

2. N. Hurley: Microstructural evolution during flash annealing of hot rolled 6061 aluminum alloys, 2006, Theses and Dissertations, Paper 915, Lehigh University, MSc thesis;

3. Heat treating of aluminum and aluminum alloys, Accessed July 2018;

4. Y. Gun Ko, K. Hamad: Annealing Behavior of 6061 Al Alloy Subjected to Differential Speed Rolling Deformation, Metals 2017, 7, 494; p.2-10, doi:10.3390/met7110494.

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